This invention relates to a multiple phase polymerization process to produce polyolefins comprising contacting olefins and a homogeneous catalyst or catalyst system at a temperature between the melting point of the dry polymer and 80.degree. C. below the melting point of the dry polymer in the presence of a fluorinated hydrocarbon. This invention also relates to a process to produce polyethylene comprising contacting a homogeneous catalyst or catalyst system with ethylene and from 0 to 40 weight % of one or more C3 to C40 olefin comonomer(s) in the presence of a fluorinated hydrocarbon and optionally a hydrocarbon solvent at a temperature below the reaction medium melting point. This invention also relates to a process to produce polypropylene comprising contacting a homogeneous catalyst or catalyst system with propylene and from 0 to 50 weight % of one or more of ethylene and C4 to C40 olefin comonomer(s) in the presence of a fluorinated hydrocarbon and optionally a hydrocarbon solvent at a temperature below the reaction medium melting point.