Process for preparing fluorine-containing polymer

Abstract

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.

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         9 . A fluorine-containing polymer, which is a copolymer comprising vinylidene fluoride and hexafluoropropylene;
 wherein the mol ratio of vinylidene fluoride:hexafluoropropylene is 9:1 to 5:5, the content of tetrafluoroethylene is 0 to 40% by mol, the weight average molecular weight is at most 140,000, weight average molecular weight/number average molecular weight is at most 3.0, iodine is essentially not contained and the compression set after vulcanization is at most 30%.   
     
     
         10 . A fluorine-containing polymer composition comprising the fluorine-containing polymer of  claim 9 , a curing agent and a vulcanization accelerator. 
     
     
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         15 . A process for preparing vulcanized fluororubber, which comprises conducting primary vulcanization of the fluorine-containing polymer composition of  claim 10 , while conducting defoaming treatment under reduced pressure.