Significant progress was reported recently with living cationic polymerization in the presence of an added base.

An aqueous solution and cross-linked gel of poly-(N-isopropylacrylamide) (PNIPAM) undergo phase separation upon heating around 32 C. 1 Heskins and Guillet 2 studied the phase separation behavior of one unfractionated PNIPAM sample using a visible observation method. Later Fujishige et al.3 studied the phase transition temperature of an aqueous solution of fractionated PNIPAM samples using 500 nm wavelength light transmission and concluded that the phase separation temperature was independent of either the molecular weight (within the range of 50,000-8,400,000) or its concentration (within the range of 0.01-1%). However, Schild and Tirrell 4 observed the effect of the molecular weight and the concentration on the phase separation temperature of aqueous PNIPAM samples. Recently, using two well characterized samples (M w ¼ 49;400 with M w =M n ¼ 1:21 and M w ¼ 101;000 with M w =M n ¼ 1:23) studied over the wide concentration range of 0.58-70 wt %, Tong et al.5 reported that the phase separation temperature inversely depends on the molecular weight and concentration of PNIPAM. Thus, there have been relatively large numbers of papers dealing with the effects of molecular weights on the phase separation of PNIPAM. It has been reported earlier that the phase separation is due to the formation of hydrophobic bonding among the side chains of a polymer in an aqueous solution.6-9 However, there is no report about the effect of tacticity on the phase separation temperature of aqueous PNIPAM solutions. One of our groups recently reported the effect of tacticity on the phase transition temperature of PNIPAM. 10Very recently, one of our groups has also reported the synthesis of stereo and molecular weight controlled PNIPAM using the RAFT polymerization in the presence and absence of Lewis acids.11 Using the same technique, we prepared a series of PNIPAM samples with different meso diad (m) values in the range of 45-72% having the molecular weight (M n ) of 37;000 AE 3000 and polydispersity (M w =M n ) in the range of 1.2-1.3. These polymers were used for the determination of the phase separation temperature of aqueous PNIPAM solutions and the novel inverse dependency of the tacticity on the phase separation temperature was observed. EXPERIMENTAL N-Isopropylacrylamide (NIPAM) (Wako, > 98%) was recrystallized twice from hexane. AIBN (Kishida, 99%) was recrystallized from methanol. Y(OTf) 3 (Aldrich, 98%) and Sc(OTf) 3 (Aldrich, 98%) were dried under vacuum before use. Dehydrated methanol (Kanto, > 99:8%) and dehydrated toluene (Kanto, > 99:5%) were used as received. 1-Phenylethyl phenyldithioacetate (PEPD) 12 was synthesized according to the literature. The polymerizations were performed in a methanol-toluene mixture at 60 C using 2.23 M NIPAM monomer, 0.80 mM AIBN as the initiator and 8.94 mM PEPD as the RAFT agent in the absence y

We examined the antibacterial and hemolytic activities in a series of amphiphilic block and random copolymers of poly(vinyl ether) derivatives prepared by base-assisting living cationic polymerization. Block and random amphiphilic copolymers with similar monomer compositions showed the same level of activity against Escherichia coli . However, the block copolymers are much less hemolytic compared to the highly hemolytic random copolymers. These results indicate that the amphiphilic copolymer structure is a key determinant of activity. Furthermore, the block copolymers induced dye leakage from lipid vesicles consisting of E. coli -type lipids, but not mammalian lipids, while the random copolymers disrupted both types of vesicles. In addition, both copolymers displayed bactericidal and hemolytic activities at concentrations 1 or 2 orders of magnitude lower than their critical (intermolecular) aggregation concentrations (CACs), as determined by light scattering measurements. This suggests that polymer aggregation or macromolecular assembly is not a requisite for the antibacterial activity and selectivity against bacteria over human red blood cells (RBCs). We speculate that different single-chain conformations between the block and random copolymers play an important role in the antibacterial action and underlying antibacterial mechanisms.

Diblock copolymers of vinyl ethers with two thermosensitive segments possessing different phase separation temperatures (TPS) have been synthesized by sequential living cationic copolymerization. Examples include 2-(2-ethoxy)ethoxyethyl vinyl ether (EOEOVE) and 2-methoxyethyl vinyl ether (MOVE), which were sequentially polymerized using a cationogen/Et1.5AlCl1.5 initiating system in the presence of tetrahydrofuran to give diblock copolymers with a very narrow molecular weight distribution. When an aqueous solution of a diblock copolymer (EOEOVE/MOVE ) 200/400) was heated, four different viscoelastic stages were observed: clear liquid (sol, e40 °C), transparent gel (42-55 °C), hot clear liquid (sol, 57-63 °C), and opaque mixture by phase separation (>63 °C). Micelle formation during physical gelation was confirmed by the change in particle diameter. The temperatures of the first and third transitions corresponded to each T PS for two segments, whereas that of the second transition was dependent on factors such as structure (sequence, composition, and molecular weight distribution) and physical properties (concentration, additives). Freeze-fracture transmission electron microscopy revealed the physical gels to consist of a regular arrangement of spherical micelles with controlled size. Experimental SectionMaterials. 2-(2-Ethoxy)ethoxyethyl vinyl ether (EOEOVE) was prepared from 2-(2-hydroxy)ethoxyethyl vinyl ether (BASF),

Au nanoclusters of less than 4 nm with a narrow size distribution were prepared and supported in thermosensitive vinyl ether star polymers obtained by living cationic polymerization. The thermosensitivity of the star polymers permitted easy separation of the clusters from the reaction mixture without any negative aggregation. Thus, the Au clusters could be recovered for reuse several times to induce alcohol oxidation with similar reactivity in each run.

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.

Star-shaped polymers (4) of alkyl vinyl ethers [ C H d H O R R = C2H5, CH2CH(CH&] were prepared in high yield on the basis of the living cationic polymerization by the HI/ZnI2 initiating system.For example, isobutyl vinyl ether (IBVE) was polymerized by HI/ZnI2 at -40 "C in toluene into a living polymer, which was subsequently allowed to react with a small amount of a divinyl ether [la; C H 4 H O C H g -CH20C~C(CH~)~C&OCH~CH~OCH=;CH2]. The resulting star-shaped polymers were completely soluble and consisted of a cross-linked core of poly(1a) to which nearly monodisperse poly(1BVE) chains were radially attached. The number of arma ranged from 3 to 60 per molecule and could be controlled by the arm's chain length, the mole ratio of the divinyl ether to the living end, and the overall concentration of the living end. When a divinyl ether with a flexible spacer ( C H F C H O C H~C H~O C H~H~, etc.) was employed, however, the yield of star polymers sharply decreased. A similar phenomenon was observed when a living polymer of a bulky vinylether (CHz=CHO-n-Cl&) was employed. These effects of reaction conditions were discussed relative to the mechanism of the star polymer formation.