Environmental Engineering Reference
In-Depth Information
Copolymerization was reported with the living cationic systems. It was reported
that the systems containing β-pinene/styrene, β-pinene/
p
-methylstyrene and
β-pinene/isobutylene all proceeded with high efficiency of β-pinene consumption
[130, 136]. These systems were capable of producing polymers with molecular
weights of several thousands. Extending this technique to incorporate chemical
functionalities capable of participating in free radical polymerizations allowed for
higher-order molecular architecture designs. Lu
et al.
reported an example where
homo- and copolymers of β-pinene were end-capped by a methacrylic group,
from which free radical polymerization of methyl methacrylate could be carried
out to give a diblock polymer [137].
5.4.4
Polymerization of Non-Pinene Terpenes
Myrcene and limonene are two terpenes that have recently gained growing
attention because of possible manipulations that afford facile polymerization. For
myrcene, an isomer of pinene obtainable by pyrolysis, it was shown that ring
closing metathesis can produce monomeric 3-methylenecyclopentene, which can
undergo radical, anionic, and cationic polymerization [138]. The study showed
that although feasible, free radical polymerization was not optimal. Both anionic
and cationic polymerization were efficient, and a cationic system of
i
BuOCH(Cl)
Me/ZnCl
2
/Et
2
O in toluene was capable of producing polymers with molecular
weights higher than 20,000 [138]. DSC characterization showed both
T
g
(11°C)
and
T
m
(65 and 105°C), suggesting the polymer was semi-crystalline [138].
Limonene, another promising terpene, was the focus of several recent studies
by Firdaus
et al.
While also obtainable by isomerization of pinene, the most abun-
dant source of limonene is citrus peel oil which is produced on an industrial scale
[125]. Firdaus utilized thio-alcohol (2-mercaptoethanol) and thio-ester (methyl
thioglycolate) to react via a free radical mechanism with the double bonds in the
terpene [125]. The reactivity differences between the terminal double bond and
the endocyclic double bond allowed manipulation of where the thiols reacted
[125]. Both double bonds could be substituted to allow difunctional monomers to
participate in facile condensation reactions to produce a linear polyester with
molecular weights around 10,000 Da [125]. Higher-molecular-weight polymers
were achievable via copolymerization between the difunctional limonene deriva-
tive, long-chain fatty-acid-based diesters, and diols [125]. Most of the polyesters
showed semi-crystalline behavior, displaying both
T
g
(-45°C) and
T
m
(-15 to
50°C) [125].
In a follow-up study, Firdaus and Meier showed how this technique could be
used to produce both polyamides and polyurethanes [139]. The polyamides pro-
duced had molecular weights up to 12,000 Da and thermal properties ranging from
amorphous to semi-crystalline [139]. The polyurethanes were obtained via an
isocyanate-free method and also had molecular weights up to 12,000 Da with
similar thermal properties ranging from amorphous to semi-crystalline [139].
