A thermosetting composition comprising a co-reactable solid, particulate mixture of (a) beta-hydroxyalkylamide crosslinking agent, e.g., bis(N,N-di-2-hydroxyethyl)adipamide, and (b) polycarboxylic acid functional polymer, is described. The polycarboxylic acid functional polymer is prepared by atom transfer radical polymerization and has well defined polymer chain architecture and polydispersity index of less than 2.5. The thermosetting compositions of the present invention have utility as powder coatings compositions.