Poly(lactic acids) (PLAs) were first synthesized by heating lactic acid and driving off water (Scheme 25.1). The product formed by this process is a gummy, tar-like material of ill-defined molecular weight and microstructure. If this polymer is heated in the presence of zinc oxide, the volatile cyclic condensation product of two lactic acid units is distilled from the polymeric mass. This product is lactide (LA), 3,6-dimethyl-1,4-dioxane-2,5-dione.1,2
There are three isomers of lactide: l-, d-, and meso (Figure 25.1; l-LA, d-LA, and meso-LA). The 50:50 racemic mixtures of l and d is known as rac-lactide (rac-LA). Lactide, in particular l-LA, can also be prepared by an enzymatic process from biomass; this is currently a major commodity material produced by Cargill.3-6 Ring-opening polymerization of lactides can be achieved by a wide variety of catalysts, many of which are metal-containing coordination complexes that are better considered as catalyst precursors.7-10 Among the most common of these are tin(II) bis(2-ethylhexanoate) (Sn(Oct)2), zinc lactate, and aluminum tris(acetylacetonate).11-16 These catalysts are typically employed in high-temperature (∼140 ◦C) melt polymerizations. Many more well-defined single-site metal alkoxide complexes are now known that can effect LA polymerization at room temperature. A number of these are discussed in detail in this chapter since they can bring about living polymerizations with control of both polymer molecular weight and stereochemistry. The reaction sequence can be described by Scheme 25.2.17
CH C OH O
CH C OH + n−1 H2O O
n HO n
SCHEME 25.1 Poly(lactic acid) formed by condensation polymerization of lactic acids.