Synthesis and characterization of poly(DL-lactide)-grafted gelatins as bioabsorbable amphiphilic polymers

A series of poly(DL-lactide) grafted gelatins, as new bioabsorbable amphiphilic polymers useful in parenteral drug delivery systems and in tissue engineering, were synthesized by the ring opening polymerization of DL-lactide onto a fractionated gelatin with the molecular weight of 1.02 x 10(5). Using tin(II) bis(2-ethylhexanoate) as catalyst, the bulk copolymerization at 140 degrees C and solution copolymerization in dimethylsulfoxide (DMSO) at 80 degrees C were firstly performed in the presence of gelatin. The results showed that the solution copolymerization in DMSO could afford the expected copolymers but the bulk copolymerization would result in an insoluble crosslinked product. The number of grafting sites on gelatin chain could be adjusted by the partial trimethylsilylation of side hydroxy, amino and carboxylic groups in gelatin. The solution copolymerization of DL-lactide on the partially protected gelatin in DMSO was also successful in providing copolymers with different molecular weights. The synthesized copolymers were characterized on the basis of elemental analysis, IR, 1H-NMR and thermal analysis. The IR and 1H-NMR data of these produced copolymers suggested that polylactide branches could be grafted onto gelatin via the side groups such as hydroxyl and amino groups in the solution copolymerization as well as carboxylic groups in bulk copolymerization. The molecular weights of the copolymers could be calculated from the difference of nitrogen contents between a copolymer and free gelatin. The results indicated that molecular weight of the copolymers and those of polylactide branches were increased with the feeding ratio of DL-lactide to gelatin in the copolymerization. However, because of the steric hindrance of some grafting sites on gelatin and the transesterifications of the propagating polylactide branches on gelatin with possibly formed homo-polymeric polylactide chains, the finally formed polylactide branches on gelatin were not very large and the highest average molecular weight of a polylactide branch was not over 4500 in any solution copolymerizations. The results from the thermal analysis of some copolymers, including thermogravimetry and differential scanning calorimetry, showed that the absorbed water in the samples could be lost at a temperature range below 150 degrees C and melting point decreased with increase of polylactide branches in the poly(DL-lactide)-grafted gelatins.