Characterization of a homologous series of D,L-lactic acid oligomers; a mechanistic study on the degradation kinetics in vitro

A series of low molecular weight polymers Of D,L-lactic acid has been synthesized. The oligomers were characterized with respect to molecular weight, glass transition temperature and solubility. The number average molecular weight of the oligomers ranged from 290 to 1320 Da. Oligomers with an (M) over bar (n) 800 Da were soluble in buffer at pH 7.4 but insoluble in water and acidic medium. Kinetic studies were performed at pH 1.5, 4.5 and 7.4 using an accelerated in vitro monomer release test. The hydrolytic rate was dependent oil molecular weight of oligomer, temperature and pH of the media, with the lowest rate found around pH 4.5. The activation energy was dependent on molecular weight and ranged from 47 to 67 kJ mol(-1). Random ester cleavage was identified as mechanism of hydrolysis in basic media whereas in acidic media chain-end cleavage (unzipping) was the mode of action. (C) 2003 Elsevier Science Ltd. All rights reserved.