This invention relates to a polymer composition of a block copolymer dispersed in water wherein the block copolymer is composed of: a) at least one first block of polymerized units of at least one first ethylenically unsaturated monomer and at least one latent thiol mercaptan chain transfer agent wherein the latent thiol mercaptan chain transfer agent has the general formula; ##STR1## wherein A is a polyvalent organic radical; x and y are positive integers such that x+y equals the valence of A; B is an organic acyl radical; and n and m are integers from 1 to 6; and b) at least one second block of polymerized units of at feast one second ethylenically unsaturated monomer, wherein said at least one second block is attached to said at least one first block by reacting with terminal thiol functional groups contained on said at least one first block.