With gegenion

Chain transfer reactions in THF polymerizations have not been considered until rather recently. Compounds known to be effective chain transfer agents include dialkyl ethers, orthoesters, and water. In addition, chain transfer to polymer and with gegenion is possible. 

With gegenion. A form of chain transfer may also occur by reaction with gegenion. For a general Lewis acid gegenion, MtX +1, the reaction would be ... 

From a chemical standpoint there is little evidence at present to prefer PF6 over SbFe or vice-versa. Both gegenions seem considerably more stable than BF4 or SbCl6". The BF4 polymerization contains considerable termination, and the SbCl6" polymerization contains considerable transfer and some termination. It is interesting to note that both results can be explained in terms of a similar reaction with gegenion. 

All our studies were made at 30°C. Data in the literature suggest that as the temperature is lowered, these differences will become less because reaction with gegenion becomes much slower. On the other hand the differences will become even more pronounced as the temperature is raised above 30 °C. Thus, for high temperature studies the use of PF6 or SbF6 is clearly indicated. 

The advent of x-ray crystallography has permitted the mapping of the atoms of molecules in the solid state. The relevance of these conformations to solution phenomena is, however, obscure. In the crystal, the molecules are closely packed, interacting with each other and with gegenions if present. This is probably not the situation normally encountered in the dilute solutions of the biological milieux. Thus, biological conclusions derived from x-ray-derived conformations must always be considered in this light. 

Quinoxaline (see Section 2.1.3) and many of its derivatives may be converted into A-alkylquimoxalinium or even A,A -dialkylquinoxalinediium salts by treatment with alkyl halides or the like. Occasionally, when the molecule bears a suitable acidic grouping, it may be possible to deprive the quaternary salt of its gegenion by treatment with a base to form an ylide (in which a carbon atom of the molecule bears the negative charge) or other zwitterionic entity, such as a quinoxaliniumolate. 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

The simplest example of carbenium ion is CH which is so unstable that it is rarely formed even in gaseous phase. The more stable ones have been prepared in solution and in some cases even in the solid state. When in solutions they are formed in polar solvents where they get solvated. They may also be associated with a negative ion called gegenion. The ion pairs are formed in non polar solvents. 

The e.s.r. spectra of phosphorus compounds have been reviewed.101 The phosphorus hyperfine splitting (ap 33.5 G) of the radical anion (77) is within the 25—36 G range of phosphorin radical anions.102 The cis- and fra/w-isomers of 1,2-bisdiphenylphos-phinoethylene gave the same radical anion (78). The unpaired electron is coupled to all the protons in the molecule as well as to the two phosphorus atoms, and shows that the electron is completely delocalized. Only when caesium was used as the gegenion in THF could a metal interaction be detected. The spectrum in this case corresponded to the association of two caesium ions with the radical anion, the third... 

The first species produced in cationic polymerizations are carbocations, and these were unknown as such prior to World War II. It is now known that pure Lewis acids, such as boron trifluoride and aluminum chloride, are not effective as initiators. A trace of a proton-containing Lewis base, such as water, is also required. The Lewis base coordinates with the electrophilic Lewis acid, and the proton is the actual initiator. Since cations cannot exist alone, they are accompanied by a counterion, also called a gegenion. 

Polymer production proceeds as described in structure 5.25. An initiator, such as sulfuric acid, produces an oxonium ion and a gegenion. The oxonium ion then adds to the oxirane, ethylene oxide, producing a macrooxonium ion with growth eventually terminated by chain transfer with water. 

All discussed Ksem s are related to the separately solvated members of a two step redox system. In solvents of low polarity the charged forms may form ion pairs. Especially prone to this association are anionic redox systems of Type C (RED + OX = 2 SEM ) since the often used gegenions K , Na and Li tend to form ion pairs with the anions. These exhibit special UV/VIS-, NMR- and ESR-spectra as well as g-values Dimeres of the type (SEM M )2 may also be formed, as demonstrated with the anion radicals of pyrazines ° heptafulvalene and tetracyano-quinodimethanes. Corresponding associations are reported for dian-... 

This past decade has seen numerous controversial studies regarding electrical conduction of DNA. Some reported high conductivity [115, 116, 118] with Crt of at most lO" S cm [115] or even superconducting properties [119], while others claimed that the carefully deionized DNA molecules are insulating [117, 120] in agreement with the old reports [121, 122] with ctri- less than 10 S cm. The controversy seems to have settled on a wide consensus that, apart from ionic conduction by the sodium gegenions, double-stranded DNA is an electrical insulator. 

Figure 9 gives the apparent rate constants of propagation (kp) as a function of the inverse square root of the living end concentration with and without an electric field for styrene with Li+ gegenions in binary... 

Like the propagation reaction, this probably occurs by a nucleophilic attack by the THF oxygen atom on a carbon alpha to the oxygen of the trialkyl oxonium ion salt. Commonly the triethyl-oxonium ion with a tetrafluoroborate gegenion has been used (3, 24, 32). Meerwein (3) has also used the hexachlorantimonate, tetrachloroferrate, and tetra-chloroaluminate gegenions. 

But at room temperature and above the BFJ gegeion is not so stable. Meerwein (3) has proposed that it is possible for the BFJ gegenion to react with the growing oxonium ion and lead to termination in the following way ... 

The possibility of this reaction probably accounts for the fact that at room temperature and above the yields reported in the bulk system using BF3 based catalysts all have led to a low ceiling temperature. It should be noted here that Rozenberg et al. (31) do not believe that the termination reaction is important with the BFJ ion. This is hard to reconcile with the fact that at 25° C and above their ultimate conversions are 5—10% lower than those obtained with the PFJ gegenion. 

There are some indications that at 70—80° C there may be some termination occurring even with the PFg gegenion. In polymers produced at 70° C. Sims (26) was able to detect some fluorine which he suggests may have formed from a reaction of the type given below ... 

Stability as used in this sense refers to the ability of the gegenion to avoid Chemical reaction with the oxonium ion itself or with other... 

With acyclic ethers. In a study of THF polymerization using PFjT gegenions Dreyfuss (25) show that in the presence of dialkyl ethers chain transfer occurs and continues to occur after equilibrium is reached. The ultimate conversion to polymer is not affected but the intrinsic viscosity of the polymer decreases with time (Fig. 6). The reaction involved is essentially the reverse of the initiation reaction with trialkyl oxonium salts (equation 5). In the case of transfer the dialkyl ether reacts with the propagating oxonium ion to give a trialkyl oxonium ion which has one long chain alkyl and two short alkyls derived from the ether. 

The cationic polymerization of isobutylene (12) and styrene (13) is initiated readily by Et2AlCl in the presence of an alkyl halide, RC1. The interaction of the catalyst and cocatalyst is presumed to produce the carbonium ion R+, which initiates polymerization, and the corresponding gegenion Et2AlCl2". Alkyl halides with low R-Cl bond dissociation energies—e.g. tertiary, substituted allylic, and benzylic halides—are among the most effective cocatalysts. 

Precursor polymer 36d, with acetate gegenions, required significantly higher elimination temperatures than did the other polymers in the series. Thermal... 

Enol sulfonates. Sulfonylation of enolate anions with 1 is markedly affected by the gegenion. Lithium enolates undergo mainly C-sulfonylation. Cesium or quaternary ammonium enolates undergo regioselective O-sulfonylation. The same behavior is observed with nonailuorobutanesulfonyl fluoride.1... 

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