Single-Site Metallocene Catalysts

It should be mentioned that many of the requirements necessary for the economic production of polyethylene and polypropylene have been achieved. However, catalysts of greater activity and of greater selectivity in the production of polymers and copolymers can be anticipated. This is of prime concern to alkene polymerisation processes in the presence of single-site metallocene catalysts. Such catalysts, undoubtedly of great scientific and commercial importance, have been developed on a large scale within recent years [29,30],... 

In the case of olefin polymerisation with homogeneous Ziegler-Natta catalysts, especially with single-site (metallocene) catalysts, the kinetic analysis may become simpler than in the case of polymerisation with heterogeneous catalysts, and in some instances can serve as a very useful tool for uncovering the true polymerisation mechanism [30,243],... 

Propagation Reaction Stereocontrol in the Presence of Single-site Metallocene Catalysts... 

Styrene, which can be treated formally as an a-olefin branched in the 3-position, forms copolymers with ethylene and a-olefins (as well as with /i-olefins, involving isomerisation copolymerisation). Both heterogeneous Ziegler-Natta catalysts and a single-site metallocene catalyst promote the copolymerisation. 

Single-site metallocene catalysts are also highly reactive vis-a-vis cycloolefins such as cyclobutene, cyclopentene or norbornene. While homopolymers of these cycloolefins have melting temperatures (>380°C), much too high for technical processability, ethylene-cycloolefin copolymers (COC s) - e.g. ethylene-norbornene copolymers - are amorphous materials with glass transition temperatures, above which they become soft and processable [W. Kaminsky, J. Polym. Sci. A, Polym. Chem., 2004, 42, 3911]. 

Equations (9-64)-(9-66) are deceptively simple, since SEC and TREE studies (Chapter 3) show that the catalytic sites do not have uniform activities (with the exception of single-site metallocene catalysts described in Section 9.5.4). The rate constants quoted in the foregoing equations are thus lumped values, which are not known a priori for untested polymerizations. 

The PS/PE blends compatibiUzed by a block copolymer have been offered commercially (Neo-polen ) and are primarily used for packaging [Utracki, 1993]. Dow has aimounced plans to commercialize ethylene-styrene copolymers employing single-site metallocene catalysts. High styrene versions of these copolymers could have interest as a compatibiUzer for PO/PS blends [Grande, 1997]. 

Materials that promote polymerization may be devided into true catalysts such as metal complexes, metal oxides, and anionic and cationic catalysts and initiators, which appear as end groups in the final polymer. It seems to be clear that the most expensive polymerization catalysts are the single-site metallocene catalysts. Sales has grown from 1 million US in 1994 to 100 million US in 2000. Ziegler catalysts used for polypropylene and polyethylene, and the dibutyl tin and special basic catalysts for polyurethanes are also expensive. 

Figure 4.3 Single-site metallocene catalyst with aluminoxane co-catalyst...

SMS styrene methylstyrene SSMC single-site metallocene catalyst... 

Fig. 18.2 Melting point versus density dependencies for LLDPE prepared using Ziegler-Natta multi-site and a single-site metallocene catalyst...

Improved clarity of PP has provided the ability for replacement of PVC with PP in applications such as blisterpacks for hardware. In addition, new PP resins are being developed that use single-site metallocene catalysts (mPP). While virtually no difference exists in the processing behavior or finished product properties between conventional PP and mPP, these new materials are easier to nucleate. The use of nucleated mPP provides for a product with the higher physical properties of PP homopolymer and the clarity of nucleated random PP copolymer. 

A new development in polyolefin synthesis is the use of homogeneous single-site metallocene catalysts [11, 12], which unlike traditional Ziegler-Natta catalysts contain... 

Figure 11 Comparison of conventional multisite catalyst and modem single site metallocene catalysts in copolymerization.

Recently, the single-site metallocene catalyst was used to prepare random and block copolymers of syndiotactic-PP (sPP) with syndiotactic-PS (sPS). These were used as compatibilizers for these syndiotactic resins. 

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). 

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