The ring-opening copolymerization (ROCOP) of cyclic anhydrides (A) and epoxides (B) is a controlled route to alternating polyesters, with the best catalysts delivering high ester sequence (AB) selectivity. In contrast, very few catalysts can selectively enchain the same monomers to produce ester-alt-ether (ABB) sequences. Here, simple, commercial, homoleptic Zr(IV) catalysts (alkoxides and chloride) show faster rates, controlled polymerization and high selectivity in epoxide/anhydride ROCOP to selectively produce poly(ester-alt-ethers) (>95%); they outperform the best prior Zr(IV) coordination catalyst. Comparing the copolymerization rates and selectivity using a systematic series of homoleptic Groups III–V metal alkoxide complexes, all d0 configurations, reveals a metal structure-polymer linkage selectivity correlation. Plots of the polymer sequence selectivity (ABB) against the metal center Lewis acidity, as assessed by its hydrolysis constant (pKh), show a linear correlation, identifying the most selective catalysts for bothpoly(ester-alt-ether) and polyester production. High Lewis acidity catalysts (1 < pKh < 0.6) are best for poly(ester-alt-ethers) (ABB), while those with lower relative Lewis acidities (8.1 < pKh < 8.5) are selective for polyesters (AB) and intermediate acidity metal catalysts form polymers with mixed linkages. The discovery of simple, commercial selective catalysts for epoxide and anhydride copolymerization is an important advance relevant to future production of elastomers, surfactants and adhesives.
Kerr et al. (Tue,) studied this question.