Synthesis and Characterization of Charge-Stabilized Poly(4-hydroxybutyl acrylate) Latex by RAFT Aqueous Dispersion Polymerization: A New Precursor for Reverse Sequence Polymerization-Induced Self-Assembly

The reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 4-hydroxybutyl acrylate (HBA) is conducted using a water-soluble RAFT agent bearing a carboxylic acid group. This confers charge stabilization when such syntheses are conducted at pH 8, which leads to the formation of polydisperse anionic PHBA latex particles of approximately 200 nm diameter. The weakly hydrophobic nature of the PHBA chains confers stimulus-responsive behavior on such latexes, which are characterized by transmission electron microscopy, dynamic light scattering, aqueous electrophoresis, and ¹H NMR spectroscopy. Addition of a suitable water-miscible hydrophilic monomer such as 2-(N-(acryloyloxy)ethyl pyrrolidone) (NAEP) leads to in situ molecular dissolution of the PHBA latex, with subsequent RAFT polymerization leading to the formation of sterically stabilized PHBA-PNAEP diblock copolymer nanoparticles of approximately 57 nm diameter. Such formulations constitute a new approach to reverse sequence polymerization-induced self-assembly, whereby the hydrophobic block is prepared first in aqueous media.

Synthesis of High Molecular Weight Water-Soluble Polymers as Low-Viscosity Latex Particles by RAFT Aqueous Dispersion Polymerization in Highly Salty Media

We report the synthesis of sterically-stabilized diblock copolymer particles at 20% w/w solids via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of N,N′-dimethylacrylamide (DMAC) in highly salty media (2.0 M (NH₄)₂SO₄). This is achieved by selecting a well-known zwitterionic water-soluble polymer, poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC), to act as the salt-tolerant soluble precursor block. A relatively high degree of polymerization (DP) can be targeted for the salt-insoluble PDMAC block, which leads to the formation of a turbid free-flowing dispersion of PDMAC-core particles by a steric stabilization mechanism. ¹H NMR spectroscopy studies indicate that relatively high DMAC conversions (>99%) can be achieved within a few hours at 30 °C. Aqueous GPC analysis indicates high blocking efficiencies and unimodal molecular weight distributions, although dispersities increase monotonically as higher degrees of polymerization (DPs) are targeted for the PDMAC block. Particle characterization techniques include dynamic light scattering (DLS) and electrophoretic light scattering (ELS) using a state-of-the-art instrument that enables accurate ζ potential measurements in a concentrated salt solution. ¹H NMR spectroscopy studies confirm that dilution of the as-synthesized dispersions using deionized water lowers the background salt concentration and hence causes in situ molecular dissolution of the salt-intolerant PDMAC chains, which leads to a substantial thickening effect and the formation of transparent gels. Thus, this new polymerization-induced self-assembly (PISA) formulation enables high molecular weight water-soluble polymers to be prepared in a highly convenient, low-viscosity form. In principle, such aqueous PISA formulations are highly attractive: there are various commercial applications for high molecular weight water-soluble polymers, while the well-known negative aspects of using a RAFT agent (i.e., its cost, color, and malodor) are minimized when targeting such high DPs.

Cationic Sterically Stabilized Diblock Copolymer Nanoparticles Exhibit Exceptional Tolerance toward Added Salt

