Provided is a process of making a polyphenylene oxide prepolymer, comprising a step of reacting a reactive cycloolefin and a vinyl-containing polyphenylene oxide in the presence of a ruthenium catalyst. The reactive cycloolefin may be selected from dicyclopentadiene monomer, dicyclopentadiene oligomer, dicyclopentadiene polymer, norbornene monomer, norbornene oligomer, norbornene polymer, and a combination thereof; the vinyl-containing polyphenylene oxide may be selected from divinylbenzyl polyphenylene oxide resin, vinylbenzyl-modified polyphenylene oxide resin, methacrylic polyphenylene oxide resin, and a combination thereof; the ruthenium catalyst may be a Grubbs catalyst. Also provided are a polyphenylene oxide prepolymer made by the process, a resin composition containing the polyphenylene oxide prepolymer, and a product made from the resin composition.