Cocatalysis, Cocatalyst

In order to generate the active Nd-species the Nd precursors are activated by cocatalysts or activators . Some aspects about cocatalysis were already addressed in the previous Sects. 2.1.1.1-2.1.1.10. In this work qualitative aspects on cocatalyst components and cocatalysis are summarized from a general point of view. Quantitative aspects and the impact of cocatalysts on polymerization rate and polymer properties are reviewed in Sect. 2.1.4. 

Titanium tetrachloride is probably one of the most extensively studied Lewis acid and some attempts at answering the questions posed alxjve can be made. The theory of cocatalysis was firmly establiriied by Polanyi s schcml and later in Plesch s laboratory with the system i )butene-titanium tetrachloride °. If one wants to draw any conclusion concerning the possible generalisation of this theory, the details of the reactions among catalyst, cocatalyst and monomer must be understood. This is by no means simple. Given the overall reactions ... 

The number of systems which have clearly been shown not to require the aid of a pro-togenic cocatalyst in the field of cationic polymerisation of olefins promoted by Lewis acids is so large that direct initiation has become an important alternative mechanism to cocatalysis. Kennedy reviewed some of these systems in Obviously, before... 

As we have already mentioned in the preceding sections, water is often a more powerful cocatalyst than hydrogen halides. This observation clearly excludes the possibility that the general cause of cocatalysis by water originates from the hydrolysis of the Lewis acid,... 

Masuda et studied the polymerisation of phenylacetylene catalysed by tungsten hexachloride and molybdenum pentachloride in benzene at 30 °C. Water cocatalysis was qualitatively established, although the authors claimed that the monomer itself could also act as a cocatalyst. This conclusion is however in apparent ccxitradiction with the fact that polymer yields were incomplete. The only way to reconcile these two phenomena would be that terminating species were produced capable of inhibiting further initiation. 

This section deals with the difficult problem of establishing the most plausible initiation mechanism for polymerisation systems in which cocatalysis is ascribed to alkyl or aralkyl halides. This type of cocatalysis is by no means a general phenomenon. We have already mentioned in preceding sections dealing with other types of cocatalysis that, for example, r-butyl chloride is not a cocatalyst in the polymerisation of isobutene induced by titanium tetrachloride and that 1-phenylethyl chloride is not a cocatalyst in conjunction vdth stannic chloride - Indeed, the reaction... 

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