Influence of Molecular Weight Distribution (MWD) onktand the Onset of the Gel Effect using the RAFT-CLD-T Method

Abstract

Understanding how the broadening of the molecular weight distribution (MWD) affects the termination rate coefficient, 〈kt〉, during free radical polymerization (FRP) is important for developing models capable of accurately predicting rates of polymerization and the resulting MWDs. In this article, we studied the evolution of 〈kt〉 for the RAFT-mediated polymerization of styrene at 90 °C. A difunctional RAFT agent was used to control the evolution of PDI with conversion and the RAFT-CLD-T method was used to determine 〈kt〉. We found that when concentration ratios of initiator to RAFT agent (i.e. [I]0:[RAFT]0) are greater than 1:1, 〈kt〉 increased due to significant amounts of short-long termination in agreement with theoretical predictions. The onset of the gel effect was examined and a surprising result was found. As the PDI increased there was a concomitant decrease in the molecular weight and weight fraction of polymer at which the onset of the gel effect was observed. This seems counterintuitive to all physical theories for the gel effect. However, when broad MWDs are accounted for using chain dynamic statistics, chain overlap (or c*) can once again account for the onset of the gel effect in agreement with previous work for narrow MWDs. These results are important and provide strong evidence for chain overlap as the cause of the onset of the gel effect. The RAFT-CLD-T method has provided valuable insight into the effects of polymer chain shape, interactions and mobility on kt.