A new styrenic monomer, 4-vinylphenoxyphthalonitrile, was synthesized by nucleophilic aromatic substitution of 4-vinylphenolate onto 4-nitrophthalonitrile. The new monomer was reacted with dimethacrylate-styrene resins (so-called vinyl esters) via free radical copolymerization at 90, then 120 degreesC with benzoyl peroxide as the initiator to form networks. The networks were further post-cured at 220, 240, or 260 degreesC for 4 h to convert the last portion of the methacrylates, and to partially cross-link the pendent phthalonitrile groups. A dimethacrylate-styrene network containing 30 wt% of the phthalonitrile-functional monomer exhibited a 70% reduction in peak heat release rate by cone calorimetry measured at an incident heat flux of 50 kW m(-2) relative to control vinyl ester networks. This still remains somewhat higher than peak heat release rates exhibited by brominated vinyl ester networks, but the CO/CO2 ratio in the smoke is much lower for these non-halogenated materials. (C) 2004 Elsevier Ltd. All rights reserved.