Valencene as a naturally occurring sesquiterpene monomer for radical copolymerization with maleimide to induce concurrent 1:1 and 1:2 propagation

Abstract

Valencene, a naturally occurring sesquiterpene that can be obtained from various citrus fruits, was copolymerized as an unconjugated 1,1-disubstituted bulky vinyl monomer by radical copolymerization with conjugated monomers possessing electron-withdrawing substituents, such as maleimides, acrylates, methacrylates, acrylonitrile, and methacrylonitrile, in toluene and PhC(CF3)2OH. The fluorinated alcohol increased the copolymerizability, as in the radical copolymerization of other unconjugated non-polar olefins with those electron-acceptor comonomers. In particular, the copolymerization of valencene and N-phenylmaleimide in PhC(CF3)2OH proceeded via concurrently occurring 1:1 and 1:2 propagation, in which the monomer reactivity ratio of maleimide and valencene to the maleimide radical with the penultimate valencene unit was close to 1 according to a kinetic analysis of the copolymerization based on the penultimate model. Therefore, the copolymerizability of valencene was higher than that of limonene, which is a similar unconjugated 1,1-disubstituted vinyl monomer with a smaller cyclohexenyl substituent that undergoes selective 1:2 radical copolymerization with maleimide derivatives under the same conditions. Reversible addition-fragmentation chain transfer RAFT copolymerization using a trithiocarbonate resulted in copolymers with controlled molecular weights and chain-end groups. The biobased copolymers of valencene and N-phenylmaleimide showed relatively high glass transition temperatures (Tg) of approximately 220–240 °C and relatively high thermal stability with 5% decomposition temperatures (Td5) of approximately 330–340 °C due to their rigid main-chain structures.