Base-stabilized carbocationic specie

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). 

However, there is a more simple way of addressing this sort of problems (viz. ionic vs non-ionic mechanism) that all chemists have learned from textbooks, which seems to have unfortunately been lost recently, at the expense of more sophisticated ones. It is based on the classical SNj (ionic) SN2 (non-ionic) distinction, performed on the basis of solvent polarity effects. Thus an SN2 reaction is almost insensitive to changes of the dielectric constant of the solvent, whereas an SNj process is highly dependent on this factor, owing to the stabilization of an intermediate carbocationic species. 

Any molecule that could specifically stabilize the cation II will be a base and combine with the proton therefore, the equilibrium is displaced away from II. For other carbocations (such as the alkyl cations), the solvents interacting specifically with the positively charged species will be nucleophiles and combine with the carbocations. This relationship does not mean that carbocations cannot be nucleophilically solvated as discussed recently (34, 35). Interestingly, though, apparently n donors interact preferentially with hydrogen atoms rather than the cationic carbon (36), thus reacting as bases rather than nucleophiles (37). Also, Sharma et al. (34) concluded that nucleophilic stabilization is not the dominant solvation for carbocations in solution. In any event, nucleophilic solvation decreases rather than enhances carbocationic character. On the contrary, any specific interaction between the solvent (SOH) and the anion, such as formation of hydrogen bonds (equation 5), displaces the equilibrium of equation 4 toward formation of carbocations ... 

One of the major drawbacks of the controlled/living cationic polymerization based on stabilization of the carbocationic intermediates is slow propagation. Because the concentration of the active propagating species is very low because of the equdibrium between active species and dormant species, overall polymerization reactions are much slower than those without the equilibrium. Another important drawback of the controlled/living polymerization is the use of additives such as Lewis bases. Such additives remain in polymer products and are generally rather difhcult to remove from the polymer products. 

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