(75c) Cascade Ring Strain Release Polymerization of Cyclohexene Oxide Derivatives to Functional, High-Tg Polyethers Using a Mono(?-alkoxo)Bis(alkylaluminum) Initiator

Cyclohexene oxide (CHO) has become an increasingly utilized building block for the synthesis of novel materials and catalyst development. Copolymerization of CHO with anhydrides, lactones, and CO₂ produces degradable polyesters while homopolymerization of CHO produces one of the few high glass transition temperature (T_g) polyethers. Although many catalysts for these polymerizations have been comprehensively studied, no studies have been conducted to understand the effect of adding substituents to the cyclohexane ring on polymerization rate or on the resulting materials properties. An advantage of the cyclohexene oxide moiety as polymerization building block is the increased ring-strain associated with the bicyclic system. The lack of functional handles attached to the CHO monomer, however, limits the ability to both pre- and post-functionalize these materials, reducing the variety of polymers that can be made. To understand the effect of substituent identity and position relative to the epoxide ring, we report the synthesis of two series of CHO derivatives with butyl, allyl, and halogen substituents alpha and beta to the epoxide ring. Polymerization was done with a mono(μ-alkoxo)bis(alkylaluminum) (MOB) initiator system that has been previously reported to polymerize a wide variety of epoxide monomers with control of molecular weight and a high degree of functional-group tolerance. Our results indicate that both the position and identity of the functional group relative to the epoxide ring can have significant impacts on the reactivity of the CHO derivatives. Larger substituents closer to the epoxide ring slowed polymerization kinetics whereas halogenated CHO monomers retained the reactivity of the cyclohexene oxide ring. Density functional theory was used to elucidate potential structural and mechanistic factors governing the differences in reactivity. These results indicate that CHO’s high reactivity relative to other epoxide monomers is due to its greater nucleophilicity and therefore improved ability to coordinate to Lewis acids. Additional thermodynamic driving force comes from the cascade release of ring strain from both the opening of the epoxide ring as well as from the conformational change of the cyclohexane ring from a locked twist-chair to a chair conformation during epoxide ring opening. The substituted poly(cyclohexene oxide)s exhibit a wide range of glass transition temperatures spanning from ca. –16 to 105 °C.