Polymer head-to-tail

FIGURE 8.16 The structure of isoprene (2-methyl-l,3-butadiene) and the structure of head-to-tail and tail-to-tail linkages. Isoprene itself can be formed by distillation of natural rubber, a linear head-to-tail polymer of isoprene units. 

Head-to-head PVC prepared by adding chlorine to c/i-polybutadiene has been found to be less stable than ordinary head-to-tail polymer [70]. 

A head-to-tail polymer, in which there is a Cl atom on every other C atom in the chain ... 

Sketch a portion of a head-to-head, tail-to-tail, and a head-to-tail polymer made from acrylonitrile, H2C=CHCN. 

Halides, 562 Hall, Charles, 3,536 Halogen An element of Group 17,31 oxidizing power of 557 oxoacids of, 567t reactivity, 559 Head-to-head polymer, 613 Head-to-tail polymer, 613 Heat A form of energy that flows between two samples because of their difference in temperature, 197,214 Heat capacity The amount of heat required to raise the temperature one degree Celsius, 199... 

Optically active polymers are potentially very useful in areas such as asymmetric catalysis, nonlinear optics, polarized photo and electroluminescence, and enantioselective separation and sensing.26 Transition metal coupling polymerization has also been applied to the synthesis of these polymers.27 For example, from the Ni(II)-catalyzed polymerization, a regioregular head-to-tail polymer 32 was obtained (Scheme 9.17).28 This polymer is optically active because of the optically active chiral side chains. 

CH3)2 C=CH2] can be polymerised by BF3 to yield polymers of molar mass well in excess of one million in just a few seconds. The first step in the reaction is between BF3 and the co-catalyst, usually water. The resulting complex readily protonates an isobutylene molecule to yield the carbenium ion C(CH3)3, which is the species that starts the polymerisation proper. For energetic reasons, the process can only yield head-to-tail polymer. 

Thus Marvel and Levesque found that from 79 to 85 percent of the oxygen was removed in this process, to be compared with the theoretically calculated figure of 81.6 percent (fraction unreacted equal to 1/c) for intramolecular reaction of this type in a head-to-tail polymer. A head-to-head, tail-to-tail arrangement, consisting of 1,4-diketone structures, should be expected to yield furan rings... 

In a polymer made of long chain like molecules, molecular motion is curtailed and rotation even around single bonds is curtailed. As a result, head to tail polymers can have varying orientation of the side groups which can be either orderly or disorderly fashion with respect to the chain. These different... 

When vinyl flouride is polymerized, a certain percentage of the flourine atoms are found on adjacent carbon atoms. When polyvinyl alcohol made by hydrolyzing polyvinyl acetate was treated with periodic acid by Flory, he found that the molecular weight was greatly reduced, indicating that there were 1,2-glycol units in the chain. But most of the polymers were regular head-to-tail polymers. 

Some polymers consisting entirely of head-to-head placements have been deliberately synthesized to determine if significant property differences exist compared to the head-to-tail polymers. The synthetic approach involves an appropriate choice of monomer for the particular H-H polymer. For example, H-H poly(vinyl chloride) is obtained by chlorination of 1,4-poly-l,3-butadiene,... 

As the active metal-carbon bond assumes more covalent character, there will be a greater tendency for hemolytic cleavage and radical-type polymerizations. This becomes favorable when the alkyl is attached to a transition metal in one of its highest valence states or to a non-transition metal of Group IV or V. One can expect such catalysts to initiate polymerizations by both the conventional simple free radical and coordinated radical mechanisms. Stereospecificity generally suffers in these systems because both mechanisms are operative and because radical addition to a double bond is less selective for producing a head-to-tail polymer structure. 

Microtubules are hollow cylinders with an internal diameter of 14 nm and an external diameter of 25 nm. A complete cylinder contains 13 parallel protofilaments each of which is a head-to-tail polymer of heterodimers forming a rod (Fig. 5-34). The heterodimers contain an a- and a (i-tubulin... 

Peter M, Heitlinger E, Haner M, Aebi U, Nigg EA. 1991. Disassembly of in vitro formed lamin head-to-tail polymers by CDC2 kinase. EMBO J 10 1535-1544. 

Even though vinyl chloride was discovered in 1835, polyvinyl chloride was not produced until 1912. It is now one of our most common polymers production in 1984 was over 6 billion pounds. The monomer is made by the pyrolysis of 1,2-dichloroethane, formed by chlorination of ethylene. Free radical polymerization follows Markovnikov s rule to give the head-to-tail polymer with high specificity ... 

The general formula [-CH(C6H5)-CH2-]n commonly used for PS corresponds to head to tail polymer (H-T), which is more common than head to head form (H-H). However, H-H polystyrene is known also, and in commercial products a certain proportion of H-H polymer can be present. Pure H-H polymer can be obtained using specific synthetic procedures [1]. 

Similarly to polystyrene, poly(a-methylstyrene) can be obtained as head to tail polymer (H-T) and also as head to head (H-H), the common material being the H-T form. The pyrolysis products of the two forms show some differences in the dimer area, the H-H form indicating the presence of 2,5-diphenylhexane and 2,5-diphenyl-1-hexene [3]. 

Addition polymerization leads preponderantly to the head-to-tail structure. Marvel (20) and coworkers carried out dechlorination of the polymer in boiling dioxane with zinc dust. In this procedure, chlorine is removed to yield cyclopropane rings and zinc chloride. The dechlorinated material contained 13-16% retained chlorine. Although this favors the head-to-tail arrangement, it does not preclude a certain amount of head-to-head or tail-to-tail product. However, the residual chlorine content corresponds to that expected from the random attack of zinc on a head-to-tail polymer structure. 

Head-to-head (H-H) PVC, prepared by adding chlorine to cis-polybutadiene, appears to be less stable than the ordinary head-to-tail polymer (26). The ordinary polymer itself is said to contain 3-3.5 H-H linkages per 1000 carbons (26), but these results were obtained with an iodometric titration technique (26) that needs to be corroborated. 

Natural rubber is almost exclusively the head-to-tail polymer of isoprene with the H atom and the CH3 group in the cis configuration while gutta-percha, the sap from another type of rubber tree, is the head-to-tail polymer having H and CH3 in the trans position. 

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