Process for continuously polymerizing trioxane

In a process for producing a polyacetal homopolymer or copolymer which comprises continuously feeding a liquid starting mixture containing molten trioxane and a catalyst and optionally a comonomer through a feed opening of a reactor, said reactor being a continuously stirring-type mixer comprising a hollow barrel having in spaced-apart relationship said feed opening for the starting materials and a discharge opening for the reaction product, said barrel having a cross-sectional surface corresponding to two overlapping eccentric circles having the same diameter, a jacket provided on the periphery of the barrel for temperature control, and stirring shafts disposed within the hollow barrel parallel to each other and extending longitudinally of the barrel through the central parts of said circles, said stirring shafts having a plurality of plate-like paddles fixed thereto in such a manner that the paddles fixed to one stirring shaft are kept in contact with each other, the cross-section of each of said paddles taken perpendicularly of the axial direction of the stirring shaft having a shape of a convex lens, an ellipse or a quasipolygon inscribing a phantom circle at each vertex, one of said paddles facing another paddle fixed to the other stirring shaft and said one paddle being positioned such that it rotates while the edges thereof keep a slight clearance from the inner surface of the barrel and the other paddle;

• polymerizing the starting mixture while moving it toward the discharge opening by the rotation of the paddles;
• withdrawing the reaction product containing more than 5% by weight but not exceeding 40% by weight of the unreacted monomer or monomers from the discharge opening as a finely divided solid;
• thereafter introducing the reaction product into a continuously stirring-type mixer of the same type as said reactor or into a horizontal continuous-type mixer of a structure in which the clearance between the outside surface of each stirring impeller and the inside surface of the barrel of the mixer, or the clearance between the outside surfaces of stirring impellers is not always narrow, said mixer being directly connected to said reactor and polymerizing the reaction mixture further;
• and withdrawing the polymerization product containing the unreacted monomer or monomers in an amount of not more than 5% by weight as a finely divided solid;
• the improvement wherein the arrangement of the paddles is changed continuously or stepwise such that with respect to one paddle on the same stirring shaft, a paddle adjacent thereto on the discharge opening side of the reactor is deviated by an angle of less than 180.degree. in a direction opposite to the rotating direction of the stirring shaft, so that the transportability of the reaction mixture becomes progressively smaller from the feed opening toward the discharge opening of the reactor.