The anionic polymerization of 4-cyano-alpha-methylstyrene 1 was carried out in THF with (diphenylmethyl)potassium (Ph2CHK) as an initiator. At the various temperature ranging from 0 to -78 degrees C, conversion and equilibrium monomer concentration, [M](e), were obtained from the GLC analysis of the residual monomer. From the plot of In [M](e) against reciprocal temperature, the thermodynamic parameters, Delta H and Delta S, and the ceiling temperature, T-c, of the anionic polymerization of 1, were determined to be -7.64 +/- 0.5 kcal mol(-1), -25.5 +/- 1.3 cal mol(-1) K-l, and 27 +/- 3 degrees C, respectively. The apparent rate constant and the activation energy of the anionic polymerization for 1 were determined as follows : In k(p)(ap) = -1.83 x 10(3)/T + 5.74 L mol(-1) s(-1) and E-a = 3.6 +/- 0.2 kcal mol(-1), respectively. Depolymerization of the living poly(1) proceeded to give a low molecular weight oligomer and starting monomer when the temperature of the polymerization system was raised from -78 to 0 degrees C. The resulting poly(1)s produced at low temperatures (-30 to -78 degrees C) possessed narrow molecular weight distributions (M-w/M-n < 1.1) and controlled molecular weights based on the molar ratios of monomer to initiator. Over -20 degrees C, after the reaction mixture stood for a period of time, the broadening of the MWD of the poly(1) was apparently observed due to the reversible equilibrium polymerization under the thermodynamic conditions.