A thermally reversible cross-linking reaction of vinyl polymers was investigated. Poly(4-vinylimidazole-co-methyl methacrylate) (2) and poly(3-isopropenyl-alpha,alpha (1)-dimethylbenzyl isocyanate-co-methyl methacrylate) (6) were prepared by radical copolymerization. Gelation was observed in a few minutes by mixing polymer solutions of 2 and 6 in DMF at room temperature. A clear solution was produced at high temperatures. Reversible formation of urea bonds in the reaction was confirmed by IR spectroscopy. No significant side reaction was observed under argon when the reaction was run repeatedly. In the model reaction of 4-vinylimidazole with 3-isopropenyl-alpha,alpha -dimethylbenzyl isocyanate in DMF-d(7), equilibrium constants were calculated at various temperatures by H-1 NMR spectroscopy. DeltaH(0) of -56 kJ mol(-1) and DeltaS(0) of -152 J mol(-1) K were determined.