ACS Publications. Most Trusted. Most Cited. Most Read
Quick View

OR SEARCH CITATIONS

My Activity
Publications
CONTENT TYPES

Figure 1Loading Img
Download Hi-Res ImageDownload to MS-PowerPointCite This:ACS Catal. 2019, 9, 2, 885-889
RETURN TO ISSUEPREVLetterNEXT

Computational Prediction and Experimental Verification of ε-Caprolactone Ring-Opening Polymerization Activity by an Aluminum Complex of an Indolide/Schiff-Base Ligand

  • Mukunda Mandal
    Mukunda Mandal
    Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
    More by Mukunda Mandal
    Orcidhttp://orcid.org/0000-0002-5984-465X
  • , 
  • Anna M. Luke
    Anna M. Luke
    Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
    More by Anna M. Luke
    Orcidhttp://orcid.org/0000-0001-9486-5635
  • , 
  • Büşra Dereli
    Büşra Dereli
    Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
    More by Büşra Dereli
  • , 
  • Courtney E. Elwell
    Courtney E. Elwell
    Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
    More by Courtney E. Elwell
    Orcidhttp://orcid.org/0000-0002-4478-9977
  • , 
  • Theresa M. Reineke
    Theresa M. Reineke
    Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
    More by Theresa M. Reineke
    Orcidhttp://orcid.org/0000-0001-7020-3450
  • , 
  • William B. Tolman*
    William B. Tolman
    Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
    Department of Chemistry, Washington University in St. Louis, One Brookings Drive, Campus Box 1134, St. Louis, Missouri 63130, United States
    *E-mail: [email protected]
    More by William B. Tolman
    Orcidhttp://orcid.org/0000-0002-2243-6409
  • , and 
  • Christopher J. Cramer*
    Christopher J. Cramer
    Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
    *E-mail: [email protected]
    More by Christopher J. Cramer
    Orcidhttp://orcid.org/0000-0001-5048-1859
Cite this: ACS Catal. 2019, 9, 2, 885–889
Publication Date (Web):December 31, 2018
https://doi.org/10.1021/acscatal.8b04540

Copyright © 2018 American Chemical Society. This publication is licensed under these Terms of Use.

Request reuse permissions
  • Open Access

Article Views

2665

Altmetric

-

Citations

19
LEARN ABOUT THESE METRICS
PDF (1 MB)
Get e-Alerts
Supporting Info (3)»
SUBJECTS:

Abstract

High Resolution Image
Download MS PowerPoint Slide

Computational screening of a series of aluminum complexes for their activity in the ring-opening transesterification polymerization (ROTEP) of ε-caprolactone (CL) was performed using a “framework distortion energy” (FDE) hypothesis. An {N,N,N,N}-aluminum complex with a bis-indolide Schiff-base ligand was predicted on the basis of that screening to be an efficient catalyst, and this prediction was tested and verified experimentally through the synthesis and characterization of the complex and evaluation of its ROTEP reactivity.

KEYWORDS:

Introduction

ARTICLE SECTIONS
Jump To
Conversion of biomass-derived cyclic esters to aliphatic polyesters offers an attractive approach for sustainable polymer production. (1) A key synthetic approach is ring-opening transesterification polymerization (ROTEP) catalyzed by metal alkoxide complexes. (2) Among the plethora of such complexes that have been studied for such purposes, salen-based aluminum complexes (salen = N,N’-bis(salicylaldimine)-1,2-ethylenediamine) are especially popular because of their high Lewis acidity, low toxicity, and high tunability, allowing systematic exploration of steric/electronic factors on ROTEP rates, molecular weight control, and selectivities. (3)
Over the past decade, experiment and theory combined have made significant progress with respect to elucidating the details of ROTEP mechanisms. (4) A “coordination–insertion” pathway (Scheme 1) is usually invoked as a preferred propagation route that proceeds through four elementary steps: (i) coordination of monomer to the penta-coordinated [Al]-OR catalyst, (ii) insertion of the alkoxide into the activated monomer ester via TS1, which is typically the turnover-limiting (TOL) step, (iii) rearrangement of the tetrahedral orthoester intermediate with respect to which oxygen atoms interact most closely with the [Al]-center, and, finally, (iv) ring-opening ester bond cleavage (via TS2) to generate the polymer chain alkoxide terminus elongated by one unit. The same mechanism is also active for initiation, with energetic details differing somewhat depending on how closely the precatalyst −OR group resembles the alkoxide terminus of the growing polymer (from the standpoint of assessing structure–activity relationships, modeling initiation can be more efficient as it avoids the complexities associated with long, flexible growing polymer chains).

Scheme 1

Scheme 1. Coordination–Insertion Mechanism for ROTEP
High Resolution Image
Download MS PowerPoint Slide
Ligand structure and coordination environment have been identified as two crucial factors that determine the reactivity of a given ROTEP catalyst. For example, an 11-fold rate increase during rac-lactide (LA) polymerization was observed using a longer backbone tether for an Al-salen catalyst, and this finding was expanded upon in later work with ε-caprolactone (CL). (4c,5) Conversely, increased rigidity in the catalyst backbone—engineered using aryl-fused tethers rather than alkyl tethers—resulted in decreased rates of LA polymerization. (6) In other recent work, an Al complex of dibenzotetramethyltetraazaannulene (TMTAA) was found to be an especially sluggish catalyst for CL polymerization, which was attributed to the high rigidity of the TMTAA ligand that made it energetically costly to reach the octahedral transition-state (TS) structure associated with the TOL step. (7) Given the significant influence that the ligand framework can have on polymerization rate, the development of efficient computational approaches to screen alternative ligands for activity has the potential to speed experimental testing through the prioritization of synthetic targets.
The Framework Distortion Energy (FDE) hypothesis (4c,8,9) can be a useful tool for just such screenings. The FDE descriptor rationalizes structure–activity variations for series of catalysts where the geometries of the TOL TS structures are all similar, at least with respect to coordination about the active site. For example, the geometries of the TOL TS structures for polymerization of CL by a series of 8 Al-based catalysts, differing only in the imine tether, were reported to have bond lengths and bond angles about the central Al-atom having mean signed deviations from their means of only ∼0.01 Å and 3°, respectively. (4c) In such instances, the energy required to distort the framework of the precatalyst so as to adopt the “optimal” TS structure, which quantitatively defines the FDE, correlates closely with the overall activation free energy (ΔG⧧) of the reaction. In practice, FDE can be computed by taking the difference in the electronic energies of two frozen ligand frameworks: one derived by the removal of the alkoxy group of the optimized “resting” precatalyst and the other by removing both the alkoxy and CL from the optimized TS1. Still more rapid screenings may in principle be accomplished by computing the energy for the latter structures by first imposing a frozen set of geometric parameters about the metal center for all putative TS structures and then simply relaxing the remaining geometric degrees of freedom, thereby obviating the need for specific TS structure optimizations, although the utility of this simplified approach will perforce degrade as explored ligand sets increase in diversity.
In this work, we report the systematic investigation of ligands screened using the FDE model with the goal of identifying catalysts predicted to have comparable or higher ROTEP activities than previously achieved for the polymerization of CL. In addition to FDE, we analyze structural variations in precatalysts as reflected by the geometry index τ5 (for a pentacoordinate atom, values of τ5 range from 0 to 1 as the local coordination environment varies from square pyramidal (sp) to trigonal bipyramidal (tbp)), (10) and also electronic structure variations associated with different coordinating heteroatoms in the ligand framework. As a result of the screening, the synthesis of one catalyst predicted to have promise was undertaken, and its catalytic activity for CL polymerization was evaluated.

Results and Discussion

ARTICLE SECTIONS
Jump To

Theory

The ROTEP of CL is notably rapid for complex 1 (Figure 1a) relative to other Al catalysts. (8) The mechanistic details associated with catalysis by 1 have been explored at the density functional level of theory (DFT), with ΔG⧧(TS1) and FDE computed to be 7.8 and 12.7 kcal/mol, respectively, at the SMD(CH2Cl2)/M06-2X-D3/6-311+G(d,p)//M06-L/6-31+G(d,p) level of theory (see the SI for full computational details). (8b) We therefore set as our goal, with respect to in silico screening of alternative catalysts, to identify ligands predicted to lead to still lower values of ΔG⧧(TS1) and/or FDE than those computed for 1.

Figure 1

Figure 1. (a) Parent salen catalyst and modifications with additional alkyl tethers. (b) Pyridine-based systems as a side arm modification to salen. (c) Pyrrole/indole-based {N,N,N,N}-complexes.

High Resolution Image
Download MS PowerPoint Slide
Our first design efforts focused on modification of the parent system 1 through the introduction of additional tethers, of varying length, bridging other atoms of the salen moiety (identified as Pn, Qn, and Rn in Figure 1a, where n refers to the number of methylene units in the tether). Our goal was to constrain the precatalyst to a geometry more closely resembling the corresponding TS1 structure, thereby reducing FDE. However, extensive surveys of frameworks 2–4 for varying values of n failed to identify any with FDE or ΔG⧧(TS1) values less than those for 1. In the interest of brevity, we summarize the results for these systems only in the Supporting Information.
As favorable constraints on the precatalyst geometries were not achieved with additional tethers, we turned next to explore incorporation of the imine functionality of the salen ligand into pyridine moieties (5 and 6, Figure 1b). In this instance, the goal was to tune both the steric and electronic aspects of the catalyst, with the hypothesis being that the pyridines would modulate the Lewis acidity of the metal center through their π-donor character while simultaneously reducing steric demand associated with the overall tether. Interestingly, both 5 and 6 are predicted to be potentially active for CL homopolymerization with ΔG⧧(TS1) ∼ 6.5 kcal/mol and FDE ∼ 10.5 kcal/mol, whose values are indeed below those computed for 1. However, we concluded that the ligands themselves and their assembly about the catalytic center were likely to pose significant synthetic challenges, and on that basis, we decided instead to explore further modifications in the ligand framework, and in particular to evaluate the replacement of O-donors with N-alternatives (7–12, Figure 1c). Complex 7 with its bis(pyrrolide) Schiff-base ligand was reported previously. (11) In contrast to the half-salen systems reported by Chen and co-workers, (12) where a five-membered-ring chelate of the ligand at Al leads to faster catalysis during CL polymerization than a six-membered one, we predict 8 to be significantly faster than 7 for the same reaction (Table 1). The larger ligand arm of 8 permits it to adopt the octahedral geometry of the TOL TS1 more readily, as reflected by its FDE of 8.5 kcal/mol compared to 12.2 kcal/mol for 7.
Table 1. Computed Geometry Indices (τ5), Activation Free Energies (kcal/mol), Framework Distortion Energies (FDE, kcal/mol), and Key Bond Lengths (Å) for N4-Donor Systemsa
catalystτ5ΔG⧧(TS1)FDEr1(Al—OMe)r2(Al—O═C)
10.717.812.71.7651.926
70.4210.612.21.7561.908
80.755.58.51.7621.912
90.807.611.61.7721.937
100.796.811.11.7711.929
110.806.211.91.7641.917
120.844.712.61.7551.908
a

Calculations performed at the SMD(CH2Cl2)/M06-2X-D3/6-311+G(d,p)//M06-L/6-31+G(d,p) level of theory.

We also explored changing the pyrrolide functionality to indolide (9), para-bromoindolide (10), benzimidazolide (11), and benzotriazolide (12). Table 1 further compares key energetic and structural parameters for catalysts 1 and 7–12. We observe that the ΔG⧧(TS1) values for 8–12 are all low, implying rapid ROTEP rates, and a comparison of structures and energetics for 1 vs 9 (Figure 2) suggests the ligand system of the latter to have significant potential. In addition, when the indolide ligands in 9 are modified to make them less electron donating, either by introducing bromo-substituents (10) or by introducing additional N atoms in the five-membered ring (11 and 12), the catalyst is predicted to be more Lewis acidic, as judged by a shortening of both r1, the metal–alkoxide bond distance in the catalyst, and r2, the metal–carbonyl bond distance in TS1, and there are corresponding improvements in predicted catalytic activity as judged by decreasing values of ΔG⧧(TS1). Interestingly, there is considerably less variation in the predicted FDE, which is consistent with the relatively limited variation in τ5 predicted for the precatalyst structures. Put differently, with τ5 values already so close to 1.0, additional acceleration must be achieved by tuning of the overall electronic structure toward enhanced Lewis acidity.

Figure 2

Figure 2. Optimized structures for 1 and 9 and their corresponding turnover-limiting TS structures for CL polymerization.

High Resolution Image
Download MS PowerPoint Slide

Experiment

Given the computational prediction of good activity for 9 coupled with its perceived synthetic accessibility, we undertook the synthesis of its benzyloxy analogue, 14 (Figure 3). Condensation of 7-indolecarboxaldehyde and 2,2-dimethylpropane-1,3-diamine gave the new ligand precursor, LH2, in high purity and good yield (81%). Characterization by NMR spectroscopy (Figure S6), high resolution mass spectrometry, and CHN analysis confirmed its formulation. Notably, the 1H NMR spectrum revealed a shift of aromatic peaks from the carboxaldehyde and the disappearance of both the CHO and NH2 proton resonances from the starting materials concomitant with the appearance of a singlet imine peak at ∼8.5 ppm. Metalation with excess diethylaluminum ethoxide (Et2AlOEt) at 70 °C for 4 days yielded 13, which was isolated as a yellow precipitate in modest yield (23%; a 1H NMR spectrum of the supernatant contained a complex mixture of peaks). The complex was identified by CHN analysis and NMR spectroscopy, with clearly shifted resonances for the ligand hydrogen atoms relative to LH2 and replacement of the N–H signals with peaks for an ethyl ligand being key indicators for the indicated structural formulation (Figure S7). Subsequent treatment of a solution of 13 in CH2Cl2 with BnOH (1 equiv) gave complex 14 as a pale-yellow solid (73%). This complex was characterized by NMR spectroscopy (Figure S8), CHN analysis, and X-ray crystallography (Figure 3).

Figure 3

Figure 3. Synthesis of ligand precursor (LH2) and complexes 13 and 14, with a representation of the X-ray crystal structure of 14 shown as 50% thermal ellipsoids (nonhydrogen atoms only). Selected distances (Å) and angles (deg): Al–N1, 1.9388(19); Al–N2, 2.0117(19); Al–N3, 2.074(2); Al–N4, 1.905(2); Al–O1, 1.7484(19); N1–Al–N2, 90.13(9); N1–Al–N3, 173.78(5); N1–Al–N4, 95.63(8); N2–Al–O1, 121.78(8); N2–Al–N4, 114.95(7).

High Resolution Image
Download MS PowerPoint Slide
The X-ray structure of 14 revealed a mononuclear, 5-coordinate complex with a distorted tbp geometry (τ5 value of 0.87). The overall geometry is similar to that calculated for methoxy analogue 9 (τ5 value of 0.80). The indolate–Al distances observed (Al–N1 and Al–N4 in Figure 3) fall well within the range of other indolide–metal bond distances in the literature (ranging from 1.88 to 2.20 Å). (13) In adopting the tbp structure shown, the chelating ligand in 14 is highly twisted, resulting in unique environments for each indolide ring. If this structure were present in solution, the NMR spectrum would be expected to exhibit distinct resonances for each indolide ring and the diastereotopic backbone hydrogen atoms. However, the 1H and 13C NMR spectra (Figure S8) show only one set of sharp indolide peaks. These findings are inconsistent with a rigid structure in solution akin to that seen in the solid state, and suggest an averaged structure, perhaps due to fluxionality that has implications for stereoselective ROTEP reactions (currently under investigation).
The kinetics for the polymerization of CL were measured by mixing precatalyst 14, internal standard (bis-para-trimethylsilylbenzene), and monomer (targeting concentrations of 0.007, 0.004, and 2.0 M, respectively, in CD2Cl2) and monitoring the loss of monomer and gain in polymer via 1H NMR spectroscopy at 300 K. Quadruplicate data were collected and integrated through use of Mestrenova software to yield concentrations of reactant and product, and the resulting concentration vs time data were fit using COPASI software (Figures S10 and S11). (14) The relative merits of both first-order and saturation (Michaelis–Menten) kinetic fits were unclear (Figures S10 and S11). In an independent test of the importance of monomer coordination (implicit in the saturation fits), we examined the 1H NMR spectra of solutions of 14 in the presence of variable concentrations (0.005–1.5 M) of γ-butyrolactone (BL, not easily ring-opened due to lack of ring strain). (15) Only slight changes in the chemical shifts of the peaks associated with 14 were observed, and these were consistent with the solvent polarity changes due to the added BL; i.e., we saw no evidence for binding of BL to the complex. We thus conclude that if CL binding occurs, it must be quite weak, and that the simpler pseudo-first-order fits of the kinetic data are preferable for obtaining useful rate constants. The fits yielded an averaged first-order rate constant (kobs) of 4.1 × 10–4 s–1. While theoretical computation would suggest that 14 (by analogy to 9) should be about as reactive as 1 for ROTEP of CL, experimentally we observed it to be 5.5-fold slower (times to reach 95% conversion are 20 min for 1 and 110 min for 14, under equivalent conditions of [CL]0 = 2 M, [cat]0 = 7 mM, 300 K). Such a discrepancy—roughly 1 kcal/mol at 298 K—is, however, well within the error one might expect for a DFT-based model, and suggests that framework distortion energy can indeed be a practical descriptor for estimating the ROTEP activity of hypothetical future catalysts, at least with respect to prioritizing (i) more complete computational characterization of reaction paths (e.g., 12, where a relatively low FDE is associated with a ΔG⧧(TS1) value that is 3.1 kcal/mol lower than that for 1) and (ii) promising targets for synthetic realization.

