ATRP in Protic Media
As described in detail in the next section both the rate of polymerization and the degree of control over the polymerization (frequently evaluated by the polydispersity index, PDI = Mw/Mn) depend on the concentration of the deactivator actually present in the system, which in turn depends upon the value of the solvent-sensitive electron affinity (KEA) and halidophilicity (KX) of the deactivator. The values of KEA in protic solvents are expected to be relatively high as the halide anions formed in KEA are stabilized. KX will likewise be affected by changes in solvent polarity and by selective solvation of ions.
In the above equation, kp is the propagation rate constant for the monomer M. Thus the values of KAE affect the rate of polymerization as well as the level of control over a polymerization. Dissociation of the ATRP deactivator can be suppressed, to a significant degree, by the addition of halide salts to the reaction medium, while the addition of complex-forming agents or co-solvents that stabilize the CuI state of the catalyst relative to CuII can suppress disproportionation.(3-4)
All side reactions have been quantitatively described(5-6) which should make it possible to predict the reaction conditions required for optimal results in specific controlled radical polymerizations of a variety of water soluble monomers in water-based or polar solvents. Indeed many monomers have been successfully polymerized in water-based media using ATRP.(7-11) There is an ongoing research effort to clarify the mechanisms of the interactions of water, or other polar solvents, with the catalyst, initiator, and monomer(s) in both homogeneous and heterogeneous systems in order to understand the multiplicity of interactions that can take place in the presence of polar monomers and solvents. The objective is to expand the utility of homogeneous and heterogeneous aqueous ATRP.(12)
All CRP processes, in particular ATRP, can be applied to the entire range of water-borne systems: from solution to suspension, indeed even dispersion.(13)
Water is an inexpensive environmentally friendly solvent with high thermal capacity, which makes it an attractive medium for exothermic radical copolymerization reactions, particularly since both solution polymerization of water-soluble polymers and biphasic polymerization of hydrophobic monomers in latexes have found direct industrial scale application in various practical fields. In heterogeneous media it is possible to prepare particles with a wide range of size, nm to mm, using different polymerization systems, as discussed in Ke Min’s thesis and recent review articles,(14-17) while employing the full spectrum of aqueous dispersed media: including suspension, dispersion, emulsion, miniemeulsion and microemulsion procedures.
An understanding of the complex interactions that can occur in polar or aqueous ATRP (shown below for the case of homogeneous aqueous solutions) is important in order to define reaction conditions that will allow preparation of well-defined polymeric materials in highly polar aqueous media.(2,7-8,10,18)
The schematic shows that several complex equilibria compete when ATRP is being conducted in protic solvents, such as alcohols or water, leading to:
- disproportionation of the CuI complex (the ATRP activator ) (Kdisp)
- reduction in the concentration of the copperII based deactivator (CuIILmX) via dissociation of the halide ligand (KX)
- complexation of the dissociated complex or ligand with the solvent and/or the polar monomers (KCu,aq or KX,aq,j), or
- disproportionation or hydrolysis of the initiator or dormant chain end.
