Several aryldiazoalkanes (M2) have been copolymerized with phenyldiazomethane (M1) in toluene‐methanol solution at 40°C, namely, the p‐chloro‐, p‐methoxy‐, p‐mesyl‐o and ‐p‐methyl‐, 2,4‐ and 3,4‐dichlorophenyldiazoalkanes, and the α‐ and β‐ naphthyl‐diazoalkanes. The copolymerization parameters r1 and r2 have been evaluated. By plotting 1/r1 against the Hammett σ values a negative ρ values was found equal to −0.88. From cationic copolymerizations carried out at −78°C in the presence of boron trifluoride‐diethyl ether as catalyst a similar plot of 1/r1 against σ gives a value of ρ equal to −0.82. The negative sign and the agreement between these ρ values demonstrates the cationie mechanism of the methanol polymerization and copolymerization of aryldiazoalkanes.