Conclusions

ARTICLE SECTIONS
Jump To
In summary, we have explored new aluminum-based ROTEP catalysts in silico by means of DFT calculations and verified the proposed FDE model by synthesizing and evaluating one such hypothetical catalyst, predicted to offer high ROTEP activity, experimentally. While additional tethers on the salen moiety in the form of Pn, Qn, and Rn series of catalysts did not prove useful in lowering either the calculated FDE, or the ΔG⧧ of the TOL TS1, ligand side arm modification with pyridine donors showed promise by modulating both the overall steric and electronic features of the catalyst. An alternative modification, involving the replacement of all O-donors with N-counterparts, provided a new class of ROTEP precatalysts having highly distorted tbp geometries—a feature that generally promotes ROTEP catalysis. Indeed, the synthesized variant 14 polymerized CL rapidly, consistent with theoretical predictions. In line with prior CL-ROTEP studies, (4b,5,16) electron withdrawing groups on the five-membered ring of the supporting ligand of 9 show promise for even faster catalysts in the form of 10–12. Agreement within 1 order of magnitude was observed when theory, using predicted ΔG⧧(TS1) values, was compared to experiment for the specific case of the ROTEP rates of 1 vs 9. Using the FDE model to screen hypothetical catalysts prior to subsequent more complete computational evaluation—and ultimate experimental realization—continues to hold promise for future catalyst design in suitably related systems.

Supporting Information

ARTICLE SECTIONS
Jump To

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b04540.

  • Computational details, spectroscopic and kinetic data, and additional figures including structures, 1H and 13C NMR spectra, aromatic region overlay, [CL] decay and [PCL] growth over time, and first-order decay plot (PDF)

  • Cartesian coordinates of stationary points (XYZ)

  • X-ray crystallography data (CIF)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

ARTICLE SECTIONS
Jump To
  • Corresponding Authors
    • William B. Tolman - Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States;  Department of Chemistry, Washington University in St. Louis, One Brookings Drive, Campus Box 1134, St. Louis, Missouri 63130, United States;  Orcidhttp://orcid.org/0000-0002-2243-6409;  Email: [email protected]
    • Christopher J. Cramer - Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States;  Orcidhttp://orcid.org/0000-0001-5048-1859;  Email: [email protected]
  • Authors
    • Mukunda Mandal - Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States;  Orcidhttp://orcid.org/0000-0002-5984-465X
    • Anna M. Luke - Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States;  Orcidhttp://orcid.org/0000-0001-9486-5635
    • Büşra Dereli - Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States
    • Courtney E. Elwell - Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States;  Orcidhttp://orcid.org/0000-0002-4478-9977
    • Theresa M. Reineke - Department of Chemistry, Center for Sustainable Polymers, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States;  Orcidhttp://orcid.org/0000-0001-7020-3450
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

ARTICLE SECTIONS
Jump To

Funding for this project was provided by the Center for Sustainable Polymers, a National Science Foundation supported Center for Chemical Innovation (CHE-1413862). The X-ray diffraction experiment was performed using a crystal diffractometer acquired through NSF MRI Award CHE-1229400. The NMR experiments were performed on Bruker Avance III 500 MHz spectrometers acquired through NIH Award S10OD011952. We thank Dr. Letitia Yao for her help with NMR experiments.

References

ARTICLE SECTIONS
Jump To

This article references 16 other publications.

  1. 1
    (a) Schneiderman, D. K.; Hillmyer, M. A. 50th Anniversary Perspective: There Is a Great Future in Sustainable Polymers. Macromolecules 2017, 50, 3733– 3749,  DOI: 10.1021/acs.macromol.7b00293
    Google Scholar
    1a
    50th Anniversary Perspective: There Is a Great Future in Sustainable Polymers
    Schneiderman, Deborah K.; Hillmyer, Marc A.
    Macromolecules (Washington, DC, United States) (2017), 50 (10), 3733-3749CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    A review. It is likely that a half-century ago even enthusiastic and optimistic proponents of the synthetic polymer industry (Mr. McGuire included) could not have predicted the massive scale on which synthetic polymers would be manufd. and used today. Ultimately, the future success of this industry will rely on the development of sustainable polymers-materials derived from renewable feedstocks that are safe in both prodn. and use and that can be recycled or disposed of in ways that are environmentally innocuous. Meeting these criteria in an economical manner cannot be achieved without transformative basic research that is the hallmark of this journal. In this Perspective we highlight five research topics-the synthesis of renewable monomers and of degradable polymers, the development of chem. recycling strategies, new classes of reprocessable thermosets, and the design of advanced catalysts-that we believe will play a vital role in the development of sustainable polymers. We also offer our outlook on several outstanding challenges facing the polymer community in the broad area of sustainable polymers.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXmvVSnsro%253D&md5=aef74ce8b1bc164a33f43c5942bc20e7
    (b) Corma, A.; Iborra, S.; Velty, A. Chemical Routes for the Transformation of Biomass into Chemicals. Chem. Rev. 2007, 107, 2411– 2502,  DOI: 10.1021/cr050989d
    Google Scholar
    1b
    Chemical Routes for the Transformation of Biomass into Chemicals
    Corma, Avelino; Iborra, Sara; Velty, Alexandra
    Chemical Reviews (Washington, DC, United States) (2007), 107 (6), 2411-2502CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. Methods for conversion of disaccharides, fats, terpenes, and other substances from vegetable biomass in industrial org. chems. are reviewed.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXlvVelsbw%253D&md5=9a15c1f2dfc5b27ac6340cd1b68847de
  2. 2
    (a) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. Polymerization of Lactide and Related Cyclic Esters by Discrete Metal Complexes. J. Chem. Soc., Dalton Trans. 2001, 2215– 2224,  DOI: 10.1039/b104197p
    Google Scholar
    2a
    Polymerization of lactide and related cyclic esters by discrete metal complexes
    O'Keefe, Brendan J.; Hillmyer, Marc A.; Tolman, William B.
    Journal of the Chemical Society, Dalton Transactions (2001), (15), 2215-2224CODEN: JCSDAA; ISSN:1472-7773. (Royal Society of Chemistry)
    A review with refs. This perspective highlights recent research on the prepn. of polyesters by the ring-opening polymn. of cyclic esters employing well-characterized metal complexes. Particular focus is placed on the prepn. of polylactide because of environmental advantages: it is biodegradable and its feedstock, lactide, is a renewable resource. A recurring theme is the correlation of precatalyst structure, often by X-ray crystallog., with polymn. activity and selectivity. Through this systematic approach to the deconvolution of catalyst structure/reactivity relationships, improved mechanistic understanding has been attained and key design criteria required for the development of new catalysts that exert control over the mol. parameters of polyesters and related copolymers have been revealed.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXlsVWntrY%253D&md5=b81ec58772251195ca0be3399fe6d408
    (b) Labet, M.; Thielemans, W. Synthesis of Polycaprolactone: A Review. Chem. Soc. Rev. 2009, 38, 3484– 3504,  DOI: 10.1039/b820162p
    Google Scholar
    2b
    Synthesis of polycaprolactone: a review
    Labet, Marianne; Thielemans, Wim
    Chemical Society Reviews (2009), 38 (12), 3484-3504CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Polycaprolactone (PCL) is an important polymer due to its mech. properties, miscibility with a large range of other polymers and biodegradability. Two main pathways to produce polycaprolactone have been described in the literature: the polycondensation of a hydroxycarboxylic acid: 6-hydroxyhexanoic acid, and the ring-opening polymn. (ROP) of a lactone: ε-caprolactone (ε-CL). This crit. review summarises the different conditions which have been described to synthesize PCL, and gives a broad overview of the different catalytic systems that were used (enzymic, org. and metal catalyst systems). A surprising variety of catalytic systems have been studied, touching on virtually every section of the periodic table. A detailed list of reaction conditions and catalysts/initiators is given and reaction mechanisms are presented where known. Emphasis is put on the ROP pathway due to its prevalence in the literature and the superior polymer that is obtained. In addn., ineffective systems that have been tried to catalyze the prodn. of PCL are included in the electronic supplementary information for completeness (141 refs.).
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsVanurrJ&md5=37be671578dc2e20e673df08d3e454cb
    (c) Thomas, C. M. Stereocontrolled Ring-Opening Polymerization of Cyclic Esters: Synthesis of New Polyester Microstructures. Chem. Soc. Rev. 2010, 39, 165– 173,  DOI: 10.1039/B810065A
    Google Scholar
    2c
    Stereocontrolled ring-opening polymerization of cyclic esters: synthesis of new polyester microstructures
    Thomas, Christophe M.
    Chemical Society Reviews (2010), 39 (1), 165-173CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Synthesis of aliph. polyesters has been studied intensively due to their biocompatible and biodegradable properties and their potential applications in medical and agricultural fields. There has been particular emphasis over the past decade on the synthesis of discrete, well-characterized complexes that are active polymn. initiators for the ring-opening polymn. (ROP) of lactide (LA) and β-butyrolactone (BBL) to give, resp., poly(lactide) (PLA) and poly(3-hydroxybutyrate) (PHB). These recent advances in catalyst design have led to a variety of polyester microstructures. This tutorial review focuses on the use of metal-based complexes for the stereoselective ROP of rac-LA and rac-BBL.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFGrsr3E&md5=f213d305dc30e00a82f5f55174d4f6cb
  3. 3
    (a) MacDonald, J. P.; Shaver, M. P. Aluminum Salen and Salan Polymerization Catalysts: From Monomer Scope to Macrostructure Control. In Green Polymer Chemistry: Biobased Materials and Biocatalysis; American Chemical Society, 2015; Vol. 1192, pp 147– 167.
    Google Scholar
    There is no corresponding record for this reference.
    (b) Wei, Y.; Wang, S.; Zhou, S. Aluminum Alkyl Complexes: Synthesis, Structure, and Application in ROP of Cyclic Esters. Dalton Trans 2016, 45, 4471– 4485,  DOI: 10.1039/C5DT04240B
    Google Scholar
    3b
    Aluminum alkyl complexes: synthesis, structure, and application in ROP of cyclic esters
    Wei, Yun; Wang, Shaowu; Zhou, Shuangliu
    Dalton Transactions (2016), 45 (11), 4471-4485CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    A review focuses on some recent developments in the design, synthesis and structure of Al(III) alkyl complexes supported by various ligands bearing N, O, S or P atoms, and their catalytic applications in the ring-opening polymn. (ROP) of cyclic esters. Al alkyl complexes have very useful applications as catalysts or reagents in small mol. transformations and as cocatalysts in olefin polymn. The coordination chem. of the Al metal center and the catalytic activity changes of the complexes caused by ligand modifications are also discussed.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVOisL8%253D&md5=79b484549379607def2900021225ebeb
  4. 4

    Selected examples and lead references:

