Prdn. of light-coloured hydrocarbon resins by polymerisation of aliphatic olefins from mixts. of conjugated and unconjugated dienes and monolefins, boiling in the -20 to +110 degrees C range, is carried out with Friedel-Crafts catalysts of the formula AlX2R.HX (where X is Cl, Br or I; R is 1-4C alkyl) and alkylbenzenes at 0-80 degrees C. The novelty is that the catalyst contains, as complexing agent, m- and/or p-diisoalkyl-substd. benzene (I), pref. having 3-8C isoalkyl substits., which is free from monisoalkyl substits. (I) give colourless complexes, which then lead to light-coloured resins, i.e. "red oil" formation is avoided. In an example, the resin obtd. using m-diisopropylbenzene as complexing agent in the polymerisation of a 5C olefin stream had a Gardner colour value of 1 and softening point of 89 degrees C.
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