BULK-POLYMERIZATION OF STYRENE IN THE PRESENCE OF POLYBUTADIENE - THEUSE OF BIFUNCTIONAL INITIATORS

This work experimentally and theoretically investigates the use of bif unctional initiators in the synthesis of high-impact polystyrene (HIPS ). The experimental design involved a series of nonisothermal bulk pol ymerizations of styrene (St) in the presence of polybutadiene (PB). Th e performance of three commercial initiators [2,5-dimethyl-2,5 bis(2-e thylhexanoyl peroxy) hexane or L-256; 2,5 dimethyl-2,5 bis(benzoyl per oxy) hexane or L-118; and ethyl 3,3 di(t-butyl peroxide) butirate or L -233] were compared to the performance of a standard monofunctional in itiator (terbutylperoctoate or TPBO), and to the blank case (i.e., wit hout initiator). From samples taken along the prepolymerization period , the phase inversion point and the 30% conversion point were estimate d. For the final product, the free polystyrene (PS) molecular weights and the St grafting efficiency were measured. A mathematical model was developed that predicts the evolution of the MWDs for the free PS, th e residual PB, and the graft copolymer, together with the chemical com position distribution for the total graft copolymer. Compared to the m onofunctional case, the L-256 initiator induces phase inversion and ru bber grafting at low conversions. Also, it shortens the prepolymerizat ion times by around 38%, without affecting the molecular characteristi cs of the final product. L-118 also shortens prepolymerization time wi th respect to TBPO; but is not as effective as L-256 or TBPO in promot ing rubber grafting. At the polymerization end, the final molecular ch aracteristics are practically independent of the initiator type becaus e most of the polymerization is induced by monomer initiation. Due to its slow decomposition rate, the L-233 initiator is less effective tha t TBPO for reducing prepolymerization times and for promoting phase in version. (C) 1996 John Wiley & Sons, Inc.