There have been reports that disproportionation can lead to a faster controlled polymerization process due to exclusive activation of an alkyl halide initiator by exceptionally active Cu0 to generate a propagating radical and a CuI species that instantaneous undergoes disproportionation of CuI into Cu0 and CuII in "catalytic" solvents such as DMSO, and that deactivation of the radical by CuII establishes an equilibrium between active and dormant polymer chains.(19) It was further postulated that the activation and deactivation processes in this technique occurred via outer-sphere electron transfer (OSET) to produce alkyl halide radical anion intermediates. However a rational investigation of the aforementioned mechanism using Cu complexes of tris[2-(dimethylamino)ethyl]amine (Me6TREN) and model studies to quantify disproportionation of CuI/Me6TREN in DMSO, DMF, and MeCN determined that comproportionation of Cu0 with CuII to form CuI was slow but dominant in all three solvents.(20-21)
The possibility of OSET among copper species and alkyl halides was evaluated on the basis of literature data and found to be negligible in comparison to an atom transfer process (i.e., inner-sphere electron transfer). Stopping or reducing disproportionation appears to be the most rational approach to a scalable robust procedure for CRP.(20-21)
Relative activation rates of alkyl halides by Cu0 and CuI with Me6TREN were studied. Reactions catalyzed by CuI/Me6TREN were significantly faster than those employing Cu0. These studies ultimately and irrefutably indicate that in addition to slowly activating alkyl halides Cu0 predominately acts as a reducing agent, regenerating the CuI activator from accumulated CuII. This procedure was initially disclosed in 1997(22-23) where conducting an ATRP with transition metal complex formed by addition of Cu0 alone, or in conjunction with CuII provided a well controlled reaction. Retrospectively this can be viewed as emulating the mechanism clarified by activators regenerated by electron transfer in atom transfer radical polymerization (ARGET ATRP).(24-25)
Another ATRP side reaction that occurs to a significant extent in water is hydrolysis of the CuII-halide complex. Because H2O solvates Br¯ ions much better than organic solvents the degree of reversible dissociation of the halide anion from the higher oxidation state metal complex will be much more significant in aqueous media. This dissociation, which is presumably followed by coordination of water to CuII, ultimately lowers the concentration of deactivator available during an aqueous ATRP. This is consistent with the observation that ATRP reactions are typically much faster and less controlled in aqueous and protic media. However, deactivator solvolysis can be suppressed, and control over the polymerization of hydrophilic polymers achieved, by addition of extra halide salts to the reaction medium(2) and adjusting the polarity of the solvent.(26)
Therefore it is possible to predict the reaction conditions leading to optimal results in the controlled radical polymerization of variety of water soluble monomers in water-based solvents; including neutral 2-hydroxyethyl methacrylate, (HEMA) and cationic 2-(N,N,N-trimethylammonio)ethyl methacrylate triflate, (TMATf) and 2-(N,N,N-dimethylethylammonio)ethyl methacrylate bromide (DMEABr) as described in reference 14 and work by Armes(8) and Dubois.(11) Polar monomers with nucleophilic groups (4VP, DMAEMA) can displace the bromine present on the chain end by nucleophilic substitution but this can be minimized by using alkyl chloride initiators and/or chloride-based catalysts.(2)
A general review describing all side reactions in protic media and steps for catalyst optimization is provided in a Chemical Review paper.(4)
The initial steps toward conducting an aqueous ATRP with lower concentrations of catalyst were taken with the development of electrochemically mediated ATRP as this provided a convenient tool to control the ratio of CuII to CuI throughout the reaction.(18) The work focused on the polymerization of oligo(ethylene glycol) methyl ether methacrylate (OEOMA475) in water with a CuII/I/TPMA (TPMA = tris(2-pyridylmethyl)amine) catalyst system and illustrated that many of the drawbacks typically associated with conventional aqueous ATRP could be overcome. This specific polymerization, OEOMA475 in water with a CuII/I/TPMA catalyst, was selected for study as the final polymer would be biocompatible and conducting such a reaction in water as the primary solvent was a critical step in defining conditions for conducting ATRP under biologically compatible conditions.(27) The primary difficulty was to ensure the presence of a sufficient concentration of deactivator in the highly active catalyst system by maintaining a high CuII/CuI ratio in the reaction medium. This initial series of experiments, in addition to addressing many of the issues of that had to be overcome in aqueous ATRP, also provided the information to extend aqueous ATRP to other activation procedures with lower concentration of catalyst; namely AGET, ARGET and ICAR ATRP with ppm catalyst.
