A novel flame retardant (SDPS) containing POSi bands was synthesized by a condensation polymerization of diphenylhydroxysilane and spirocyclic pentaerythritol di(phosphate monochloride). The structure of SDPS was confirmed by Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance (1H NMR). Limiting oxygen index (LOI) test showed that when loading 22.5 wt % SDPS and 7.5 wt % in polypropylene, LOI value markedly increased to 31.4% in comparison with PP/SDPS (30 wt %) and PP/MP (30 wt %) systems. Combusting experiment further proved a synergistic effect of SDPS and melamine polyphosphates (MP) in flame retarding of polypropylene. Thermogravimetric analysis (TGA) and FTIR-TG results revealed that the synergistic effect was attributed to a catalyzed breaking down of POSi bands in the presence of MP, which acted as an acid catalyst, consequently leading to a facilitated char and silica forming process. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013