Anionic Polymerization of the Terpene-Based Diene β‑Ocimene: Complex Mechanism Due to Stereoisomer Reactivities

Motivated by the need for sustainable, bio-based materials, the living anionic polymerization of the under-explored terpene monomer β-ocimene (Oc) was investigated for the first time. Homopolymers with Mn up to 50 kg mol–1 of the Oc isomeric mixture (E:Z = trans:cis = 70:30) were synthesized in cyclohexane and analyzed with respect to molecular weight control, dispersity, microstructure, and glass transition temperature, Tg. Employing styrene as a comonomer, diblock copolymers, and a series of statistical copolymers with Mn up to 20 kg mol–1 with varying comonomer compositions offered the opportunity to tailor the glass transition of the copolymers. Real-time 1H nuclear magnetic resonance (NMR) kinetics indicated a remarkably divergent reactivity of the trans and cis isomers. This unveiled the unique observation that the homopolymerization of Oc is in fact a copolymerization of the cis and trans isomers, which one might name as “stereo-copolymerization” (rtrans = 3.16; rcis = 0.32). Kinetic studies of the statistical copolymerization of the Oc isomeric mixture with styrene revealed an astonishingly contradictory reactivity of the two isomers (rtrans rcis). The reactivity differences of the cis and trans isomers in the polymerization were utilized to isolate the individual isomers for the first time. Subsequently, they were independently homo- and copolymerized with styrene. The complex mechanism of these polymerizations and the rather high polymer dispersities (Đ ≈ 1.6–2) are discussed using various kinetic models supported by density functional theory modeling. The surprisingly different behavior of the two isomers with styrene was validated experimentally via a 1H NMR-monitored chemical titration