Synthesis And Properties Of Multi-Responsive Miktoarm Star Copolymers With Y Junctions

Synthesis and properties of topological polymers are important topics in polymer science.Among them,multicomponent polymers have tunable compositions and integrated functions and can form aggregates with complex microphase separation and potential functional regions.Meanwhile,stimuli-responsive polymers can be subjected to significant changes in conformations and amphiphilicity and destruction or recombination of chemical structures,and thus they hold great potential in nanotechnology and biomedical engineering.More recently,gas-responsive polymers have attracted increasing attention in addition to traditional smart polymers sensitive to external stimuli such as temperature,p H and redox.For instance,CO2 and O2 have some advantages such as large reserves,non-toxicity,environmental friendliness,reversibility,and easy removal,and they can act as promising external stimuli for novel smart polymers.This study aims at controlled synthesis of multi-responsive multicomponent miktoarm star copolymers(MMS)with multifunctional Y junctions and its thermoresponsive and aggregation behaviors upon external stimuli such as CO2,O2,thermo,p H and redox.To this end,controlled polymerizations involving reversible addition-fragmentation chain transfer(RAFT)polymerization,atom transfer radical polymerization(ATRP),and ring-opening polymerization(ROP)were combined to synthesize thiolactone-bearing 3-arm ABC-type miktoarm star quaterpolymer via “core first” method,followed by tandem amine-thiol-ene reactions to generate the desired MMS with Y junctions.The resultant star copolymers have thermoresponsive N-isopropylacrylamide(NIPAM)units,p H/CO2-responsive tertiary amine groups or p H-sensitive poly(acrylic acid)segment,O2-responsive trifluoroethyl groups,and oxidation-sensitive thioether moieties.In this research,a new strategy of enhanced compositions with reduced miktoarms(ECRM)was adopted to synthesize some ABC-type star quaterpolymers,and the introduction of a single or dual gas responsiveness could render efficiently tunable cloud points and morphologies of copolymer aggregates.Main contents of this dissertation are listed below.In part 1,the study aimed at synthesis and properties of an ABC star with O2 and redox dual-responsive Y junctions.A heterotrifunctional initiator 2-bromoisobutyryl amido-3-hydroxypropyl 4-(benzodithioyl)-4-cyanopentanoate(BHBCP)was designed and used to synthesize poly(N-acryloylhomocysteine thiolactone-co-N-isopropyl acrylamide)-arm-poly(?-caprolactone)-arm-poly(acrylic acid)(P(ATL-co-NIPAM)-PCL-Pt BA)star,followed by successive amine-thiol-ene reactions using 2-amino ethanol and N-(2,2,2-trifluoroethyl)acrylamide(TFEAM)and hydrolysis to afford poly(4-(2-(trifluoroethylcarbamoyl)ethylthio)-2-(acrylamido)-N-(2-hydroxyethyl)buta namide-co-N-isopropylacrylamide)-arm-poly(?-caprolactone)-arm-poly(acrylic acid)(P(TAHB-co-NIPAM)-PCL-PAA)star.The resultant star copolymer possessed thermoresponsive NIPAM units,biodegradable PCL segment,and p H-sensitive PAA segment,and multifunctional TAHB units comprised redox-sensitive thioether linkages,O2-responsive trifluoroethyl moieties and H-bond related hydroxyl and triple amide groups.1H NMR and GPC analyses confirmed that star quaterpolymer and its precursors had tunable compositions,well-controlled molecular weights and relatively low dispersity(? = 1.14-1.22).On this basis,self-assembly was performed to form aggregates,and the phase transitions were affected by some factors such as p H,O2 bubbling and oxidation.With increasing p H,the lower critical solution temperature(LCST)increased from 33.5 ?(p H 3.0)to 39.8(p H 5.9)and 50.1 ?(p H 7.4)and vanished at p H 10.Upon bubbling with O2,the enhanced LCST could reach up to around 11(p H 3.0),14(p H 5.9)and 30 ?(p H 7.4).After oxidation using excess H2O2,the LCST was larger than 88 ?.In addition,it was found that the sizes and morphologies of copolymer aggregates were strongly dependent on external stimuli.The star quarterpolymer could self-assemble into multicompartment vesicles(p H 3.0),vesicles(p H 5.9)and small micelle/vesicle mixtures(p H 10)at ambient temperature.Upon thermo,redox and O2 stimuli,the assemblies formed at different p Hs could be further converted into some other morphologies such as large compound micelles,vesicles,and nanocapsules with hollow caves.In part 2,the research aimed at synthesis and properties of an ABC star with CO2 and O2 dual-gas-responsive Y junctions.P(ATL-co-NIPAM)-b-PCL copolymer obtained in part 1 was used as a macroinitiator to initiate ATRP of 2-(diisopropylamino)ethyl methacrylate(DPA)to generate P(ATL-co-NIPAM)-PCLPDPA,followed by amine-thiol-ene reactions with 2-aminoethanol and TFEAM to afford a poly(4-(2-(2,2,2-trifluoroethylcarbamoyl)ethylthio)-2-(acrylamido)-N-(2-hydroxyethyl)butanamide-co-N-isopropylacrylamide)-arm-poly(?-caprolactone)-armpoly(2-(diisopropylamino)ethyl methacrylate)(abbreviated as P(TAHB-co-NIPAM)-PCL-PDPA)star bearing O2 and redox-responsive Y junctions.On this basis,the thermoresponsive and self-assembly behaviors of star copolymer upon external stimuli were investigated.The LCST varied from 59.6 ?(p H 3.0)to 49.4(p H 7.1)and 69.0 ?(p H 10)with increasing p H,and gas bubbling led to enhanced LCST(52.4 ? for CO2 bubbling,and higher than 68.6 ? for O2 bubbling).After oxidation using excess H2O2,the phase transition disappeared due to significantly enhanced hydrophilicity.The cloud point upon different stimuli was higher than that of the initial aggregates formed in deionized water(p H 7.1),revealing the enhanced hydrophilicity in distinct degree.These results could be ascribed to some factors such as quaternization of tertiary amine groups,O2-trifluoroethyl interaction and enhanced anion-dipole interaction under basic conditions.Meanwhile,the morphologies of copolymer aggregates were strongly dependent on external stimuli.At 25 ?,the star quarterpolymer could self-assemble into mixtures of small and compound micelles(p H 3.0),vesicles(p H 7.1)and compound micelles(p H 10).A single stimulus or dual stimuli related to gas could lead to remarkable changes in sizes and morphologies of copolymer aggregates,where the morphologies were flake-like micelles(O2),spherical micelles(CO2-O2),vesicles(CO2)and nanocapsules with hollow caves(O2-CO2).In addition,the application of thermo,p H and oxidation stimuli could induce the formation of other morphologies such as ellipsoidal micelles and hyperbranched vesicles.In summary,O2 and CO2-based gas stimulation was firstly introduced into MMS in this study,two-types of multi-responsive ABC-type star quaterpolymers were achieved,and thermoresponsive and aggregation behaviors were dependent on external stimuli.Upon a single stimulus or combined stimuli,the changes in amphiphilicity,chemical composition and intra/intermolecular interactions could induce widely tunable cloud points and sizes and morphologies of copolymer aggregates.This study develops a robust method to generate multicomponent miktoarm stars with rich compositions and tunable properties,and it is meaningful in enriching the types of MMS and stimuli-responsive polymers.This study affords new opportunity for synthesis,properties and applications of multifunctional MMS with Y junctions,and multi-triggered phase transitions and morphological transformations may endow multi-responsive star copolymers with great potential in smart materials.