Retardation and inhibition of polymerization

There is no adequate kinetic treatment for this aspect of coordination polymerization although many compounds have a major influence on the reactions. Some have a retarding effect, e.g., water, alcohols, carbon dioxide [101], while others accelerate polymerization. Examples of the 

Donor molecules such as thioethers and tertiary amines, may accelerate or retard the rate [106, 107], and there may be changes in the microstructure of the polymers [63, 108]. 

Those additives which are chemically transformed by the catalyst have effects which depend on the order of addition to the system. For example, water in small amounts reacts with aluminium alkyls to form aloxanes, (R2A10)2, which are effective catalysts, but if added first to a-TiCl3 it results in an induction period proportional to the amount added followed by acceleration up to the normal rate as the coordinated water is removed from the catalyst sites. A detailed examination of the kinetics of the effect of water has been carried out by Kissin et al. [109]. The rate of removal of adsorbed water follows the law 

Methanol has an effect which is more complex. There is an initial retardation resulting from sites on the surface being blocked, giving rise to proportionality between ((Ep)o p)/(-Rp)o and [D]/[T] (Fig. 7), [D] 

Vesely [93] has expressed the effects of a number of additives in terms of competitive adsorption at the catalyst site by the insertion of a term Ki, [D] in the denominator of eqn, (6). Values of quoted for propene polymerization by a-TiCl3/AlEt3 are given in Table 2. However, if the additives are removed by chemical reaction the use of an equilibrium constant as a measure of their ability to coordinate with the catalytic centre would appear to be a considerable over-simplification. 

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