AbstractCationic polymerizations of various 5‐alkyl‐2‐norbornenes (RNB, where RH, CH3, C2H5, C5H11, C7H15, and C10H21) were carried out by the AlEtCl2/tert‐butyl chloride catalyst system, and the effect of the length of the alkyl substituents on the monomer reactivity and polymerizability, polymer structure, and softening point (SP) were investigated. The introduction of an alkyl substituent into 5‐position of the norbornene ring remarkably reduces the monomer reactivity and polymerizability, although the length of alkyl group does not affect them.13C NMR structural analysis of poly(RNB)'s revealed that not only the normal 5‐or 6‐alkyl‐2,3‐unit but also other structures such as 5‐ or 6‐alkyl‐2,7‐, 4‐alkyl‐3,6‐units, etc., are the most plausible structures for the repeating units. Furthermore, the existence of the 4‐alkyl‐3,6‐repeating unit and of the alkylidene type end‐group indicates the presence of an intermediate tertiary cation (5‐alkyl‐5‐norbornyl cation), which causes the low reactivity and polymerizability of the monomers. The SP of oligomers of equal molecular weight (DPn= 3–5) decreases in the following order: HNB, CH3NB,C2H5NB>C5H11NB>C7H15NB>C10H21NB. The effect the alkyl substituent length on the SP is discussed on the basis of the13C spin