Stepwise Cleavable Star Polymers and Polymeric Gels Thereof

A multifunctional initiator 1 with chemically labile disulfide and ester groups bearing four bromoisobutyryl groups was prepared by esterification of bis(2-hydroxyethyl)disulfide with 2,2-bis(2-bromoisobutyroyloxymethyl)propionic acid. Then well-defined four-arm star polystyrenes, (S n )4, were synthesized by atom transfer radical polymerization of styrene initiated with 1. Owing to the presence of the −S−S− and the ester linkages in precisely controlled position of the (S n )4 core inherited from the initiator, such a star polystyrene demonstrated interesting stepwise cleavage properties dependent upon the applied conditions. When treating samples with tri-n-butylphosphine or dithiothreitol 2,3-dihydroxy-1,4-butanethiol, the disulfide linkage which located in the center of the star polymers was cleaved and, as a result, one star molecule (S n )4 was cut into two linear polystyrenes, 2(S n )2, with a −SH group situated in the middle of chains. Then, the resulted (S n )2 could be further cleaved into two shorter polystyrenes, 2(S n )1, by hydrolysis to cut two ester groups located in the middle of the PS chains with KOH in a solution of THF/EtOH. Moreover, the (S n )4 stars may also be decomposed into the shortest segment, 4(S n )1, directly by applying the hydrolysis condition. Furthermore, a polymer organogel was obtained from the cleavable star polymers (S n )4 by subjecting atom transfer radical coupling reaction which occurred between their −Br arm ends that preserved during formation of the star polymers. This gel could be also decomposed into the linear polystyrene segments of different length by breaking disulfide and ester linkages, respectively.