Synthesis and characterization of 4-arm star side-chain liquid crystalline polymers containing azobenzene with different terminal substituents via ATRP

4-Arm star side-chain liquid crystalline (LC) polymers containing azobenzene with different terminal substituents were synthesized by atom transfer radical polymerization (ATRP). Tetrafunctional initiator prepared by the esterification between pentaerythritol and 2-bromoisobutyryl bromide was utilized to initiate the polymerization of 6-[4-(4-methoxyphenylazo)phenoxy] hexyl methacrylate (MMAzo) and 6-[4-(4-ethoxyphenylazo)phenoxy]hexyl methacrylate (EMAzo), respectively. The 4-arm star side-chain LC polymer with p-methoxyazobenzene moieties exhibits a smectic and a nematic phase, while that with p-ethoxyazobenzene moieties shows only a nematic phase, which derives of different terminal substituents. The star polymers have similar LC behavior to the corresponding linear homopolymers, whereas transition temperatures decrease slightly. Both star polymers show photoresponsive isomerization under the irradiation with UV-vis light.