Isomers atactic

Huels and Mobil developed technologies473 to manufacture isotactic poly (1-butene), a less important and more expensive polymer by Ziegler-Natta catalysts. The Mobil process474 is carried out in excess 1-butene and produces highly isotactic polymer. The Huels technology475 is a slurry operation and requires removal of the atactic isomer. 

Strictly speaking, the foregoing discussion of segmental correlation has assumed that the polymers are isotactic. When the chemical repeat units along the polymeric chains are not all stereo-identical, and syndiot ctic or even atactic isomers are present, the simple molecular concept of a segmental correlation factor no longer applies. A more elaborate treatment is then required (Volkenstein, 1963). Riande and Saiz (1992) discuss examples in more detail. 

Because of their orderly arrangements, the chains of the tactic polymers (syndiotactic and isotactic) can lie closer together and the polymers are partially crystalline, whereas atactic polymers are amorphous and soft indicating the absence of all crystalline order. Isotactic pol q)ropy-lene is highly crystalline with a melting point of 160°C, whereas the atactic isomer is an amorphous (noncrystalline) soft polymer with a melting... 

Polybutadiene. Unlike cis- and trans-1,4-polybutadiene, high vinyl 1,2-polybutadiene has a chiral center which can exist in one of three different stere-ochemically related forms. The material can either be atactic, leading to an amorphous elastomer, or it can be isotactic or syndiotactic, both of which are crystalline. The elastomeric amorphous form has found utility in tire tread applications (271) and although certain molybdenum (272) coordination catalysts can produce this material, commercialization has focused on anionic alkali metal initiators modified with Lewis bases. Of the two crystalline forms, isotactic 1,2-polybutadiene with a melting temperature of 126° C is the most elusive isomer. A few chromium systems based on soluble salts and aluminum alkyls have been reported to give 45% of the isotactic polymer in a mixture of the atactic isomer (273,274). 

The regularity of the polymer will be shown by the distribution of the D and L configurations. Polymers with highly regular distributions are referred to as tactic isomers, while those with random distribution are said to be atactic isomers. 

Figure 2.3 Sketch of projections onto the plane of atactic, Isotactic and syndiotactic poiypropyiene molecules.The latter two are crystalllzable, whereas the atactic isomer does not crystallize.

For the 1,2- and 3,4-addition, a chiral carbon (marked by an asterisk) is formed which has an R or 3 configuration, but there is no net optical activity, because equal amounts of the R and S configurations are formed. The R and S configurations along the polymer chains lead to diastereomeric isomers called isotactic, syndiotactic, and atactic. In isotactic polyisoprene all monomer units have the same configuration as illustrated for isotactic... 

Of these three PP isomers (called that because they all have the same formula, just different stereoconfigurations), isotactic makes the best plastic. Atactic polypropylene is soft, elastic, and rubbery but not as good as rubber, natural, or synthetic. It is usually separated from the isotactic propylene and discarded as waste, which adds considerable cost to the remaining isotactic. The iso tactic form has a high degree of crystallinity with the chains packed... 

Stereochemistry. The field of organic chemistry devoted, to three-dimensional spatial arrangements of molecules. Deals with stereoisomers, compounds having identical chemical formulas but different spatial arrangement of their atoms, such as geometric (cis/trans) isomers and optical (isotactic, atactic, and syndiotactic) isomers. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

Bis(2-arylindene)zirconium dichlorides have been studied for the purpose of synthesizing isotactic-atactic stereoblock polymers [Busico et al., 2001 Lin et al., 2000 Lin and Way-mouth, 2002 Nele et al., 2000], Without the phenyl substituents, bisindenylzirconium dichloride yields atactic polypropene because there is rapid rotation of the r 5-ligands. The 2-phenyl substituents in bis(2-arylindene)zirconium dichloride interfere with each other suf-ficently that rotation is slowed to produce isotactic-atactic stereoblock polypropene. Three conformational isomers (conformers) are possible in this metallocene (Eq. 8-54). There is... 

