C1 or Br and (B) a vanadium compound selected from (1) VO(OR) X Where R is an alkyl or aryl radical, m is not less than 1 or more than 3, and X is C1 or Br and (2) vanadium oxyhalides soluble in the reaction medium; provided that when the vanadium compound is selected from (1) the vanadium concentration in the reaction zone is not greater than 0.260 millimole/liter of solution and the Al/ V ratio