Selective and quantitative cyclotrimerization of p-tolylisocyanate proceeded by using sodium p-toluenesulfinate as a catalyst and 1,3-dimethylimidazolidinone as a solvent. Exploitation of this system to the cyclotrimerization of methylene diphenyl 4,40-diisocyanate (MDI) permitted formation of the corresponding networked polymer, which was selectively consisted of isocyanurate moiety and thus exhibited excellent thermal stability. Utilization of phenyl isocyanate (PhNCO) as a comonomer with varying feed ratio [ MDI](0)/[PhNCO](0) allowed successful control of flexibility of the networked polymers, while retaining its high thermal stability. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 5186-5191, 2011