Bernard François

Dicumylbarium. A novel initiator in anionic polymerization. Its physicochemical properties in tetrahydrofuran and tetrahydropyran

Abstract The preparation of dicumylbarium by reaction of methyl cumyl ether with finely-divided barium in tetrahydrofuran (THF) and tetrahydropyran (THP) is described. The salt was characterized by spectrophotometric and chemical methods. Conductance studies for 10−3 to 10−5M concentrations of the salt in THF at temperatures from 15 to −70°C and in THP from 15 to −30°C demonstrated the presence of triple ions in thermodynamic equilibrium with free ions and pairs. In THF the dissociation constant varies from 1.0 × 10−9M at 15°C to 2.3 × 10−9M at −70°C, the corresponding enthalpy and entropy of reaction being −1.1 kcal mol−1 and −45 e.u. respectively. In THP, however, dissociation of the salt is too weak to be determined directly. Kinetic studies of the protonation reaction between dicumylbarium and triphenylmethane in THF showed that the apparent rate of reaction is 1.65 × 10−5 sec−1, independent of carbanion concentration. This indicates that protonation proceeds exclusively via free ions and hence provides further evidence for the presence of triple ions in dicumylbarium solutions.

Solvation of ion pairs of barium and strontium salts of α, ω-bis(9-fluorenyl)polymethylenes in tetrahydrofuran

Abstract The conversion of tight ion pairs into loose (solvent separated) ion pairs for strontium and barium salts of the dicarbanions Fl − (CH 2 ) n Fl − (where Fl − is 9-fluorenyl and n equals 2, 4 or 6) was studied spectrophotometrically in tetrahydrofuran as a function of temperature, and compared with the behavior of the barium and strontium salt of the monocarbanion 9-n-butylfluorenyl. On cooling to −80°C the barium salts, F l − , Ba 2+ , F l − , for n equals 4 and 6 are converted to the mixed tight-loose ion pair F l − ∥Ba 2+ , F l − (ΔH =−8 kcal/mol, ΔS = −36 eu). The strontium salts of these two compounds are already fully separated ion pairs below −30°C, i.e., F l − ∥Sr + ∥ F l − . However, the dicarbanion salt with n equals 2 remains a tight ion pair down to −110°C, at least when barium is the counter ion. With strontium this compound is also difficult to solvate, but at −100°C the tight ion pair slowly converts to a fully solvated loose ion pair complex, a process which is followed by precipitation of the salt. The possible cause of this peculiar behavior is being discussed.