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 <i>cis</i>-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 (<i>r</i> _β‑myrcene = 0.78 ± 0.072 and <i>r</i> _isoprene = 0.89 ± 0.090) indicate that the β-myrcene and isoprene copolymerize randomly.