Reversible modulation of polymer chain mobility by selective cage opening and closing of pendant boratrane units

Abstract

As a simple and versatile methodology for large modulation of polymer chain mobility, we report the selective opening and closing reactions of cage-shaped triethanolamine borate (TEAB) at polymer side chains and their effect on the thermal properties of the parent polymers. A series of TEAB-pendant polymethacrylates was synthesized by common free-radical polymerization using TEAB-containing methacrylate. Boron vertex removal from the pendant TEAB groups proceeded quantitatively via hydrolysis at room temperature to afford triethanolamine (TEA), an open cage structure. Differential scanning calorimetry revealed that the pendant TEAB/TEA interconversion induces a large variation in the glass transition temperature (Tg) of the parent polymers up to 166 °C, as a result of the contrasting conformational flexibility of the rigid TEAB and flexible TEA. Regeneration of the pendant TEAB group (cage closing) also proceeded in a highly selective manner by adding a commercially available borate ester to the TEA-pendant polymer. The mechanism underlying the high reaction selectivity was investigated using density functional theory calculations. The borated product showed a comparable Tg to that of the pristine TEAB-pendant polymer, demonstrating the efficiency of TEAB/TEA interconversion for reversible and large modulation of polymer chain flexibility. Selective opening and closing reactions of cage-shaped triethanolamine borate (TEAB) at polymer side chains and their effect on the thermal properties of the parent polymers are reported. The conversion of TEAB-pendant polymethacrylates into the triethanolamine (TEA) counterparts was quantitatively achieved by hydrolysis of the pendant TEAB groups. The side chain conversion resulted in a large variation in glass transition temperature (Tg). Regeneration of pendant TEAB group also proceeded in a highly selective manner and almost recover the original Tg, demonstrating the reversible and large modulation of polymer chain flexibility.