Controlled Synthesis,Topological Transformation And Properties Of Multisite Copolymers

Controlled synthesis and structural transition of topological polymers have been important topics in polymer science.Inspired by intrachain folding of linear and cyclic polypeptides,many synthetic strategies of sing-chain folding nanoparticles have been developed.Despite tremendous progresses,examples of successive secondary and multiple intrachain folding of polymers are very scarce,and limited attention has been paid to simultaneous folding and grafting systems.To further extend subjects on topological transformation,multisite polymers can be adopted to prepare single-chain polymers as well as polymers with enhanced types and numbers of chemical compositions and functionalities via polymerization or post-modification reaction.Consequently,the goal to obtain multi-tunable properties and elevate potential applications of polymer materials can be made.In this context,this thesis is aimed at design of multisite polymers with reactive functionalities such as anthryl,thiolactone and alkyl bromide groups to achieve topological transformation such as linear-to-cyclic-to-intrachain folding-to-graft polymer transitions.On this basis,the influence of topology,chemical composition and external stimuli on physical properties involving glass transition temperature,phase transition temperature and macromolecular self-assembly is revealed.Main contents are shown below.The first part is intended to synthesis topological transformation and properties of linear block copolymers with multiple anthryl groups.First,2,3-di(2-bromopropionyloxy)propyl anthracene-9-carboxylate was used as the initial initiator to conduct atom transfer radical polymerization(ATRP)of styrene(St)and chain extension polymerization of tert-butyl acrylate(tBA)to generate anthracene-mid-functionalized PtBA-b-PSt-b-PtBA.Second,end group transformation and double-strain-promoted azide-alkyne cycloaddition reaction was combined to synthesize(PtBA-b-PSt-b-PtBA)n-type linear multiblock copolymer with pendant anthryl groups,followed by UV-induced intrachain anthracene dimerization to achieve sing-chain folding copolymer.Finally,Diels-Alder(DA)reaction between anthracene and maleimide functionalities were adopted to attach poly(N-isopropylacrylamide)(PNIPAM)segment onto triblock and multiblock copolymers,hydrolysis was performed to convert PtBA to poly(acrylic acid)(PAA)chain,and(PAA-b-PSt’)2PNIPAM-type three-arm star terpolymer and(PAA-b-PSt-b-PAA)n-g-PNIPAM-type graft terpolymer with dual pH and thermo-responsiveness were obtained.The resultant polymers had well controlled molecular weight and composition and relatively low dispersity.The combination of anthracene dimerization and DA reaction could efficiently convert linear copolymer into star,single-chain folding and graft copolymers.Single-chain folding could lead to different hydrodynamic volume,glass transition temperature and diffusion coefficient.In addition,star copolymer and graft copolymer with similar compositions and different molecular weight were chosen as nonlinear analogues to reveal the effects of topology and external stimuli on phase transition and aggregation behavior of polymer aqueous solution.Copolymer solutions formed at pH 3.0 and 5.8 could exhibit a lower critical solution temperature,while the solution at pH 10.0 lacked obvious phase transition.Compared with graft copolymer,star copolymer could exhibit higher phase transition temperature in aqueous solution at the same pH,indicating the role of topology in thermo-responsiveness.With increasing temperature,copolymer assemblies could exhibit morphological transformations such as lamella-vesicle-nanoribbon/micelle cluster and micelle cluster-nanoribbon transitions.Lamellar nanostructures formed from star copolymer at 25℃ showed excellent stability upon pH variation,and morphological transition order upon heating was quite different,indicating that aggregation behaviors could be efficiently adjusted by some factors such as topology,temperature and pH.Under different conditions,the changes of amphipathy,hydrogen bonding and electrostatic interactions could induce reaggregation of copolymer assemblies,leading to the formation of distinct nanostructures.The second part is aimed at synthesis,topological transformation and properties of multisite cyclic block copolymer.First,3-(2-formyl-3-methylphenoxy)propyl 4-(benzodithioyl)-4-cyanopentanoate was adopted to mediate RAFT copolymerization of styrene with 2-maleimidyl-4-thiobutyrolactone(MTL),N-(2-hydroxyl)ethyl maleimide(HEMI),N-(2-(2-bromoisobutyryloxy))ethyl maleimide(BEMI)to synthesize linear block copolymer/-[P(MTL-co-St)-b-PSt-b-P(HEMI-co-St)-b-PSt’-bP(BEMI-co-St)-b-PSt"](l-PSM)via one-pot strategy,followed by UV-induced DA ring-closure reaction to obtain cyclic block copolymer c-[P(MTL-co-St)-b-PSt-b-P(HEMI-co-St)-b-PSt’-b-P(BEMI-co-St)-b-PSt"](c-PSM).Second,tandem amine-thiol-ene reactions were adopted to convert c-PSM into macrocycle-derived intrachin-folding polymer F1,followed by ring-opening polymerization of ε-caprolacotne(CL)to synthesize the corresponding graft copolymer F2.Finally,three-step reactions involving successive ATRP of tBA using c-PSM macroinitiator,tandem amine-thiol-ene reaction to graft methyloxy-terminated poly(ethyl glycol)chains,and CL polymerization were conducted to achieve cyclic graft copolymers G1-G3,and hydrolysis was used to obtain cyclic graft copolymers G1’-G3’comprising pH-sensitive PAA segments.The resultant polymers were of well controlled molecular weight and composition and relatively low dispersity.Starting from linear multisite block copolymer,reaction-induced topological transformations from linear polymer to monocycle and quasi-twin cycle polymers occurred,and graft reaction enabled synthesis of cyclic graft copolymers with one to three types of side chains.Single-chain folding could lead to distinct hydrodynamic volume,glass transition temperature and diffusion coefficient.In addition,self-assembly of PAA-bearing copolymers was investigated.The results indicated that pH variation could change amphipathy,relative strength of hydrogen bonding and electrostatic interactions,and crystallinity of PCL segments,leading to the formation of distinct nanostructures such as spherical micelles,micelle clusters,cylindrical micelles and lamellae.In summary,two kinds of linear multisite polymers were designed to achieve multiple topological transformations,in which sing-chain folding copolymers were synthesized by intrachain crosslinking via anthracene dimerization and thio-bromo click reaction,and star,and linear/cyclic graft copolymers were prepared by polymerization or coupling reaction.They were used as model samples to reveal dependence of physical properties on topology and afford multi-tunable thermoresponsive and aggregation behaviors.The progress of this research favors to afford general methods to synthesize multifunctional materials,enrich types of topological polymers via sing-chain folding and graft modification,enable property variations via some factors such as topology,chemical composition,external stimuli and crystallization,and thus underlies systematic researches on developing novel polymeric materials.