The N-[3-(dimethoxy-methyl-silanyl)-propyl]N'-(9-methyl-3,9-dioxo-2,4,8,10-tetraoxa-3,9-diphospha-spiro[5.5]undec-3-yl )-ethane-1,2-dian-dne/dimetl-ioxy dimethyl silane copolymer (PSiN II), which simultaneously contains silicon, phosphorus, and nitrogen, is synthesized and incorporated into polypropylene (PP). The flame retardancy is evaluated by the limiting oxygen index value, which is enhanced to 29.5 from 17.4 with 20% total loading of PSiN II. The thermal degradation behavior of PP/PSiN II is investigated by thermogravimetric analysis under N-2 and air. The PP/PSiN II sample degrades at 400 degrees C for different amounts of time, and the process of degradation is studied by Fourier transform IR. The morphology of the char formed at 400 degrees C for 10 min is investigated by scanning electron microscopy. The swollen inner structure, close, and smooth outer surface provide a much better barrier for the transfer of heat and mass during fire and good flame retardancy. The thermal stability of PP is improved by incorporation of PSiN II. (c) 2005 Wiley Periodicals, Inc.