Polymers 2022, 14, 1520. https://doi.org/10.3390/polym14081520 www.mdpi.com/journal/polymers Article Thermal and Calorimetric Investigations of Some Phosphorus‐Modified Chain Growth Polymers 2: Polystyrene Malavika Arun 1, *, Stephen Bigger 1 , Maurice Guerrieri 1 , Paul Joseph 1, * and Svetlana Tretsiakova‐McNally 2 1 Institute of Sustainable Industries and Liveable Cities, Victoria University, P.O. Box 14428, Melbourne, VIC 8001, Australia; stephen.bigger@vu.edu.au (S.B.); maurice.guerrieri@vu.edu.au (M.G.) 2 Belfast School of Architecture and the Built Environment, Ulster University, Newtownabbey BT37 0QB, UK; s.tretsiakova‐mcnally@ulster.ac.uk * Correspondence: malavika.arun@vu.edu.au (M.A.); paul.joseph@vu.edu.au (P.J.); Tel.: +61‐410‐279‐828 (M.A.) Abstract: In this paper, we report on the thermal degradation behaviours and combustion attributes of some polymers based on polystyrene (PSt). Here, both additive and reactive strategies were em‐ ployed, through the bulk polymerization route, where the modifying groups incorporated P‐atom in various chemical environments. These included oxidation states of III or V, and the loading of phosphorus was kept at ca. 2 wt.% in all cases. The characterization techniques that were employed for the recovered products included spectroscopic, thermal, and calorimetric. It was found that the presence of different modifying groups influenced the degradation characteristics of the base poly‐ mer, and also exerted varying degrees of combustion inhibition. In all cases, the modification of the base matrix resulted in a noticeable degree of fire retardance as compared to that of the virgin ma‐ terial. Therefore, some of the modifications presented have the potential to be explored on a com‐ mercial scale. Keywords: polystyrene; phosphorus‐containing compounds; additive and reactive routes; thermal stability; combustion attributes 1. Introduction Polystyrene (PSt) is a relatively inexpensive, readily available, and transparent ther‐ moplastic polymer that is used for a wide number of applications [1]. The monomer, sty‐ rene, may also be copolymerised with other monomers to produce co‐ and/or ter‐poly‐ mers, often having improved properties [2]. Some of the modified systems can be utilized for specific functions, such as removing heavy metals from water [3–6]. Some of the mul‐ ticomponent systems include styrene acrylonitrile (SAN), acrylonitrile‐butadiene‐styrene (ABS) polymers, styrene‐butadiene rubber (SBR), etc. Furthermore, there are two types of polystyrene foams that are commercially available: expanded polystyrene (EPS) and ex‐ truded polystyrene (XPS). Expanded polystyrene is generally used for food packaging applications and XPS, which is a higher density foam, is used in the building sector [7]. One of the disadvantages of PSt, despite its useful characteristics, is its relatively high flammability. When ignited, polystyrene and its copolymers often burn quickly with a visible flame, releasing volatiles including styrene monomer, oligomers, lower hydrocar‐ bons such as benzene, lower alkylbenzenes [8], etc. During the burning process, polysty‐ rene can also melt, flow, and drip, which can lead to an increased fuel load feeding into enhanced flame spread [9]. Generally, combustion of unmodified polystyrene produces a minimal amount of char residue. Polystyrene homopolymer generally starts degrading at a temperature around 270 °C and continues until 425 °C under normal conditions in air. Through random main‐ Citation: Arun, M.; Bigger, S.; Guerrieri, M.; Joseph, P.; Tretsiakova‐McNally, S. Thermal and Calorimetric Investigations of Some Phosphorus‐Modified Chain Growth Polymers 2: Polystyrene. Polymers 2022, 14, 1520. https://doi.org/10.3390/polym14081520 Academic Editor: Bob Howell Received: 22 February 2022 Accepted: 6 April 2022 Published: 8 April 2022 Publisher’s Note: MDPI stays neu‐ tral with regard to jurisdictional claims in published maps and institu‐ tional affiliations. Copyright: © 2022 by the authors. Li‐ censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con‐ ditions of the Creative Commons At‐ tribution (CC BY) license (http://crea‐ tivecommons.org/licenses/by/4.0/).