A facile approach for preparing tadpole and barbell-shaped cyclic polymers through combining ATRP and atom transfer radical coupling (ATRC) reactions
The ring-forming behavior of polymers with an odd number of arms in highly dilute solutions has been studied using tri-arm polystyrene (PSt3) as a model through combining atom transfer radical polymerization (ATRP) and atom transfer radical coupling (ATRC) technologies. The star polymer, PSt3, with three arms and a central core was obtained by ATRP of styrene using a designed initiator with three active Br terminals and ester groups, followed by the Br terminal–Br terminal radical coupling of PSt3, which was implemented via intramolecular ATRC under high dilution conditions. A tadpole-shaped cyclic polymer (tadpole-PSt) was successfully prepared, proving that there no intermolecular side reactions that occur during the intramolecular ATRC of PSt3. Interestingly, the tadpole-PSt not only possesses a cyclic topology, but also contains a living chain, in contrast to other cyclic analogues that have been previously reported. Finally, the tadpole-PSt was used as a building block to construct a type of barbell polymer (barbell-PSt) by removing most of the solvent via vacuum distillation at the end of the intramolecular ATRC of PSt3. Size exclusion chromatography, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, 1H nuclear magnetic resonance spectroscopy and hydrolysis experiments proved that high-purity cyclic polymers (tadpole-PSt and barbell-PSt) were successfully prepared. The thermal properties of the cyclic and star polymers were analyzed using differential scanning calorimetry and thermogravimetric analysis. The results demonstrated that the formation of a cyclic topology makes a huge contribution to the increase in the glass transition temperature (Tg) and thermal stability of the polymer.