Tacticity of Polypropylene

Similarly, a pentad and hexad of syndiotactic PP (sPP) would have the structures  

As well as this, the monomer units in a polymer can exist in head-to-head, head-to-tail and tail-to-tail configurations as illustrated below in the case of iPP (discussed later under regioisomerism . Chapter 7)  

Processes for the manufacture of ethylene-propylene copolymer can produce several distinct types of polymer which, although they may contain similar proportions of the two monomer units, differ appreciably in their physical properties. The differences in these properties lie not only in the ratio of the two monomers present but also, and very importantly, in the detailed microstructure of the two monomer units in the polymer molecule. Ethylene-propylene copolymers may consist of mixtures of the following types of polymer  

Although molecular symmetry is well understood, until the development of proton nuclear magnetic resonance (NMR) spectroscopy, and later C-NMR, a study of this aspect of polymer structure presented problems. 

The advantages so clearly evident when applying C-NMR to polymer configurational analyses are not devoid of difficulties. The sensitivity of C-NMR to subtle changes 

An interesting aspect of the PP chain structure is the existence of distinct configurational isomers resulting from the presence of a pseudo asymmetric carbon atom. The actual polymer stereogularity or tacticity, as it is termed, is quite variable, being dependent on the nature of the catalyst, the presence or absence of additives, and other parameters such as temperature or reaction medium. Since the polymer morphology and hence physical properties are crucially dependent on the tacticity of the PP, the measurement of these properties is of considerable importance both in commercial production and fundamental investigations. 

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How do metallocene catalysts define the molecular weight distribution and tacticity of polypropylene ... 

Nuclear magnetic resonance is also used to determine the tacticity of polypropylene and vinyl polymers. For example, high-resolution NMR... 

The most pronoimced decrease in the melting temperature is observed with isotactic polypropylene, indicating that the crystalline perfection of polypropylene is more highly affected by ionizing irradiation than that of polyethylene. Obviously, the process includes not only the main chain scissions but also the change in tacticity of polypropylene macromolecules. 

When polymer scientists discuss the stereochemical features of PP, they usually discuss it in terms of tacticity or percent tacticity of polypropylene, and in the marketplace the term polypropylene is generally used to refer to a material that has high tacticity, meaning high isotactic content. The high tacticity PP materials have desirable physical, mechanical, and thermal properties in the solid state. Atactic material is a soft, sticky, gummy material that is mainly used in sealants, caulks, and other applications where its stickiness is desirable. Syndiotactic PP, not a large volume commercial material, is far less crystalline than isotactic PP. 

The chlorination of polypropylene has been the subject of several fundamental studies and a variety of products is obtainable according to the tacticity of the original polymer and to the extent of chlorination. 

Figure 31.1 Isotactic, syndio-tactic, and atactic forms of polypropylene.

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

Name the three forms of tacticity. For polypropylene, which form crystallizes fastest and why ... 

How does the type and level of tacticity of the polypropylene chain affect the degree of crystallinity of the solid polymer ... 

Figure 14 Tacticity of (A) polypropylene and (B) poly(methyl-phenyl) silane.

Tacticities of the polypropylenes above 60% [mmmm] highlights the tendency of the backward oriented substitution (4a) to produce plastomeric materials. During experiments in liquid propylene, no similarity is observed between the two isomeric catalysts. Catalyst 4b/borate does not follow the same trend in the catalytic performance as previously mentioned, but leads to polypropylenes with sufficient amount of isotactic sequences and relatively high molecular weight for the design of plastomeric polypropylenes. 

The measurement of polymer configuration was difficult and sometimes speculative until the early 1960 s when it was shown that proton NMR could be used, in several instances, to define clearly polymer stereochemical configuration. Bovey was able to identify the configurational structure of poly(methylmethacrylate) in terms of the configurational triads, mm, mr and rr, in a classic example (3). In the case of polypropylene, configurational information appeared available but was not unambiguously accessible because severe overlap complicated the identification of resonances from the mm, mr and rr triads (4). Several papers appeared on the subject of polypropylene tacticity but none totally resolved the problem (5). 

They are based on various metals. Such as zirconium, complexed with cyclopentadienide anions. This type of compound is called a zirconocene and is used with organoalu-minum to make highly regular polymers. The catalyst has the ability to flip back and forth from making atactic to isotactic polypropylene in the same polymerization. The alternating tacticity of the polymer breaks up the crystallinity of the chains and yields an elastomer. Metallocene catalysts are currently very expensive and cannot yet polymerize dienes such as butadiene, so they have only enjoyed limited commercial success in elastomers. However, this is one of the most intense fields of polymer research and many new product breakthroughs are expected in the near future. 

Two examples clearly illustrate the relationship between molecular structures of the metallocene catalysts on the one hand, and the tacticity of the resultant polymers on the other. As shown in Fig. 6.9, complexes 6.32, 6.33, and 6.34 have very similar structures. In 6.33 and 6.34 the cyclopentadiene ring of 6.32 has been substituted with a methyl and a f-butyl group, respectively. The effect of this substitution on the tacticity of the polypropylene is remarkable. As already mentioned, 6.32, which has Cs symmetry, gives a syndiotactic polymer. In 6.33 the symmetry is lost and the chirality of the catalyst is reflected in the hemi-isotacticity of the polymer, where every alternate methyl has a random orientation. In other words, the insertion of every alternate propylene molecule is stereospecific and has an isotactic relationship. In 6.34 the more bulky t-butyl group ensures that every propylene molecule inserts in a stereospecific manner and the resultant polymer is fully isotactic. 

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