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 ₁ZnOEt)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 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 ₁ZnOEt to be distinguished and the empirical rate law to be elucidated, -d[LA]/dt = K_p[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.