Metallocenes heterogenization

Belleli, P. G., Eberhardt, A., Dos Santos, J. H. Z., Ferreira, M. L., and Damiani, D. E. 2003. Metallocene heterogenization on acid supports. Recent Research Developments in Polymer Science 7 223-245. 

Most catalysts for solution processes are either completely soluble or pseudo-homogeneous all their catalyst components are introduced into the reactor as Hquids but produce soHd catalysts when combined. The early Du Pont process employed a three-component catalyst consisting of titanium tetrachloride, vanadium oxytrichloride, and triisobutjlalurninum (80,81), whereas Dow used a mixture of titanium tetrachloride and triisobutylalurninum modified with ammonia (86,87). Because processes are intrinsically suitable for the use of soluble catalysts, they were the first to accommodate highly active metallocene catalysts. Other suitable catalyst systems include heterogeneous catalysts (such as chromium-based catalysts) as well as supported and unsupported Ziegler catalysts (88—90). 

Eluidized-bed reactors are highly versatile and can accommodate many types of polymerization catalysts. Most of the catalysts used for LLDPE production are heterogeneous Ziegler catalysts, in both supported and unsupported forms. The gas-phase process can also accommodate supported metallocene catalysts that produce compositionaHy uniform LLDPE resins (49—51). 

In the early 1990s, solution processes acquired new importance because of their shorter residence times and abiUty to accommodate metallocene catalysts. Many heterogeneous multicenter Ziegler catalysts produce superior LLDPE resins with a better branching uniformity if the catalyst residence time in a reactor is short. Solution processes usually operate at residence times of around 5—10 min or less and are ideal for this catalyst behavior. Solution processes, both in heavy solvents and in the polymer melt, are inherently suitable to accommodate soluble metallocene catalysts (52). For this reason, these processes were the first to employ metallocene catalysts for LLDPE and VLDPE manufacture. 

B. C. Childress, Properties of homogeneous and heterogeneous polyolefins Metallocene catalyzed versus Ziegler-Natta catalyzed resins, MetCon 94 Proceedings, USA, May 1994. 

Polystyrene (PS) is the fourth big-volume thermoplastic. Styrene can be polymerized alone or copolymerized with other monomers. It can be polymerized by free radical initiators or using coordination catalysts. Recent work using group 4 metallocene combined with methylalumi-noxane produce stereoregular polymer. When homogeneous titanium catalyst is used, the polymer was predominantly syndiotactic. The heterogeneous titanium catalyst gave predominantly the isotactic. Copolymers with butadiene in a ratio of approximately 1 3 produces SBR, the most important synthetic rubber. 

The sterically unencumbered catalyst active site allows the copolymerization of a wide variety of olefins with ethylene. Conventional heterogeneous Ziegler/Natta catalysts as well as most metallocene catalysts are much more reactive to ethylene than higher olefins. With constrained geometry catalysts, a-olefins such as propylene, butene, hexene, and octene are readily incorporated in large amounts. The kinetic reactivity ratio, rl, is approximately... 

A characteristic of polyolefins synthesized with metallocene catalysts is their significantly lower polydispersity compared to one obtained by using heterogeneous Ziegler-Natta catalysts. Such narrower molecular mass distributions can lead to different mechanical properties of the resulting material. 

These are some key advantages that the metallocene catalysts have over conventional Ziegler-Natta catalysts and hence it is highly probable that inter-and intra-chain heterogeneity expected in ethylene-a-olefins copolymers can be controlled through the use of the metallocene system. 

The features of homogeneous metallocene/M AO catalysts distinguishable from the classical heterogeneous Ziegler-Natta catalysts can be summarized as follows ... 

Section 3 will deal with catalytic systems whose stereospecificity is mainly controlled by the chirality of the environment of the transition metal, independently of the possible chirality of the growing chain (chiral site stereocontrol). In particular, in Section 3.1 the chirality and stereospecificity of homogeneous catalytic systems based on metallocenes of different symmetries and in different experimental conditions will be reviewed. In Section 3.2 the chirality of model catalytic sites, which have been supposed for isospecific first-generation TiCl3-based and high-yield MgC -supported catalysts, is described. In Section 3.3 we will present a comparison between model catalytic sites proposed for heterogeneous and homogeneous stereospecific site-controlled catalysts. 

In general, these stereochemical events are completely independent. For instance, isospecific catalysts like the heterogeneous ones (based on TiCl3 or on TiCU supported on MgCI2) as well as C2 symmetric metallocenes [e.g., based on rac-C2H4(H4-l-Ind)2ZrCl2] can present extremely poor cis trans... 

Copolymerization. See also Copolymers of acrylonitrile, 11 202-204 anionic, 7 624-626 catalytic, 7 627-632 cationic, 7 626-627 chloroprene-sulfur, 19 833-834 of cyclic olefins, 16 112-113 with depropagation, 7 617-619 free-radical, 7 611-624 heterogeneous, 11 203-204 homogenous, 11 202-203 with metallocene catalysts, 16 111... 

Zirconocene and Half-Sandwich Zirconium Derivatives The development of a single-site heterogeneous catalyst for metallocene-based polymerization catalysis has also been explored extensively with zirconocene and half-sandwich zirconium derivatives [32, 75, 91, 92]. 

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