    (a) Marshall, E. L.; Gibson, V. C.; Rzepa, H. S. A Computational Analysis of the Ring-Opening Polymerization of rac-Lactide Initiated by Single-Site β-Diketiminate Metal Complexes: Defining the Mechanistic Pathway and the Origin of Stereocontrol. J. Am. Chem. Soc. 2005, 127, 6048– 6051,  DOI: 10.1021/ja043819b
    Google Scholar
    4a
    A computational analysis of the ring-opening polymerization of rac-lactide initiated by single-site β-diketiminate metal complexes: defining the mechanistic pathway and the origin of stereocontrol
    Marshall, Edward L.; Gibson, Vernon C.; Rzepa, Henry S.
    Journal of the American Chemical Society (2005), 127 (16), 6048-6051CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The ring-opening polymn. of rac-lactide at a β-diketiminate magnesium center, [HC{CMeN-2,6-iPr2C6H3}2]Mg(OMe)(THF), has been investigated using a B3-LYP d. functional procedure employing three different layers of basis set: 6-311G(3d) at the Mg center, 6-31G(d) for both the ligand skeleton and the monomer, and a STO-3G basis set at the bulky 2,6-diisopropylphenyl substituents. By studying the consecutive ring-opening of two lactide mols., clear conclusions are drawn regarding both the mechanism of ring-opening and the origin of heterotactic stereocontrol obsd. with such initiators. Polymn. proceeds via two major transition states, an observation applicable to other coordinative initiator systems, with the highest energy transition state dictating the stereochem. of monomer insertion. In the β-diketiminate magnesium system, a detailed examn. of the rate-limiting second transition state geometries reveals that heterotactic poly(lactic acid) arises via the minimization of several steric interactions, possibly reinforced by an attractive CH···π interaction.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXis1Ojtbg%253D&md5=89397c4be30ad25df3764c199eeb1943
    (b) Miranda, M. O.; Deporre, Y.; Vazquez-Lima, H.; Johnson, M. A.; Marell, D. J.; Cramer, C. J.; Tolman, W. B. Understanding the Mechanism of Polymerization of ε-Caprolactone Catalyzed by Aluminum Salen Complexes. Inorg. Chem. 2013, 52, 13692– 13701,  DOI: 10.1021/ic402255m
    Google Scholar
    4b
    Understanding the Mechanism of Polymerization of ε Caprolactone Catalyzed by Aluminum Salen Complexes
    Miranda, Maria O.; DePorre, Yvonne; Vazquez-Lima, Hugo; Johnson, Michelle A.; Marell, Daniel J.; Cramer, Christopher J.; Tolman, William B.
    Inorganic Chemistry (2013), 52 (23), 13692-13701CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Studies of the kinetics of polymn. of ε-caprolactone CL by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics R = OMe, Br, or NO2 were performed using high initial monomer concns. 2 M < [CL]0 < 2.6 M in toluene-d8 at temps. ranging from 20 to 90 °C. Satn. behavior was obsd., enabling detn. of monomer equil. consts. Keq and catalytic rate consts. k2 as a function of R and temp. While Keq varied only slightly with the electron donating properties of R Hammett ρ = +0.16(8), k2 showed a more significant dependence reflected by ρ = +1.4(1). Thermodn. parameters ΔGo assocd. with Keq and ΔG assocd. with k2 were detd., with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO2. D. functional theory DFT calcns. were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is not an energy min. on the reaction pathway, arguing against Keq being assocd. with coordination of monomer according to the typical coordination-insertion mechanism. An alternative hypothesis is presented assocg. Keq with "nonproductive" coordination of substrate in a manner that inhibits the polymn. reaction at high substrate concns.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhslOms7rO&md5=327bc62d0de5ff34a35f80a76a5c2c5f
    (c) Marlier, E. E.; Macaranas, J. A.; Marell, D. J.; Dunbar, C. R.; Johnson, M. A.; DePorre, Y.; Miranda, M. O.; Neisen, B. D.; Cramer, C. J.; Hillmyer, M. A.; Tolman, W. B. Mechanistic Studies of ε-Caprolactone Polymerization by (Salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations. ACS Catal. 2016, 6, 1215– 1224,  DOI: 10.1021/acscatal.5b02607
    Google Scholar
    4c
    Mechanistic Studies of ε-Caprolactone Polymerization by (salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations
    Marlier, Elodie E.; Macaranas, Joahanna A.; Marell, Daniel J.; Dunbar, Christine R.; Johnson, Michelle A.; DePorre, Yvonne; Miranda, Maria O.; Neisen, Benjamin D.; Cramer, Christopher J.; Hillmyer, Marc A.; Tolman, William B.
    ACS Catalysis (2016), 6 (2), 1215-1224CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Aluminum alkoxide complexes (2) of salen ligands with a three-carbon linker and para substituents having variable electron-withdrawing capabilities (X = NO2, Br, OMe) were prepd., and the kinetics of their ring-opening polymn. (ROP) of ε-caprolactone (CL) were investigated as a function of temp., with the aim of drawing comparisons to similar systems with two-carbon linkers investigated previously (1). While 1 and 2 exhibit satn. kinetics and similar dependences of their ROP rates on substituents X (invariant Keq, similar Hammett ρ = +1.4(1) and 1.2(1) for k2, resp.), ROP by 2 was significantly faster than for 1. Theor. calcns. confirm that, while the reactant structures differ, the transition state geometries are quite similar, and by analyzing the energetics of the involved distortions accompanying the structural changes, a significant contribution to the basis for the rate differences was identified. Using this knowledge, a simplified computational method for evaluating ligand structural influences on cyclic ester ROP rates is proposed that may have utility for future catalyst design.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhs1Oksr0%253D&md5=c5d5b373ba3c9fa20b7c70bb4a2e0a76
    (d) Wei, J.; Riffel, M. N.; Diaconescu, P. L. Redox Control of Aluminum Ring-Opening Polymerization: A Combined Experimental and DFT Investigation. Macromolecules 2017, 50, 1847– 1861,  DOI: 10.1021/acs.macromol.6b02402
    Google Scholar
    4d
    Redox Control of Aluminum Ring-Opening Polymerization: A Combined Experimental and DFT Investigation
    Wei, Junnian; Riffel, Madeline N.; Diaconescu, Paula L.
    Macromolecules (Washington, DC, United States) (2017), 50 (5), 1847-1861CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    The synthesis, characterization, and reactivity of an aluminum alkoxide complex supported by a ferrocene-based ligand, (thiolfan*)Al(OtBu) (1red, thiolfan* = 1,1'-di(2,4-di-tert-butyl-6-thiophenoxy)ferrocene), are reported. The homopolymers of L-lactide (LA), ε-caprolactone (CL), δ-valerolactone (VL), cyclohexene oxide (CHO), trimethylene carbonate (TMC), and their copolymers were obtained in a controlled manner by using redox reagents. Detailed DFT calcns. and exptl. studies were performed to investigate the mechanism. Mechanistic studies show that after the insertion of the first monomer, the coordination effect of the carbonyl group, which has usually been ignored in previous reports, can significantly change the energy barrier of the propagation steps, thus playing an important role in polymn. and copolymn. processes.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXjs1SnsLY%253D&md5=d96e11bf00be01dfc55dac05dac2a464
    (e) Robert, C.; Schmid, T. E.; Richard, V.; Haquette, P.; Raman, S. K.; Rager, M.–N.; Gauvin, R. M.; Morin, Y.; Trivelli, X.; Guérineau, V.; del Rosal, I.; Maron, L.; Thomas, C. M. Mechanistic Aspects of the Polymerization of Lactide Using a Highly Efficient Aluminum(III) Catalytic System. J. Am. Chem. Soc. 2017, 139, 6217– 6225,  DOI: 10.1021/jacs.7b01749
    Google Scholar
    4e
    Mechanistic Aspects of the Polymerization of Lactide Using a Highly Efficient Aluminum(III) Catalytic System
    Robert, Carine; Schmid, Thibault E.; Richard, Vincent; Haquette, Pierre; Raman, Sumesh K.; Rager, Marie-Noelle; Gauvin, Regis M.; Morin, Yohann; Trivelli, Xavier; Guerineau, Vincent; del Rosal, Iker; Maron, Laurent; Thomas, Christophe M.
    Journal of the American Chemical Society (2017), 139 (17), 6217-6225CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    We report here a unique example of an in situ generated aluminum initiator stabilized by a C2-sym. salen ligand which shows a hitherto unknown high activity for the ROP of rac-lactide at room temp. Using a simple and robust catalyst system, which is prepd. from a salen complex and an onium salt, this convenient route employs readily available reagents that afford polylactide in good yields with narrow polydispersity indexes, without the need for time-consuming and expensive processes that are typically required for catalyst prepn. and purifn. In line with the exptl. evidence, DFT studies reveal that initiation and propagation proceed via an external alkoxide attack on the coordinated monomer.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXlvFWhtro%253D&md5=a3ced1ab872ed92cd35af7c378a1ec51
    (f) Jitonnom, J.; Molloy, R.; Punyodom, W.; Meelua, W. Theoretical Studies on Aluminum Trialkoxide-Initiated Lactone Ring-Opening Polymerizations: Roles of Alkoxide Substituent and Monomer Ring Structure. Comput. Theor. Chem. 2016, 1097, 25– 32,  DOI: 10.1016/j.comptc.2016.10.009
    Google Scholar
    4f
    Theoretical studies on aluminum trialkoxide-initiated lactone ring-opening polymerizations: Roles of alkoxide substituent and monomer ring structure
    Jitonnom, Jitrayut; Molloy, Robert; Punyodom, Winita; Meelua, Wijitra
    Computational & Theoretical Chemistry (2016), 1097 (), 25-32CODEN: CTCOA5; ISSN:2210-271X. (Elsevier B.V.)
    Four aluminum (III) trialkoxides, namely, Al(III) ethoxide (1), Al(III) isopropoxide (2), Al(III) tert-butoxide (3) and Al(III) sec.-butoxide (4) have been evaluated their efficiency as initiators for bulk ring-opening polymns. (ROP) of some lactones (γ-butyrolactone (GBL), γ-valerolactone (GVL), δ-valerolactone (VL) and ε-caprolactone (CL)). The influences of the initiator's alkoxide substituent and monomer ring structure on the initiating activity and mechanism of ROP were computationally analyzed by means of d. functional theory method. Upon the activation energies, the relative activities of the initiators toward CL ROP follow the trend 4 > 2 > 3 > 1, which were also confirmed by expt. All initiators were found to follow the coordination-insertion mechanism. It was shown that 2 and 4 were efficient initiators for the reaction and the calcns. demonstrate that their branch substituents play a key role in stabilizing intermediates and transition states that leads to lower reaction energies and activation barriers. The rate-detg. step of the initiation was the formation of penta-O-coordinated Al and their computed activation energies (24.5-34.8 kcal/mol) were in agreement with available kinetic data.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhslWjurnO&md5=0aade88d17a506a579e3829cf8fe49db
  5. 5
    Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; Pugh, R. I.; White, A. J. P. Study of Ligand Substituent Effects on the Rate and Stereoselectivity of Lactide Polymerization Using Aluminum Salen-Type Initiators. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15343– 15348,  DOI: 10.1073/pnas.0602765103
    Google Scholar
    5
    Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators
    Hormnirun, Pimpa; Marshall, Edwarrd L.; Gibson, Vernon C.; Pugh, Robert I.; White, Andrew J. P.
    Proceedings of the National Academy of Sciences of the United States of America (2006), 103 (42), 15343-15348CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    A series of aluminum salen-type complexes [where salen is N,N'-bis(salicylaldimine)-1,2-ethylenediamine] bearing ligands that differ in their steric and electronic properties have been synthesized and investigated for the polymn. of rac-lactide. X-ray crystal structures on key precatalysts reveal metal coordination geometries intermediate between trigonal bipyramidal and square-based pyramidal. Both the phenoxy substituents and the backbone linker have a significant influence over the polymn. Electron-withdrawing groups attached to the phenoxy donor generally gave an increased polymn. rate, whereas large ortho substituents generally slowed down the polymn. The vast majority of the initiators afforded polylactide with an isotactic bias; only one exhibited a bias toward heteroselectivity. Isoselectivity generally increases with increased flexibility of the backbone linker, which is presumed to be better able to accommodate any potential steric clashes between the propagating polymer chain, the inserting monomer unit, and the substituents on the phenoxy donor.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XhtFCjtLvP&md5=f3ae59ed4f32da542f44122538dee8f1
  6. 6
    Agatemor, C.; Arnold, A. E.; Cross, E. D.; Decken, A.; Shaver, M. P. Aluminium Salophen and Salen Initiators in the Ring-Opening Polymerisation of rac-Lactide and rac-β-Butyrolactone: Electronic Effects on Stereoselectivity and Polymerisation Rates. J. Organomet. Chem. 2013, 745–746, 335– 340,  DOI: 10.1016/j.jorganchem.2013.08.023
    Google Scholar
    6
    Aluminium salophen and salen initiators in the ring-opening polymerization of rac-lactide and rac-β-butyrolactone: Electronic effects on stereoselectivity and polymerization rates
    Agatemor, Christian; Arnold, Amy E.; Cross, Edward D.; Decken, Andreas; Shaver, Michael P.
    Journal of Organometallic Chemistry (2013), 745-746 (), 335-340CODEN: JORCAI; ISSN:0022-328X. (Elsevier B.V.)
    Three aluminum salophen and two aluminum salen complexes were synthesized, characterized and screened in the ring-opening polymn. (ROP) of rac-lactide and rac-β-butyrolactone. The focus was on controlling the apparent polymn. rate (kp) and stereoselectivity of poly(lactic acid) and poly(3-hydroxybutyrate) by modulating the electron d. at the aluminum center or by switching from an alkyl backbone (salen complex) to an aryl backbone (salophen complex). The salen complexes generally showed higher kp as well as isoselectivity compared to the salophen complexes. For instance, salophen and salen complexes biased the microstructure of poly(3-hydroxybutyrate) towards syndiotacticity and isotacticity, resp. Electron-withdrawing or electron-donating backbones on a salophen complex tuned kp, with electron-donating backbones offering faster kp.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhs1Cru7bF&md5=c282e306e3cf4b9116fc87a87b6f139e
  7. 7
    Stasiw, D. E.; Mandal, M.; Neisen, B. D.; Mitchell, L. A.; Cramer, C. J.; Tolman, W. B. Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε-Caprolactone. Inorg. Chem. 2017, 56, 725– 728,  DOI: 10.1021/acs.inorgchem.6b02849
    Google Scholar
    7
    Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε-Caprolactone
    Stasiw, Daniel E.; Mandal, Mukunda; Neisen, Benjamin D.; Mitchell, Lauren A.; Cramer, Christopher J.; Tolman, William B.
    Inorganic Chemistry (2017), 56 (2), 725-728CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Polymn. of ε-caprolactone (CL) using an Al alkoxide (OEt) dibenzotetraazaannulene catalyst (1) designed to prevent unproductive trans binding was monitored at 110° in toluene-d8 by 1H NMR and the concn. vs. time data fit to a 1st-order rate expression. A comparison of t1/2 for 1 to values for many other Al alkyl and alkoxide complexes shows much lower activity of 1 toward polymn. of CL. D. functional theory calcns. were used to understand the basis for the slow kinetics. The optimized geometry of the ligand framework of 1 was found indeed to make CL trans binding difficult: no trans-bound intermediate could be identified as a local min. Nor were local min. for cis-bound precomplexes found, suggesting a concerted coordination-insertion for polymer initiation and propagation. The sluggish performance of 1 is attributed to a high-framework distortion energy required to deform the resting ligand geometry to that providing optimal catalysis in the corresponding transition-state structure geometry, thus suggesting a need to incorporate ligand flexibility in the design of efficient polymn. catalysts.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XitFagsLjF&md5=4d2fa0354e53f704392fea1e233dc8df
  8. 8
    (a) Macaranas, J. A.; Luke, A. M.; Mandal, M.; Neisen, B. D.; Marell, D. J.; Cramer, C. J.; Tolman, W. B. Sterically Induced Ligand Framework Distortion Effects on Catalytic Cyclic Ester Polymerizations. Inorg. Chem. 2018, 57, 3451– 3457,  DOI: 10.1021/acs.inorgchem.8b00250
    Google Scholar
    8a
    Sterically Induced Ligand Framework Distortion Effects on Catalytic Cyclic Ester Polymerizations
    Macaranas, Joahanna A.; Luke, Anna M.; Mandal, Mukunda; Neisen, Benjamin D.; Marell, Daniel J.; Cramer, Christopher J.; Tolman, William B.
    Inorganic Chemistry (2018), 57 (6), 3451-3457CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Aluminum alkoxide complexes supported by salen ligands [salen = N,N'-bis(salicylaldimine)-2-methylpropane-1,2-diamine or N,N'-bis(salicylaldimine)-2,2-dimethylpropane-1,3-diamine] with o-adamantyl substituents have been synthesized and investigated for the polymn. of ε-caprolactone. Geometric anal. of the catalysts used for the reaction reveals the metal coordination geometries to be intermediate between square-pyramidal and trigonal-bipyramidal. A detailed kinetic study accompanied by d. functional theory modeling of key mechanistic steps of the reaction suggest that, in addn. to the length of the backbone linker, the o-aryl substituents have a significant impact on the catalyst's reactivity. Bulky ortho substituents favorably distort the precatalyst geometry and thereby foster the achievement of the rate-limiting transition-state geometry at low energetic cost, thus accelerating the reaction.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXjtleisr0%253D&md5=810e25bce8527c59efd6fe4d8a2daeb6
    b

    In the original article, the ΔG⧧(TS1) and FDE for 1 were computed to be 10.3 and 12.4 kcal/mol respectively as opposed to 7.8 and 12.7 kcal/mol reported here. This is attributed to three factors: (i) CH2Cl2 solvation effects instead of toluene, (ii) inclusion of Grimme’s D3-dispersion correction term, and (iii) a more favorable orientation of the CL fragment in TS1 identified here. For details on how CL orientation affects TS1 energetics, see section S5 in the Supporting Information.

