A catalyst system for the polymerization of at least one 1- olefin comprising a mixture of a bis-trihydrocarbyltinchromate essentially of the formula: ##STR1## and cocatalytic amounts of an alkyl aluminum compound of the formula R.sub.a AlX.sub.b in each of which formula each R is a hydrocarbyl radical containing from 1 to about 14 carbon atoms and all R's are the same or different, X is a halogen or an alkoxy group, a is an integer of 1-3, b is an integer of 0-2 and the sum of a and b is 3. The disclosure also includes the method of polymerizing at least one such olefin which comprises contacting the olefin under polymerizing conditions with the above catalyst system.
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