The thermally annealed solid state polymerization of 2,3-dicyano-5,7-dimethyl-6H-1,4-diazepine(A) was studied by H-1, C-13, and N-15 CP/MAS solid state NMR spectroscopy, and by a H-1 solution state NMR deuterium exchange experiment of A Collectively, the NMR data suggests that a tautomeric form of the monomer is responsible for the solid state reactivity of the crystalline monomer. The solid state polymerization of crystalline A proceeds as a heterogeneous decomposition of the crystalline lattice resulting in an insoluble, amorphous material. A plausible reaction scheme involving a tautomeric form of A is proposed. The scheme is consistent with the NMR spectra.