  9. 9
    (a) Jin, R.; Liu, S.; Lan, Y. Distortion-Interaction Analysis along the Reaction Pathway to Reveal the Reactivity of the Alder-Ene Reaction of Enes. RSC Adv. 2015, 5, 61426– 61435,  DOI: 10.1039/C5RA10345B
    Google Scholar
    9a
    Distortion-interaction analysis along the reaction pathway to reveal the reactivity of the Alder-ene reaction of enes
    Jin, Rui; Liu, Song; Lan, Yu
    RSC Advances (2015), 5 (75), 61426-61435CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    The reactivity of hetero-substituted propylene in uncatalyzed Alder-ene type reactions was investigated using CBS-QB3, G3B3, M11, and B3LYP methods, and the results are interpreted by distortion-interaction anal. of both the transition states and the complete reaction pathways. The reactivity trend for third-period element substituted ene reactants (ethylidenesilane, ethylidenephosphine, and ethanethial) is higher than that of the corresponding second-period element substituted ene reactants (propylene, ethanimine, and acetaldehyde). Theor. calcns. also indicate that for the same period element substituted ene reactants, the reactivity trend is ethylidenesilane > ethylidenephosphine > ethanethial, and propylene > ethanimine > acetaldehyde. Application of distortion-interaction anal. only of the transition states does not give a satisfactory explanation for these reactivities. Using distortion-interaction anal. along the reaction pathways, we found that the reactivity is mainly controlled by the interaction energy. A lower interaction energy along the reaction pathway leads to an earlier transition state and a lower activation energy, which also can be attributed to orbital interaction, closed-shell repulsion, and static repulsion. In some cases, the distortion energy also influences the reactivity.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtFCktbvO&md5=934cb3f5868d09a064e457e3e3e3f184
    (b) Bickelhaupt, F. M.; Houk, K. N. Analyzing Reaction Rates with the Distortion/Interaction-Activation Strain Model. Angew. Chem., Int. Ed. 2017, 56, 10070– 10086,  DOI: 10.1002/anie.201701486
    Google Scholar
    9b
    Analyzing Reaction Rates with the Distortion/Interaction-Activation Strain Model
    Bickelhaupt, F. Matthias; Houk, Kendall N.
    Angewandte Chemie, International Edition (2017), 56 (34), 10070-10086CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The activation strain or distortion/interaction model is a tool to analyze activation barriers that det. reaction rates. For bimol. reactions, the activation energies are the sum of the energies to distort the reactants into geometries they have in transition states plus the interaction energies between the two distorted mols. The energy required to distort the mols. is called the activation strain or distortion energy. This energy is the principal contributor to the activation barrier. The transition state occurs when this activation strain is overcome by the stabilizing interaction energy. Following the changes in these energies along the reaction coordinate gives insights into the factors controlling reactivity. This model has been applied to reactions of all types in both org. and inorg. chem., including substitutions and eliminations, cycloaddns., and several types of organometallic reactions.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtFOlt7nL&md5=6e86c7cfaf7b09e58c0a50a589601578
  10. 10
    Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen–sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc., Dalton Trans. 1984, 1349– 1356,  DOI: 10.1039/DT9840001349
    Google Scholar
    10
    Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen-sulfur donor ligands: the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2'-yl)-2,6-dithiaheptane]copper(II) perchlorate
    Addison, Anthony W.; Rao, T. Nageswara; Reedijk, Jan; Van Rijn, Jacobus; Verschoor, Gerrit C.
    Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1984), (7), 1349-56CODEN: JCDTBI; ISSN:0300-9246.
    The complexes [CuL(OH2)][ClO4]2 (I) and CuL(ClO4)2.2H2O (II) of the linear quadridentate N2S2 donor ligand III (L) were prepd.; they are significantly more stable towards autoredn. than the nonmethyl analogs. The structure of I was detd. by x-ray crystallog.; results were refined to an R of 0.047 for 3343 reflections. The Cu coordination is intermediate between trigonal bipyramidal and square pyramidal. In the solid state the coordination sphere in II may be a topoisomer of I. A new angular structural parameter, τ, is defined as an index of trigonality, a general descriptor of 5-coordinate centric mols. By this criterion the coordination of I in the solid state is described as 48% along the path of distortion from square pyramidal to trigonal bipyramidal. The S → Cu charge-transfer bands in the electronic spectrum of I are assigned. ESR and ligand field spectra show that the Cu compds. adopt a tetragonal structure in donor solvents.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL2cXmtVeitb8%253D&md5=dffdd1312bd477bfe37d8fabe3509990
  11. 11
    (a) Du, H.; Velders, A. H.; Dijkstra, P. J.; Zhong, Z.; Chen, X.; Feijen, J. Polymerization of Lactide Using Achiral Bis(Pyrrolidene) Schiff Base Aluminum Complexes. Macromolecules 2009, 42, 1058– 1066,  DOI: 10.1021/ma802564s
    Google Scholar
    11a
    Polymerization of Lactide Using Achiral Bis(pyrrolidene) Schiff Base Aluminum Complexes
    Du, Hongzhi; Velders, Aldrik H.; Dijkstra, Pieter J.; Zhong, Zhiyuan; Chen, Xuesi; Feijen, Jan
    Macromolecules (Washington, DC, United States) (2009), 42 (4), 1058-1066CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    A series of aluminum ethyls and isopropoxides based on a bis(pyrrolidene) Schiff base ligand framework has been prepd. and characterized. NMR studies of the dissolved complexes indicate that they adopt a sym. structure with a monomeric, five-coordinated aluminum center core. The aluminum ethyls used as catalysts in the presence of 2-propanol as initiator and the aluminum isopropoxides were applied for lactide polymn. in toluene to test their activities and stereoselectivities. All polymns. are living, as evidenced by the narrow polydispersities and the good fit between calcd. and found no.-av. mol. wts. of the isolated polymers. All of these aluminum complexes polymd. (S,S)-lactide to highly isotactic PLA without epimerization of the monomer, furnished isotactic-biased polymer from rac-lactide, and gave atactic polymer from meso-lactide. The study of kinetics indicated that the activity of the bis(pyrrolidene) Schiff base aluminum initiator systems toward lactide polymn. decreases in the following order: (S,S)-lactide > rac-lactide > meso-lactide. The Me substituents on the diimine bridge or on the pyrrole rings both exert significant influence on the course of the polymns., affecting both the stereoselectivity and the polymn. rate. Kinetics using [L2AlEt]/2-propanol (2a/2-propanol) and [L2AlOiPr] (2b) indicated that the polymns. are both first-order with respect to rac-lactide monomer and catalyst. The higher polymn. rate const. (kp) values for [L2AlOiPr] (2b) compared with those of [L2AlEt]/2-propanol (2a/2-propanol) revealed that in this case the overall polymn. rate was influenced by the relatively slow in situ alcoholysis reaction of aluminum ethyls. Polymn. expts. with [L2AlOiPr] (2b) revealed that with this complex much faster (kp = 13.0 L/mol-1/min-1) lactide polymns. can be achieved compared with other aluminum complexes.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXpsl2msA%253D%253D&md5=d0395d36cbe7a1d572eee1c9d0fe1bad
    (b) Tabthong, S.; Nanok, T.; Sumrit, P.; Kongsaeree, P.; Prabpai, S.; Chuawong, P.; Hormnirun, P. Bis(Pyrrolidene) Schiff Base Aluminum Complexes as Isoselective-Biased Initiators for the Controlled Ring-Opening Polymerization of rac-Lactide: Experimental and Theoretical Studies. Macromolecules 2015, 48, 6846– 6861,  DOI: 10.1021/acs.macromol.5b01381
    Google Scholar
    11b
    Bis(pyrrolidene) Schiff Base Aluminum Complexes as Isoselective-Biased Initiators for the Controlled Ring-Opening Polymerization of rac-Lactide: Experimental and Theoretical Studies
    Tabthong, Sittichoke; Nanok, Tanin; Sumrit, Pattarawut; Kongsaeree, Palangpon; Prabpai, Samran; Chuawong, Pitak; Hormnirun, Pimpa
    Macromolecules (Washington, DC, United States) (2015), 48 (19), 6846-6861CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
    Bis(pyrrolidene) Schiff base Al complexes Al{κ4-((C4H3NCH:N)2R)}Me (R = CH2CH2 (1), CH2CH2CH2(2), CH2C(CH3)2CH2(3), o-C6H4CH2 (4), CH2CH2CH2CH2 (5), o-C6H4 (6), trans-1,2-C6H10 (7)) were synthesized and characterized by NMR spectroscopy and elemental anal. All complexes were efficient initiators for the ring-opening polymns. of L-LA and rac-LA in toluene at 70°. Kinetic studies revealed 1st-order kinetics in monomer and the rates of L-LA and rac-LA polymns. decreased in the order of 1,2-benzylene (4) » 1,3-propylene (2) > 2,2-dimethyl-1,3-propylene (3) > 1,4-butylene (5) > trans-1,2-cyclohexylene (7) > 1,2-ethylene (1) » 1,2-phenylene (6). Microstructure analyses of the resulting polylactides by homonuclear decoupled 1H NMR spectroscopy disclosed the isotactic-biased stereocontrol of all synthesized complexes, except 5. Isotactic stereoblock polylactide with a high Pm value of 0.80 was produced by 3. A systematic DFT study on the rac-lactide ring-opening mechanism initiated by the initiators synthesized in this study revealed the correlation between the structure of backbone linker and the polymn. activity and stereoselectivity.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs1ajsb7I&md5=40d4791805c40c51e8d9cc31a99fd45a
    (c) Rufino-Felipe, E.; Lopez, N.; Vengoechea-Gómez, F. A.; Guerrero-Ramírez, L.-G.; Muñoz-Hernández, M.-Á. Ring-Opening Polymerization of rac-Lactide Catalyzed by Al(III) and Zn(II) Complexes Incorporating Schiff Base Ligands Derived from Pyrrole-2-Carboxaldehyde. Appl. Organomet. Chem. 2018, 32, e4315,  DOI: 10.1002/aoc.4315
    Google Scholar
    There is no corresponding record for this reference.
  12. 12
    Lee, C. L.; Lin, Y. F.; Jiang, M. T.; Lu, W. Y.; Vandavasi, J. K.; Wang, L. F.; Lai, Y. C.; Chiang, M. Y.; Chen, H. Y. Improvement in Aluminum Complexes Bearing Schiff Bases in Ring-Opening Polymerization of ε-Caprolactone: A Five-Membered-Ring System. Organometallics 2017, 36, 1936– 1945,  DOI: 10.1021/acs.organomet.7b00068
    Google Scholar
    12
    Improvement in Aluminum Complexes Bearing Schiff Bases in Ring-Opening Polymerization of ε-Caprolactone: A Five-Membered-Ring System
    Lee, Chieh-Ling; Lin, Ya-Fan; Jiang, Man-Ting; Lu, Wei-Yi; Vandavasi, Jaya Kishore; Wang, Li-Fang; Lai, Yi-Chun; Chiang, Michael Y.; Chen, Hsuan-Ying
    Organometallics (2017), 36 (10), 1936-1945CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    A series of five- and six-membered-ring Al complexes bearing Schiff bases was synthesized and their application to the ring-opening polymn. of ε-caprolactone (CL) was studied. The five-membered-ring Al complexes have been shown to have a significantly higher polymn. rate than six-membered-ring Al complexes (2-3 fold for CL polymn.). The X-ray data revealed that the Al center of a five-membered-ring Al complex is farther from the Schiff base ligand than is that of a six-membered-ring Al complex. The results of d. functional theory calcns. also suggest that more space around the Al center of five-membered-ring Al complexes may reduce the steric repulsion in CL polymn. and increase the catalytic activity of five-membered-ring Al complexes.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXntFarsrw%253D&md5=63e2e09693ff1b820164669a4c3676bf
  13. 13
    (a) Black, D. S. C.; Craig, D. C.; Kumar, N.; Wong, L. C. H. Nickel(II) Complexes of Imine Ligands Derived from 7-Formylindoles. J. Chem. Soc., Chem. Commun. 1985, 1172– 1173,  DOI: 10.1039/c39850001172
    Google Scholar
    There is no corresponding record for this reference.
    (b) Liu, S.–F.; Wu, Z.; Schmider, H. L.; Aziz, H.; Hu, N.–X.; Popovic, Z.; Wang, S. Syntheses, Structures, and Electroluminescence of New Blue/Green Luminescent Chelate Compounds:  Zn(2-Py-in)2(THF), BPh2(2-Py-in), Be(2-Py-in)2, and BPh2(2-Py-Aza) [2-Py-in = 2-(2-Pyridyl)Indole; 2-Py-Aza = 2-(2-Pyridyl)-7-Azaindole]. J. Am. Chem. Soc. 2000, 122, 3671– 3678,  DOI: 10.1021/ja9944249
    Google Scholar
    13b
    Syntheses, Structures, and Electroluminescence of New Blue/Green Luminescent Chelate Compounds: Zn(2-py-in)2(THF), BPh2(2-py-in), Be(2-py-in)2, and BPh2(2-py-aza) [2-py-in = 2-(2-pyridyl)indole; 2-py-aza = 2-(2-pyridyl)-7-azaindole]
    Liu, Shi-Feng; Wu, Qingguo; Schmider, Hartmut L.; Aziz, Hany; Hu, Nan-Xing; Popovic, Zoran; Wang, Suning
    Journal of the American Chemical Society (2000), 122 (15), 3671-3678CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Four novel blue/green luminescent compds., Zn(2-py-in)2(THF) (1), BPh2(2-py-in) (2), Be(2-py-in)2 (3), and BPh2(2-py-aza) (4) (2-py-in = 2-(2-pyridyl)indole and 2-py-aza = 2-(2-pyridyl)-7-azaindole) were synthesized and fully characterized. The 2-py-in ligand and 2-py-aza ligand in the new compds. are chelated to the central atom. Compds. 2-4 are air stable and readily sublimable, with a m.p. >250°. In the solid state, compds. 1-4 have an emission max. at λ 488, 516, 490, and 476 nm, resp. The structures of compds. 2 and 4 are similar. The blue shift of emission energy displayed by compd. 4, in comparison to that of 2, is attributed to the presence of an extra N atom in the 2-py-aza ligand as confirmed by ab initio calcns. on compds. 2 and 4. Electroluminescent devices of compds. 3 and 4 were fabricated by using N,N'-di-1-naphthyl-N,N'-diphenylbenzidine (NPB) as the hole transporting layer, Alq3 (q = 8-hydroxyquinolinato) as the electron transporting layer, and compd. 3 or 4 as the light emitting layer. At 20 mA/cm2 the EL device of 3 has an external efficiency of 1.06 cd/A while the EL device of 4 has an external efficiency of 2.34 cd/A, demonstrating that compds. 3 and 4 are efficient and promising emitters in electroluminescent devices.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3cXhvFyms78%253D&md5=06b8dd480e60f368ab8a884096bda6d0
    (c) Mason, M. R.; Fneich, B. N.; Kirschbaum, K. Titanium and Zirconium Amido Complexes Ligated by 2,2‘-Di(3-Methylindolyl)Methanes:  Synthesis, Characterization, and Ethylene Polymerization Activity. Inorg. Chem. 2003, 42, 6592– 6594,  DOI: 10.1021/ic0347236
    Google Scholar
    There is no corresponding record for this reference.
    (d) Kingsley, N. B.; Kirschbaum, K.; Mason, M. R. Confirmation of Bridging N-Indolides in 3-Methylindole and Di- and Tri(3-Methylindolyl)Methane Complexes of Dimethyl-, Diethyl-, and Diisobutylaluminum. Organometallics 2010, 29, 5927– 5935,  DOI: 10.1021/om100781k
    Google Scholar
    13d
    Confirmation of Bridging N-Indolides in 3-Methylindole and Di- and Tri(3-methylindolyl)methane Complexes of Dimethyl-, Diethyl-, and Diisobutylaluminum
    Kingsley, Nicholas B.; Kirschbaum, Kristin; Mason, Mark R.
    Organometallics (2010), 29 (22), 5927-5935CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    Reactions of 3-methylindole with one equiv. of R3Al produce the N-indolide-bridged dimers [(3-CH3C8H5N)AlR2]2 (R = Me (3a), Et (3b), iBu (3c)). NMR spectroscopy (1H, 13C) demonstrates that compds. 3a, 3b, and 3c each exist in soln. as a mixt. of interconverting syn and anti isomers. Reactions of di(3-methylindolyl)methane or tri(3-methylindolyl)methane with one equiv. of Me3Al or Et3Al similarly produce the N-indolide-bridged compds. [(Ph)HC(3-CH3C8H4NAlR2)2] (R = Me (4)) or [HC(3-CH3C8H4NAlR2)3] (R = Me (5a), Et (5b)), resp. Reaction of tri(3-methyl-2-indolyl)methane with one equiv. of iBu3Al yielded the N-indolide-bridged compd. [HC(3-CH3C8H4NAliBu2)2(3-CH3C8H4NAl(H)iBu)] (6), apparently the result of β-H elimination of isobutene from one iso-Bu group per complex. The mol. structures of compds. 3a, 4, and 5a were detd. by x-ray crystallog. Crystallog. data confirmed the presence of μ2-η1:η1-N-3-methylindolide groups in each compd., the 1st examples of bridging N-indolide groups in compds. of the Group IIIB elements.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtlWksbzF&md5=0fafb7f1987f4b9ac06ec138ef7daa92
    (e) Peng, K.-F.; Chen, Y.; Chen, C.-T. Synthesis and Catalytic Application of Magnesium Complexes Bearing Pendant Indolyl Ligands. Dalt. Trans. 2015, 44, 9610– 9619,  DOI: 10.1039/C5DT01173F
    Google Scholar
    13e
    Synthesis and catalytic application of magnesium complexes bearing pendant indolyl ligands
    Peng, Kuo-Fu; Chen, Yun; Chen, Chi-Tien
    Dalton Transactions (2015), 44 (20), 9610-9619CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    Three novel indole-based ligand precursors [HIndPhR, R = methoxy, HIndPhOMe (2a); thiomethoxy, HIndPhSMe (2b); and N,N'-dimethylamino, HIndPhNMe2 (2c)] were synthesized via Sonogashira and cyclization reactions with moderate to high yield. Reactions of these ligand precursors with 0.6 equiv of MgBu2 in THF afforded the magnesium bis-indolyl complexes 3a-3c, resp. All the ligand precursors and related magnesium complexes were characterized by NMR spectroscopy and elemental analyses. The mol. structures are reported for 3a and 3b. Under optimized conditions, compd. 3a demonstrates efficient catalytic activities towards the ring opening polymn. of L-lactide and ε-caprolactone in the presence of BnOH.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXms1yhs7g%253D&md5=fa75b765d4980de4d778fc77bfa123b0
  14. 14
    Hoops, S.; Sahle, S.; Gauges, R.; Lee, C.; Pahle, J.; Simus, N.; Singhal, M.; Xu, L.; Mendes, P.; Kummer, U. COPASI—a COmplex PAthway SImulator. Bioinformatics 2006, 22, 3067– 3074,  DOI: 10.1093/bioinformatics/btl485
    Google Scholar
    14
    COPASI - A COmplex PAthway SImulator
    Hoops, Stefan; Sahle, Sven; Gauges, Ralph; Lee, Christine; Pahle, Juergen; Simus, Natalia; Singhal, Mudita; Xu, Liang; Mendes, Pedro; Kummer, Ursula
    Bioinformatics (2006), 22 (24), 3067-3074CODEN: BOINFP; ISSN:1367-4803. (Oxford University Press)
    Motivation: Simulation and modeling is becoming a std. approach to understand complex biochem. processes. Therefore, there is a big need for software tools that allow access to diverse simulation and modeling methods as well as support for the usage of these methods. Results: Here, we present COPASI, a platform-independent and user-friendly biochem. simulator that offers several unique features. We discuss numerical issues with these features; in particular, the criteria to switch between stochastic and deterministic simulation methods, hybrid deterministic-stochastic methods, and the importance of random no. generator numerical resoln. in stochastic simulation.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28Xht1OgsrvK&md5=ff340a6c0c48f525a92a50c983aa1ddd
  15. 15
    Houk, K. H.; Jabbari, A.; Hall, H. K., Jr.; Alemán, C. Why δ-Valerolactone Polymerizes and γ-Butyrolactone Does Not. J. Org. Chem. 2008, 73, 2674– 2678,  DOI: 10.1021/jo702567v
    Google Scholar
    15
    Why δ-Valerolactone Polymerizes and γ-Butyrolactone Does Not
    Houk, K. N.; Jabbari, Arash; Hall, H. K.; Aleman, Carlos
    Journal of Organic Chemistry (2008), 73 (7), 2674-2678CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    γ-Butyrolactone, unlike δ-valerolactone, does not polymerize despite a strain energy of ∼8 kcal mol-1 which could be relieved by opening the s-cis lactone ester bond to an s-trans ester bond in the polymer. To explain this anomaly, we have applied quantum mech. methods to study the thermochem. involved in the ring-opening reactions of γ-butyrolactone and δ-valerolactone, the conformational preferences of model mols. that mimic their corresponding homo-polyesters, and the variation of enthalpy assocd. to the polymerizability of such two cyclic lactones. The overall results indicate that the lack of polymerizability of γ-butyrolactone should be attributed to the low strain of the ring, which shows much less geometric distortion in the ester group than δ-valerolactone, and the notable stability of the coiled conformations found in model compds. of poly-4-hydroxybutyrate.
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXivV2gsrw%253D&md5=4c8d631874c54f7c5762a1cd8a631fa3
  16. 16
    Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Stereoselective Ring-Opening Polymerization of a Racemic Lactide by Using Achiral Salen– and Homosalen–Aluminum Complexes. Chem. - Eur. J. 2007, 13, 4433– 4451,  DOI: 10.1002/chem.200601308
    Google Scholar
    16
    Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen- and homosalen-aluminum complexes
    Nomura, Nobuyoshi; Ishii, Ryohei; Yamamoto, Yoshihiko; Kondo, Tadao
    Chemistry - A European Journal (2007), 13 (16), 4433-4451CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Highly isotactic polylactide or poly(lactic acid) is synthesized in a ring-opening polymn. (ROP) of racemic lactide with achiral salen- and homosalen-aluminum complexes (salenH2 = N,N'-bis(salicylidene)ethylene-1,2-diamine; homosalenH2 = N,N'-bis(salicylidene)trimethylene-1,3-diamine). A systematic exploration of ligands demonstrates the importance of the steric influence of the Schiff base moiety on the degree of isotacticity and the backbone for high activity. The complexes prepd. in situ are pure enough to apply to the polymns. without purifn. The crystal structures of the key complexes are elucidated by x-ray diffraction, which confirms that they are chiral. However, anal. of the 1H and 13C NMR spectra unambiguously demonstrates that their conformations are so flexible that the chiral environment of the complexes cannot be maintained in soln. at 25° and that the complexes are achiral under the polymn. conditions. The flexibility of the backbone in the propagation steps is also documented. Hence, the isotacticity of the polymer occurs due to a chain-end control mechanism. The highest reactivity in the present system is obtained with the homosalen ligand with 2,2-di-Me substituents in the backbone (ArCH=NCH2CMe2CH2N=CHAr), whereas tBuMe2Si substituents at the 3-positions of the salicylidene moieties lead to the highest selectivity (Pmeso = 0.98; Tm = 210°). The ratio of the rate consts. in the ROPs of racemic lactide and L-lactide is found to correlate with the stereoselectivity in the present system. The complex can be utilized in bulk polymn., which is the most attractive in industry, although with some loss of stereoselectivity at high temp., and the afforded polymer shows a higher melting temp. (Pmeso = 0.92, Tm up to 189°) than that of homochiral poly(L-lactide)(Tm = 162-180°). The "livingness" of the bulk polymn. at 130° is maintained even at a high conversion (97-98%) and for an extended polymn. time (1-2 h).
    >> More from SciFinder ®
    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXmt12gt7g%253D&md5=cc12a44282eb59acbd5a605168e84aa0

