Polymerization, activation coordination

Conjugation between the imino and acyl [121], or nitrile (in complex 1.47) [122] moieties permitted the remote activation of nickel. Another catalyst (1.48) exhibiting coordination via an alkenyl moiety is noteworthy because of its sterically small size, which should prohibit the production of high molecular weight polymers [123], This is believed to be possible because of the catalyst s unique electronic properties. As bulkier imino-aryl substituents are introduced, polymerization activity and polymer molecular weight increases, as expected [124],... 

Since a positive correlation between the electrophilicity of the active sites and ethylene-polymerization activity can be expected and has actually been observed in a metallocene activated with homologous perfluoroarylboranes, counteranions that are sterically and electronically less coordinative have been pursued in order to render the active species more electrophilic. To this end, many sterically encumbered activators incorporating extended conjugation... 

Solvent polarity has a significant impact on the nature of ion pairing. The large acceleration of polymerization activity that is sometimes observed in halogenated solvents can be attributed to enhanced ion separation in more polar media, and to possible weak coordination of the solvent to the metal cation. ... 

There is evidence that each active center which initiates a polymeric chain (coordination complex between the titanium salt and a metal-ulkyl... 

In this equilibrium the Nd-species to which a diene is coordinated is active in polymerization, whereas the Nd-species to which an arene is coordinated is inactive. According to the authors the experimentally determined ranking of activities toluene > mesitylene > toluene (+ 7% hexamethylbenzene) correlates with the electron richness (i.e. Lewis basicity) of the aromatic compounds. The polymerization activity decreases with increasing Lewis basicity of the aromatic compound as the equilibrium is shifted and the concentration of the active species is reduced. These considerations were supported by the following experimental results (Table 18). 

Presently, the importance of Nd allyl compounds as intermediates in Nd carboxylate- and other Nd-based catalyst systems is widely accepted. As various Nd allyl compounds have been synthesized, characterized and successfully tested as polymerization catalysts this view is supported by solid experimental evidence (Sect. 2.1.1.5 and the references therein). Selected Nd allyl compounds exhibit significant polymerization activities without the addition of cocatalysts. In these cases the active species is neutral. But also cationic active Nd species are taken into consideration (Sect. 2.1.1.5) [288,291]. Cationic species also prevail in the presence of non-coordinating anions. 

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

The main conclusions are that (i) Fivefold coordinated Cr3+ sites exposed on the (0112) faces, although reactive in the formation of weak molecular Cr3+-ethene complexes, are not active in catalytic polymerization, and (ii) polymerization (and oligomerization) activity is attributed only to Cr2+ centers, located at structural defects (such as edges, steps, and corners). This last conclusion strongly suggests that a highly coordinatively unsaturated state is a necessary prerequisite for the polymerization activity of Crx+ centers. 

Flere MAO first generates the dimethyl complex 6.26 from 6.25. This reaction, of course, can also be brought about by Me3Al. It is the subsequent reaction (i.e., the conversion of 6.26 to 6.27 that is of crucial importance. The high Lewis acidity of the aluminum centers in MAO enables it to abstract a CH3 group from 6.26 and sequesters it in the anion, [CH3-MAO]. Although 6.27 is shown as ionically dissociated species, probably the anion, [CH3-MAO], weakly coordinates to the zirconium atom. It is this coordinatively unsaturated species, 6.27, that promotes the alkene coordination and insertion that are necessary for polymerization activity. 

Decay of ionic and coordination centres always leads to the formation of some end groups and centre residues. The centres usually lose their polymerizing activity on contact with atmospheric humidity. A residue of very active centres, which are rare, is usually not removed from the polymer (e.g. of the order of one ppm of the transition metal in low-pressure polyethylene). Larger residues have to be washed out (some types of polypropylene are still washed at the present time). 

The amino-pyrrolate complex 99 is another case of a complex with coordination 4 and two coordinating N atoms. In the temperature range 0-50 °C, the 99/MAO catalyst oligomerizes ethylene (/t/w < 8000, Mw//l/n < 2.8) with high activity (< 8x 102gpK(mmol M)-Ih bar ), whereas the monoligated imino-pyrrolate complex 100 shows moderate polymerization activity (19g (mmol M)-1 h 1 bar-1).1086... 

A few other complexes with N-N coordinating atoms have been tested as olefin polymerization catalysts. The aminopyridinato complex 181 is only able to oligomerize propylene and 1-butene (Mn < 3500) with low activity.1222 The /3-diketiminato Zr complex 182 exhibits moderate ethylene polymerization activity ( 50gPE(mmol M)-1 h-1 bar-1), but molecular masses were disappointingly low (Mn up to 30000).1117 Finally, at 100°C, the MAO-activated Ti complex 183, with a tridentate triazacyclononane ligand, yields PE with very high activity (104gPE(mmol M)-1 h-1 bar-1) but rather low molecular masses and broad polydispersity (Mn = 39500 Mw/Mn = 7.0).1223... 

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