mixing elements but is gently pushed by the kneading bars 90 and 94 with positive discharge out of exit slot 98 into extruder 62 the apparatus 30 is in no sense an extruder because the mixing kneading elements are not capable of compressing the polymer for the apparatus to act as an extruder an alternative apparatus in which the process of this invention may also be practised is a form of continuous reactor extruder one commercial such apparatus is the polycon from baker perkins such an apparatus is a co rotating intermeshing twin screw extruder in which the screw has an l d ratio of about 5 1 to 20 1 preferably about 8 1 to about 15 1 and in which the screw design is modular in form and has a limited feed section a significant kneading mixing section and a limited discharge section the design of the screw flight is different in the three sections to provide the appropriate action needed in that section referring to fig6 it is seen that the apparatus 130 has one vent port 140 located at the top near the upstream end to this vent port vapor conduit 44 may be connected for the operation of this process at the downstream end a die plate 162 is provided for the discharge of the product such apparatus will now be described in detail with reference to fig6 to 8 it consists of three internal zones 1 feed zone 163 2 mixing kneading zone 164 and 3 discharge zone 165.