Diacetylene solid-state polymerization

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

An important example of the solid state polymerization of disubstituted diacetylenes is 1,6-bis(2,4-dinitro-phenoxy)-2,4-hexadiyne (DNP) shown in Figure 9 - DNP is unique among known diacetylenes... 

Figure 8. Solid-state polymerization of diacetylenes. A crystalline array of monomer units polymerizes through intermediate states to the final crystalline polymer chain.

The solid-state polymerization of diacetylenes is an example of a lattice-controlled solid-state reaction. Polydiacetylenes are synthesized via a 1,4-addition reaction of monomer crystals of the form R-C=C-CeC-R. The polymer backbone has a planar, fully conjugated structure. The electronic structure is essentially one dimensional with a lowest-energy optical transition of typically 16 000 cm-l. The polydiacetylenes are unique among organic polymers in that they may be obtained as large-dimension single crystals. 

The second reaction we will treat here that in many cases proceeds by a homogeneous mechanism is the solid-state polymerization of diacetylenes. The photolability of these compounds has long been known (189), but it was not until the appearance of the pioneering works of Hirshfeld and Schmidt (168) and of Wegner (190) that some real understanding of the process was introduced. This led to an explosive interest in the subject, and more than 200 diacetylene derivatives have now been studied, by a wide variety of techniques (191). 

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... 

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

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... 

Poly diacetylenes. The polydiacetylenes (PDA s) are unique among highly conducting polymers discovered in the past years in that they can be obtained as highly perfect macroscopic single crystals.68 Upon solid-state polymerization of... 

X-Ray structures of bis(thiophenes) and bis[benzo(7)thiophenes] have been determined. In compounds 1,4-bis-(2-thienyl)butadiyne 34a, both the thiophene rings are disordered. Thus the position of sulfur and carbon atoms in thiophene rings, adjacent to the diacetylenic backbone, are interchangeable but with unequal occupations. The thiophene rings are planar and the dihedral angle between them is 65.6°. The diacetylene chains are inclined to the shortest axis, that is, the -axis, by 40.5°, and the perpendicular distance between the adjacent chains is 3.823 A, as against the respective values of 45° and 3.4 < Aj < 4.0 A, required for solid-state polymerization. 

Fibers produced in this manner have some shortcomings. For example, these fibers are of very short length, no more than 5-10 cm long, and are produced rather slowly. One should also add that the only single crystal fiber produced is based on solid state polymerization of certain substituted diacetylenes and the technique does not seem to be applicable to other systrans. 

One approach to the determination of intrinsic properties, which has been utilized since the earliest interest in conjugated polymers, is to study the properties of related oligomers, as in the preceding paper ( ). It is, however, also possible to study model macromolecules, the polydiacetylene. The existence of solid state polymerization in diacetylene monomers has a long history (22, 23, 2 ), but it was not thoroughly studied until... 

Figure 1. Solid-state polymerization of diacetylenes shown schematically (left) an array of monomer molecules in the crystal lattice frightj the resulting polydiacetylene chain.

This contribution gives a review of recent spectroscopic investigations concerning the photophysical and photochemical primary and secondary processes of the solid state polymerization reaction in diacetylene single crystals. It will be shown, that diacetylenes are an unique model system for the study of the reaction mechanism of a solid state chemical reaction which is characterized by a variety of reaction intermediates. The polymerization reaction in these crystals is of special importance, due to the resulting polymer single crystals, which exhibit extraordinary anisotropic physical properties. 

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