Metallocene catalyst systems

Fig. 3. Metallocene catalyst systems for LLDPE synthesis (a) Kaminsky catalyst (b) cationic catalyst and (c) Dow catalyst.

The focus of commercial research as of the mid-1990s is on catalysts that give desired and tailored polymer properties for improved processing. Development of metallocene catalyst systems is an example. Exxon, Dow, and Union Carbide are carrying out extensive research on this catalyst system for the production of polyethylene and polypropylene. 

Novel polyethylene materials have been synthesized by copolymerization of ethylene with 1-butene, 1-hexene, and/or 1-octene using metallocene catalyst systems [9]. 

Figure 7 GPC curve of LLDPE obtained with metallocene catalyst system. (From Ref. 13.)...

A final example of homogeneous catalysis is the use of metallocene catalyst systems in chain growth polymerization processes. The metallocene, which consists of a metal ion sandtviched between two unsaturated ring systems, is activated by a cocatalyst. The activated catalyst complexes with the monomer thereby reducing the reaction s energy of activation. This increases the rate of the reaction by up to three orders of magnitude. 

Figure 21 Structure of methylaluminoxane (extreme left), and some metallocene catalyst systems. Reproduced from Kaminsky [128], with permission of The Royal Society of Chemistry.

It has also been demonstrated that a PP-fc-EPR block copolymer was prepared with a living polymerization catalyst composed of V(acac)3/AlEt2Cl at -78 °C [ 145]. Also, PP-fo-EPR was produced with metallocene catalyst systems at -78 °C [146],... 

Supported metallocene catalyst systems are preferred to soluble versions in conventional polyolefin plants, which were designed to use supported Ziegler-Natta or Cr203-based catalysts. Metallocenes can be supported on a number of substrates, such as Si02, MgC or AI2O3. Supported catalysts also provide polypropylene with fewer stereochemical defects. 

Recent progress made in the development of novel metallocene catalyst systems for polyolefin polymerizations represents a true technological breakthrough with wide ramifications in the plastics industry [Wigotsky, 1995 Schut, 1996 Zamora,... 

The copolymerization of Tim and Tia with ethylene has been examined using metallocene catalyst systems. Of the two protected carboxylates, Tia appears to be preferred in terms of its incorporation into the copolymer (eq 5). 

Studies of temperature effects on propene polymerization of a number of metallocene catalyst systems show that the stereoregularity, streospecificity, and molecular weight of polypropene decrease with increasing polymerization temperature, whereas the polymerization activity increases. 

Natta et al. [20,21] noted that the rate of propylene polymerization is directly proportional to the catalyst concentration if all other conditions remain unchanged and the effect of monomer diffusion is not considered. The metal alkyls and the Al metal molar ratio of a catalyst system also affect the rate of polymerization [22- 24]. The polymerization rate increases rapidly with the concentration of aluminum trialkyl (AIR3) to a maximum value and then decreases. It is generally considered that a certain AIR3 concentration is required for the active centers to be effective. For the metallocene catalyst system, the rate of polymerization is also affected by the Al-metal molar ratio [25]. 

There have been numerous formulations of the metalloeene catalysts in the last 20 years. Table 3.4 presents a few examples of metallocene catalyst systems of interest (see all preceding references) [58-66]. It can be seen that earlier efforts were focused on catalyst efficiency and polymer stereospecificity. More recently, some interests seemed to have turned to syndiotactic polypropylene for various polymer and fiber properties. 

Table 3.5 compares the key features of Ziegler-Natta and metallocene catalyst systems. 

Vaughan, G.A. Szul, J.F. McKee, M.G. Farley, J.M. Lue, C.T. Mixed metallocene catalyst systems containing a poor comonomer incorporator and a good comonomer incorporator, U.S. Patent 6,828,394, December 7, 2004. 

A useful concept pointing the way for the description of these metallocene catalyst systems goes back to the kinetic studies of Reichert and Meyer [8-10]. In [8] in the summary Reichert wrote ... 

Table 5 Comonomer effect in propene/a-olefin copolymerization using heterogeneous ZN catalyst and homogeneous and supported metallocene catalyst systems...

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