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

Abstract

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.

Amphiphilic gradient copolymer of [poly(ethylene glycol) methyl ether] methacrylate and styrene via atom transfer radical polymerization

“…4,6,[8][9][10][11][12][13][22][23][24] Gradient copolymers have also been synthesized using other living or pseudo-living polymerization methods such as ring-opening metathesis polymerization 7,14,25 and cationic polymerization. 26 The simple synthetic procedures involved give gradient copolymers a major advantage over block copolymers.…”

Statistical copolymers of 2‐(trimethylsilyloxy)ethyl methacrylate and methyl methacrylate synthesized by ATRP

“…However, in contrast to block copolymers, the repulsive interactions between monomer units and/or sequences are gradually distributed along the chains in gradient copolymers, resulting in differences in interfacial behavior and thermal properties of gradient and block copolymers. 7,8 There are a few applications of gradient copolymers. For instance, these polymeric structures have recently been found to act as effective compatibilizers for polymers blends 9,10 and pressure sensitive adhesives.…”

Control of gradient copolymer composition in ATRP using semibatch feeding policy

“…18,22,23 Composition gradient as a new variable to fine-tune chain structure has attracted increased attention in the recent years. [24][25][26] In theory, the composition gradient can have any profile from uniform to block. The influence of some special composition distributions on polymer properties has been investigated.…”