Bulk and suspension polymerization of styrene in the presence of n‐pentane. An evaluation of monofunctional and bifunctional initiation

Abstract The plasticizing effect of n ‐pentane on the rate of bulk free radical polymerization of styrene and molecular weight distribution development has been modeled on the basis of the free volume theory for both monofunctional and bifunctional initiation. A strong decrease in the reaction rate in the late stages of the polymerization, due to the displacement of the onset of the gel effect, has been observed for both types of initiation. This decrease in the polymerization rate limited the terminal conversion to values well below 100% for mono‐functional initiation. However, in bifunctionally initiated polymerization, terminal conversions close to 100% were obtained in spite of the decrease in reaction rate. Contrary to what was expected, the molecular weight distribution obtained at terminal conversion was almost completely insensitive to these changes in polymerization rate. This phenomenon is explained in terms of limited transfer to monomer reactions when n ‐pentane is present in the system. In suspension polymerization, the limiting conversion and plasticizing effects of n ‐pentane in monofunctionally initiated systems, caused enhanced coalescence leading to suspension set‐up. In bifunctionally initiated systems this enhanced coalescence was completely overcome by the short duration of the particle growth stage, owing to high polymerization rates, and stable suspensions were achieved. For these systems the particle size distributions obtained were similar to that of suspension polystyrene without n ‐pentane. © 1993 John Wiley & Sons, Inc.