Aqueous Reversible Addition-fragmentation Chain Transfer Research Articles

Bioconjugation of D-glucuronic acid sodium salt to well-defined primary amine-containing homopolymers and block copolymers

The synthesis of well-defined carboxylic acid-functionalized glycopolymers prepared via one-step postpolymerization modification of poly(N-[3-aminopropyl] methacrylamide) (PAPMA), a water-soluble primary amine methacrylamide, in aqueous medium is demonstrated. PAPMA was first polymerized via aqueous reversible addition-fragmentation chain transfer polymerization in aqueous buffer using 4-cyanopentanoic acid dithiobenzoate as the chain transfer agent and 4,4′-azobis(4-cyanovaleric acid) (V-501) as the initiator at 70 °C. The resulting well-defined PAPMA was then conjugated with D-glucuronic acid sodium salt through reductive amination in alkaline medium (pH 8.5) at 45 °C. The successful bioconjugation was proven through proton (1H) and carbon (13C) nuclear magnetic resonance spectroscopy and matrix-assisted laser desorption/ionization time of flight mass spectrometry analysis, which indicated near quantitative conversion. A similar bioconjugation reaction was conducted with poly(2-aminoethyl methacrylate) (PAEMA) and poly(2-aminoethyl methacrylate-b-poly(N-[2-hydroxypropyl]methacrylamide) (PAEMA-b-PHPMA). For the PAEMA homopolymers and block copolymers, however, lower conversion was obtained, most likely because of degradation reactions of PAEMA in alkaline medium. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3052–3061, 2010

“Schizophrenic” Self-Assembly of Block Copolymers Synthesized via Aqueous RAFT Polymerization: From Micelles to Vesicles†Paper number 143 in a series on Water-Soluble Polymers.

Dually responsive poly[(N,N-diethylaminoethyl methacrylate)-b-(N-isopropyl acrylamide)]s (P(DEAEMA-b-NIPAM)s) capable of “schizophrenic” aggregation in aqueous solution were synthesized via aqueous reversible addition−fragmentation chain transfer (RAFT) polymerization. The nanoassembly morphologies, dictated by the hydrophilic mass fraction, can be controlled by the polymer block lengths, solution pH, and temperature. Both P(DEAEMA98-b-NIPAM209) (52.5 wt % NIPAM) and P(DEAEMA98-b-NIPAM392) (70.8 wt % NIPAM) self-assemble into PDEAEMA-core PNIPAM-shell spherical micelles with a hydrodynamic radii (Rh) of 21 and 25 nm, respectively, at temperatures below the lower critical solution temperature of PNIPAM and at solution pH values greater than the pKa of PDEAEMA. The two block copolymers, however, display quite different temperature-responsive behaviors at pH < 7.5. At elevated temperatures (>42 °C) P(DEAEMA98-b-NIPAM209) forms spherical micelles (Rh = 28 nm) with hydrophobic PNIPAM cores stabilized by a hydrophilic PDEAEMA shell. By contrast, P(DEAEMA98-b-NIPAM392) assembles into vesicles (Rh = 99 nm) above 38 °C. The nanostructures were characterized by a combination of dynamic and static light scattering as well as transmission electron microscopy and are being investigated for their potential application as drug delivery vehicles.

RAFT cryopolymerizations ofN,N-dimethylacrylamide andN-isopropylacrylamide in moderately frozen aqueous solution

Aqueous reversible addition-fragmentation chain transfer (RAFT) cryopolymerizations of N,N-dimethylacrylamide (DMA) and N-isopropylacrylamide (NIPAM) with potassium persulfate/sodium ascorbate as redox initiators were performed at −15 °C. For the homopolymerizations, water-soluble chain transfer agents (CTAs) of 2-(1-carboxy-1-methylethyl-sulfanylthiocarbonylsulfanyl)-2-methylpropionic acid and 2-dodecylsulfanylthiocarbonylsulfanyl-2-methylpropionyl-capped methoxy poly(ethylene glycol) were used. For the sequential block copolymerizations, the obtained trithiocarbonate-functionalized polymers were used as macro-CTAs. Although well-defined homo and block polymers of DMA and NIPAM were synthesized and these RAFT cryopolymerizations were well controlled, their behavior depended on the monomers and CTAs. The polymerization kinetic and polymer structure were studied by proton nuclear magnetic resonance analysis and gel permeation chromatography measurement. Poly(N,N-dimethylacrylamide)-based cryogels crosslinked with reductively cleavable disulfide-containing diacrylamide, N,N′-bisacryloylcystamine, were synthesized via RAFT cryopolymerization. Scanning electron microscopy observation revealed that the porous structure of cryogels depended on the CTA used. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009

