Syndiotactic polypropene

Modification of the cyclopentadienyl ligands has led to a very rich chemistry and today a great variety of microstructures and combination thereof can be synthesised as desired including isotactic polymer with melting points above 160 °C, syndiotactic polypropene [16], block polymers, hemi-isotactic polymers etc. 

The chain conformation of syndiotactic polypropene in the crystalline state is ... 

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],... 

Both isotactic and syndiotactic polypropenes are achiral as a result of a series of mirror planes (i.e., planes of symmetry) perpendicular to the polymer chain axis. Neither exhibits... 

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. 

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. 

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.

Table 9. Syndiotactic polypropenes prepared by different metallocene catalysts. Polymerisations were carried out at 60 °C in 11 of liquid propene...

Commercial product of syndiotactic polypropene utilizes a silica supported metallocene in a bulk suspension process at 50-70 °C and a pressure of 30 kg/cm2 [157]. 

The combination of flexibility, clarity and tensile set and low heat seal temperatures (Table 18) enables syndiotactic polypropenes to be applied instead of PVC, EVA and LLDPE in films, foils and extruder products. 

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).

A series of elegant experiments which support this mechanism involve the use of isopropyl(l-fluorenyl-cyclopentadienyl) ligands [51]. This complex is not chiral (i.e the dichloride precursor of Fig. 6.20), but has a plane of symmetry instead. The catalyst was found to give syndiotactic polymer. Coordination of propene at either site now leads to mirror images. Migration of the alkyl chain will create carbon atoms with the opposite absolute configuration (i.e syndiotactic polypropene). This is an important result, since hitherto it was thought that syndiotactic polymers could only be obtained via 2,1-insertion, controlled by the stereochemistry of the chain end. 

Chapter 11 also explores polymerization reactions that are used to make giant molecules with many practical uses. Organotransition metal complexes play key roles in these transformations. Ziegler and Natta (who shared the 1963 Nobel Prize in Chemistry) were pioneers in the use of early transition metal compounds to catalyze the polymerization of ethene and propene. Stereoregular polymers resulted in the latter case, such as syndiotactic polypropene. 

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. 

The first metallocene -and the first catalyst in general- able to produce highly syndiotactic polypropene (s-PP), was the Q-symmetric Me2C (Cp) (9-Flu)-ZrCp (Cs-1 in Chart 18)." The behavior of this catalyst and the characterization of 5 pp397-407 have been extensively reviewed. A number of studies on the thermal behavior, crystal structures, and morphology of 5-PP have appeared in the litera-... 

The development of Ziegler-Natta-type catalysts (see Section 27.8) 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 pol5mierization. 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 polypropene, while use of catalyst C results in syndiotactic polypropene (see Section 27.8 for definitions of... 

User Dubaj-commonswiki (talk/contributes) (2006) File Syndiotactic polypropene.png. This work is in the public domain. https //commons.wikimedia.0rg/wiki/File Syndiotactic polypropene.png. https //en.wikipedia.org/wiki/polypropylene... 

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. 

Kaminsky, W. Hopf, A. Piel, C. Q-symmetiic hafnocene complexes for synthesis of syndiotactic polypropene. J. Orgaromet. Chem. 2003, 684, 200-205. 

Natta, in collaboration with the Italian company, Montecatini, extended Ziegler s method to propene. He found that polymerization occurs almost entirely head to tail and that crystalline, stereoregular products can result. Using EtjA1 in conjunction with TiCl (active species TiClj) isotactic polypropene is formed in which all the methyl groups are all on the same side of the carbon backbone (Fig. 12.9). With Et AlCl/VCl, syndiotactic polypropene results in which the configurations at the carbon atoms alternate along the chain. 

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