Trans-Poly butadiene point

The l,4-poly(butadienes) corresponding to the butene-2 compounds, however, differ in their melting points by almost 140°C (Table 1-2). 1,4-c/s-Poly(butadiene) is an elastomer l,4-trans-poly(butadiene) is thermoplastic. The other isomeric poly(butadienes) likewise show a marked difference in their properties. Aromaticized l,2-poly(butadiene), although not an isomer of poly(butadiene), shows semiconductor properties because of its conjugated double bonds. 

The great influence exerted by the structure of macromolecular compounds on properties, in contrast to low-molecular-weight structures, becomes obvious in a comparison of the two 1,2-dimethylethylenes with the l,4-poly(butadienes). The melting point of the trans-butene-2 is about 34°C higher than that of the cis-butene-2, while the boiling points Tbp only differ by about 3°C. While butene-2 shows two isomers, poly-(butadiene) can occur in five isomeric forms. 

It can be seen from the data points for poly (1,4-butadiene) and polyisoprene in Figure 12.9 that the CM of polymers with all-c/.v double bonds in their backbones deviates only slightly (-10%) from Equation 12.27, while the of polymers with ail-trans double bonds shows a very large systematic positive deviation. These results are also consequences of Equation 12.3. The correlation between o2 and CM for silicone-type polymers (see Figure 12.9) is given by... 

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. 

It was pointed out in Chapter 6 that the polymorphic transitions from one crystalline form to another can be induced by the application of an external stress on an axially oriented crystalline system. Anisotropic dimensional changes usually accompany the transformation. Typical examples are the classical a-p transition of the keratins,(21) and the crystal-crystal transition in poly(l,4-trans-butadienes).(22) The dimensional changes in these cases reflect the different axial or fiber repeat distances of the two polymorphs. The dimensional change would be expected to be... 

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