Amorphous and semi-crystalline poly(arylene ether sulfone) homo- and copolymers and related structures were systematically synthesized via nucleophilic, aromatic substitution reactions using the potassium carbonate/N-methylpyrrolidone synthesis route. In particular, deuterated poly(arylene ether sulfones) were synthesized in order to investigate molecular motion via deuterium NMR. The mechanical, dynamic mechanical and fracture properties of these materials were also examined. Polysulfones were characterized to be tough, ductile materials and it was found that polysulfone-poly(dimethylsiloxane) block copolymers blends with homopolysulfone serves to further increase their ductility as indicated by fracture toughness K_I,C measurements. In addition, it was demonstrated that functionalized polysulfone was an excellent thermoplastic modifier for brittle epoxy networks. The morphology and properties of these copolymers and copolymer-homopolymer blends were studied as a function of molecular weight and chemical structure. A number of techniques were utilized to elucidate the possible toughening mechanisms that are operative.