Cited By

ARTICLE SECTIONS
Jump To

This article is cited by 19 publications.

  1. Chutikan Nakornkhet, Tanin Nanok, Worawat Wattanathana, Pitak Chuawong, Pimpa Hormnirun. Titanium Complexes of Salicylbenzoxazole and Salicylbenzothiazole Ligands for the Ring-Opening Polymerization of ε-Caprolactone and Substituted ε-Caprolactones and Their Copolymerizations. Inorganic Chemistry 2022, 61 (20) , 7945-7963. https://doi.org/10.1021/acs.inorgchem.2c00577
  2. Xiao-Lu Chen, Bin Wang, Li Pan, Yue-Sheng Li. Synthesis of Unsaturated (Co)polyesters from Ring-Opening Copolymerization by Aluminum Bipyridine Bisphenolate Complexes with Improved Protonic Impurities Tolerance. Macromolecules 2022, 55 (9) , 3502-3512. https://doi.org/10.1021/acs.macromol.2c00034
  3. Farihah M. Haque, Jacob S. A. Ishibashi, Claire A. L. Lidston, Huiling Shao, Frank S. Bates, Alice B. Chang, Geoffrey W. Coates, Christopher J. Cramer, Paul J. Dauenhauer, William R. Dichtel, Christopher J. Ellison, Ethan A. Gormong, Leslie S. Hamachi, Thomas R. Hoye, Mengyuan Jin, Julia A. Kalow, Hee Joong Kim, Gaurav Kumar, Christopher J. LaSalle, Stephanie Liffland, Bryce M. Lipinski, Yutong Pang, Riffat Parveen, Xiayu Peng, Yanay Popowski, Emily A. Prebihalo, Yernaidu Reddi, Theresa M. Reineke, Daylan T. Sheppard, Jeremy L. Swartz, William B. Tolman, Bess Vlaisavljevich, Jane Wissinger, Shu Xu, Marc A. Hillmyer. Defining the Macromolecules of Tomorrow through Synergistic Sustainable Polymer Research. Chemical Reviews 2022, 122 (6) , 6322-6373. https://doi.org/10.1021/acs.chemrev.1c00173
  4. Christopher A. DeRosa, Anna M. Luke, Kendra Anderson, Theresa M. Reineke, William B. Tolman, Frank S. Bates, Marc A. Hillmyer. Regioregular Polymers from Biobased (R)-1,3-Butylene Carbonate. Macromolecules 2021, 54 (13) , 5974-5984. https://doi.org/10.1021/acs.macromol.1c00828
  5. Anna M. Luke, Appie Peterson, Sina Chiniforoush, Mukunda Mandal, Yanay Popowski, Hussnain Sajjad, Caitlin J. Bouchey, Dimitar Y. Shopov, Brendan J. Graziano, Letitia J. Yao, Christopher J. Cramer, Theresa M. Reineke, William B. Tolman. Mechanism of Initiation Stereocontrol in Polymerization of rac-Lactide by Aluminum Complexes Supported by Indolide–Imine Ligands. Macromolecules 2020, 53 (5) , 1809-1818. https://doi.org/10.1021/acs.macromol.0c00092
  6. Chuanzhi Wei, Binghao Han, Dejuan Zheng, Quande Zheng, Shaofeng Liu, Zhibo Li. Aluminum Complexes Bearing Bidentate Amido–Phosphine Ligands for Ring-Opening Polymerization of ε-Caprolactone: Steric Effect on Coordination Chemistry and Reactivity. Organometallics 2019, 38 (19) , 3816-3823. https://doi.org/10.1021/acs.organomet.9b00502
  7. S. Gesslbauer, R. Savela, Y. Chen, A. J. P. White, C. Romain. Exploiting Noncovalent Interactions for Room-Temperature Heteroselective rac-Lactide Polymerization Using Aluminum Catalysts. ACS Catalysis 2019, 9 (9) , 7912-7920. https://doi.org/10.1021/acscatal.9b00875
  8. M. V. Zabalov, B. N. Mankaev, M. P. Egorov, S. S. Karlov. Copolymerization of l-lactide and ε-caprolactone using aluminum aminobisphenolate as an initiator: a quantum chemical study. Russian Chemical Bulletin 2023, 72 (3) , 602-616. https://doi.org/10.1007/s11172-023-3824-6
  9. Yi-Liang Hsieh, Wachara Benchaphanthawee, Hsu-Hung Teng, Ning Huang, Jia-Hao Yang, Jyun-Ru Sun, Gene-Hsiang Lee, Nawee Kungwan, Chi-How Peng. Ring-opening polymerization of cyclic esters mediated by zinc complexes coordinated with benzotriazo-based imino-phenoxy ligands. Polymer 2023, 267 , 125687. https://doi.org/10.1016/j.polymer.2023.125687
  10. Mingqian Wang, Zhiqiang Ding, Bin Wang, Yuesheng Li. (Bipyridine bisphenolate)-aluminum/onium salt pair: a highly active binary catalyst for ring-opening polymerization of lactide with improved thermostability and protic tolerance. Polymer Chemistry 2022, 14 (1) , 45-54. https://doi.org/10.1039/D2PY01273A
  11. L. Kannammal, B. Meeenarathi, V. Parthasarathy, R. Anbarasan. Synthesis and characterization of metal-mercaptoacetate hybrids and its application towards ring-opening polymerization of ε-caprolactone: a comparative study. Polymer Bulletin 2022, 79 (10) , 9001-9023. https://doi.org/10.1007/s00289-021-03939-9
  12. Chi-Tien Chen, Zi-Ling Lai. Aluminium complexes containing indolyl-phenolate ligands as catalysts for ring-opening polymerization of cyclic esters. RSC Advances 2022, 12 (43) , 28052-28058. https://doi.org/10.1039/D2RA05112E
  13. R. Anbarasan, B. Meenarathi, V. Parthasarathy. Schiff base-Cu2+ complex catalyzed and initiated ring opening polymerization of ɛ-Caprolactone: Synthesis and characterization. Journal of Polymer Research 2021, 28 (8) https://doi.org/10.1007/s10965-021-02549-3
  14. Alexandria M. McCollum, Angela M. Longo, Amanda E. Stahl, Abigail S. Butler, Arnold L. Rheingold, Thomas R. Cundari, David B. Green, Kelsey R. Brereton, Joseph M. Fritsch. Synthesis, spectroscopy, and crystallography of mononuclear, five-coordinate aluminum complexes that act as cyclic ester polymerization initiators. Polyhedron 2021, 204 , 115233. https://doi.org/10.1016/j.poly.2021.115233
  15. Nattawut Yuntawattana, Chutikan Nakornkhet, Tanin Nanok, Kanokon Upitak, Pimpa Hormnirun. Dinuclear aluminum complexes bearing methylene-bridged phenoxy-imine ligands and their application in the ring-opening polymerization of rac -lactide. New Journal of Chemistry 2020, 44 (17) , 6965-6978. https://doi.org/10.1039/D0NJ00154F
  16. Francisco M. García-Valle, Tomás Cuenca, Marta E.G. Mosquera, Stefano Milione, Jesús Cano. Ring-Opening Polymerization (ROP) of cyclic esters by a versatile aluminum Diphenoxyimine Complex: From polylactide to random copolymers. European Polymer Journal 2020, 125 , 109527. https://doi.org/10.1016/j.eurpolymj.2020.109527
  17. Xiaofeng Yu, Cheng Zhang, Zhongxia Wang. Controlled Polymerization of ϵ‐Caprolactone using Aluminum and Zinc Complexes with Iminophosphorane Ligands. ChemistrySelect 2020, 5 (1) , 426-429. https://doi.org/10.1002/slct.201904185
  18. Yun Wei, Lulu Song, Ling Jiang, Zeming Huang, Shaowu Wang, Qingbing Yuan, Xiaolong Mu, Xiancui Zhu, Shuangliu Zhou. Aluminum complexes with Schiff base bridged bis(indolyl) ligands: synthesis, structure, and catalytic activity for polymerization of rac -lactide. Dalton Transactions 2019, 48 (40) , 15290-15299. https://doi.org/10.1039/C9DT02724F
  19. Lin, Jheng. Mechanistic Insight into the Ring-Opening Polymerization of ɛ-Caprolactone and L-Lactide Using Ketiminate-Ligated Aluminum Catalysts. Polymers 2019, 11 (9) , 1530. https://doi.org/10.3390/polym11091530
Download PDF

  • Abstract

    High Resolution Image
    Download MS PowerPoint Slide

    Scheme 1

    Scheme 1. Coordination–Insertion Mechanism for ROTEP
    High Resolution Image
    Download MS PowerPoint Slide

    Figure 1

    Figure 1. (a) Parent salen catalyst and modifications with additional alkyl tethers. (b) Pyridine-based systems as a side arm modification to salen. (c) Pyrrole/indole-based {N,N,N,N}-complexes.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 2

    Figure 2. Optimized structures for 1 and 9 and their corresponding turnover-limiting TS structures for CL polymerization.

    High Resolution Image
    Download MS PowerPoint Slide

    Figure 3

    Figure 3. Synthesis of ligand precursor (LH2) and complexes 13 and 14, with a representation of the X-ray crystal structure of 14 shown as 50% thermal ellipsoids (nonhydrogen atoms only). Selected distances (Å) and angles (deg): Al–N1, 1.9388(19); Al–N2, 2.0117(19); Al–N3, 2.074(2); Al–N4, 1.905(2); Al–O1, 1.7484(19); N1–Al–N2, 90.13(9); N1–Al–N3, 173.78(5); N1–Al–N4, 95.63(8); N2–Al–O1, 121.78(8); N2–Al–N4, 114.95(7).

    High Resolution Image
    Download MS PowerPoint Slide

  • References

    ARTICLE SECTIONS
    Jump To

    This article references 16 other publications.