ICAR ATRP in Water
Low catalyst concentrations and benign solvents are desirable to reduce the environmental impact of ATRP.(28) For the first time, Cu catalyst concentrations of 100 ppm and lower were used in aqueous media to prepare well-defined macromolecules.(29) The effect of various reaction parameters including: the concentration of a halide salt, the correct catalyst complex and the targeted degree of polymerization for the homopolymerization of A(EO)XOMe were investigated in an ICAR initiated system. As noted above, one challenge associated with copper based ATRP in water is the dissociation of deactivator complexes to give bromide ions and Cu(II)/L complexes that cannot deactivate growing radicals.(2,30) Adding a large excess of a salt with a complementary halide anion, e.g. tetraethylammonium bromide (TEABr) shifts the equilibrium back towards the deactivator complex and actually led to products with narrower dispersity. The results should be independent of the cation used which implies that cations such as sodium or potassium could be used, or even polymerizable cations such as those from quarentized 2-(dimethylamino)ethyl methacrylate.
The copper halide catalyst with tris(pyridin-2-ylmethyl)amine (TPMA) ligand gave stable CuI complexes in water, without any significant disproportionation, (3,18,21) and polymers of oligo(ethylene oxide) methyl ether acrylate were synthesized with low dispersity (Mw/Mn = 1.15-1.28) using 20-100 ppm of an active CuBr/TPMA based catalyst in the presence of excess bromide anions. Higher copper concentrations lead to slower polymerization rates, as seen in earlier reports on ICAR ATRP.(31) All polymerizations were performed at 44 C, which corresponds to a 10 h half-life for the 2,2'-azobis[2-(2-imidazolin-2-yl)propane] dihydrochloride (VA-044) azo-initiator, higher temperatures are expected to be too high for the majority of polymer protein conjugate applications(27) which are a target for controlled grafting from polymerizations. Chains grow uniformly with Cu catalyst concentrations between 20 and 100 ppm. However when 5 ppm catalyst was used, the molecular weights were higher than in the other polymerizations, indicating a loss of control at approximately 5 ppm due to a lower concentration of deactivator, and longer transient radical lifetimes.(32-33) The procedure was suitable for the preparation of thermo-responsive block copolymers and grafting from a protein where the grafted chains displayed values of Mn ≈ 55,000 and Mw/Mn = 1.15.
The ICAR method disclosed in the paper uses ca. 500 times lower catalyst concentrations than other ATRP methods recently optimized for the synthesis of well-defined polymers from proteins,(27) facilitating purification of the PPH. Nevertheless, since ICAR like RAFT, generates new chains due to the use of a thermal initiator which broadens Mw/Mn and introduces homopolymer into block copolymer synthesis this provided an additional incentive to optimize aqueous ARGET ATRP.(34)
A well-controlled polymerization of oligo(ethylene oxide) methyl ether methacrylate (OEOMA) was conducted with 100-300 ppm of a copper catalyst with tris(2-pyridylmethyl)amine (TPMA) ligand in the presence of an excess of an excess of ligand and an excess of halide salts at room temperature.(34) The Cu/TPMA catalyst was used, since this complex provides significant stability at high dilutions in aqueous media with minimal disproportionation,(3) and does not denature proteins.(27) Catalyst concentrations as low as 100 ppm were shown to provide a well controlled polymerization. In order to maintain an appropriate concentration of CuII in the reaction medium it was necessary to continuously add low amounts of ascorbic acid to the reaction mixture to generate and then regenerate a low fraction of the the activator complex. The effect of the halide salt concentration, ligand concentration, feeding rate of the reducing agent, and copper concentration were systematically studied to identify conditions that provide both an acceptable rate of polymerization and good control over the polymer properties. The optimized polymerization conditions provided linear first-order kinetics, linear evolution of the molecular weight with conversion and polymers with narrow Mw/Mn, below 1.3, at high monomer conversions (~70%) with retention of chain end functionality which was confirmed by chain extension to form a block copolymer. In the chain extension of a p(OEOMA475) macroinitiator formed with 100 ppm copper; a Cu/L = 1/8, in the presence of 100nM NaCl and addition of an ascorbic acid solution at a rate of 16nmol/min with OEOA480 under similar conditions, but with higher rate of feeding of ascorbic acid to compensate for the lower KATRP of acrylates, there was a clean shift of MW in the GPC traces.
