## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Reactive Modifications of Styrenic Polymers
#### 4.2.2 DOPO Derivatives (P+5)
This confirmed that the oxygen-containing atmosphere had an accelerating effect on the char formation by DOPO groups, since the phosphonic acid is oxidised to phosphoric acid, \begin{table}
\begin{tabular}{c c c c c} \hline
**Copolymer** & **UL-94 Rating** & **Dripping** & **LOI** & **1 CR (wt\%) (N2/Air)** \\ \hline St/PEPAAA & V-0 & No & 27.0 & 36.0/34.6 \\ St/DOPOAA & V-2 & Yes & 24.2 & 6.2/19.3 \\ \hline \end{tabular}  
* Char residue obtained in TGA runs at 500 °C.  
\end{table} 
Table 1: Comparative data of PS/PEPA-AA and PS/DOPOAA (DOPO–methylacrylate) copolymers from fire tests [114]. LOI: limiting oxygen index, TGA: thermogravimetric analysis, CR: char residue. Figure 21: Syntheses of different DOPO derivatives from ODOPM precursor [48,113]. Figure 22: Synthesis route of 1-oxo-2,6,7-trioxa-1-phorsphabicyclo [2,2,2]oct-4-yl methyl acrylate (PEPA-AA) [114]. which works as an excellent char-forming agent. Meanwhile, during the thermal oxidative degradation of the St/DOPOAA polymer, the cross-linking of phosphate structures increased [113]. Recently, the use of synergistic reactive FRs with P- and/or N-containing moieties has gained great attention owing to their effectiveness and lower negative impact on the environment [115]. The P-N synergism can accelerate the phosphorylation of the polymer, during thermal decomposition/combustion, by enhancing the in situ production of phosphoric acid due to catalysing _cis_-elimination (Figure 23) [90; 116]. This reaction retains phosphorus in the condensed phase, thereby promoting a char formation and thermal stabilisation of the polymeric substrate [116]. The _cis_-elimination process resulted in the formation of different compounds containing P-N linkages, thus enabling the higher retention of phosphorus in the condensed phase and more enhanced char formation during decomposition [116]. As the content of P in the char increased, the flame inhibition slowed, since the phosphorus in the condensed phase competed with the P-species release to the gaseous phase [116; 117]. The nucleophilic substitution of the above-mentioned ODOPM with \(N\)-hydroxymethylacrylamide and methyldichlorophosphate is behind the synthesis of novel P-N-containing reactive monomer 2-[(6-oxido-6H-dibenzo[c,e][1,2]oxphosphinin-6-yl)-methoxy]methyl phosphate \(N\)-methyl acrylamide (DMPMA) (Figure 21b) [48]. The free radical bulk polymerisation of St with DMPMA resulted in the copolymer having better fire retardance (Figure 24) than the homopolymer [48]. Indeed, the MCC measurements showed that the values of the PHRR were decreased from 840 W/g for the unmodified PS to 360 W/g for the copolymer containing 10 wt% of the incorporated DMPMA units. The combined effect of the condensed phase action and the gaseous phase activity in modified PS symbolised the synergistic effect of P- and N-bearing groups [48]. Meanwhile, the toxicity studies (with the aid of a steady-state tube furnace) established that the copolymers of St with DMPMA produced more CO and had higher smoke density compared to the same parameters measured for the neat PS [48]. Figure 23: Decomposition mechanisms of the reactive P–N-containing FRs [116]. [PERSON] et al. (2017) reviewed the available synthetic methods to design new organophosphorus FRs [118]. One example is the Kabachnik-Fields reaction involving three components with carbonyl and amino groups coupled with dialkyl phosphonate (Figure 25). This reaction can convert the P-H bond in the organophosphorus compound to the P-C bond, forming \(\alpha\)-aminophosphonates [118]. The P-N-containing compound (HOC\({}_{2}\)NP\({}_{2}\)), prepared through the Kabachnik-Fields process, can react further with acryloyl chloride to obtain ethyl-\(N\),\(N\)-tetramethyl-bis(phosphonate)-bis(methylene)amine acrylate (AC\({}_{2}\)NP\({}_{2}\)) [119]. The solution polymerisation of St with AC\({}_{2}\)NP\({}_{2}\), at 65 \({}^{\circ}\)C, using dioxane as a solvent, resulted in the formation of the polymer with enhanced fire performance.