Herein we report the synthesis and characterization of a series of pH reversible shell cross-linked micelles. A series of novel pH/salt-responsive block copolymers of poly(sodium 2-acrylamido-2-methyl-1-propanesulfonate-block-N-acryloyl-l-alanine) (P(AMPS-b-AAL)) were synthesized utilizing aqueous reversible addition−fragmentation chain transfer (RAFT) polymerization. Micellization of P(AMPS-b-AAL) is induced by rendering the PAAL block hydrophobic through protonation of the carboxylic acid (pH 1−3). The pH at which micelle formation occurs and the hydrodynamic diameters of the resultant micelles are dictated by block copolymer composition and electrolyte concentration. The anionic PAMPS micelle shells were subsequently cross-linked with a RAFT synthesized cationic homopolymer of either poly(N-[3-(dimethylamino)propyl]acrylamide) (PDMAPA, pK a = 8.5) or poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA, pK a = 7.3). Upon deprotonation of the PAAL block, these cross-linked micelles swell but remain stable and intact. Significantly, the reversibility of the cross-linking was determined to be tunable by utilizing the different cationic homopolymers for cross-linking. This was demonstrated by increasing the pH above the pK a of the cationic homopolymer cross-linker, resulting in deprotonation of the cationic cross-linker and dissociation of the cross-linked micelles.
A comparative study of pH-responsive polyzwitterions (PZs) with polyampholyte or polybetaine architectures was conducted with well-defined model polymer systems. Low-charge-density PZs, including ampholytic terpolymers composed of acrylamide (AM), sodium 3-acrylamido-3-methylbutanoate, and (3-acrylamidopropyl-)trimethylammonium chloride and carboxybetaine copolymers composed of AM and 3-(3-acrylamidopropyldimethylammonio)propionate, were prepared via free-radical polymerization in 0.5M NaCl to yield ter-and copolymers with random termonomer and comonomer distributions. Sodium formate was used as a chain-transfer agent during the polymerizations to eliminate the effects of the monomer feed composition on the degree of polymerization (DP) and to suppress gel effects and broadening of the molecular weight distributions. The polymer compositions were determined via 13 C-NMR spectroscopy, and the residual counterion content was determined via elemental analysis for Na ϩ and Cl Ϫ . The molecular weights (MWs) and polydispersity indices (PDIs) were determined via size exclusion chromatography/multi-angle laser light scattering (SEC-MALLS); the polymer MWs ranged from 1.4 to 1.5 ϫ 10 6 g/mol, corresponding to DPs of 1.6 -1.9 ϫ 10 4 repeat units, with all the polymers exhibiting PDIs less than or equal to 2.1. The intrinsic viscosities determined from SEC-MALLS data and the Flory-Fox relationship agreed with the intrinsic viscosities determined via low-shear dilute-solution viscometry. Data from the SEC-MALLS analysis were used to analyze the radius of gyration/molecular weight (R g -M) relationships and the Mark-Houwink-Sakurada intrinsic viscosity/ molecular weight ([]-M) relationships for the PZs. The R g -M and []-M relationships and viscometric data revealed that under size exclusion chromatography conditions, the poly[acrylamide-co-3-(3-acrylamidopropyldimethylammonio)propionate] betaine copolymers had more open, random-coil conformations and greater polymer-solvent interactions than the ampholytic poly[acrylamide-co-sodium 3-acrylamido-3-methylbutanoate-co-(3-acrylamidopropyl)-trimethylammonium chloride] terpolymers. The pH-and salt-responsive dilute-solution viscosity behavior of the PZs was examined to assess the effects of the polymer structure and composition on the solution properties. The polyampholyte terpolymers had greater solution viscosities and more pronounced stimuli-responsiveness than the polybetaine copolymers because of their stronger intramolecular interactions and increased chain stiffness.
Low‐charge‐density ampholytic terpolymers composed of acrylamide, sodium 3‐acrylamido‐3‐methylbutanoate (NaAMB), and (3‐acrylamidopropyl)trimethylammonium chloride were prepared via free‐radical polymerization in 0.5 M NaCl to yield terpolymers with random charge distributions. NaOOCH was used as a chain‐transfer agent during the polymerization to eliminate the effects of the monomer feed composition on the degree of polymerization (DP) and to suppress gel effects and broadening of the molecular weight distribution. The terpolymer compositions were obtained via 13C NMR spectroscopy, and the residual counterion content was determined via elemental analysis for Na+ and Cl−. The molecular weights (MWs) and polydispersity indices (PDIs) were determined via size exclusion chromatography/multi‐angle laser light scattering (SEC–MALLS); the terpolymer MWs ranged from 1.3–1.6 × 106 g/mol, corresponding to DPs of 1.6–1.9 × 104 repeat units, with all terpolymers exhibiting PDIs of less than 2.0. Intrinsic viscosities determined from SEC–MALLS data and the Flory–Fox relationship were compared to intrinsic viscosities determined via low‐shear dilute‐solution viscometry and were found to agree rather well. Data from the SEC–MALLS analysis were used to analyze the radius of gyration/molecular weight (Rg–M) relationships and the Mark–Houwink–Sakurada intrinsic viscosity/molecular weight ([η]–M) relationships for the terpolymers. The Rg–M and [η]–M relationships revealed that most of the terpolymers exhibited little or no excluded volume effects under size exclusion chromatography conditions. Potentiometric titration of terpolymer solutions in deionized water showed that the apparent pKa value of the poly[acrylamide‐co‐sodium 3‐acrylamido‐3‐methylbutanoate‐co‐(3‐acrylamidopropyl)trimethylammonium chloride] terpolymers increased with increasing NaAMB content in the terpolymers and increasing ratios of anionic monomer to cationic monomer at a constant terpolymer charge density. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3236–3251, 2004
The solution properties of low‐charge‐density ampholytic terpolymers of acrylamide, sodium 3‐acrylamido‐3‐methylbutanoate, and (3‐acrylamidopropyl)trimethylammonium chloride were studied as functions of the solution pH, ionic strength, and polymer concentration. Terpolymers with low charge densities, large charge asymmetries, or both exhibited excellent solubility in deionized (DI) water, and higher charge density terpolymers were readily dispersible in DI water; however, the higher charge density terpolymer solutions separated into polymer‐rich and polymer‐poor phases upon standing over time. Charge‐balanced terpolymers exhibited antipolyelectrolyte behavior at pH values greater than or equal to the ambient pH (6.5 ± 0.2); the same terpolymers behaved increasingly as cationic polyelectrolytes with decreasing solution pH because of the protonation of the 3‐acrylamido‐3‐methylbutanoate (AMB) repeat units. Unbalanced terpolymers generally exhibited polyelectrolyte behavior, although the effects of intramolecular electrostatic attractions (i.e., polyampholyte effects) on the hydrodynamic volume of the unbalanced terpolymer coils were evident at certain values of the solution pH and salt concentration. The dilute‐solution behavior of the terpolymers correlated well with the behavior predicted by several polyampholyte solution theories. In the semidilute regime, solution viscosities increased with increasing terpolymer charge density, and this indicated a significant enhancement of the solution viscosity by intermolecular electrostatic associations. Upon the addition of NaCl, semidilute‐solution viscosities tended to decrease because of the disruption of the intermolecular electrostatic associations. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3252–3270, 2004
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