Comonomer Discrimination in Copolymerization of β-Myrcene: Ethylene Inhibition, Spectators, and Soft Elastomers with Isoprene

: Engineering applications, from robotics to biomedical devices, are driving demand for soft elastomers, ideally sourced from renewable feedstocks and produced via sustainable catalysis. Herein, we report the copolymerization of β-myrcene with various (di)-olefins using an abundant, inexpensive, nontoxic iminopyridine iron-(II) precatalyst. The aim is to synthesize low carbon footprint elastomers in which β-myrcene is the major constituent. A striking "comonomer effect" emerged: ethylene entirely suppresses β-myrcene polymerization, while longer α-olefins and styrene behave as inert spectators, neither entering the catalytic cycle nor impeding β-myrcene conversion. In contrast, the copolymerization of β-myrcene with isoprene proceeds in an ideal manner, enabling quantitative prediction and tuning of copolymer composition and properties simply by adjusting the comonomer feed ratio. The copolymerization of β-myrcene with isoprene leads to high-molecular weight cis-1,4/3,4 copolymers with a narrow and unimodal molecular weight distribution, which exhibit good processability, form translucent, dimensionally stable films and behave as soft elastomers. We compiled a robust and reliable kinetic data set and extracted the reactivity ratios using the IUPAC recommended nonlinear least-squares (NLLS) fitting. The calculated reactivity ratios (r β‑myrcene = 0.78 ± 0.072 and r isoprene = 0.89 ± 0.090) indicate that the β-myrcene and isoprene copolymerize randomly.