Dormant site

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. 

This mechanism, although understandable in a conceptual sense, is not fully understood in a mechanistic sense. For instance, the exact nature of active species and the role of the activator and/or counterion is a subject of debate this concerns methylaluminoxane-activated group 4 metallocene systems in particular. Methylaluminoxane may act to generate the active species and remove impurities from the polymerisation system as well as playing a more fundamental role such as assisting in the insertion of each monomer unit or reactivating dormant sites [358]. 

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). 

Figure 17.11 Proposed structure of active site and dormant site for styrene polymerization (a) Active site (b) dormant site by the coordination error of monomer (c) dormant site by the polymer chain rotation...

It is well known that in Ziegler-Natta and related polymerizations it is virtually impossible to intercept reactions intermediates. High field techniques represent a promising tool also in the direct investigation of catalytic species. Along this line we were able to count active and dormant sites in metallocene-catalyzed polymerization 14). We hope to extend these studies in the next future. 

Figure 1 Catioiuc metallocene alkyls are catalytically active and can form temporarily inactive ( dormant ) sites.

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]. 

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 structure of the dormant sites might be an irregular coordination of the monomer (Rg. 4.10b) or a change of the direction of the monomer coordination (Rg. 4.10c). Tlie polymerization reaction may be stopped after an irregular coordination of styrene. This also supports the chain-end controlled mechanism of stereospecilicity. 

A major benefit of the use of DMCs compared with that of conventional base-catalyzed systems in these reactions is that they produce high-molecular-weight polymers, with very narrow molecular weight distributions, very low levels of unsaturation, and lower viscosities. The downside to using DMCs however is that they require an activation period at elevated temperatures in the presence of initiator molecules, which causes an induction period at the start of the polymerization process [10]. After this induction period, the polymerization process proceeds very rapidly, making it a very important parameter to control. The CAs play an important role in the catalytic activity of the DMCs and in the activation procedure. The zinc sites with bound CAs can be seen as dormant catalytic sites [11]. After exchange of the CA, preferably rerr-butanol, with initiator molecules such as polyfpropylene glycol), the dormant sites are converted into the very active catalytic sites. 

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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