Technical pure styrene monomer was cationically polymerized either alone or with alpha -metylstyrene to prepare tackifying resins. Anhydrous aluminum chloride dispersed in toluene was used as the catalyst. The polymerization was optimized with emphasis on practical viewpoints, and incremental monomer addition method was used in all the syntheses. Certain parameters affected on the reaction and on the resin characteristics, i.e., starting reaction temperature, catalyst percent, monomer and comonomer concentration, and solvent type were investigated. Post-heating of the resins was briefly studied as well. Number average molecular weight (M-n) and polydispersity (PDI) of the resins were determined by size-exclusion chromatography. Changes of M-n and PDI, as well as the reaction rate and yield versus variation of the mentioned parameters, were basically investigated according to the mechanism of carbocationic polymerization. Finally, the structure of the synthesized resins was confirmed by IR and NMR spectroscopy. A graphical method was proposed to quantify comonomer composition of the copolymeric resins by using of the ratio of aromatic to aliphatic protons peak area from the corresponding NMR spectra.