Aqueous RAFT polymerization of 2‐aminoethyl methacrylate to produce well‐defined, primary amine functional homo‐ and copolymers

AbstractWe report the direct homopolymerization and block copolymerization of 2‐aminoethyl methacrylate (AEMA) via aqueous reversible addition‐fragmentation chain transfer (RAFT) polymerization. The controlled “living” polymerization of AEMA was carried out directly in aqueous buffer using 4‐cyanopentanoic acid dithiobenzoate (CTP) as the chain transfer agent (CTA), and 2,2′‐azobis(2‐imidazolinylpropane) dihydrochloride (VA‐044) as the initiator at 50 °C. The controlled “living” character of the polymerization was verified with pseudo‐first order kinetic plots, a linear increase of the molecular weight with conversion, and low polydispersities (PDIs) (&lt;1.2). In addition, well‐defined copolymers of poly(AEMA‐b‐HPMA) have been prepared through chain extension of poly(AEMA) macroCTA with N‐(2‐hydroxypropyl)methacrylamide (HPMA) in water. It is shown that the macroCTA can be extended in a controlled fashion resulting in near monodisperse block copolymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5405–5415, 2009

Facile synthesis of multivalent folate-block copolymer conjugates via aqueous RAFT polymerization: targeted delivery of siRNA and subsequent gene suppression.

Cell specific delivery of small interfering ribonucleic acid (siRNA) using well-defined multivalent folate-conjugated block copolymers is reported. Primary amine functional, biocompatible, hydrophilic-block-cationic copolymers were synthesized via aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization. N-(2-hydroxypropyl)methacrylamide) (HPMA), a permanently hydrophilic monomer, was copolymerized with a primary amine containing monomer, N-(3-aminopropyl)methacrylamide (APMA). Poly(HPMA) confers biocompatibility, while APMA provides amine functionality, allowing conjugation of folate derivatives. HPMA-stat-APMA was chain extended with a cationic block, poly(N-[3-(dimethylamino)propyl]methacrylamide), to promote electrostatic complexation between the copolymer and the negatively charged phosphate backbone of siRNA. Notably, poly(HPMA) stabilizes the neutral complexes in aqueous solution, while APMA allows the conjugation of a targeting moiety, thus, dually circumventing problems associated with the delivery of genes via cationically charged complexes (universal transfection). Fluorescence microscopy and gene down-regulation studies indicate that these neutral complexes can be specifically delivered to cancer cells that overexpress folate receptors.

Aqueous RAFT Synthesis of pH-Responsive Triblock Copolymer mPEO−PAPMA−PDPAEMA and Formation of Shell Cross-Linked Micelles

