PURPOSE: To obtain a title oligomer useful for a coupling agent or the like by a method wherein a high molecular material is subjected to heat decomposition at a high temperature in an inert gas stearm and a property of a high molecular material as a feedstock is highly maintained by subjecting a heat decomposed reaction product to a solvent-nonsolvent-reprecipitating purification.

CONSTITUTION: A high molecular material such as polypropylene, polyisobutylene, poly-butene, polymethylmethacrylate, ethylene-propylene copolymer or the like is subjected to heat decomposition at a reaction temperature of 300W450°C for 0.5W 4 hrs. in an inert gas stream such as nitrogen, helium or the like and the obtained heat decomposed product is subjected to a solvent-nonsolvent-reprecipitating purification to obtain an α,ω-diene-oligomer, having double bonds at both terminal ends of a molecule with an average M.W. range of about 600W10,000. These oligomers are widely utilized by carrying out various chemical reactions utilizing reactive groups of both terminal ends, by highly maintaining a specific property of a feed polymer.