As outlined in Chapters 1 and 2, it is difficult to incorporate phosphorus into linear polymers by (co)polymerization or (c0)polycondensation. Only in rare cases is it possible to obtain products of high phosphorus content, high molecular weight, satisfactory mechanical properties, and satisfactory hydrolytical stability. Therefore, it is natural that the subsequent incorporation of phosphorus into commercial polymers has also been investigated. The macromolecular character of these products remains unaffected as long as no degradationof the main chain occurs. The phosphorus is always found in the side groups. The problem inherent in the subsequent introduction of phosphorus is the fact that there are only a few suitable phosphorylation reactions. Phosphorylation should be simple to carry out and should proceed rapidly even with insoluble polymers to give high yields; the incorporated phosphorus should be linked to the polymers by a stable bond, preferably a P-C bond, but above all the phosphorylation should not give rise to chain breakdown or cross-linking of the polymer. Understandably, the fulfillment of these requirements depends not only on the method of phosphorylation but also on the polymer used. Good results have been achieved so far, especially with polyolefins and polystyrene.