Self-assembled structures using CABC multi-block copolymers

Abstract

This thesis describes the self-assembly of block copolymers and their applications. Controlled radical polymerizations, in particular, the reversible complexation mediated polymerization (RCMP), were introduced as well. By applying a new temperature-selective dual initiator, the CABC multi-block copolymer was prepared using RCMP. Taking advantage of the asymmetric structure of the CABC multi-block copolymer, novel structural nanoparticles, interesting micellar morphological transformation, and the toroidal nanostructure were achieved in the thesis. In Chapter 1, I described the block copolymer self-assembly properties and their applications, especially in the drug delivery and stimuli-responsive systems. The assembly morphology influence factors were discussed as well. Controlled radical polymerizations, especially the reversible complexation mediated polymerization (RCMP), have been introduced. I also introduced the motivations and purposes of Chapters 2, 3 and 4. In Chapter 2, temperature-selective radical generation from a newly designed alkyl diiodide (I–R2–R1–I) was studied. R1–I and I–R2 had different reactivities for generating alkyl radicals in the presence of a tetraoctylammonium iodide (ONI) catalyst. Taking advantage of the temperature selectivity, I used the alkyl diiodide as a dual initiator in ONI-catalyzed living radical polymerization to uniquely synthesize CABC non-symmetric multi-block copolymers. Because of their non-symmetric structure, CABC multi-block copolymers form unique assemblies, i.e., Janus-type particles with hetero-segment coronas and flower-like particles with hetero-segment petals. In Chapter 3, I developed a temperature-directed micellar morphological transformation using CABC multi-block copolymers with a hydrophobic block A, a hydrophilic block B, and a thermally responsive block C with a lower critical solution temperature (LCST). The micellar structure was switched from a star (below LCST) to a flower (above LCST). The transition-temperature was tuneable in a wide range (11-90 oC) by varying the C monomer composition. The large difference in the loading capacity between the star and flower enabled efficient encapsulation and controlled release of external molecules. Unlike conventional systems, the present star-to-flower transformation keeps micellar structures and hence does not liberate polymers but only external molecules selectively. Another application is a hidden functional segment. A functional segment is hidden (shielded) below LCST and exposed to interact with external molecules or surfaces above LCST, which may serve as a new temperature-directed interface for, e.g., biological and sensing applications. Chapter 4 described the assembled morphologies evolution using CABC block copolymers with different length of each segment. The CABC block copolymer composing of hydrophobic block A, hydrophilic block B, and thermo-responsive block C with LCST. Varying the lengths of A and C segments (varying the hydrophobicity), the assembly morphological change from spherical micelles to discs, toroids, oval toroids, and cage-like structures. I also crosslinked toroids and studied their size change below and above LCST with keeping the toroid structures.