Hemi-isotacticity

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

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. 

There are three principal stereochemical types of poly(l-alkene)s, illustrated in Scheme 8.38 for polypropylene. In isotactic polypropylene 80 (i-PP) all methyl substituents have the same relative orientation (m). The scheme shows the stereochemistry with the usual Fischer projection underneath. In syndiotactic PP (81, s-PP) every second CHMe unit has the opposite stereochemistry to the first, while in atactic PP (82, a-PP) the orientation of the methyl substituents is random. In some polymers there is partial order, i. e. only every second monomer orientation is random (83, hemi-isotactic PP). 

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. 

With the advent of metallocene catalysis, a range of tacticities and structures are possible see Figure 14.2 (4). Thus a range of compositions not easily prepared before, such as syndiotactic, hemi-isotactic, and isoblock copolymers, are now possible. 

Metallocene catalysts are remarkably versatile for stereochemical control they can be used to polymerize [71] propylene to atactic, isotactic, syndiotac-tic, isotactic stereoblock and hemi-isotactic polypropylene (Fig. 52). 

Cl symmetric metallocene with j6-Me introduced into the Cp ligand of catalyst 12 (catalyst 13) behaves differently with differing properties of each coordination site, and produces hemi isotactic PP [91]. 

Thus, the principle of manipulating the ligand structure is extremely versatile, and rational tailoring of the polymer structure (e.g., atactic PP, isotactic PP, hemi isotactic PP, syndiotactic PP) would be applied more widely in the future. 

Many other catalysts have been introduced for the polymerization of propylene to produce a range of different structures ranging from isotactic, syndiotactic, to hemi-isotactic and isotac-tic/syndiotactic sequences. These include catalysts based on a bridged cyclopentadiene/fluorene methyl-substituted ligand framework that can alternate the substitution of ethylene and propylene in copolymers. 

The ease of ligand design in metallocene catalysts has made possible novel stereoregulated polymers that have not been seen with conventional Ziegler-Natta catalysts. Hemi-isotactic polyolefins, and isotactic-atactic block polymers are examples that will be discussed in Sect. 2.5. 

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