A series of blends is prepared comprising 2,2-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine)propane (BA-a) with variously 5, 10, or 20 wt % of a selected oligomer represented by poly(arylsulfone) (PSU) or poly(arylethersulfone) (PES). The oligomers, comprising either chloro-, hydroxyl- or benzoxazinyl- (Bz) terminal functionality, are of low molecular weight (3000-12000 g mol(-1)). The introduction of the oligomers is shown to initiate the, polymerization of a bisbenzoxazine monomer where the terminal functionality of the oligomer is coreactive (e.g., hydroxyl or benzoxazine) without having a detrimental effect on the polymerization kinetics (similar values for the activation energy and orders of reaction are obtained). The introduction of the nonreactive chloro-terminated oligomer appears to favor the formation of an interpenetrating network (IPN) with a higher energy of activation. The thermal stability of the blends is generally increased compared with the polybenzoxazine homopolymer, regardless of the molecular weight or thermoplastic loading. Aside from the aforementioned PSUCI-containing IPN, the nature of the resulting network is slightly modified by the addition of the thermoplastic with similar or slightly elevated cross-link densities recorded (compared with the polybenzoxazine homopolymer). The heterogeneity of the network increases with a broadening of the tan delta response, suggesting an improvement in the toughness of the resulting blend.