Syndiotactic arrangement

In a syndiotactic arrangement, the substituents are in an ordered alternating sequence, appearing alternately on one side and then on the other side of the chain, thus... 

What is it about the monomer that promotes a syndiotactic arrangement of the pendant styrene group Why is the polymer still considered atactic if the syndiotactic addition is more energetically favorable ... 

In a syndiotactic arrangement, the substituents are in an ordered alternating sequence, appearing alternately on one side and then on the other side of the chain, as shown for a segment of a vinyl chloride polymer. If the sidechains are not in any particular order with respect to each other (random), the polymer is said to be atactic. 

The new ternary compound SrMgSi2 shows a considerable disproportionation of the silicon atoms into terminal (lb)Si , bridging (2b)Si, and branching (3b)Si" species (Fig. 2b, [31]). As shown in figure 2b the whole polyanion contains a one dimensional planar chain of silicon atoms with syndiotactically arranged side chains [Si(Si3)] . The Si-Si distances vary somewhat between 235 and 249 pm, with the shortest distances between the side chain bridging atoms and those of the main backbone. 

When the propylene (monomer) polymerises, the pendent CH can lie in either direction.The randomly disposed CHj group (atactic form) does not form good fibre. In the isotactic form the methyl groups are on the same side of the polymer backbone, but in syndiotactic arrangement the methyl groups lay attemately on either side of the polymer chain (Fig. 1-30). To produce this form of polypropylene, special catalysts are used. 

The determination of polyethylene crystallinity includes all polyethylene present in the copolymer, whereas measurements on the regularity bands of C3 give only the number of units either isotactically or syndiotactically arranged. Amorphous polypropylene, on the other hand, cannot be distinguished from the C3 portion of the copolymer by IR spectroscopy alone and is included in the total C3 percentage11. 

Clearly, the 3-dimensional periodicity of a crystal could not be achieved without some kind of regularity in the tacticity. Consequently, only the isotactic and syndiotactic arrangements can form crystals, whilst the atactic arrangement will make amorphous materials. 

The rate of propagation and consequent polyene sequence length are also enhanced by syndiotactic arrangements, since polymers with high syndiotactic content produce abnormally long polyene sequences [1484]. 

In Figure 19.3, there are two aliphatic proton peaks at 1.32 and 1.85ppm and two sharp aromatic proton peaks at 6.68 and 7.15ppm present in all compositions, indicating a high syndiotactic arrangement [1] of styrene units in all... 

There are two sets of major peaks at 1.45 and 2.10 ppm, corresponding to the CH2 and CH protons in the SPS backbone, and at 6.82 and 7.21 ppm, corresponding to the protons of the phenyl groups that have a syndiotactic arrangement. In addition, two minor peaks at 0.15 and 3.81 ppm correspond to the protons of the -0-Si(CFl3)3 and -CH(< ))-OSi(CFl3)3 groups, respectively. The combination of peak intensity ratio between silane and phenyl... 

The single glucose molecules are rotated at 180° (syndiotactic arrangement) (Fig. 2.25) within the polysaccharide. Multiple hydrogen bridge bonds act between the polymer chains. Because of this intense sterical gearing, the final cellulosic fibers lack plasticity. Therefore, cellulosic fibers cannot be texturized and are difficult to iron. 

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