Polarisability relation with reactivity

It now remains to place the concept of an ester as an active species into a wider chemical context, with special reference to polymerisation catalysts. Sinn and Patat [39] have emphasised the distinction between monofunctional and bifunctional catalytic systems and this distinction is obviously and necessarily related to the idea, explained above, that there is a difference in kind between polarised molecules and the ions which can be formed from them. Whereas the carbonium and other cations as reactive species are monofunctional, the esters evidently belong to the class of bifunctional catalysts their mode of action - the addition of their constituent parts across a double bond - is, in modern terminology, an insertion reaction. In this context, we must note the important... 

Nucleophilic aromatic substitution reactions have been examined in terms of a model based on the combination of a cation with an anion. The reactivities of 2,4-dinitro-halogenobenzenes with nucleophiles have been shown to be related to the basicity and polarisability of the nucleophile and the polarisability of the substrate only atoms and bonds at or near the reaction centre are involved in producing the polarisability effects. Hammett cr values have been measured for unsaturated groups —CH=NX in the 4-position for nucleophilic displacement of chlorine in 2-nitrochlorobenzene derivatives and compared with those for established activating groups. ... 

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