This paper describes a new method for rubber toughening brittle powder coatings. The design involves the use of toughening agents with specific physical and chemical properties. The modifiers are low-to-medium molecular weight polymers with polymerizable end groups (macromonomers). To achieve the physical properties required for fabrication into fine powders for subsequent deposition using conventional powder coating equipment, semi-crystalline polymers with Tg's well below room temperature and melting points above ≈70°C were required. Upon copolymerization with a thermosetting resin, crystallization of the modifier was precluded provided that low ethylene oxide compositions were employed. This scheme, in principle, yields an amorphous low Tg modifier chemically bound to the cured network. Poly(ethylene oxide), PEO, was found to be an excellent candidate, since it has a Tg of -68°C and a Tm of 70°C, sufficient crystallinity for friability and functional end groups for copolymerization. Free radical polymerization of 2,2-bis[4-vinylbenzoyloxyphenyl] hexafluoropropane and step-growth polymerization of 4,4′-(hexafluoroisopropylidene)diphenyl cyanate were utilized to demonstrate the feasibility of this approach. An electrostatic powder coater was used to co-deposit the PEO macromonomers with either of the thermosetting precursor monomers, followed by thermal curing to produce the modified networks. The resulting networks showed multiphase morphologies with improved toughness.