Atactic structures

In this representation the X indicates the substituent other bonds involve only hydrogens. This formalism also applies to 1,1-disubstituted ethylenes in whicli the substituents are different. With these symbols, the isotactic, syndiotactic, and atactic structures shown in Fig. 1.2 are represented by structures [VI]-[VIII], respectively ... 

The 29Si resonance is therefore a single narrow line. However for dialkylpolysilanes with two different alkyl groups on each silicon, (RR Si)n, each silicon atom is a chiral center and the resonance for a particular silicon will depend upon the relative stereochemistry of other nearby silicon atoms. For such polymers, a rather symmetrical cluster of peaks is observed (Figure 5). These results are consistent with atactic structures, having a statistical (Bernoullian) distribution of relative configurations.(32,33)... 

Samples of the poly(dialkylphosphazenes) 1 and 2 displayed X-ray powder diffraction patterns characteristic of crystalline regions in the materials. The peaks in the diffraction pattern of 1 were of lower amplitude and greater angular breadth than those of 2. These data indicate that poly(diethylphosphazene) (2) is highly crystalline while poly(dimethyl-phosphazene) (1) is more amorphous with smaller crystalline zones. This high degree of crystallinity is probably responsible for the insolubility of 2 as noted above. All of the phenyl substituted polymers 3-6 were found to be quite amorphous in the X-ray diffraction studies, a result that is further evidence for an atactic structure of the poly(alkylphenylphosphazenes) 3 and 4 and for a random substitution pattern in the copolymers 5 and 6. 

In an atactic structure the substituent groups are arranged irregularly above and below the plane of the main chain. 

Ziegler-Natta, the oldest process leading to linear and isotactic polypropylene with a low level of atactic structure. 

Stereoselective polymerizations yielding isotactic and syndiotactic polymers are termed isoselective and syndioselective polymerizations, respectively. The polymer structures are termed stereoregular polymers. The terms isotactic and syndiotactic are placed before the name of a polymer to indicate the respective tactic structures, such as isotactic polypro-pene and syndiotactic polypropene. The absence of these terms denotes the atactic structure polypropene means atactic polypropene. The prefixes it- and st- together with the formula of the polymer, have been suggested for the same purpose it-[CH2CH(CH3)] and st-[CH2 CH(CH3)] [IUPAC, 1966],... 

The situation is exactly analogous to the polymerization of monosubstituted alkenes the various polymer structures would be those in Fig. 8-1 with R = — CH=CH2. With chloroprene and isoprene, the possibilities are enlarged since the two double bonds are substituted differently. Polymerizations through the 1,2- and 3,4-double bonds do not yield the same product as they would in 1,3-butadiene polymerization. There are, therefore, a total of six structures possible—corresponding to isotactic, syndiotactic, and atactic structures for both 1,2- and... 

An atactic structure is in both cases not crystallisable. Atactic PP is because of its glass-rubber transition temperature (Tg = -15 °C) rubbery and technically of no use. Isotactic PP is able to crystallise and can, therefore, be used in practice. For PS atacticity is no objection its properties as a glassy polymer are retained up to its Tg (95 °C). 

Atactic PVAc prepared in a free-radical polymerization is crosslinked by means of benzoyl peroxide. The resulting elastomertic networks are studied in elongation, both unswollen and swollen with triethylbenzene, over the range 273 - 363 K. The most important experimental results obtained are values of the network birefringence, which is negative. Calculations carried out to interpret the birefringence are based on Monte-Carlo simulations of the atactic structure, and on the RIS theory. 

Any of the four monomer residues can be arranged in a polymer chain in either head-to-head, head-to-tail, or tail-to-tail configurations. Each of the two head-to-tail vinyl forms can exist as syndiotactic or isotactic structures because of the presence of an asymmetric carbon atom (marked with an asterisk) in the monomer unit. Of course, the random mix of syndiotactic and isotactic, ie, atactic structures also exists. Of these possible structures, only... 

Soluble catalysts based on vanadium compounds polymerize propylene to polypropylene of syndiotactic or atactic structure 73 sc,. Both types of polypropylenes are soluble in hydrocarbons, independent of the molecular weight of the polymers, even at a low temperature of —78 °C. Therefore, the polymerization system remains homogeneous during the polymerization. 

