Divergent Catalytic Strategies for the Cis/Trans Stereoselective Ring-Opening Polymerization of a Dual Cyclic Carbonate/Olefin Monomer

A dual seven-membered cyclic carbonate/olefin monomer was synthesized from CO2 and cis-1,4-butenediol and polymerized. The properties of the polymer were controlled using divergent catalytic strategies toward the stereochemistry of the olefin. Ring-opening polymerization of the cyclic carbonate using an organocatalytic approach retained the cis-stereoconfiguration of the olefin and yielded a hard semicrystalline polymer (Tm 115 °C). Ring-opening metathesis polymerization using Grubbs’ catalyst proceeded with high trans-stereoregularity (95%) and produced a soft amorphous polymer (Tg −22 °C). Cis to trans isomerization of the polymer was possible using Cu(I) salts under UV light. In all polymers, the C=C double bond remained available for postpolymerization modification and thermoset resins were formed by cross-linking. From this single monomer, cis-trans-cis triblock copolymers, with potential applications as thermoplastic elastomers, were synthesized by combining both strategies using cis-1,4-butenediol as a chain transfer agent.


* S Supporting Information
ABSTRACT: A dual seven-membered cyclic carbonate/ olefin monomer was synthesized from CO 2 and cis-1,4butenediol and polymerized. The properties of the polymer were controlled using divergent catalytic strategies toward the stereochemistry of the olefin. Ringopening polymerization of the cyclic carbonate using an organocatalytic approach retained the cis-stereoconfiguration of the olefin and yielded a hard semicrystalline polymer (T m 115°C). Ring-opening metathesis polymerization using Grubbs' catalyst proceeded with high transstereoregularity (95%) and produced a soft amorphous polymer (T g −22°C). Cis to trans isomerization of the polymer was possible using Cu(I) salts under UV light. In all polymers, the CC double bond remained available for postpolymerization modification and thermoset resins were formed by cross-linking. From this single monomer, cis-trans-cis triblock copolymers, with potential applications as thermoplastic elastomers, were synthesized by combining both strategies using cis-1,4-butenediol as a chain transfer agent. C ontrolling polymer stereochemistry is a powerful strategy to manipulate its physical properties. 1−8 The influence of the stereochemistry of alkene bonds in polymer chains is particularly remarkable. In nature, poly(cis-isoprene) (natural rubber) has thus vastly superior elastomeric properties compared with poly(trans-isoprene) (gutta percha). 9−11 Catalysis has been an efficient way to control synthetic polymers' stereochemistry by generating chain tacticity, yet examples of its application in cis/trans isomerism remain comparatively rare. Hillmyer and co-workers for example demonstrated the stereoselective ring-opening metathesis polymerization of cis-cyclooctene monomers using Ru catalysts (trans-selective) 12 and Mo catalysts (cis-selective). 13 Becker and Dove reported an organocatalytic stereocontrolled synthesis of unsaturated polyesters, resulting in a tunable cis/trans ratio, crystallinity, and mechanical properties. 14 Postpolymerization catalysis was also exploited to isomerize poly(propylene maleate) into poly(propylene fumarate) toward 3D printing and medical applications. 15−17 The ability to drastically change polymer properties through cis/trans isomerization, e.g. from hard to soft materials, is particularly appealing in the context of synthetic thermoplastics elastomers (TPEs), which have a wide range of applications including 3D printing, elastomers (e.g., in footwear), pressuresensitives adhesives, and coatings. 18,19 TPEs have similar properties to chemically cross-linked rubbers, 20 but they can be melt-(re)processed like thermoplastics. TPEs are usually ABA triblock copolymers in which hard/crystalline end-blocks (A) can microphase-separate from the soft/amorphous midblock (B) and act as physical cross-links to strengthen the elastic matrix. To the best of our knowledge, these TPEs are always made from two different monomers and may require purification of the middle block. Motivated by the desire to prepare hard or soft materials, as well as triblock TPEs, from one single monomer and in one pot, we anticipated that cis/ trans stereoselective polymerization of a selected monomer would provide the desired platform. Here we describe such a monomer, which is a dual cyclic carbonate/olefin. We also demonstrate that these polymers are amenable to cross-linking and thermosets formation.
cis-1,4-Butenediol is a 1,4-diol which may be derived from erythritol, 21 a fermentation product of glucose ( Figure 1). The corresponding cyclic carbonate, 4,7-dihydro-1,3-dioxepin-2one (1), was readily prepared in one step from CO 2 , in 51% crystalline yield, following a procedure reported by our group (Figure 1). 22 CO 2 insertion was achieved using 2 equiv of 2,2,6,6-tetramethylpiperidine (TMP) with cyclization facilitated by 1 equiv of para-toluenesulfonate chloride (TsCl). 1 is a seven-membered cyclic compound with both a carbonate and an alkene functionality, providing opportunities for ringopening polymerization using metathesis (ROMP) or transcarbonation (referred to here as ROP) methodologies. We reasoned that ROMP and ROP would both produce a polycarbonate, but could show opposite stereoselectivity toward the configuration of the polymer alkene bonds.
First, various organo-and metal-based catalysts were screened for the ROP of 1 in solution (Table S1). When using 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) as the catalyst (Table 1), ROP proceeded rapidly at room temperature with 100% conversion after 10 min, at a monomer/catalyst/initiator ([1] 0 :[cat.] 0 :[I] 0 ) feed ratio of 100:1:1 (Table 1, entry 2). The quantitative conversion of the monomer to polymer is consistent with the high ring strain of 1 calculated by DFT ( Figure S56). For each of the ROP catalysts tested, the cis configuration of the monomer's alkene was maintained in the polymer as evidenced by 1 H NMR spectroscopy ( Figure 2). Using TBD led to a good correlation between theoretical and experimental M n values measured by Size-Exclusion Chromatography (SEC), with dispersities (Đ M ) of 1.3−1.8 (Tables 1  and S1). ROP could also occur at 60°C in molten monomer using Sn(Oct) 2 or TBD as the catalyst (Table 1 entry 4 and  Table S1 entry 5, respectively). MALDI-ToF spectrometry and 1 H NMR spectroscopy confirmed the presence of 4-methyl benzyl alcohol and OH chain ends ( Figure S27). Initial rate kinetic studies carried out with TBD indicated first-order kinetics (k obs = 0.117 s −1 ) with respect to monomer concentration ( Figures S30−S31). M n was found to increase linearly with conversion ( Figures S32−S33) with an M n,SEC of 22 100 g mol −1 achievable (degree of polymerization (DP) ≈ 200, Figure S18).
Next, the ROMP of 1 was carried out (Scheme 1), catalyzed by Grubbs second generation catalyst (GII). Quantitative conversion was achieved at 22°C in solution after 30 min at the [1] 0 :[GII] 0 150:1 ratio. Good control over M n and Đ M was observed across the range of loadings tested (Table 1, entries  7−10 and Table S2). An M n,SEC of up to 34 300 g mol −1 (DP ≈ 300) was achieved after 50 min (Table 1, entry 9). Satisfyingly, GII favored the formation of the trans isomer with stereoregularities of up to 95% obtainable in 30 min under UV, as observed by 1 H NMR spectroscopy ( Figure 2 and Table 1, entry 8). The trans selectivity of the reaction was found to be sensitive to time, monomer concentration, and temperature (Table S2). Longer reaction times and high dilution favored formation of the trans isomer, with a maximum selectivity of 95% observed after 92 h (Table S2, entry 9). At 120°C, trans selectivity decreased to 88% and 80% when performing reactions in 1,2-tetrachloroethane or neat monomer, respectively (Table S2, entry 6, Table 1, entry 10).
Postpolymerization isomerization was also explored as DFT indicated the trans isomer was thermodynamically more stable ( Figure S57). Isomerization of the double bond was possible

