Kinetics and Mechanism of ε-Caprolactone Polymerization Using Yttrium Alkoxides as Initiators

Abstract

ε-Caprolactone polymerization 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 1.65 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-propanol. In the second step the polar alkoxide is able to attack the carbonyl group and form 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 1H and 13C 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.