Thermochromic transitions of the polydiacetylenes (PDAs) of the bis(ethyl)- and bis(isopropyl)urethanes of 5,7-dodecadiyne-1,12-diol (poly(ETCD) and poly(IPUDO)) were investigated by solid-state C-13 cross-polarization and magic angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy and differential scanning calorimetry (DSC). Side group conformational changes resulting from the polymerization and thermochromic transition were monitored by solid-state C-13 CP/MAS NMR. Consecutive heating/cooling cycles in DSC experiments in the temperature range that includes the thermochromic transition reveal that poly(IPUDO) displays better reversibility than poly(ETCD). In solid-state C-13 CP/MAS NMR spectra, poly(IPUDO) showed more restricted side group movement during the transition than poly(ETCD), which may be the major factor for better reversibility. A nonthermochromic form of poly(ETCD) was prepared and compared to its thermochromic counterpart by means of X-ray powder diffraction, DSC, Fourier transform infrared spectroscopy (FT-IR), and solid-state C-13 CP/MAS NMR spectroscopy. Nonthermochromic poly(ETCD) maintains intramolecular hydrogen bonding between neighboring urethane functions as in thermochromic poly(ETCD). The C-13 NMR chemical shifts, sensitive to mechanical strain, of nonthermochromic poly(ETCD) showed an upfield shift of 2 ppm. from the red phase alkyne carbon resonance of thermochromic poly(ETCD), which may be an indicator of the removal of thermochromic interactions.