Synthesis And Self-assembly Of Novel Block Copolymers

This thesis consists of the following parts:1. Synthesis of multiblock copolymers by coupling reaction based on self- assembly and click chemistry.It is well known in polymer physics that A-B diblock or A-B-A triblock copolymers with a proper comonomer composition in a selective solvent can self-assemble to form polymeric core-shell-like micelles. Such a self-assembly forces and concentrates the reactive end-groups of the soluble block to stay on the periphery of each micelle, which should make the coupling or linking reaction much easier. Such a combination of polymer physics and synthetic chemistry has enabled us to develop a new methodology for the synthesis of long multiblock copolymers with an ordered chain sequence and controllable block lengths. Using such a self-assembly assisted coupling reaction, we have so far had a limited success in the preparation of multi-block copolymers by starting with A-B-A triblock copolymers terminated with two reactive end-groups. However, a commonly encountered drawback, when using multiple coupling reaction using a linking agent, is a low yield of multiblock products due to the slow and inefficient reactions between the di-ends-functionalized triblock copolymer chain ends and di-ends-functionalized linking agent. First, the addition of a right amount of linking agent to link each two of the reactive chain ends is always a problem, and insufficient or excessive amount of linking agent will reduce the linking efficiency. Second, highly efficient coupling reactions require reactive end-groups having very high activity, which are always sensitive to impurities such as moisty, carbon dioxide and oxygen. Therefore, a large amount of selective solvent for the self-assembly of triblock copolymers inevitably lose the coupling reactivities because of impurities. In addition, it is difficult to ensure that the linking agents can be dissolved by the selective solvents. As compared with linking reactions, directly self-assembly assisted coupling reaction among di-ends-functionalizaed block copolymers without linking agent should overcome above shortcomings. Click chemistry has been extensively used in polymer chemistry due to the high efficiency and technical simplicity of the reaction. Moreover, this procedure can be conducted in aqueous or organic media with little or no side reactions in a wide temperature range. Herein, we demonstrate the synthesis of multi-block copolymers by multiple coupling procedure using a combination of self-assembly and click chemistry. Di-ends-azido- terminated and di-ends-alkynyl-terminated poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymers (N3-PEO-b-PPO-b-PEO-N3 and HC≡C-PEO-b-PPO-b-PEO-C≡CH) were respectively prepared and used together as the precursors for multiblock copolymer synthesis through coupling reaction by combination of self-assembly and click chemistry. The self-assembly of equimolar resultant triblock copolymers in water, a selective solvent for PEO, results in a core-shell structure with the insoluble and collapsed PPO blocks as the core and the soluble and swollen PEO blocks as the shell. The self-assembly concentrates and exposes the azido and alkynyl end-groups on the periphery. The multiple click coupling reactions between the self-assembled di-ends-functionalized triblock copolymers were performed, leading to the highly efficient formation of ?(?PEO-b-PPO-b-PEO?)n? multiblock copolymer chains. In comparison, the click coupling reaction was also presented in N, N-dimethylformamide (DMF) solution without self-assembly. It was found that the efficiency of the coupling reaction was very low and no long multi-block copolymer chains were produced.2. Synthesis and self-assembly in water of novel double-crystalline amphiphilic polyethylene-b-poly[oligo(ethylene glycol) methyl ether methacrylate] linear- brush diblock copolymer.We study the synthesis and self-assembly of a novel well-defined linear-brush diblock copolymer with linear crystalline polyethylene (PE) as the coil block and hydrophilic poly[oligo(ethylene glycol) methyl ether methacrylate] (POEGMA) as rod brush via a combination of chain shuttling ethylene polymerization (CSEP) and atom transfer radical polymerization (ATRP). Initially, bromine end-terminated PE macroinitiator (PE-Br) was synthesized through the esterification of 2-bromo-2- methylpropionyl bromide with monohydroxyl-terminated PE (PE-OH) which was prepared by means of CSEP with 2,6-bis[1-(2,6-dimethylphenyl) imino ethyl] pyridine iron (II) dichloride / methylaluminoxane (MAO) / ZnEt2 and subsequent in situ oxidation with oxygen. The resultant PE-b-POEGMA linear-brush diblock copolymer was synthesized by ATRP of monomethoxy-capped oligo(ethylene glycol) methacrylate (OEGMA) using PE-Br as macroinitiator. The self-assembly of the double-crystalline PE-b-POEGMA in aqueous solution were investigated by dynamic light scattering, transmission electron microscopy and cryofield emission scanning electron microscopy. It was found that, in water, a solvent selectively good for the POEGMA brush, PE-b-POEGMA chains could self-assemble to form sandwich-like micelles with the insoluble and crystallized PE blocks as the interlayer cores and the soluble and swollen POEGMA brush as the outer-layer shell. The crystallization of both PE and POEGMA blocks in self-assembled structure formed from aqueous solution was investigated by differential scanning calorimetry.3. Synthesis and self-assembly in water of novel double-crystalline amphiphilic polyethylene-b-poly(ethylene oxide)-b- polyethylene triblock copolymerNarrowly distributed polyethylene-b-poly(ethylene oxide)-b-polyethylene triblock copolymers (PE-b-PEO-b-PE) with a linear PE block were successfully synthesized by click chemistry between polymer precursors of akynyl-terminated polyethylene (PE-≡) and di-ends-azido-terminated poly(ethylene oxide) (N3-PEO-N3). PE-≡was synthesized via the esterification of pentynoic acid with hydroxyl-terminated PE (PE-OH), which was prepared using chain shuttling ethylene polymerization. N3-PEO-N3 derived from tosylation and subsequent substitution by sodium azide of dihydroxyl-terminated PEO (HO-PEO-OH). The self-assembly of three double-crystalline PE-b-PEO-b-PE samples (PE-b-PEO2k-b-PE, PE-b-PEO6k-b-PE, PE-b-PEO8k-b-PE) with different PEO block length in water were investigated by laser light scattering (LLS) and transmission electron microscopy (TEM). It was found that, in water, a solvent selectively good for the middle PEO block, PE-b-PEO-b-PE chains could form flower-like micelles with the two insoluble and crystallized PE blocks as the core and cyclic swollen PEO blocks as the shell. Differential scanning calorimetry (DSC) experiments showed that the crystallization of both PE and PEO blocks was intensely confined by the previously self-assembled structure of PE-b-PEO-b-PE in aqueous solution.