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.