Journal of the American Chemical Society
Communication under UV light at room temperature, using 1 alkene equivalent of CuI (52% conversion of poly(cis-1) to trans isomer, 68 h) ( Figure S10). No degradation of the polymer was observed by SEC ( Figure S13) or NMR spectroscopy ( Figure S12).
The appearance and solubility of poly(trans-1) are significantly different from those of poly(cis-1). At room temperature poly(trans-1) is a viscoelastic solid, soluble in THF, whereas poly(cis-1) is a powder, insoluble in THF but soluble in CHCl 3 ( Figure S8). Thermogravimetric analysis (TGA) indicated that the onset of thermal degradation (T d,onset ) for both polymers occurred at 152°C (Table S4, entries 1 and 2). Differential scanning calorimetry (DSC) revealed for the cis polymer the presence of a minor glass transition at −24°C (T g ) and a melting transition at 115°C (T m ) ( Figure S46). For the 5:95 (cis/trans) polymer, a major single T g at −22°C was observed, attributed to the cis regime ( Figure 3 and Figure S47). A T m was not observed. The significant difference in crystallinity of the trans and cis polymers highlights the impact of the double-bond isomerism on material properties.
The synthesis of a cis-trans-cis triblock was then attempted, utilizing a sequential ROMP/ROTEP concept originally demonstrated by Hillmyer and co-workers, 27 Table 2, entry 2). 13 C{ 1 H} NMR spectroscopy gave some insight into the copolymer chain microstructure, revealing limited transcarbonation reactions and well-defined blocks ( Figure S25). This was supported by DSC analysis, which showed phase separation and retention of the crystallinity of the cis end-blocks ( Figure  S55). 18 In conclusion, we have demonstrated that from novel dual monomer 1, divergent stereoselective polymerization catalytic strategies can direct polymer properties between a soft amorphous material (T g −22°C) and a hard semicrystalline material (T m 115°C). These thermoplastics can further be transformed into thermosets via cross-linking of the alkene polymer backbone. A CTA has also been employed to synthesize triblock ABA polymers from this single monomer, with potential applications as thermoplastic elastomers. Our ongoing efforts are now aimed at investigating the effect of composition and molar mass of these triblock copolymers on nanoscales morphologies and mechanical properties. Finally, the carbonate function of these polymers introduces some

Scheme 2. Sequential ROMP/ROTEP Strategy for the Formation of Cis-Trans-Cis Triblock Copolymers
Journal of the American Chemical Society Communication opportunities for chemical and biological degradation. We anticipate that understanding the degradability of these materials will facilitate their adoption as sustainable alternatives in various commodity and specialty applications.

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
Analytical facilities were provided through the Material and Chemical Characterisation Facility (MC 2 ) at the University of Bath. We thank the University of Bath HPC for computing resources, the UK EPSRC, and the University of Bath (studentship for TMG), as well as the Royal Society (RG/ 150538, UF/160021 fellowship to AB) for research funding. Sequential procedure outlined in Scheme 2, with [1] 0 = 1 mol L −1 . Monomer conversion quantitative for each step. Values in brackets were obtained after the ROMP step. b Calculated as M r (CTA) + (2 × n × M r (1)). c Calculated by the relative integration of the methylene protons in the CTA (δ = 4.19−4.27 ppm) and the polymer (δ = 4.60 − 4.80 ppm). d Calculated by SEC relative to polystyrene standards in THF eluent.

Journal of the American Chemical Society
Communication