Kinetics and Mechanism of l-Lactide Polymerization Using Two Different Yttrium Alkoxides as Initiators

Abstract

Polymerization of l-lactide in dichloromethane at 25 °C using an initiating system comprising tris(2,6-di-tert-butylphenoxy)yttrium and 2-propanol was first order in monomer and initiator. A propagation rate constant of 1.92 × 103 L mol-1 min-1 was determined using in situ UV spectroscopy. In the first step the large 2,6-di-tert-butylphenoxy ligands are exchanged for the sterically less demanding 2-propoxide ligands. In the second step the alkoxide is able to attack the carbonyl group with formation of the ring-opened product. After the first two initiating steps the polymerization can proceed until the equilibrium conversion of monomer is reached. Evidence for these three steps is provided by 1H and 13C NMR spectroscopy of the reaction products of each step. The kinetic behavior of commercially available yttrium isopropoxide was more complicated.