Radiation-induced polymerization of β(−)pinene: A further insight

The radiolysis of β(−)pinene has been studied at five different radiation dose, namely at 50, 100, 150, 300 and 600 kGy with a dose rate of 2.2 kGy/h. At lower radiation dose, β(−)pinene showed a reduction of the optical activity, hence, the expected radioracemization appeared predominant. At higher radiation dose, an opposite and unexpected trend was observed: the optical activity increased almost linearly with the radiation dose. The increase of [α]_D of radiolyzed β(−)pinene was due to the formation oligomers, mainly a dimer and/or a trimer of β(−)pinene which remain soluble in the monomer but which displayed a higher optical activity than the starting monomer, contributing to the increase in the specific optical rotation. Simultaneously to the formation of the dimer/trimer the radiolysis of β(−)pinene induced also its polymerization into a poly(β(−)pinene) resin which was insoluble in the monomer and which was characterized by a high structural order and by the preservation of the chiral centers so that it showed an enhanced specific optical rotation which is ≈2 times that of the monomer. It is shown that poly(β(−)pinene) radiopolymer can be easily racemized in presence of a Friedel-Crafts catalyst. The kinetics of β(−)pinene radiation-induced oligomerization and polymerization can be described by a pseudofirst order rate constant k = 1.9·10⁻⁷·s⁻¹. The radiation chemical yield for the same reaction is G = 1.93·10⁻⁶ mol/J. The work shows that also the radiation-induced polymerization of chiral monomers may be a tool for the preservation and amplification of the optical activity of the monomer with implications for abiotic mechanisms of chiral amplification.