    1. 1
      (a) Schneiderman, D. K.; Hillmyer, M. A. 50th Anniversary Perspective: There Is a Great Future in Sustainable Polymers. Macromolecules 2017, 50, 3733– 3749,  DOI: 10.1021/acs.macromol.7b00293
      Google Scholar
      1a
      50th Anniversary Perspective: There Is a Great Future in Sustainable Polymers
      Schneiderman, Deborah K.; Hillmyer, Marc A.
      Macromolecules (Washington, DC, United States) (2017), 50 (10), 3733-3749CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      A review. It is likely that a half-century ago even enthusiastic and optimistic proponents of the synthetic polymer industry (Mr. McGuire included) could not have predicted the massive scale on which synthetic polymers would be manufd. and used today. Ultimately, the future success of this industry will rely on the development of sustainable polymers-materials derived from renewable feedstocks that are safe in both prodn. and use and that can be recycled or disposed of in ways that are environmentally innocuous. Meeting these criteria in an economical manner cannot be achieved without transformative basic research that is the hallmark of this journal. In this Perspective we highlight five research topics-the synthesis of renewable monomers and of degradable polymers, the development of chem. recycling strategies, new classes of reprocessable thermosets, and the design of advanced catalysts-that we believe will play a vital role in the development of sustainable polymers. We also offer our outlook on several outstanding challenges facing the polymer community in the broad area of sustainable polymers.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXmvVSnsro%253D&md5=aef74ce8b1bc164a33f43c5942bc20e7
      (b) Corma, A.; Iborra, S.; Velty, A. Chemical Routes for the Transformation of Biomass into Chemicals. Chem. Rev. 2007, 107, 2411– 2502,  DOI: 10.1021/cr050989d
      Google Scholar
      1b
      Chemical Routes for the Transformation of Biomass into Chemicals
      Corma, Avelino; Iborra, Sara; Velty, Alexandra
      Chemical Reviews (Washington, DC, United States) (2007), 107 (6), 2411-2502CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Methods for conversion of disaccharides, fats, terpenes, and other substances from vegetable biomass in industrial org. chems. are reviewed.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXlvVelsbw%253D&md5=9a15c1f2dfc5b27ac6340cd1b68847de
    2. 2
      (a) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. Polymerization of Lactide and Related Cyclic Esters by Discrete Metal Complexes. J. Chem. Soc., Dalton Trans. 2001, 2215– 2224,  DOI: 10.1039/b104197p
      Google Scholar
      2a
      Polymerization of lactide and related cyclic esters by discrete metal complexes
      O'Keefe, Brendan J.; Hillmyer, Marc A.; Tolman, William B.
      Journal of the Chemical Society, Dalton Transactions (2001), (15), 2215-2224CODEN: JCSDAA; ISSN:1472-7773. (Royal Society of Chemistry)
      A review with refs. This perspective highlights recent research on the prepn. of polyesters by the ring-opening polymn. of cyclic esters employing well-characterized metal complexes. Particular focus is placed on the prepn. of polylactide because of environmental advantages: it is biodegradable and its feedstock, lactide, is a renewable resource. A recurring theme is the correlation of precatalyst structure, often by X-ray crystallog., with polymn. activity and selectivity. Through this systematic approach to the deconvolution of catalyst structure/reactivity relationships, improved mechanistic understanding has been attained and key design criteria required for the development of new catalysts that exert control over the mol. parameters of polyesters and related copolymers have been revealed.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXlsVWntrY%253D&md5=b81ec58772251195ca0be3399fe6d408
      (b) Labet, M.; Thielemans, W. Synthesis of Polycaprolactone: A Review. Chem. Soc. Rev. 2009, 38, 3484– 3504,  DOI: 10.1039/b820162p
      Google Scholar
      2b
      Synthesis of polycaprolactone: a review
      Labet, Marianne; Thielemans, Wim
      Chemical Society Reviews (2009), 38 (12), 3484-3504CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Polycaprolactone (PCL) is an important polymer due to its mech. properties, miscibility with a large range of other polymers and biodegradability. Two main pathways to produce polycaprolactone have been described in the literature: the polycondensation of a hydroxycarboxylic acid: 6-hydroxyhexanoic acid, and the ring-opening polymn. (ROP) of a lactone: ε-caprolactone (ε-CL). This crit. review summarises the different conditions which have been described to synthesize PCL, and gives a broad overview of the different catalytic systems that were used (enzymic, org. and metal catalyst systems). A surprising variety of catalytic systems have been studied, touching on virtually every section of the periodic table. A detailed list of reaction conditions and catalysts/initiators is given and reaction mechanisms are presented where known. Emphasis is put on the ROP pathway due to its prevalence in the literature and the superior polymer that is obtained. In addn., ineffective systems that have been tried to catalyze the prodn. of PCL are included in the electronic supplementary information for completeness (141 refs.).
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsVanurrJ&md5=37be671578dc2e20e673df08d3e454cb
      (c) Thomas, C. M. Stereocontrolled Ring-Opening Polymerization of Cyclic Esters: Synthesis of New Polyester Microstructures. Chem. Soc. Rev. 2010, 39, 165– 173,  DOI: 10.1039/B810065A
      Google Scholar
      2c
      Stereocontrolled ring-opening polymerization of cyclic esters: synthesis of new polyester microstructures
      Thomas, Christophe M.
      Chemical Society Reviews (2010), 39 (1), 165-173CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Synthesis of aliph. polyesters has been studied intensively due to their biocompatible and biodegradable properties and their potential applications in medical and agricultural fields. There has been particular emphasis over the past decade on the synthesis of discrete, well-characterized complexes that are active polymn. initiators for the ring-opening polymn. (ROP) of lactide (LA) and β-butyrolactone (BBL) to give, resp., poly(lactide) (PLA) and poly(3-hydroxybutyrate) (PHB). These recent advances in catalyst design have led to a variety of polyester microstructures. This tutorial review focuses on the use of metal-based complexes for the stereoselective ROP of rac-LA and rac-BBL.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhsFGrsr3E&md5=f213d305dc30e00a82f5f55174d4f6cb
    3. 3
      (a) MacDonald, J. P.; Shaver, M. P. Aluminum Salen and Salan Polymerization Catalysts: From Monomer Scope to Macrostructure Control. In Green Polymer Chemistry: Biobased Materials and Biocatalysis; American Chemical Society, 2015; Vol. 1192, pp 147– 167.
      Google Scholar
      There is no corresponding record for this reference.
      (b) Wei, Y.; Wang, S.; Zhou, S. Aluminum Alkyl Complexes: Synthesis, Structure, and Application in ROP of Cyclic Esters. Dalton Trans 2016, 45, 4471– 4485,  DOI: 10.1039/C5DT04240B
      Google Scholar
      3b
      Aluminum alkyl complexes: synthesis, structure, and application in ROP of cyclic esters
      Wei, Yun; Wang, Shaowu; Zhou, Shuangliu
      Dalton Transactions (2016), 45 (11), 4471-4485CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      A review focuses on some recent developments in the design, synthesis and structure of Al(III) alkyl complexes supported by various ligands bearing N, O, S or P atoms, and their catalytic applications in the ring-opening polymn. (ROP) of cyclic esters. Al alkyl complexes have very useful applications as catalysts or reagents in small mol. transformations and as cocatalysts in olefin polymn. The coordination chem. of the Al metal center and the catalytic activity changes of the complexes caused by ligand modifications are also discussed.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVOisL8%253D&md5=79b484549379607def2900021225ebeb
    4. 4

      Selected examples and lead references:

      (a) Marshall, E. L.; Gibson, V. C.; Rzepa, H. S. A Computational Analysis of the Ring-Opening Polymerization of rac-Lactide Initiated by Single-Site β-Diketiminate Metal Complexes: Defining the Mechanistic Pathway and the Origin of Stereocontrol. J. Am. Chem. Soc. 2005, 127, 6048– 6051,  DOI: 10.1021/ja043819b
      Google Scholar
      4a
      A computational analysis of the ring-opening polymerization of rac-lactide initiated by single-site β-diketiminate metal complexes: defining the mechanistic pathway and the origin of stereocontrol
      Marshall, Edward L.; Gibson, Vernon C.; Rzepa, Henry S.
      Journal of the American Chemical Society (2005), 127 (16), 6048-6051CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The ring-opening polymn. of rac-lactide at a β-diketiminate magnesium center, [HC{CMeN-2,6-iPr2C6H3}2]Mg(OMe)(THF), has been investigated using a B3-LYP d. functional procedure employing three different layers of basis set: 6-311G(3d) at the Mg center, 6-31G(d) for both the ligand skeleton and the monomer, and a STO-3G basis set at the bulky 2,6-diisopropylphenyl substituents. By studying the consecutive ring-opening of two lactide mols., clear conclusions are drawn regarding both the mechanism of ring-opening and the origin of heterotactic stereocontrol obsd. with such initiators. Polymn. proceeds via two major transition states, an observation applicable to other coordinative initiator systems, with the highest energy transition state dictating the stereochem. of monomer insertion. In the β-diketiminate magnesium system, a detailed examn. of the rate-limiting second transition state geometries reveals that heterotactic poly(lactic acid) arises via the minimization of several steric interactions, possibly reinforced by an attractive CH···π interaction.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXis1Ojtbg%253D&md5=89397c4be30ad25df3764c199eeb1943
      (b) Miranda, M. O.; Deporre, Y.; Vazquez-Lima, H.; Johnson, M. A.; Marell, D. J.; Cramer, C. J.; Tolman, W. B. Understanding the Mechanism of Polymerization of ε-Caprolactone Catalyzed by Aluminum Salen Complexes. Inorg. Chem. 2013, 52, 13692– 13701,  DOI: 10.1021/ic402255m
      Google Scholar
      4b
      Understanding the Mechanism of Polymerization of ε Caprolactone Catalyzed by Aluminum Salen Complexes
      Miranda, Maria O.; DePorre, Yvonne; Vazquez-Lima, Hugo; Johnson, Michelle A.; Marell, Daniel J.; Cramer, Christopher J.; Tolman, William B.
      Inorganic Chemistry (2013), 52 (23), 13692-13701CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Studies of the kinetics of polymn. of ε-caprolactone CL by salen-aluminum catalysts comprising ligands with similar steric profiles but different electron donating characteristics R = OMe, Br, or NO2 were performed using high initial monomer concns. 2 M < [CL]0 < 2.6 M in toluene-d8 at temps. ranging from 20 to 90 °C. Satn. behavior was obsd., enabling detn. of monomer equil. consts. Keq and catalytic rate consts. k2 as a function of R and temp. While Keq varied only slightly with the electron donating properties of R Hammett ρ = +0.16(8), k2 showed a more significant dependence reflected by ρ = +1.4(1). Thermodn. parameters ΔGo assocd. with Keq and ΔG assocd. with k2 were detd., with the former being ∼0 kcal/mol for all catalysts and the latter exhibiting the trend R = OMe > Br > NO2. D. functional theory DFT calcns. were performed to characterize mechanistic pathways at a microscopic level of detail. Lowest energy transition-state structures feature incipient bonding of the nucleophile to the lactone carbonyl that is approaching the metal ion, but a distinct CL adduct is not an energy min. on the reaction pathway, arguing against Keq being assocd. with coordination of monomer according to the typical coordination-insertion mechanism. An alternative hypothesis is presented assocg. Keq with "nonproductive" coordination of substrate in a manner that inhibits the polymn. reaction at high substrate concns.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhslOms7rO&md5=327bc62d0de5ff34a35f80a76a5c2c5f
      (c) Marlier, E. E.; Macaranas, J. A.; Marell, D. J.; Dunbar, C. R.; Johnson, M. A.; DePorre, Y.; Miranda, M. O.; Neisen, B. D.; Cramer, C. J.; Hillmyer, M. A.; Tolman, W. B. Mechanistic Studies of ε-Caprolactone Polymerization by (Salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations. ACS Catal. 2016, 6, 1215– 1224,  DOI: 10.1021/acscatal.5b02607
      Google Scholar
      4c
      Mechanistic Studies of ε-Caprolactone Polymerization by (salen)AlOR Complexes and a Predictive Model for Cyclic Ester Polymerizations
      Marlier, Elodie E.; Macaranas, Joahanna A.; Marell, Daniel J.; Dunbar, Christine R.; Johnson, Michelle A.; DePorre, Yvonne; Miranda, Maria O.; Neisen, Benjamin D.; Cramer, Christopher J.; Hillmyer, Marc A.; Tolman, William B.
