Biopolymers - Food and Industrial Bioproducts and Bioprocessing - page 267

The polymerization rate, molecular weight control, and stereo control are the key
parameters for PLA synthesis (Williams and Hillmyer, 2008). The physical, mechanical,
and degradation properties of PLAs are strongly dependent on the chain stereochemistry.
Isotactic PLLA is a highly crystalline polymer with a T m of 170 °C (Zhong et al ., 2003 ). It
has excellent mechanical properties and degrades rather slowly. Atactic poly( rac -lactic
acid) is amorphous and degrades faster. Therefore, it is very important to control the chain
stereochemistry for PLA homopolymers and copolymers. Much effort has focused on the
development of new catalysts for the controlled polymerization of lactide, including chiral
aluminum isopropoxides based on enantiopure or racemic cyclohexylsalen ligands (Jacobsen
ligand), imidazole (Kricheldorf et al ., 2008 ), aluminum-methyl complexes supported by
tetradentate phenoxy-amine ligands (Tang and Gibson, 2007), Group 3 metal complexes
supported by dianionic alkoxy-amino-bisphenolate ligands (Amgoune et al ., 2006 ), primary
and secondary alcohol adducts of 1,3-dimesitylimidazolin-2-ylidene (Csihony et al ., 2005 ),
as well as tin(II) complexes supported by

-diketiminate ligands (Dove et al ., 2006 ).
Moreover, Lohmeijer and co-workers (2006) reported some guanidine and amidine
organocatalysts for ring-opening polymerization of lactide, which avoid heavy metal
contaminants from catalyst residues in the polymer. It was shown that
1,5,7-Triazabicyclo[4.4.0]dec-5-ene (TBD), N -methyl-TBD (MTBD), and
1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) are very efficient catalysts for ring-opening
polymerization with excellent control of molecular weight and polydispersity.
Since PLAs are generally very brittle and exhibit lower elongation at break, they need to
be toughened for some applications. Anderson and co-workers (2008) reviewed the tough-
ening methods as adjusting stereochemistry, crystallinity, and processing, plasticization,
and blending with other degradable or non-degradable polymers. For instance, when
poly(l-lactic acid)(PLLA) is toughened with polymerized soybean oil, the blends exhibit a
tensile toughness as high as four times and strain at break values as high as six times greater
than those of unmodified PLLA (Robertson et al ., 2010 ).
In 2008 Pounder and co-workers took advantage of thiol-ene reactions, which are
considered to be one of the click reactions, and used a furan protected alcohol functional
maleimide as an initiator for the polymerization of lactide. After polymerization and
deprotection, some aromatic and aliphatic thiol compounds with various functionalities were
reacted to yield PLAs with different chain-end functionalities under very mild conditions
without any degradation. Another interesting approach to synthesize PLA grafted with
poly(ethylene oxide)(PEO), containing various repeating ethylene oxide units, was reported
by Jiang and co-workers in 2008. Mono-methoxy PEGs were functionalized with a lactic
acid moiety through a three-step process and the subsequent cyclization of these lactic acid
moieties into lactide was carried out with the aid of an organic acid. Finally, this adduct was
subjected to ring-opening polymerization using Sn(2-ethylhexanoate) 2 , a very common
catalyst for PLA synthesis. The resulting polymers, containing one and two PEO repeat units
in the pendant chain, were hydrophilic but not water soluble. Those having longer PEO
chains were water soluble. Moreover, a lower critical solution temperature was detected for
the two polymers (19 and 37 °C, respectively) indicating their thermo responsive behavior.
PLA can also be used for medical applications owing to its biocompatibility and
biodegradability. PLA was shown to degrade via hydrolytic erosion (Suggs et al ., 2007 ), in
which the mechanical properties remain similar during the degradation progress. The
degradation rate and mechanism, as well as other properties of PLA, depend on molecular
weight and distribution, pH, T g , crystallinity, and temperature. For further enhancement of
the physical and mechanical properties, PLA was copolymerized with glycolic acid,

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