Living radical polymerization of bicyclic dienes: Synthesis of thermally cross-linkable block copolymers

A new thermal cross-linking strategy based on retro-Diels-Alder reaction of bicyclic diene polymers was developed. 2,3-Dimethylenenorbornane (1) and its derivatives polymerized under free-radicalconditions predominantly via the 1,4-addition mode. Monomer 1 had a much higher reactivity toward propagating polymer radicals than isoprene despite having larger substituents, which was attributed to the rigid s-cis coplanar conformation of the diene group in 1. Nitroxide-mediated polymerization at 110 °Cprovided bicyclic diene polymers with controlled molecular weights and narrow molecular weight distributions,as well as block copolymers of 1 and methyl acrylate. Poly(1) underwent retro-Diels-Alder reaction with a concurrent release of ethylene upon heating at 245 °C, which ultimately led to the formation of crosslinkedstructures. Block copolymers of 1 and methyl acrylate phase-separated into lamellae and cylindricalmicrostructures when annealed at 180 °C. Upon further heating to 245°C, the poly(1) domain in the block copolymer cross-linked to provide rigid polymer nanostructures, which was monitored by a gradual increasein its glass transition temperature. The morphology formed by block copolymer phase separation was preserved during the cross-linking process, as confirmed by a small-angle X-ray scattering analysis.