Carbocation stabilization nucleophilic counteranions

Except for some heterocycles (2), living cationic polymerization has been considered almost impossible, particularly for vinyl monomers, which generate unstable carbocationic intermediates. Despite this pessimistic view, we have recently found that living cationic vinyl polymerization is indeed possible by stabilizing the growing carbocations with nucleophilic counteranions or with externally added bases (3). 

As already reported (11), various end-functionalized polymers of vinyl ethers have been prepared through the living cationic polymerization that is based on the carbocation stabilization by the nucleophilic iodide counteranion (Eqs. 7 and 8). The first method (Eq. 7) starts from the addition of hydrogen iodide to a vinyl ether ( ) with... 

The living cationic polymerizations discussed above are invariably based on the nucleophilic iodide counteranion (activation of the carbon-iodine terminal bond Eq. 3). It is expected, however, that similar living processes are equally possible with other counteranions that can exert, as the iodide anion does, a suitably strong nucleophilic interaction with the growing carbocation. We have in fact found the phosphate anions to meet this requirement (10). Similarly to hydrogen iodide, monoacidic phosphate esters [H0P(0)R 2 R alkyl, alkoxyl, etc.] like diphenyl phosphate ( ) form a stable adduct 5) with a vinyl ether (Eq. 5). Zinc chloride or iodide then activates the phosphate bond in 5 by increasing its polarization (as in 6), and living cationic polymerization proceeds via an intermediate (7) where the carbocationic site is stabilized by a phosphate anion coupled with the zinc halide activator. 

When a counteranion is not so nucleophilic as the iodide anion, the propagating carbocation may be stabilized instead by adding an base (Z) so that living polymerization proceeds (Eq. 9 see also section 1.1) (4). This method is particularly effective for the polymerization initiated with ethylaluminum dichloride (EtAlCl2) (13) and typically, the bases may be 1,4-dioxane and related ethers that form a "base-stabilized" carbocationic species like where Z is an ether oxygen. We have recently synthesized end-functionalized polymers via these base-stabilized living species (14). 

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