Condensation polymerization polylactic acids

Polylactic acid (PLA) has been produced for many years as a high-value material for use in medical applications such as dissolvable stitches and controlled release devices, because of the high production costs. The very low toxicity and biodegradability within the body made PLA the polymer of choice for such applications. In theory PLA should be relatively simple to produce by simple condensation polymerization of lactic acid. Unfortunately, in practice, a competing depolymerization process takes place to produce the cyclic lactide (Scheme 6.10). As the degree of polymerization increases the rate slows down until the rates of depolymerization and polymerization are the same. This equilibrium is achieved before commercially useful molecular weights of PLA have been formed. 

Polylactic acid (PEA) is probably the best-known biobased polymer. It is made from glucose by fermentation to its monomer lactic acid. Two molecules of lactic acid are then condensed into the dimer lactide, which is subsequently ring-opened and polymerized to PLA in the presence of a catalyst. 

Ajioka, M., Enomoto, E., Suzuki, K. and Yamaguchi, A. (1995) Basic properties of polylactic acid produced by the direct condensation polymerization of lactic acid. Bulletin of the Chemical Society of Japan, 68,2125-2131. 

Polylactic acid (PLA) is the world s most popular synthetic biodegradable polymer and has a widespread use in the biomedical field. It maybe obtained directly from lactic acid by condensation polymerization or, more commonly, by ring-opening polymerization from the cyclic dimer of lactic acid lactide. Lactide is a chiral molecule that exists in three isomeric forms D(-), L(+) and racemic (D,L) lactide. Consequently, the polymerization of this monomer can lead to the formation of three different forms of polylactide poly-L-lactic acid (PLLA), poly-D-lactic acid (PDLA), and poly-D,L-lactic acid (PDLLA). The general representation of the chemical structure of PLA is presented in Figure 16.10. 

Poly(lactic acid), produced by Nature Works LLC, is another example for a hybrid process, in which the monomer, lactic acid is produced by fermentation of com using lactobacilli. The subsequent polymerization is accomplished either by anionic ring-opening polymerization of the lactide dimer, or more recently by an azeotropic dehydration condensation, a chemical process. Cargill Dow now has shown that the polylactic acid produced by it s subsidiary NatureWorks LLC, is fully competitive with its synthetic counterparts, and fibers can also be melt-spun from this polymer. These Ingeo fibers are world s first man made fiber made from 100% atmually renewable resources. [12]. 

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