An upper critical solution temperature (UCST)type phase separation in water was achieved using well-defined polymeric ionic liquids (ILs) with imidazolyl groups in their side chains, prepared based on living cationic polymerization using a cationogen/Et 1.5 AlCl 1.5 initiating system with 1,4-dioxane as an added base. Aqueous solutions of the polymers with tetrafluoroborate as counteranions showed sharp and reversible UCST-type phase separation at 5−15 °C. The effect of polymer concentration, chain-end groups, and molecular weight on the phase separation temperature suggests that the phase separation resulted from interpolymer electrostatic interactions. Other polymeric ILs with SbF 6− also showed a lower critical solution temperature-type phase separation in various organic solvents.

Cationic polymerization of isobutyl vinyl ether (IBVE) was examined using a variety of metal halides. In the presence of an appropriate added base, ester or ether, the living polymerization of IBVE proceeded for almost all Lewis acids (MCl n ; M: Fe, Ga, Sn, In, Zn, Al, Hf, Zr, Bi, Ti, Si, Ge, Sb) used in conjunction with an IBVE-HCl adduct in toluene at 0°C. The difference in the polymerization activity of these Lewis acids was significant. As examples, polymerization with some acids, such as FeCl 3 , proceeded in the order of seconds, whereas it took more than a few weeks with others such as SiCl 4 and GeCl 4 . The difference in activity is based on the strength of the interaction between the Lewis acid and the propagating end chloride anion and/or the basic carbonyl (or ether) oxygen atom of the added base, that is, the chlorophilic or oxophilic nature of each metal halide is a decisive factor. In addition, a suitable combination of a Lewis acid and an additive was indispensable for living polymerization. With metal pentachlorides, NbCl 5 and TaCl 5 , addition of a salt (nBu 4 NCl) resulted in superior control of the reaction over that for addition of a base. Lewis acids for living cationic polymerization of vinyl ether were categorized into groups depending on the preferences for these additives.

Alkyl vinyl ethers and isobutylene oxide were concurrently copolymerized through cationic vinyl addition and ring opening using B(C6F5)3 as a catalyst. NMR analyses and acid hydrolysis of the products demonstrated that the copolymerization successfully proceeded through crossover reactions between vinyl and cyclic monomers to yield multiblock-like copolymers. Appropriate catalyst and monomer combinations with suitable reactivities were key for copolymerization.

Our recent extensive research on Lewis acid catalysts with a weak base for the cationic polymerization of vinyl ethers led to unprecedented living reaction systems: fast living polymerization within 1-3 s; a wide choice of metal halides containing Al, Sn, Fe, Ti, Zr, Hf, Zn, Ga, In, Si, Ge, and Bi; and heterogeneously catalyzed living polymerization with Fe 2 O 3 . The use of added bases for the stabilization of the propagating carbocation and the appropriate selection of Lewis acid catalysts were crucial to the success of such new types of living polymerizations. In addition, the base-stabilized living polymerization allowed the quantitative synthesis of star-shaped polymers with a narrow molecular weight distribution via polymerlinking reactions and the precision synthesis and self-assembly of stimuli-responsive block copolymers.

A metal-free initiating system for the living cationic polymerization of alkyl vinyl ethers (VEs) and styrene derivatives was developed using diaryliodonium salts as an organic Lewis acid catalyst. Unlike many past examples of their use as a photoinitiator, diaryliodonium salts were demonstrated to function as a Lewis acid catalyst for cationic polymerization. The cationic polymerization of isobutyl VE smoothly proceeded using a diaryliodonium salt in conjunction with a cationogen that generates a carbon−halogen propagating end, yielding polymers with predictable molecular weights and very narrow molecular weight distributions. The central iodine atom of the diaryliodonium salt most likely exhibited Lewis acidity, thereby generating a dormant−active equilibrium through the reversible generation of a carbocation via the abstraction of the halogen anion from the propagating end. The role of the diaryliodonium salts as Lewis acid catalysts was confirmed by a series of experiments that focused on the effects of concentrations, substituents on the aryl rings, and counteranions. Polymerizations of styrene derivatives and VEs with polar groups also proceeded using diaryliodonium salts in a controlled manner.