Microcalorimetry was used to study the thermal polymerization and the structural instabilities in hydrogenated (p TS-H) and deuterated (p TS-D) diacetylene 2,4-hexadiynylene bis(p-toluenesulfonate). In the induction period, the polymerization reaction proceeds about seven times faster in p TS-D than in p TS-H. Whereas the phase transitions of pure p TS-H and p TS-D monomer crystals and of pure p TS-H and p TS-D polymer crystals are very similar, the situation is completely different for mixed monomer-polymer crystals of p TS-H and p TS-D. The results are interpreted by studying comparative influence of the chain lengths for PTS-H and p TS-D oligomers. A comparison is made between these results and previous experimental results obtained from dielectric measurements, UV-spectroscopy, X-ray, and elastic neutron scattering.