Polyanhydrides: V. Branched polyanhydrides

The objective of the present study was to investigate the properties of branched polyanhydrides and compare them to the corresponding linear polymers. Sebacic acid was polymerized with 1,3,5 benzenetricarboxylic acid and poly(acrylic acid) to yield random and graft-type branched polyanhydrides. The polymerization was followed until the gel point and the resulting polymers were evaluated for their physico-chemical properties and degradation behaviour. Drug release from these polymers was studied using morphine as a model drug. The experiment showed that the molecular weights of branched polyanhydride were significantly higher (mol wt 250 000) than the molecular weight of linear poly(sebacic anydride) (mol wt 80 000). In the case of poly(acrylic acid) branched polymers, the molecular weight increased linearly with increasing concentration of poly(acrylic acid). The specific viscosities of the branched polyanhydrides were lower than linear polyanhydrides with similar molecular weights. Except for the difference in molecular weights, there were no noticeable changes in the physico-chemical or thermal properties of the branched polymers and the linear poly(sebacic anhydride). The degradation of the branched polyanhydride was triphasic and the degradation rates were faster than for linear poly(sebacic anhydride). The release of morphine from the branched polymers was lower than the corresponding poly(sebacic anydride). Release of morphine was much higher from the poly(acrylic acid) branched polymers compared to the 1,3,5 benzenetricarboxylic acid branched polymers and increased with increasing concentrations of the branching agent. However, in both cases the release rates and the total amounts of morphine released approached that of poly(sebacic anhydride).