A melt devolatilization extrusion process, comprising a) feeding a heat-plasticized polymer mixture that includes component 1): at least one first thermoplastic polymer containing at least 25% by weight of aliphatically bound halogen; component 2): at least one second thermoplastic organic polymer, which is compatible with component 1) at the relative amounts thereof that are present, wherein the second organic polymer is devoid of polymerizable carbon-carbon unsaturation, contains less than 5% by weight of halogen, and has a weight average molecular weight of from 25,000 to 175,000 as measured by GPC against a polystyrene standard; and component 3): from 1 to 60% by weight, based on the total weight of the polymer mixture of at least one volatile compound; into a devolatilizing extruder while at a temperature above the boiling temperature of the at least one volatile compound; b) separating the heat-plasticized polymer mixture in the devolatilizing extruder into a devolatilized, heat-plasticized polymer blend containing not more than 3,000 ppm of volatile compounds and a separate vapor phase containing the at least one volatile compound; and c) 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: A) the devolatilized polymer blend discharged from the extruder outlet has a melt shear viscosity at 180°C of no greater than 500 Pa·s at a shear rate of 100 s -1 and of no greater than 300 Pa·s at a shear rate of 1000 s -1 as measured by capillary rheometry; and B) during steps a), b) and c), the polymer mixture and the devolatilized polymer blend are maintained at a temperature of no greater than 100°C above the glass transition temperature of the first thermoplastic polymer. a) feeding a heat-plasticized polymer mixture that includes component 1): at least one first thermoplastic polymer containing at least 25% by weight of aliphatically bound halogen; component 2): at least one second thermoplastic organic polymer, which is compatible with component 1) at the relative amounts thereof that are present, wherein the second organic polymer is devoid of polymerizable carbon-carbon unsaturation, contains less than 5% by weight of halogen, and has a weight average molecular weight of from 25,000 to 175,000 as measured by GPC against a polystyrene standard; and component 3): from 1 to 60% by weight, based on the total weight of the polymer mixture of at least one volatile compound; into a devolatilizing extruder while at a temperature above the boiling temperature of the at least one volatile compound; b) separating the heat-plasticized polymer mixture in the devolatilizing extruder into a devolatilized, heat-plasticized polymer blend containing not more than 3,000 ppm of volatile compounds and a separate vapor phase containing the at least one volatile compound; and c) 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: A) the devolatilized polymer blend discharged from the extruder outlet has a melt shear viscosity at 180°C of no greater than 500 Pa·s at a shear rate of 100 s -1 and of no greater than 300 Pa·s at a shear rate of 1000 s -1 as measured by capillary rheometry; and B) during steps a), b) and c), the polymer mixture and the devolatilized polymer blend are maintained at a temperature of no greater than 100°C above the glass transition temperature of the first thermoplastic polymer. A) the devolatilized polymer blend discharged from the extruder outlet has a melt shear viscosity at 180°C of no greater than 500 Pa·s at a shear rate of 100 s -1 and of no greater than 300 Pa·s at a shear rate of 1000 s -1 as measured by capillary rheometry; and B) during steps a), b) and c), the polymer mixture and the devolatilized polymer blend are maintained at a temperature of no greater than 100°C above the glass transition temperature of the first thermoplastic polymer.