By using standard bromination conditions, the insoluble 1,2,1,2-polypropadiene (formed by Ni(acac)2 or Co(acac)2 or 3, C3H4, (iBu)3Al catalysts) is transformed into a soluble bromopolypropadiene. Using this technique, determination of molecular weight becomes possible. It was found that the molecular weight increases with polymerization time until a steady value is reached. As the polymer yield continues to increase when a constant molecular weight has been reached, chain transfer must occur. The molecular weight of polybromopropadiene was independent of the concentrations of the catalyst components. From experiments with crosslinked polymers and from theoretical considerations, it was deduced that the low solubility of the original 1,2,1,2-polypropadiene is due to its high crystallinity.