## 4 Fire Retardation of Styrenic Polymers with Phosphorous-Containing Compounds
### Reactive Modifications of Styrenic Polymers
#### 4.2.2 DOPO Derivatives (P+5)  
Other commercially important P-containing FRs are based on DOPO or its derivatives prepared by the chemical transformation of the P-H bond within its molecule. These compounds have been employed as the additive modifications in a variety of polymeric materials [62; 86; 111; 112]. However, as a relatively small molecule, the DOPO-based additive can be easily lost, thus limiting its application in styrene polymers. An effective method to overcome this shortcoming is to modify the structure of DOPO with other known FRs or to incorporate them during polymerisation processes in the presence of other reactive monomers. The most commonly used method is the chemical modification of DOPO through the reaction with paraformaldehyde, which resulted in the formation of 2-(6-oxido-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)methanol (ODOPM), as shown in Figure 20[48; 113; 114]. In several studies, ODOPM has been utilised as a precursor compound having a hydroxyl (OH) group that is able to undergo further chemical transformation (e.g., condensation reactions), producing different unsaturated compounds suitable for the reactive modifications [48; 113; 114]. For instance, [PERSON] et al. (2020) functionalised ODOPM with acryloyl chloride, leading to the formation of DOPO-methylacrylate denoted as DOPOAA (Figure 21a) [113]. Then, this DOPO-bearing monomer was incorporated into the PS chains through the emulsion polymerisation. The contents of DOPOAA units integrated into the chains of the copolymer were found to be 3.30 and 5.97 wt%. The TGA results of both St/DOPOAA copolymers showed no significant increase in the values of T\({}_{i}\), T\({}_{\max}\) and  
Figure 19: Chemical structures of unsaturated phosphates and phosphonates [38]. Figure 20: Synthesis of 2-(6-oxido-6H-dibenz[c,e][1,2]oxaphosphorin-6-yl)methanol (ODOPM) from DOPO [48; 113; 114]. the amounts of a TG char formed. However, the maximum degradation rate (Rmax) was found to be reduced by 32% and 46% for the copolymers with 3.30 and 5.97 wt% DOPOAA, respectively [113]. This trend can possibly indicate better thermal stability of the reactively modified PS. However, the condensed phase activity was limited, as the productivity of char was not improved significantly and remained below 1 wt% as for the homopolymer. Meanwhile, the microcone calorimetric (MCC) measurements revealed that the PHRR values dropped significantly, from 972 W/g in the neat PS to 354 W/g, in the copolymer [113]. At the same time, the LOIs increased from 18 vol% for the PS to 26 vol% for the St/DOPOAA copolymer. These results confirmed the improved fire performance of PS with built-in DOPOAA monomeric units. It was also suggested that during thermal decomposition, the DOPO groups produced PO radicals, which entered the gaseous phase and eliminated the free radicals generated during combustion of the polymers [113]. In another study, [PERSON] et al. (2010) reactively modified PS with DOPOAA and with 1-oxo-2,6,7-trioxa-1-phorsphabicyclo [2,2,2]oct-4-yl methyl acrylate (PEPA-AA, Figure 22) [114]. The comparative studies proved that the PEPA-AA moieties were more efficient in promoting the condensed phase mechanism of fire retardation in the copolymer than the actions of the DOPO derivative. The results of the different flammability parameters of both copolymers are summarised in Table 1.  
The TG char residue obtained in the inert atmosphere for the copolymer St/PEPAAA was increased to 36 wt%, while the residue from the St/DOPOAA was only 6.2 wt% under the same conditions. The lower char residue generated by the polymer with the covalently attached DOPO groups was explained by the formation of the weaker phosphonic acid instead of the stronger phosphoric acid during combustion [114]. The phosphonic acid seemed to be less effective in promoting acid-catalysed char production on the polymer surface. The similar trend was observed by [PERSON] et al. (2020) [113]. Nevertheless, in the air atmosphere, the amount of char residue formed by the St/DOPOAA was increased up to 19.3 wt%.