Method for producing polyacetal copolymer

A method of manufacturing a polyacetal copolymer, the method comprising:

• a raw material-introducing step of introducing a raw material comprising a trioxane, a comonomer copolymerizable with the trioxane and a nonvolatile protonic acid to a reaction apparatus of continuous stirring mixer-type having two parallel axes rotating in the same direction or in different directions relative to each other, a large number of paddles attached on each of the axes and a barrel located close to the outer peripheries of the paddles, wherein ends of long axes of the paddles are configured to periodically approach ends of short axes of their counterparts, and the raw material is to be introduced through an inlet provided at one end in the axial direction, and a reaction mixture and unreacted monomers are to be withdrawn through an outlet provided at the other end;
• a polymerization reaction step of performing a polymerization reaction of the raw material to obtain the reaction mixture;
• an unreacted-monomer re-introducing step of performing vapor separation of the unreacted monomers from the reaction mixture, and supplying the unreacted monomers to the raw material-introducing step;
• a polyacetal copolymer-recovery step of recovering a polyacetal copolymer from the reaction mixture into a recovery section provided downstream of the outlet and adjusted to a gage pressure of 0.2 kPa or more;
• and
• a deactivation step of adding a basic compound to the polyacetal copolymer recovered at the polyacetal copolymer-recovery step, and performing melt-kneading treatment to deactivate the nonvolatile protonic acid.