The invention is directed to biodegradable, thermoplastic, phase separated segmented multi-block copolymers. The copolymers of the present invention find use in various biomedical applications as well as in pharmaceutical applications. Provided is a composition for the controlled release of at least one biologically active polypeptide to a host, comprising the at least one biologically active polypeptide encapsulated in a matrix comprising at least one phase separated, thermoplastic multi-block copolymer, the copolymer being characterized in that (i) it comprises at least two hydrolysable segments chosen from prepolymer (A) and prepolymer (B), prepolymer (A) having a Tg lower than 37°C and prepolymer (B) having a Tm of 40°C - 100°C under physiological conditions； (ii) the segments being linked by a multifunctional chain-extender； (iii) the segments are randomly distributed over the polymer chain； and (iv) prepolymer (A) contains a segment that is derived from a water soluble polymer.
