A novel petroleum resin having excellent characteristics as a tackifier for use in a hot-melt adhesive is produced by copolymerizing a monomer mixture comprising 20 to 80 % by weight of aliphatic monoolefins mainly of four and five carbon atoms and 80 to 20 % by weight of styrene and/or a styrene derivative with a catalyst system comprising an organoaluminum compound represented by the general formula AlRnX3-n (wherein R is an alkyl group having 1 to 4 carbon atoms, X is a halogen atom, and n is 2 or 1.5) as main catalyst and an alkyl halide or a hydrogen halide as co-catalyst in a hydrocarbon solvent at a temperature of 0 DEG to 60 DEG C.
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