Syntheis And Hierarchical Self-assembly Of Heterografted Toothbrush-like And Cyclic Graft Copolymers

Macromolecular self-assembly is an important topic in the field of polymer science and materials,and it has become an effective way to prepare functional materials at micro and nano scales.Starting from amphiphilic homopolymers or copolymers,a variety of aggregates have been achieved by macromolecular self-assembly.Besides,various nanomaterials can be directly prepared by polymerization-induced self-assembly,but it remains challenging to achieve reversible transformation between different morphologies.Therefore,it is extremely urgent to develop new efficient macromolecular self-assembly systems to prepare hierarchical nanostructures and achieve reversible transformations among different morphologies via adopting multiple factors.Based on macromolecular design,a new strategy defined as multi-tunable hierarchical self-assembly(MHSA)is proposed and developed in this study.Given topological confinement effect of highly dense graft system and macrocycle architecture and change of macromolecular shape via chain conformation upon external stimuli,this thesis aims at design,synthesis and properties of diblock toothbrush-like and cyclic graft copolymers with stimuli-responsive heterografts.Based on direct self-assembly in aqueous solution,subsequent programmable self-assembly is available by changing polymer concentration,chemical composition and adopting external stimuli such as temperature,pH and oxidation.Main contents are listed below.In part one,synthesis,thermoresponsivity and multi-tunable hierarchical selfassembly of multi-responsive(AB)mC-type miktobrush-coil terpolymers was investigated.Four examples of toothbrush-like copolymers with different chain length of poly(2-(dimethylamino)ethyl methacrylate)(PDMAEMA)were synthesized by combination of reversible addition-fragmentation chain transfer(RAFT)process,atom transfer radical polymerization(ATRP),end-capping reaction and amine-thiol-telomerization reaction.In addition to linear block of poly(N,N-Dimethylacrylamide)(PDMA),the heterografted block comprised the backbone of poly(styrene-co-maleimide)and two types of side chains,namely,poly(N-isopropylacrylamide)(PNIPAM)and PDMAEMA.Starting from 2-(2-cyanopropyl)dithiobenzoate(CPDB),RAFT copolymerization between 2-maleimidyl-4-thiobutyrolactone(MTL)and 4-vinylbenzyl 2-bromo-2-methylpropanoate(VBP),RAFT chain extension polymerization of DMA,ATRP of NIPAM,end-capping reactions,and one-pot aminolysis of thiolactone units and telomerization of DMAEMA were combined to generate toothbrush-like copolymers P((VBP-g-PNIPAM)-co-(MTL-g-PDMAEMA))-b-PDMA.Various copolymers were characterized by H NMR spectra,GPC and FT-IR spectra,and the resultant heterografted toothbrush-like copolymers had tunable chain length of each segment and dispersity in the range of 1.39-1.72.Influence of chain length of PDMAEMA,pH and oxidation on phase transition behaviors was investigated,and it was found that various factors could slightly affect the cloud points of polymer aqueous solutions,in which the cloud points were in the range of 38.6-43.0?(before oxidation)and 41.6-46.0?(after oxidation).On this basis,they acted as basic building blocks to construct an MHSA system,and the mechanism on formation and transformation of hierarchical nanostructures was discussed.As revealed by dynamic light scattering(DLS),transmission electron microscopy(TEM)and scanning electron microscopy(SEM)analyses,some factors such as chain length of PDMAEMA grafts,polymer concentration,chemical composition and external stimuli could efficiently induce hierarchical self-assembly,leading to the formation of a wide range of nanostructures-Under different conditions,self-assembly could be directly conducted in water,and various nanostructures such as spherical micelles,disk-like micelles,vesicles,disk-like compound vesicles,nanosheets,nanoribbons,cylindrical micelles and polyhedron micelles could be formed.As confirmed by temperature-variable 1H NMR analysis,some factors involving pH and temperature could induce the changes in degree of hydration of heterografts,chain conformation and amphipathy,leading to subsequent changes in macromolecular shape,packing parameter and mutual interactions.Consequently,the initial copolymer assemblies could be subjected to hierarchical reassembly and form distinct low-dimensional nanostructures.In part two,synthesis and properties of multi-responsive c-AmBn-type diblock cyclic graft copolymers were investigated.Three examples of cyclic graft copolymers bearing polyacrylate backbone and PNIPAM/PDMAEMA heterografts were synthesized by combination of RAFT polymerization,UV-induced Diels-Alder cycloaddition reaction and ATRP.First.3-(2-formyl-3-methylphenoxy)propyl 4-(benzodithioyl)-4-cyanopentanoate(FBCP)was used as a chain transfer agent to mediate RAFT polymerization of 2-(2-bromo-2-methylpropanoyloxy)ethyl acrylate(BEA)?followed by chain extension polymerization of 2-hydroxyethyl acrylate(HEA)to give a diblock copolymer PBEA-b-PHEA.Second,Diels-Alder cycloaddition reaction between terminal o-benzaldehyde and dithioester groups was induced by UV light to obtain cyclic PBEA-b-PHEA.Finally,successive ATRP of NIPAM,end group transformation and ATRP of DMAEMA were conducted to generate cyclic P(BEA-g-PNIPAM)-b-P(BEA-g-PDMAEMA).Based on 1H NMR and GPC analyses,the target polymers and their precursors had well-controlled molar mass,with dispersity up to 1.3.Influence of chain length of PDMAEMA and pH on phase transition behaviors was investigated,and it was found that various factors could slightly affect the cloud points of polymer aqueous solutions,in which the cloud points varied between 34.9 and 39.4?.Owing to high hydrophilicity,they could directly form copolymer assemblies in aqueous solution.As revealed by DLS and TEM analyses,different nanostructures such as vesicles,nanoribbons and nanosheets could be formed under various conditions,and the sizes of copolymer assemblies could be roughly tuned.The preliminary results indicated some factors such as chain length of side chains and solution pH could efficiently induce the hierarchical self-assembly to form distinct nanostructures.In summary,two types of novel diblock toothbrush-like and cyclic graft copolymers have been synthesized,and the influence of different factors such as polymer concentration,chemical composition and external stimuli on aqueous self-assembly has been investigated.A wide range of low-dimensional nanostructures have been directly prepared in aqueous solution,and a multifunctional platform to achieve multi-tunable hierarchical self-assembly that depends on composition,concentration and external stimuli has been constructed.More importantly,different nanostructures can be reversibly transformed under suitable conditions.With the progress of this study,topological polymers based on highly dense graft systems have been enriched,and some fields such as macromolecular self-assembly have been promoted.In addition,this research underlies systematic research on shape-dependent physical properties and potential applications of nanomaterials.