Atactic and syndiotactic polypropylene

Atactic polypropylene may be obtained either as a by-product of the manufacture of isotactic polypropylene or by specific processes designed for its direct production. 

Whilst completely atactic material would be amorphous, commercial materials have a small measure of crystallinity. This is often assessed in terms of insolubility in n-heptane which is usually of the order of 5—10%. Viscosity average molecular weights are in the range 20 000-80 000 and specific gravities are about 0.86 g/cm.  

In appearance and on handling the material is somewhat intermediate between a wax and a rubber. It is also semi-tacky. Like isotactic polypropylene it is attacked by oxygen but unlike the isotactic material it swells extensively in aliphatic and aromatic hydrocarbons at room temperature. It is also compatible with mineral fillers, bitumens and many resins. 

For many years atactic polypropylene was an unwanted by-product but today it finds use in a number of markets and is specially made for these purposes rather than being a by-product. In Europe the main use has been in conjuction with bitumen as coating compounds for roofing materials, for sealing strips where it confers improved aging properties and in road construction where it improves the stability of asphalt surfaces. Less important in Europe but more important in USA is its use for paper laminating for which low-viscosity polymers are used, often in conjunction with other resins. Limestone/atactic 

High molecular weight atactic polyropylene is now available (Rexene-Huntsman). This is miscible with isotactic polypropylene in any proportion to give transparent blends of interest in packaging applications. 

268 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers 

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Carbon-13 spectroscopy is particularly effective in the study of paraffinic polymers because the chemical shifts are very sensitive to structure, being spread over a ca. ppm range as compared only about 2 ppm for protons. In Fig. 5 are shown the spectra of isotactic, atactic and syndiotactic polypropylenes. Although all three carbons, the CH2 ot-CH, and CH3, are responsive to configuration, the CH3 carbon is clearly the most sensitive. 

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

Polypropylene made by free-radical polymerization is generally atactic , that is to say, there is no pattern to the stereochemistry. On the other hand, both isotactic polypropylene (in which all the stereocenters are the same) and syndiotactic polypropylene (in which the stereocenters alternate) may be made via the Ziegler-Natta process (see Chapter 18, Problem 4). Experimentally, both isotactic and syndiotactic polypropylene generally have higher melting points than atactic polypropylene. 

Examine three different strands ofpolypropylene. For each strand, assign R/S stereochemistry to each stereocenter. (All three strands have as their terminal monomer perfluoropropane in order to facilitate assignment of stereochemistry.) Which of the three strands corresponds to atactic polypropylene, isotactic polypropylene and syndiotactic polypropylene ... 

Chiral titanocenes, zirconocenes, and hafnocenes in combination with methylalu-minoxane [A1(CH3)—0] , can lead to highly isotactic propylene. Nonchiral metallocenes like (Cp)2ZrCl2 or other similar compounds produce only pure atactic polypropylenes. Molecular mass of 590,000 for atactic polypropylenes can be achieved by low polymerization r. The activities of these hydrocarbon soluble catalysts are extremely high. Different structures of polypropylenes are obtained when the rr-bonded ligand of the transition metal is varied (Fig. 1). With no other catalyst can atactic, isotactic, stereoblock, isoblock, and syndiotactic polypropylene of such purity be produced. 

Tacticity describes the symmetry of the monomer orientation or stereochemistry of the monomer. In polybutadiene, atactic polymers have no steric order along the polymer. Isotactic polymers have the monomer pendent groups at the same relative position. Syndiotactic 1,2- polybutadiene has pendent groups alternating along the polymer chain. For illustrative purposes atactic, isotactic, and syndiotactic polypropylene are shown in Figure 4.2 [2, 4]. 

Ansa-metallocene catalysts for the formation of atactic, isotactic, and syndiotactic polypropylene. 

Figure 3.1 The 25-MHz spectra of three preparations of polypropylene isotactic, atactic, and syndiotactic. (From Bovey, F.A., Chain Structure and Conformation (f Macromolecules, Academic Press, New York, 1982. With permission.)...

The IR spectra of (a) isotactic, (b) syndiotactic, and (c) atactic PP are compared in Fig. 19. The poor stereoregularity of atactic polypropylene (c) is reflected in the broadening of the absorption bands compared to the corresponding bands in isotactic and syndiotactic polypropylene. 

Both isotactic and syndiotactic polypropylene are stereoregular polymers each is characterized by a precise stereochemistry at the carbon atom that bears the methyl group. The third possibility, called atactic, has a random orientation of its methyl groups it is not stereoregular. 

Free-radical polymerization arranges functional groups, such as alkyls, in a random manner, whereas coordination polymerization can exercise stereochemical control over functional groups. With the proper choice of experimental conditions, such as temperature, solvent, and catalyst, monomers can polymerize to any of three arrangements isotactic, syndiotactic, and atactic. (For polyethylene, there are no such stereoisomers, since the monomeric units are identical, —CH2—.) Isotactic and syndiotactic polypropylenes are highly crystaUine atactic polypropylene is a soft, elastic, and rubbery material. Following are the three stereoisomers of polypropylene ... 

Fig. 13.5. The regularity of a polypropylene chain structure decides its c stallizability. The isotactic and syndiotactic polypropylene c stallize, but atactic polypropylene does not.

Different behavior can be found for each of the three stereoisomers. Isotactic and syndiotactic polypropylene can pack into a regular crystalline array, giving a polymer with more rigidity. Both materials are crystalline however, syndiotactic polypropylene has a lower than the isotactic polymer. The isotactic polymer is the most commercially used form with a melting point of 165°C. Atactic polypropylene has a very small amount of crystallinity (5 to 10 percent), because its irregular structure prevents crystallization thus, it behaves as a soft flexible material. It is used in applications such as sealing strips, paper laminating, and adhesives. 

Various chiral metallocenes with methylaluminox-ane cocatalysts (MAO) are specific for the preparation of isotactic, syndiotactic, atactic and hemiisotactic polypropylenes and higher polyolefins [72]. In hemiisotactic polypropylene (hit-PP) every other methyl is placed isotactically, the remaining methyls randomly. This type of polypropylene has served as a keypoint in the elucidation of the polymerization mechanism with metallocene catalysts [73]. The chiral metallocene catalysts are not as stereorigid as the conventional heterogeneous systems. Consequently under some... 

When Rj is different from R2 in Formula (1.1) the carbon atom is asymmetric and may have d or 7 forms. If all the asymmetric carbon atoms have either d or 7 forms, the polymer chain is said to be isotactic. If these carbon atoms are instead alternating d and 7 , the polymer chain is said to be syndiotactic. If the d and 7 assignments are random along the chain, it is said to be atactic [8,18-20] (see Fig. 1.1). Isotactic polypropylene, poly(butene-l) and poly(4-methylpentene-l) are commercially available. Both isotactic and syndiotactic polypropylene and polystyrene have been synthesized, subjected to extensive investigation. The two isomeric polymers have different crystal structures and their atactic forms do not crystallize. Isotactic and syndiotactic polymers were originally developed by Natta and his coworkers [18, 19] at Milan Polytechnic and Montecatini. In recent years, there has been interest in producing polyolefins with controlled intermediate tacticities [20]. 

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