Counteranions, living polymerization

Controlled/Living Polymerizations with Nucleophilic Counteranions... 

Controiied/Living Polymerizations with Added Salts The two approaches discussed above are primarily useful in nonpolar solvents (like toluene and n-hexane) where the interactions of carbocations with nucleophiles are strong and favored. In relatively polar solvents like methylene chloride, these methods often fail to give controlled polymerizations, most likely because the interaction is weaker between the growing carbocations and nucleophiles [whether they are built-in (counteranions) or externally added (esters, etc.)], which facilitates dissociation of the carbocation. The effect of solvent in the latter system, however, is much weaker. 

Initiating Systems with Nucleophilic Counteranions The first generation of initiating systems for vinyl ether controlled/living polymerizations consists of a protonic acid (HB, initiator) with a Lewis acid (MtX , activator), [63,69,70]. The reported examples ofHB and MtX ... 

J. Initiating Systems with Nucleophilic Counteranions For isobutene, this group refers almost exclusively to the BCh-based initiating systems without external additives. As listed in Table 3.A, combinations of BCl3 with tertiary esters, ethers (methoxides), or alcohols induce controlled/living polymerization of isobutene in CH3C1 or other solvents at temperatures below -30° C. Scheme 10 illustrates the proposed pathway for the polymerization initiated with the cumyl acetate (12)/BC13 system [35] ... 

The first reported controlled/living polymerization of p-methylstyrene with acetyl perchlorate in the presence of HBU4NCIO4 [221] was based on the added salt method. Later, it was reported that cumyl acetate/ BC13 and related initiating systems induce controlled/living polymerizations of p-methylstyrene [117,222] and 1,3,5-trimethylstyrene [223]. The HI/ZnC(2 system, suited for vinyl ethers and p-alkoxystyrenes, can also be used for p-methylstyrene [224], but the lower reactivity of the monomer requires a much higher concentration of the zinc activator (ca. 100 mJW for 10 mM HI) to obtain a sufficient polymerization rate. These systems function without added nucleophilic additives and can be classified under the counteranion method. 

As illustrated earlier, the inherent problem in controlled cationic polymerizations is the instability of the macrocarbocations. However, living polymerizations can be realized by stabilizing the growing carbenium ions with a suitable nucleophilic counterion (Be), 9-19, or an added Lewis base (X) containing a weakly nucleu-cleophilic counteranion (Be), 9-20 [7]. That is ... 

A final method to induce living polymerization of vinyl ethers is based on halide exchange reactions by the addition of a salt. The addition of a tetrabutyl ammonium salt with a nonnucleophilic counteranion, such as perchlorate, to an a-halide ether results in an exchange equilibrium between the halide and the perchlorate. The formed perchlorate adduct is also in equilibrium with the carbocationic species inducing living polymerization of various vinyl ethers as depicted in Scheme 8.12 [53-55]... 

As indicated in the above examples, for a specific monomer, the rate of exchange and the position of the equilibrium and, to some extent, the zero-order monomer transfer constants depend on the nature of the counteranion in addition to temperature and solvent polarity. Therefore, initiator/coinitiator systems that bring about controlled and living polymerization under a certain set of experimental conditions are largely determined by monomer reactivity. 

The stabilization of a counteranion was a key to eliminate chain transfer reaction with the BCI3 system for IB polymerization. A tertiary ester was used as an initiator for polymerization of IB using BCI3. Cumyl acetate and 2,4,4-ttimethylpentyl acetate induced living polymerization in CH2Cl2/hexane at-30°C. ... 

Control of polymerization can be difficult with a nucleophilic counteranion if a strong Lewis add is used. An active catalyst would produce more free ionic spedes, some of which cause side reactions. The addition of an appropriate additive such as a Lewis base and a quaternary ammonium salt into a reaction with a strong Lewis acid shifts the equilibrium to induce living polymerization. 

Scheme 3 Living polymerization using a weak Lewis acid and a nucleophilic counteranion.

The addition of a quaternary ammonium salt attains an appropriate equilibrium between the active and dormant species for living polymerization (Scheme 5). The anions of ammonium salts are not necessarily the common ions of the counteranions generated from the initiator and the dormant bond. However, nucleophilic anions (usually halide anions) are required for achieving living polymerization. For example, less nucleophilic anions such as CIO4 have no effea on controlling... 

The success of the HI/I, system suggested the effectiveness of a combination of a nudeophilic counteranion and a relatively mild Lewis add for living polymerization. Thus, cationic polymerization of alkyl VEs was examined using various protonic adds (or an addud of a VE with a protonic acid) with weak Lewis adds. Living polymerization was achieved using hydrogen halides " and acetic adds in conjunction with zinc... 

Lewis acids tend to induce nonliving polymerization even if a nucleophilic counteranion is employed, as mentioned above. Living polymerization was also achieved with some strong Lewis acids in the presence of additives, such as Lewis bases and organoammonium salts. 

This review first emphasizes the strong dependence of the nature of propagating species on their counteranions the last two chapters describe that this characteristic of cationic polymerization in turn enables selective linear dimerization and living polymerization,... 

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). 

As discussed in the preceding sections of this chapter, the key to living cationic polymerization is to reduce the effect of chain transfer reactions (Scheme 4) because termination is much less important in the cationic polymerization of vinyl monomers. The primary reason for frequent chain transfer reactions of the growing carbocation (1) is the acidity of the /3-H atoms, next to the carbocationic center, where a considerable part of the positive charge is localized. Because of their electron deficiency, the protons can readily be abstracted by monomers, the counteranion (B ), and other basic components of the systems, to induce chain transfer reactions. It is particularly important to note that cationically polymerizable monomers are, by definition, basic or nucleophilic. Namely, they have an electron-rich carbon-carbon double bond that can be effectively poly-... 

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