Synthesis of Bottlebrush Block Copolymers Containing Polystyrene and Polymethacrylates and Their Self-Assembly

Abstract

Bottlebrush polymers are comb-like macromolecules with polymeric side chains densely grafted to a linear backbone. Due to their chemical structures, these polymers exhibit low chain entanglement and ability to rapidly self-assemble to highly ordered nanostructures with large domain size compared to their linear counterparts. For the synthesis of well-defined bottlebrush block copolymers (BBCPs), high-quality macromonomers (MMs) are required without any undesired side reaction during polymerization. Living anionic polymerization (LAP) and ring opening metathesis polymerization (ROMP) are well-known as brilliant techniques for synthesis of MMs and bottlebrush polymers with controlled molecular weights (MWs) and low dispersity in high conversions. Recently, various structural types of BBCPs with the combination of polystyrene (PS) and polylactic acid or polyisocyanates or polystyrene and polyethylene oxide were published for photonic crystal applications. However, only a few PS and polymethacrylates-based BBCPs with relatively low MWs have been studied. For diverse and specific investigation of this kinds of BBCPs, I chose PS and three kinds of polymethacrylates MMs (poly(tert-butyl methacrylate), poly(methyl methacrylate), and poly(benzyl methacrylate)). The BBCPs containing symmetric block ratios or bottlebrush random copolymers of polymethacrylates as one block were synthesized through ROMP with “grafting through” strategy. Chapter 1 contains outline of bottlebrush polymers and their synthetic strategies through living controlled radical/anionic polymerizations for well-defined MMs. Chapter 2 introduces overview of photonic crystals and studies of BBCPs for photonic crystal applications through their self-assembly. Herein, the first synthesis of well-controlled ultrahigh molecular weight BBCPs which have PS and polymethacrylates (poly(tert-butyl methacrylate) (PtBMA), poly(methyl methacrylate) (PMMA), or poly(benzyl methacrylate) (PBzMA)) as side brushes, was studied by combining LAP and ROMP. (PNB-g-PSt)-b-(PNB-g-PtBMA) was well-controlled with high molecular weight and a low dispersity (Mn = 3002 kDa, Ð = 1.17) with a total DP of 976. The self-assembly of the resulting BBCP films revealed periodic lamellar morphologies with characteristic photonic crystal properties. Chapter 3 introduces some studies of synthesis/self-assembly of bottlebrush random copolymers. In chapter 2, the self-assembly of BBCPs based on PS and PBzMA showed disordered morphologies because the side chains of each block are more compatible relatively, despite the total degree of polymerization of the BBCP being scaled up to 1045. Herein, I explored the synthesis and self-assembly of nine different BBCPs (A-b-(B-co-C) containing bottlebrush polystyrene (A) and bottlebrush random copolymers (B-co-C) of polymethacrylates. To induce the phase separation of bottlebrush polystyrene (PNB-g-PS) (A) and bottlebrush poly(benzyl methacrylate) (PNB-g-PBzMA) (C)-based BBCP with an extremely low Flory-Huggins interaction parameter (χ), three kinds of bottlebrush polymethacrylates (B): polynorbornene-g-poly(methyl methacrylate) (PNB-g-PMMA), polynorbornene-g-poly(tert-butyl methacrylate) (PNB-g-PtBMA), and polynorbornene-g-poly(methacrylic acid) (PNB-g-PMAA), respectively were randomly combined with C. An order-disorder phase transition of the BBCPs (A-b-(B-co-C)) was observed with a change in mole ratios of PMMA, PtBMA or PMAA to PBzMA of 25, 50, and 75% in random copolymer blocks using field-emission scanning microscopy (SEM). Chapter 4 covers phase separation of the BBCPs based on PS and PBzMA with extremely low χ value by hydrogen bonding with 3-n-pentadecylphenol (PDP). PDP formed hydrogen bonding with carbonyl group of PBzMA. Self-assembly with PDP induced phase transition of lamellar structure with increasing domain size and red-shift of structural colors depending on molecular weight of the BBCPs. The prepared films of the supramolecular complex revealed reversible phenomena of photonic crystals and crystallization of long alkyl chains of PDP on the film surface between 70 ℃ and room temperature. The morphologies of all samples were disordered at 180 ℃.