Structural Aspects of the Thermochromic Transition in Urethane-Substituted Polydiacetylenes

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Abstract

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 13C 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 13C 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 13C 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 13C CP/MAS NMR spectroscopy. Nonthermochromic poly(ETCD) maintains intramolecular hydrogen bonding between neighboring urethane functions as in thermochromic poly(ETCD). The 13C 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.

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