(Bipyridine bisphenolate)-aluminum/onium salt pair: a highly active binary catalyst for ring-opening polymerization of lactide with improved thermostability and protic tolerance

Abstract

Developing aluminum-based catalysts with high thermostability and good protic tolerance in the ring-opening polymerization (ROP) of lactide (LA) remains a great challenge, because of the decomposition of the active species under these conditions. In this contribution, a series of bipyridine bisphenolate aluminum (BpyBph-Al) complexes were synthesized and used to catalyze the ROP of LA with the assistance of an onium salt as a cocatalyst in epoxide. We disclosed the relationship between the structures of (BpyBph)Al and their catalytic performances, and thoroughly investigated the effects of polymerization conditions (including polymerization temperature, catalyst/cocatalyst and monomer/catalyst feed ratios) on LA polymerization behaviors. The (BpyBph)Al/onium binary catalytic system is of particular interest as it exhibits high thermostability and excellent tolerance to a wide range of protonic impurities, compared with the (salen)Al/onium salt. These features allowed a rapid LA polymerization under bulk and melt conditions with a very high feed ratio of [LA] : [catalyst pair] : [expoxide] = 5000 : 1 : 2. Finally, the chain initiation mechanism was further clarified by in situ NMR experiments. It was shown that a six-coordinated bis(alkoxide) aluminum complex will be generated by the (BpyBph)Al/onium salt in the presence of epoxide, which can be reversibly converted to a five coordinated (BpyBph)Al alkoxide and an alkoxide salt. The cooperative effects between the (BpyBph)Al alkoxide and the alkoxide salt were mainly responsible for rapid polymerization.