Halogen-bond-directed polymer synthesis and structure formation in solid-phase, in liquid-phase, and on surface and applications to functional materials

• Main Author: Le, Hong Tho
• Other Authors: Atsushi Goto
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Science::Chemistry::Organic chemistry::Polymers; Science::Chemistry::Crystallography
• Online Access: https://hdl.handle.net/10356/164056
• Institution: Nanyang Technological University

This thesis describes the utilization of halogen bonding in free-radical solid-phase polymerization (SPP) and its applications to synthesize conjugated polymers and emissive polymers, which are inaccessible to the radical polymerization in solution. Halogen bonding was further employed to create unique polymeric self-assembly structures in three (solid, surface, and solution) phases. The obtained polymers are applied to pre-shaped and stimuli-responsive materials, metal-ion adsorbents, host-guest interactive polymers, polymeric carriers and emissive materials. In Chapter 1, the principles of halogen bonding and SPP are introduced. The fundamentals of free radical polymerizations and reversible complexation mediated polymerization are explained. The concept of halogen-bond-driven supramolecular structures, stimuli-responsive polymers, and polymer brushes, as well as aggregation-induced emission enhancement (AIEE) and crosslink-enhanced emission (CEE) are discussed. The thesis objective and aims of each chapter in this thesis are also described. Chapter 2 describes the first free-radical SPP of vinyl monomers via halogen bonding. Nitrogen-containing and oxygen-containing vinyl monomers were cocrystallized with iodine-containing linkers to form monomer cocrystals via halogen bonding. The resultant monomer cocrystals were subsequently polymerized via free-radical SPP. The polymers with high molecular weights and relatively low dispersities were obtained owing to the high degree of the monomer alignment in the cocrystals. Interestingly, liquid monomers were converted into solid monomers, enabling to pre-shape desired structures before the polymerization. The structures were retained after SPP to generate a two-layer polymer sheet and a three-dimensional (3D) house-shaped polymer material. The two-layer polymer sheet had a dual porous network after the linker removal and acted as a solvent-responsive shape memory polymer because of the different swelling properties of the two layers. Chapter 3 describes the first free-radical SPP of acetylenes. Acetylene monomers were cocrystallized via halogen bonding, followed by the SPP, yielding polyacetylenes (not oligo-acetylenes (oligomers)). The proximity of acetylene monomers in the cocrystals enables effective propagation to generate conjugated polyacetylenes with high molecular weights, which is inaccessible in solution-phase radical polymerizations. The SPPs of a diacetylene monomer followed by the linker removal yielded two-dimensional conjugated microporous polyacetylenes (2D CMPs), in which the polymer nanosheets cumulated in layer-by-layer manners. The pore sizes were modulated by the linkers. Notably, the resultant 2D CMP exhibited the highest ever absorption of lithium-ion (Li+) and high adsorption of boronium-ion (B3+). The 2D CMPs are purely organic compounds and can be environmentally friendly materials. This technique might be amenable to a variety of electron-donating acetylenes and diacetylenes and may open up new materials. Chapter 4 describes the first dual use of halogen bonding in driving AIEE and CEE. Weak luminophores were used as halogen bonding linkers. Upon the cocrystallization with vinyl monomers via halogen bonding, the linkers were rigidly aligned in the monomer cocrystals to drive AIEE. The obtained cocrystal monomers subsequently underwent SPP, yielding crosslinked polymers. The linkers were incorporated into the polymer matrix, and the resultant restriction of the intramolecular motions of the luminophores led to CEE. By modulating the polymer formation in the SPP (CEE), emission-patterned polymer sheets were fabricated. Halogen bonding is a reversible bond and its strength is tuneable. Taking advantage of those nature of halogen bonding, stimuli-responsive (temperature, pH, and solvent-responsive) emissive polymeric materials were created. Host-guest interactive polymers were also obtained, in which the emissions were tuneable subjecting to the embedded luminophores. Chapter 5 describes the use of halogen bonding in directing self-assemblies of quaternary ammonium iodide (QAI)-containing polymers, generating unique assemblies in three (solution, surface, and solid) phases. In solutions, QAI-containing block copolymers generated unique self-assembly structures including giant micrometre-sized vesicles and a one-dimensional (1D) structure via inter-vesicular linkages. External molecules were successfully loaded and unloaded in the assemblies owing to the temperature dependence of the halogen bonding strength. On surfaces, reversible halogen bonding crosslinking and decrosslinking of QAI-containing polymer brushes were achieved. The wettability of the polymer brushes was facilely tuned depending on the halogen-bond-crosslinking density (or the QAI unit fractions). In solids, QAI-containing monomers were cocrystallized via halogen bonding and were subsequently polymerized via free-radical SPP, generating a QAI-containing polymer sheet. The resultant polymer sheet allowed capturing and releasing of different halogen bonding linkers as guests via host-guest interaction, thereby may serve as a polymeric host for a wide range of halogen-bond-coordinating molecules.