The title toughening master batch (TMB) was synthesized in a low‐viscosity reaction system by using dynamic vulcanization technique starting from polypropylene (PP) as the matrix resin and ethylene–propylene or butadiene–styrene elastomer as the toughening agent through a polymer–bridge conjunction derived from a monomer containing a carbonate group in the presence of a free radical initiator. The chemical structure of the TMBs and the effects of technological conditions on structural parameters were investigated using fractional extraction and infrared spectroscopy. The prepared TMBs consisted of unreacted PP, unreacted elastomers, graft copolymer of PP and/or elastomers containing branched chains formed by bridging agent, and crosslinked copolymer of PP and/or elastomers in conjunction with polymer bridge chains derived from bridging agent. Results showed that the PP existed in graft and crosslinked forms was in the range of 3–21 wt% and that of the elastomer toughening agent was in the range of 50–70 wt%, grafting and bridging efficiency of bridging agent was in the range of 62–88 wt%, graft copolymer content in the total TMB was in the range of 0.18–3.65 wt% and crosslinked copolymer content was in the range of 22–42 wt%. Copyright © 2000 John Wiley & Sons, Ltd.