Intermediates, diacetylene spectroscopy

Sixl, H. Spectroscopy of the Intermediate States of the Solid State Polymerization Reaction in Diacetylene Crystals. Vol. 63, pp. 49 — 90. 

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. 

In all experiments described in this work only extremely low concentrations of intermediates are considered. This is due to our interest which is primarily focussed on the most important initial steps of the polymerization reaction, which are characteristic of the overall polymerization reaction mechanism. Consequently only low final polymer conversion is exp>ected and, therefore, complications arising from the interaction between the intermediate oligomer states can be neglected. It will be shown that the low temperature conventional optical absorption and ESR spectroscopy are powerful spectroscopic methods which yield a wealth of information concerning structural and dynamical aspects of the intermediate states in the photopolymerization reaction of diacetylene crystals. Therefore, this contribution will center on the photochemical and photophysical primary and secondary processes of this... 

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. 

In a solid state polymerization reaction monomer crystals of diacetylene molecules (R-CiC-C=C-R) are converted to polydiacetylene crystals (1,2). The primary photochemical processes during the low-temperature photopolymerization reaction have been investigated by ESR (3,4) and optical absorption spectroscopy (5,6). A review ofthe spectroscopy of the intermediate states has been given by Sixl (V. A simple reaction scheme is shown in Figure 1. The reaction is characterized by the uv-photolnitiation of dira-dlcal dimer molecules. Chain propagation is performed by thermal addition of monomer molecules. Thus trimer, tetra-mer, pentamer etc. molecules are obtained. 

Siegel D., H. Sixl, V. Enkelinann and G. Wenz. Polymerization of TS-12 Diacetylene Crystals Crystal Structures of Monomer and Polymer and Spectroscopy of Reaction Intermediates, Chem Phys. 72 (1982)201-212. 

H. Sixl Spectroscopy of the intermediate state of the solid state polymerization reaction in diacetylene crystals, in H.-J. Cantow (ed.) Polydiacetylene, Springer, 49 (1984)... 

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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