Green Route to Prepare Renewable Polyesters from Monomers: Enzymatic Polymerization 223
LLA showed now reactivity [58]. However, N435 was able to catalyze the ROP of LLA
when the polymerization reaction was carried out in supercritical CO 2 , which resulted
in PLLA (Poly-L-Lactide) with goodMwof 12,900 Da but with low yield (≤12%) [59].
7.4 Lipase-Catalyzed Polycondensation
Despite the many advantages of using ROP to synthesize polyesters, polycondensation
is considered as the main synthetic route to prepare commercial polyesters [13]. In fact,
this polymerization technique allows direct synthesis of wide spectrum of polyesters
form naturally derived monomers. Polyesters can be synthesized by condensation poly-
merization using either AA- and BB-type monomers or AB-type monomers by either
esterification or transesterification reaction route (Figure 7.3). In both cases, however,
small molecular weight compound, for example, water, hydrochloric acid, alcohol, and
so on, is usually produced besides the main product during the condensation reaction.
Elimination of this by-product during the polymerization process is necessary in order
to shift the equilibrium reaction toward products, which can be achieved by carrying
out polymerization reaction (i) in bulk under driven conditions, for example, high
temperature, vacuum or gas stream [60], (ii) in the presence of solvent that is able to
form an azeotropic mixture with the resulting by-product, which is then constantly
eliminated by passing the mixture through Soxhlet apparatus packed with molecular
sieveofsuitableporesize[61].
Monomers with activated acyl donors, such as thioester, oxime ester, and anhydrides,
have been used for lipase-catalyzed polycondensation of polyesters as they own higher
reactivity toward polymerization process and due to ease of elimination the resulting
by-products. However, utilization of monomers with enol ester, such as vinyl esters,
seems to be the most effecting synthetic strategy as the resulting enol is not stable
and thus tautomerizes readily to give the corresponding aldehydes or ketones, which
makes the polymerization process irreversible [12, 62]. A lot of commercial available
monomers obtained from renewable resources have been tested for lipase-catalyzed
polycondensation. While the lipase-catalyzed polymerization of lactic acid gave a
low-molecular-weight PLA [63], polyesters with higher molecular weight have been
enzymatically produced by utilization of another monomer.
HO
R
O
HO OH + RHO
R
Lipase R
O
O
O
HO R 2 OR′
R 1
OO
O
OR′
(a)
(b)
2
HO OH +
Esterification
R′: H
Transesterification
R′: CH 3 , CH 2 CH 3 , CH 2 CH, CICH 2 CH2, CF 3 CH 2 etc.
R 1
R 2 OR′
OO
R′O
Lipase
Figure 7.3Lipase-catalyzed polycondensation to obtain polyester via (a) AB-type monomers, (b) AA-,
BB-type monomers.
