Anionic polymerization with alkaline earth counterions—I. Propagation kinetics of living poly-2-vinylpyridine

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Propagation kinetics of the homopolymerization of 2-vinylpyridine initiated with cumyl barium and with difunctional dimeric 1,1-diphenylethyl barium in tetrahydrofuran (THF) are reported. Conductance studies on the monofunctional living polymer solution, at concentrations from 10 −3 to 10 −5 M over the range 15 to −70°C, revealed the presence of triple ions in thermodynamic equilibrium with free ions and ion pairs. The dissociation constant of ion pairs decreases from 1 × 10 −11 M at 15°C to 0.7 × 10 −11 M at −70°C with an enthalpy of dissociation of 2.5 kJ/mol. Comparison with the analogous system of monofunctional polystyryl barium led to the conclusion that the living polymer of 2-vinylpyridine is 10 times less dissociated. Occurrence of intramolecular complexation, as observed with sodium and magnesium cations, thus appears much less pronounced in the presence of the barium cation on account of its relatively large radius. Kinetic studies showed that living poly-2-vinylpyridine, irrespective of its functionality, propagates essentially via ion pairs. The corresponding constant of propagation ranges from 294 M −1 sec −1 at 22°C to 20.7 M −1 sec −1 at −50°C with an activation energy of 20 kJ/mol. The polymers contain more than 50%, isotactic triads.