Head-to-tail structure

A-Substituted polypyrazoles can also be obtained by using A-alkylhydrazines, and it should be noted that these polymers consist of a random mixture of head-to-head and head-to-tail structures. Other syntheses of polypyrazoles have been described in the literature. Thus polyphenylene pyrazoles (742) and (743) occurred when m- or p-diethynyl-benzene (DEB) reacted with 1,3-dipoles such as sydnones or bis(nitrilimines) (Scheme 64). 

It is an observed fact that with most synthetic polymers the head-to-tail structure is formed. In the case of diene polymers differences may arise in the point of addition. Reaction can take place at the 1 and 4 positions, the 1 and 2 positions or the 3 and 4 positions to give the structures indicated in Figure 4.9. 

It was found that the amount of chlorine that could be removed (84-87%) was in close agreement to that predicted by Flory on statistical grounds for structure Figure 12.10(a). It is of interest to note that similar statistical calculations are of relevance in the cyclisation of natural rubber and in the formation of the poly(vinyl acetals) and ketals from poly(vinyl alcohol). Since the classical work of Marvel it has been shown by diverse techniques that head-to-tail structures are almost invariably formed in addition polymerisations. 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

Thermogravimetric analysis and other studies made on low-molecular weight model compounds such as 1,3, 5,-trichlorohexane [7,8] corresponding to the idealized head-to-tail structure of PVC show these structures to be considerably more stable than the polymer. This abnormal instability of the polymer is attributed to structural irregularities or defects in the polymer chain, which serve as initiation sites for degradation. 

Since that time, many studies by NMR and other techniques on the microstructure of acrylic and methacrylic polymers formed by radical polymerization have proved their predominant head-to-tail structure. 

Introduction of bulky lateral substituents on monomer units to increase interchain distance and prevent close packing in polymer crystal. The use of unsymmetrically substituted monomers, resulting in a random distribution of head-to-head and head-to-tail structures in polymer chains, further helps in disrupting regularity. Some examples of substituent effects are given in Table 2.16. 

PVC has been shown to have a head-to-tail structure. Typical experimental evidence for this is that when dissolved in dioxan and treated with zinc dust, it undergoes a Wurtz-type reaction to yield a product containing a small amount of chlorine and no detectable unsaturation. The alternative possible structure, the head-to-head arrangement, would yield unsaturated sites where adjacent chlorine atoms had been removed (Reaction 1.4). 

If the head-to-tail structure is assumed for the polymer of methyl vinyl ketone... 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

Direct chemical evidence for the head-to-tail structure was furnished in 1959 by Applequist et al. using a stereospecific elimination/21 ... 

All of the mejo-substituted photodimers have been found to have the head-to-tail structure. 

When poly (vinyl chloride) in dioxane solution is made to treat with zinc dust the resulting polymer gets saturated and is having a small chlorine content. This reveals that it is having a head-to-tail structure. 

Polyisobutene has a regular head-to-tail structure, and the crowding of the methyl groups is so great that the molecule can only be built from conventional atomic models with great... 

It was the latter that was produced as shown by the fact that only about 86% of the chlorine was removed by treating the polymer with copper. Later it was shown that this treatment produced cyclopropane units in the chain so that the question of head-to-tail structure was definitely established. A few polymers do have structures containing both head-to-tail and head-to-head, tail-to-tail structures. 

The complicated pattern for the methylene carbon of the polymers indicates the presence of an irregular structure of head to head and tail to tail linkages. On the other hand, the uniformly head to tail structure of polyepichlorohydrin elastomers made by the coordination catalyst shows a doublet for the methylene carbon at 70.2 and 70.0 ppm (21). No peak corresponding to the terminal methine or chloromethyl carbons is detected in elastomers. 

The polymer has a predominantely head-to-tail structure with propagation occurring almost exclusively by attack at the P-carbon—the less sterically hindered site (Eq. 7-11), that is, an Sn2 attack [Kasperczyk and Jedlinski, 1986 Oguni et al., 1973 Price and Osgan, 1956 Quirk, 2002], Propylene oxide and other substituted epoxides polymerize more slowly than does ethylene oxide because of steric hindrance. 