For certain commercial applications such as enhanced oil recovery, sterically stabilized colloidal dispersions that exhibit high tolerance toward added salt are desirable. Herein, we report a series of new cationic diblock copolymer nanoparticles that display excellent colloidal stability in concentrated aqueous salt solutions. More specifically, poly(2-(acryloyloxy)ethyltrimethylammonium chloride) (PATAC) has been chain-extended by reversible addition-fragmentation chain transfer aqueous dispersion polymerization of diacetone acrylamide (DAAM) at 70 °C to produce PATAC₁₀₀-PDAAM_x diblock copolymer spheres at 20% w/w solids via polymerization-induced self-assembly. Transmission electron microscopy and dynamic light scattering (DLS) analysis confirm that the mean sphere diameter can be adjusted by systematic variation of the mean degree of polymerization of the PDAAM block. Remarkably, DLS studies confirm that highly cationic PATAC₁₀₀-PDAAM₁₅₀₀ spheres retain their colloidal stability in the presence of either 4.0 M KCl or 3.0 M ammonium sulfate for at least 115 days at 20 °C. The mole fraction of PATAC chains within the stabilizer shell was systematically varied by the chain extension of various binary mixtures of non-ionic poly(N,N-dimethylacrylamide) (PDMAC) and cationic PATAC with DAAM to produce ([n] PATAC₁₀₀ + [1 - n] PDMAC₆₇)-PDAAM_z diblock copolymer spheres at 20% w/w. DLS studies confirmed that a relatively high mole fraction of cationic PATAC stabilizer chains (n ≥ 0.75) is required for the dispersions to remain colloidally stable in 4.0 M KCl. Cationic worms and vesicles could also be synthesized using a binary mixture of PATAC and PDMAC precursors, where n = 0.10. However, the vesicles only remained colloidally stable up to 1.0 M KCl, whereas the worms proved to be stable up to 2.0 M KCl. Such block copolymer nanoparticles are expected to be useful model systems for understanding the behavior of aqueous colloidal dispersions in extremely salty media. Finally, zeta potentials determined using electrophoretic light scattering are presented for such nanoparticles dispersed in highly salty media.

Synthesis of block cationic polyacrylamide precursors using an aqueous RAFT dispersion polymerization

Synthesis of cationic polyacrylamides (CPAMs) by introducing cationic polymer precursors followed by chain extension of acrylamide (AM) homopolymer blocks via RAFT polymerization is a promising approach for engineering high-performance CPAMs. However, the aqueous solution polymerization of AM usually leads to high viscosity, thus limiting the solid content in the polymerization system. Herein a novel approach is introduced that uses a random copolymer of AM and methacryloxyethyltrimethyl ammonium chloride (DMC) as a macro RAFT chain transfer agent (mCTA) and stabilizer for aqueous RAFT dispersion polymerization of AM. The AM/DMC random copolymers synthesized by RAFT solution polymerization, having narrow dispersities (Đ _s) at different molecular weights and cationic degrees (C _s), could serve as the mCTA, which was confirmed by mCTA chain extension in aqueous solution polymerization of AM under different C _s, solid contents, AM addition contents, extended PAM block lengths, and mCTA chain lengths. The block CPAMs had a Đ value of less than 1.2. A model was developed using the method of moments with consideration of the diffusion control effect, for further understanding the chain extension kinetics. Predicted polymerization kinetics provided an accurate fit of the experimental data. The AM/DMC random copolymers were further used for aqueous RAFT dispersion polymerization of AM under different polymerization temperatures, C _s, and mCTA chain lengths. The resulting products had a milky appearance, and the block copolymers had Đ _s of less than 1.3. Higher C _s and longer chain lengths on mCTAs were beneficial for stabilizing the polymerization systems and produced smaller particle sizes and less particle aggregation. The products remained stable at room temperature storage for more than a month. The results indicate that aqueous RAFT dispersion polymerization using random copolymers of AM and DMC at moderate cationic degrees as a stabilizer and mCTA is a suitable approach for synthesizing CPAM block precursors at an elevated solid content.

Synthesis of High Molecular Weight Poly(glycerol monomethacrylate) via RAFT Emulsion Polymerization of Isopropylideneglycerol Methacrylate

High molecular weight water-soluble polymers are widely used as flocculants or thickeners. However, synthesis of such polymers via solution polymerization invariably results in highly viscous fluids, which makes subsequent processing somewhat problematic. Alternatively, such polymers can be prepared as colloidal dispersions; in principle, this is advantageous because the particulate nature of the polymer chains ensures a much lower fluid viscosity. Herein we exemplify the latter approach by reporting the convenient one-pot synthesis of high molecular weight poly(glycerol monomethacrylate) (PGMA) via the reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization of a water-immiscible protected monomer precursor, isopropylideneglycerol methacrylate (IPGMA) at 70 °C, using a water-soluble poly(glycerol monomethacrylate) (PGMA) chain transfer agent as a steric stabilizer. This formulation produces a low-viscosity aqueous dispersion of PGMA-PIPGMA diblock copolymer nanoparticles at 20% solids. Subsequent acid deprotection of the hydrophobic core-forming PIPGMA block leads to particle dissolution and affords a viscous aqueous solution comprising high molecular weight PGMA homopolymer chains with a relatively narrow molecular weight distribution. Moreover, it is shown that this latex precursor route offers an important advantage compared to the RAFT aqueous solution polymerization of glycerol monomethacrylate since it provides a significantly faster rate of polymerization (and hence higher monomer conversion) under comparable conditions.

