Indene (ID) was found to copolymerize spontaneously with polar vinyl monomers containing a nitrile or ester group in the presence of ZnCb and to give rise simultaneously to its cationic homopolymer in the latter comonomer. The formation of a 1:1-charge transfer complex between ID with acrylonitrile coordi-nated to ZnCl2((AN)c) was confirmed by UV-spectroscopic studies and the equilibrium constant for it was estimated to be 0.121 L/mol in AN at 25°C. The overall activation energy for the copolymerization with (AN)c was obtained to be ∼9.8 kcal/mol. An increasing amount of ZnCl2in AN resulted in increases in the copolymerization rate, viscosity, and alternating tendency of the copolymer. The addition of 1, 1-diphenyl-2-picrylhydrazyl to the System ID-(AN)c retarded the copolymerization and induced a cationic polymerization of ID. Further, terpolymers containing ∼50 mol % AN were formed spontaneously in the System ID-styrene-(AN)c. Comparing these results with the corresponding ones obtained for 1,3-cyclodienes as donor monomer reported previously, a discussion is given on the reactivity of ID-(AN)c complex in the initiation and the propagation of the copolymerization. The mechanism of the attendant cationic polymerization of ID is briefly considered.