Herein we report the synthesis and characterization of polymeric cross-linking agents and novel shell cross-linked (SCL) micelles utilizing reversible addition−fragmentation chain transfer (RAFT) polymerization. A series of pH-responsive ABC triblock copolymers consisting of α-methoxypoly(ethylene oxide)-b-poly[N-(3-aminopropyl)methacrylamide]-b-poly[2-(diisopropylamino)ethyl methacrylate] (mPEO−PAPMA−PDPAEMA) have been synthesized via RAFT polymerization in aqueous media at 70 °C employing a PEO-based macro-chain transfer agent (macro-CTA). These triblock copolymers molecularly dissolve in aqueous solution at low pH (<5.0) due to protonation of primary amine residues on the PAPMA block and tertiary amine residues on the PDPAEMA block. Above pH 6.0, the polymers self-assemble into micelles consisting of PDPAEMA cores, PAPMA shells, and mPEO coronas. Hydrodynamic dimensions of the triblock copolymer micelles depend on both triblock copolymer composition and solution pH. Narrowly dispersed poly(N-isopropylacrylamide) was synthesized utilizing the difunctional CTA, 2-(1-carboxy-1-methylethylsulfanylthiocarbonylsulfanyl)-2-methylpropionic acid (CMP). The chain ends of the PNIPAM were converted from carboxylic acids to N-hydroxysuccinimidyl esters (NHS) through dicyclohexylcarbodiimide (DCC) coupling, yielding an amine-reactive polymeric cross-linking agent, NHS−PNIPAM−NHS. SCL micelles were attained via reaction of PAPMA (shell) amine functionality with NHS-functionalized PNIPAM. These SCL micelles swell when the solution pH is lowered below the pKa of the PDPAEMA block. The polymeric cross-linking agent NHS−PNIPAM−NHS synthesized in this work has inherent temperature-responsive segments and a cleavable trithiocarbonate unit which have future potential in mediating drug delivery from SCL micelles.

Aqueous RAFT Synthesis of Micelle-Forming Amphiphilic Block Copolymers Containing N-Acryloylvaline. Dual Mode, Temperature/pH Responsiveness, and “Locking” of Micelle Structure through Interpolyelectrolyte Complexation

Temperature- and pH-responsive, micelle-forming, amphiphilic block copolymers were prepared from N,N-dimethylacrylamide (DMA), N-isopropylacrylamide (NIPAM), and N-acryloylvaline (AVAL) utilizing aqueous reversible addition−fragmentation chain transfer (RAFT) polymerization. A series of block copolymers were synthesized by employing DMA as a macro-chain transfer agent to mediate the statistical copolymerization of NIPAM with AVAL. Structural organization and solution behavior were investigated utilizing dynamic light scattering, two-dimensional NMR spectroscopy, and transmission electron microscopy. It has been demonstrated that the critical micellization temperature for the block polymers can be tuned to range from ≈10 to 36 °C by adjusting the solution pH. Micelles with apparent hydrodynamic diameters from 45 to 86 nm are formed between pH 2 and 5. Above pH 5, a sufficient number of the AVAL units are deprotonated which prevents micellization. The extent of pH and temperature changes on the apparent hyd...

Effect of Sequential Layer-by-Layer Surface Modifications on the Surface Energy of Plasma-Modified Poly(dimethylsiloxane)

Surface-initiated grafting of N,N-dimethylacrylamide, styrenesulfonate (SS), and (ar-vinylbenzyl)trimethylammonium chloride (VBTAC) from microwave plasma carboxylated, initiator-functionalized poly(dimethylsiloxane) (PDMS) surfaces was accomplished utilizing reversible addition-fragmentation chain transfer (RAFT) polymerization. Surface spectroscopic attenuated total reflectance (ATR) FT-IR analysis and atomic force microscopy (AFM) measurements were utilized to determine surface grafting and morphological surface features. The VBTAC-grafted PDMS provided a smooth, hydrophilic cationic surface for creating layer-by-layer (LBL) surfaces via alternating deposition of well-defined poly(SS) and poly(VBTAC), also prepared via aqueous RAFT. Comparisons of the ATR FT-IR spectra of the LBL assemblies and those of respective anionic poly(SS) and cationic poly(VBTAC) components confirmed strong electrostatic complexation of a fraction of the sulfonate and quarternary ammonium species in the layers as well as the existence of noncomplexed species. AFM images of surface topology indicated the presence of domains, likely phase-separated segments of the respective homopolymers, as well as interlayer mixing. The employed LBL methodology results in formation of stable, highly hydrophilic surfaces on a PDMS substrate. To our knowledge, this is the first study that illustrates surface functionalization of PDMS using microwave plasma and RAFT polymerization, followed by LBL deposition of polyelectrolytes.

