Synthesis and glass transition behavior of high molecular weight styrene/4-acetoxystyene and styrene/4-hydroxystyrene gradient copolymers made via nitroxide-mediated controlled radical polymerization

Styrene (S)/4-acetoxystyrene (AS) gradient copolymers were synthesized by moderate-temperature, nitroxide-mediated controlled radical polymerization. Hydrolysis of styrene/4-acetoxystyrene gradient copolymers produced styrene/4-hydroxystyrene (HS) gradient copolymers. Molecular weight (MW) characterization via gel permeation chromatography demonstrated that these materials were made in a "controlled" manner, while intrinsic viscosity measurements revealed that the MWs exceeded 100 000 g/mol, with apparent viscosity-average MW values ranging between 100 000 and 385 000 g/mol, making these materials the first high MW gradient copolymers ever synthesized. Characterization of the glass transition temperature, T _g, revealed different behavior depending on the type of gradient copolymer produced. Using a normal thermal history for measuring T _g by differential scanning calorimetry (DSC), linear gradient copolymers exhibited one T _g, with a value intermediate to the T _gs of polystyrene (PS) and poly(4-acetoxystyrene) (PAS) or poly(4-hydroxystyrene) (PHS). In contrast, "blocky" gradient copolymers with overall S content ≥55 mol % yielded two T _gs, one near the T _g of PS and the other intermediate to the T _gs of PS and PAS or PHS, indicating microphase separation. When the cumulative composition of the "blocky" gradient copolymer was majority AS or HS, only one T _g was resolved, with a value near the T _g of PAS or PHS. For S/AS and S/HS gradient copolymers of identical chain length, overall fractional S content, and strength of gradient, physical aging at 90°C provided resolution of a second T _g via DSC, allowing comment on how manifestations of microphase separation depend on gradient structure and the strength of the comonomer repulsive interactions.