Development of Industrial Production of High Molecular Weight Poly-L-lactate from Renewable Resources
Hitomi OHARA*, Hisashi OKUYAMA, Seiji SAWA, Yasuhiro FUJII and Keiichiro HIYAMA†
Shimadzu Corporation Technology Research Laboratory; 1-8-1 Tsukinowa, Otsu-shi 520-2152 Japan
† Osaka Municipal Technical Research Institute; 1-6-50 Morinomiya, Joto-ku, Osaka-shi 536-8553 Japan
This development is related to the method of the industrial production of high molecular weight poly-L-lactate having the applicable properties as commodity plastics by use of sugars which are derived from renewable resources. To realize the effect of drawing orientation of films and fibers, high cristallinity of the poly-L-lactate is required. In other words, L-lactic acid preparation having high optical purity must be used as the raw material to produce such high crystal poly-L-lactate. Further, the price of the raw material should be reasonable one to make the price of such poly-L-lactate reasonable. For this purpose, we have developed a technology to produce L-lactic acid preparation of which optical purity is 99.0% by a strain of Bacillus species which has low auxotrophic characteristic. For the industrial production of poly-L-lactate, LL-dilactide was produced from the pre-polymers of low molecular weight poly-L-lactate through the intramolecular transesterification. In the preparation of the pre-polymers, polycondensation takes place where the linear mono-, di-, and trimer of lactic acid are totally refluxed so that the yield of LL-dilactide can be improved, then the temperature of the reactor is raised up in three steps, 135, 150, 160 °C to prevent the racemization of L-lactic acid. The high purity of LL-dilactide is also required to produce high molecular weight poly-L-lactate. The melting crystallization method without a solvent was developed for the purification of LL-dilactide after consideration of lowering the load on the environment. Among various intramolecular transesterification catalysts which we have examined, tin(II) 2-ethylhexanoate showed highest activity and obtained high optical purity LL-dilactide. Ring-opening polymerization of LL-dilactide was continuously took place without a solvent resulting in the production of high molecular weight poly-L-lactate of which weight-average molecular weight is 180000 to 200000. In addition, the properties of poly-L-lactate and the behavior of its biodegradation in a compost were revealed in this work.
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