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

The reversible addition−fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) was carried out successfully in the absence and presence of Lewis acid Y(OTf)3 to synthesize controlled molecular weight atactic and isotactic poly(NIPAM)s using both 1-phenylethyl phenyldithioacetate (PEPD) and cumyl phenyldithioacetate (CPDT) as the RAFT agents. The polymerization rate is about 16 times faster in the presence of the Lewis acid than that in the absence and 1.4 times faster with PEPD than with CPDT as the RAFT agent. The polymer with a higher polydispersity was obtained when prepared in the presence of the Lewis acid than that in the absence. A longer induction period was observed using CPDT than PEPD. The chain-end structure of isotactic poly(NIPAM) was determined by 1H NMR and MALDI−TOF mass spectrometry. The RAFT agent derived isotactic poly(NIPAM) was the main product as expected from the well-accepted mechanism of RAFT polymerization. Moreover, a series of stereoblock [atactic(a)-b-isotactic(i)] poly(NIPAM) with different block lengths were synthesized via a one-pot synthesis procedure:  synthesis of the atactic block in the absence of the Lewis acid followed by the addition of the Lewis acid to synthesize the isotactic block. The longer is the isotactic block length, the higher is the meso dyad value of the stereoblock polymer as expected. We also successfully synthesized the diblock copolymers, a-poly(NIPAM)-b-polystyrene and i-poly(NIPAM)-b-polystyrene, starting with the atactic and isotactic poly(NIPAM) macro-RAFT agents, respectively.

A new versatile method for conducting living radical polymerization has been developed in which organostibines induce consecutive group-transfer radical reactions with alkenes. The method has been successfully applied, for the first time, to the controlled polymerization of both conjugated and unconjugated vinyl monomers, and the desired polymers with predetermined molecular weight and low polydispersity index were obtained in excellent yields. This characteristic feature of this method is exemplified in the first synthesis of block copolymers composed of conjugated and unconjugated monomers, which would be of great importance as functional smart organic nanomaterials.

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Poly(N-vinylpyrrolidone)s (PNVPs) with well-defined macromolecular structure were prepared by organostibine-mediated living radical polymerization. PNVPs with expected number-average molecular weight (M n ) 3000-84 000) and low polydispersity indexes (PDIs ) 1.1-1.3) were formed by heating a solution of organostibine mediator and NVP in the presence of AIBN at 60 °C. The polymer structure was analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI TOF) mass and 2 H NMR spectroscopies after reduction of the organostibanyl polymer end by tributyltin deuteride. The analyses reveal that, while the addition of AIBN considerably enhances the rate of the polymerization, the effect of azobis(isobutyronitrile) (AIBN)derived radical to the ω-end structure is negligible. The analyses also reveal the existence of "dead" dormant species due to head-to-head addition followed by the organostibanyl group transfer. However, since the probability of the head-to-head addition is small (0.02-0.10%) compared to the normal head-to-tail addition, its effect on the controllability was negligible under the current conditions. Diblock copolymers poly(styrene [St]-block-NVP), poly(methyl methacrylate [MMA]-block-NVP), and poly(NVP-block-MMA) were successfully prepared by successive addition of corresponding monomers to the organostibine macromediators.

Dispersion polymerization of acrylamide has been successfully carried out in aqueous tert-butyl alcohol (TBA) media (TBA = 50 − 80 vol %) using poly(vinyl methyl ether) (PVME) as the polymeric stabilizer and ammonium persulfate as the initiator. Polydisperse spherical as well as oval particles are formed. The simultaneous presence of both spherical and oval particles suggests the coalescence of similar size particles (homocoalescence) takes place, leading to polydispersity. The increase of stabilizer concentration leads to a decrease in particle size and an increase in molecular weight. An increase of initiator concentration results in an increase in particle size and a decrease in molecular weight. These observations are in conformity with other dispersion polymerizations reported in the literature. Particle size decreases as the TBA concentration increases, i.e. as the medium polarity decreases. TBA−water mixtures exhibit cosolvency toward PVME, the solvency becoming highest at about 70 vol % TBA. This solvency aspect cannot account for the monotonous decrease of particle size with an increase in TBA concentration. A plot of particle diameter (D̄ n) vs the initial solubility parameter (δ) of the medium gives a straight line unlike the linear relationship between D̄ n and 1/δ reported for dispersion polymerization of nonpolar monomers in polar solvents. This difference in behavior for the two systems can be reconciled if the grafted polymer acts as the true stabilizer.

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