Diradicals diacetylene polymerization

In this article it has been shown, that the low temperature photopolymerization reaction of diacetylene crystals is a highly complex reaction with a manifold of different reaction intermediates. Moreover, the diacetylene crystals represent a class of material which play a unique role within the usual polymerization reactions conventionally performed in the fluid phase. The spectroscopic interest of this contribution has been focussed mainly on the electronic properties of the different intermediates, such as butatriene or acetylene chain structure, diradical or carbene electron spin distributions and spin multiplicities. The elementary chemical reactions within all the individual steps of the polymerization reaction have been successfully investigated by the methods of solid state spectroscopy. Moreover we have been able to analyze the physical and chemical primary and secondary processes of the photochemical and thermal polymerization reaction in diacetylene crystals. This success has been largely due to the stability of the intermediates at low temperatures and to the high informational yield of optical and ESR spectroscopy in crystalline systems. 

Two different reaction mechanisms have been postulated for the topochemical polymerization of diacetylenes involving diradical or carbene chain ends The first mechanism leads to a butatriene structure (I), the latter to the acetylene structure (II) of polydiacetylene chains. 

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. 

Such studies did, however, suggest that the initial step in the polymerization reaction is a diradical (26), see Figure 5, apparently contrary to the evidence of the existence of carbene radicals in thermally polymerizing crystals (27), This apparent conflict has been resolved by an extensive series of studies of the photo- and thermal polymerization of diacetylenes at low temperatures conducted at the Universities of Stuttgart and Bayreuth- A review of this worK is in the course of publication and references to the extensive literature will be found there (20 - 30)- The most thorough studies have been made of the monomer TS, similar but less detailed results have been obtained for a number of other diacetylene monomers (31). In most of the experiments monomer crystals at 4 K are irradiated with ultraviolet radiation and studied by optical, EPR and ENDOR spectroscopy. At 4 K broad band irradiation produces stable oligomeric intermediates since polymerization cannot proceed by thermal activation. 

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