Synthesis of Cross-Linked Poly(acrylamide) Microspheres by Dispersion Polymerisation in Aqueous Ammonium Sulfate Solution

Cross-linked poly(acrylamide) microspheres, i.e. PAMBA, with mean diameters ranging from 169.7 to 525.2 nm were prepared by dispersion polymerisation of acrylamide in aqueous ammonium sulfate (AS) solution. N,N′-methylenebis(acrylamide) (MBA), sodium dodecyl sulfate (SDS), and potassium persulfate (KPS) were selected as the cross-linking agent, stabiliser, and initiator, respectively. The basic conditions for producing PAMBA microspheres, such as the salt concentration and monomer concentration, were optimised based on the precipitation behaviour of the polymer and the state of the product obtained after polymerisation. The optimum AS concentration and monomer concentration were determined as 300 and 88 g L−1, respectively. The effects of parameters, such as SDS concentration, MBA concentration, initiator concentration and temperature, on the product morphology and particle size were investigated by dynamic light scattering and transmission electron microscopy. The results show that the optimum conditions for the generation of microspheres are concentrations of 2.2–8.8 g L−1 for SDS, 4–6 g L−1 for MBA, 0.3–1.0 wt-% based on acrylamide for KPS, and the temperature should be kept at 35–45°C. The mean diameter of the microspheres decreases with an increase in SDS concentration and increases with an increase in MBA concentration. The polydispersity of the microspheres increases when SDS concentration exceeds 6.6 g L−1 as well as when MBA concentration increases. The formation mechanism of the products was discussed based on the results.

Stimuli-responsive poly(N-vinylcaprolactam-co-2-methoxyethyl acrylate) core–shell microgels: facile synthesis, modulation of surface properties and controlled internalisation into cells

Poly(N-vinylcaprolactam-co-2-methoxyethyl acrylate) core–shell microgels as imaging/diagnostic system.

Thionine-Modified Poly(glycidyl methacrylate) Nanospheres as Labels of Antibodies for Biosensing Applications

Monodisperse poly(glycidyl methacrylate) (PGMA) nanospheres were obtained by emulsifier-free emulsion polymerization and characterized by physicochemical methods. The effects of various reaction parameters on the particle properties were investigated. The particle size was controlled in the range of 350-420 nm. To introduce carboxyl groups, the PGMA nanospheres were hydrolyzed and oxidized with KMnO4. Subsequently, the enzyme horseradish peroxidase (HRP) and the electron mediator thionine were covalently attached to the PGMA nanospheres to obtain an antibody indicator suitable for enzyme-based electrochemical immunosensors. Combined HRP and thionine binding to the nanospheres had beneficial effects for the labeling efficiency and at the same time prevented the formation of soluble electron mediators.

Synthesis and characterization of a novel in situ forming gel based on hydrogel dispersions

This report describes the synthesis and characterization of hydrogel dispersions for the potential use as an in situ gel-forming wound sealant. A series of polyacrylamide (PAAm) particles (solid content 5-10 wt %) were prepared by dispersion polymerization of acrylamide (AAm) in a solvent mixture of ethanol/water (90/10 w/w). These particles are characterized by the presence of a "core" [ethylene glycol dimethacrylate (EGDMA) crosslinked PAAm] surrounded by a "shell" [acetoacetoxyethyl methacrylate (AAEMA) or 2-aminoethylmethacrylate-containing PAAm, named as AAEMA-xPAAm and NH(2)-xPAAm, respectively; x represents "crosslinked"]. Two "2-in-1" working systems were prepared: (1) AAEMA-xPAAm mixed with NH(2)-xPAAm and (2) AAEMA-xPAAm mixed with 1,6-diaminohexane [AAEMA:NH(2) (1:1 mol/mol in (1) and (2)]. These systems appeared to possess essential characteristics for their potential use as wound sealants. For example, at 37 degrees C (and/or room temperature), both systems (1) and (2) formed transparent films, within 2 min upon applying them to a substrate; a rapid, concomitant crosslinking reaction occurred between AAEMA and -NH(2) on their outer "shells," knitting the EGDMA-crosslinked PAAm micro-"gel" together into a macro one. In contrast to other reported wound sealants, which are in either solution form or powder form, the advantages of hydrogel dispersion-based sealant may include ease of application (sprayable due to its low viscosity), species encapsulation and protection, and so forth. To our best knowledge, this is the first report on designing a wound sealant based on micro-particle dispersions.

