Isomers isotactic

More recent catalysts for syndiotactic polypropylene are complexes, like -propyl(cyclopen-tadienyl-l-fluorenyl)haihium dichloride with methyl aluminoxane [70]. Another, similar catalyst is i-propyl(ri -cyclopentadienyl-r -fluorenyl)zirconium dichloride with methyl aluminoxane. These catalysts yield polymers that are high in syndiotactic material (the zirconium-based compound yields 86% of racemic pentads) [70, 71]. Commercial production of syndiotactic polypropylene is in the early stages. What catalytic system is used, however, is not disclosed at this time. Some of the properties of the two isomers, isotactic and syndiotactic polypropylenes, are compared in Table 6.5. 

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... 

There are two formulae for butene according to the position of the double bond (see Figure 4.15). Only the butene-1 isomer is used to polymerize an engineering material in its 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. 

An even simpler spectrum (a single resonance for sterically pure samples) was obtained from polymers of the two [(Z) and ( )] isomers of propylene-1,2,3,3,3-[Pg.35]

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... 

Enantiomers, nonsuperimposable mirror-image isomers, could also be designated as dd or // instead of it dldl instead of st and, for example, dllddl-dlldld instead of at. The isotacticity of commercially produced PP may be increased by the hexane extraction of solvent extraction step. 

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. 

The general case of nonzero k o can be approximately treated by averaging M over the triad population. The probability M for the isotactic, syndiotactic, and heterotactic triads is taken equal to that for a polymer which is purely isotactic, purely syndiotactic, and alternately iso- and syndiotactic, respectively. It can be shown that M is given exactly for a purely isotactic polymer by Eq. (3), provided that kiio = k iD, me o a°d that kJjM = 2. A purely syndiotactic polymer can be treated in a similar way by utilizing rates from the dl isomer. A strictly alternating syndiotactic-isotactic polymer has the following exact expression for M ... 

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. 

Dihydrofurans are known to polymerize readily by cationic means but not by free radical initiation (77MI11102). 2,3-Dihydro-5-methylfuran (40), for example, has been polymerized to a stereoregular, optically active polymer (Scheme 9) by the appropriate choice of a catalyst system. Interestingly, the double bond isomer 2-methylenetetrahydrofuran (41) can be cationically polymerized (74CL499) to the isomeric polymer (42 Scheme 10). Under the polymerization conditions, no isomerization of (41) to (40) occurred, and the resultant polymer was shown to be isotactic and crystalline by NMR and X-ray analysis, respectively. 

Obviously, the polymerization of ethylene does not involve isotactic steric control, since no isomers are possible in the linear polyethylene polymer. However, as the simpliest member of the vinyl monomer series, ethylene should be considered along with the substituted ethylenes. Where does the Ziegler catalyst, which produces polyethylene, lie in relation to the polypropylene catalysts ... 

Natta (67) has studied the effect of two isomeric forms of titanium trichloride on the polymerization of dienes and of isotactic polypropylene. He has found that the alpha isomer, which gave the higher isotactic polypropylene, gave greater amounts of trans 1.4-polydiene than the beta, which gave cis 1.4-diene. 

Since isomerically pure polymers were not available, three different kinds of BR, each relatively high in one of the three kinds of base units were used as standards [35]. The band near 1308 cm 1 was identified [38,39] with the cis isomer and used for analyses [43]. The 1308 cm 1 band is weak and relatively broad, with the appearance of an unresolved doublet (1306,1311 cm 1). The cis band at 730 cm 1 is more frequently used in spite of some difficulties. Relatively pure, crystalline stereoregular polymers have been prepared and structures were determined by X-ray diffraction for cis [44], trans [45] and syndiotactic vinyl [46] and isotactic vinyl [47]. Infrared spectra [48-50] have been published for the four stereoregular polybutadienes, with detailed analyses of the spectra and band assignments for cis [51], trans [51] and syndiotactic vinyl [51] polymers. For the spectrum of isotactic vinyl BR, bands at 1232, 1225, 1109, 943, 876, 807 and 695 cm"1... 

The determination of percentage of styrene and butadiene isomer distribution in copolymers is an extension of the methods for the analysis of polybutadiene. The styrene band at 700 cm 1 is largely independent of the sequence distribution and therefore useful in styrene content determination [76]. A series of bands in the IR spectrum of crystalline isotactic polystyrene at 758, 783, 898, 920, 1053, 1084, 1194, 1261, 1297, 1312 cm"1 have been attributed to the helical structure [77]. The absorption bands for butadiene in SBR are similar to BR structures (Table 3.2a). 

Both isomers are polymerised by some coordination catalysts, whereas with others only the (E) isomer is polymerised. Of the various possible stereoregular poly(l,3-pentadiene)s, only five have been prepared so far cis- 1,4-isotactic [11,12], trans- 1,4-isotactic [13], cA-l,4-syndiotactic [14,15], 1,2-cA-syndiotactic [16] and 1,2-traro-syndiotactic [17] polymers. In addition to these stereoregular polymers, amorphous polymers of various types, consisting of different monomeric units in variable proportions, have been obtained [7]. 

However, cz s-isotactic and cz s-syndiotactic polymers have been obtained predominantly from common cycloolefins such as cyclopentene the 1,3-insertion isomerisation-polymerisation of cycloalkenes such as cyclopentene farely yields polymers with a trans structure (usually in an amount not exceeding 3%) [15,19,20], Figure 6.2 shows both cw-isomers of poly(l,3-cyclopentylene). 

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

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