The thermal degradation of the biodegradable bacterial polyesters poly(3-hydroxybutyrate), PHB, and poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHB/V produces a macromer, especially a 1000-6000 g/mol macromolecule, which contains an unsaturated end group as well as a carboxylic acid end group. The macromers may be polymerized to produce homopolymers and copolymers for different applications in which amphiphilic and biocompatible properties are required, for example, drug delivery systems.