220 Introduction to Renewable Biomaterials
mostly to be also safely recycled or degraded [6]. In that regard, polyesters represent an
important class of polymers that can meet a lot of green chemistry requirements as they
can be mostly synthesized starting from monomers obtained from biomass feedstocks
[2]. Additionally, their biodegradability causes no environmental pollution compared
to that of nondegradable plastic and makes them suitable for biomedical applications
[7–10]. The wide spectrum of monomers, which can be used for polyester synthesis,
with their various chemical structures allows for the synthesis of polyester with tailored
chemical structure able to meet specific properties needed for different applications.
Some of the polyesters synthesized completely or partially from renewable feedstock
have already found their way to the market, that is, poly(lactic acid) (PLA), poly(butylene
succinate) (PBS), poly(butylene adipate-co-terephthalate), and poly(hydroxyalkanoates)
with worldwide production of 195,000, 100,000, 75,000, 32,000 tons in 2013, respectively
[1, 11]. Polymer synthesis starting from sustainable monomers, however, is not enough
to meet the demands of green chemistry, as industrial synthetic processes involve
utilization of toxic catalyst as well as using high reaction temperature. Utilization of
enzymes in organic synthesis has emerged as an attractive green synthetic pathway
alternative to conventional chemical catalyst [12]. Scientists have investigated the
potential application of enzymes as catalyst for polymers synthesis. In fact, utilization
of enzymes as catalyst for polyester synthesis supports the requirement of green
principles for polymer production from many aspects [2, 13] including the following:
- Enable the synthesis of polyesters under mild conditions, which minimize the amount
of energy needed for polymer production and avoid undesirable side products.
- Allow the synthesis of functional polyester in one step, without the need for
protection–deprotection steps, due to the chemo-, enantio-, and regioselectivity of
enzymes.
- Contrary to most catalysts used currently for polyester synthesis, enzymes are
classified as nontoxic.
- Enzymes are derived from renewable resources (sustainability).
- Enzymes are recyclable when they are immobilized, which reduces the catalyst costs.
This chapter presents an overview of the application of enzymatic polymerization
to catalyze the synthesis of polyesters using different types of monomers prepared
from biomass, where some recently and interesting published reports are highlighted.
A deeper insight, however, into enzymatic polymerization topic is available from other
published excellent reviews [13–17].
(Lactones and Lactides) 7.3 Lipase-Catalyzed Ring-Opening Polymerizations of Cyclic Monomeric Esters
Monomeric Esters (Lactones and Lactides)
Lactones are an important class of monomers for the synthesis of aliphatic polyesters
via chain-growth mechanism, which offers a lot of attractive advantages over monomers
that react via step-growth mechanism [18]; for example, the polymerization reaction
proceeds in one direction without generating leaving groups [19], synthesizing high
molar mass and low polydispersity polyesters within shorter reaction time and the
capability to control end groups of resulting polyester [20]. Many reports have appeared
recently on the synthesis of useful lactones from biomass (Figure 7.1).γ-Butyrolactone
