Cascade Ring Strain Release Polymerization of Cyclohexene Oxide and Derivatives Using a Mono(μ-Alkoxo)bis(alkylaluminum) Initiator

Abstract

Cyclohexene oxide (CHO) is a useful building block for the synthesis of novel materials and is a model substrate for polymerization catalyst development. The driving force for CHO polymerization is derived from its bicyclic structure, which combines the release of the enthalpy from epoxide ring-opening (ca. −15 kcal/mol) and a twist-chair-to-chair conformation shift in the cyclohexane ring (ca. −5 kcal/mol) upon enchainment. The lack of regio-defined functional handles attached to the CHO monomer limits the ability to both pre-and post-functionalize the resultant materials and establish structure−property relationships, which reduces the versatility of currently accessible materials. We report the synthesis of two series of CHO derivatives with butyl, allyl, and halogen substituents in the α and β positions relative to the epoxide ring. Adding substituents to the CHO ring was found to affect polymerization kinetics, with 4-substituted (β) CHO being more reactive than 3-substituted (α) CHO analogs when initiated with a mono(μ-alkoxo)bis(alkylaluminum) pre-catalyst. Polymer thermal properties depended on substituent location and identity. Halogenated CHO rings were most reactive and produced the highest glass transition temperatures in the resultant polymers (up to 105 °C). Density functional theory revealed a possible mechanistic explanation consistent with the observed differences in polymerization rate for the 3-and 4-substituted CHOs derived from a combination of steric and thermodynamic considerations.