A Highly Active Zinc Catalyst for the Controlled Polymerization of Lactide

We report the preparation, structural characterization, and detailed lactide polymerization behavior of a new Zn(II) alkoxide complex, (L1ZnOEt)2 (L1 = 2,4-di-tert-butyl-6-{[(2'-dimethylaminoethyl)methylamino]methyl}phenolate). While an X-ray crystal structure revealed the complex to be dimeric in the solid state, nuclear magnetic resonance and mass spectrometric analyses showed that the monomeric form L1ZnOEt predominates in solution. The polymerization of lactide using this complex proceeded with good molecular weight control and gave relatively narrow molecular weight distribution polylactide, even at catalyst loadings of <0.1% that yielded Mn as high as 130 kg mol-1. The effect of impurities on the molecular weight of the product polymers was accounted for using a simple model. Detailed kinetic studies of the polymerization reaction enabled integral and nonintegral orders in L1ZnOEt to be distinguished and the empirical rate law to be elucidated, −d[LA]/dt = kp[L1ZnOEt][LA]. These studies also showed that L1ZnOEt polymerizes lactide at a rate faster than any other Zn-containing system reported previously. This work provides important mechanistic information pertaining to the polymerization of lactide and other cyclic esters by discrete metal alkoxide complexes.