ethylbenzene 0 03 parts by weight of di t amyl peroxide and 0 2 parts by weight of di t dodecyl mercaptane are mixed to form a raw material which is then continuously fed into a stainless steel reactor having a volume of 2 l capable of controlling reaction temperature and agitation the reactor is maintained at 130 u00b0 c with an average retention time of 2 hours followed by continuously discharging the resultant polymer the conversion in the reactor is controlled to 35 the resultant discharged polymer is continuously charged to a second reactor in the series simultaneously 3 8 parts by weight of styrene based on 100 parts by weight of the monomer mixture is continuously added to the second reactor the second reactor is maintained at 130 u00b0 c with an average retention time of 2 hours followed by continuously discharging the resultant polymer the conversion in the second reactor is controlled to 35 so that the final conversion becomes 70 the final resultant polymer from the second reactor is introduced to a devolatilizer controlled at a temperature of 220 u00b0 c and a pressure of 20 torr to remove unreacted monomers and solvent and then pelletized using a pelletizer to obtain a thermoplastic resin in pellet form the refractive index of the thermoplastic resin measured using a prism coupler manufactured by metricon corp.