AbstractThis paper describes a re‐examination of the kinetics and molecular weights formed in the zwitterionic polymerization of butyl cyanoacrylate by triphenylphosphine (Ph3P) and pyridine (Py) in THF, and an extension to include the solvents, diethyl ether (Et2O), dimethoxyethane (DME) and hexane. In THF, the previously observed kinetics were confirmed, yielding evidence for absence of termination, and values for rate constants for initiation (ki) and propagation (kp) with substantial agreement between the values for Kpobtained from both systems. At 20°C, the mean value found forkpin THF was 3 · 105l · mol−1· s−1; in Et2O, some 10 times lower, and in hexane, 10 times lower again. The temperature dependence ofkpin THF was confirmed to be extremely low and anomalous; in Et2O rather greater and near‐normal, corresponding to an energy of activation of ca. 5 kcal · mol−1. A correction procedure enabled the large experimental scatter in thekpvalues to be reduced sufficiently to reveal trends downwards at higher concentrations of propagating species, i.e. evidence for their dissociation on dilution. In Py‐initiated polymerizations, excess of pyridinium salts did not depresskp; evidence that the propagating species are preponderately paired ions, not free. It is concluded that at 20°C these species are mainly ‘tight’ ion‐pairs in all solvents, and remain so in Et2O at all temperatures down to −80°C. In THF, over this temperature range, there is increasing solvation producing an increasing proportion of more reactive ‘solvent‐separated’ ion‐pairs. The molecular weights produced under all conditions were high (Mnusually 1 to 3 · 106). Those produced by high concentration of Ph3P did not conform to theory; others followed the ‘no‐termination’ theory as modifie