Synthesis And Temperature-sensitive Gelling ABA Type Block Copolymer

The block from the stimulus-response (environmental sensitivity) doublehydrophilic block copolymer becomes hydrophobic, resulting the formation ofcopolymer micelle, when temperature, pH, ionic strength changes. This stimulusresponsive polymeric micelle and the gel formed by this micelle have a wideapplication prospect in in the fields of biology, medicine, catalysis, separation.Poly(N-isopropylacrylamide)(PNIPAM) exhibits a reversible thermoresponsivephase transition in aqueous solution. The solubility decreases with increasingtemperature, the polymer chains occur phase separation and then precipitate from thesolution. The temperature of phase transition is the lower critical solution temperature(LCST), but it reversibly restored to the original of the state at low temperature whenthe temperature decreases. Studies have shown that the copolymerization withhydrophilic monomer can enhance the hydrogen bonding and affect the phaseseparation behavior of polymer in solution, which can increase the LCST of thethermosensitive polymer close to the human physiological temperature.N-hydroxymethylacrylamide (HMAM) can bind some chlorine-containing chromosomekeys together, which enhances the interaction of HMAM copolymer with biologicalmacromolecule.The copolymerization of NIPAM with other monomers or graftingPNIPAM onto other substrates can change the phase transition temperature andmechanical properties of PNIPAM, even with the other functional such as pH, light,pressure-sensitive features and expanding the application of PNIPAM. There are threecategories polymers of PNIPAM including linear, gel, substrate grafted polymersdepending on the different purposes. PNIPAM-based thermo-sensitive material is abiocompatible smart material by the temperature-controlled leading to volume phasetransition. Due to phase transition temperature closes to the human physiologicaltemperature (37°C), there is potential use in drug load.Poly(ethylene glycol)(PEG), as a typical hydrophilic segment, has severalbiomedical and pharmaceutical applications due to its specific properties such asnontoxicity, biodegradability, biocompatibility, and resistance to recognition by theimmune system. In addition, polyethylene glycol (PEG) hydrophilic chain can improve water absorption, increase the LCST of copolymer by adjusting the dosage and enhancethe formation of water channel which quickly reaches the swelling balance in order toimprove its response in the copolymer cross-linked network. While the PEG can formhydrogen bonds between the amide and increase physical crosslinking points andstrength mechanical properties of gel.ABA triblock copolymer, composed of a permanently water soluble PEG B blockand thermo-sensitive A block, can self-assemble into “flower” micelle in a diluteaqueous solution with the outer blocks associating into the core and the central blocksforming loops in the corona layer upon application of an external stimulus. These“flower” micelle can undergo reversible structural transition under the aid of externalstimulis such as temperature, have a wide application prospect in in the fields of drugdelivery, tissue engineering, and protein modification. When the polymer concentrationis sufficiently high, above the critical gelation concentration (CGC), a3-dimensionalmicellar network is formed, in which the central block forming bridges amongneighboring micelles. This physical gel is called smart gel due to responding to outsidetemperature change.On the basis of the studies above, the biocompatible, temperature-responsive ABAtriblock copolymers in which the outer A blocks comprised P(NIPAM-co-HMAM) andthe central B blocks comprised PEG were synthesized using atom transfer radicalpolymerization. The properties of aqueous solutions and the micellization behavior ofthe aqueous solutions of triblock copolymers were studied. The copolymer which beable to form hydrogel was obtained by changing the synthesis conditions and itsrheological behaviors were studied. This dissertation mainly includes two parts asfollows:(1)ABA typical triblock copolymer P(NIPAM-co-HMAM)-b-PEO-b-P(NIPAM-co-HMAM) was synthesized via atom transfer radical polymerization (ATRP) in methanolusing CuCl/PMDETA as the catalyst system and Br-PEO-Br as the initiator. Thetemperature sensitivity of the triblock copolymer aqueous solution was investigated bytransmittance and viscosity measurement and surface tension measurement. Themicellization of the triblock copolymer aqueous solution was investigated byfluorescence probe and transmission electron microscope. The critical micelleconcentration(CMC) of the copolymer in aqueous solution was determined.(2) A high degree of polymerization triblock copolymer P(NIPAM-co-HMAM)-b -PEO-b-P(NIPAM-co-HMAM) was synthesized by changing the catalyst system ofCu(I)Br/Me4Cyclam and the use of Schlenk technology. The temperature sensitivityand the micellization of the triblock copolymer aqueous solution was investigated bytransmittance, fluorescence probe and transmission electron microscope. Thetemperature-sensitive reversible hydrogel was obtained by selecting the appropriatedegree of polymerization and polymer concentration, the rheological behavior of thehydrogel was studied by rheometry and phase diagram.