Homopolymers of sodium 4-styrenesulfonate have been synthesized directly in aqueous media by reversible addition−fragmentation chain transfer polymerization (RAFT). The resulting homopolymers have narrow molecular weight distributions, with polydispersity indices in the range 1.12−1.25, as determined by aqueous size exclusion chromatography. Using a dithioester-capped sodium 4-styrenesulfonate homopolymer as a macro chain transfer agent, a block copolymer with sodium 4-vinylbenzoate has been prepared in aqueous media. Additionally, a block copolymer of (ar-vinylbenzyl)trimethylammonium chloride with N,N-dimethylvinylbenzylamine has been synthesized, using the same methodology. We believe these represent the first examples of AB diblock copolymers prepared directly in aqueous media via the RAFT process. Both block copolymers are stimuli-responsive and undergo reversible pH-induced micellization in aqueous solution. Micelles with hydrodynamic diameters in the range 18−38 nm were observed by dynamic light scattering.

Introduction. Controlled radical polymerization 1 (CRP) has been the focus of intense research in recent years. Atom transfer radical polymerization (ATRP), 2 nitroxide-mediated polymerization (NMP), 3 and more recently, reversible addition-fragmentation chain transfer polymerization (RAFT) 4 have allowed for the synthesis of (co)polymers with designated molecular weights and narrow molecular weight distributions.Considering the advantages of precisely controlled structures and the need for environmentally viable technologies, we have recently centered our efforts on conducting CRP in aqueous media. We first reported the synthesis, via RAFT, of homopolymers and block copolymers based on the water-soluble styrenic monomers, sodium 4-styrenesulfonate, sodium 4-styrenecarboxylate, N,N-dimethylvinylbenzylamine, and (ar-vinylbenzyl)trimethylammonium chloride. 5 Subsequently, we reported the first example of CRP of anionic acrylamido monomers in aqueous media. 6 As well as ionic monomers, we are interested in polymerizing nonionic hydrophilic/water-soluble species, such as N,N-dimethylacrylamide (DMA) via RAFT. Herein, we report the CRP of DMA, in aqueous media, utilizing two RAFT CTAs, Table 1. As far as we are aware, this constitutes the first report outlining the CRP of this monomer via RAFT in water.The CRP of DMA, and acrylamido monomers in general, has proven to be challenging using techniques such as ATRP and NMP. Li and Brittain reported the polymerization of DMA by NMP using TEMPO, but the process was shown to be uncontrolled. 7 However, with the development of more universal nitroxides, Benoit et al. demonstrated the ability to (co)polymerize DMA via NMP. 8 Teodorescu and Matyjaszewski reported the ATRP of several (meth)acrylamides. 9 However, the authors concluded that these systems were not "living". This was subsequently confirmed by Rademacher et al. 10 Senoo et al. reported the ATRP synthesis of PDMA, employing the RuCl 2 (PPh 3 ) 3 -based initiating system, although resulting polydispersities were typically >1.60. 11 With the discovery of RAFT, a wider range of monomers are now amenable to CRP. Significantly, the CRP of DMA 12 and N-isopropylacrylamide, 13 via RAFT, in organic media have already been demonstrated.In the work reported here, DMA homopolymers were synthesized in water via RAFT. Both sodium 4-cyanopentanoic acid dithiobenzoate (CTPNa) and N,N-dimethyl-s-thiobenzoylthiopropionamide (TBP) were employed as the RAFT chain transfer agents (CTAs). CTPNa was chosen due to its inherent water-solubility

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Well-defined poly(sodium 2-(acrylamido)-2-methylpropanesulfonate-block-sodium 6-acrylamidohexanoate) (pNaAMPS-AaH) was synthesized by reversible addition−fragmentation chain transfer (RAFT) radical polymerization of sodium 6-acrylamidohexanoate (AaH) using the sodium 2-(acrylamido)-2-methylpropanesulfonate-based macrochain transfer agent. The “living” polymerization of AaH was evidenced by the fact that the number-average molecular weight increased linearly with monomer conversion while the molecular weight distribution remained narrow independent of the conversion. pH-induced association and dissociation behavior of the diblock copolymers was investigated by quasi-elastic light scattering (QELS), static light scattering (SLS), 1H NMR spin−spin relaxation time, and fluorescence probe techniques. At pH < 4, the diblock copolymers exhibited large values of the hydrodynamic radius and small values of the 1H NMR spin−spin relaxation time. These observations indicated that micellization occurred to form polymer micelles comprising hydrophobic protonated AaH cores and hydrophilic NaAMPS coronas at pH < 4. On the other hand, these diblock copolymers dissolved in aqueous solutions as a state of unimer under high-pH conditions. 8-Anilino-1-naphthalenesulfonic acid, ammonium salt hydrate (ANS), as a fluorescence probe could be incorporated into the protonated AaH core of diblock copolymer micelles at low pH and released upon dissociation of the micelles at high pH, which was completely reversible.

