- Thermal polymerization of pentabromobenzylacrylate (PBBMA) in a polypropylene (PP) composite that contains glass fibers and magnesium hydroxide has been studied using scanning and transmission electron microscopy techniques coupled with energy-dispersive spectrometry. The addition of PBBMA imparts flame retardant (FR) properties to the PP composite but also affects adversely its mechanical properties. It is of practical importance to determine the spatial distribution and the extent of polymerization of the FR in the PP composite in order to understand better its role in the system. The methods presented here allow the distinction between the monomeric and polymeric forms of the FR and to determine their spatial distributions. PP itself shows poor adhesion to the glass fibers, which may be improved by the addition of the reactive PBBMA. The latter is polymerized during reactive extrusion through an antimony-catalyzed reaction. Antioxidant hinders self-thermal polymerization but the presence of antimony overcomes this interference. PP shows good adhesion to sized Mg(OH) 2 as expected from a properly surface-treated filler.