In recent years, interest in environmental protection is increasing. In Japan, purchase promotion of environment-friendly materials by Law of Promoting Green Purchasing, recycling promotion of plastic materials by Containers and Packaging Recycling Law and of electric appliance by Home Appliance Recycling Law, and the like are its manifestation. In such a series of a current, polymers such as an aliphatic polyester and an aliphatic polycarbonate, having low environmental load are attracting attention.
A polylactic acid which is one of aliphatic polyesters has the characteristics to have high transparency, be tough, and easily hydrolyze in the presence of water. Therefore, in the case of using as a general-purpose resin, it decomposes without environmental pollution after disposal, and thus is environment-friendly. Further, in the case of remaining in a living body as a medical material, it is decomposed and absorbed in a living body without exerting toxicity to the living body after accomplishment of the purpose as the medical material, and thus is living body-friendly.
However, from the standpoint of heat resistance, it is difficult to say that the polylactic acid is adequate to use the same as a general-purpose resin. For this reason, emergence of homopolymers or copolymers of an aliphatic polyester or an aliphatic polycarbonate, having adequately high heat resistance and molecular weight is eagerly desired.
As one of aliphatic polycarbonates, a polycarbonate prepared from isosorbide and phosgene is disclosed in Non-Patent Document 1. According to this, it is described that this polycarbonate has Mw of 32,000, but a high molecular weight polycarbonate having Mw of 35,000 or more is not described. Further, in Patent Document 1, there is the description regarding a copolymer of 1,2-O-isopropylidene-D-xylofuranose-3,5-cyclic carbonate (IPXTC) which is an aliphatic cyclic carbonate, and a polylactic acid. According to this, it is described that the copolymer of 60 mol % of IPXTC as an aliphatic polycarbonate component, with a polylactic acid has Tg of 90° C. However, a compositional ratio of the copolymer of PLA and IPXTC, containing 60 mol % of IPXTC is converted into a weight ratio of PLA/IPXTC=18/82 (wt %), and an expensive IPXTC must be used in a large amount. Further, there are the problems that where the amount of IPXTC added is increased, the yield decreases, and the molecular weight of a copolymer obtained becomes low.
From the above facts, an aliphatic polymer having adequately high heat resistance and molecular weight and capable of producing economically is not known.    Patent Document 1: U.S. Pat. No. 6,093,792, pages 5-10    Non-Patent Document 1: Journal Fur Praktische Chemie Chemiker-Zeitung (1992), 334 (4), pages 298-310