Most cationic ring-opening polymerizations are highly regioselective with the formation of head-to-tail structures, although varying amounts of head-to-head and tail-to-tail structures are found in some systems [Dreyfuss and Dreyfuss, 1987]. 

In general, the head-to-tail structure is the by far most predominant motif. The proportion of head-to-head structure is small and can only be determined experimentally in some specific cases. Further types of structural isomerism are found in polymeric conjugated dienes addition of a monomer to the chain end can occur in 1,2- and in 1,4-position. Moreover, in the case of nonsymmetric dienes, 3,4-addition is a further possibility ... 

An earlier paper on the thermal cyclodimerization of 3 (R1 = F R2 = Ph) also reported the formation of a cis/trans mixture which was separated by gas chromatography.3 This structure contradicted an earlier assignment of the head-to-tail structure for this dimer.4... 

It is interesting to note that the yield of 39 can be increased to 95 % in the presence of a Ni(0) catalyst (see Section I.3.I.2.).43 In unsymmetrically substituted buta-l,2,3-trienes, head-to-head dimerization takes place as well, as seen in the examples below.44 The intermediate cyclopropy-lidenecumulenes 41 are formed from vinylidene insertions into alkenes and reactions occur at the terminal unsaturated site of the cyclopropylidene group. Structures 42 were confirmed by X-ray crystallographic analysis and revised the original assignment of the head-to-tail structures for these derivatives.45... 

Irradiation of 4-stilbazole (25) in aqueous HC1 gave predominantly the syn head-to-tail dimer 21 and minor amounts of dimer 22. The syn head-to-tail structure was also assumed for the solid state photodimer of 4-stilbazole methiodide 26 (58). 

Finally, Wenger feels that the dimer formed on addition of styrene to styrene- radical-ion has a head-to-tail structure, and that this structure characterizes the corresponding di-anion, S. S-. This is again erroneous. Such dimers were produced, converted into corresponding dicarboxylic acid and proved to have the structure HOOC. CH(Ph). CH2. CH2. CH(Ph). COOH. (See also refs. 21 and 25.)... 

Styrene and ring-substituted styrenes also form copolymers with carbon monoxide, similarly to ethylene and a-olefins. These styrene/carbon monoxide copolymers are of alternating, highly regioregular head-to-tail structure and are characterised by different stereoregularity (syndiotactic, isotactic) [115-117]. 

In this simplified scheme (the participation of the adjacent zinc atom as the nucleophilic attack carrier is omitted), the formation of poly(propylene ether-carbonate) characterised by a regioregular head-to-tail structure is shown. This is possible because of the nucleophilic attack on the Cp atom of the neighbouring ring to that coordinating with its more basic oxygen atom to the metal atom. Actually, the predominant head-to-tail structure of the poly(propylene ether carbonate) obtained at 80 °C with the diethylzinc-pyrogallol-water (3 1 0.5) catalyst has been proved [146]. 

The head-to-tail structure of poly (vinyl chloride) permits the continuous regeneration of an allylic chloride moiety as hydrogen chloride elimination proceeds along a chain. Thus, once initiated, loss of hydrogen chloride may proceed along a polymer chain without abatement. 

Dechlorination of head-to-head, tail-to-tail structure can be expected to go to 100% completion. If dechlorination of head-to-tail structure starts at random positions, then 13.5% of the chlorine should remain at the end of reaction. Dilute solutions of PVC treated with zinc removes 87% of the chlorine, proving the head-to-tail structure of PVC (71). 

The above conclusions were indirectly corroborated by NMR spectra of isotactic, Ziegler type poly(3-methylbutene-l) having a bonafide conventional 1,2 head-to-tail structure (167) shown in Fig. 4. The NMR spectra of the cationic and isotactic polymers are completely different (160). 

Isotactic polypropylene is a stereoregular polymer with all methyl groups pointing in the same direction when the backbone is stretched. In syndiotactic polypropylene the methyl groups alternate along the chain. The atactic polymer lacks regularity. All three forms retain a perfect head-to-tail structure. 

Functional groups which can be formally reduced by litanium(II) to the corresponding dianion will produce melallacycles of the head-to-tail structure or products whose genesis is explained by a precursor of this structure. An example is given in Equation (64) and shows the reaction of Cp Ti(CO)2 and diphcnylkciene. 

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