Use of deuterium to investigate extent of branching in polystyrene

The extent of branching in polystyrene is attributed to a polymer transfer step occurring during polymerization. In this process a radical abstracts a hydrogen atom from along a chain which then adds monomer at the site of the abstraction. The rate of this process should be decreased if the hydrogen is replaced by deuterium, and consequently deuterated monomers should polymerize to form polymers less branched than those from the nondeuterated monomer. Polymers of α‐ and β‐deuterostyrene were synthesized and thier intrinsic viscosity and molecular weight by light scattering determined. In the case of the α‐deuterostyrene, the polymer was also fractionated and measurements were made on the fractions. The data for the deuterated styrenes fit quite closely the same relationships as a normal styrene sample prepared simultaneously and under the same conditions, i.e., 70°C. and without catalyst. The results indicate, therefore, that branching is quite small in polystyrene prepared under such typical conditions. A check of the isotope effect on the transfer process was made utilizing S‐deuterobutyl mercaptan. A value of 4.0 was obtained at 60°C. for the relative rate of transfer with butyl mercaptan to that for the deuterated mercaptan.