      ACS Catalysis (2016), 6 (2), 1215-1224CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Aluminum alkoxide complexes (2) of salen ligands with a three-carbon linker and para substituents having variable electron-withdrawing capabilities (X = NO2, Br, OMe) were prepd., and the kinetics of their ring-opening polymn. (ROP) of ε-caprolactone (CL) were investigated as a function of temp., with the aim of drawing comparisons to similar systems with two-carbon linkers investigated previously (1). While 1 and 2 exhibit satn. kinetics and similar dependences of their ROP rates on substituents X (invariant Keq, similar Hammett ρ = +1.4(1) and 1.2(1) for k2, resp.), ROP by 2 was significantly faster than for 1. Theor. calcns. confirm that, while the reactant structures differ, the transition state geometries are quite similar, and by analyzing the energetics of the involved distortions accompanying the structural changes, a significant contribution to the basis for the rate differences was identified. Using this knowledge, a simplified computational method for evaluating ligand structural influences on cyclic ester ROP rates is proposed that may have utility for future catalyst design.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xhs1Oksr0%253D&md5=c5d5b373ba3c9fa20b7c70bb4a2e0a76
      (d) Wei, J.; Riffel, M. N.; Diaconescu, P. L. Redox Control of Aluminum Ring-Opening Polymerization: A Combined Experimental and DFT Investigation. Macromolecules 2017, 50, 1847– 1861,  DOI: 10.1021/acs.macromol.6b02402
      Google Scholar
      4d
      Redox Control of Aluminum Ring-Opening Polymerization: A Combined Experimental and DFT Investigation
      Wei, Junnian; Riffel, Madeline N.; Diaconescu, Paula L.
      Macromolecules (Washington, DC, United States) (2017), 50 (5), 1847-1861CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      The synthesis, characterization, and reactivity of an aluminum alkoxide complex supported by a ferrocene-based ligand, (thiolfan*)Al(OtBu) (1red, thiolfan* = 1,1'-di(2,4-di-tert-butyl-6-thiophenoxy)ferrocene), are reported. The homopolymers of L-lactide (LA), ε-caprolactone (CL), δ-valerolactone (VL), cyclohexene oxide (CHO), trimethylene carbonate (TMC), and their copolymers were obtained in a controlled manner by using redox reagents. Detailed DFT calcns. and exptl. studies were performed to investigate the mechanism. Mechanistic studies show that after the insertion of the first monomer, the coordination effect of the carbonyl group, which has usually been ignored in previous reports, can significantly change the energy barrier of the propagation steps, thus playing an important role in polymn. and copolymn. processes.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXjs1SnsLY%253D&md5=d96e11bf00be01dfc55dac05dac2a464
      (e) Robert, C.; Schmid, T. E.; Richard, V.; Haquette, P.; Raman, S. K.; Rager, M.–N.; Gauvin, R. M.; Morin, Y.; Trivelli, X.; Guérineau, V.; del Rosal, I.; Maron, L.; Thomas, C. M. Mechanistic Aspects of the Polymerization of Lactide Using a Highly Efficient Aluminum(III) Catalytic System. J. Am. Chem. Soc. 2017, 139, 6217– 6225,  DOI: 10.1021/jacs.7b01749
      Google Scholar
      4e
      Mechanistic Aspects of the Polymerization of Lactide Using a Highly Efficient Aluminum(III) Catalytic System
      Robert, Carine; Schmid, Thibault E.; Richard, Vincent; Haquette, Pierre; Raman, Sumesh K.; Rager, Marie-Noelle; Gauvin, Regis M.; Morin, Yohann; Trivelli, Xavier; Guerineau, Vincent; del Rosal, Iker; Maron, Laurent; Thomas, Christophe M.
      Journal of the American Chemical Society (2017), 139 (17), 6217-6225CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      We report here a unique example of an in situ generated aluminum initiator stabilized by a C2-sym. salen ligand which shows a hitherto unknown high activity for the ROP of rac-lactide at room temp. Using a simple and robust catalyst system, which is prepd. from a salen complex and an onium salt, this convenient route employs readily available reagents that afford polylactide in good yields with narrow polydispersity indexes, without the need for time-consuming and expensive processes that are typically required for catalyst prepn. and purifn. In line with the exptl. evidence, DFT studies reveal that initiation and propagation proceed via an external alkoxide attack on the coordinated monomer.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXlvFWhtro%253D&md5=a3ced1ab872ed92cd35af7c378a1ec51
      (f) Jitonnom, J.; Molloy, R.; Punyodom, W.; Meelua, W. Theoretical Studies on Aluminum Trialkoxide-Initiated Lactone Ring-Opening Polymerizations: Roles of Alkoxide Substituent and Monomer Ring Structure. Comput. Theor. Chem. 2016, 1097, 25– 32,  DOI: 10.1016/j.comptc.2016.10.009
      Google Scholar
      4f
      Theoretical studies on aluminum trialkoxide-initiated lactone ring-opening polymerizations: Roles of alkoxide substituent and monomer ring structure
      Jitonnom, Jitrayut; Molloy, Robert; Punyodom, Winita; Meelua, Wijitra
      Computational & Theoretical Chemistry (2016), 1097 (), 25-32CODEN: CTCOA5; ISSN:2210-271X. (Elsevier B.V.)
      Four aluminum (III) trialkoxides, namely, Al(III) ethoxide (1), Al(III) isopropoxide (2), Al(III) tert-butoxide (3) and Al(III) sec.-butoxide (4) have been evaluated their efficiency as initiators for bulk ring-opening polymns. (ROP) of some lactones (γ-butyrolactone (GBL), γ-valerolactone (GVL), δ-valerolactone (VL) and ε-caprolactone (CL)). The influences of the initiator's alkoxide substituent and monomer ring structure on the initiating activity and mechanism of ROP were computationally analyzed by means of d. functional theory method. Upon the activation energies, the relative activities of the initiators toward CL ROP follow the trend 4 > 2 > 3 > 1, which were also confirmed by expt. All initiators were found to follow the coordination-insertion mechanism. It was shown that 2 and 4 were efficient initiators for the reaction and the calcns. demonstrate that their branch substituents play a key role in stabilizing intermediates and transition states that leads to lower reaction energies and activation barriers. The rate-detg. step of the initiation was the formation of penta-O-coordinated Al and their computed activation energies (24.5-34.8 kcal/mol) were in agreement with available kinetic data.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhslWjurnO&md5=0aade88d17a506a579e3829cf8fe49db
    5. 5
      Hormnirun, P.; Marshall, E. L.; Gibson, V. C.; Pugh, R. I.; White, A. J. P. Study of Ligand Substituent Effects on the Rate and Stereoselectivity of Lactide Polymerization Using Aluminum Salen-Type Initiators. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15343– 15348,  DOI: 10.1073/pnas.0602765103
      Google Scholar
      5
      Study of ligand substituent effects on the rate and stereoselectivity of lactide polymerization using aluminum salen-type initiators
      Hormnirun, Pimpa; Marshall, Edwarrd L.; Gibson, Vernon C.; Pugh, Robert I.; White, Andrew J. P.
      Proceedings of the National Academy of Sciences of the United States of America (2006), 103 (42), 15343-15348CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      A series of aluminum salen-type complexes [where salen is N,N'-bis(salicylaldimine)-1,2-ethylenediamine] bearing ligands that differ in their steric and electronic properties have been synthesized and investigated for the polymn. of rac-lactide. X-ray crystal structures on key precatalysts reveal metal coordination geometries intermediate between trigonal bipyramidal and square-based pyramidal. Both the phenoxy substituents and the backbone linker have a significant influence over the polymn. Electron-withdrawing groups attached to the phenoxy donor generally gave an increased polymn. rate, whereas large ortho substituents generally slowed down the polymn. The vast majority of the initiators afforded polylactide with an isotactic bias; only one exhibited a bias toward heteroselectivity. Isoselectivity generally increases with increased flexibility of the backbone linker, which is presumed to be better able to accommodate any potential steric clashes between the propagating polymer chain, the inserting monomer unit, and the substituents on the phenoxy donor.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XhtFCjtLvP&md5=f3ae59ed4f32da542f44122538dee8f1
    6. 6
      Agatemor, C.; Arnold, A. E.; Cross, E. D.; Decken, A.; Shaver, M. P. Aluminium Salophen and Salen Initiators in the Ring-Opening Polymerisation of rac-Lactide and rac-β-Butyrolactone: Electronic Effects on Stereoselectivity and Polymerisation Rates. J. Organomet. Chem. 2013, 745–746, 335– 340,  DOI: 10.1016/j.jorganchem.2013.08.023
      Google Scholar
      6
      Aluminium salophen and salen initiators in the ring-opening polymerization of rac-lactide and rac-β-butyrolactone: Electronic effects on stereoselectivity and polymerization rates
      Agatemor, Christian; Arnold, Amy E.; Cross, Edward D.; Decken, Andreas; Shaver, Michael P.
      Journal of Organometallic Chemistry (2013), 745-746 (), 335-340CODEN: JORCAI; ISSN:0022-328X. (Elsevier B.V.)
      Three aluminum salophen and two aluminum salen complexes were synthesized, characterized and screened in the ring-opening polymn. (ROP) of rac-lactide and rac-β-butyrolactone. The focus was on controlling the apparent polymn. rate (kp) and stereoselectivity of poly(lactic acid) and poly(3-hydroxybutyrate) by modulating the electron d. at the aluminum center or by switching from an alkyl backbone (salen complex) to an aryl backbone (salophen complex). The salen complexes generally showed higher kp as well as isoselectivity compared to the salophen complexes. For instance, salophen and salen complexes biased the microstructure of poly(3-hydroxybutyrate) towards syndiotacticity and isotacticity, resp. Electron-withdrawing or electron-donating backbones on a salophen complex tuned kp, with electron-donating backbones offering faster kp.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhs1Cru7bF&md5=c282e306e3cf4b9116fc87a87b6f139e
    7. 7
      Stasiw, D. E.; Mandal, M.; Neisen, B. D.; Mitchell, L. A.; Cramer, C. J.; Tolman, W. B. Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε-Caprolactone. Inorg. Chem. 2017, 56, 725– 728,  DOI: 10.1021/acs.inorgchem.6b02849
      Google Scholar
      7
      Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε-Caprolactone
      Stasiw, Daniel E.; Mandal, Mukunda; Neisen, Benjamin D.; Mitchell, Lauren A.; Cramer, Christopher J.; Tolman, William B.
      Inorganic Chemistry (2017), 56 (2), 725-728CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Polymn. of ε-caprolactone (CL) using an Al alkoxide (OEt) dibenzotetraazaannulene catalyst (1) designed to prevent unproductive trans binding was monitored at 110° in toluene-d8 by 1H NMR and the concn. vs. time data fit to a 1st-order rate expression. A comparison of t1/2 for 1 to values for many other Al alkyl and alkoxide complexes shows much lower activity of 1 toward polymn. of CL. D. functional theory calcns. were used to understand the basis for the slow kinetics. The optimized geometry of the ligand framework of 1 was found indeed to make CL trans binding difficult: no trans-bound intermediate could be identified as a local min. Nor were local min. for cis-bound precomplexes found, suggesting a concerted coordination-insertion for polymer initiation and propagation. The sluggish performance of 1 is attributed to a high-framework distortion energy required to deform the resting ligand geometry to that providing optimal catalysis in the corresponding transition-state structure geometry, thus suggesting a need to incorporate ligand flexibility in the design of efficient polymn. catalysts.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XitFagsLjF&md5=4d2fa0354e53f704392fea1e233dc8df
    8. 8
      (a) Macaranas, J. A.; Luke, A. M.; Mandal, M.; Neisen, B. D.; Marell, D. J.; Cramer, C. J.; Tolman, W. B. Sterically Induced Ligand Framework Distortion Effects on Catalytic Cyclic Ester Polymerizations. Inorg. Chem. 2018, 57, 3451– 3457,  DOI: 10.1021/acs.inorgchem.8b00250
      Google Scholar
      8a
      Sterically Induced Ligand Framework Distortion Effects on Catalytic Cyclic Ester Polymerizations
      Macaranas, Joahanna A.; Luke, Anna M.; Mandal, Mukunda; Neisen, Benjamin D.; Marell, Daniel J.; Cramer, Christopher J.; Tolman, William B.
      Inorganic Chemistry (2018), 57 (6), 3451-3457CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Aluminum alkoxide complexes supported by salen ligands [salen = N,N'-bis(salicylaldimine)-2-methylpropane-1,2-diamine or N,N'-bis(salicylaldimine)-2,2-dimethylpropane-1,3-diamine] with o-adamantyl substituents have been synthesized and investigated for the polymn. of ε-caprolactone. Geometric anal. of the catalysts used for the reaction reveals the metal coordination geometries to be intermediate between square-pyramidal and trigonal-bipyramidal. A detailed kinetic study accompanied by d. functional theory modeling of key mechanistic steps of the reaction suggest that, in addn. to the length of the backbone linker, the o-aryl substituents have a significant impact on the catalyst's reactivity. Bulky ortho substituents favorably distort the precatalyst geometry and thereby foster the achievement of the rate-limiting transition-state geometry at low energetic cost, thus accelerating the reaction.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXjtleisr0%253D&md5=810e25bce8527c59efd6fe4d8a2daeb6
      b

