Synthesis and Properties of Poly(Styrene‐co‐Phosphonate‐Containing Maleimide)

Soluble copolymers of diethyl(p‐(N‐phenyl)maleimide)phosphonate (4P), diphenyl (p‐(N‐phenyl)maleimide)phosphonate (PP), diethyl(p‐(N‐(2′,6′‐dibromophenyl)maleimide) phosphonate) (BrP) with styrene (St) were synthesized by radical polymerization in toluene at 70°C using 2,2′‐azobisisobutyronitrile (AIBN) as initiator. The comonomer reactivity ratios were calculated by the conventional Fineman‐Ross, Joshi‐Joshi, and Kelen‐Tüdos methods. The glass transition temperatures (T gs) and thermal degradation of copolymers were determined by differential scanning calorimetric (DSC) and thermo‐gravimetric analysis (TGA) methods, respectively. The introduction of phosphonate side‐chain into maleimide (MI) monomers changed the comonomer reactiviy ratio with St from an alternating to azeotropic copolymerization. The curves of T gs vs. the different compositions of the above synthesized MI–St copolymers matched the modified Johnston's equation as increasing the affect of weight ratio on alternating‐segment, and exhibited an S‐shaped curve of deviation in comparison with the Fox's equation. As the azeotropic composition of the above copolymers nearby Fox's equation, their thermal stability and flame retardancy could be enhanced simultaneously by the synergy effect of their comonomers. Furthermore, a rigid structure or polar bromine atom within the phosphonate‐containing MI segment of copolymer also could further promote its thermal stability and flame retardancy.