We report a series of biocompatible and biodegradable block copolymers of poly(epsilon-caprolactone) with "clickable" polyphosphoester (PPE). The block copolymers are synthesized through controlled ring-opening polymerization of five-membered cyclic phosphoester monomer, propargyl ethylene phosphate (PAEP), initiated with poly(epsilon-caprolactone) macroinitiator. The polymerization followed first-order kinetics with living polymerization characteristics, thus the molecular weight and composition of copolymers are tunable by adjusting the feed ratio of PAEP monomer to macroinitiator. Azide-functionalized poly(ethylene glycol) has been grafted to the copolymer to demonstrate the reactive feasibility by Cu(I)-catalyzed "click" chemistry of azides and alkynes, generating "brush-coil" polymers. The mild conditions associated with the click reaction are shown to be compatible with poly(epsilon-caprolactone) and PPE backbones, rendering the click reaction a generally useful method for grafting numerous types of functionality onto the block copolymers. The block copolymers also show good biocompatibility to cells, suggesting their suitability for a range of biomaterial applications. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 487-494, 2011