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 1 H and 13 C 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.