Anionic Polymerization of the Terpene-Based Diene β-Ocimene: Complex Mechanism Due to Stereoisomer Reactivities
Shivani P. Wadgaonkar, Manfred Wagner, Luis A. Baptista, Robinson Cortes–Huerto, Holger Frey, Axel H. E. Müller
Abstract
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 M n 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, T g . Employing styrene as a comonomer, diblock copolymers, and a series of statistical copolymers with M n up to 20 kg mol –1 with varying comonomer compositions offered the opportunity to tailor the glass transition of the copolymers. Real-time 1 H 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” ( r trans = 3.16; r cis = 0.32). Kinetic studies of the statistical copolymerization of the Oc isomeric mixture with styrene revealed an astonishingly contradictory reactivity of the two isomers ( r trans < r cis ). The r eactivity 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 1 H NMR-monitored chemical titration.