Abstract~-Vinylpyridine was polymerized by cumyl barium in THF at 0°C. Detailed conductance studies at various concentrations of the living oligomer solution gave similar experimental results as for 2-vinylpyridine, i.e. triple ions exist in thermodynamic equilibrium with free ions and ion pairs. The dissociation constant of ion pairs increases from 1.96 x 10-~0 M at 15°C to 4.35 x 10-~0 M at -70°C with enthapy and entropy of dissociation of -3.9 kJ/mol and -200 J mol-L K -~ respectively. The extent of dissociation of ion pairs of living oligo-4-vinylpyridine is comparable to that of living polystyrene (K~ ~-10 ~0) but higher than that of living poly-2-vinylpyridine (K~ = 10-"). This result is interpreted in terms of intramolecular complexation which is no longer favourable when the nitrogen atom is situated at the 4-position of the pyridine nucleus. Studies of propagation kinetics revealed that 4-vinylpyridine polymerizes by the same reaction mechanism as the 2-isomer, the active sites being essentially ion pairs. At 0°C, its propagation constant was found to be 211.1 M-~ sec -~ compared with 151.04 M-I sec-~ for living poly-2-vinylpyridine. As for free radical polymerization, the polymers contain only about 26~o isotactic triads.