An ESR method was used to study the structure of the macroradical of the propagating chain R(p) in the low-temperature, solid-phase polymerization of p-diethynylbenzene (DEB). The ESR spectra for gamma-irradiated DEB samples and those of DEB deuterated in the ethynyl group showed that in the range 77-230 K, the unpaired electron of the macroradical was localized on one of the monomer links. At 230-310 K, its delocalization in a polyconjugated system took place because of addition of a linear macroradical to a double bond of a polymer molecule. The encounter of the macroradical with double bond probably occurs as polymer chain propagation.