Attempts were made to prepare 2,6-naphthoquinodimethanes with different substituents such as cyano and alkylthio groups at the 11 and 12 positions, 11,12-bis(alkylthio)-11,12-dicyano-2,6-n quinodimethanes (2a-e). Compounds 2a-e could not be obtained as isolable crystals at room temperature but as their oligomers. It was found that when their oligomers were warmed in benzene, the oligomers depolymerized to give monomeric compounds and on cooling the monomeric compounds polymerized to form oligomers. Moreover, in the range of 20-40 degrees C polymerization-depolymerization equilibrium was found to be set up without side reaction. On the basis of analysis of the equilibrium polymerization behavior, thermodynamic parameters for the polymerizations of 2a-e were determined. Their values were compared with the substituent parameters of the alkyl groups of the alkylthio substituents by means of linear free energy relationships, which exhibited that their enthalpy changes of polymerization are predominantly related with Taft’s E(s) values of the alkyl groups of alkylthio substituents and independent of their sigma* values (rho* = 0), indicating that homopolymerizabilities of 2a-e are determined exclusively by a steric hindrance effect of the alkylthio substituents.