Biomedical Engineering Reference
In-Depth Information
Starch
Enzyme
OH
O
HO
OH
OH
OH
d-Glucose
Fermentation
O
HO
OH
d, l-lactic acid
Azeotropic polycondensation
“Step growth”
Cyclization
O
O
O
O
Ring-opening polymerization
“chain growth”
O
O
O
*
*
+
+
O
O
O
O
n
O
O
O
PLA
d -lactide
meso -lactide
l -lactide
Figure 11.4 Synthesis of lactic acid, lactide and poly(lactic acid).
glucose fermentation, enantiomerically pure monomer and polymer synthesis, a wide range
of catalyst systems, and final product properties and enhancements.
PLAs have been industrially fabricated into fibers, films, and surgical implants and
sutures. Currently, most PLA is produced by Natureworks ® (Dow-Cargill) in an amount of
136 000 tonnes per year in its plant in Nebraska, USA (Mooney, 2009). The advantageous
properties of PLA include being renewable, biodegradable, recyclable, compostable,
biocompatible, processable, and energy saving. Nevertheless, PLA has poor toughness,
slow degradation, hydrophobicity, and lack of reactive side-chain groups, which needs to be
improved (Rasal et al ., 2010 ).
Currently PLA is industrially synthesized via chemical pathways (e.g. ring-opening
polymerization of lactide or solvent-based azeotropic condensation). Although azeotrotopic
distillation overcomes the limitation of the molecular weight, a typical drawback of step-
growth polymerization kinetics, to some extent, the ring-opening polymerization of lactide
allows better control of the polymerization and remains by far the most widely used method
for the synthesis of well-defined materials (Dechy-Cabaret et al ., 2004 ). In contrast, Yang
and co-workers (2010) studied one-step fermentative in vivo production of PLA in E. coli .
The results led to the biosynthesis of PLA and its copolymers with 3-hydroxybutyrate
containing various lactate fractions within the range 8.7-64.4 mol%.
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