LACTIC-ACID BASED POLY(ESTER-URETHANE)S - THE EFFECTS OF DIFFERENT POLYMERIZATION CONDITIONS ON THE POLYMER STRUCTURE AND PROPERTIES

A two-step process for lactic acid polymerization is studied: in the f irst step the lactic acid is condensation-polymerized to a low-molecul ar-weight hydroxyl-terminated prepolymer; and then the molecular weigh t is raised by joining prepolymer chains together using diisocyanate a s the chain extender. The resulting polymer is a thermoplastic poly(es ter-urethane). In this study, we synthesized three different prepolyme rs and used three different diisocyanates as chain extenders. All of t he prepolymers were hydroxyl-terminated, and their weight average mole cular weights were 5,500 g/mol, 11,900 g/mol, and 26,000 g/mol. One of the diisocyanates was aliphatic, and the other two were stiff cycloal iphatic diisocyanates. The results indicate that of the tested diisocy anates, high weight average molecular weight can be achieved only by u sing aliphatic 1,6-hexamethylene diisocyanate. The cycloaliphatic diis ocyanates produced poly(ester-urethane)s with weight average molecular weights which were quite low, but due to the stiffness of the polymer chains the glass transition temperatures can be as high as 60 degrees C. The 1,6-hexamethylene diisocyanate (HDI) results also indicate tha t the molecular weight and network formation can be controlled indepen dently by the amount of diisocyanate used and the polymerization condi tions. Only the poly(ester-urethane)s which were produced with HDI had good mechanical properties, while the stiff diisocyanates produced ve ry brittle polymers. (C) 1997 John Wiley & Sons, Inc.