Active centres ionic nature

Chain reactions do not continue indefinitely, but in the nature of the reactivity of the free radical or ionic centre they are likely to react readily in ways that will destroy the reactivity. For example, in radical polymerisations two growing molecules may combine to extinguish both radical centres with formation of a chemical bond. Alternatively they may react in a disproportionation reaction to generate end groups in two molecules, one of which is unsaturated. Lastly, active centres may find other molecules to react with, such as solvent or impurity, and in this way the active centre is destroyed and the polymer molecule ceases to grow. 

The situation has already been described in the preceding paragraph where one of the combining ions comes from the added electrolyte. When both combining ions are natural components of the polymerizing system we have a case of pure termination. Combination of the counter-ion with the active centre is the reason why many sufficiently strong acids and bases cannot be used for the initiation of ionic polymerizations. 

A key problem of the catalytic polymerisation theory is known to be that of the AC structure. Experimental studies and theoretical developments in the ionic and ionic-coordination polymerisation of unsaturated compounds revealed a considerable amount of new information about the nature of AC, for example [50-53]. The most important concept is the polycentrism of cataljnic systems as this problem is currently becoming much more important [49]. Researches have advanced from single examples of active centre types, in some catalytic systems, to an awareness of the fact that in some rare cases, the catalytic system can be considered to include only one type of active centre. 

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