MyrDOL, a Protected Dihydroxyfunctional Diene Monomer Derived from β-Myrcene: Functional Polydienes from Renewable Resources via Anionic Polymerization
Christoph Hahn, Manfred Wagner, Axel H. E. Müller, Holger Frey
Abstract
The protected functional diene monomer 2,2,4,4-tetramethyl-5-(3-methylenepent-4-en-1-yl)-1,3-dioxolane (myrcene dioxolane, MyrDOL) is introduced, based on β-myrcene. The monomer is suitable for carbanionic polymerization because its acetal functionality as a protective group for diols is stable under carbanionic conditions. The polymerization of MyrDOL in cyclohexane at 25 °C using sec-butyllithium as an initiator resulted in homopolymers with well-controlled molecular weights in the range 4.0–31 kg mol–1 (SEC, PMMA calibration, and MALDI-TOF) and low dispersities, Đ, between 1.07 and 1.13. In pronounced contrast to polymyrcene, which contains 95% 1,4-myrcene microstructure (synthesis in cyclohexane by anionic polymerization, Tg = −67 °C), microstructure characterization of P(MyrDOL) shows 30–33% of 3,4-units and a Tg of 11 °C. The acetal groups can be quantitatively removed under mild conditions by using acidic deprotecting agents (e.g., DOWEX resin), resulting in well-defined poly(myrcene-2,3-diol). The copolymerization of MyrDOL with the dienes myrcene, styrene, and isoprene was investigated in great detail via in situ 1H NMR kinetics. The substitution pattern of the 1,3-diene in combination with the polarity of the monomer has a significant influence on the copolymerization behavior, resulting in disparate reactivity ratios and formation of tapered copolymers in statistical copolymerizations. Myrcene copolymers with varying MyrDOL content, in the range 10–100 mol % MyrDOL, were synthesized (Đ ≤ 1.15) and characterized regarding their glass transition temperatures and polydiene microstructure. An increase in the 3,4-microstructure content was observed as a consequence of both increasing MyrDOL content and polarity of the polymerization medium, resulting in an increase in Tg from −67 to 9 °C. The protected, functional MyrDOL monomer is promising with respect to polar, hydroxyl-functional rubbers.