Homopolymers bearing terminal azide and alkyne groups can be coupled via click chemistry to yield diblock copolymers. When performed in solvents that dissolved both homopolymers, the click reaction was found in this study to be inefficient, probably due to the embedding of the reactive end groups inside the random coils of the polymers. The efficiency was only slightly affected by the addition of a small amount of water into the reaction mixture. However, the reaction efficiency increased dramatically near the water volume fraction where one or both of the reacting polymers began to precipitate. Further increases in water content caused the polymer(s) to undergo macroscopic phase separation and the click reaction efficiency decreased once again. A possible explanation for the observed effect of water on the polymer coupling reaction is proposed. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4922-4928, 2010