This work examines the effect of Na montmorillonite (Na MMT) and cation exchanged layered montmorillonite on the polymerization of a cyanate ester monomer (1,1-bis(4-cyanatophenyl)ethane). The cyanate ester polymerization reaction is examined using a traditional catalyst and several montmorillonites. Thermal properties of the liquid monomer mixtures and final polymers were investigated, along with the montmorillonite structural changes that occur during the various stages of polymerization. It was found that Na montmorillonite catalyzed the polymerization reaction and that the catalytic activity increased upon sodium cation exchange in the montmorillonite gallery. Montmorillonite catalyzed systems were found to have superior properties (cross-link homogeneity and extent of polymerization) compared to traditionally catalyzed cyanate ester-montmorillonite polymers. Evidence suggests that the montmorillonite structure is a mixture of exfoliated layers and intercalated tactoids. The intercalated montmorillonite was found to undergo a two-step layer expansion process beginning with monomer permeation into the gallery, followed by expansion possibly caused by monomer/polymer reorientation within the gallery. (C) 2004 Published by Elsevier Ltd.