The invention relates to a gelable copolymer composition comprising a copolymer in a solvent. The copolymer has the structure A(B)n, wherein the core (A) is soluble in the solvent, the arms (B) are convertible between soluble and insoluble in the solent depending on an environmental condition, and n>1. The composition forms a gel under environmental conditions in which B is insoluble, through formation of B aggregates. Block copolymers comprising polyethylene glycol (PEG) and poly(N-iso-propylacrylamide) (PNIPAAm) having a liquid form at ambient temperature under aqueous conditions and a gel form at body temperature are disclosed. Copolymer compositions according to the invention can be used to form in situ implants useful in slow-release formulations of biologically active molecules