Polymerisation with Heterogeneous

The polymerisation of olefins in the presence of Ziegler Natta catalysts (with an organometallic activator, e.g. AIR3) involves individual reaction steps, which may be presented as follows [240]  

In the case of a-olefin polymerisation, the monomer insertion into the Mt C bond in the propagation step can proceed by two pathways - via monomer 1,2-enchainment (primary insertion) or 2,1-enchainment (secondary insertion)  

Where no specific chain transfer agent has been added to the polymerisation system, three chain transfer reactions are usually considered transfer with the monomer [scheme (36)], transfer with the trialkylaluminium activator [scheme (37)] and spontaneous transfer [scheme (38)]. The transfer with the monomer and the spontaneous transfer involve a reaction of /1-hydrogen elimination from the growing polymer chain, whereas the transfer with the activator is the exchange reaction of substituents at two metals [240,241]. 

The relative importance of the chain transfer process depends on the polymerisation conditions and on the kind of catalyst used. For the MgC /TiCU AIR3 catalyst, over a large range of activator and monomer concentrations, chain transfer with the monomer [scheme (36)] is by far the most important chain transfer reaction. Other chain transfer reactions, particularly the spontaneous transfer, become detectable only under extreme conditions [241,248]. With TiCU—ZnR2 catalysts, chain transfer with the activator appeared to be the most significant chain transfer reaction [249]. 

The chain transfer reaction with hydrogen is as follows  

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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],... 

Termination of the olefin polymerisation with heterogeneous Ziegler-Natta catalysts by the addition of carbon monoxide to the system is often used in the laboratory to determine the active centres of the catalyst. 

For the majority of olefin polymerisations with heterogeneous Ziegler Natta catalysts, the polymerisation rates, Rv, are proportional to the concentrations of procatalyst (MtX ) and monomer (M), but do not depend on the concentration of alkylaluminium activator (A) as long as a threshold concentration is maintained [37] ... 

Polymerisation with Heterogeneous Ziegler-Natta Catalysts... 

The mechanism that is commonly considered to operate in the polymerisation of ethylene and a-olefins in the presence of group 4 metallocene-based catalysts is that devised by Cossee [268, 276, 277] for propylene polymerisation with heterogeneous Ziegler-Natta catalysts, though modifications invoking effects such as a-agostic hydrogen interactions with the metal centre have been proposed [343,344]. 

The described chain migratory insertion mechanism, which operates in olefin polymerisation with metallocene-based single-site catalysts, follows that proposed by Cossee [268,277,278] for olefin polymerisation with heterogeneous catalysts there is, however, no back skip of the polymer chain to the previously occupied position prior to the coordination of the next monomer molecule, but rotation of the chain around the axis of the Mt-CH2 bond takes place (Figure 3.19) [358],... 

Activities of a-olefins in insertion polymerisation with coordination catalysts are generally related to the monomer structure both electronic and steric factors influence the activity of a-olefins in stereospecific polymerisation, but the steric factor primarily influences the rate of polymerisation, which has been especially shown for isospecific polymerisation with heterogeneous catalysts [46, 250],... 

Styrene polymerisation with heterogeneous Ziegler Natta catalysts activated by alkylaluminium compounds generally produces a mixture of isotactic and non-stereoregular polymer. For example, polystyrene produced with the... 

Table 4.1 Relative reactivity of vinylaromatic monomers in coordination polymerisation with heterogeneous Ziegler-Natta catalyst ...

In view of these results, it was suggested that syndiospecific and isospecific polymerisations with heterogeneous catalysts containing a chlorine atom in the titanium compound or in the support are promoted by homogeneous species formed in the polymerisation system and by heterogeneous species respectively. 

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