Metallocene catalysis isotactic

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. 

Isotactic polypropylenes produced by metallocene catalysis are now being produced by a number of different manufacturers and because different systems are used there is some variation in properties. Typically however such materials have similar density, hardness and tensile strength to conventional homopolymers but differ in having... 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

A foamable isotactic polypropylene homopolymer is obtained by metallocene catalysis and has a molecular weight distribution and density, which fall within broad ranges. It may be prepared in a multiple stage polymerisation process nsing the same metallocene component in at least two stages. 

In research with Ziegler catalysts, Cossee (11) and Arlmann and Cossee (12) hypothesized that the insertion of propylene monomer takes place in a cis conformation into a titanium-carbon bond. Natta et al. (8) postulated that in the stereospecific polymerization, chiral centers on the surface are needed to produce isotactic polymers. These and other issues regarding the nature of the active sites have helped to increase the interest in investigations of homogeneous metallocene catalysis. 

Yang, K., Huang, Y, and Dong, J.-Y. 2007. Efficient preparation of isotactic polypropylene/ montmorillonite nanocomposites by in situ polymerization technique via a combined use of functional surfactant and metallocene catalysis. Polymer 48 6254-6261. 

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. 

Work in the application of metallocene-based catalysis to olefin polymers has become a research topic of growing interest in recent years. A great number of symmetrie and chiral zirconocenes have been synthesized to give totally different structures of isotactic, syndiotactic, atactic or block polymers. The isotactic sequence length of polypropylene is influenced by the nature of the ligands of the metallocene. New ring or bridge substituted metallocene/methylalumoxane catalysts for the olefin polymerization are described. 

By using Brintzinger s ansa-titanocene C2H4(1-Ind)2TiCl2, Ewen first proved the correlation between metallocene chirality and isotacticity, a textbook example of shape selective catalysis. The C2-symmetric, racemic form yields isotactic polypropene while the achiral, meso form produces low molecular weight atactic polypropene. However, this titanocene is unstable at normal temperatures and has a quite low activity and a low stereoselectivity, producing... 

As will be discussed later, the polymerization of propylene is catalyzed by either Ziegler-Natta catalyst or metallocene catalyst. In the Ziegler-Natta catalyzed polymerization, the propylene monomer accesses the reaction site between the metal and carbon atoms in two ways the head-to-tail and tail-to-head insertion. The head-to-tail insertion occurs primarily in isotactic polymerization [8-10], while the tail-to-head insertion occurs in the low-temperature catalysis with VCl4-Et2AlCl catalyst [11]. There are mostly head-to-tail insertions and very few head-to-head or tail-to-tail inversions in the main chain of polypropylene [12,13]. 

Industrial applications of metallocene catalysts are a recent development AU of them possess in their reaction center two aromatic rings, between which a complex bond holds a metal atom, in most cases zirconium. This type of catalysis produces polymers with exceptionally uniform structures. The chain lengths of the individual molecules closely approximate one another. The spatial structure is therefore well-defined. Polypropylene, for instance, is completely isotactic. It is even possible to produce polypropylene in which the pendant group orientation alternates between right and left. The resulting substance is known as a syndiotactic polypropylene (see above) [3]. 

In comparison with the extensive and fruitful investigations in the area of homogeneous syndiose-lective styrene polymerization, research in the field of isoselective polymerization is still trudging. Isotactic polystyrene, known for almost a half century, is best produced by heterogeneous Ziegler-Natta catalysis. Recently, the homogeneous isoselective polymerization of styrene by soluble metallocenes and nonmetallocene transition metal complexes has been gradually developed. 

---