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The ease of ignition and associated combustion hazards of modern synthetic polymers are major concerns, and often limit their wider applicability, especially in construction and transport sectors. It is quite evident that many of the chain-growth polymers that are widely used as commodity plastics in the modern era need adequate flame resistance before they can be put in the market. Whilst there are many known and tried-out methods, to achieve acceptable levels of fire proofing of polymers that undergo extensive random chain scission and/or unzipping, like polymethyl methacrylate (PMMA) and polystyrene (PSt), can be still problematic. The main impetuses behind the chosen chain-growth polymers (PMMA and PSt) emanated from their known desirable properties, such as the ability of PMMA to form transparent plaques and thermal-insulation properties of foamed polystyrene. Furthermore, the use of certain condensation polymers as fire proofing coatings are also under explored. Whilst there are a number of reports in the literature describing the use of phosphorus- containing compounds to flame retard PMMA and PSt, none of these have so far attempted a systematic and detailed investigation. In addition, the elucidation of mechanisms of flame retardance, operating in condensed and vapour phases, are also far from complete. Therefore, through the present work, an attempt to fill the current knowledge gap in this context is carried out. The primary aim of the present project was to design different methods to modify some commercially important polymers with phosphorus-containing groups with a view to exploring their passive fire protection attributes. In this context, the thermal and calorimetric properties of polyaniline (PA), polypyrrole (PPe) and polydopamine (PDA) were also examined. The corresponding attributes of some modified styrenic- and acrylic-based systems were investigated, in detail, which also included their combustion behaviours. Furthermore, in the case of chain-growth polymers, both additive and reactive strategies were adopted, where the modifying species consisted of phosphorus- and/or phosphorus/nitrogen-containing compounds/groups. Through the current programme, polymerizations of styrene- and methyl methacrylate-based systems were carried out through a host of polymerization routes, such as: solution, bulk, aqueous-slurry, suspension and emulsion. These processes, except the solution technique, are common practices in the industry owing to their environmentally benign features where monomer(s) are used as neat, or an aqueous-based heterogeneous medium is employed. The structural characterization of both the low-molecular weight compounds (i.e. precursors, additives and monomers) and polymeric systems was achieved through some routine spectroscopic techniques. Furthermore, the products obtained through the bulk polymerization route were chosen for detailed investigations in terms of their thermal degradation behaviours and combustion characteristics. Whilst the synthetic methodology employed to prepare step-growth polymers was found to be successful, and resulted in materials that exhibited superior thermal properties and combustion characteristics, all of them failed to produce uniform and coherent coatings onto mild steel and some common polymeric surfaces. It was also found that, generally with PMMA-based systems, the modifying groups only exhibited co-operative interaction during forced flaming combustion, whereas PSt-based systems also showed varying degrees of combustion inhibition in other tests (i.e. programmed heating: DSC, and forced non-flaming combustion: PCFC and ‘complete’ combustion: ‘bomb’ calorimetry). Some probable elements of mechanism(s) operating in the condensed- and gaseous-phases of the PMMA- and PSt-based systems were also formulated. In the case of PMMA-based systems, there is evidence that some of the P- bearing compounds/groups, upon thermal cracking during the early stages of flaming combustion, produce ‘phosphorus’ acid species. These acidic species can subsequently initiate the chemical pathway for producing char precursors. In the case of solid additives, it was found to be more likely that they produce phosphorus- and/or oxygenated phosphorus-containing volatiles that can act in the gaseous-phase. On the other hand, with PSt-based systems, a probable mechanistic pathway involving the phosphorylation of the phenyl rings leading to crosslinking and char formation is proposed. Furthermore, preliminary investigations were conducted with a view to gauging the corrosion inhibiting attributes of some of the modified-acrylic systems. In addition, two software programs, primarily developed in-house, were successfully utilized to analyse the kinetic parameters from TGA thermograms, and to follow the extent of corrosion of mild steel substrates with time. A relatively straightforward analytical technique (inductively-coupled optical emission spectroscopy: ICP/OES) was also explored with a view to following the extents of corrosion on mild steel surfaces, both coated and uncoated, of various sizes and shapes. In conclusion, the current research programme opened up potential and commercially viable routes to adequately flame retard two of the most important thermoplastics.