Effect of the terminal substituent of azobenzene on the properties of ABA triblock copolymers via atom transfer radical polymerization

The effect of the terminal substituent of azobenzene on the properties of ABA triblock copolymers was investigated. For this study, three kinds of azobenzene-containing monomers with different terminal substituents - 6-[4-(4-methoxyphenylazo)phenoxy] hexyl methacrylate, 6-[4-(4-ethoxyphenylazo) phenoxy]hexyl methacrylate, and 6-[4-(4-nitrophenylazo)phenoxy]hexyl methacrylate - were used to synthesize ABA triblock copolymers PMMAzo ₂₅-PEG₁₃-PMMAzo₂₅/PMMAzo₁₂-PEG ₁₃-PMMAzo₁₂, PEMAzo₁₄-PEG₁₃- PEMAzo₁₄, and PNMAzo₁₄-PEG₁₃-PNMAzo ₁₄, respectively, by atom transfer radical polymerization (PMMAzo is poly{6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate], PEMAzo is poly{6-[4-(4-ethoxyphenylazo)phenoxy]hexyl methacrylate}, and PNMAzo is poly{6-[4-(4-nitrophenylazo)phenoxy]hexyl methacrylate)). These copolymers were characterized with ¹H NMR spectroscopy and gel permeation chromatography and exhibited controlled molecular weights and narrow molecular weight distributions. Differential scanning calorimetry and polarizing optical microscopy showed that these copolymers had mesophases. PMMAzo ₂₅-PEG₁₃-PMMAzo₂₅ and PMMAzo ₁₂-PEG₁₃-PMMAzo₁₂ had a smectic mesophase and a nematic mesophase, whereas both PEMAzo₁₄-PEG₁₃- PEMAzo₁₄ and PNMAzo₁₄-PEG₁₃-PNMAzo₁₄ had a nematic mesophase. This demonstrated that the liquid-crystalline properties of these copolymers highly depended on the terminal substituent of azobenzene. The photoresponsive behavior of these copolymers was also investigated in tetrahydrofuran solutions, and the influence of the terminal substituents attached to azobenzene was studied.