Diacetylene structure model

Fig. 11. Structure model of monomeric and polymerized multilayers of cadmium salts of diacetylene fatty acids. Polymer chains grow within the layer plane, the aliphatic chains are tilted with regard to...

Figure 5.52 (a) Space-filling model of three-dimensional structure of diacetylenic lipid DCg PC... 

A kinetic model for single-phase polymerizations— that is, reactions where because of the similarity of structure the polymer grows as a solid-state solution in the monomer crystal without phase separation—has been proposed by Baughman [294] to explain the experimental behavior observed in the temperature- or light-induced polymerization of substimted diacetylenes R—C=C—C=C—R. The basic feature of the model is that the rate constant for nucleation is assumed to depend on the fraction of converted monomer x(f) and is not constant like it is assumed in the Avrami model discussed above. The rate of the solid-state polymerization is given by... 

An alternative view of the polysilane structure is depicted using the worm-like model as proposed for poly(diacetylene)s59, where the linear chain has a large number of small twists without sharp twists playing a special role60-62. In this model, a Gaussian distribution of site energies and/or exchange interactions and the coherence of the excitation is terminated by any of the numerous usual random deviations from perfect symmetry. 

Polydiacetylenes are produced by the solid-state polymerization of single crystalline diacetylenes of the form RC=C—C=CR by 1,4-addition (see Fig. 32). The fully conjugated backbone of the polydiacetylenes is a model quasi one-dimensional electronic system being capable of representation by two extremes of bonding, sequence (a) an acetylene structure -fRC—C=C—CR)- and sequence (b) a butatriene structure -fRC=C=C=CR)-A,. There is great current debate as to the best method of representation but, in line with the belief that the nature of the side groups affect the bond lengths (and hence the... 

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 Fig. 5 the values of the packing parameters d and d> are plotted for constant separations R between the reacting atoms Cl and C4. The relevance of the model considerations can be tested using crystal structure data, which have become available recently for a number of reactive and unreactive diacetylene monomers. Reactivity is only observed in a small area of the map. The distribution of the points for highly reactive structures suggest the criterion for which the separation R should be less than 4 A to be a more critical condition than the requirement of a least motion pathway as calculated by Baughman Figure 5 shows that all but one reactive diacetylene... 

A polymerizing diacetylene crystal can be considered as a composite material with large differences in the mechanical properties of both components. In such a material the mechanical properties will not only depend on the relative amount of the components but also very strongly on the geometrical arrangements of the structural elements Two limiting cases can be considered The first model consists... 

The diacetylenic acids (9,10) have also been widely investigated because of their polymerizability. Here the interesting diacetylenic entity is normally incorporated into the structure of an alkanoic acid to give a compound such as CH3(CH2)ioC=C-C=C(CH2)7COOH. The topochemical polymerization proceeds within the LB layer but results in an array of two-dimensional domains whose size is influenced by material purity. Diacetylenes have also been incorporated into lipidlike molecules and polymerized as model membranes 11, 12). Because of the structural flexibility of LB films, there is likely to be continued interest in polymer LB films research, some of which will involve preformed polymers (13). However, the rigidity of many polymer films prevents this approach being used generally. 

In recent years interest in these materials has grown mainly for physical reasons. The layer perovskites are looked at as model compounds for the study of magnetic properties in two-dimensional systems (J2) and as models for the study of structural phase transitions in lipid bilayer-type arrays ( ). The use of layer perovskites as a matrix for organic solid state reactions represents a fairly new research topic. First experiments were carried out studying the photolysis of butadiyne (diacetylene) derivatives (li-ZSl) For a corresponding study of the butadiene derivatives the compounds listed in Table I were synthesized. 

Many aspects of the preparation and properties of polydiacetylenes are the subject of lively debate. This review presents recent results that bear on some of these controversies. First the relationship of diacetylene monomer crystal structure and solid-state reactivity is discussed. Secondly the temporal evolution of solvato-chromio transitions of soluble polydiacetylenes is displayed. Optical and Raman spectra reveal the occurrence of an intermediate form of the polymer. A model compatible with these results is described. 

Brief reviews of the structure-reactivity relationship of dlacetylene monomers and the evolution of solution spectra after changes in solvent composition have been presented. Much remains to be done to formulate a precise set of guidelines for the production of reactive diacetylenes. The presence of multiple substitution of conjugated rings in the end groups of monomers has been shown to be unfavourable. The presence of intermediate partially ordered PDA chains in solution has been shown to occur for several nBCMU substituted polymers. A simple model consistent with the spectroscopic data is introduced. 

Finally, it was found that assumption (V) was needed to adequately model systems where the substituents are bulkier in one direction than another, i.e. rings. In many of the crystal structures of substituted diacetylenes, this assumption is valid. Certainly it should be for the reactive diacetylenes because if the rings were parallel to the stacking axis, the monomers would be pushed farther apart, leading to large d values. 

The restricted delocalization of the p electron is essentially due to the energy difference of 2e = 0.4 eV per unit cell between the butatriene and acetylene chain structure. The chain length dependencies of the fine structure and hyperfine structure constants of the ESR spectra as well as the chain length dependencies of the absorption series have been described by theoretical model calculations. The complexity of the polymerzation reaction in diacetylene crystals has been demonstrated by the manifold of different reaction products observed in the experiments. The large amount of detailed information concerning the electronic structures and concerning the reaction mechanisms is essentially due to the crystalline structure of this model system and to the thermal stability of the reaction intermediates at low temperatures. 

Under favourable packing conditions, diacetylene single crystals exposed to heat, radiation, or pressure react to form high quality polydiacetylene single crystals. Structural aspects of monomer reactivity are fairly well understood (at least at a qualitative level) via models based on least motion principles. However, there remain many unanswered questions regarding the detailed chemistry and physics of reaction initiation, propagation, and termination. 

A model system which can be used is the diacetylene polymer for which the stress-strain curve was given in Fig. 5.34. By controlling the polymerization conditions it is possible to prepare single crystal fibres which contain both monomer and polymer molecules. The monomer has a modulus of only 9 GN m along the fibre axis compared with 61 GN m" for the polymer and the partly polymerized fibres which contain both monomer and polymer molecules are found to have values of modulus between these two extremes as shown in Fig. 5.36. The variation of the modulus with the proportion of polymer (approximately equal to the conversion) can be predicted by two simple models. The first one due to Reuss assumes that the elements in the structure (i.e. the monomer and polymer molecules) are lined up in series and experience the same stress. 

The approach to a similar level of understanding for polymers has been hindered by the structural complexity of the majority of polymeric materials, A notable exception to this is a subset of the polymers based on disubstituted diacetylenes, which are available in the form of macroscopic single crystals. The fact that these polymers have conjugated backbones adds considerably to their value as model materials, particularly in view of the interest in other less perfect conjugated polymers, such as polyacetylene, which can be doped to obtain high electrical conductivity,... 

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