The controlled radical polymerization (CRP) technique reversible addition−fragmentation chain transfer (RAFT) has potential for preparing functional (co)polymers directly in an aqueous environment. Hydrolysis and aminolysis can eliminate the active end groups necessary for maintaining “livingness” in water. These reactions have not previously been evaluated with respect to their effect on aqueous RAFT polymerizations. Herein we determine rate constants of hydrolysis and aminolysis for representative water-soluble chain transfer agents (CTAs) cyanopentanoic acid dithiobenzoate (CTP) and the macro-chain-transfer agents (macro-CTAs) of poly(sodium 2-acrylamido-2-methylpropanesulfonate) (AMPSX) and poly(acrylamide) (AMX) at selected pH values. Rates of hydrolysis and aminolysis both increase with increasing pH and decrease with increasing molecular weight of the dithioester. On the basis of these rate constants, mathematical relationships have been developed to predict the number of living chain ends and the molecular weight with competitive hydrolysis. Utilizing this approach, predictions of molecular weight at specific conversions are in agreement with experimental values determined by SEC/MALLS.
We discuss the synthesis and characterization of polyacrylamide (PAM) homopolymers with carefully controlled molecular weights (MWs). PAM was synthesized via free-radical solution polymerization under conditions that yield highly linear polymer with minimal levels of hydrolysis. The MW of the PAM homopolymers was controlled by the addition of sodium formate (NaOOCH) to the polymerization medium as a conventional chain-transfer agent. MWs and polydispersity indices (PDIs) were determined via size exclusion chromatography/multi-angle laser light scattering analysis; for polymerizations carried out to high conversion, PAM MWs ranged from 0.23 to 6.19 ϫ 10 6 g/mol, with most samples having PDI Ϸ2.0. Zero-shear intrinsic viscosities of the polymers were determined via low-shear viscometry in 0.514 M NaCl at 25°C. Data derived from the polymer characterization were used to determine the chaintransfer constant to NaOOCH under the given polymerization conditions and to calculate Mark-Houwink-Sakurada K and a values for PAM in 0.514 M NaCl at 25°C.
The relationships between drag reduction performance and polymer parameters including chemical structure, molecular weight, hydrodynamic volume, associations, and solvent nature were examined using synthetic water-soluble copolymers. Copolymer models were tailored to be systematically responsive to changes in electrolyte addition and included polyelectrolytes, polyampholytes, hydrophobically modified polymers, and uncharged, hydrophilic polymers. Commercial poly(ethy1ene oxide) (PEO) and copolymers of acrylamide with the comonomers sodium 3-(acrylamido)-3-methylbutanoate (NaAMB), sodium 2-(acrylamido)-2-methylpropanesulfonate (NaAMPS), [2-(acrylamido)-2-methylpropyl]dimethylammonium chloride (AMPDAC), and diacetone acrylamide (DAAM) synthesized in our laboratories were tested for drag reduction effectiveness using a rotating disk and a tube flow apparatus. Hydrodynamic volume as determined by viscometry and light scattering was monitored in deionized water and 0.514 M NaCl for particular compositions and molecular weights. Drag reduction performance was greatly affected by the nature of polymer/polymer and polymer/solvent interactions, generally increasing with hydrodynamic volume. Enhanced drag reduction behavior observed for the associating DAAM copolymers is proposed to be due to changes in water structuring in turbulent flow. IntroductionThe reduction of drag in turbulent flow produced by addition of small concentrations of high molecular weight polymers has been studied for over 40 years.' Studies of polymers of varying structures have shown that drag reduc-
Experimental solution intrinsic viscosity responses to temperature and polymer molecular weight variations were used to test the modeling capability of a simplified intrinsic viscosity equation. The multiple linear equation contains three parameters that are related to the thermodynamic properties of a polymer solution. Simple linear regression was used to produce an intrinsic viscosity equation containing unique fitted parameters for each of three solutions. These parameters describe the polymer coil size at unperturbed conditions and the polymer coil expansion capabilities of the solvent as a function of fluid temperature and molecular weight.
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
A series of highly purified galacturonate polysaccharides have been extracted from the Aloe vera plant and analyzed in terms of chemical composition and molecular weight. This Aloe vera polysaccharide (AvP) has been found to exist as a high molecular weight species and possess a unique chemical composition, including a high galacturonic acid (GalA) content and low degree of methyl ester substitution. These factors facilitate gel formation upon exposure to low concentrations of calcium ions, leading to potential application in formulations designed for in situ nasal or subcutaneous protein delivery. Thorough examination of classic dilute solution properties, the [eta]-M(w), and R(g)-M(w) relationships, persistence length (L(p)), and inherent chain stiffness (B parameter), indicate an expanded random coil in aqueous salt solutions. The critical concentration for transition from dilute to concentrated solution, C(e), was determined by measuring both the zero shear viscosity (eta(o)) and fluorescence emission of the probe molecule 1,8-anilino-1-naphthalene sulfonic acid (1,8-ANS) as a function of polymer concentration. Examination of zeta potential and C(e) as a function of ionic strength indicates that the shift in C(e) from 0.60 to 0.30 wt % is related to an increased occurrence of intermolecular interactions at high salt concentrations. Additionally, dynamic rheology data are presented highlighting the ability of AvP to form gels at low polymer and calcium ion concentrations, exemplifying the technological potential of this polysaccharide for in situ drug delivery.
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