A mechanochemical reactor providing strong shear-force has been developed in order to graft the polar groups onto the polymers. The extent of polymers' chain scission is a key factor in grafting process. In order to evaluate the scission extent, the shearing experiments were carried out for GPPS152P, which were produced by SECCO Petrochemical Company, Shanghai, China, at different revolving speeds, and temperatures. The decrease in molecular weight of GPPS152P was evaluated through gel permeation chromatography method and was estimated to be 13.8%. Additionally, the molecular weight distribution (MWD) of this sample became narrower. The chain scission theory and the shear rate, simulated through POLYFLOW software, were simultaneously employed to conduct quantitative analysis of molten PS' chain scission process. According to this method, the decrease in molecular weight was estimated to be 12.88%. Therefore, this method can be taken as a guideline for estimating molecular weight and MWD of products in manufacturing industry. POLYM. ENG. SCI., 58:913-919, 2018. (c) 2017 Society of Plastics Engineers