Environmental Engineering Reference
In-Depth Information
the polymerization of modified rosin using relatively new controlled free-
radical polymerization techniques of ATRP and RAFT [157-160]. The work
utilized an acrylated dehydroabietic acid, where the acylation occurred on the
carboxylic acid and the dehydroabietic acid form was used for the stabilized
aromatic ring. The acrylic functionality can be further removed from the
rosin by using an aliphatic spacer between the acrylate and the carboxylic acid
[158]. Polymers with molecular weights ranging from 10 to 100 kDa were
produced and a relationship between the length of the aliphatic spacer and the
steric effect observed was established [118, 158]. The Tang group has reported
the production of interesting antimicrobial polymers from similar thermo-
plastics [161].
The Tang group explored the combination of poly(dehydroabietic acid)
(PDA) and PCL. PCL is a well-known biodegradable polyester, synthesized by
ring-opening polymerization (ROP) of ε-caprolactone. By combining ROP and
ATRP, Wilbon
et al.
reported the first rosin-containing block copolymer of
PDA and PCL [160]. As expected, the composition of the blocks dictated the
thermal properties of the final polymer and a decrease in
T
m
was associated
with a higher content of PDA, because the PDA interfered with the crystallization
of PCL [160].
An interesting follow up to the PDA-PCL block copolymer study investigated
the effects of rosin acids as side chain pendants to PCL. Yao
et al.
synthesized a
PCL polymer backbone and, using azide-alkyne click chemistry, coupled alkyne-
modified dehydroabietic acid to the PCL backbone [162]. The
T
g
of PCL prior to
coupling ranged between -40 and -60°C, and after coupling the
T
g
increased to
55-85°C [162]. The resulting polymer had improved thermal properties while
maintaining the excellent ability of PCL to biodegrade, suggesting promise for a
number of applications including food packaging, drug delivery, and other
biomedical applications [118, 162].
5.5.5
Rosin Conclusion
The use of rosin acids as a renewable monomer has, like many biorenewable
chemistries, received renewed interest over the past decade. The tricyclic, multi-
functional acids, illustrated in Figure 5.4, are capable of competing with their
fossil-based counterparts in a number of areas, including as epoxy resins and as
thermosetting polyesters [147, 148, 153, 156]. More importantly, new polymerization
techniques have shown that rosin acids are useful in the design of novel biomedical
materials and biodegradable thermoplastics [118, 162]. These biorenewable mon-
omers have the potential to be used in a number of film and coating applications.
With continued research into the behavior of their block copolymers, these stiff
monomers may be the renewable answer to styrene in the search for an all-natural
substitute for the ubiquitous styrene-butadiene-styrene triblock polymer used in
so many applications.
