Origin of the Two Major Types of Repeating Units in Poly(β-pinene) Obtained by Cationic Polymerization

β-pinene is a very reactive substrate that isomerizes rapidly in the presence of protic catalysts and polymerizes in the presence of Lewis acid catalysts and initiators. Because of the sustainable perspective and high reactivity, precise polymerization of β-pinene has been given great attention, but limited examples of microstructure analyses have been done. Here, we comprehensively characterized the 1,3-cyclohexenyl sequence besides the conventional 1,4-cyclohexenyl sequence by various NMR techniques. The structure was confirmed by comparing NMR spectra with those of separately synthesized poly(β-phellandrene). The ratio of 1,3-sequence was increased in the polymer obtained at a lower temperature, in a more polar solvent, or by using a stronger Lewis acid catalyst. Considering the result of the theoretical study, a thermodynamically stable allyl cation that gives the 1,3-cyclohexenyl sequence would more frequently be formed when the ion pair recombination of the carbocation and counteranion during the polymerization is less frequent. The microstructure change affected the thermal property of poly(β-pinene); that is, more 1,4-cyclohexenyl sequence raised the glass transition temperature of the polymer.