A water-soluble diblock copolymer was prepared from sodium 2-(acrylamido)-2-methylpropanesulfonate (NaAMPS) and N-isopropylacrylamide (NIPAM) via reversible addition-fragmentation chain transfer (RAFT) controlled radical polymerization. The RAFT "living" radical polymerization process of NIPAM using an NaAMPS-based macrochain transfer agent was confirmed by the fact that the numberaverage molecular weight increased linearly with monomer consumption while the molecular weight distribution remained to be narrow for the polymerization. The NIPAM block exhibited a lower critical solution temperature (LCST) in water. Both the NaAMPS and NIPAM blocks are soluble in water at room temperature. At temperatures above the LCST, the NIPAM blocks associated into a polymer aggregate. The polymer aggregate was assumed to be an elongated micelle or a multiple aggregate due to intermicellar association of the spherical core-corona micelles based on characterization data obtained from 1 H NMR, turbidity, light scattering, and fluorescence probe experiments. A hydrophobic compound such as 8-anilino-1-naphthalenesulfonic acid, ammonium salt hydrate (ANS), was incorporated into the hydrophobic aggregate of the NIPAM blocks above LCST and released from the aggregate when temperature was reduced below LCST. The capture and release of ANS triggered by temperature change were completely reversible.

Polyelectrolyte gels comprising fixed ions exhibit swelling behaviors because of external solution conditions. Such behaviors are usually explained by using the Flory–Rehner model that considers the Donnan equilibrium. However, this model assumes a homogeneous distribution for fixed ions; therefore, its applicability to the case of heterogeneous distributions remains unclear. Here, we successfully designed a hydrogel with alternating neutral/highly charged sequences (i.e., tetrapoly­(acrylic acid)–poly­(ethylene glycol) gel). The Flory–Rehner model with the Donnan equilibrium was found to predict the swelling ratio only below pH 5.6. The distance between two neighboring fixed ions is larger than the Bjerrum length. The swelling behaviors above pH 5.6, where the fixed ions are more closely located than the Bjerrum length, can be explained by considering additionally the counterion condensation in the poly­(acrylic acid) units. These results indicate that the ionic interactions within the length scale of the network strand can determine the counterion condensation and that it is vital to estimate accurately the distance between neighboring fixed ions for predicting the swelling behaviors of polyelectrolyte gels.

Superabsorbent polymer particles, consisting of partly neutralized, slightly crosslinked poly(acrylic acid), have been surface-crosslinked photochemically. Surface crosslinking is required for many applications of superabsorbent polymers, such as disposable diapers, to control the flow and absorption of liquids in the gel bed. Photoinduced surface crosslinking has been achieved under UV irradiation (200-300 nm) with (NH 4 ) 2 S 2 O 8 as a photoactivated crosslinking agent. In comparison with the currently used thermal ester bridging method for surface crosslinking, the new photochemical method generates superabsorbent particles with superior properties, such as an improved flow of liquid through the gel bed, which utilizes the entire gel bed. These improved properties have been shown by water absorption capacity studies, fluid flow dynamics, environmental scanning electron microscopy, and low-energy ion-scattering studies.

A series of poly(sodium acrylate) gels was synthesized via end-linking reaction between tetra-arm poly(tert-butyl acrylate)s by copper-catalyzed azide–alkyne cycloaddition followed by deprotection of tert-butyl group and neutralization. Characterization of the gels using gravimetry, IR measurement, and stretching test revealed that they have only small amounts of sol fractions, dangling chains, and trapped entanglements. The relationship between polymer weight fraction (ϕe) and shear modulus (G′) at equilibrium swelling in saline was expressed as G′ ∝ ϕe 2.5. The power index (2.5) was comparable with the theoretical value for nonionic gels swollen in good solvents (2.3). The relationship between G′ and ϕe was superior to that of poly(sodium acrylate) gels synthesized by conventional radical copolymerization. The difference may be partly attributed to the difference in the ratio of chemical cross-links to trapped entanglements.