A set of intrinsically flame retardant, halogen-free styrenic copolymers: Synthesis, characterization, processing, and properties

• New styrenic copolymers obtained by radical copolymerization of styrene, sulfur, and phosphorus comonomers. • The copolymers achieve excellent UL 94 V classifications without the use of additional flame retardant additives. • Halogen-free materials, which are suitable for applications with high environmental standards. Halogen-free, intrinsically flame retardant copolymers with glass transition temperatures in the range of 93 - 97 °C were synthesized from styrene, elemental sulfur, and organophosphorus comonomers in a facile radical bulk polymerization process. The copolymers were characterized using multidetector size exclusion chromatography (SEC), elemental analysis (EA), thin layer chromatography (TLC), UV/Vis spectroscopy, and 1 H, 13 C, and 31 P nuclear magnetic resonance (NMR) spectroscopy. SEC revealed number average molar masses (M n ) ranging from 14 - 18 kg/mol, and weight average molar masses (M w ) from 41 - 62 kg/mol. The phosphorus contents ranged from 0.1 - 0.6 wt.%, while the sulfur contents were 0.4 - 0.7 wt.%, with covalently bound sulfur being predominantly present in the lower molar mass fractions. Structural details of the formed sulfur-containing moieties in the copolymers were elucidated with homonuclear correlation spectroscopy (COSY) and 1 H/ 13 C heteronuclear single-quantum correlation spectroscopy (HSQC) techniques, indicating the presence of PS-substituted 1-phenylethyl sulfanyl units with a mono-, di- or trisulfidic arrangement. Thermogravimetric analyses (TGA) showed slightly decreased decomposition onset temperatures for all copolymers compared to that for polystyrene, and no or only minimal formation of residual masses above 500 °C. Selected copolymers were extruded and injection molded on a laboratory scale. UL 94 vertical flame testing demonstrated a good flame retardant performance for most copolymer types, alongside with short after-flaming times and intensive melt dripping. Increased melt flow caused by polymer decomposition, probably initiated by the sulfur moieties in the copolymer structures, and cooling are considered the main condensed-phase flame retardant processes involved. Two copolymers of the set display a UL 94 V-0 classification.