Development of Biodegradable Engineering Plastics

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Biodegradable ahphatic polyesters are used for both biomedical and general purposes. However, their mechanical and thermal properties are insufficient in comparison with aromatic polyesters. It is well known that the introduction of rigid rings into the polymer backbone improves both thermal and mechanical properties, although small aromatic compounds produced by the hydrolysis of such a material can be harmful. Therefore, to improve thermal properties of ahphatic polyesters, dimethyl 3, 6-diketopiperazine-2, 5-diacetate (DDK), which has a rigid 6-membered ring, was synthesized from aspartic acid and used as the dicarboxylic acid component for aliphatic polyesters.First, a homopolymer (P(DDK-TEG)) was prepared from DDK and tetraethylene glycol (TEG). The molecular weight of P(DDK-TEG) was low because of the low solubility of DDK as well as the low thermal stability of the resulting homopolymer.Next, DDK was introduced to PBS by the copolycondensation with dimethyl succinate and 1, 4-butanediol. The products were soluble in chloroform as far as their DDK content did not exceed 10 mol%. The copolymer showed improved thermal stability: its 5% weight loss temperature was found to be 341℃ under nitrogen, while that of PBS was 307℃. The introduction of the rigid unit into the copolymer backbone reduces the mobility of the chain above the melting point, which is considered to retard its thermal decomposition. A film was prepared from a copolymer, whose DDK content was 6.8 mol% and Mw was 100, 000. This sample was successfully hydrolyzed by Lipase PS^【○R】, although the degradation rate was lower than that of PBS. The non-enzymatic degradation of the copolymer proceeded much faster than that of PBS did, which is explained by the highly polar nature of the DDK unit: it is so hydrophilic that it assists the penetration of water into the sample which results in the acceleration of non-enzymatic hydrolysis.