Solid‐state polymerization of a diacetylene crystal: Thermal, ultraviolet, and γ‐ray polymerization of 2,4‐hexadiyne‐1,6‐diol bis‐(p‐toluene sulfonate)

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Abstract
Results are presented for the thermal, ultraviolet, and γ‐ray polymerization of 2,4‐hexadiyne‐1,6‐diol bis‐(p‐toluene sulfonate) (PTS). Monomer extraction is used to obtain polymer conversion‐vs.‐time curves at 30, 50, and 80°C. In agreement with previous work over a narrower temperature range, the curves all display a dramatic autocatalytic effect with an onset at about 10% 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 (E) of 22.2 ± 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, E = 22.5 ± 0.8 kcal/mole is determined from measurements of the time required to go from 10 to 50% polymer at 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 E = 22.8 ± 0.6 kcal/mole. Thus E 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 γ‐ray polymerization. In the case of γ‐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 γ‐ray polymerization are quite low (2‐3 kcal/mole) and confirm that the chain initiation event makes the major energetic contribution to E. 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.

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