Green Route to Prepare Renewable Polyesters from Monomers: Enzymatic Polymerization 227
higher molecular weights (7900–11,600 Da) were produced. Both double bonds and
epoxy groups remained stable during the polymerization process due to the chemos-
electivity of the enzyme and the ability, which was given by them, to carry out the
polymerization process under mild conditions. Completely green synthetic strategy
to synthesize both monomers and corresponding polyesters has been suggested
later by Gross and his coworkers [87], where they prepared theα,ω-dicarboxylic
acid monomers from fatty acids usingCandida tropicalisATCC20962 or related
engineered strains. Accordingly, three bio-based monomers, 18-cis-9-octadecenedioic,
1,22-cis-9-docosenedioic, and 1,18-cis-9,10-epoxy-octadecanedioic acids, were synthe-
sized from oleic, erucic, and epoxy stearic acids, respectively. The resulting unsaturated
and epoxidizedα,ω-dicarboxylic acid were then used as a building block besides various
diols,1,3-propanediol, 1,8-octanediol, and 1,16-hexadecanediol, to produce aliphatic
polyesters containing double bond- or epoxy-functionalized polyesters using N435 as a
catalyst. The resulting polyesters had a molecular weight in the range from 25 to 57 kDa
based on the utilized monomers and/or reaction conditions.
7.4.3.2 Lipase-Catalyzed Polycondensation of Hydroxy Fatty Acids
Ricinoleic acid (RA), unsaturated hydroxyl fatty acid, is the main component of the
seed oil obtained from castor plant. In 2005, RA was used as a monomer to synthesize
lipophilic star-shaped polyester in two steps [88]. First, poly(ricinoleic acid) was
synthesized in bulk using N435 as a catalyst at 70∘Cfor10days.Onlyoligomers
of molecular weight 1040 Da was obtained due to the high viscosity of RA and its
chemical structure, which contains only secondary hydroxyl group accessible for the
polymerization reaction. In the next step, the branched star-shaped polyester was
enzymatically prepared by the reaction of resulting poly(ricinoleic acid) and polyols,
for example, trimethylolpropane or pentaerythritol, using immobilized lipase from
C. antarcticaorRhizomucor mieheiat 70∘C for 14 days. The resulting polymer had an
average molecular weight up to 4850 Da and owned high viscosity and high viscosity
index, which makes it a good candidate to apply in the field of environmentally friendly
lubricant materials. Using enzymes to produce the previous polymer enabled its
production under mild conditions in order to avoid the discoloration, odor, dehydra-
tion. Further attempts to enzymatically synthesize poly(RA) were carried out later by
Matsumura and his coworkers (Figure 7.6) [89]. The authors screened the activity of
many enzymes to test their reactivity toward enzymatic polycondensation of RA or
methyl ricinoleate. Among the tested enzymes, immobilized lipase PC showed the best
activity and was able to produce poly(RA) withMw5600 Da.
Poly(RA) with molecular weight of 100,600 Da could be achieved by utilization of
methyl ricinoleate instead of the RA, placing 4 Å molecular sieves in the vapor phase
HO
R: H or CH 3
OR
O
O
O
OO
Dicumyl peroxide Cross-linked
Curing reaction poly(RA)
n
Lipase
Figure 7.6Lipase-catalyzed polycondensation of ricinoleic acid or methyl ricinoleate followed by
cross-linking reaction using dicumyl peroxide [89].
