In this study, we present an approach for the synthesis of novel phosphorous-or phenyl-containing polymers, 2phenyl, 3phospho, and 2phenyl-co-3phospho, derived from ring-opening metathesis polymerization (ROMP), to reduce the flammability of polypropylene (PP). The composites were processed by melt-blending ROMP polymers and octaphenyl-polyhedral oligomeric silsesquioxane with PP/intumescent flame retardant (IFR) compounds at different compositions. The composites were characterized by limiting oxygen index (LOI), UL-94, and mechanical tests as well as thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The molecular structures of phosphorous-and phenyl-containing polymers were proved by 1 H-NMR, 13 C-NMR, and Fourier transform infrared spectroscopy. The online rheological measurements indicated that the addition of additives to the PP/IFR system increased the melt viscosity of the compounds regardless of the type. The DSC analysis showed that the addition of ROMP polymers to the PP/IFR system influenced the crystal perfection and degree of crystallization. TGA analysis of the composites revealed that the addition of ROMP polymers to PP/IFR compounds deteriorated the thermal stability as the amount of phosphorus increased in the matrix. Dynamic mechanical properties such as storage modulus (E') and loss modulus (E'') of the composites were lowered by the addition of ROMP polymers. The LOI and UL-94 rating of PP/IFR were enhanced by the addition of ROMP polymers. It was successfully demonstrated that the novel phosphorous-or phenyl-containing polymers were highly potent additives in optimizing the flammability of PP composites. (C) 2017 Wiley Periodicals, Inc.