The semi-continuous bulk copolymerization of N -phenylmaleimide and styrene was carried out in the presence of polybutadiene(PB). The phase inversion

refractive index and the rheological behavior of the polymerizing mass

and the grafting ratio of N -phenylmaleimide-styrene copolymer onto PB in the course of polymerization were investigated. The results indicated that the phase inversion occurred in a relatively wide range of time. In the phase inversion region

the refractive index changed irregularly and the grafting ratio increased sharply

while the viscosity of the polymerizing mass declined.On-line monitoring of agitating torque for the reactor is very useful to identify the region of phase inversion.