Synthesis And Responsive Behaviors Of Macrocycles And Linear-Cyclic Conjugates Bearing Y Junctions

Synthesis and structure-property correlations of architectural polymers have been important topics in polymer science.By virtue of "living"/controllable polymerization techniques and highly efficient linking reactions,scientists have successfully prepared a wide range of linear,cyclic and branched polymers and revealed the dependence of properties and applications on architectures.Among them,cyclic polymers without end groups have attracted much attention due to their great promise in some fields such as biomedicine,catalysis and surface modification.Despite tremendous progress,there are still some problems.For instance,examples of multifunctional cyclic polymers and their derivatives comprising Y juncitons are relatively scarce,researches on properties are mostly limited to linear and monocyclic polymers,and hierarchical/multi-scale selfassembly remains at the early stage.Therefore,the efficient synthesis of multifunctional macrocycle-bearing polymers and their linear analogues,the programmed transformation of various nanostructures and macroscopic phase separation,and insight into the internal relationships between structures and properties have become urgent problems to be solved in the current research field of cyclic polymers.To address these questions,linear and star-shaped polymers with reactive functionalities were designed in this thesis.Diels-Alder(DA)reaction was adopted to achieve cyclization of main chain,double strain-promoted azide-alkyne click reaction(DSPAAC)was chosen to induce topological transformation from star to tadpoleshaped and hyperbranched polymers,and thiolactone chemistry was used to introduce stimuli-responsive or cleavable groups and polymer chains.A series of monocyclic,cyclic toothbrush-like and tadpole-shaped polymers with rich groups/compositions and single/multiple Y junctions were successfully achieved.On this basis,some approaches such as linking reaction,hydrolysis of ester group and cleavage of thioketal and dynamic covalent bond was used to achieve topological transformation,physical properties were tuned by some factors such as topology,chemical composition and external stimuli,and the topology effects were revealed.Specific research contents are listed as follows:Part one was aimed at multi-Tunable of thermoresponsive properties and aggregation behaviors of linear and cyclic polyacrylamide copolymers.A series of linear and cyclic P(DMA-co-ATL)were successfully prepared by reversible additionfragmentation chain transfer(RAFT)copolymerization of N,N-dimethylacrylamide(DMA)and N-thiolactoneacrylamide(ATL)and Diels-Alder ring-closure reaction.Using DEDA-triggered ring-opening reaction of thiolactone unit and thiol-Michael addition reaction using excess acrylate monomer(M)to prepare a series of linear and cyclic copolymers P(DMA-co-ATL)-g-DEDA/M(Lx-M/Cx-M,x=1-6)bearing Y junctions of temperature,pH and oxidation responsiveness.The test results showed that as the size of substituents(R=CmH2m+1)further augmented,the phase transition type of the polymers solution shifted from dual upper critical solution temperature(UCST)and lower critical solution temperature(LCST)(m=1-3)to LCST(m=4-8)until lack of phase transition(m=10).During temperature-induced self-assembly(TISA)process,L6-EA and L6-tBA could realize spherical micelles-vesicles-nanosheets-nanoribbonsspherical micelles/micelle clusters transitions,the morphologies of C6-tBA appeared successively spherical micelles-nanosheets-nanoribbons transitions,meanwhile,the formed two-dimensional lamellar structure showed improved thermal stability.Multiple morphological transitions could be primarily attributed to the differences in degree of hydration of distinct subunits and topological confinement of cyclic architecture during heating.Therefore,a convenient synthesis method of multiresponsive cyclic polymers was developed,a temperature-induced hierarchical selfassembly platform of cyclic polymers was preliminarily established,and physical properties were strongly dependent on some factors especially substituent and topology.Part two was aimed at synthesis and properties of linear and cyclic polyacrylamide copolymers with trifluoroethyl groups.Multi-step reactions were adopted to synthesize linear and cyclic P(NIPAM-co-TADP)(PNTA)and P(NIPAM-co-ADTB)(PNAD)with CO2 and O2 dual-gas responsive Y-junctions,in which NIPAM,TADP and ADTB represent N-isopropylacrylamide,2,2,2-trifluoroethyl 3-((3-acry lamido-4-((2(diethylamino)ethyl)amino)-4-oxobutyl)thio)propanoate and 2-acrylamido-N-(2(diethylamino)ethyl)-4-((3-oxo-3-((2,2,2-trifluoroethyl)amino)propyl)thio)butanamide,respectively.The LCST values of polymer solutions upon gas stimuli decreased in the order of O2-CO2>CO2-O2>O2>N2,and the application of CO2 stimulus led to absence of LCST due to enhanced hydrophilicity.l-PNTA and l-PNAD could form spherical micelles and vesicles by self-assembly,respectively.After being stimulated by different gases,l-PNTA was liable to form nanoribbons,while l-PNAD formed nanoribbons(O2),disk micelles(CO2),nanosheets(CO2-O2)and micellar