We describe the use of organic catalysis for the ring-opening polymerization of functionalized lactones and conversion of the resulting aliphatic polyesters into crosslinked nanoparticles that carry additional functional groups amenable to further modification. Specifically, highly functional aliphatic polyester homopolymers, as well as random and block copolymers, were prepared by 1,5,7-triazabicyclo[4.4.0]dec-5-ene catalysis, giving polyesters with pendent alkene and alkyne groups. Azide-alkyne click and thiol-ene chemistries were used for postpolymerization modification of diblock copolymers possessing alkene groups on one block and alkyne groups on the other block. The polyesters were crosslinked using azide/alkyne cycloaddition, by reaction of alpha,omega-diazides with the pendent alkynes on the polyester backbone. This gave polyester nanoparticles possessing alkene functionality, which were subjected to further modification using thiol-ene reactions to introduce additional functionality. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012