Unsymmetrical 1,4-diene monomers such as isoprene and chloroprene can also undergo both 1,2 addition and 3,4 addition, and each of these structural isomers can be atactic, isotactic, or syndiotactic, giving right different (possible) isomeric forms (Figure 1.10). 

Tacticity and geometric isomerism affect the tendency toward crystallization the tendency increases as the tacticity (stereoregularity) is increased and when the geometric isomers are predominantly trans. Thus isotactic PS is crystalline, whereas atactic PS is largely amorphous and c/s-polyisoprene is amorphous, whereas the more easily packed trans isomer is crystalline. 

Scheme 1 Poly(butadiene) isomers (at = atactic, it = isotactic, st = syndiotactic)...

The polyinsertion reaction of conjugated dienes can proceed in three modes which yield three different isomers 1,2-polymers, 3,4-polymers and 1,4-polymers [297]. The situation is more complicated as the 1,4-isomers either exhibit as-1,4 or trans-, 4 configuration. In addition, the 1,2-/3,4-polymers can have an atactic, isotactic or syndiotactic structure (Scheme 1 in Sect. 1.2) [623]. The various moieties are either randomly distributed along the polymer chain or are aligned block wise. 

Unsubstituted polymer chains cannot form different stereo isomers, while substituted polymers can have a large number of different possible isomeric forms. As a result it is possible to have various configurations for substituted polymers. For example polystyrene produced by radical polymerization is atactic which means the phenyl groups bound to every second C-atom are randomly distributed on both sides of the polymer chain. Polymers produced using Ziegler catalysts, made from monomers like styrene, propene and others are isotactic (Figure 2-2) ... 

Poly (methylstyrene), mixed isomers Polystyrene, atactic Poly (formaldehyde)... 

Figure 6.10 Temperature-dependent equilibrium between chiral and achiral isomers of a metallocene catalyst and the resultant polypropylene with isotactic and atactic polymer-blocks.

The fact that the polymers obtained from the cis isomer are optically active also proves that these polymers, notwithstanding that they are amorphous by x-ray, are not atactic. In fact if they were atactic in the proper meaning of the word—that is if the configurations of the asymmetric carbon... 

For the purpose of synthesizing carbosiloxane copolymers with organo-cyclopentasiloxane fragments in the dimethylsiloxane backbone, hydride polyaddition of a,co-dihydridedimethylsiloxanes to l,5-ivinyl-l,5-dimethylhexaphenylcyclopentasiloxane in the presence of platinum hydrochloric acid as a catalyst was studied at temperatures below 100°C at 75°C, 80°C and 85°C. Forasmuch as the initial 1,5-divinyl-1,5-dimethylhexaphenylcyclopentasiloxane represents a mixture of cis- and /ram-isomers (at the ratio copolymers derived from them are atactic. Preliminary heating of initial compounds within the temperature range of 80 - 95°C in the presence of the catalyst indicated that under these conditions organocyclopentasiloxane fragments are not polymerized. 

Forasmuch as 1,7- and 1,5-divinylcyclohexasiloxanes, used in polyaddition, represent mixtures of cis- and tram-isomers of the approximate 52 48 ratio, synthesized copolymers are atactic. Reprecipitation of copolymers from toluene solution by methyl alcohol has given viscous or solid (with regard to the value of flexible junction) transparent products with T sPec=0.09-0.29, well soluble in different organic solvents. It is found that at short length of dimethylsiloxane unit (n < 4), copolymer yields are slightly decreased that may be explained by partial proceeding of hydride polyaddition by intramolecular cyclization mechanism (see Tables 3 and 4). 

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. 

Four cyclolinear PBPCS-6 polymers of different structures were synthesized from pure cis- and trans-isomers of dichloro- and dihydroxyderivatives of diphenyloctamethyl-cyclohexasiloxanes the first one is enriched with tt-sequences in the absence of cc-combinations (trans-tactic polymer) the second one is enriched with cosequences in the absence of tt ones (c/.v-tactic polymer) the third one is mostly enriched with tosequences from co-dichloro derivative and /ra ,v-diol (due to partial inversion of dichloride, tt-sequences are present that is why spatial structure of the third polymer is similar to the first one) the fourth one represents atactic polymer. 

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