The ring-opening copolymerization of 3,6-bis(chloromethyl)-1,4-dioxane-2,5-dione and L-lactide affords a chloro-substituted polylactide copolymer (chloro-PL). Base-promoted dehydrochlorination of chloro-PL provides a copolymer (ene-PL) that contains electrophilic alpha,beta-unsaturated ester units. The copolymer undergoes conjugate addition with a variety of thiols in solution under mildly basic conditions and also in the presence of AIBN. Reaction on the surface of films of the unsaturated copolyester is demonstrated by the addition of a thiol-substitutedfluorescent dye.