A thermosetting composition useful as a powder coating comprises a co-reactable solid, particulate mixture of (a) an epoxy functional polymer containing at least one of the polymer chain structures -[(M)p-(G)q]x- or -[(G)q-(M)p]x- wherein M is an oxirane-free residue of at least one ethylenically unsaturated monomer, G is a glycidyl-functional ethylenically unsaturated monomer, p and q each independently has a value of 0 to typically less than 100, and x has a value of at least 1 to typically less than 100, the total value of p, q and x selected such that said epoxy functional polymer has a number average molecular weight of at least 250, and (b) a co-reactant having functional groups reactive with the epoxy groups of (a), e.g., dodecanedioic acid. The epoxy functional polymer is prepared by atom transfer radical polymerization and a polydispersity index of less than 2.5.