Polymers Large molecules isotactic

Various published reports show a relatively large variability in the yield of the fragment molecules [102], The relative amount of fragment molecules depends on the polymer stereoreguiarity and on the experimental conditions of pyrolysis. The yield of fragments with different numbers of atoms as a function of equilibrium temperature (T q) is shown for a commercial polypropylene 85% isotactic in Figure 6.1.15 [82]. 

In vinyl polymers (Structure III, Chapter 1) the molecules in the aystal form helices. The way in which this is adiieved by rotation from the planar zigzag is illustrated for isotactic pofypropylene in Figure 23. The interference between the large CHj groups is relieved by a rotation of 120° in... 

Chlorination of poly(vinyl chloride) destroys the regularity of the tacticity. Dor-restijn et al. [123] executed chlorination in two different ways. First, pol>tvinyl chloride) was chlorinated in solution so that no preference existed for the kind of hydrogen atom which was replaced by chlorine atoms the tacticity of the polymer molecules is destroyed more or less completely. Second, the polymer was chlorinated in powder form in this case there is a large preference for the amorphous parts, whereas the crystalline parts are mudi less available for chlorination and remain more or less intact Even if the polymer is attacked prrferentially at the isotactic sequences, the tendency to form gels will decrease, as the [nresence of an even number of isotactic sequences in between syndiotactic sequences causes an enormous increase in the crystallinity of polyfviny chloride) [82,84]. Dorrestijn et al. showed that chlorination had a dramatic influence on the tendency to form gels in the case of solution chlorination, whereas powder chlorination had only a minor effect despite much higher chlorine contents. 

Due to the large size of the calixarene molecules, they were also used for the synthesis of stereoregular polymers. Thus, neodymium complex containing the calixarene 32 was active in the formation of isotactic polypropylene oxide (Scheme 4.20) [80]. 

Finally, it may be noted that some polymers have been obtained in which optical activity is ascribed mainly to conformational asymmetry. In these cases there is a predominance of either right-handed or left-handed enantiomorphs of helical polymer molecules, in contrast to the more usual situation wherein equal amounts of the two enantiomorphs are produced and there is no resultant optical activity. Optically active polymers of this type have been obtained from a-olefins possessing optically active side chains, e.g., 3-methylpent-l-ene, 4-methylhex-l-ene and 5-methylhept-l-ene. Isotactic polymers from these monomers have greatly enhanced optical activity compared to the monomer. Since these polymers are vinyl polymers this optical activity cannot be associated with the asymmetry of the carbon atom in the polymer backbone (for the reasons given above). Thus it is supposed that the presence of optically active side groups favours a particular screw sense of the helix so that the resultant polymer shows a large optical rotation. Optical activity of this type has not been observed when the side groups are not asymmetric. 

When the polymerization of EMA was carried out in toluene with 1,1-diphenylhexyllithium as initiator at -78 C, a large amount of methanol-soluble, isotactic polymer formed with a very small amount of methanol-insoluble fraction. The latter fraction was also isotactic in this case. The results indicate the possibility that in the polymerization of EMA in toluene with BuLi the propagating species for syndiotactic polymer exist in the form of ion pairs complexed with one or more molecules of lithium ethoxide, and the isotactic active species are less connected with the ethoxide or free from it. At higher polymerization temperatures the coordination of ethoxide weakens and the rate of propagation at the chain ends becomes very rapid, which results in a decrease in the formation of methanol-insoluble, syndiotactic fraction. 

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