Topotactic reactions, polymerization

Keywords Crystal engineering Solid-state photoreaction Topochemical polymerization Controlled radical polymerization Dimerization Isomerization Topotactic reaction... 

Several reviews of early work on topotactic polymerizations and isomeriza-tions are available, and the reader is referred to the summaries of Morawetz [88] and Gougoutas [8] for a more complete account. The earliest study of a topotactic reaction appears to have been the observation, in 1932, of the polymerization of trioxane to poly-oxy-methylene [89]. Similar polymerizations of tetraoxane [90] and of trithiane [91 ] have also been reported to show retention of crystallographic axes from the monomer lattice. Other examples are discussed below. The topo-tacticity of a reaction can be determined solely by x-ray crystallographic analysis at the reactant and product endpoints. Thus a simple classification of a reaction as topotactic tells very little about how the structure of the crystal lattice changed in the course of reaction. 

The discussion above left aside an entire class of CPs, the polydiacetylenes (PDAs) (see Table 1). Their study has followed a path similar to that of other CPs, but with only occasional intersections. Indeed, they differ greatly from other CPs in many respects. PDAs have a feature that is almost unique in the entire polymer field, and not only among CPs single crystals of macroscopic size can be obtained. They owe that to their peculiar polymerization mechanism, which is a topotactic reaction occurring in the mon-... 

Reactivity in the solid-state is always connected with specific motions which allow the necessary contact between the reacting groups. In most cases solid-state reactions proceed by diffusion of reactions to centers of reactivity or by nucleation of the product phase at certain centers of disorder. This leads to the total destruction of the parent lattice. If the product is able to crystallize it is highly probable that nucleation of the crystalline product phase at the surface of the parent lattice will lead to oriented growth under the influence of surface tension. In such topotactic reactions certain crystallographic directions of parent and daughter phases will coincide. Typical examples for this behaviour are the solid-state polymerizations of oxacyclic compounds such as trioxane, tetroxane or 3-propiolactone... 

Crystalline trithiane, the sulfur containing analog of trioxane polymerizes in a topotactic reaction after irradiation on subsequent heating to 180° C 93). Again the crystal structure is twinned and differential thermal analysis has shown a higher melting point for polymers produced by solid state polymerization than for solution polymerized trithiane... 

These characteristics can be derived from a polymerization of crystalline monomers controlled topochemically. Topotactic reaction geno-ally involve a strong correspondence between the lattices of the monomer crystal and the resulting polymer crystal. Hence, the topochemical solid state reaction occurs when sufficiently intense thermal mobility of molecules takes place in the lattice, and the distance between active centers for the polymerization in the neighboring molecules should not exceed 3.7 A for C — O interactions All monomers mentioned above show quite close C — O interactions. 

Solid-state chemistry is less widely used than solution chemistry but the particular molecular configuration and interaction imposed by the crystal lattice leads to a well defined reaction product. The very specific nature of the lattice packing required for reaction, however, makes the solid-state route less predictable than solution chemistry. Solid-state polymerization has been observed to occur by bulk, topotactic and topochemical reactions. In bulk reactions the product is amorphous because the crystal lattice is destroyed during the reaction. Topotactic reactions produce a product phase which has a small number of orientations, possibly only one, with respect to the initial crystal. Topochemical reactions are those in which the reaction path is determined by the crystal structure and may lead to a topotactic product. 

Convincing evidence for phase separation was obtained from the photopolymerization behavior of 6 in the mixed 6/DSPE monolayer films. Photopolymerization of diacetylenes is a topotactic process which requires the proper alignment of the 1,3-diyne moieties [35]. Thus diacetylenes typically polymerize rapidly in the solid state but not in solution. Polymerization is triggered by ultraviolet irradiation and proceeds via a 1,4-addition mechanism yielding a conjugated ene-yne backbone (Fig. 5). The reaction can be followed by the growth of the visible absorption band of the polymer. 

A topotactic solid state polymerization reaction has been reported for the Ni complex with tris(2-cyanoethyl)phosphine, P(CH2CH2CN)3 the square planar NiBr2 P(CH2CH2CN)3 2 com-... 

The range of potential sensitizers for photopolymerization of diacetylenes widened considerably when Wegner and coworkers discovered sensitized photopolymerization in LB-multilayer systems doped with certain dyes. It is known for long that dye molecules carrying long aliphatic substituents can easily be incorporated into LB-multilayer assemblies without destroying the sample architecture Therefore, the topotactic requirements for the polymerization reaction to proceed are retained. 

In summary, crystallization during ionic and coordination polymerization may give rise to a large variety of morphologies. Some crystalline cyclic monomers seem to be capable of topotactic polymerization reactions which must be simultaneous polymerization and crystallization. A direct influence of nucleation and ordered structure on the polymerization rate was observed for helical polymers in solution giving an... 

Free radical polymerization from crystalline monomers is even more difficult. Besides the general conditions for a topotactic polymerization to provide the proper geometry for monomer approach for reaction and... 

The final group of solid state monomers which have been polymerized are ring type monomers. There polymerizations showns two favorable factors. On ring opening and polymerization the number of covalent bonds stays constant and the heat of reaction and the voliune change are smaller. It seems that in this group of reactions the only topotactic polymerization reactions have been discovered. 

Another claim of a thermal and photochemical homogeneous (or topotactic) polymerization of the diacetylene 30 a (Scheme 2.1.9) is questionable. The claim had to be modified several times, it was admitted later that the polymorph required crystal solvent (e.g. 0.5 dioxane) [57], and 30 a was no longer discussed in the review of the same research group [58] that dealt with 30 b and listed 32 further diacetylenes. First order reaction was claimed in order to try to substantiate topochem-ical behavior [58]. However, inspection of the published kinetic curves indicates zero order up to 90 % conversion after an induction period instead of first order for... 

The reversibility of these reactions suggests that they are topotactic or intercalation reactions. We believe that these reactions represent oxidation of the polymeric [Ni(TTL)]a, or [Co2(CO)2,TTL]a chains... 

Scheme 7.8 CuAAC reaction of preassembled organogelators - a case of pseudo-topotactic polymerization.

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