Polypropene, atactic isotactic

It should be noted that other polymer structures can be postulated—those where one substituent is atactic while the other is either isotactic or syndiotactic or those where one substituent is isotactic while the other is syndiotactic. However, these possibilities are rarely observed since the factors that lead to ordering or disordering of one substituent during polymerization generally have the same effect on the other substituent. An exception is the formation of hemiisotactic polypropene where isotactic placements alternate with atactic placements [Coates, 2000]. 

In the mid-1980s the first metallocene/MAO catalysts for the isotactic polymerization of propene were described. Ewen found Cp2TiPh2/MAO to produces isotactic polypropene at low temperatures by chain end control mechanism (stereoblock structure). When using a mixture of racemic and meso [En(Ind)2] TiCl2 in combination with MAO, he obtained a mixture of isotactic and atactic polypropene, the isotactic polymer having a microstructure in accordance with... 

Giller, C., Gururajan, G., Wei, J., et al., 2011. Synthesis, characterization, and electrospinning of architecturally-discrete isotactic-atactic-isotactic triblock stoeoblock polypropene elastomers. Macromolecules 44 (3), 471 82. 

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 polymer chain end control model is supported by the observation that highly syndiotactic polypropene is obtained only at low temperatures (about —78°C). Syndiotacticity is significantly decreased by raising the temperature to —40°C [Boor, 1979]. The polymer is atactic when polymerization is carried out above 0°C. 13C NMR analysis of the stereoerrors and stereochemical sequence distributions (Table 8-3 and Sec. 8-16) also support the polymer chain end control model [Zambelli et al., 2001], Analysis of propene-ethylene copolymers of low ethylene content produced by vanadium initiators indicates that a syndiotactic block formed after an ethylene unit enters the polymer chain is just as likely to start with an S- placement as with an R-placement of the first propene unit in that block [Bovey et al., 1974 Zambelli et al., 1971, 1978, 1979]. Stereocontrol is not exerted by chiral sites as in isotactic placement, which favors only one type of placement (either S- or R-, depending on the chirality of the active site). Stereocontrol is exerted by the chain end. An ethylene terminal unit has no preference for either placement, since there are no differences in repulsive interactions. 

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... 

There are striking differences in physical properties between the atactic and isotactic forms. The atactic material is soft, elastic, somewhat sticky, and rather soluble in solvents such as 1,1,2,2-tetrachloroethane. Isotactic polypropene is a hard, clear, strong crystalline polymer that melts at 175°. It is practically insoluble in all organic solvents at room temperature, but will dissolve to the extent of a few percent in hot 1,1,2,2-tetrachloroethane. That the difference between the atactic and isotactic polymers arises from differences in the configurations of the methyl groups on the chains is shown in a... 

Figure 29-9 Proton-decoupled 13C spectra of different polypropene samples taken in CHCI2CHCI2 solution at 150° at 15.9 MHz. The upper spectrum is of a highly isotactic polypropene, which shows only the faintest indication of lack of stereoregularity. The middle spectrum is of atactic polypropene, which shows a variety of chemical shifts for the CH3 groups as expected from the different steric interactions generated by random configurations of the methyl groups. The lower spectrum is of a sample of so-called stereoblock polymer, which is very largely isotactic. The 13C spectrum of syndiotactic polypropene looks exactly like that of the isotactic polymer, except that the CH3— peak is about 1 ppm upfield of the position of the isotactic CH3 peak and the CH2 peak is about 1 ppm downfield of the isotactic CH2 peak.

Ci symmetric metallocenes are able to produce new microstructures if consecutive insertions take place on the same active site in addition to chain migratory insertion. Polypropenes containing blocks of atactic and isotactic sequences are produced, the block lengths depending on the rate of chain stationary insertion or site isomerization vs chain migratory insertion [126-133], Thus hemiisotactic polypropene represents a special case, having a chain stationary/chain migratory ratio of 1 1. 

Most recent efforts by Coates and Waymouth have shown that 2-phenylin-denyl-groups are well suited for this purpose [139]. They oscillated between the enantiomeric and meso arrangements giving rise to a stereoblock polypropene containing atactic (produced by the meso rotamer) and isotactic (produced by the chiral rotamer) sequences. The block length was strongly dependent on the temperature. 

Table 12. Comparison of low molecular weight atactic polypropene from solvent extraction of a polymer produced using a conventional TiCU/MgCb catalyst with two high molecular weight atactic polypropenes of different molecular weight prepared by [Me2Si(Flu)2)ZrCl2MAO. For comparison typical data of an isotactic polypropene prepared by a conventional catalyst are also given [150]...

Figure 22-11 Stereochemistry of successive propane insertion steps into M—R bonds to give isotactic polypropene (a) and syndiotactic polypropene (b). In the absence of a stereocontrol mechanism atactic polypropene is formed. Note that in (a) the prochiral monomers coordinate to the metal with the same ff-face, so that an isotactic polymer is formed. This stereochemistry is favored by C2-symmetric ligands of type (22-XXVIII). In reaction (b) the second monomer coordinates with the opposite w-face to the first this stereochemistry is enforced by metallocenes of Cs-symmetry (22-XXX, R = H).

Fig. 8 Methine pentad region from the 100 MHz NMR spectra of isotactic (top), atactic (middle), and syndiotactic (bottom) polypropenes. (From Ref.. )...

A quite different interpretation of polypropene radiolysis has been given recently by Veselovskii et al. [321]. The crosslinking of isotactic polypropene is strongly inhibited in the presence of 2-methylbutene-l which contains a vinylidene double bond, whereas inhibition is far less important in the presence of other olefins. This indicates, according to the authors, that most of the intermolecular bonds produced by irradiation of isotactic polypropene are formed by reactions of the vinylidene-type double bonds formed by scission of the main chain. Since the inhibiting effect of hydroquinone, anthracene and stannous chloride on the quantity of gel in isotactic polypropene is small, network formation would not involve free radicals, but reaction of the excited vinylidene double bonds with the polymer chain. No gel is, however, found even after a 150 Mrad dose in atactic amorphous polypropene irradiated at room temperature in the presence of the radical acceptors. In this case, radicals play the main role in the crosslink formation. 

While both isotactic and syndiotactic polypropenes are partially crystalline materials with relatively high melting points (up to 160—170 °C for i-PP, and 150 °C for 5-PP), atactic polypropene (a-PP) is a fully amorphous polymer, since it lacks long-range stereochemical regularity. 

By using Brintzinger s ansa-titanocene C2H4(1-Ind)2TiCl2, Ewen first proved the correlation between metallocene chirality and isotacticity, a textbook example of shape selective catalysis. The C2-symmetric, racemic form yields isotactic polypropene while the achiral, meso form produces low molecular weight atactic polypropene. However, this titanocene is unstable at normal temperatures and has a quite low activity and a low stereoselectivity, producing... 

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