Statistical Copolymerization of Epoxides and Lactones to High Molecular Weight

Copolymerization provides a modular strategy for compositional control of structure-property relationships in polymeric materials. However, this versatility is typically limited to structurally homologous comonomers. To further expand the scope of copolymerization in heterocyclic systems, we explored the copolymerization of structurally distinct lactones and epoxides utilizing the classical Vandenberg catalyst. Copolymerizations were conducted between monomer pairs selected from among two common lactones (dl-lactide, ε-caprolactone) and four epoxides (epichlorohydrin, butylene oxide, propylene oxide, ethylene oxide). The resultant materials had molecular weights of up to 16 Mg/mol. Reactivity ratios were determined for the copolymerization of dl-lactide and propylene oxide, which were consistent with a gradient copolymer with propylene oxide (PO) being the preferred monomer: r_PO = 2.81 ± 0.27 and r_LA = 0.36 ± 0.02. The copolymerization between ε-caprolactone and propylene oxide was also monitored by ¹H NMR spectroscopy. A greater preference for propylene oxide was evident, but incomplete consumption of the ε-caprolactone under these reaction conditions complicated determination of reactivity ratios. Meyer-Lowry analysis of the time-dependent compositional data provided estimates of r_PO = 2.17 ± 0.04 and r_CL = 0.08 ± 0.01. Distinct ester-ether dyad signals were observed in the ¹H NMR spectra of the copolymers, and thermal properties of the copolymers were distinct from those of the respective pure homopolymers. The expected hydrolytic degradability of poly[(dl-lactide)-co-(ethylene oxide)] was demonstrated under neutral and basic conditions.