Synthesis And Thermoresponsive Properties Of Multisensitive Linear And Graft Copolymers With Amido Sulfide Units

Motivated by highly adaptive biological systems in nature,a wide range of smart materials have been created.Among them,stimuli-responsive polymers with integrated chemical compositions and functions have attracted much attention and have a great potential in various fields involving nanomaterials and biomedical materials.By the virtue of macromolecular design,we can not only introduce different types of functionalities,polymer segments and architectures into the desired polymers but achieve multi-responsive polymer materials with improved properties and applications.At present,some strategies such as modular synthesis,postpolymerization modification and multicomponent polymerization have become efficient methods to synthesize functional polymers,by which thiolactone chemistry has acted as a promising tool to construct some topological polymers.However,most of cyclic graft polymers lack multisensitive properties,and the examples of thermoresponsive polymers with dual low(LCST)and upper(UCST)critical solution temperature-type phase transitions are very scarce.In this context,this study aims at the influence of various factors such as topology,chemical composition,external stimuli,deuterated solvent and solution blending on tunable inter/intramolecular interactions and eventual control over the thermoresponsive behaviors of polymer solutions.To this end,postpolymerization modification and multicomponent polymerization based on thiolactone chemistry were adopted to synthesize linear/cyclic graft copolymers.On this basis,the target polymers were fully characterized,and the effects of various factors on phase transitions of polymer solutions were investigated.Main contents and results are listed below.In Part 1,the research was focused on synthesis and thermoresponsive behaviors of multi-responsive linear/cyclic graft copolymers with multifunctional Y junctions,polyacrylamide backbone and PEG grafts.The target graft copolymers were synthesized by RAFT copolymerization,UV-induced Diels-Alder cycloaddition reaction and tandem amine-thiol-ene reactions.Linear poly(NIPAM-co-ATL)(l-PNA)was initially synthesized by 3-(2-formyl-3-methylphenoxy)propyl 4-(benzodithioyl)-4-cyanopentanoate(FBCP)mediated RAFT copolymerization of N-isopropylacrylamide(NIPAM)andN-acryloylhomocysteine thiolactone(ATL),and followed by Diels-Alder reaction to generate cyclic poly(NIPAM-co-ATL)(c-PNA).Last,linear(l-PDP)and cyclic(c-PDP)graft copolymers poly(NIPAM-co-ATL)-g-DEDA/PEGA(where DEDA means N,N-diethylethylenediamine)were synthesized by tandem reactions involving ring-opening reaction between thiolactone group and DEDA and subsequent thiol-ene Michael addition reaction.Various polymers were characterized by 1H NMR,GPC,IR and DSC,and the results revealed that the target copolymers had well-controlled composition,molecular weight and architecture,with dispersity less than 1.2.The effect of cyclization was confirmed by the elevated cloud point in H2O up to 12.8 °C under same conditions.The PEG-connecting Y junctions with dual amide,thioether and tertiary amine groups allowed thermo-,solvent-,pH/CO2-,and oxidation-switchable inter-/intramolecular hydrogen bonding interactions and hence resulted in unusual solvent(H2O or D2O)and pH(about 6.8-8.5)dependent phase transition and irreversible CO2-responsive behavior.Owing to the formation of more stable intramolecular hydrogen bonds within the dual amide groups of l-PDP and NHCH2CH2N moieties of c-PDP in D2O,they could exhibit remarkably distinct cloud points in H2O and D2O.The presence of peripheral PEG grafts and pH/CO2-switchable interactions in the Y junctions were critical to exhibit LCST-type phase transition in a narrow pH window(pH ? 