Single-Site Metallocene Catalyst Systems

The introduction of metallocene and single-site polyethylene catalyst systems may eventually change the demand for higher olefins used as comonomers. Some sources indicate that their use will increase the demand for comonomers, but others feel that they will reduce comonomer use. At any rate, it is not expected that their introduction will have a significant effect on comonomer growth rates for the period 1992—1996 (see Olefin polymers). 

The chemistries utilized in gas-phase technologies employ the same Ziegler-Natta (314,315) and single-site (metallocene) catalysts (313) described in the processes included below. In gas-phase systems, however, the catalysts are generally solid-supported, but produce the same range of polybutadiene microstructures inherent to the nonsupported catalyst. Several patents also include anionic polymerization systems as useful in gas-phase processes (378). Kinetic modeling work has also been done to better predict the gas-phase polymerization behavior of 1,3-butadiene (379). 

The active site in single-site metallocene catalysts is tetrahedrally coordinated while the active sites in heterogenous Ziegler type systems are octahedrally coordinated in the TiCh or MgCl2 lattice. This difference leads to a greater openness of the active site, facilitating incorporation of higher alkenes in the production of LLDPE. 

To better understand the polymerization process and to be able to rationally design better catalyst systems, single-site homogeneous catalyst systems were developed as models of the heterogeneous systems employed commercially. Of particular interest were metallocene-type catalysts, which had been discovered to be slightly active for polymerizations in the early work of Natta et Until the accidental discovery of MAO by Sinn, Kaminsky and coworkers in 1980, however, the activity of metallocene catalysts was far too low to be useful. Activation of a metallocene precursor such as (ri -C5H5)2ZrCl2 with MAO was found to yield extremely active polymerization catalysts, and the number of metallocene-type catalysts reported in the literamre has since exploded. ... 

In catalyst systems developed for polypropylene like the single site metallocene catalysts, highly isotactic polypropylene structures with configurational defects are obtained by a chain-end mechanism of stereocontrol. For catalytic polymerizations where the tacticity is high, configurational defects can be recognized. 

Possible pathways for monomer insertion in olefin polymerisation systems with homogeneous single-site metallocene-based catalysts and heterogeneous Zieg-ler-Natta catalysts are shown in Figure 3.17 [122],... 

The classical heterogeneously catalyzed propene polymerization as discovered hy Natta is a stereospecific reaction forming a polymer with isotactic microstructure. During the development of single-site polymerization catalysts it was found that C2-symmetric chiral metallocene complexes own the same stereospecificity. An analysis of the polymer microstructure hy means of NMR spectroscopy revealed that misinsertions are mostly corrected in the next insertion step, which suggests stereocontrol (Figure 6) hy the coordination site, as opposed to an inversion of stereospecificity hy control from the previous insertion steps (chain-end control). In addition, it was found that Cs-symmetric metallocene catalysts lead to syndio-tactic polymer since the Cosee-Arlmann chain flip mechanism induces an inversion of the stereospecificity at every insertion step. This type of polymer was inaccessible by classical heterogeneous systems. 

The issues raised in Section 7.2 are best illustrated by considering several real-world examples. Isotactic polymerization is considered first, followed by syndiotactic polymerization. These classic examples are followed by two studies on novel polymerization systems. The third case study concerns a single-site metallocene system that produces a stereoblocky material with novel elastomeric properties. The last example involves an ethylene-propylene copolymerization catalyst that exhibits nonrandom monomer incorporation. Common themes of these four examples include monomer-polymer chain control, van der Waals attraction, and the importance of the counteranion. 

Last but not least, Tl-based MOFs were tried in the Zieglff-Natta polymerization of ethylene and propylene, though these systems were infaior to the well-known single-site metallocene polymerization catalysts with their trCTiendous turnover frequencies and space-time yields [107]. 

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... 

Fig. 1.8 Structural comparisons of i-PP metallocene catalyst precursors and two recently reported new classes of competitive single-site systems discovered using HT-R. D.

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