Synthesis of Amphiphilic and Double Hydrophilic Star-like Block Copolymers and the Dual pH-Responsiveness of Unimolecular Micelle

A series of amphiphilic star-like triblock copolymers with well-defined molecular architectures, molecular weight, and narrow polydispersity were successfully synthesized by sequential atom transfer radical polymerization (ATRP) using beta-cydodextrin as the initiator. These star-like triblock copolymers are composed of poly(4-vinyl pyridine) (P4VP) as the first block, poly(tert-butyl acrylate) (PtBA) as the second block, and either polystyrene (PS), poly(methyl methacrylate) (PMMA), or poly(ethylene oxide) (PEO) as the third block (denoted as P4VP-b-PtBA-b-PS, P4VP-b-PtBA-b-PMMA, and P4VP-b-PtBA-b-PEO, respectively). Notably, by deliberate addition of a linear initiator [i.e., ethyl a-bromoisobutyrate (EBiB)] into the polymerization reaction when growing the second PtBA block (i.e., synthesis of star-like P4VP-block-PtBA; denoted as P4VP-b-PtBA), a suitable concentration of Cu2+ can be achieved. This suppresses further termination reactions between active chain ends of star-like polymer, thus rendering the ability to better control the growth of star-like P4VP-b-PtBA diblock copolymer (polydispersity index; PDI < 1.2). Kinetic studies of PtBA polymerization with and without the addition of linear initiators of EBiB were conducted. Importantly, the living nature of ATRP of star-like P4VP-b-PtBA diblock copolymer can only be attained when the linear initiator was added. Subsequently, star-like P4VP-b-PtBA-b-PS, P4VP-b-PtBA-b-PMMA, and P4VP-b-PtBA-b-PEO were synthesized by using star-like P4VP-b-PtBA as the macroinitiator via ATRP (for hydrophobic PS and PMMA) and dick reaction (for hydrophilic PEO), respectively, further confirming the tunability of surface chemistry (i.e., the outer block of star-like triblock copolymers) in this system. Finally, after hydrolyzing PtBA into poly(acrylic acid) (PAA), the dual pH-responsive behaviors of double hydrophilic star-like P4VP-b-PAA diblock copolymer were explored. In addition to dual pH-responsiveness, the strategy we developed based on sequential ATRP via cyclodextrin as the initiator in this study may conveniently enable the synthesis of a rich variety of other stimuli-responsive unimolecular star-like block copolymers comprising dissimilar blocks that are either pH-responsive, thermo-responsive, or photo-responsive. As such, they may afford a unique platform for fundamental research and applications in smart delivery vehides, sensors, and tunable templates for nanomaterials.