the invention provides a method comprising the steps of providing a poly having one terminal hydroxyl group ; reacting the terminal hydroxyl group of the poly with dicarbonate to form a 1 - benzotriazolylcarbonate ester of the poly ; reacting the 1 - benzotriazolylcarbonate ester of the poly with an amino acid to form an amino acid derivative ; and reacting the amino acid derivative with a biologically active agent under conditions to form a polymer - active agent conjugate .