Bio-sourced and degradable polymers are critical materials that can reduce our dependence on fossil-fuel based polymer production. The ring opening polymerization (ROP) of lactide to make polylactide (PLA) is an efficient route in the synthesis of high molecular weight PLA. Intense research in catalyst design has led to the development of highly stereospecific catalysts; however, the design of systems which display high activity at room temperature and tolerance to protic impurities have remained challenging. Recently, we reported the synthesis, characterization, and stoichiometric reactivity of a novel class of homoleptic rare-earth bis(phosphine-oxide) methanides, HRL− (R = Me, Ph; RE(HPhL)3 and RE2(HMeL)6). These complexes are highly dynamic and can act as both a strong nucleophiles and Brønsted-bases. Herein we report the catalytic ROP activity of homoleptic yttrium and lanthanum complexes of bis(phosphine-oxide) methanides. The strong Brønsted-basicity of the methanides coupled with facile access to the strong Lewis acids generates catalysts capable of unprecedented activity. Our mechanistic studies have revealed that these complexes operate through divergent initiation mechanisms for the ROP of lactide depending on the presence or absence of a chain transfer agent (CTA). Notably, a rare deprotonation mechanism was observed in the absence of CTA, which leads to the facile synthesis of ultrahigh molecular weight PLA. These high-activity catalysts are capable of operating at extremely low catalyst loadings, (0.01 mol %) and polymerizing unpurified lactide.

Divergent initiation mechanisms in lactide polymerization with rare-earth bis(phosphine-oxide) methanides

ACS Spring 2022

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