ESR observations of single crystals of bis(p-toluene sulphonate) of 2.4.hexadiyne-1,6dioI during solid state thermal polymerization at 60ยฐC reveal spectra characteristic of pammagnetic triplet species. One ccntre has large values of the fine structure tensor and maximum intensity at maximum polymerization rate. The other has small fine structure values and anne& out dowly after the period of rapid polymerization. The nature of these centres is discussed. : The solid state polymerization of many disubstituted diacetylene monomers lead to the production of macroscopic polymer single crystals [ 143. The polymer molecular structure as revealed by X-ray diffraction studies is close to the valence bond structure shown in fig. I. a. Two possible mechanisms have been suggested for the polymerization reaction [ 1.51; fust a radical polymerization with a subsequent indirect change in bond structure and secondly a carbene centre at a bent monomer terminal carbon bond inyolving a direct change from monomer to polymer structure. Compare figs. lb and lc. In the carbene case the unpaired spins can be parallel or antiparallel giving states of total spin S = 1 or 0. In many situations the spin triplet (S = 1) state lies lowest in energy and is characterized by large values of the zero-field param-