Model and Terpenoid-Derived exo-Methylene Six-Membered Conjugated Dienes: Comprehensive Studies on Cationic and Radical Polymerizations of Substituted 3-Methylenecyclohexenes

This work was designed as a comprehensive study of cationic and radical polymerizations of exo-methylene six-membered conjugated dienes, i.e., 3-methylenecyclohexenes, which were prepared from bio- and petroleum-based compounds, to determine the effects of substituents on monomer reactivities and thermal properties of the polymers. To clarify the effect of a methyl group on the conjugated diene as well as those of isopropyl substituents at other positions in the biobased monomers, a series of model monomers with or without a methyl group at the 3- or 4-position of the exo-methylene moiety were prepared from petroleum-derived compounds. Cationic homopolymerizations of the model monomers were extremely fast, but radical processes were very slow, which was consistent with results reported for terpenoid-derived exo-methylene-conjugated dienes. Kinetic studies of living cationic copolymerizations involving the model monomers and isobutyl vinyl ether indicated that the methyl group on the conjugated diene moiety strongly influenced the cationic reactivity of the monomers; a methyl group at the 4-position provided the highest reactivity due to formation of a stable conjugated tertiary cationic propagating species, whereas a methyl group at the 3-position resulted in the lowest reactivity due to steric hindrance around the exo-methylene moiety. The monomer reactivity ratios observed for radical copolymerizations of exo-methylene-conjugated dienes with methyl acrylate or styrene revealed that a methyl group at the 3-position decreased the reactivities of the exo-methylene-containing dienes due to steric hindrance around the exo-methylene moiety. In addition, the effects of isopropyl substituents at the other positions in the terpenoid-derived monomers depended on the specific position. The polymers obtained from the terpenoid-derived monomers showed higher glass transition temperatures than those obtained from the corresponding model monomers due to additional isopropyl substituents originating from the biobased compounds, and this indicated that unique naturally occurring structures enhanced the thermal properties of the resulting polymers.