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

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 × 10 3 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 1 H and 13 C NMR spectroscopy of the reaction products of each step. The kinetic behavior of commercially available yttrium isopropoxide was more complicated.