Lactic acid stereochemical purity

What is interesting, however, is some of the chemistry that is not present. For example, the petrochemical industry does not have a basic feedstock in the five-carbon area and thus we see few products derived from or based on five-carbon chemistry. Optical active compounds are also missing from the petrochemical-derived product list. For example, lactic acid is now made exclusively from glucose, with the reason being that the fermentation route provides stereochemical purity that is difficult to achieve from petrochemical building blocks. 

Although increasing the catalyst concentration results in the faster overall reaction rate, enhanced rate of racemization, which is unwanted in the production of optically pure lactide may also occur. Higher synthesis temperatures, longer reaction times and presence of some metal cations such as sodium and potassium have the same effect on the stereochemical purity of the crude lactide [14, 15, 19]. Released metals through corrosion and carboxylic acid impurities formed during lactic acid fermentation are other sources of impurities [10,14, 20-22]. 

The properties of PLA, as indeed those of other polymers, depend on its molecular characteristics, as well as on the presence of ordered structures, such as crystalline thickness, crystallinity, spherulite size, morphology and degree of chain orientation. The physical properties of polylactide are related to the enantiomeric purity of the lactic acid stereo-copolymers. Homo-PLA is a linear macromolecule with a molecular architecture that is determined by its stereochemical composition. PLA can be produced in a totally amorphous or with up to 40 per cent crystalline. PLA resins containing more than 93 per cent of L-lactic acid are semi-crystalline, but, when it contains 50-93 per cent of it, it is entirely amorphous. Both meso- and D-lactides induce twists in the very regular PLLA architecture. Macromolecular imperfections are responsible for the decrease in both the rate and the extent of PLLA crystallization. In practise, most PLAs are made up of L-and D,L-lactide copolymers, since the reaction media often contain some meso-lactide iir turities. 

Stereochemical Purity In order to make semicrystalline, high-melting PLA, stereochemically pure lactic acid is needed. Not all microorganisms yield such stereochemically pure lactic acid and some even produce a racemic mixture [29]. Therefore, a strain must be chosen that meets the quality demands. Finding such a strain that produces L-lactic acid in an economically feasible manner is relatively easy. Producing D-lactic acid by bacterial fermentation on an industrial scale is far more difficult. 

The dehydration of lactic acid to make the prepolymer should start with an —OH to —COOH ratio of 1 1. All other components with —OH and —COOH functionality disrupt the stoichiometric balance and may be incorporated as comonomers during prepolymerization, which limits the final lactide production yield from lactic acid. Little public information is available on the technical and economic relationship between lactic acid quality and lactide synthesis. Only a few patents mention the effect of metal impurities on racemization [68,69]. Stereochemical purity is one of the key parameters determining lactic acid purity. 

A lactide synthesis reactor invariably produces a crude lactide stream that contains lactic acid, lactic acid oligomers, water, mc50-lactide, and further impurities. The specifications for lactide are stringent mainly for free acid content, water, and stereochemical purity. Basically, two main separation methods, distillation and crystallization, are currently employed for lactide purification ... 

Poly-/-lactic acid has been extensively studied. By cationic polymerization of Mactide, a highly crystalline isotactic polymer can be obtained with Fridel-Craft initiators, or better with zinc or lead oxide ones [130]. The basicity of the catalyst seems to have a substantial effect on the optical purity of the polymer obtained. A stereochemical study is reported by Schultz and Schwaad [131] on a poly-5-lactic acid (LII). 

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