## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Reactive Modifications of Styrenic Polymers
#### 4.2.1 Organic Phosphates and Phosphonates (P\({}^{+5}\))  
The _co-_polymerisation of St with P-containing unsaturated monomers (see Figure 1) can be an effective way to increase the thermal stability and fire retardance of styrene polymers. There is a wide range of organic P-containing FRs available including phosphates, phosphonates, polyphosphonates, hybrid metal phosphonate salts, etc. [5; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124; 125]. The mechanism and the efficiency of FRs depend not only on the chemical structure of a P-bearing compound itself, but also on its interaction with the polymeric backbone during pyrolysis and combustion. The use of synergists also has an impact on the predominant mode of fire retardation [34; 35]. Another factor affecting the fire retardance is the position of P-containing functionalisation within the polymer chain. For example, the free radical polymerisation of St with different P-based compounds such as phosphorylated acrylates and methacrylates, 2-(acryloyloxy)ethyldiethylphosphate (ADP), and phosphonamide (Figure 17) showed that only chain-end and chain-middle phosphorylation resulted in the improved thermal resistance and fire retardance [109]. There are limited reports about the _ter_-polymerisation of St being employed as a route to improve the fire resistance of styrene polymers. For example, [PERSON] et al. (2007) used diethyl(methacryloyloxymethyl)phosphonate (DEMMP) as well as 1,4,6-trioxaspiro[4,4]-2-nonylmethyl acrylate (SOE-AC) as the monomers (Figure 18, Scheme a) for the radical _ter_-polymerisation of St [110]. The obtained _ter_-polymer had higher LOI values of 25.2 vol% and higher thermal stability as indicated by the amounts of TG char residue raised by 18.4 wt% compared to the corresponding _co_-polymer of St with SOE-AC [110]. The presence of the cationic cross-linking agent, ytterbium triflate, Yb(OTf)\({}_{3}\), initiated the double ring-opening of the pendant SOE moieties to develop a cross-linked network (Figure 18, Scheme b) that prevented the polymer from shrinkage and promoted the formation of an insulating protective layer by DEMMP fragments. Unfortunately, the authors of [110] did not consider the mechanism of fire retardance of the proposed formulations. The impact of the chemical environment of the P atom on the efficiency and fire-retarding mechanism of reactively modified polymers was studied by [PERSON] et al. (2007) [38]. In this study, the influence of phosphate and phosphonate FRs on the fire behaviours of PS systems was compared. The reactively modified PS (with 3.5 wt% of P) contained the following monomeric units: diethyl(acryloyloxyethyl)phosphate (DEAEP), diethyl (methacryloyloxy ethyl) phosphate (DEMEP), diethyl(methacryloyloxymethyl)phosphonate (DEMMP), and diethyl (acryloyloxymethyl)phosphonate (DEAMP) (Figure 19a-d). The reactive modifications within the PS copolymers acted both in the condensed phase and in the gaseous phase. Moreover, the chemical environment of the P atom affected the mechanism of fire retardance to varying extents. During thermal decomposition, the copolymers with the integrated phosphates, DEAEP and DEMEP, formed phosphor  
Figure 17: Free radical copolymerisation of styrene (St) with 2-(acryloyloxy)ethyldiethylphosphate (ADP) [109]. Figure 18: Schemes of _ter_-polymerisation of St and a ring opening of 1,4,6-trioxaspiro[4,4]-2-nonylmethyl acrylate (SOE-AC) moieties [110]. phosphonate moieties, DEMMP and DEAMP, generated phosphonic acid in the condensed phase. These acidic species were believed to catalyse the Friedel-Crafts cross-linking of aromatic rings, thus increasing the thermal stability of the modified polymers. The stronger phosphoric acid was found to be promoting the cross-linking at a more rapid rate than the weaker phosphonic acid [38]. Thus, thermal degradation and the release of volatile products were much inhibited with phosphorus modification of the parent polymeric chains.