Synthesis of biocompatible sterically-stabilized poly(2-(methacryloyloxy)ethyl phosphorylcholine) latexes via dispersion polymerization in alcohol/water mixtures

Poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC) is soluble in either 2-propanol or water but becomes insoluble in certain alcohol-rich 2-propanol/water mixtures. We have exploited this unusual cononsolvency behavior in order to prepare new biocompatible sterically stabilized PMPC latexes via nonaqueous dispersion polymerization in 2-propanol/water mixtures. All polymerizations were conducted in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) as a reactive stabilizer, with some formulations including ethylene glycol dimethacrylate (EGDMA) as a cross-linker. Under optimized conditions, unimodal size distributions could be obtained with a mean latex diameter of approximately 1 microm, as judged by laser diffraction and DLS. The mean latex diameter depended on both the PEGMA and initiator concentration but was almost independent of the cross-linking density. Smaller PMPC latexes were obtained by increasing the alcohol content of the dispersion medium. On dilution with water, these latexes acquired microgel character. The microgel solution viscosity was insensitive to added salt due to the so-called "antipolyelectrolyte" effect, which is characteristic of polyzwitterions. Finally, copolymerization of MPC with a fluorescein-based methacrylic comonomer produced fluorescently labeled PMPC latexes, which may have potential biomedical applications.

Synthesis of poly(2-hydroxypropyl methacrylate) latex particles via aqueous dispersion polymerization

Poly(2-hydroxypropyl methacrylate) latexes were prepared by aqueous dispersion polymerization at 60 °C using poly(-vinylpyrrolidone) [PNVP] as a steric stabilizer. The mean latex diameter can be controlled over a wide range by varying the synthesis parameters (initiator type, stabilizer concentration, addition of co-surfactant or comonomer), and narrow size distributions were observed in most cases. These sterically-stabilized latex particles were characterized by electron microscopy, dynamic light scattering, Malvern Mastersizer and FT-IR spectroscopy. The presence of the PNVP stabilizer at the surface of the latex particles was confirmed by X-ray photoelectron spectroscopy and the stabilizer content was assessed by H NMR spectroscopy and nitrogen microanalyses. Colloidally stable surfactant-stabilized poly(2-hydroxypropyl methacrylate) latexes could also be prepared in the absence of any PNVP stabilizer. Since 2-hydroxypropyl methacrylate contains a small amount of dimethacrylate impurity, these latexes are actually lightly cross-linked; their degree of swelling in DO, d-methanol and d-pyridine was investigated using dynamic light scattering and H NMR spectroscopy. Finally, three ionic water-soluble comonomers were successfully copolymerized with 2-hydroxypropyl methacrylate under aqueous dispersion conditions, as judged by aqueous electrophoresis studies.

Molecularly imprinted polymer particles: synthetic receptors for future medicine

Molecular imprinting is a relatively new and rapidly evolving technique used to create synthetic receptors; it also possesses great potential in a number of applications in the life sciences. Traditionally, molecularly imprinted polymers are prepared by bulk polymerization, followed by crushing and sieving to obtain polymer beads. However, several methods can be used to synthesize polymer micro- and nano-particles directly, thereby avoiding the time- and labor-consuming process of crush sieving. Possible applications are foreseen in enhanced drug loading, controlled drug delivery and drug targeting. This review describes the different methods of synthesis of molecularly imprinted micro- and nano-particles and discusses how these methods challenge the outstanding issues that molecular imprinting is facing today, thereby facilitating biomedical applications in the future.