A plug flow reactor having an inner shell 27 surrounded by outer shell 21 and having at least one annular flow passage 35 therebetween can be used to prepare compositions, including polymers. The plug flow reactor also includes inlet port 36, an outlet port 37 and a plurality of exchanger tubes 26 wherein the exchanger tubes are in fluid communication to the at least one annular flow passage. Polystyrene and high impact polystyrene can be prepared using the reactor.