Thermal reactivity diacetylenes

The aim of this article is to provide an overview of the polymerization of diacetylenes. The focus will be on optical excitation, although some results on thermal reactivity will also be quoted to illustrate analogies. Comprehensiveness is not intended, instead, emphasis will be placed on model considerations. Structural aspects of the polymerization process, as well as the low temperature spectroscopy of reaction intermediates will only briefly be addressed since they are treated in detail in the contributions of V, Enkelmann and H. Sixl in this volume. 

While diacetylene polymerization is justifi U3ly regarded (18) as better investigated and imderstood than any other polymerization reaction, several fundamental uncertainties remain. With respect to initiation emd propagation, the lack of thermal reactivity of certain monomers (e.g., urethanes) which require radiation for polymerization contrasts with a monomer such as PTS (la), which polymerizes both thermally and with radiation. Equally... 

PDA solutions have provided the first determination of the molecular weight of a PDA and the discovery of interesting thermally- and solvent-induced chromic transitions of the individual PDA chains in solution. Finally, the polymerization of the monomers in the solid state is interesting for two reasons 1) they display the highest photochemical reactivity of studied diacetylenes and 2) they show almost no thermal reactivity. A possible explanation of the difference in thermal and photochemical reactivity is discussed in the next section. 

It appears that the reaction mechanism and the intermediates involved in the solid-state polymerization of diacetylenes are reasonably well understood. However, experimental results obtained with special monomers should not be generalized. It is not possible to design a monomer with desired properties. Inspection of Table 1 shows that on the basis of the crystallographic data and the monomer packing the absolute reactivity and the polymerization kinetics caimot be quantitatively predicted, e.g. it is not possible, to date, to explain why certain diacetylenes can be polymerized thermally whereas others with equal packing are thermally inactive. A more realistic kinetic model should include the various energy transport processes and the complex side group motions which are connected to the reaction. 

Table 2.5 A selection of substituents R with which the topochemical reaction of the diacetylenes has been investigated. The right-hand columns indicate the reactivities qualitatively for the case of thermal activation and for / irradiation. Other substituents have been given by Enkelman. After [10].

In a solid state polymerization reaction monomer diacetylene crystals are transformed to polymer crystals in successive reaction steps. Nearly perfect polymer single crystals are obtained thermally (kT) or by UV- or X-ray irradiation (hv) of the monomer crystals [1-3]. Within the class of diacetylene molecules (R-C=C-C=C-R) which show this unusual chemical reaction, the TSHD (with side groups R = -CH2SO2-0-CH2) is the best known representative, which is characterized by a variety of reaction intermediates [4-19]. The unconventional reactivity and the unusual properties of the polymer crystal have attracted the interest of both, physicists and chemists. The general feature of the low temperature photopolymerization reaction is shown schematically in Fig. 1 by example of the diradical DR-intermediates. 

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