| 摘要 |
PROBLEM TO BE SOLVED: To efficiently granulate a polylactic acid under conditions hardly causing thermal deterioration, oxidative deterioration or the like and to obtain a high mol.wt. polylactic acid in a high yield by preparing a solid solution comprising a polylactic acid and a specific amount of diphenyl ether by the solution polymerization. SOLUTION: A solid solution is obtained by the solution polymerization of lactic acid using diphenyl ether as a solvent. The mixing ratio of a polylactic acid to diphenyl ether is 100 pts.wt., in terms of a lactic monomer, of the polylactic acid to 2-100 pts.wt. of diphenyl ether. The melting point of the solid solution is determined only by the ratio of diphenyl ether. The polylactic acid is prepared by granulating a polymer obtained by the solution polymerization (a molten solid solution) at a temperature lower than the melting point of the sole polylactic acid by at least 10 deg.C and subjecting it to a solid state polymerization. The solid solution is cooled down to the melting point or lower to solidify, crushed and granulated, or alternatively, a part of diphenyl ether in the solid solution is volatilized and removed so as to raise the melting point of the solid solution causing solidification followed by granulation.
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