Branching in free radical polymerization due to chain transfer, 3. Polymerization of styrene in the presence of a tetrafunctional chain transfer agent

The effect of neopentanetetrayl tetrakis(2‐mercaptoacetate) on the molecular weight of polystyrene in free radical polymerization was studied. Polymerization experiments were carried out at three different concentrations of the tetrafunctional transfer reagent and were followed to monomer conversions up to β = 25%. For comparison, runs of polymerization were made without added transfer reagent and with a monofunctional 2‐mercaptoacetate, respectively. An apparent extent of branching was calculated from the molecular weights Mw obtained with the tetrafunctional and the monofunctional transfer reagent at fixed monomer conversion β and fixed concentration of reagent groups. This apparent extent of branching passes through a maximum at very small β and approaches the value 1 for large β. The decrease in the extent of branching is shown to be caused by the increasing fraction of linear polystyrene as the transfer reagent becomes exhausted. The data observed were compared with curves calculated from Cascade Theory with the known kinetic constants for polystyrene. A very good agreement between theory and experiment is observed at medium concentration of transfer groups. At smaller concentrations the increase in Mw is less than that predicted, and at large concentrations the increase in Mw is stronger than expected by theory. The deviations are attributed to negative and positive substitution effects.