The reaction rate, and hence any exotherm, could be directly controlled by stopping or starting the continuous feeding of the reducing agent.
Finally, the aqueous ARGET ATRP technique was applied to biological systems by synthesizing a well-defined protein-polymer hybrid by the “grafting from” method.
ATRP has been extended to all aspects of aqueous dispersed media for the preparation of polymer latexes containing well-defined polymers with various compositions, topologies, and well-maintained functionalities.(17) However, preparation of polymer particles with both good colloidal stability and multiple site-specific functional groups introduced to the polymer surface remained a challenge until recent advances in the use of reactive surfactants.(35-37) Selection of a suitable ligand is a critical first step in order to conduct a controlled ATRP in any bi-phasic aqueous system. The choice of surfactant is also important in all heterogeneous emulsion systems. However ligand selection remains of primary importance since the ligand determines the solubility of the metal complex in the monomer phase and partitioning of the metal complexes between the different phases.(17)
The earliest work directed at conducting a successful ATRP in an emulsion system required relatively low solids content and high surfactant levels (13% solids, 13.5 wt% surfactant based on monomer)(38-39) for production of stable latexes. However by moving to a miniemulsion process and using a reverse ATRP initiation procedure,(40-41) we were able to increase the solids content of the system and significantly reduce the level of surfactant (30 % solids, 2.3 wt% surfactant based on monomer) in addition to starting with an oxidatively stable catalyst complex. One critical requirement is selection of an appropriate ligand; dNbpy, BMODA and tNtpy are sufficiently hydrophobic ligands to retain sufficient CuII in the monomer droplets dispersed in a miniemulsion polymerization to provide a controlled polymerization, while complexes formed with bpy, PMDETA and Me6TREN allow migration of CuII to the aqueous phase and control is lost.(6,40-41) Suitable commercially available surfactants include CTAB, Brij 98 and Tween 80 whereas PEG 100, PEG 4600, Brij 97 and Tween 20 provided less stable emulsions.
While reverse initiation simplified setting up a miniemulsion ATRP it limited the range of materials that could be prepared and the catalysts that could be employed in the reaction.(40-41) In a reverse initiation procedure one cannot independently adjust catalyst level and DPtarget since formation of lower oxidation state catalyst complex and an ATRP initiator are dependent on the amount of standard free radical initiator added to the reaction. Therefore a new initiation system was developed: one that was called Simultaneous Reverse and Normal Initiation (SR&NI)(42) that involved addition of both an ATRP initiator and a free radical initiator. SR&NI allowed the preparation of block and star copolymers in a miniemulsion with low levels of an active catalyst and lower levels of surfactant.(43-44) The use of SR&NI provided stable latexes with high solids content. As shown in the 2D-GPC plot below, only 4.5% of linear homopolystyrene was present in the final polymer when a tri-arm poly(methyl acrylate) macroinitiator was chain extended by styrene in a SR&NI miniemulsion process. The homopolymer results from use of a free radical initiator to activate the catalyst complex which simultaneously generated a monofunctional initiator.
This particular 2D-GPC plot provided the incentive to work on a further improvement in the procedures used for initiation of an ATRP and directly led to the development of Activator Generated by Electron Transfer (AGET) ATRP. In an AGET ATRP miniemulsion polymerization process a water soluble reducing agent is used to activate the catalyst and control the fraction of deactivator present in the suspending medium.(45,46) The following 2D-GPC plot shows the final result of an AGET ATRP using the same tri-arm macroinitiator.
No homopolymer can be detected, however, as noted in reference 45, the amount and addition rate of the reducing agent affects the polymerization rate and the level of control attained in the polymerization. However, as the image shows, when the selection is made appropriately no homopolymer and no coupling product are detected.