If there is no regular arrangement of vertical lines in the adapted Fischer projection or alkyl substituents in the planar zigzag projection (Figure 3.2), then an atactic structure occurs. Atactic poly(a-olefin)s are characterized by the appearance in their chains of m diads and r diads in equal amounts. These diads constitute heterotactic mr triads and rm triads as well as isotactic mm triads and syndiotactic rr triads which also appear in equal amounts. 

It should also be mentioned that atactic polymers may be formed from monodeuteropropylene and pentadeuteropropylene, similarly to the case of the polymerisation of non-deuterated propylene. However, atactic structures with respect to one stereogenic tertiary carbon atom, with the other possibly arranged to form tactic polymer sequences, are not known. Also, ditactic polymers, which might be characterised by the appearance of two distinct series of tacticity as regards both stereogenic tertiary carbon atoms, i.e. the isotactic with respect to one stereogenic carbon atom and syndiotactic with respect to the other one, and vice versa, are not known. 

Monomers such as E-l,3-pentadiene (piperylene) and E-2-methyl-l,3-pentadiene give 1,4-polymers with an asymmetric carbon atom. Therefore, these monomers in principle will result in polymers with an isotactic, syndio-tactic or atactic structure. 

Side groups in atactic structures and chain branching hinder crystallization of the thermoplastic polymers in contrast to unbranched and isotactic configurations which lead to increased crystallinity. With increasing crystallinity the density, strength and stiffness are increased but the transparency and processability of the plastic decrease. 

Since initial dihydride- and dihydroxyorganosiloxanes, used in dehydrocondensation, represent a mixture of cis- and traws-isomers, synthesized copolymers possess atactic structure. As observed from the data, catalytic dehydrocondensation proceeds at a deeper level with formation of higher molecular products, than in the case of homofunctional products. 

Forasmuch as neither of initial compounds was separated into spatial isomers (though the affinity to enrichment of a mixture of trans-isomers was observed), synthesized polymers possessed atactic structure. 

Because compounds representing the mixture of cis- and trans-isomers were used in polyaddition reactions, carbosiloxane polymers should possess the atactic structure. Such conclusion is also pro-ved by the above-mentioned NMR spectra data. In this connection, the absence of ability to crystal-lize in the majority carbosiloxane polymers is quite explainable. However, the absence of mesomor-phous properties in polymer with decamethylcyclohexasiloxane fragments is the unexpected result. The case is that at the polymerization degree P>5, atactic non-crystallizing cyclolinear polyorganosi-loxanes with... 

Composition and structure of synthesized oligomers is proved on the basis of the ultimate analysis and by IR, H and 29Si NMR spectral data. Spectral data (oligomers No 1-3, Table 2.18) indicate that the presence of cis-trans, trans-trans and trans-cis combinations of rings in the polymer chain is si-milar to polydecaorganocyclohexasiloxanes [26] in the case of trans-dihydroxyorganocyclohexasi-loxane, czs-czs-combination in the polymer chain is absent. They display atactic structure of cycloli-near chain, enriched with trans-trans-sequences. 

A monosubstituted vinyl monomer yields polymers having three regioregular arrangements of configuration (Fig. 5), described by triad stereosequences. The isotactic structure has all R groups on the same side of the backbone (mm) the syndiotactic structure has R groups on alternate sides of the polymer s backbone (rr) and the heterotactic or atactic structure has R groups randomly oriented on either side of the polymer s backbone. 

In specifying the tacticity of the polymer, the prefixes it and st are placed before the name or structure to indicate isotactic and syndiotactic structures, respectively. For example, it-polystyrene means that the polystyrene is isotactic. The absence of these terms denotes that the polymer has an atactic structure. 

Account for the fact that atactic structures commonly predominate in free-radical polymerization in contrast to ionic and Ziegler-Natta polymerizations. 

Transition metal catalysts can polymerize propylene and other higher a-olefins in a stereoregular way to isotactic structures (1), syndiotactic structures (2), or to the statistical atactic structure (3). 

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