Monomers and Resulting Polymers from Biomass - Introduction to Chemicals from Biomass - page 178

The majority of terpenes produced annually are α-pinene and β-pinene. Through
thermal or acidic isomerization, any number of monoterpenes can be produced
from these feedstocks. A review by Corma et al. describes the vast array of manip-
ulations possible and the final products obtained from these pinene starting
materials. Isomerization, hydrogenation, epoxidation, oxidation, and other techniques
are utilized to produce a large variety of compounds, including terpenoids and
terpenols [119]. More recently, thiol addition to limonene and β-pinene has been
explored, which allows the modified terpene to participate in the facile thiol-ene
click reaction [125].

5.4.2 Cationic Polymerization of Pinenes

It is generally accepted that monoterpenes do not participate in radical
homopolymerization [117]. Monoterpenes can undergo cationic polymeriza-
tion as long as an electron-donating group is on the double bond so that the
cation is obtainable. Despite the prevalence of α-pinene, the majority of the
research over the past century has centered on β-pinene because of its reactive,
exocyclic double bond [117]. Only oligomers of α-pinene are obtained
using cationic polymerization because of the steric hindrance of the necessary
endocyclic addition [126].
Early work with β-pinene showed that Lewis acid metal halides such as
AlCl 3 and ZrCl 4 are able to initiate homopolymerization reactions, though low
temperatures (-40°C) are needed to achieve polymers with molecular weights
of a few thousand [127]. Of the initiators explored, the most efficient is EtAlCl 2
which has been used to produce poly(β-pinene)s with molecular weights up to
40,000 at low temperatures and of a few thousands at room temperature [128,
129]. In addition, living cationic polymerization systems were developed for
β-pinene, allowing for the synthesis of block copolymers [130]. Lu et al.
reported a system of 2-chloroethylvinyl ether, isopropoxytitanium chloride and
tetra- n -butylammonium chloride in dichloromethane at low temperatures that
enabled a living cationic homopolymerization of β-pinene [131].

5.4.3 Copolymerization of Pinenes

The copolymerization of α-pinene and β-pinene with several synthetic mono-
mers has been investigated, including styrene, α-methylstyrene, and isobutene
[117, 132-135]. Increased polymerization efficiency was observed in the
copolymerized systems to the point copolymers between α-pinene and isobu-
tene, with molecular weights ranging from a few thousands up to 29,000
reported [133]. While this system eludes the goal of a completely renewable
backbone, it is evidence that these monomer systems can be incorporated
into diverse polymer backbones in order to increase the renewable carbon
content.