## 5 Conclusions
The comparative study of PS with phosphorus compounds containing various pendant groups revealed that those with diphenyl phosphonates and phosphates had much higher thermal stability compared to the materials with the incorporated diethyl phosphorus compounds [130]. The mode of action, which is essential for an adequate FR selection, varies from one formulation to another and depends on several factors. Given a complex nature of chemical components and mechanistic pathways that underpin the combustion cycle of a polymeric material, in general, and the relative predominance of condensed-, gaseous- and inter-phase, in particular, it is not often easy to identify the underlying physico-chemical processes unambiguously. Phosphorus compounds could function either as char promoters in the condensed phase or as fire inhibitors in the gaseous phase. The majority of the P-containing formulations were found to rely on a combination of both mechanisms. For the syrenic materials, the increasing of char content during combustion would be an important contribution to the improved fire retardance. The oxidation state of the phosphorus atom in a FR compound may influence the char-forming ability and the overall thermal stability of the parent polymer matrix. In general, the higher oxidation state of the phosphorus atom, as in the phosphate-based compounds, enables more efficient char production and improves the condensed phase activity of FR [34]. However, this trend can also vary widely, and often depends strongly on other factors including the chemical nature and constitution of the parent polymer matrix and modifying groups. Thus, more scientific data are required to draw conclusions about the effect of the oxidation number of P on the fire performance of the modified syrenic polymers. The synergistic phenomena between P and several other elements have been postulated by some authors. Normally, synergism is taken into account when the fire retardance of a polymer is improved by the combined effect of P and another element in the comparison to using each element separately, at the same loading levels. Among various synergistic systems explored so far, the P-N synergism was shown to be very beneficial for PS, though it is not systematically studied and clearly explained. The _ter_-polymerisation of styrene with two different monomers containing P and N atoms is another important route to improve the fire-retarding properties of PS. However, it is also a less studied approach and could be considered as an upcoming and promising technique. The analysis of publications in the present review revealed a variety of analytical techniques and test methods employed to assess the fire retardance of syrenic polymers. In the majority of studies, the evaluation of fire retardance was confined to TGA, LOI, CC, MCC, UL-94 and Gas Chromatography-Mass Spectrometry (GC-MS). Thus, it would be very beneficial for researchers to explore the capabilities of 'hyphenated' techniques such as TGA-MS, TGA-FTIR, laser pyrolysis/time of flight-mass spectrometry (LP/TOF-MS) and a steady-state tube furnace test, which would provide a clearer picture about the mechanisms of fire retardance. In addition, there is a need for researchers to adapt other characterisation techniques such as rheometry or Dynamic Mechanical Analysis (DMA) to determine the important attributes, which would help to forecast the behaviour of fire retarded polymers in real-scale fire scenarios. [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] contributed to the manuscript by writing different sections of the review. [PERSON] led the preparation of the manuscript. [PERSON] structured the manuscript and drafted the text. [PERSON], [PERSON], [PERSON], and [PERSON] reviewed and edited the text of the manuscript. All authors have read and agreed to the published version of the manuscript. This research was not supported any external funding.