Dormant catalyst sites

It turns out that the insertion of a next molecule of propene in the branched alkyl metal complex is much slower than the insertion of propene in a regular chain formed after a 1,2 insertion. In several catalysts studied this leads to a situation in which a great deal of the catalyst sites are "dormant", i.e. the metal is tied up in unreactive secondary alkyl metal complexes. If eventually an... 

Another way to recover the catalyst from the dormant site is the copolymerisation of ethene, but this is slower and less attractive than the use of hydrogen. Furthermore the use of ethylene inevitably results in the formation of propylene-ethylene copolymers with all the consequent effects on polymer properties. 

More recently, Landis et al. studied the polymerisation kinetics of 1-hexene with (EBI)ZrMe( t-Me)B(C5F5)3 64 as catalyst in toluene [EBI = rac-C2H4(Ind)2]. Catalyst initiation was defined as the first insertion of monomer into the Zr-Me bond, 65 (Scheme 8.30). Deuterium quenching with MeOD was used to determine the number of catalytically active sites by NMR. The time dependence of the deuterium label in the polymer was taken as a measure of the rate of catalyst initiation. This method also provides information of the type of bonding of the growing polymer chain to zirconium, as n-or sec-alkyl, allyl etc. Hexene polymerisation is comparatively slow, with high regio- and stereoselectivity there was no accumulation of secondary zirconium alkyls as dormant states [96]. 

Cr(VI)/Silica develops polymerization activity only gradually when exposed to ethylene at 100°C in a slurry autoclave. An example is shown in Fig. 5, which depicts an experiment in which the catalyst was not immediately active upon introduction into the reactor, but first underwent a dormant period or induction time. The rate of polymerization then increased during the remainder of the experiment. This is thought to be due to the slow reduction of Cr(VI) by ethylene to the Cr(II) active site, or perhaps to the desorption of by-products such as formaldehyde (32). Thus, the concentration of active sites is probably not constant but increases with time. Below 100°C the induction time becomes longer until at about 60°C there is almost no activity. Conversely, increasing the temperature shortens the induction time. At 150°C the catalyst exhibits an immediate and constant activity in solution phase polymerization. 

Jiingling, S., Mulhaupt, R., Stehling, U., Brintzinger, H.-H., Fischer, D. and Langhau-ser, F., The Role of Dormant Sites in Propene Polymerization using Methylalumox-ane Activated Metallocene Catalysts , Macromol. Symp., 97, 205-216 (1995). 

Catalyst preparation and decomposition 9.3.5 Formation of dormant sites... 

In rhodimn catalyzed hydroformylation the effect is less drastic and often remains imobserved, but surely diene impurities obscure the kinetics of alkene hydroformylation [42]. Because the effect is often only temporary we summarize it here under dormant sites . Hydroformylation of conjugated alkadienes is much slower than that of alkenes, but also here alkadienes are more reactive tiian alkenes toward rhodium hydrides [43, 44]. Stable tc-allyl complexes are formed that undergo very slowly insertion of carbon monoxide (Figure 17). The resting state of the catalyst wBl be a Ji-allyl species and less rhodium hydride is available for alkene hydroformylation. Thus, alkadienes must be thoroughly removed as described by Garland [45], especially in kinetic studies. It seems likely that 1,3- and 1,2-diene impurities in 1 -alkenes will slow down, if not inhibit, the hydroformylation of alkenes. 

Ligand metallation. In early transition metal polymerization catalysis often metalation of the ligand occurs leading to inactive catalysts. In late transition metal chemistry the same reactions occur, but now the complexes formed represent a dormant site and catalyst activity can often be restored. Work-up of rhodium-phosphite catalyst solutions after hydroformylation often shows partial formation of metallated species, especially when bulky phosphites are used [50]. Dihydrogen elimination or alkane elimination may lead to the metallated complex. The reaction is reversible for rhodium and thus the metallated species could function as a stabilized form of rhodium during a catalyst recycle. Many metallated phosphite complexes have been reported, but we mention only two, one for triphenyl phosphite and rhodium [51, 52] (see Figure 19) and one for a bulky phosphite and iridium [53]. 

Deactivation. One of the factors that complicates the quantification of active-site concentration (135) is the fact that metallocene cations are subject to equilibria between catalytically active and inactive forms. In situations in which intramolecular coordination of an arene group can occur, this process competes with monomer coordination in styrene (136) and possibly olefin polymerization. Another dormant state invoked to explain catalyst decay is the dimeric structure [Cp2Zr(CH3)(/u.-CH3)Zr(CH3)Cp2]+ in which a methyl group bridges two metallocene fragments. This has been characterized by NMR for the reaction of Cp2Zr( CH3)2 with MAO and other cocatalysts (136). 

An effective way to lower polymer molecular weights is to add hydrogen as a chain transfer agent (Scheme 1.6c)." " This results in saturated -propyl and isobutyl chain ends for the polymer. For catalysts that produce dormant sites through 2,1-insertion vide supra), hydrogen can also serve to increase productivity. ... 

The Cossee-Arlman mechanism for the polymerization of olefins is the most widely accepted theory but as yet it is not complete. Cossee developed his early ideas of polyethylene growth at a titanium-carbon bond and supported the theory by molecular orbital calculations. The role of the alkyl aluminium co-catalyst was in the generation of the active species, via the alkylation of the titanium chloride bonds, and to remove impurities in both the gas stream and catalyst preparative procedure. There was also the suggestion that it might be involved in the insertion of each monomer molecule, and also in the regeneration of dormant sites or the formation of new active sites. 

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