clusters(O2CO2),respectively.As temperature increased,PNTA assemblies could exhibit hierarchical transformation from spherical micelles to vesicles and nanosheets.In constrast,l-PNAD showed a multi-stage spherical micelles-nanosheets-vesiclesnanosheets-nanobowls-nanosheets transitions,and c-PNAD also showed a multi-stage transitions from nanosheets to vesicles,nanosheets and spherical micelles.PNTA and PNAD could exhibit different TISA processes,in which PNAD assemblies were of alternating transitions of spheres/vesicles and lamella morphological by crossing multiple phase interfaces.Chemical composition and topology played important roles in physical properties,and gas/thermo-induced hierarchical self-assembly systems were successfully established.Part three was aimed at synthesis and properties of pH/oxidation-responsive toothbrush-like copolymers.Linear(P2)and cyclic(P3)P(St12-co-(MTL-gDMDA/PEGA)6)-b-PSt40 toothbrush-like copolymers were prepared by multi-step reactions.The polymer consists of hydrophobic polystyrene(PSt)backbone,hydrophilic poly(ethylene glycol)(PEG)side chain and multi-reactive Y-junction at the hydrophilic-hydrophobic interface.The test results showed that the cyclization of backbone could significantly increase the glass transition temperature,and reduce the solution viscosity and critical aggregation concentration.Both toothbrush-like copolymers could form unilamellar vesicles structure by aqueous self-assembly,and metastable double-lamellar vesicle structure could be formed by external factors such as acidic/basic conditions,heating and ultrasound treatment.In addition,long-term acidic and basic conditions could lead to hydrolysis/cleavage of ester and thioketal groups,leading to interesting hierarchical self-assembly.Unilamellar vesicles-large composite micelles-multicompartment vesicles(P2 assemblies)and unilamellar vesicles-double lamellar vesicles-disk micelles-large composite micelleshyperbranched micelles(P3 assemblies)transitions were observed at pH 3,and unilamellar vesicles-spherical micelles-sea urchin-like micelles-large composite micelles(P2 assemblies)unilamellar vesicles-double lamellar vesiclesmulticompartment vesicles(P3 assemblies)transitions occurred at pH 10.Upon oxidation stimulus and pH/oxidation stimuli,unilamellar vesicles only transformed into spherical micelles and their aggregates.In this chapter,pH could induce the cleavage of ester bond and thioketone bond of toothbrush-like polymers,and two types of topological transformation with reduced molecular weight,namely,linear toothbrush to starlike to linear polymers and cyclic toothbrush to linear-cyclic mixture to linear polymers were realized.In addition,an in-situ hydrolysis-induced hierarchical selfassembly platform was established,which broadened the preparation methods of various nanostructures.Part four was aimed at synthesis and properties of three-arm star,tadpole and hyperbranched polystyrenes containing succinimide thioether bond.The desired polymers were prepared by combining ATRP,azidation and DSAPPC ring-closure reaction.Both cyclic structure and enhanced molecular weight of the polymer could increase the glass transition temperature.In the presence of small molecular mercaptan,succinimide thioether bond was broken by heat,and two types of topological transformations,namely,star-to-linear and tadpole-to-linear/cyclic mixture transitions were realized.In addition,the effects of solvent types,topological structure and chemical composition on self-assembly in organic solvents were investigated.It was found that star-shaped and tadpole-shaped polystyrenes containing succinimide thioether bonds could self-assemble into spherical micelles in THF,and the tadpoleshaped polymers could undergo macroscopic phase separation within a certain period of time.Multi-scale self-assembly could be mainly ascribed to strengthened intra/intermolecular π-π interactions and topologically confined macrocycle,leading to the formation of larger aggregates by reassembly of nano-sized spherical micelles.In this chapter,three kinds of topological transformations,namely,star-to-tadpole,star-tohyperbranched,star-to-linear and tadpole-to-linear/cyclic mixture transitions were realized by chemical reactions,and multi-scale self-assembly behavior from nano to micron and larger size aggregates was preliminarily realized.In conclusion,cyclic polymers,cyclic toothbrush-like polymers,tadpole-shaped polymers and their linear analogues were synthesized,and they could act as model samples to afford insight into physical properties related to topology,chemical composition and external stimuli.Meanwhile,hierarchical and multi-scale selfassembly systems were successfully constructed.The research favors to enrich compositions and types of macrocycle-based polymers and extend synthetic methodology of cyclic polymers and their derivatives.In addition,the research is beneficial to reveal unique properties derived from cyclic architecture,achieve multitunable phase transition and aggregation behaviors.Consequently,the research progress can underlie the insight into potential applications of macrocycle-bearing polymers in material fields.