The present invention provides a process for preparing a
fluorine-containing polymer having few branched chains and little weight
change in high temperatures, which is a new process wherein composition
distribution substantially does not occur. The fluorine-containing
polymer is prepared by a batch copolymerization process conducted under
conditions of reduced temperature of at least 0.95 and reduced pressure
of at least 0.80 of the critical constant calculated from critical
temperature, critical pressure and composition ratio of each monomer in
the gaseous phase of the reaction vessel using the Peng-Robinson formula.