Influences of a Dizinc Catalyst and Bifunctional Chain Transfer Agents on the Polymer Architecture in the Ring-Opening Polymerization of ε-Caprolactone

Abstract

The polymerization of ε-caprolactone is reported using various bifunctional chain transfer agents and a dizinc catalyst. Conventionally, it is assumed that using a bifunctional chain transfer agent (CTA), polymerization will be initiated from both functional groups; however, in this study this assumption is not always substantiated. The different architectures and microstructures of poly(ε-caprolactone) samples (PCL) are compared using a series of bifunctional and monofunctional alcohols as the chain transfer agents, including trans -1,2-cyclohexanediol (CHD), ethylene glycol (EG), 1,2-propanediol (PD), poly(ethylene glycol) (PEG), 2-methyl-1,3-propanediol (MPD), 1-hexanol, 2-hexanol, and 2-methyl-2-pentanol. A mixture of two architectures is observed when diols containing secondary hydroxyls are used, such as cyclohexanediol or propanediol; there are chains that are both chain-extended and chain-terminated by the diol. These findings indicate that not all secondary hydroxyl groups initiate polymerization. In contrast, chain transfer agents containing only primary hydroxyl groups in environments without steric hindrance afford polymer chains of a single chain extended architecture, whereby polymer chains are initiated from both hydroxyl groups on the diol. Kinetic analyses of the polymerizations indicate that the propagation rate constant ( k p ) is significantly higher than the initiation rate constant ( k i ): k p / k i > 5. A kinetic study conducted using a series of monof…