Covalent Stabilization of Self-assembled Structures of Block Copolymers Having Various Molecular Designs and Their Applications

Abstract

Self-assembly of amphiphilic block copolymers into various nanostructures continues to be a significant topic in modern research on biomimetics such as smart delivery vehicles, nanoreactors, and cellular mimics. Furthermore, to utilize self-assembled nanostructures in physically and chemically harsh environments without loss of structural integrity, cross-linking of hydrophobic blocks within the bilayer membrane should be required to provide mechanical stability. It allows the covalently stabilized nanoparticles to be used as potential candidate for the formation of nanoreactors and drug delivery systems in chemically and mechanically harsh conditions. In chapter 2, polymersomes composed of amphiphilic block copolymers containing polydimethylsiloxane with side-chain pendant vinyl groups were constructed. A reversibly deformable polymersome compartmentalizing membrane was obtained by cross-linkage of PEG-b-poly(dimethyl-r-methylvinyl)silane in a self-assembled bilayer via photo-radical generation in aqueous media. Polymersome-encapsulated hemoglobin bound oxygen reversibly, indicating the polymersomes could be used as O2 carriers that reversibly deform without sacrificing structural integrity or oxygen transportability. In chapter 3, a series of block copolymers synthesized by joining two structural modules, a branched poly(ethylene glycol) (PEG) hydrophilic block and linear/branched polyisoprene (PI) hydrophobic blocks. In addition to the block ratio and architecture of the block copolymer, PEG-b-PI, the length of the hydrophobic PI chain by adjusting its microstructure as a key structural parameter for the self-assembly of PEG-b-PI to form inverse cubic mesophases. The polymer cubosomes of these block copolymers can be covalently stabilized by the photo-radical-induced cross-linking of PI chains, resulting in the formation of rubbery hydrophobic domain. In chapter 4, we reported the synthesis of block copolymers (BCPs) incorporating a branched polyethylene glycol (PEG) hydrophilic block and randomly copolymerized styrene and isoprene units (p(St-co-Ip)) of different molecular weights as the hydrophobic component. The morphological transitions of self-assembled BCP structures are dependent on changes in the chain length of the hydrophobic block, which is determined by the regiospecificity of the isoprene units. The self-assembled structures could be covalently stabilized via intermolecular cyclization between the hydrophobic blocks. Notably, the cross-linked structures displayed a reversible swelling/deswelling ability in response to the type of solvent medium. In chapter 5, block copolymers (BCPs) composed of hydrophilic polyethylene glycol (PEG) blocks and discrete poly(phenyllactic acid) (dPPLA) blocks having hydrazone-based photoswitches in the specific positions: (1) middle of hydrophobic PPLA chains and (2) junction of amphiphilic chains. The nanostructures were constructed via solution self-assembly or direct hydration of BCPs and placed under light source to induce E/Z isomerization of hydrazone-based photoswitches. We found that the configurational switching of this system can attribute the reduced hydrodynamic volume of hydrophobic chains, leading to the shape transformations of self-assembled nanoparticles.