1. Field of the Invention
The present invention relates to a process for emulsion polymerization. More particularly, the present invention relates to a process for emulsion polymerizing a monomer which is hardly soluble in water.
2. Description of the Related Art
In general emulsion polymerization, at least one monomer and a polymerization initiator are added in an amount of water and polymerized, if desired, in the presence of an emulsifier and a buffer while keeping a polymerization system in an emulsified state.
When a monomer which is hardly soluble in water is to be emulsion polymerized, its polymerization rate is very low, since it diffuses into an aqueous phase at an extremely slow diffusion rate. In order to efficiently polymerize such a monomer on a commercial scale, it is necessary to increase the reaction rate and the diffusion rate through the increase of the reaction temperature. However, polymerization at a high temperature is not economical. Alternatively, it may be possible to accelerate the polymerization of monomer which is hardly soluble in water with the use of a larger amount of the emulsifier so as to improve the solubility and dispersibility of the monomer in water. However, the use of an emulsifier in a larger amount makes it difficult to separate the emulsifier from the resulting polymer. When the monomer which is hardly soluble in water is copolymerized with another monomer, the other monomer is preferentially polymerized so that the copolymer produced contains only very small amount of the monomer which is hardly soluble in water.
Various proposals have been made to increase the polymerization rate in an emulsion polymerization or to solve the problem of non-uniform consumption of the monomers during copolymerization. For example, Japanese Pat. Kokai Publication No. 223007/1986 discloses copolymerization of a perfluorovinyl ether with other fluoroolefins in the presence of a specific emulsifier. Japanese Pat. Kokai Publication No. 89713/1987 proposes the use of hexafluoropropylene as a third monomer so as to stably emulsion polymerize a perfluorovinyl ether and tetrafluoroethylene with good reproducibility.