A petroleum resin is prepared by a process which comprises polymerizing using a Friedel-Crafts catalyst
A. a petroleum resin feed comprising C.sub.5 and C.sub.5 diolefins, C.sub.6 olefins and C.sub.6 diolefins or a mixture of C.sub.5 and C.sub.6 olefins and diolefins, said feed being obtained from the cracking of petroleum feedstock and
B. at least 0.5 wt.% based on the petroleum resin feed of a tertiary hydrocarbyl halide.
The resulting resin usually has a softening point of 20.degree. to 125.degree. C. These resins have an extremely narrow molecular weight distribution and hence have good wax compatibility, viscosity, flexibility and tackifying properties for rubber.