6.8-8.5).With increasing basicity,the gradual deprotonation of tertiary amine moiety favored to form intramolecular hydrogen bonds within the Y junction surrounding by the hydrophilic PEG segment,and the anion-dipole interactions between OH-and polar groups was liable to increase,leading to gradually elevated cloud points until the phase transition vanished.The irreversible CO2-responsive behavior could be ascribed to some factors involving the strong interaction between CO2 and diethylamino group,the Lewis acid-base interaction between CO2 and thioether/ether groups,and the relatively high pKa of copolymer solution.The oxidation from sulfide into sulfoxide and sulfone could enhance the hydrophilicity and hinder the thermoresponse of graft copolymers.By virtue of solution blending,the mixtures could exhibit one or two phase transition(s)strongly dependent on types and molar compositions of the blends.With increasing molar fraction of cyclic graft copolymer c-PDP,the linear non-grafted/cyclic grafted mixtures were liable to exhibit a single LCST(r ? 0.2)and two independent phase transitions(r = 0.25-0.95)with LCST difference ranging between 22 and 32 ?.The appearance of dual LCST transitions of linear non-grafted/cyclic grafted mixtures using suitable feed ratios could be ascribed to thermo-induced coaggregation and reaggregation,as evidenced from DLS measurements at different temperatures.In Part 2,the research was focused on synthesis and thermoresponsive behaviors of multi-responsive linear poly(amido sulfide)with hydroxyl and diethylamino functionalities.A series of novel poly(amido sulfide)copolymers were readily synthesized using ATL,ethanolamine and DEDA as starting materials via tandem thiol-amine-ene reactions,and the polymers comprised N-diethylaminoethylamide unit(P1),N-hydroxyethylamide unit(P14)or both units(P2-P13).The resultant copolymers were characterized by 1H NMR,GPC and IR,and the molar fraction of N-hydroxyethylamide unit(fOH)was determined by 1H NMR analysis.The phase transition behaviors of copolymer solutions could be efficiently tuned by some factors involving chemical composition,deuterated solvent,concentration,pH/CO2,oxidation,and solution blending.The higher cloud points in D2O,the decreased cloud point with elevated pH,CO2-irreversible phase transition and lack of notable phase transition after oxidation into sulfoxide revealed that deuterated solvent and external stimuli could efficiently tune the thermoresponsive properties.Depending on the hydroxyl/diethylamino ratio,various copolymer solutions could exhibit a single LCST(fOH<0.84)or UCST(fOH>0.89)transition and LCST/UCST double transitions(fOH =0.84-0.89)resulting from the balance of inter/intramolecular interactions involving hydrogen bonds and electrostatic interactions.Moreover,the solution blending of two copolymers with hydroxyl or diethylamino groups allowed for the formation of one or two phase transition(s)dependent on molar compositions of the blends,and two independent phase transitions were noted as suitable feed ratios(fOh = 0.60-0.85)were adopted.As compared with LCST/UCST transitions observed in the copolymers with both hydroxyl and diethylamino functionalites,the solution blending afforded relatively broad fOH range within 0.65 and 0.80 to exhibit double phase transitions.Owing to the presence of bulky diethylamino groups,the hydrogen bonding association was not strong enough and could be broken at a relatively low temperature,followed by formation of more hydrophilic solution until a second-stage aggregation occurred at higher temperature,and thus dual transtions of UCST first then LCST were noted during solution blending.In summary,a series of thermo,pH,CO2 and oxidation-responsive graft and linear polymers were achieved by the virture of rational macromolecular design,and some factors such as topology,deuterated solvent,external stimuli,chemical composition,and solution blending could be efficiently used to tune the thermoresponsive properties.Under suitable conditions,a single cloud point(LCST or UCST)or two independent cloud points(either two LCSTs or both LCST and UCST)could be available.This study affords a versatile method to prepare multi-responsive linear/cyclie graft copolymers and linear copolymers,and the introduction of multifunctional Y junctions and the utilization of multicomponent polymerization have a great potential to synthesize a series of topological polymers with variable compositions and properties.Moreover,the progress in topology,solvent,stimuli and composition-dependent phase transitions can accelerate the development of stimuli-sensitive polymers and underlie the research on exploring practical applications of smart materials toward bio materials and nanotechnology.