Corona-Stabilized Interpolyelectrolyte Complexes of SiRNA with Nonimmunogenic, Hydrophilic/Cationic Block Copolymers Prepared by Aqueous RAFT Polymerization

The complexation of small interfering ribonucleic acid (siRNA) with a series of specifically designed block copolymers consisting of the hydrophilic, nonimmunogenic monomer N-(2-hydroxypropyl)methacrylamide (HPMA) and the cationic monomer N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA) has been investigated for potential siRNA stabilization and delivery applications. Specific compositions of poly(HPMA-b-DMAPMA) copolymers were synthesized via aqueous reversible addition−fragmentation chain transfer (RAFT) polymerization and characterized using aqueous size exclusion chromatography with multiangle laser light scattering (SEC-MALLS) and 1H NMR spectroscopy. The degree of soluble complex formation between a model siRNA and the polymers was determined by centrifugal membrane filtration experiments and quantitated by scintillation counting of 32P ATP-labeled siRNA to determine complex solubility and to estimate the degree of complexation relative to cationic and neutral block lengths. Dynamic and static light scattering methods were employed to determine the hydrodynamic radii, molecular weights, and second virial coefficients of the complexes and to demonstrate their unimodal size distributions. In vitro enzymatic degradation studies of selected siRNA/block copolymer complexes were conducted to demonstrate the enhanced stability of the siRNA/poly(HPMA-b-DMAPMA) complexes. Furthermore, the siRNA/polymer complexes dissociate slowly under gel electrophoresis conditions. Therefore, the siRNA/polymer complexes demonstrate some highly desirable properties for potential applications in therapeutic siRNA stabilization and delivery.

Effect of an added base on (4-cyanopentanoic acid)-4-dithiobenzoate mediated RAFT polymerization in water

The effect of an added base on the aqueous reversible addition–fragmentation chain transfer polymerization of a methacrylic glycomonomer with (4-cyanopentanoic acid)-4-dithiobenzoate was investigated. When sodium carbonate or sodium bicarbonate were used to dissolve the RAFT agent in aqueous solution at room temperature, an inhibition period of 60–90 min was observed at the beginning of the polymerization together with a marked decrease in the overall polymerization rate. Also, experimental M n values were much higher than the calculated ones in both cases. When sodium carbonate was used, control over the polymerization process was lost within 43% conversion. Better results were obtained with sodium bicarbonate, in which case the molecular weight distribution remained narrow and unimodal up to 81% conversion. At that point, a higher molecular weight shoulder developed that kept growing in intensity at the proceeding of the reaction. Dramatically improved results were obtained by adding circa 10% ethanol to the polymerization mixture to facilitate the dissolution of (4-cyanopentanoic acid)-4-dithiobenzoate. Following this protocol, narrow polydispersity poly(methyl 6- O-methacryloyl-α- d-glucoside) was obtained possessing a molecular weight close to the predicted value.

Chiroptical Properties of Homopolymers and Block Copolymers Synthesized from the Enantiomeric Monomers N-Acryloyl-L-Alanine and N-Acryloyl-D-Alanine Using Aqueous RAFT Polymerization

Chiral homo- and block copolymers based on the enantiomeric monomers N-acryloyl-l-alanine (ALAL) and N-acryloyl-d-alanine (ADAL) were prepared directly in water using controlled reversible addition–fragmentation chain transfer (RAFT) polymerization. The polymerization of the chiral monomers proceeded in a controlled fashion producing the respective homopolymers, block copolymers, and a statistical copolymer with targeted molecular weights and narrow molecular weight distributions. The chiroptical activity of these biomimetic polymers and their analogous model compounds was investigated using circular dichroism (CD). P(ALAL) and P(ADAL) were shown to be optically active exhibiting mirror image CD spectra. In addition, statistical and enantiomeric block copolymers prepared at 1:1 stochiometric ratios exhibited virtually no optical activity.

Aqueous RAFT Polymerization: Recent Developments in Synthesis of Functional Water‐Soluble (Co)polymers with Controlled Structures

AbstractFor Abstract see ChemInform Abstract in Full Text.

