The effect of various aluminium alkoxides in match with the same molybdenum alkoxide on the activity in the polymerization of butadiene has been studied. The order of activity of R2AlOR' with varied R is as follows:(i-Bu)2AlOPh > Et2AlOPh ≥ Me2AlOPh

when R' = Ph; (i-Bu)2AlOC8Hl7 > Et2AlOC8H17

when R' = C8H17. The order of activity of R2AlOR' with avried R' is as follows:(i-Bu)2AlOPh > (i-Bu)2AlOC8H17

when R = i-Bu; Et2AlOPh > Et2AlOC8Hl7

when R = Et.The active valence state of molybdenum as determined by ESR is three.