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, (L (1) ZnOEt) (2) (L (1) = 2, 4-di-tert-butyl-6- (2'-dimethylaminoethyl) methylaminomethylphenolate). 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 L (1) ZnOEt 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 M (n) 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 L (1) ZnOEt to be distinguished and the empirical rate law to be elucidated, -dLA/dt = k (p) L (1) ZnOEtLA. These studies also showed that L (1) ZnOEt 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.