Aqueous RAFT polymerization: recent developments in synthesis of functional water-soluble (co)polymers with controlled structures.

Reversible addition-fragmentation chain transfer (RAFT) polymerization has been the focus of intensive research over the past few years since this methodology allows the synthetic tailoring of macromolecules with complex architectures including block, graft, comb, and star structures with predetermined molecular weight, terminal functionality, and narrow molecular weight distribution. In this paper we recount significant milestones in achieving controlled free radical homopolymerization and block copolymerization of water-soluble and amphiphilic monomers including nonionic, cationic, anionic, and zwitterionic species. It is shown that under aqueous conditions, control of homopolymerization and further blocking to extend the molecular weight or to produce precisely structured block copolymers require not only careful selection of reagents (initiator, chain transfer agent, and monomer) but also regulation or elimination of hydrolysis of the omega-terminal thiocarbonylthio functionality. The technological potential of such systems is illustrated for the stimuli (pH) reversible micellization of amphiphilic block copolymers and for stabilization and stimuli responsive aggregation of gold nanoparticles bearing covalently tethered co(polymers). Given the advantages of RAFT over other controlled free radical techniques for preparation of water-soluble architectures, it may be anticipated that this technology will be at the forefront of nano- and microscale self-assembly in electronics and biotechnology.

Aqueous solution properties of pH‐responsive AB diblock acrylamido–styrenic copolymers synthesized via aqueous reversible addition–fragmentation chain transfer

AbstractHere we report the synthesis and solution characterization of a novel series of AB diblock copolymers with neutral, water‐soluble A blocks consisting of N,N‐dimethylacrylamide and pH‐responsive B blocks of N,N‐dimethylvinylbenzylamine. To our knowledge, this represents the first example of an acrylamido–styrenic block copolymer prepared directly in a homogeneous aqueous solution. The best blocking order [with poly(N,N‐dimethylacrylamide) as a macro‐chain‐transfer agent] yielded well‐defined block copolymers with minimal homopolymer impurities. The reversible aggregation of these block copolymers in aqueous media was studied with 1H NMR spectroscopy and dynamic light scattering. Finally, an example of core‐crosslinked micelles was demonstrated by the addition of a difunctional crosslinking agent to a micellar solution of the parent block copolymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1724–1734, 2004

Hydrolytic Susceptibility of Dithioester Chain Transfer Agents and Implications in Aqueous RAFT Polymerizations

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 ...

The direct polymerization of 2-methacryloxyethyl glucoside via aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization

A preliminary study on the direct controlled radical polymerization of a glycomonomer, namely 2-methacryloxyethyl glucoside (MAGlu), under reversible addition-fragmentation chain transfer (RAFT) polymerization conditions in aqueous media has been conducted. This represents the first example detailing the direct polymerization of a sugar monomer via RAFT and, significantly, has been conducted without protecting group chemistry. 4-Cyano-4-methyl-4-thiobenzoylsulfanyl butyric acid (CTP) was employed as the RAFT chain transfer agent (CTA) due to its inherent water-solubility and its applicability for methacrylic monomers. The homopolymerization displays all the characteristics of a controlled/‘living’ polymerization—linear increase in Mn with conversion, pseudo-first order kinetics, the final polymers have narrow molecular distributions and novel block copolymers can be prepared.

Facile preparation of transition metal nanoparticles stabilized by well-defined (co)polymers synthesized via aqueous reversible addition-fragmentation chain transfer polymerization.

Herein we disclose a facile method for the preparation of (co)polymer-stabilized transition metal colloids, via the "grafting-to" approach. The method takes advantage of the fact that (co)polymers synthesized in aqueous media, in a controlled fashion, via reversible addition-fragmentation chain transfer bear thiocarbonylthio end groups. These are readily reduced to yield (co)polymers with thiol end groups. When the reduction is performed, with NaBH(4), for example, in aqueous media in the presence of an appropriate transition metal species, (co)polymer-stabilized metal nanoparticles are formed in which the size and size distribution are dependent upon the individual transition metals. Colloid formation is confirmed by transmission electron microscopy and UV-vis spectroscopy.