Cationic copolymerization of isobutylene and bio-renewable β-myrcene towards sustainable elastomers: synthesis and mechanism

The use of renewable bio-based monomers to prepare polyisobutylene-based polymers can not only reduce the dependence on petroleum resources but also promote the green transformation of the cationic polymerization industry. Herein, 2-chloro-2,4,4-trimethylpentane (TMPCl) was synthesized as the main initiator and then cooperated with co-initiator TiCl₄ to construct a cationic initiation system, which was successfully used in preparing poly(isobutylene-co-β-myrcene). The effects of co-initiator concentration, monomer concentration and monomer feed ratio on the copolymerization of isobutylene (IB) and β-myrcene were investigated. The results showed that the molecular weight (M_n) first increased and then decreased with elevating the concentration of TiCl₄. When the monomer concentration was 15 wt% and the monomer feed ratio (mole ratio between IB and β-myrcene) was 98:2, an IB-co-β-myrcene random copolymer with a molecular weight of 7.3 × 10³ g/mol and a lower T_g (-67.97 ℃) was obtained. According to the ¹H NMR analysis of the polymer structure and terminal groups, it was suggested that IB and β-myrcene mainly formed the copolymer with 1,4 structural unit under this initiation system, thus the cationic polymerization mechanism of IB and β-myrcene was further proposed. This work can provide basic data and theoretical guidance for the manufacture of greener polyisobutylene-based polymers in the future.