Research Projects
Recently, well-designed stimuli-responsive polymers have attracted much interest as precursors of advanced polymer materials such as nano-organized self-assemblies, intelligent hydrogels, and DDS. In particular, recent progress in living (or precision) polymerization has encouraged us to design various types of amphiphilic block copolymers or hydrophobically modified water-soluble polymers in order to examine their stimuli-induced self-association. In the field of polymer micelles, for example, micelles with multiple morphologies, covalently cross-linked micelles such as SCKs, and thermosensitive micelles have been prepared by the special living polymerization techniques. Furthermore, the characteristic micellization and gelation properties of PEO-based block copolymers with relatively low molecular weights such as PEO-b-PPO-b-PEO and PEO-b-poly(alkyleneoxide) have been investigated in detail by many physical chemists.
We have prepared a variety of functional polymers with well-controlled structures and molecular weights by living cationic polymerization in the presence of an added base (see below, "stabilization of the growing . . . "). For example, poly(vinyl ether)s with side oxyethylene units and ω-alkyl groups obtained by living cationic polymerization were found to exhibit thermally-induced phase separation in water. Further investigation showed that (i) the temperatures of phase separation (TPS) could be controlled by substituting different side oxyethylene units or ω-alkyl groups, (ii) the narrow molecular weight distribution (MWD) of the polymer produced the high sensitivity of phase separation, and (iii) block copolymers exhibited multistage phase separation at the TPS of each block segment, which is considerably different behavior to homopolymers or random copolymers. On the basis of these results, we have designed a new strategy for preparing block copolymers with various types of stimuli-responsive properties such as thermosensitive physical gelation.
(1) Syntheses of well-defined polymers with various characteristic properties by living cationic polymerization
(2) Syntheses of advanced stimuli-responsive block copolymers
(3) Investigation of polymerization mechanism and design of novel living polymerization
(4) Syntheses of Various Functinal Polymers by Coordination Polymerization or Radical Polyaddition
"Stabilization of the Growing Propagating Carbocation by Externally Added Bases"
Living polymerization is an important tool to design and synthesize new polymer materials with controlled architecture (molecular weight, structure etc.). During the last two decades, a concept of the living polymerization has expanded into various polymerization systems, including cationic polymerization, and their recent advances are especially impressive in quantity and quality. We have reported on the living cationic polymerization of vinyl ethers in the presence of added bases . For example, the polymerization of isobutyl vinyl ether (IBVE) with EtAlCl2 in conjunction with 1-(isobutoxy)ethyl acetate in the presence of a large excess of ethyl acetate or 1,4-dioxane as an added base, yields well-defined living polymers with a very narrow MWD (Mw/Mn < 1.1) even at higher polymerization temperature such as +40°C. In this system, the unstable growing carbocations are stabilized with a nucleophilic interaction of the added basic compounds. Recently, on the basis of this principle, various cationic polymerization systems have been developed to give living polymers by using externally added bases (electron donors, nucleophiles). This concept differs fundamentally from that for the previous living systems expressed as "stabilization of the growing propagating carbocation by counteranions".