CONJUGATED DIENES ARE POLYMERIZED BY A NEW CATALYST SYSTEM WHICH PERMITS CONTROL OF THE MOLECULAR WEIGHT AND GIVES A MORE EASILY PROCESSED PRODUCT, TOGETHER WITH AN INCREASED PROPORTION OF 1,2-CONFIGURATION IN THE POLYMER REPEATING UNIT STRUCTURE. THIS CATALYST SYSTEM COMPRISES (1) A SODIUM ALLYL COMPOUND HAVING 3-10 CARBON ATOMS AND (2) A LITHIUM ALKYL, A LITHIUM ARYL OR A LITHIUM FERROCENYL, COBALTCENYL OR NICKELCENYL COMPOUND. THE DIENE POLYMERS PRODUCED BY THIS PROCESS HAVE CONTROLLABLE MOLECULAR WEIGHTS IN THE RANGE OF 5,000 TO 1,000,000, PREFERABLY 100,000 TO 500,000, BROAD MOLECULAR WEIGHT DISTRIBUTION, GLASS TRANSITION TEMPERATURES HIGHER THAN NORMALLY OBTAINED, HIGH DEGREE OF BRANCHING, HIGH PROPORTION OF 1,2-REPEATING UNIT CONFIGURATION, AND ARE MORE EASILY PROCESSED IN THE PRODUCTION OF RUBBER AND OTHER COMPOSITIONS FOR COMMERCIAL USE.