Isotactic, Syndiotactic, and Atactic Polypropenes

While the properties and applications of isotactic polymers have been extensively studied, those of syndiotactic polymers received less attention until relatively recently. The reason is the relative ease of forming isotactic polymers. Syndioselective polymerizations were less frequently encountered or proceeded with less efficiency compared to isoselective polymerizations. But the situation is changing fast as initiators and reaction conditions have been developed for syndioselective polymerizations. In the case of polypropene, the properties of the syndiotactic polymer have been examined [Youngman and Boor, 1967]. Syndiotactic polypropene, like its isotactic counterpart, is easily crystallized, but it has a lower Tm by about 20°C and is more soluble in ether and hydrocarbon solvents. 

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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],... 

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.

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).

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. 

Syndiotactic polypropene has a regular alternation of 50% of hydrogen/methyl groups in front of/ behind the —C—C—C—chain viewing plane as shown in Figure 1.13. Its properties are similar to isotactic polypropene rather than the atactic form, i.e., the regular polymer structure produces stronger intermolecular forces and a more crystalline form than the atactic polypropene. 

The development of Ziegler-Natta-type catalysts (see Section 25.8) has, since the 1980s, included the use of zirconocene derivatives. In the presence of methylaluminoxane [MeAl(p-0)] as a co-catalyst, compounds A, B and C (shown below) are active catalysts for propene polymerization. Compounds A and B are chiral because of the relative orientations of the two halves of the organic ligand. A racemic mixture of A facilitates the formation of isotactic pol3fpropene, while use of catalyst C results in syndiotactic polypropene (see Section 25.8 for definitions of syndiotactic, isotactic and atactic). If... 

Stereoregularity We have already discussed the stereoregularity of the IR spectrum of polystyrene. Figure 17.23 shows three IR spectra of polypropene. The spectra of atactic, syndiotactic, and isotactic polypropene are virtually the same at wave numbers of 1000cm and above. Remarkable spectral differences are observed, however, in the spectra at wave numbers below 1000 cm . This is consistent with our observations on the IR spectra of atactic, syndiotactic, and isotactic polystyrene as mentioned previously. 

FIGURE 17.23 IR spectra of atactic, syndiotactic, and isotactic polypropene. [Source Siesler and Holland-Mortiz (1980). By permission of Marcel Dekker, Inc.]... 

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. 

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

Stereochemical relationships play an especially important role in the properties of polymers. Consider the three polypropylene segments illustrated as Fischer projections in Figure 2.35. In the isotactic polymer, all of the methyl-substituted carbon atoms have the same configuration. In the syndiotactic polymer, the methyl-substituted carbon atoms have alternating configurations. In the atactic polymer there is a random pattern of configurations at the methyl-substituted carbon atoms. As with smaller molecules, the physical properties of diastereomeric polymers differ. Table 2.2 shows file relationship between tacticity and properties of polypropene. ... 

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