Metallocene-based Olefin Polymerization

We will discuss briefly the mechanism of metallocene catalysis and the role of methylaluminoxanes, as follows. The methylaluminoxanes are formed by the controlled reaction of AlMes and water, with elimination of CH4, and have the approximate composition [MeA10-]n with a molecular mass in the range 1000-1500 g/mol. They contain hnear, cyclic and cross-linked compounds. The following formula shows a simphfied picture of the MAO structure  

Excess of MAO is normally required, typical Al metallocene ratios ranging between 50-100 for supported systems and 400-20000 in solution. The optimum ratio depends on the metallocene used and the experimental conditions. 

The metallocene reacts with the MAO, and methyl groups replace chlorine on the metallocene. MAO then acts as a Lewis acid taking one of the methyl groups from the Zr to give the active catalyst  

For Ti, Zr, and Hf, the resulting catal5hically active species is therefore a 14-elec-tron cationic alkyhnetallocenium ion formed by dissociation of the metallocene alu-minoxane complex. The [aluminoxane-Me] anion is considered to be weakly coordinated or even non-coordinating. The positive Zr ion is stabilised by sharing electrons from a C-H bond. 

The reaction mechanism is believed to involve successive additions and insertions of ethylene at each Zr centre comprising the single-site catalyst. Due to this special structure, these highly active catalysts produce uniform homo- and co-polymers with narrow molecular mass distributions. Furthermore, the polymer structure may be controlled by the symmetry of the catalyst precursors. 

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Since the mid-1980s bilhons of dollars have been invested worldwide in research focusing on modifying and improving the metallocene-based olefin polymerization catalysts. Key structures are summarized in Figure 2. 

Cocatalysts for metallocene-based olefin polymerization catalyst systems Metallocenes supported on ion exchange resins Olefin polymerization catalysts Process and a catalyst for preventing reactor fouling Supported metallocene catalysts for the production of polyolefins... 

While group 4 metallocene-based olefin polymerization catalysts have dominated the field of homogenous olefin polymerization catalysis since the late 1950s, the development of complexes bearing non-Cp ligands as potential olefin polymerization catalysts has become a rapidly expanding area over... 

Mulhaupt and coworkers have reported the details of several studies related to the preparation of block copolymers from thiol, maleic acid and hydroxy-functional polypropylene prepared by a metallocene catalyst [157, 158]. The same group also reported the transformation of metallocene-mediated olefin polymerization to anionic polymerization by a novel consecutive chain-transfer reaction for the preparation of polypropylene-based block copolymers [159]. The latter were also... 

Figure 6. The spectrum of iPP shows only one methyl group signal typical for the meso (mmmm) pentad, whereas the racemic (rrrr) pentad is typical for syndiotactic polypropylene (sPP). The remarkable history of polyolefins and the development of the stereoselective 1-olefin polymerization is described in reviews by Pino and Miilhaupt [5], covering the first 25 years, and by Brintzinger et al. [6, 7], covering modem aspects of metallocene-based propylene polymerizations. PP technology is the subject of several books [8-11] including the chapter Industrial polymerization processes in this book.

The advent of homogeneous olefin polymerization based on metallocenes has also ushered in a new era in olefin polymerization. In fact, the recent commercialization of numerous poly(olefins) prepared using metallocene strategies is testament to the importance of this field from both an academic and an industrial viewpoint. One of the main intellectual driving forces is the realization that as a specific example of single site catalysts, these systems are also amenable to... 

Brookhart and coworkers [1] have recently developed Ni(II) and Pd(II) bis-imine based catalysts of the type (ArN=C(R)-C(R)=NAr)M-CH3+ (la of Figure 1) that are promising alternatives to both Ziegler-Natta systems and metallocene catalysts for olefin polymerization. Traditionally, such late metal catalysts are found to produce dimers or extremely low molecular weight oligomers due to the favorability of the P-elimination chain termination process [2],... 

For industrial applications for the polymerization of norbomenes into COCs, zirconium based metallocene catalysts are most popular. However, other metal based catalysts have been described too. Catalysts for olefin polymerization have been reviewed in the literature (10,23,24). 

Patents have been reported for a series of bimetallic metallocene catalysts based on lanthanide and group IV metals. The bridging ligand is a substituted ethyl-linked fluorenyl indenyl bearing a substituent of varying length [26]. The complexes are reported to act as good olefin polymerization catalysts in the presence of MAO.52,53... 

Fig. 5-18i,13 has been followed in recent years by studies of early transition metal-carboranes as catalysts for olefin polymerization.14 In these applications, the metalla-carborane species resemble their metallocene-based analogues however, there are other areas, such as the construction of new types of electronic, magnetic, and optical materials, in which the unique structural and other characteristics of metalla-carboranes may be put to use. Venus flytrap linked-dicarbollide ligands such as that shown encapsulating a Co3+ ion in Fig. 5-18f have been prepared as a possible way to bind radiotransition metals to tumor-associated monoclonal antibodies for purposes of diagnosis and therapy.15... 

Based on these kinetic and microscopic observations, olefin polymerization by supported catalysts can be described by a shell by shell fragmentation, which progresses concentrically from the outside to the centre of the support particles, each of which can thus be considered as a discrete microreactor. A comprehensive mathematical model for this complex polymerization process, which includes rate constants for all relevant activation, propagation, transfer and termination steps, serves as the basis for an adequate control of large-scale industrial polymerizations with Si02-supported metallocene catalysts [A. Alex-iadis, C. Andes, D. Ferrari, F. Korber, K. Hauschild, M. Bochmann, G. Fink, Macromol. Mater. Eng. 2004, 289, 457]. 

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