Cyanate ester or triazine networks are attaining increasing importance as potential candidates for high temperature adhesives and composite matrices. Low toughness is a major drawback with most crosslinked thermosetting materials, including the cyanate ester networks. Considerable attention has been devoted to the aspect of toughening such brittle networks in our laboratories. Reactive functional thermoplastics not only enhance toughness but also impart highly desirable stability to solvent stress cracking without seriously affecting the moderately high modulus. Various aspects of this technology, have earlier been successfully applied to epoxy and bismaleimide systems. Careful control of the heterophase morphological structure is necessary to achieve significant toughening. This thesis has focused on modifications of a specific cyanate ester network system based on Bisphenol-A with thermoplastic modifiers, which were systematically varied with respect to back-bone molecular weight and chemistry. Hydroxyl or cyanato functional Bisphenol-A based amorphous poly(arylene ether)s have been successfully utilized to toughen the cyanate ester networks. Blends of reactive and non-reactive Bisphenol-A based amorphous poly(arylene ether sulfone)s were also demonstrated to be useful tougheners, apparently by allowing phase size control. The use of Bisphenol-A based amorphous polyarylene ether ketones (which are of lower polarity relative to the Bisphenol-A based polyarylene ether sulfones) resulted in larger, well defined morphologies which in turn resulted in tougher networks. It was demonstrated that either hydroxyl or cyanato reactive end-groups could be effectively utilized. Both were superior to non-reactive systems in terms of mechanical performance as well as solvent stability. One of the major drawbacks of this effort was that 3-4 fold improvements in toughness were attained but this was at the expense of the upper use temperature which dropped to a significant extent. Hydroxyl functional phenolphthalein based amorphous poly(arylene ether)s have also been successfully utilized to toughen the cyanate ester networks. This is significant in that toughened multi phase networks were generated without a sacrifice in either the Tg or the moderately high modulus of the unmodified cyanate ester networks. It has been demonstrated that the heterophase morphological structure which strongly influences mechanical performance is in turn influenced by the back-bone chemistry, molecular weight and end-functionality of the thermoplastic modifier. In addition, the kinetics of network formation also significantly influences the microphase separated morphologies. Generation and control of such microphase separated morphologies employing both thermal and microwave radiation has been investigated. An interdisciplinary investigation was undertaken to explore the feasibility of hydroxy functionalized phenolphthalein based poly(arylene ether sulfone) modified cyanate ester networks as potential candidates for high performance adhesive and composite matrix applications. Investigations into composite matrix applications, involved establishing models for the experimentally determined time and temperature dependent kinetics of cure as well as melt rheology. It is expected that these models will consequently complement efforts in establishing an optimized cure protocol for the fabrication of composite panels. Preliminary studies concerning aspects of fiber-matrix interfacial adhesion and the viability of thermoplastic modified cyanate ester networks as a structural adhesive have been conducted. / Ph. D.