Tropylium hexachloroantimonate

Polymerizations initiated by ionizing radiation or stable carbocation salts such as trityl or tropylium hexachloroantimonate are useful for evaluating the free-ion propagation rate constant. Ionizing radiation yields free ions (in the absence of ion pairs) whose concentrations... 

When using a cation source in conjunction with a Friedel-Crafts acid the concentration of growing centers is most often difficult to measure and remains unknown. By the use of stable carbocation salts (for instance trityl and tropylium hexachloroantimonate) the uncertainty of the concentration of initiating cations is eliminated. Due to the highly reproducible rates, stable carbocation salts have been used in kinetic studies. Their use, however, is limited to cationically fairly reactive monomers (eg, IV-vinylcarbazole,/>-methoxystyrene, alkyl vinyl ethers) since they are too stable and therefore ineffective initiators of less reactive monomers, such as isobutylene, styrene, and dienes. 

The salt formulated crystallizes from nitromethane in yellow prisms, m.p. 218°. Reaction of antimony pentachloride with cycloheptatriene in carbon disulfide similarly affords tropylium hexachloroantimonate, m.p. 190°. 

Tropylium hexachloroantimonate, 330 Tsugaeff reaction, 123 Tungsten carbonyl, 207 Twistane, 33... 

Two-step hydride shifts to perchlorocarbenium ions. The expected hydride-ion transfer (150) between cycloheptatriene and PDM occurs rapidly at room temperature, a//-undecachlorodiphenylmethane (PDM—H) and tropylium hexachloroantimonate being isolated (Ballester et al., 1971c, 1982a). 

Correspondingly, the reaction of cycloheptatriene with the carbenium ion PTM, which is much more shielded at the a-carbon than PDM, gives a quantitative yield of radical PTM- and tropylium hexachloroantimonate no a-H compound (PTM—H) is formed. This result, which implies the formation of radical-cation C7Hgt is undoubtedly due to severe steric shielding toward C—H bond formation (Ballester et ai, 1971c, 1982a Carreras, 1989). 

The initiator cation formed in this way can, in many cases then add directly on to the monomer to form the monomer cation. Examples of this are the polymerization of p-methoxystyrene or epoxides with trityl hexa-chloroantimonate, the polymerization of tetrahydrofuran with acetyl perchlorate, and the polymerization of vinyl ethers and A -vinyl carbazole with tropylium hexachloroantimonate, for example ... 

Trichloromethylmesitylene allowed to react with SbClg in CCI4 under anhydrous conditions carhonium salt (Y 80-85%) treated at -8° with cycloheptatriene dichloromethylmesitylene (Y 85.2%) and tropylium hexachloroantimonate. F. e. s. H. Volz and M. J. Volz de Lecca, Tetrah. Let. 1965, 3413. 

Not all organic salts are able to initiate the polymerization of NVK. While stable ammonium salts such as benzyltrimethylammonium hexafluoroantimonate do not initiate the polymerization, stable ammonium and tropylium salts will. A kinetic study of NVK polymerization initiated by stable tropylium hexachloroantimonate and perchlorate salts provided the first propagation rate constants for that monomer [440]. The authors showed that initiation involves direct addition of the tropylium ion to NVK via formation of a charge transfer complex [441]. This mechanism was proved by isolation of the corresponding methanol adduct. In this particular system the initiation reaction was almost instantaneous and quantitative, and no chain termination was observed. Under these conditions the propagation rate constants can be determined from conversion-time... 

A number of years ago triphenylmethyl cation, Ph3C, formed in situ by dissociation of triphenylmethyl chloride, was shown [73] to initiate the polymerization of 2-ethylhexyl vinyl ether in m-cresol solvent. More recently certain stable carbonium ion salts, notably hexachloroantimonate (SbCls) salts of cycloheptatrienyl (tropylium, C7H7) and triphenylmethyl cations have been shown [74, 50] to be very efficient initiators of the cationic polymerization of many reactive monomers [27, 29, 75]. Since the discovery of the effectiveness of the SbClg salt, triphenylmethyl salts with different anions have also been used [76—78]. The most detailed kinetic studies using these initiators have been carried out on alkyl vinyl ethers [27, 30] and A-vinylcarbazole [39] in homogeneous solution in methylene chloride. 

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