Abstract: High molecular weight polyester resins are sometimes produced from low molecular weight polyester prepolymers having the same composition by solid state polymerization. Polyester prepolymers are generally converted from the amorphous state to the crystalline state prior to solid state polymerization in order to raise their sticking temperature. This is done in order to keep the polyester prepolymer from sticking together as a solid mass in the solid state polymerization reactor. Such polyester prepolymers have traditionally been crystallized by heating them to elevated temperatures. It has been found that polyester prepolymers can be crystallized by exposing them to the vapors of certain volatile organic compounds. By utilizing such vapors to crystallize polyester prepolymers the need for a thermal crystallization step can be eliminated.

Abstract: This invention comprises an improvement in the preparation of high molecular weight polyester, said improvement comprising simultaneously and continuously crystallizing, drying and heating low molecular weight particulated, melt polymerized linear condensation polyester prepolymer resin by subjecting said prepolymer resin to a high frequency energy field in the range from about 20.0 to about 300 megahertz for a period of time ranging from about 20 to 90 seconds.