Lactide, formation

Polymerization through lactide formation is currently used by Cargill (NatureWorks ), which patented a continuous process [17,18]. 

Tsuji, H., Fukui, I., Daimon, H. tmd Fujie, K. (2(X)3) Poly(L-lactide) XI. Lactide formation by theimal depolymerization of poly(L-lactide) in a closed system. Polymer Degradation and Stability, 81, 501-509. 

Fan, Y, Nishida, H., Mori, T. et al. (2004) Thermal degradation of poly(L-lactide) effect of alkali earth metal oxides for selective L,L-lactide formation. Polymer, 45. 1197-1205. 

Polylactic acid (PLA) has been known for many years but use of this biodegradable plastic polymer was largely confined to internal surgical stitching due to the relatively high cost. Recent advances in the process for fermentation of com starch to produce lactate, coupled with chemical lactide formation, have resulted in a cost that is economically attractive. Cargill Dow Polymers have announced investments of over 300 million in a new facility in... 

In one study, poly(L-lactide) (M =70,000, M /M =3.2) and PCL (M = 55,000, MJM = 2) were blended in a twin-screw extruder at 200 °C in an attempt to prepare block copolymers through transesterification reactions in the presence of various catalysts [116], Catalysts were added as 20 wt % solution in toluene. Poly(L-lactide) decomposes slightly above its melting point (180 °C) and the processing window is small processing with limited residence times at 200 °C was considered satisfactory although there was some decomposition [116]. Coextrusion of the polymers (50/50) with a residence time of 5 min without catalyst gave no transesterification and no lactide formation i.e. residual catalysts left in the polymers on synthesis did not catalyse reactions. 

Racemization. In the production of stereochemically pure lactide, formation of the other lactic acid enantiomer and wcjo-lactide is unwanted. Higher temperatures, longer reaction times, and increased catalyst levels result in increased rates of racemization [4,6,69]. Since temperature and catalyst influence the rate of lactide formation as well, controlhng the racemization rate can become quite complex. 

This underlines the need for a low me o-lactide content in the monomer mixture for semicrystalline PLA, because we o-lactide formation by racemization cannot be avoided during melt polymerization of lactides. According to Gruber and coworkers, racemization, which lowers the stereochemical purity of the PLA, is believed to be driven by factors such as temperature, pressure, time at a given temperature or pressure, the presence of catalysts or impurities, and relative concentrations of the two enantiomers at any given time during the polymerization process [88]. 

It was earlier mentioned that the reversible lactide formation from polycondensated lactic acid was initially explored by Carothers. He furthermore observed that manipulation of the temperature and pressure could be utilized for pushing the equilibrium toward the lactide product. This was utilized later for the preparation of lactide, but the presence of other species (e.g., lactic acid, water, lactoyllactic acid, lactoyl-lactoyllactic acid, and higher oligomers) necessitates further purification of the crude lactide to make it useful for polymerization purposes. 

Noda, M. (1999). LL-Lactide formation from thermal depolymerization reaction of poly(L-lactic acid) oligomer (I), Shimadzu Hyoron, 56, 83-86 (in Japanese). 

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