Kinetics and mechanism of epsilon-caprolactone polymerization using yttrium alkoxides as initiators

epsilon-Caprolactone polymerization using an initiating system comprising tris(2,6-di-tert-butylphenoxy) yttrium and 2-prapanol was first-order in monomer and initiator. A propagation rate constant of 1.65 L . mol(-1). s(-1) was determined in dichloromethane. The reaction proceeds via a three-step mechanism. In the first step the large 2,6-di-tert-butylphenoxy ligands are exchanged for the smaller 2-propanal. In the second step the polar alkoxide is able to attack the carbonyl group and ii,lm the thermodynamically favored ring-opened product. After the first two initiating steps, the polymerization can proceed until complete 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 single step. The kinetic behavior of commercially available yttrium isopropoxide was more complicated. 2-Propanol could be used as an effective chain transfer agent for this initiator.