Polymerization of L-lactide in dichloromethane at 25 degrees 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 x 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 H-1 and C-13 NMR spectroscopy of the reaction products of each step. The kinetic behavior of commercially available yttrium isopropoxide was more complicated.