In this study, the synthesis of a star-shaped polymer with a narrow molecular weight distribution and subsequent formation of a homogeneous polymer network composed of the star-shaped polymer were realized by combining single-electron transfer living radical polymerization, click reaction, and amide bond formation with a condensing agent, which are toolized reactions. First, a 4-armed star polymer consisting of Nisopropylacrylamide was synthesized by living radical polymerization using N,N′-ethylenebis (2,2-dichloroacetamide) as a 4branched initiator. By this polymerization method, a terminal Cl 4branched star poly(N-isopropylacrylamide) (PNIPA) with a narrow molecular weight distribution could be obtained, but it was found that the reaction activity of the polymer terminal was lost in the process of purification and isolation. Therefore, after obtaining the terminal Cl 4-branched star PNIPA by living radical polymerization, an azide reaction was carried out in one pot without purification and isolation of the star-shaped polymer. As a result, the azide group was successfully introduced to all ends of the star-shaped polymer. By introducing a carboxyl group or an amino group into the 4-branched star polymer with azide groups using a click reaction, two types of 4branched star polymers with different end groups were obtained. Equal amounts of both 4-branched star polymers were mixed at a polymer concentration equal to or higher than the overlapping concentration, and as a result of forming an amide bond with a condensing agent, a polymer gel was obtained. The swelling behaviors of the polymer gel indicate that almost no unreacted carboxyl group or amino group was present in the obtained polymer gel. That is, it was found that both 4-branched star polymers reacted efficiently to form a polymer network. In addition, structural observation of the polymer network by the smallangle X-ray scattering method showed that a polymer gel consisting of a network of uniform size was obtained. As mentioned above, we succeeded in constructing a polymer gel consisting of a homogeneous network structure using a temperatureresponsive 4-branched star polymer as the building block. The living radical polymerization method, the click reaction, and the amide formation by condensation reaction used in this study can be applied not only to the NIPA used here but also to various other monomers. If the construction of a precise network structure is realized by many polymers and the relation with the functional expression derived from the structure is clarified, it will be possible to design the network structure in accordance with the usage of the polymer gel.

Temperature-responsive polymer gel can rapidly and reversibly switch its various physical properties and has been used in practical applications including reservoirs for drug delivery systems, cell sheet preparation base materials for regenerative medicine, and switching elements for microchannels. However, the sudden volume shrinkage in the gel with rapid temperature change may cause the formation of a skin layer and the macrophase separation phenomenon of the polymer network. Due to this phenomenon, it usually takes a very long time to reach a thermodynamically stable shrunken state of the gel. By adjusting the heterogeneity of the network structure of the conventional temperature-responsive polymer gel and the monomer arrangement of the polymer, the polymer gel may shrink faster without causing skin layer formation or macrophase separation phenomenon. In this study, almost ideal homogeneous temperatureresponsive polymer gels could be synthesized by combining two types of synthetic methods for well-defined star-shaped polymers, i.e., the core-first method with a multifunctional initiator and linking method with a divinyl compound as the cross-linker, using a polymerization system with extremely high living fashion to afford polymer chains with narrow molecular weight distributions. As a result, the temperature-responsive polymer gels derived from N-isopropylacrylamide (NIPA) with almost uniform network structure were successfully prepared, in which the number of polymer chains bonded to the cross-linking points and the molecular weight between them are quite uniform. Due to the high living fashion of the polymerization used in this method, a temperature-responsive copolymer gel composed of NIPA and other hydrophilic monomers could also be synthesized. The polymer gel consisting of starshaped block polymers of NIPA and dimethylacrylamide (DMA) responded much faster than that with an uncontrolled network structure and uncontrolled monomer sequence distribution, which shrank completely in less than 1/60 time as fast with optical transparency and without either skin layer formation or macrophase separation of the polymer network during shrinkage.

We have developed a method to obtain a polymer gel consisting of a relatively homogeneous network structure composed of polymers with a narrow molecular weight simply by mixing the necessary compounds. In this method, a radical chain polymerization reaction with a fast reaction rate is combined with a radical addition cross-linking reaction that has a sufficiently slow reaction rate compared with the polymerization reaction. For the polymerization reaction, a multifunctional initiator with a strictly controlled number of polymerization initiation sites was used to prepare a star polymer with a chain number and length determined by living radical polymerization. In the cross-linking reaction, a multifunctional terminating agent was used to prepare a star polymer from a polymer with a narrow molecular weight via living radical polymerization followed by coupling with multiple polymer chains using a termination reaction. As a result, two kinds of reactions with greatly different kinetics occurred sequentially only when all of the compounds were mixed and allowed to stand at a constant temperature, and a relatively homogeneous network structure comprising cross-linked polymers with a low molecular weight distribution was inherently constructed.