A process for producing and devolatilizing a brominated polymer, comprising a) brominating a starting organic polymer by reaction of the organic polymer with a brominating agent in solution to produce a brominated polymer solution that contains at least 65% by weight volatile compounds and not more than 35% by weight of the brominated organic polymer; b) combining the brominated polymer solution with at least one second thermoplastic organic polymer to reduce the concentration of volatile compounds in the solution to at most 60% by weight, wherein the second thermoplastic organic polymer is substantially devoid of polymerizable carbon-carbon unsaturation and contains less than 5% by weight of halogen, and c) separating the product obtained in step b) in a devolatilizing extruder that includes at least one back vent to form a devolatilized polymer blend containing the brominated organic polymer, the second thermoplastic organic polymer, and not more than 3,000 ppm of volatile compounds, and a separate vapor phase containing separated volatile compounds; and discharging the vapor phase through at least one vent in the devolatilizing extruder barrel and discharging the devolatilized polymer blend from the devolatilizing extruder through an outlet downstream of the vent(s), wherein (i) the devolatilizing extruder includes a main inlet port through which the brominated polymer solution or combination thereof with some of the second thermoplastic polymer is introduced into the devolatilizing extruder; (ii) the back vent is upstream of the main inlet port, and (iii) the devolatilizing extruder further includes a first inlet port upstream of the back vent, and at least a portion of the second thermoplastic polymer is introduced into the devolatilizing extruder through the first inlet port. a) brominating a starting organic polymer by reaction of the organic polymer with a brominating agent in solution to produce a brominated polymer solution that contains at least 65% by weight volatile compounds and not more than 35% by weight of the brominated organic polymer; b) combining the brominated polymer solution with at least one second thermoplastic organic polymer to reduce the concentration of volatile compounds in the solution to at most 60% by weight, wherein the second thermoplastic organic polymer is substantially devoid of polymerizable carbon-carbon unsaturation and contains less than 5% by weight of halogen, and c) separating the product obtained in step b) in a devolatilizing extruder that includes at least one back vent to form a devolatilized polymer blend containing the brominated organic polymer, the second thermoplastic organic polymer, and not more than 3,000 ppm of volatile compounds, and a separate vapor phase containing separated volatile compounds; and discharging the vapor phase through at least one vent in the devolatilizing extruder barrel and discharging the devolatilized polymer blend from the devolatilizing extruder through an outlet downstream of the vent(s), wherein (i) the devolatilizing extruder includes a main inlet port through which the brominated polymer solution or combination thereof with some of the second thermoplastic polymer is introduced into the devolatilizing extruder; (ii) the back vent is upstream of the main inlet port, and (iii) the devolatilizing extruder further includes a first inlet port upstream of the back vent, and at least a portion of the second thermoplastic polymer is introduced into the devolatilizing extruder through the first inlet port.