Thermal polymerization of diphenyldiacetylene in the absence of a catalyst is a second-order reaction. The rate of polymerization is proportional to the square of monomer concentration, and the resultant molecular weight of the polymer is independent of the monomer concentration. The results suggest a mechanism in which bimolecular initiation and bimolecular termination steps are involved. The IR, NMR, UV, EPR, and mass spectral properties of the polymer are described.