BULK AND SUSPENSION POLYMERIZATION OF STYRENE IN THE PRESENCE OF N-PENTANE - AN EVALUATION OF MONOFUNCTIONAL AND BIFUNCTIONAL INITIATION

The plasticizing effect of n-pentane on the rate of bulk free radical polymerization of styrene and molecular weight distribution developmen t has been modeled on the basis of the free volume theory for both mon ofunctional and bifunctional initiation. A strong decrease in the reac tion rate in the late stages of the polymerization, due to the displac ement of the onset of the gel effect, has been observed for both types of initiation. This decrease in the polymerization rate limited the t erminal conversion to values well below 100% for monofunctional initia tion. However, in bifunctionally initiated polymerization, terminal co nversions close to 100% were obtained in spite of the decrease in reac tion rate. Contrary to what was expected, the molecular weight distrib ution obtained at terminal conversion was almost completely insensitiv e to these changes in polymerization rate. This phenomenon is explaine d in terms of limited transfer to monomer reactions when n-pentane is present in the system. In suspension polymerization, the limiting conv ersion and plasticizing effects of n-pentane in monofunctionally initi ated systems, caused enhanced coalescence leading to suspension set-up . In bifunctionally initiated systems this enhanced coalescence was co mpletely overcome by the short duration of the particle growth stage, owing to high polymerization rates, and stable suspensions were achiev ed. For these systems the particle size distributions obtained were si milar to that of suspension polystyrene without n-pentane. (C) 1993 Jo hn Wiley & Sons, Inc.