Amphiphilic/Thermoresponsive Block/Graft Copolymers: Synthesis And Aqueous Solution Properties

The aggregation and self-assembly behavior of amphiphilic/thermoresponsive polymers have attracted great academic interests there years, due to the potential applications related to nanocatalyst, biomedicine, advanced materials, etc. The molecular structure and composition have appeared to be the fundamental factors that regulate the aggregation behavior, especially the structural characteristics and physical properties of the polymeric aggregates. Therefore, it is necessary to synthesize copolymers with various well-defined molecular structures, narrow molecular weight distributions and tunable molecular compositions, so as to establish structure/properties relationships for the aggregation behavior in aqueous solutions. In our work, we have prepared series of amphiphilic/thermoresponsive block/graft copolymers with well-defined molecular structures, via anionic polymerization and living radical polymerization. Additionally, we have also investigated the physical properties, especially the aggregation behavior in aqueous solutions, of these copolymers. The main work and results are summarized as follows:1. We first present the synthesis of polystyrene-block-poly(p-hydroxystyrene-graft-ethylene oxide), PS-b-(PHOS-g-PEO), amphiphilic block-graft copolymers. The backbone diblock copolymers (PS-b-PHOS) were prepared by lithium-based anionic polymerization, followed by post polymerization acid hydrolysis of the poly(p-tert-butoxystyrene), PtBOS, precursor block. The PEO side chains were synthesized by metal-free anionic ring-opening polymerization of ethylene oxide (EO), using the phosphazene base (t-BUP4) and the phenolic hydroxyl groups (PhOH) in the backbones as the complex multifunctional initiating system. In all cases, star-like block-graft copolymers with high molecular weights and low polydispersities were synthesized. Well-controlled polymerization was achieved even with the molar ratio of t-BuP4 to PhOH being equal to 0.2. Dynamic and static light scattering and fluorescence spectroscopy studies were carried out to investigate the solution behavior of the amphiphilic block-graft copolymers, including the critical micelle concentration and structural characteristics of the aggregates formed in aqueous solutions. Because of the high PEO content and the star-like macromolecular architecture, the PS-b-(PHOS-g-PEO) block-graft copolymers form highly swelled aggregates with low aggregation numbers, having nanostructures resembling hyperbranched clusters.2. Thermoresponsive brush copolymers with poly(propylene oxide)-block-poly(ethylene oxide) side chains were synthesized via a "grafting from" technique. Near monodisperse poly(p-hydroxystyrene) was used as the backbone, and the brush copolymers were prepared by sequential metal-free anionic ring-opening polymerization of the oxyalkylene monomers, using the phosphazene base (t-BuP4) and the phenolic hydroxyl groups in the backbone to generate the complex multifunctional initiating system. The length and composition of the side chains were varied by changing the feed ratios of the backbone and the side chain monomers. By inverting the sequence of the monomer addition, two different molecular structures were achieved, with either poly(propylene oxide) or poly(ethylene oxide) linked to the backbone. In all cases, brush copolymers with high molecular weights and low molecular weight distributions were synthesized. The thermoresponsive behavior of the brush copolymers in dilute aqueous solutions was investigated by dynamic/static light scattering and fluorescence measurements. Temperature-induced intramolecular chain contraction/association and intermolecular aggregation could both be observed at different stages of the heating process. Intermolecular aggregation was more pronounced for the sample with the poly(propylene oxide) blocks located at the periphery. The results from fluorescence spectroscopy indicate the incompletely solvated state of the brush copolymer in aqueous solution at low temperature and the absence of compact hydrophobic domains in some of the aggregates, due to the core-shell brush-like molecular structure of the copolymers.3. Thermoresponsive brush copolymers with poly(propylene oxide-ran-ethylene oxide) side chains were synthesized via the "grafting from" technique discussed above. Poly(p-hydroxystyrene) was used as the backbone, and the brush copolymers were prepared by random copolymerization of mixtures of oxyalkylene monomers, utilizing metal-free anionic ring-opening polymerization, with the phosphazene base (t-BuP4) being the polymerization promoter. By controlling the monomer feed ratios in the graft copolymerization, two samples with the same side chain length and different compositions were prepared, both of which possessed high molecular weights and low molecular weight distributions. The results from light scattering and fluorescence spectroscopy indicated that the brush copolymers in their dilute aqueous solutions were near completely solvated at low temperature, and underwent slight intramolecular chain contraction/association and much more profound intermolecular aggregation at different stages of the step-by-step heating process. Above 50℃, very turbid solutions, followed by macrophase separation, were observed for both of the samples, which implied that it was difficult for the brush copolymers to form stable nanoscopic aggregates at high temperature. All these observations were attributed, at least partly, to the distribution of the oxyalkylene monomers along the side chain and the overall brush-like molecular architecture.4. Double hydrophilic poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) block copolymers were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization, using a PEO-based chain transfer agent (PEO-CTA). The molecular structures of the copolymers were designed to be asymmetric with a short PEO block and long PNIPAM blocks. Temperature-induced aggregation behavior of the block copolymers in dilute aqueous solutions was systematically investigated by a combination of static and dynamic light scattering. The effects of copolymer composition, concentration (Cp), and heating rate on the size, aggregation number, and morphology of the aggregates formed at temperatures above the LCST were studied. In slow heating processes, the aggregates formed by the copolymer having the longest PNIPAM block, were found to have the same morphology (spherical "crew-cut" micelles) within the full range of Cp. Nevertheless, for the copolymer having the shortest PNIPAM block, the morphology of the aggregates showed a great dependence on Cp. Elongation of the aggregates from spherical to ellipsoidal or even cylindrical was observed. Moreover, vesicles were observed at the highest Cp investigated. Fast heating leads to different characteristics of the aggregates, including lower sizes and aggregation numbers, higher densities and different morphologies. Thermodynamic and kinetic mechanisms were proposed to interpret these observations, including the competition between PNIPAM intrachain collapse and interchain aggregation.5. The effect of sonication on the size and structure of polymeric aggregates formed by amphiphilic block copolymers was studied by the combination of dynamic and static light scattering. Poly(ethylene oxide)-b-polyisoprene, poly(ethylene oxide)-polystyrene diblock copolymers and poly(ethylene oxide)-b-polyisoprene-b-poly(ethylene oxide) triblock copolymer were used as typical polymeric amphiphiles. Sonication was found to be an effective method to break up intermicellar associations and split large polymeric aggregates, present initially in the aqueous solutions, into monodisperse micelles. The content and type of hydrophobic block, copolymer solution-preparation protocol, and copolymer concentration were also investigated as co-factors in conjunction to the effect of sonication time.