Trans conformation poly dienes

All the possible line repetition groups for cis and trans poly dienes compatible with the isotactic or syndiotactic configurations are reported in Figure 2.15,47,68 In order to consider only the possible conformations assumed in the crystalline state, the torsion angle of the central single bond is assumed to be 180° trans) in both the cis and trans polydienes. This condition produces conformations sufficiently extended to be packed in a crystalline lattice for each value of the torsion angles 0i and 02 (Figure 2.15). 

In the preceding section the behavior of the catalyst at Al/ Ti 1.0 was examined. Next, lower ratios will be discussed, but first it is instructive to include some description of a-TiCla, another crystalline modification of the trichloride (Natta et al., 1961a). In combination with trialkylaluminum or dialkylalumi-num chloride the a form produces trans-l,4-poly dienes with butadiene or isoprene (Natta et al., 1959b). The reason for the difference in behavior between the /3 and a modifications has not definitely been established, but it is thought to be related to the different Ti-Ti ionic distances (Saltman, 1963). In /3-Ti-CI3 this is 2.9 A, about the same as the 1-4 carbon-carbon distance for isoprene in the cis conformation. The a-TiCls has a Ti-Ti distance of 3.54 A, more in line with the 1-4 carbon-carbon distance for isoprene in the tram conformation (3.7 A). Perhaps these atomic distances are fortuitously similar, but if one assumes two-point coordination of monomer on the surface the difference between the allotropic forms can be explained. 

In the case of crystals, both intramolecular (conformational) and packing energies should be taken into account simultaneously. Such a total energy minimization method, with suitable crystallographic constraints, has been applied in different steps of the analysis of crystalline structures of three different synthetic polymers. Structures of these molecules, namely, isotactic trans-1,4-poly-penta-1,3-diene (ITPP), poly-pivalolactone (PPVL), and isotactic cis-1,4-poly(2-methyl-penta-1,3-diene)(PMPD), do not have troublesome features such as charged groups, counterions, and solvent molecules. 

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