Selective Transesterification to Control Copolymer Microstructure in the Ring-Opening Copolymerization of Lactide and ε-Caprolactone by Lanthanum Complexes

A series of novel lanthanum amido complexes, supported by ligands designed around the salan framework (salan = N,N′-bis(o-hydroxy, m-di-tert-butylbenzyl)-1,2-diaminoethane) were synthesized and fully characterized in the solid and solution states. The ligands incorporate benzyl or 2-pyridyl substituents at each tertiary amine center. The complexes were investigated as catalysts in the ring-opening homopolymerization of lactide (LA) and ε-caprolactone (ε-CL) and copolymerization of equimolar amounts of LA and ε-CL at ambient temperature. Solvent (THF or toluene) and the number of 2-pyridyl groups in the complex were found to influence the reactivity of the catalysts in copolymerization reactions. In all cases, complete conversion of LA to PLA was observed. The use of THF, a coordinating solvent, suppressed ε-CL polymerization, while the presence of one or more 2-pyridyl groups promoted ε-CL polymerization. Each copolymer gave a monomodal trace in gel permeation chromatography–size-exclusion chromatography (GPC-SEC) experiments, indicative of copolymer formation over homopolymerization. Copolymer microstructure was found to be dependent on catalyst structure and reaction solvent, ranging from blocky to close to alternating. Experiments revealed rapid conversion of LA in the initial stages of the reaction, followed by incorporation of ε-CL into the copolymer by either transesterification or propagation reactions. Significantly, the mode of transesterification (TI or TII) that occurs is determined by the structure of the metal complex and the reaction solvent, leading to the possibility of controlling copolymer microstructure through catalyst design.


NMR Spectra of Ligands
Figure S1: 1 H NMR Spectrum of H2L 1 in CDCl3           Data can be obtained from: www.ccdc.cam.ac.uk a The structure of 3 was found to contain a high level of disorder, particularly of the dimethylsilyl groups, and efforts to model this were unsuccessful, leading to large thermal ellipsoids, a high R1 and high residual electron density.Additionally, a solvent mask was applied to account for 1.5 molecules of disordered toluene per unit cell.

Additional Experiments
Scheme S1: Copolymerisation of active PLA and PCL chains promoted by complex 3 in THF.
Figure S45: 1 H NMR spectrum of the methine and methylene region (CDCl3) of the copolymerisation of active PCL and PLA chains using complex 3 in THF.Table S4: Thermal characterisation data for selected copolymers.

Figure
FigureS59: 1 H NMR spectrum of a copolymer prepared by complex 3 in THF* for DOSY analysis .

Figure S46 :
Figure S46: Quantitative 13 C NMR spectrum of the carbonyl region (CDCl3) of the copolymerisation of active PCL and PLA chains using complex 3 in THF.

Figure
Figure S48: (A) The carbonyl region of the 13 C NMR spectrum of the NMR scale reaction between 3 and 1 equiv (S)-ethyl-lactate in benzene-d6 and (B) The carbonyl region of the 13 C NMR spectrum of (S)-ethyl-lactate in benzene-d6 for comparison.

Figure
Figure S49: 1 H DOSY NMR spectrum of 1 Data processed using Dynamics Centre software

Figure
Figure S50: 1 H DOSY NMR spectrum of 2 Data processed using Dynamics Centre software

Figure
Figure S51: 1 H DOSY NMR spectrum of 3 Data processed using Dynamics Centre software

Figure S52 :
Figure S52: Showing 1 H NMR spectra from the NMR scale reaction of complex 1 with 1 equivalent Ph3P=O in C6D6.

Figure S53 :
Figure S53: Showing 1 H NMR spectra from the NMR scale reaction of complex 2 with 1 equivalent Ph3P=O in C6D6.

Figure S54 :
Figure S54: Showing 1 H NMR spectra from the NMR scale reaction of complex 3 with 1 equivalent Ph3P=O in C6D6.
NMR spectroscopy at 400 MHz in CDCl3.c Determined by DSC analysis.Data was obtained from the second heating cycle using heating rate of 10 °C min -1 d Tgtheo values calculated using the Fox equation.

Figure S56: 1 H*
FigureS56: 1 H NMR spectrum of a copolymer prepared by complex 2 in toluene* for DOSY analysisCopolymer prepared with 2 in toluene.esp

Figure
Figure S57: 1 H DOSY NMR spectrum of a copolymer prepared by complex 2 in toluene

Figure S59: 1 H*
FigureS59: 1 H NMR spectrum of a copolymer prepared by complex 3 in THF* for DOSY analysisCopolymer prepared with 3 in THF.esp

Figure
Figure S60: 1 H DOSY NMR spectrum of a copolymer prepared by complex 3 in THF

Figure*
Figure S62: 1 H NMR spectrum of a copolymer prepared by complex 3 in toluene* for DOSY analysis Copolymer prepared with 3 in toluene.esp

Figure
Figure S63: 1 H DOSY NMR spectrum of a copolymer prepared by complex 3 in toluene

Figure S65 :
Figure S65: Natural bond orbitals representing interactions between the lanthanum ion and the ligand framework showing character from both the donating ligand atom and the lanthanum for complex 1 (top), 2 (middle), and 3 (bottom).

Table 1
Figure S30: Carbonyl region of the 13 C NMR Spectrum (quantitative) of polymer fromTable 1, entry 11 GPC Trace of sample from Table 1 entry 1 Figure S33: GPC Trace of sample from Table 1 entry 2 Figure S34: GPC Trace of sample from Table 1 entry 3 Figure S35: GPC Trace of sample from Table 1 entry 4 Figure S36: GPC Trace of sample from Table 1 entry 5 Figure S37: GPC Trace of sample from Table 1 entry 6 Figure S38: GPC Trace of sample from Table 1 entry 7 Figure S39: GPC Trace of sample from Table 1 entry 8 Figure S40: GPC Trace of sample from Table 1 entry 9 Figure S41: GPC Trace of sample from Table 1 entry 10 Figure S42: GPC Trace of sample from Table 1 entry 11 Figure S43: GPC Trace of sample from Table 1 entry 12 5. X-ray data

Table S3 :
Single Crystal X-ray Data Entry Initiator [I] Solvent CL/LA (mol %) b