Branching in polybutadiene

A detailed study has been made of the intrinsic viscosity‐molecular weight relationship for a polybutadiene prepared at 50°C. and one prepared at 5°C. Original fractions were at least twice refractionated from very dilute solutions in an attempt to obtain fairly sharp distributions. Fox‐Flory K constants are observed to decrease with increasing molecular weight in each case, and the intrinsic viscosity‐molecular weight plots show a curvature. This is interpreted to be a result of branching. A method of plotting the data is proposed which simultaneously yields the K for unbranched material and an estimate of the degree of branching. It is estimated that, in polybutadiene prepared at 50°C., one out of 6500 monomer units is involved in a branch, and that in 5°C. polybutadiene, one out of 10,000.