High molecular weight poly(lactic acid) produced by an efficient iron catalyst bearing a bis(amidinato)-N-heterocyclic carbene ligand

Abstract

An iron bis(alkoxide) complex containing a bis(amidinato)-N-heterocyclic carbene ancillary ligand has been synthesized and utilized for the polymerization of (rac)-lactide. This complex was an excellent precatalyst for the controlled polymerization of (rac)-lactide leading to polymer with a number average molecular weight that is approximately four times larger than the analogous bis(imino)pyridine iron complexes while maintaining a similar reaction rate. The unusually high molecular weight observed in these reactions was explained by a fraction of the added complex being the active catalyst for lactide polymerization due to slow initiation versus propagation rates. Mechanistic investigations corroborated this hypothesis revealing an induction period that can be shortened by increasing the concentration of the lactide. This feature of the catalyst system led to a situation where larger observed rate constants were measured as the amount of lactide relative to iron was increased. At the very high lactide to complex ratio of 5000:1 (0.02mol% catalyst loading), the bis(amidinato)-N-heterocyclic carbene iron bis(alkoxide) complex produced very high molecular weight polymer (Mn>350kg/mol, PDI=1.2) within 8h (85% conversion). Kinetic studies revealed that the bis(amidinato)-N-heterocyclic carbene iron complex is approximately seven times more active than the corresponding bis(amino)pyridine iron complex.