Solutions of certain urethane substituted polydiacetylenes undergo dramatic, reversible color changes on addition of a nonsolvent. In some cases, the lowest energy optical transition of the polymer shifts by more than 5000 cm−1 (21 300 cm−1 to 15 900 cm−1 when the nonsolvent is added). The color changes are shown to be due to a planar–nonplanar conformational transition of the polymer backbone. The planar, fully-conjugated conformation is stabilized by intramolecular hydrogen bonding between urethane functionalities on adjacent substituent groups.

The optical properties of single crystals and solution-cast films of two polydiacetylenes, poly3BCMU and poly4BCMU where the substituent group is-(CH2h .. OCONHCH2COOC~, are characterized. Visible absorption and reflection spectra for the polymer crystals are typical of those observed for other urethane substituted polydiacetylenes. The optical properties of the polymer films are controlled primarily by intramolecular hydrogen bonding between the N-H and C=O of the urethane functionalities on adjacent substituent groups. Hydrogen bonding stabilizes the planar, fully conjugated conformation of the individual polymer chains in the films. Increased temperature causes a disruption of the hydrogen bond network and a destabilization of the planar polymer conformation. Dramatic color changes result because of the sensitivity of the optical properties to backbone conformation.

SynopsisResults are presented for the thermal, ultraviolet, and y-ray polymerization of 2,4-hexadiynel,6-diol bis-(p-toluene sulfonate) (PTS). Monomer extraction is used to obtain polymer conversion-vs.-time curves at 30,50, and SOT. In agreement with previous work over a narrower temperature range, the curves all display a dramatic autocatalytic effect with an onset a t about 1Wo conversion to polymer. Although the polymerization rate undergoes a 200-fold change over this temperature range, the shape of the conversion curves does not change. These data yield an activation energy (Ekh) of 22.2 f 0.4 kcal/mole when interpreted in terms of the time required to reach 50%polymer. An annealing technique is used to provide a closer look at the autocatalytic region. In that case, ELh = 22.5 f 0.8 kcal/mole is determined from measurements of the time required to go from 10 to 50% polymer a t temperatures ranging from 23 to 80°C (a 500-fold change in rate). Thermal polymerization rates measured in the low-conversion limit using a spectroscopic method based on diffuse reflectance yield Eih = 22.8 f 0.6 kcal/mole. Thus Ekh is independent of polymer conversion and the autocatalytic effect can be best understood as arising from a large increase in the propagation length of the polymer chains. The autocatalytic effect is shown to be present in both UV and y-ray polymerization. In the case of y-ray polymerization, conversion-vs.-time and spectroscopic measurements are consistent with inhomogeneities in the polymer concentration caused by particle tracks. Activation energies for UV and y-ray polymerization are quite low (2-3 kcal/mole) and confirm that the chain initiation event makes the major energetic contribution to Ekh. The polymerization mechanism is discussed in detail. The photopolymerization experiments can be consistently interpreted with a model based on the triplet excited state of the diacetylene monomer as the chain initiation species. R is --CH2SO3C6H4CH3. A great deal of recent work has been concerned with the optical3-5 and electrical propertiesG8 of fully and partially polymerized PTS, as well as other polydiacetylenes.9-l1The thermal polymerization of PTS is particularly interesting because of a dramatic "autocatalytic" effect evident in the conversion-vs. time curves.l At about 10% polymer conversion, the polymerization rate increases dramatically and the conversion rapidly approaches 100%. The polymerization rate in the autocatalytic region is at least 10 times the rate in the low-conversion limit. However, the measured activation energies at low conversion and at high conversion are the same within experimental error (22 kcal/mole). Since these activation energies are primarily associated with the chain initiation event, there

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A novel planar ↔ nonplanar visual thermochromic conformational transition of polydiacetylene molecules in poor solvents is reported. The conformational transition is associated with both a color change (blue or red ↔ yellow) and a change in the state; the yellow solution (liquid) transforms to a blue or red gel (solid). The color transition occurs within a narrow range of temperature and has a large associated hysteresis. The enthalpy of the conformational transition is 29 kJ/mole of repeat unit. Fourier‐transform infrared studies show that molecules acquire a planar conformation in red or blue gels by formation of intramolecular H bonds between the adjacent substituent groups. Virtually all H bonds break (a nonplanar conformation) when the gels turn into yellow solutions.