Abstract: The effectiveness of an alfin catalyst and the quality of the rubber it produces are increased by carrying out the polymerization in a closed container in which the ratio of vapor to liquid space is less than 0.2, this ratio being responsible for a reduction of the boiling (desorption of monomer) that occurs on each particle because of the heat of polymerization. In general the effectiveness of the catalyst and the quality of the polymer are improved by any method or combination of methods that reduces the boiling per particle to a level less than that achieved by carrying out the reaction in a closed container in which the volume of vapor space to liquid is less than 0.2. The more this reduction, the better the product as a rule.The product obtained is a millable rubber of high molecular weight, well above that of natural rubber and primarily with a trans structure of the diene component.