Synthesis And Solution Properties Of Poly(N-isopropylacrylamide)-Based Water-Soluble Polymers With Varying Chain Architectures

Poly(N-isopropylacrylamide)(PNIPAM) has been extensively studied due to its unique thermoresponsive property.It is well-known that chain topology of polymers can exhibit significant effects on their physical properties.However,it has been difficult to synthesize PNIPAM polymers with varying chain architectures (including cyclic,hyperbranched,star-shaped,dendrimer-like),thus our understanding of the relationship between thermoresponsive properties and chain topologies of PNIPAM has been quite limited.In this dissertation,a series of PNIPAMs with varying topologies such as linear PNIPAM,cyclic PNIPAM, hyperbranched PNIPAM,star-shaped and dendrimer-like PNIPAM polymers were synthesized via a combination of controlled radical polymerizations(CRP) and click chemistry.Their supramolecular self-assembly and thermoresponsive properties in aqueous solution were studied in detail.The dissertation includes the following six parts:1.Narrow-disperse pyrene-labeled PNIPAM polymers were prepared by conventional free-radial copolymerization and subsequent fractionation.The cononsolvency-induced coil-to-globule transition kinetics of pyrene-labeled linear PNIPAM chains was studied by stopped-flow technique.We have,for the first time,observed that the coil-to-globule transition of individual long linear PNIPAM chains(DP=3100) labeled with pyrene molecules is a two-stage process,namely,the fast crumpling of a random coil and the slow collapsing of the crumpled chain to the compact globule,with two distinct characteristic relaxation times(～12 and～270 ms).It agrees well with previous theoretical considerations.2.We synthesized well-defined cyclic-PNIPAM polymer via a combination of atom transfer radical polymerization(ATRP) and click chemistry,and investigated its unique thermal phase transition behavior as compared to that of linear precursor. The subsequent end-to-end intramolecular coupling reaction ofα-alkyne-ω-azido heterodifunctional PNIPAM precursor(linear-PNIPAM-N₃) under high dilution and click conditions leads to efficient preparation of narrow-disperse cyclic-PNIPAM.The thermal phase transition behavior of cyclic-PNIPAM was investigated and compared to that of linear-PNIPAM-N₃ with the same molecular weight.The former possesses lower critical solution temperatures (LCSTs),more prominent concentration dependences of LCST values and cloud points(CPs),broader thermal phase transition range,and prominently lower enthalpy changes(â–³H).The above differences in thermal phase transition behaviors between cyclic- and linear-PNIPAM should be due to the absence of chain ends and stringent restrictions on backbone conformations in the former.3.Successive reversible addition-fragmentation transfer(RAFT) polymerizations of NIPAM and 2-(dimethylamino) ethyl methacrylate(DMA) were conducted using fractionated fourth-generation hyperbranched polyester(Bolton H40)-based macroRAFT agent.At lower temperatures(＜20℃),hyperbranched macromolecules H40-PNIPAM-PDMA exist as unimolcular core-shell-corona nanostructures with hydrophobic H40 as the core,swollen PNIPAM as the inner shell,and swollen PDMA as the corona.PNIPAM and PDMA homopolymers undergo phase transitions at their LCSTs,which are found to be 32℃for PNIPAM and 40-50℃for PDMA,respectively.Upon continuously heating through the LCSTs of PNIPAM and PDMA,such hyperbranched unimolecular micelles exhibit two-stage thermally induced collapse.This process is reversible with a two-stage reswelling upon cooling.4.Three types of PNIPAM-based star-polymers based onβ-cyclodextrin(β-CD) cores were reported.Firstly,the synthesis of well-defined 7-arm and 21-arm star polymers of NIPAM-based onβ-CD were achieved via the combination of ATRP and click reactions,namelyβ-CD-(PNIPAM)₇ andβ-CD-(PNIPAM)₂₁. We observed that lower critical phase separation temperatures(T_c) ofβ-CD-(PNIPAM)₂₁ considerably increased with increasing DP of PNIPAM arms, which was contrary to that of linear PNIPAM.On the other hand,forβ-CD-(PNIPAM)₂₁ star polymer with larger arm length,the collapse of inner chain segments can be well-stabilized by the outer layer,the collapse and aggregation of which occurred at elevated temperatures due to its much lower local chain density.Secondly,we prepared thermo- and pH-responsive double hydrophilic 'Janus' A₇B₁₄-sharp star copolymer of NIPAM and 2-(diethylamino) ethyl methacrylate(DEA),(PDEA)₇-CD-(PNIPAM)₁₄,withβ-cyclodextrin core, employing ATRP and click chemistry,respectively.In acidic solution and at elevated temperatures,it sef-assembes into vesicles with hydrophilic PDEA arms forming the corona and PNIPAM arms locating in the membrane walls.Whereas in slightly alkaline solution and at room temperature,vesicles with inverted structure form.Finaly,we prepared well-defined amphiphilic 'Janus' A₇B₁₄-sharp star copolymer of NIPAM and methyl methacrylate(MMA), (PMMA)₇-CD-(PNIPAM)₁₄ and(PNIPAM)₇-CD-(PMMA)₁₄,bearingβ-cyclodextrin cores,employing ATRP and click chemistry.5.Dendrimer-like star PNIPAMs of one and two generations were synthesized via the "click" ligation technique.Theα-azido-ω-chloro-terminated PNIPAM precursor(G0-Cl) was synthesized by the ATRP of NIPAM using 3-azidopropyl-2-chloropropionate as an initiator.The PNIPAM chains were then reacted with tripropargylamine to produce the first generation dendrimer [G1-(Cl)₃]with chlorine chain ends,which could be easily converted to azide groups.Then the azide groups was substituted for chlorine atom from the end of the polymer chain[G1-(N₃)₃].The second generation dendrimer[G2-(Cl)₆]was synthesized by first coupling(in large excess) tripropargylamine to the G1-(N₃)₃ to form the G1-(alkyne)₆ with dialkyne chain ends.This polymer was then reacted with G0-Cl.The thermal phase transitions of these novel PNIPAMs samples in aqueous solutions were contrary to that of linear PNIPAM.6.To further understand the relationship between the chemical topology of acrylamide-based polymers and their solution properties,we synthesized a novel monomer,N-ethylmethylacrylamide(EMA),which is structurally similar to that of N-isopropylacrylamide(NIPAM).Well-defined poly(N-ethylmethylacrylamide) (PEMA) samples were prepared by ATRP and RAFT techniques.Thermal phase transitions of low-polydispersity PEMA homopolymers with varying molecular weights in aqueous solution were systematically investigated.The cloud point of PEMA lies between those of structurally similar poly(N,N-dimethylacrylamide) and poly(N,N-diethylacrylamide) homopolymers,and varies in the temperature range of 58-68℃,depending on the molecular weights,polymer concentrations, and the hydrophobicity/hydrophilicity of end groups.