A method was investigated for preparing butyl-rubber modified polystyr
ene. Styrene was polymerized free-radically in the presence of dissolv
ed butyl rubber and the process was monitored by dark-field optical mi
croscopy. A phase inversion occurred during the polymerization process
leading to a system consisting of rubber particles dispersed in a pol
ystyrene matrix. The particles had a multiple-inclusion structure simi
lar to that found in some polybutadiene toughened polystyrenes.