Organic Cation Salts

Organic Cation Salts.—In general, organic cation salts, which can be preformed or generated in situ, operate through relatively simple initiation mechanisms. Consequently the efficacy of these initiators can often be traced directly to structural features in the cation, the counterion, or the monomer. 

Significant differences in reactivity were observed between p-methylbenzyl bromide and benzyl bromide in the formation of carbenium ions by reaction with silver hexafluorophosphate. The former compound in conjunction with AgPFj was found to be a highly efficient initiator for the cationic polymerization of tetrahydrofuran (THF) at —10 °C. In situ formation of the benzyl ion was much slower, rendering syntheses of monodisperse poly(THF) impossible. 

Theoretical calculations of the stabilities of various hexahaloantimonates have shown that the rupture of an Sb-Cl bond requires more energy than that of an Sb-Br bond, and the relative positions of chlorine and bromine atoms within a mixed halogen counterion can affect the ease of bond breakage. Model compound studies of the reaction between p-methyl benzyl chloride, triethyl aluminium, and 2,4,4-trimethylpent-l-ene (dimeric isobutylene) indicated that the principal chain termination reactions involve hydridation and ethylation resulting from counterion decomposition.  

Pulse radiolytic techniques facilitated estimates of the rate constants for reactions between benzyl and diphenylmethyl carbenium ions and a series of alkenes and 

Coilomb, A. Gandini, and H. Cheraidame, Markomol Chem., Rapid Commun., 1980,1,489. 

Organic Cation Salts.—Cationic salts can be generated in solvents of low polarity from a wide variety of organic substrates and either isolated or used in situ as polymerization initiators. Recent work has concentrated on the development of carbenium and oxonium ion salts. 

Several alkyl, allyl, and benzyl halides were found to be ef dent initiators for tetrahydrofuran (THF) polymerization when allowed to react with AgPF, or AgC10 . The time required for reaction (2) to occur was measured and the 

Oxonium ion salts can be generated by the interaction of carbenium ion salts and heterocyclic monomers, and this technique has been employed in the synthesis of 4,5- and 2,4,5-dioxolan-2-ylium salts according to equation (3).  

The structure of the substituent R has been observed to exert a strong effect on the rate of trioxane polymerizations induced by these salts.  

The nature of the counterion in an oxonium ion salt initiation affects the stability of the active centres during polymerization. Even relatively stable counterions such as SbCle undergo decomposition in polymerizations of cyclic ethers.  

---

Ledwith, A. and Sherrington, D. C. Stable Organic Cation Salts Ion Pair Equilibria and Use in Cationic Polymerization. Vol. 19, pp. 1 — 56. 

In cationic polymerisation no generalised picture has yet emerged, though the use of stable organic cation salts (15) has made a useful contribution. 

Stable Organic Cation Salts C.2. Relevant Results... 

Stable Organic Cation Salts F. Oxazolines and Oxazines... 

Stable Organic Cation Salts Ion Pair Equilibria and Use in... 

---