Cobalt catalysed controlled copolymerization: an efficient approach to bifunctional polyisoprene with enhanced properties

Abstract

Preparation of functional polyisoprene via direct catalytic insertion polymerization has been preferred because the polymers’ regularity, molar mass and functionality can be facilely controlled by the catalyst. However, this method poses critical challenges including loss of activity, poor control of the functional degree and chain transfer and termination reactions associated with the detrimental effect of the functional group. In this contribution, hydroxyl, furan and pyridine bifunctional dienes have been synthesized from bio-sourced myrcene and their controlled copolymerization with isoprene via a catalytic insertion mechanism has been demonstrated. Copolymerization followed the first-order kinetics without obvious chain transfer and termination, and this feature was not disturbed by the nature and amount of functionality and the excess masking cocatalyst. High molecular weight functional copolymers in the range of 152–194 kg mol⁻¹ with a narrow PDI of 1.1–1.3 were obtained. The catalytic efficiency was calculated to be up to 92.8%–74.5%. The reactivity ratios r_ip and r_f were determined to be 1.14–1.21 and 0.82–0.87, respectively, indicative of a nearly random copolymerization. The functionality rate increased with increasing monomer feeding, which additionally allowed the copolymer to be custom designed for tailoring specific incorporation. A combination of NMR spectroscopy techniques including ¹H, ¹³C, HSQC, HMBC and DOSY verified that the copolymerization approach for the generation of functionalized polyisoprene was feasible. The bulky OP(^t-Bu)₂(–N) moiety in the ligand is assumed to be chelated with Al³⁺, electronically and sterically stabilizing the active species and importantly, inhibiting the chain transfer reaction. The functional copolymers showed significantly improved hydrophilicity as indicated by the reduced water contact angle. The good thermal properties and good tolerance towards low temperature provided platforms to fabricate advanced functional elastomers applied in the rubber industry.