228 Introduction to Renewable Biomaterials
to capture the resulting methanol and thus shifting the equilibrium reaction toward the
product, and increasing the amount of utilized lipase to a concentration of 150 wt%. The
final polymer was a viscous liquid with a glass transition temperature of−74.8∘C.
Cis-9,10-epoxy-18-hydroxyoctadecanoic acid (EHC) is another interesting renewable
monomer, which forms about 100 g kg−^1 of dry outer bark ofBetula verrucosa[90].
In contrast to RA, the chemical structure of EHC is too suitable for the enzymatic
polymerization as it owns primary hydroxyl group, which should be more accessible
to enzyme than the secondary one. EHC was used as a monomer to prepare linear
epoxy-functionalized polyesters using N435 as a catalyst [91]. The effect of solvent on
polymerization process was investigated first using three solvents: acetonitrile, dioxane,
and toluene. The highest molecular weight, 20 kDa, was obtained when toluene was
used as a solvent for 68 h. Similar molecular weight, 16 kDa, in a much shorter time,
6 h, could be achieved when the polymerization process was carried out in bulk.
7.4.3.3 Fatty Acids as Side Chains to Modify Functional Polyesters
Utilization of sustainable saturated and unsaturated fatty acids as side chains to modify
linear functional polyester, that is, prepared enzymatically by copolymerization of
dicarboxylic acids or their esters with polyols, have attracted increasing attention
due to the potential applications of the resulting polymers in many interesting fields.
Linear reactive polyester grafted with unsaturated fatty acid chains was prepared by
lipase-catalyzed polycondensation of divinyl sebacate and glycerol in the presence
of unsaturated higher fatty acids, for example, oleic, linoleic, and linolenic acids
[79, 92]. The reaction was carried out in bulk for 24 h using equivalent amount of
each component. The polymer yield and molecular weight increased by increasing
the reaction temperature, while the polymer composition was nearly the same. The
currying process of the resulting polyester was induced either by oxidation with
cobalt naphthenate catalyst or by thermal treatment to yield finally cross-linked
transparent film. Comb-like, epoxide-containing polyesters were prepared later via two
synthetic routes [93]. In the first one, aliphatic polyester owing unsaturated groups
in the side chain was prepared in one step by lipase-catalyzed polycondensation of
divinyl sebacate, glycerol, and the unsaturated fatty acids. The double bond in the side
chains were then epoxidized using hydrogen peroxide in the presence of lipase. In the
second route, unsaturated fatty acid was first epoxidized, using hydrogen peroxide
and lipase, subsequently used in the enzymatic reaction to prepare the final epoxide
comb-like polyester. Currying of the resulting polymer was thermally achieved, which
resulted in transparent polymeric film owing glassy surface and pencil hardness higher
than the cured film obtained from polyester own unsaturated fatty acids. In fact,
both utilized routes are considered as green synthetic strategy as no toxic catalyst
was needed to prepare the polyester main chain and to epoxidize the fatty acid side
chains. Poly(glycerol adipate) (PGA) is a linear aliphatic polyester with free pendant
OH groups and can be obtained by lipase-catalyzed polycondensation of divinyl/or
dimethyl adipate with glycerol under mild conditions. A library of comb-like polyesters
has been prepared by acylation of PGA with various fatty acids, for example, laurate,
stearate, and behenate, with various degrees of substitution. The properties of the
resulting polymers were investigated in bulk, water, and at air/water interface. In water,
the resulting polymers were able to form well-defined particles of small size and high
homogeneity [94]. By using interfacial deposition method, Mäderet al.wereable
