Topochemical polymerization calculation

Wegner, however, established that radiation-induced solid-state polymerization of BCMO leads to a polymer morphology, which is incompatible with the so-called topochemical polymerization, i.e., a process in which monomer molecules are transformed into polymer without destruction of the crystal lattice 36). Electron microscopy, X-ray analysis and electron diffraction studies, have shown that polymerization starts at the edges and imperfections of the monomer crystals and that amorphous polymer is formed initially. Further transition from the amorphous state leads to the thermodynamically unstable monoclinic p-form. Density measurements indicate that the polymer is only 45-50% crystalline. The density of the amorphous poly-BCMO is 1.368 g/cm3 the density calculated for the crystalline polymer from crystallographic data of the p-form is 1.456 g/cm3. The density of the product of the radiation-induced solid-state polymerization is 1.41 g/cm3 36). 

The spontaneous topochemical polymerization of (SN)2 to (SN), at 0°C , the most prominent example of a polymerization reaction in S—N chemistry, has been reviewed. Crystals of (SN)j belong to the space group PZ /c and it has been proposed that the a axis of the dimer converts into the b axis of the polymer with chain extension occurring parallel to this axis . On the basis of EHMO calculations, this process is both thermally and photochemically allowed , and the photoinduced polymerization occurs at — 65°C in THF . ESR studies indicate a radical process, but ab initio MO and configuration interaction studies suggest that the polymerization involves a cascade effect initiated by only a few radical centers . 

A theoretical model for predicting which diacetylene monomers will undergo topochemical polymerization has been developed. The semiempirical SCF-MO methods NfrroO, AMI, and PM3 have been used to carry out calculations based on this model. These methods were used to minimize the monomer geometries and the intermolecular distances and angles (R and a) between the monomer pairs. They have been applied to reactive and unreactive derivatives as well as some unknown derivatives. The results from these calculations suggest that this method may be applicable to a large variety of substituted diacetylene monomers. Specific examples of cases which show both the utility and the limitations of the model are discussed. 

The most critical structural parameter shown in Figure 1 is the transitional repeat distance d of the diacetylene. The ideal value for d is about 0.49 nm. This is a necessary structural parameter for a topochemically controlled polymerization. If this structural condition is achieved and the diacetylene functionalities close pack then a simple calculation demonstrates that the angle will be about 45° and the 1-4 carbons of the diyne will be in close contact. 

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