Provided herein is a bioresorbable polyester wherein the metal content of the polyester is >；0 ppm and <；40 ppm, a controlled release pharmaceutical composition containing such polyesters, and a kit for injecting a controlled release pharmaceutical composition. Also provided herein is a method of purifying a bioresorbable polyester with a metal content >；0 ppm comprising the steps of: i) dissolving the polyester in an organic solvent to form a polyester-solvent solution, wherein the organic solvent comprises at least one heteroatom selected from oxygen, nitrogen, sulphur and phosphorus； ii) precipitating the polyester from the polyester-solvent solution by combining the polyester-solvent solution with an organic non-solvent, said non-solvent being an alcohol； and (iii) separating the precipitated polyester from the solvent and non-solvent to obtain a purified polyester.
