Homogeneous Single-site Metallocene

Methylaluminoxane-activated zirconocenes were first reported to exhibit extraordinarily high activities in ethylene polymerisation, much higher than those of heterogeneous catalysts, and retained over prolonged periods of time. The productivity of the zirconocene-methylaluminoxane catalysts, which were 

Catalyst activity increases with an increasing Al/Mt ratio [95,101,106]. A large excess of methylaluminoxane (Al/Mt ratio 500) is generally required for acceptable activity a particularly high catalyst productivity, reaching 2260 kg of polyethylene per mmol Zr/h atm (24775 kg per gram Zr/h atm), was obtained at 90 °C using an Al/Zr ratio of ca 67 000 [22], 

A decrease in the catalyst Al/Mt ratio and an increase in polymerisation temperature both result in a lower average polymer molecular weight and, in the case of stereospecific a-olefin polymerisation, decreased stereo regularity [22,107]. 

The catalyst activity generally decreases in the following order Zr Hf Ti [29,30,108]. Titanium is rapidly deactivated above -50 °C owing to the reduction of Ti4+ to Ti3+ [93]. 

In connection with the above characteristic features of metallocene methy-laluminoxane catalysts, it must be emphasised that alternative, potentially cheaper alkylaluminoxanes, such as ethyl or z-butyl derivatives, which are more soluble in aliphatic hydrocarbons than methylaluminoxane, or other alkylaluminium compounds used as activators for metallocene procatalysts, show inferior activity. 

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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],... 

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

Examples of catalytic reactions and processes relevant to hydrocarbon chemistry are numerous. The technologies of the oil refinery with extremely low (<0.1) E factors are excellent examples demonstrating the possibilities that can be achieved by the development of selective catalytic methods, particularly by the use of various solid acids (see detailed discussions in Chapter 2). Further examples of commercially highly successful processes are the oxidation catalyst TS-1 developed by Enichem researchers160 161 (see Sections 9.1.1, 9.2.1, and 9.4.1), the homogeneous aqueous-phase Rh-catalyzed hydroformylation (see Sections 7.1.3 and 7.4.1), and single-site metallocene polymerization catalysts, which allow the preparation of tailored polymers with new properties (see Sections 13.3.2).162-164... 

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],... 

In the late 1980s and in the 1990s, homogeneous metallocene-based Ziegler Natta and related aluminium-free catalysts as well as other non-metallocene-based homogeneous single-site catalysts, which are active in the syndiospecific polymerisation of styrene, were found also to promote the polymerisation of conjugated dienes [16,38 13],... 

Advanced reactor polymerization processes Single-site metallocene catalyzed POs reportedly can produce PP with a fine, homogeneous dispersion of rubber regions and a soft, saatch-resistant surface. For example, LyondellBasell s Catalloy process and its Softell PP is said to provide improved saatch resistance and lower gloss, compared with standard TPO, and it can be compounded with glass fiber rather than talc [17-7]. 

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... 

Polymerisation with Homogeneous Metallocene Single-site Catalysts... 

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