      In the original article, the ΔG⧧(TS1) and FDE for 1 were computed to be 10.3 and 12.4 kcal/mol respectively as opposed to 7.8 and 12.7 kcal/mol reported here. This is attributed to three factors: (i) CH2Cl2 solvation effects instead of toluene, (ii) inclusion of Grimme’s D3-dispersion correction term, and (iii) a more favorable orientation of the CL fragment in TS1 identified here. For details on how CL orientation affects TS1 energetics, see section S5 in the Supporting Information.

    9. 9
      (a) Jin, R.; Liu, S.; Lan, Y. Distortion-Interaction Analysis along the Reaction Pathway to Reveal the Reactivity of the Alder-Ene Reaction of Enes. RSC Adv. 2015, 5, 61426– 61435,  DOI: 10.1039/C5RA10345B
      Google Scholar
      9a
      Distortion-interaction analysis along the reaction pathway to reveal the reactivity of the Alder-ene reaction of enes
      Jin, Rui; Liu, Song; Lan, Yu
      RSC Advances (2015), 5 (75), 61426-61435CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      The reactivity of hetero-substituted propylene in uncatalyzed Alder-ene type reactions was investigated using CBS-QB3, G3B3, M11, and B3LYP methods, and the results are interpreted by distortion-interaction anal. of both the transition states and the complete reaction pathways. The reactivity trend for third-period element substituted ene reactants (ethylidenesilane, ethylidenephosphine, and ethanethial) is higher than that of the corresponding second-period element substituted ene reactants (propylene, ethanimine, and acetaldehyde). Theor. calcns. also indicate that for the same period element substituted ene reactants, the reactivity trend is ethylidenesilane > ethylidenephosphine > ethanethial, and propylene > ethanimine > acetaldehyde. Application of distortion-interaction anal. only of the transition states does not give a satisfactory explanation for these reactivities. Using distortion-interaction anal. along the reaction pathways, we found that the reactivity is mainly controlled by the interaction energy. A lower interaction energy along the reaction pathway leads to an earlier transition state and a lower activation energy, which also can be attributed to orbital interaction, closed-shell repulsion, and static repulsion. In some cases, the distortion energy also influences the reactivity.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtFCktbvO&md5=934cb3f5868d09a064e457e3e3e3f184
      (b) Bickelhaupt, F. M.; Houk, K. N. Analyzing Reaction Rates with the Distortion/Interaction-Activation Strain Model. Angew. Chem., Int. Ed. 2017, 56, 10070– 10086,  DOI: 10.1002/anie.201701486
      Google Scholar
      9b
      Analyzing Reaction Rates with the Distortion/Interaction-Activation Strain Model
      Bickelhaupt, F. Matthias; Houk, Kendall N.
      Angewandte Chemie, International Edition (2017), 56 (34), 10070-10086CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The activation strain or distortion/interaction model is a tool to analyze activation barriers that det. reaction rates. For bimol. reactions, the activation energies are the sum of the energies to distort the reactants into geometries they have in transition states plus the interaction energies between the two distorted mols. The energy required to distort the mols. is called the activation strain or distortion energy. This energy is the principal contributor to the activation barrier. The transition state occurs when this activation strain is overcome by the stabilizing interaction energy. Following the changes in these energies along the reaction coordinate gives insights into the factors controlling reactivity. This model has been applied to reactions of all types in both org. and inorg. chem., including substitutions and eliminations, cycloaddns., and several types of organometallic reactions.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXhtFOlt7nL&md5=6e86c7cfaf7b09e58c0a50a589601578
    10. 10
      Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C. Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen–sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc., Dalton Trans. 1984, 1349– 1356,  DOI: 10.1039/DT9840001349
      Google Scholar
      10
      Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen-sulfur donor ligands: the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2'-yl)-2,6-dithiaheptane]copper(II) perchlorate
      Addison, Anthony W.; Rao, T. Nageswara; Reedijk, Jan; Van Rijn, Jacobus; Verschoor, Gerrit C.
      Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1984), (7), 1349-56CODEN: JCDTBI; ISSN:0300-9246.
      The complexes [CuL(OH2)][ClO4]2 (I) and CuL(ClO4)2.2H2O (II) of the linear quadridentate N2S2 donor ligand III (L) were prepd.; they are significantly more stable towards autoredn. than the nonmethyl analogs. The structure of I was detd. by x-ray crystallog.; results were refined to an R of 0.047 for 3343 reflections. The Cu coordination is intermediate between trigonal bipyramidal and square pyramidal. In the solid state the coordination sphere in II may be a topoisomer of I. A new angular structural parameter, τ, is defined as an index of trigonality, a general descriptor of 5-coordinate centric mols. By this criterion the coordination of I in the solid state is described as 48% along the path of distortion from square pyramidal to trigonal bipyramidal. The S → Cu charge-transfer bands in the electronic spectrum of I are assigned. ESR and ligand field spectra show that the Cu compds. adopt a tetragonal structure in donor solvents.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL2cXmtVeitb8%253D&md5=dffdd1312bd477bfe37d8fabe3509990
    11. 11
      (a) Du, H.; Velders, A. H.; Dijkstra, P. J.; Zhong, Z.; Chen, X.; Feijen, J. Polymerization of Lactide Using Achiral Bis(Pyrrolidene) Schiff Base Aluminum Complexes. Macromolecules 2009, 42, 1058– 1066,  DOI: 10.1021/ma802564s
      Google Scholar
      11a
      Polymerization of Lactide Using Achiral Bis(pyrrolidene) Schiff Base Aluminum Complexes
      Du, Hongzhi; Velders, Aldrik H.; Dijkstra, Pieter J.; Zhong, Zhiyuan; Chen, Xuesi; Feijen, Jan
      Macromolecules (Washington, DC, United States) (2009), 42 (4), 1058-1066CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      A series of aluminum ethyls and isopropoxides based on a bis(pyrrolidene) Schiff base ligand framework has been prepd. and characterized. NMR studies of the dissolved complexes indicate that they adopt a sym. structure with a monomeric, five-coordinated aluminum center core. The aluminum ethyls used as catalysts in the presence of 2-propanol as initiator and the aluminum isopropoxides were applied for lactide polymn. in toluene to test their activities and stereoselectivities. All polymns. are living, as evidenced by the narrow polydispersities and the good fit between calcd. and found no.-av. mol. wts. of the isolated polymers. All of these aluminum complexes polymd. (S,S)-lactide to highly isotactic PLA without epimerization of the monomer, furnished isotactic-biased polymer from rac-lactide, and gave atactic polymer from meso-lactide. The study of kinetics indicated that the activity of the bis(pyrrolidene) Schiff base aluminum initiator systems toward lactide polymn. decreases in the following order: (S,S)-lactide > rac-lactide > meso-lactide. The Me substituents on the diimine bridge or on the pyrrole rings both exert significant influence on the course of the polymns., affecting both the stereoselectivity and the polymn. rate. Kinetics using [L2AlEt]/2-propanol (2a/2-propanol) and [L2AlOiPr] (2b) indicated that the polymns. are both first-order with respect to rac-lactide monomer and catalyst. The higher polymn. rate const. (kp) values for [L2AlOiPr] (2b) compared with those of [L2AlEt]/2-propanol (2a/2-propanol) revealed that in this case the overall polymn. rate was influenced by the relatively slow in situ alcoholysis reaction of aluminum ethyls. Polymn. expts. with [L2AlOiPr] (2b) revealed that with this complex much faster (kp = 13.0 L/mol-1/min-1) lactide polymns. can be achieved compared with other aluminum complexes.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXpsl2msA%253D%253D&md5=d0395d36cbe7a1d572eee1c9d0fe1bad
      (b) Tabthong, S.; Nanok, T.; Sumrit, P.; Kongsaeree, P.; Prabpai, S.; Chuawong, P.; Hormnirun, P. Bis(Pyrrolidene) Schiff Base Aluminum Complexes as Isoselective-Biased Initiators for the Controlled Ring-Opening Polymerization of rac-Lactide: Experimental and Theoretical Studies. Macromolecules 2015, 48, 6846– 6861,  DOI: 10.1021/acs.macromol.5b01381
      Google Scholar
      11b
      Bis(pyrrolidene) Schiff Base Aluminum Complexes as Isoselective-Biased Initiators for the Controlled Ring-Opening Polymerization of rac-Lactide: Experimental and Theoretical Studies
      Tabthong, Sittichoke; Nanok, Tanin; Sumrit, Pattarawut; Kongsaeree, Palangpon; Prabpai, Samran; Chuawong, Pitak; Hormnirun, Pimpa
      Macromolecules (Washington, DC, United States) (2015), 48 (19), 6846-6861CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      Bis(pyrrolidene) Schiff base Al complexes Al{κ4-((C4H3NCH:N)2R)}Me (R = CH2CH2 (1), CH2CH2CH2(2), CH2C(CH3)2CH2(3), o-C6H4CH2 (4), CH2CH2CH2CH2 (5), o-C6H4 (6), trans-1,2-C6H10 (7)) were synthesized and characterized by NMR spectroscopy and elemental anal. All complexes were efficient initiators for the ring-opening polymns. of L-LA and rac-LA in toluene at 70°. Kinetic studies revealed 1st-order kinetics in monomer and the rates of L-LA and rac-LA polymns. decreased in the order of 1,2-benzylene (4) » 1,3-propylene (2) > 2,2-dimethyl-1,3-propylene (3) > 1,4-butylene (5) > trans-1,2-cyclohexylene (7) > 1,2-ethylene (1) » 1,2-phenylene (6). Microstructure analyses of the resulting polylactides by homonuclear decoupled 1H NMR spectroscopy disclosed the isotactic-biased stereocontrol of all synthesized complexes, except 5. Isotactic stereoblock polylactide with a high Pm value of 0.80 was produced by 3. A systematic DFT study on the rac-lactide ring-opening mechanism initiated by the initiators synthesized in this study revealed the correlation between the structure of backbone linker and the polymn. activity and stereoselectivity.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs1ajsb7I&md5=40d4791805c40c51e8d9cc31a99fd45a
      (c) Rufino-Felipe, E.; Lopez, N.; Vengoechea-Gómez, F. A.; Guerrero-Ramírez, L.-G.; Muñoz-Hernández, M.-Á. Ring-Opening Polymerization of rac-Lactide Catalyzed by Al(III) and Zn(II) Complexes Incorporating Schiff Base Ligands Derived from Pyrrole-2-Carboxaldehyde. Appl. Organomet. Chem. 2018, 32, e4315,  DOI: 10.1002/aoc.4315
      Google Scholar
      There is no corresponding record for this reference.
    12. 12
      Lee, C. L.; Lin, Y. F.; Jiang, M. T.; Lu, W. Y.; Vandavasi, J. K.; Wang, L. F.; Lai, Y. C.; Chiang, M. Y.; Chen, H. Y. Improvement in Aluminum Complexes Bearing Schiff Bases in Ring-Opening Polymerization of ε-Caprolactone: A Five-Membered-Ring System. Organometallics 2017, 36, 1936– 1945,  DOI: 10.1021/acs.organomet.7b00068
      Google Scholar
      12
      Improvement in Aluminum Complexes Bearing Schiff Bases in Ring-Opening Polymerization of ε-Caprolactone: A Five-Membered-Ring System
      Lee, Chieh-Ling; Lin, Ya-Fan; Jiang, Man-Ting; Lu, Wei-Yi; Vandavasi, Jaya Kishore; Wang, Li-Fang; Lai, Yi-Chun; Chiang, Michael Y.; Chen, Hsuan-Ying
      Organometallics (2017), 36 (10), 1936-1945CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      A series of five- and six-membered-ring Al complexes bearing Schiff bases was synthesized and their application to the ring-opening polymn. of ε-caprolactone (CL) was studied. The five-membered-ring Al complexes have been shown to have a significantly higher polymn. rate than six-membered-ring Al complexes (2-3 fold for CL polymn.). The X-ray data revealed that the Al center of a five-membered-ring Al complex is farther from the Schiff base ligand than is that of a six-membered-ring Al complex. The results of d. functional theory calcns. also suggest that more space around the Al center of five-membered-ring Al complexes may reduce the steric repulsion in CL polymn. and increase the catalytic activity of five-membered-ring Al complexes.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXntFarsrw%253D&md5=63e2e09693ff1b820164669a4c3676bf
    13. 13
      (a) Black, D. S. C.; Craig, D. C.; Kumar, N.; Wong, L. C. H. Nickel(II) Complexes of Imine Ligands Derived from 7-Formylindoles. J. Chem. Soc., Chem. Commun. 1985, 1172– 1173,  DOI: 10.1039/c39850001172
      Google Scholar
      There is no corresponding record for this reference.
      (b) Liu, S.–F.; Wu, Z.; Schmider, H. L.; Aziz, H.; Hu, N.–X.; Popovic, Z.; Wang, S. Syntheses, Structures, and Electroluminescence of New Blue/Green Luminescent Chelate Compounds:  Zn(2-Py-in)2(THF), BPh2(2-Py-in), Be(2-Py-in)2, and BPh2(2-Py-Aza) [2-Py-in = 2-(2-Pyridyl)Indole; 2-Py-Aza = 2-(2-Pyridyl)-7-Azaindole]. J. Am. Chem. Soc. 2000, 122, 3671– 3678,  DOI: 10.1021/ja9944249
      Google Scholar
      13b
      Syntheses, Structures, and Electroluminescence of New Blue/Green Luminescent Chelate Compounds: Zn(2-py-in)2(THF), BPh2(2-py-in), Be(2-py-in)2, and BPh2(2-py-aza) [2-py-in = 2-(2-pyridyl)indole; 2-py-aza = 2-(2-pyridyl)-7-azaindole]
      Liu, Shi-Feng; Wu, Qingguo; Schmider, Hartmut L.; Aziz, Hany; Hu, Nan-Xing; Popovic, Zoran; Wang, Suning
      Journal of the American Chemical Society (2000), 122 (15), 3671-3678CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Four novel blue/green luminescent compds., Zn(2-py-in)2(THF) (1), BPh2(2-py-in) (2), Be(2-py-in)2 (3), and BPh2(2-py-aza) (4) (2-py-in = 2-(2-pyridyl)indole and 2-py-aza = 2-(2-pyridyl)-7-azaindole) were synthesized and fully characterized. The 2-py-in ligand and 2-py-aza ligand in the new compds. are chelated to the central atom. Compds. 2-4 are air stable and readily sublimable, with a m.p. >250°. In the solid state, compds. 1-4 have an emission max. at λ 488, 516, 490, and 476 nm, resp. The structures of compds. 2 and 4 are similar. The blue shift of emission energy displayed by compd. 4, in comparison to that of 2, is attributed to the presence of an extra N atom in the 2-py-aza ligand as confirmed by ab initio calcns. on compds. 2 and 4. Electroluminescent devices of compds. 3 and 4 were fabricated by using N,N'-di-1-naphthyl-N,N'-diphenylbenzidine (NPB) as the hole transporting layer, Alq3 (q = 8-hydroxyquinolinato) as the electron transporting layer, and compd. 3 or 4 as the light emitting layer. At 20 mA/cm2 the EL device of 3 has an external efficiency of 1.06 cd/A while the EL device of 4 has an external efficiency of 2.34 cd/A, demonstrating that compds. 3 and 4 are efficient and promising emitters in electroluminescent devices.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3cXhvFyms78%253D&md5=06b8dd480e60f368ab8a884096bda6d0
      (c) Mason, M. R.; Fneich, B. N.; Kirschbaum, K. Titanium and Zirconium Amido Complexes Ligated by 2,2‘-Di(3-Methylindolyl)Methanes:  Synthesis, Characterization, and Ethylene Polymerization Activity. Inorg. Chem. 2003, 42, 6592– 6594,  DOI: 10.1021/ic0347236
      Google Scholar
      There is no corresponding record for this reference.
      (d) Kingsley, N. B.; Kirschbaum, K.; Mason, M. R. Confirmation of Bridging N-Indolides in 3-Methylindole and Di- and Tri(3-Methylindolyl)Methane Complexes of Dimethyl-, Diethyl-, and Diisobutylaluminum. Organometallics 2010, 29, 5927– 5935,  DOI: 10.1021/om100781k
      Google Scholar
      13d
      Confirmation of Bridging N-Indolides in 3-Methylindole and Di- and Tri(3-methylindolyl)methane Complexes of Dimethyl-, Diethyl-, and Diisobutylaluminum
      Kingsley, Nicholas B.; Kirschbaum, Kristin; Mason, Mark R.
      Organometallics (2010), 29 (22), 5927-5935CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      Reactions of 3-methylindole with one equiv. of R3Al produce the N-indolide-bridged dimers [(3-CH3C8H5N)AlR2]2 (R = Me (3a), Et (3b), iBu (3c)). NMR spectroscopy (1H, 13C) demonstrates that compds. 3a, 3b, and 3c each exist in soln. as a mixt. of interconverting syn and anti isomers. Reactions of di(3-methylindolyl)methane or tri(3-methylindolyl)methane with one equiv. of Me3Al or Et3Al similarly produce the N-indolide-bridged compds. [(Ph)HC(3-CH3C8H4NAlR2)2] (R = Me (4)) or [HC(3-CH3C8H4NAlR2)3] (R = Me (5a), Et (5b)), resp. Reaction of tri(3-methyl-2-indolyl)methane with one equiv. of iBu3Al yielded the N-indolide-bridged compd. [HC(3-CH3C8H4NAliBu2)2(3-CH3C8H4NAl(H)iBu)] (6), apparently the result of β-H elimination of isobutene from one iso-Bu group per complex. The mol. structures of compds. 3a, 4, and 5a were detd. by x-ray crystallog. Crystallog. data confirmed the presence of μ2-η1:η1-N-3-methylindolide groups in each compd., the 1st examples of bridging N-indolide groups in compds. of the Group IIIB elements.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtlWksbzF&md5=0fafb7f1987f4b9ac06ec138ef7daa92
      (e) Peng, K.-F.; Chen, Y.; Chen, C.-T. Synthesis and Catalytic Application of Magnesium Complexes Bearing Pendant Indolyl Ligands. Dalt. Trans. 2015, 44, 9610– 9619,  DOI: 10.1039/C5DT01173F
      Google Scholar
      13e
      Synthesis and catalytic application of magnesium complexes bearing pendant indolyl ligands
      Peng, Kuo-Fu; Chen, Yun; Chen, Chi-Tien
      Dalton Transactions (2015), 44 (20), 9610-9619CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      Three novel indole-based ligand precursors [HIndPhR, R = methoxy, HIndPhOMe (2a); thiomethoxy, HIndPhSMe (2b); and N,N'-dimethylamino, HIndPhNMe2 (2c)] were synthesized via Sonogashira and cyclization reactions with moderate to high yield. Reactions of these ligand precursors with 0.6 equiv of MgBu2 in THF afforded the magnesium bis-indolyl complexes 3a-3c, resp. All the ligand precursors and related magnesium complexes were characterized by NMR spectroscopy and elemental analyses. The mol. structures are reported for 3a and 3b. Under optimized conditions, compd. 3a demonstrates efficient catalytic activities towards the ring opening polymn. of L-lactide and ε-caprolactone in the presence of BnOH.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXms1yhs7g%253D&md5=fa75b765d4980de4d778fc77bfa123b0
    14. 14
      Hoops, S.; Sahle, S.; Gauges, R.; Lee, C.; Pahle, J.; Simus, N.; Singhal, M.; Xu, L.; Mendes, P.; Kummer, U. COPASI—a COmplex PAthway SImulator. Bioinformatics 2006, 22, 3067– 3074,  DOI: 10.1093/bioinformatics/btl485
      Google Scholar
      14
      COPASI - A COmplex PAthway SImulator
      Hoops, Stefan; Sahle, Sven; Gauges, Ralph; Lee, Christine; Pahle, Juergen; Simus, Natalia; Singhal, Mudita; Xu, Liang; Mendes, Pedro; Kummer, Ursula
      Bioinformatics (2006), 22 (24), 3067-3074CODEN: BOINFP; ISSN:1367-4803. (Oxford University Press)
      Motivation: Simulation and modeling is becoming a std. approach to understand complex biochem. processes. Therefore, there is a big need for software tools that allow access to diverse simulation and modeling methods as well as support for the usage of these methods. Results: Here, we present COPASI, a platform-independent and user-friendly biochem. simulator that offers several unique features. We discuss numerical issues with these features; in particular, the criteria to switch between stochastic and deterministic simulation methods, hybrid deterministic-stochastic methods, and the importance of random no. generator numerical resoln. in stochastic simulation.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28Xht1OgsrvK&md5=ff340a6c0c48f525a92a50c983aa1ddd
    15. 15
      Houk, K. H.; Jabbari, A.; Hall, H. K., Jr.; Alemán, C. Why δ-Valerolactone Polymerizes and γ-Butyrolactone Does Not. J. Org. Chem. 2008, 73, 2674– 2678,  DOI: 10.1021/jo702567v
      Google Scholar
      15
      Why δ-Valerolactone Polymerizes and γ-Butyrolactone Does Not
      Houk, K. N.; Jabbari, Arash; Hall, H. K.; Aleman, Carlos
      Journal of Organic Chemistry (2008), 73 (7), 2674-2678CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      γ-Butyrolactone, unlike δ-valerolactone, does not polymerize despite a strain energy of ∼8 kcal mol-1 which could be relieved by opening the s-cis lactone ester bond to an s-trans ester bond in the polymer. To explain this anomaly, we have applied quantum mech. methods to study the thermochem. involved in the ring-opening reactions of γ-butyrolactone and δ-valerolactone, the conformational preferences of model mols. that mimic their corresponding homo-polyesters, and the variation of enthalpy assocd. to the polymerizability of such two cyclic lactones. The overall results indicate that the lack of polymerizability of γ-butyrolactone should be attributed to the low strain of the ring, which shows much less geometric distortion in the ester group than δ-valerolactone, and the notable stability of the coiled conformations found in model compds. of poly-4-hydroxybutyrate.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXivV2gsrw%253D&md5=4c8d631874c54f7c5762a1cd8a631fa3
    16. 16
      Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Stereoselective Ring-Opening Polymerization of a Racemic Lactide by Using Achiral Salen– and Homosalen–Aluminum Complexes. Chem. - Eur. J. 2007, 13, 4433– 4451,  DOI: 10.1002/chem.200601308
      Google Scholar
      16
      Stereoselective ring-opening polymerization of a racemic lactide by using achiral salen- and homosalen-aluminum complexes
      Nomura, Nobuyoshi; Ishii, Ryohei; Yamamoto, Yoshihiko; Kondo, Tadao
      Chemistry - A European Journal (2007), 13 (16), 4433-4451CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Highly isotactic polylactide or poly(lactic acid) is synthesized in a ring-opening polymn. (ROP) of racemic lactide with achiral salen- and homosalen-aluminum complexes (salenH2 = N,N'-bis(salicylidene)ethylene-1,2-diamine; homosalenH2 = N,N'-bis(salicylidene)trimethylene-1,3-diamine). A systematic exploration of ligands demonstrates the importance of the steric influence of the Schiff base moiety on the degree of isotacticity and the backbone for high activity. The complexes prepd. in situ are pure enough to apply to the polymns. without purifn. The crystal structures of the key complexes are elucidated by x-ray diffraction, which confirms that they are chiral. However, anal. of the 1H and 13C NMR spectra unambiguously demonstrates that their conformations are so flexible that the chiral environment of the complexes cannot be maintained in soln. at 25° and that the complexes are achiral under the polymn. conditions. The flexibility of the backbone in the propagation steps is also documented. Hence, the isotacticity of the polymer occurs due to a chain-end control mechanism. The highest reactivity in the present system is obtained with the homosalen ligand with 2,2-di-Me substituents in the backbone (ArCH=NCH2CMe2CH2N=CHAr), whereas tBuMe2Si substituents at the 3-positions of the salicylidene moieties lead to the highest selectivity (Pmeso = 0.98; Tm = 210°). The ratio of the rate consts. in the ROPs of racemic lactide and L-lactide is found to correlate with the stereoselectivity in the present system. The complex can be utilized in bulk polymn., which is the most attractive in industry, although with some loss of stereoselectivity at high temp., and the afforded polymer shows a higher melting temp. (Pmeso = 0.92, Tm up to 189°) than that of homochiral poly(L-lactide)(Tm = 162-180°). The "livingness" of the bulk polymn. at 130° is maintained even at a high conversion (97-98%) and for an extended polymn. time (1-2 h).
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXmt12gt7g%253D&md5=cc12a44282eb59acbd5a605168e84aa0

  • Supporting Information

    Supporting Information

    ARTICLE SECTIONS
    Jump To

    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b04540.

    • Computational details, spectroscopic and kinetic data, and additional figures including structures, 1H and 13C NMR spectra, aromatic region overlay, [CL] decay and [PCL] growth over time, and first-order decay plot (PDF)

    • Cartesian coordinates of stationary points (XYZ)

    • X-ray crystallography data (CIF)


    Terms & Conditions

    Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

PDF [ 1MB]

Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

Pair your accounts.

Export articles to Mendeley

Get article recommendations from ACS based on references in your Mendeley library.

You’ve supercharged your research process with ACS and Mendeley!

STEP 1:
Click to create an ACS ID

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

Please note: If you switch to a different device, you may be asked to login again with only your ACS ID.

MENDELEY PAIRING EXPIRED
Your Mendeley pairing has expired. Please reconnect