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 (kp) is significantly higher than the initiation rate constant (ki): kp/ki > 5. A kinetic study conducted using a series of monofunctional chain transfer agents shows that the initiation rate, ki, is dependent on the nature of the hydroxyl group, with the rates decreasing in the order ki(primary) > ki(secondary) > ki(tertiary). It is proposed that two polymer architectures are present as a consequence of slow rates of initiation from the secondary hydroxyl groups, on the diol, compared to propagation which occurs from a primary hydroxyl group. In addition to the reactivity differences of the alcohols, steric effects also influence the polymer architecture. Thus, even if a chain transfer agent with only primary hydroxyl groups, such as 2-methyl-1,3-propanediol, is applied, a mixture of different polycaprolactone architectures results. The paper highlights the importance of analyzing the polymer architecture in the ring-opening polymerization of ε-CL, using a combination of NMR spectroscopic techniques, and refutes the common assumption that a single chain extended structure is produced in all cases.