Nucleophile-vinyl cation reactivity orders

Four different probes gave short reactivity orders toward vinyl cations (1) common ion rate depression in solvolysis (2) competitive capture of solvolytically generated ions (3) direct reaction of a vinyl cation with nucleophiles and (4) competition between intra- and intermolecular nucleophilic capture. A short reactivity order is obtained in each case, but because of the different solvents and conditions the orders cannot be combined to a single series. However, a selectivity rule that governs the relative reactivities toward different vinyl cations in terms of a constant selectivity or a reactivity-selectivity relationship can be determined. 

In principle, for a successful crossover reaction in the sequential monomer addition process, the monomers have to be added from the more reactive monomer (more nucleophilic) to the less reactive monomer (e.g., VE>aMeSt>St>IB). The monomer reactivity order in cationic polymerization is based on the electron-donating ability of the pendant groups of the vinylic double bond. However, this is not sufficient, and in most of the cases tuning of the Lewis acidity is also required. The synthesis of poly(a-methylstyrene)-l7-polyisobutylene (PaMeSt-l -PlB) is a characteristic example (Scheme 13). 

The same authors chose another very reactive nucleophilic function, the silyl enol ether group, which upon reaction with living cationic chain ends of poly(vinyl ether)s, also leads to a carbon-carbon bond with formation of a ketone (Scheme 4). Model reactions of living poly(IBVE) with various monofunctional silyl enol ethers [47] showed that the a-substituent R should have electron-donating properties in order to increase the electron density on the double bond. 

Alkenes polymerize cationically by electrophilic addition of the monomer to a growing carbenium ion. Therefore, the monomer must be nucleophilic and capable of stabilizing the resulting positive charge. In addition, the double bond must be the most nucleophilic functionality in the monomer. Some vinyl monomers which can polymerize cationically are listed in Eq. (34) in their order of reactivity, which corresponds to the electron-donating ability of their substituents. 

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