Polymerization of Ethylene and a-Olefins

Surface Ln-silylamido moieties lla- obtained via grafting of homoleptic silyl-amides Ln[N(SiMe3)2]3 on dehydroxylated silica Vulkasil S (BAYER AG, a = 175  

Supported precatalyst Activator (ratio) Activit/ 10 M PDP Reference 

Conditions O.lOOmmol precatalyst, toluene (400mL), 7bar ethylene 1 h, 80°C. Conditions 0.2 mmol precatalyst, toluene (400mL), 7bar ethylene Ih, 80 °C. 

Conditions 0.113mmol precatalyst, toluene (20mL), l.lbar ethylene 2h, 90°C. 

A similar study with materials [Ln N(SiMe3)2 3] AS-380. (T = 250, 500, 700°C) revealed the implications of thermal pretreatment of the support for ethylene polymerization [117]. Materials featuring predominantly monopodal neodymium silylamido surface species produced the most achve catalysts [12b (100% monopodal) 13b (40%) 14b (25%), Table 12.6 cf, Nd[N(SiMe3)2]3 0.03 kg-PE molm h bar ]. Apparently, extensive replacement of silylamido by (surface) siloxo ligands, that is, an increase of podality of the supported species at 

---

Protonation of the TMM complexes with [PhNMe2H][B(C6Fs)4] in chlorobenzene at —10 °C provided cationic methallyl complexes which are thermally robust in solution at elevated temperatures as determined by NMR spectroscopy. In contrast, addition of BfCgFsls to the neutral TMM precursors provided zwitterionic allyl complexes (Scheme 98). Surprisingly, it was found that neither the cationic nor the zwitterionic complexes are active initiators for the Ziegler-Natta polymerization of ethylene and a-olefins. °°... 

The coordination polymerization of ethylene and a-olefins with Ziegler-Natta catalysts involves, in general, many elementary reactions, such as initiation (formation of active centers), chain propagation, chain transfers and chain terminations. The length of growing polyolefin chains is limited by the chain-terminating processes, as schematically represented (for ethylene) by 21,49 51)... 

Radioactive, 4C0 has been used to determine the number of active centers in Ziegler-Natta catalysts for the polymerization of ethylene and a-olefins 31 33>. However, the reliability of this method for active center determination is a matter of discussion 105,106). The argument arises from an uncertainty in the mechanism of the reaction of CO with a metal-polymer bond in the active center. Some model systems have been employed to investigate the mechanism. 

In addition to the familiar and extensive role perfluoroaryl boranes and borates fill as co-catalysts for the coordination polymerization of ethylene and a-olefins, these compounds can act as initiators for other polymerization reactions due to their high Lewis acidity. 

Table 10. Maximum concentration of active species and elementary rate constants for the polymerization of ethylene and a-olefins with supported catalysts... 

Broadly, the polymerization of ethylene and a-olefins by organometallic catalysts is one of the most important catalytic processes in the chemical industry and the Ziegler-Natta catalysis an impressive example of polymerization on and with molecular catalysts per se or on a support. The detailed understanding of this fundamental catalysis on all relevant chemical and physical scales, the molecular level of the catalyst structure, the microkinetic mechanistic reaction paths, and the macrokinetic reaction engineering level of the total polymerization process, reflects a story of innovations in science and technology that is still today undergoing further development. 

Gas-phase polymerization of ethylene and a-olefins using a dual-reactor process the CSTR for the preparation of pre-polymer and FBR for the polymerization step ... 

Scheme 10. Some types of catalytic centers proposed for the stereospecific polymerization of ethylene and a-olefins...

---