A further advantage of AGET ATRP is that by adding an excess of reducing agent the reaction can be successfully carried out in the presence of small amounts of air thereby simplifying set up procedures.(46)
ATRP was expanded to microemulsion polymerization systems(47) and as always the proper selection of ligand and surfactant remain keys to success. A hydrophobic ligand is preferred for mini- and micro-emulsion and selection of a non-ionic surfactant with a suitable HLB value results in good colloidal stability and good control. AGET initiation was preferred for microemulsion polymerizations as this provided copolymers with narrow PDI, 1.28, and narrow particle size distribution. Ascorbic acid was selected as the reducing agent since it is soluble in the aqueous phase and ensures rapid reduction of any Cu(II) that migrates to the aqueous phase or is present at the interphase.
The expansion of dispersed media ATRP to microemulsion and development of a process for addition of pure monomer to the system to increase the % solids in the reaction was initially employed to prepare a forced gradient copolymer(48) in a heterogeneous aqueous controlled copolymerization. An extension of the concept resulted in the development of an ab initio emulsion polymerization process capable of directly preparing block copolymers in a stable latex.(49)
Schematic illustration of an ab initio emulsion ATRP in (from Ke Min's PhD thesis)
The critical requirement for these advances in dispersed media ATRP was the ability to encapsulate all agents required for an ATRP in the first formed micelles. This allowed pure monomer to be added to the reaction medium. The added monomer was then able diffuse to the active micelles allowing an increase in micelle size and concomitant increase in the percent solids and decrease in % surfactants in the system. This two-step approach to an emulsion ATRP proved that a microemulsion ATRP could be successfully transformed into an emulsion polymerization. Addition of a second monomer after a major proportion of the first monomer had polymerized leads to preparation of a block copolymer with a “gradient” second block, see above schematic.
Preparation of "hairy" nanoparticles
An extension of the concept that led to the development of the ab initio emulsion polymerization resulted in an efficient one-pot synthesis of hairy nanoparticles. The procedure was developed by applying ATRP to a two-step sequential heterogeneous copolymerization where the first step was a microemulsion ATRP involved copolymerization of a monomer with cross-linker, leading to formation of uniform stable nanogels.(15) Addition of a second monomer to the initial microemulsion system successfully converted the system into an emulsion polymerization and the second monomer diffused to the crosslinked nanoparticles and arms grew from the retained initiating sites in the crosslinked nanogels, forming hairy nanoparticles in situ. The use of the “ab initio emulsion” procedure allowed the preparation of hairy nanoparticles with uniform size. The particle size increased as the second monomer was converted to polymer. Direct visualization of the hairy nanoparticles by AFM provided additional evidence for the successful synthesis of uniform-sized particles.
The images were taken from particle dispersions in chloroform (0.2 mg/mL) spin-coated onto mica surface.
At the other end of the particle size spectrum, there is another heterogeneous process where particles > 1 μm are prepared. Atom transfer radical dispersion polymerization of styrene in ethanol was successfully carried out using a “two-stage” polymerization technique, in which the first stage, conducted to less than 5% monomer conversion, involves a conventional FRP and the second an ATRP.(13) When styrene was employed as the exemplary monomer polystyrene particles with size 1.5-3 μm were obtained. The large particles, with narrow size distribution, contained polymers possessing relatively low polydispersity (Mw/Mn=1.4-1.8, compared with Mw/Mn= 4-5 for polystyrene prepared exclusively in a conventional radical dispersion polymerization) indicating a high fraction of retained chain end functionality within the particle. This accessible functionality can be readily employed for further modification of the core or the surface of the particle.(13)
A further advance in this procedure for the preparation of uniform sized polymer particles is the use of commercially available poly(ethylene oxide) macromonomers as reactive stabilizers for atom transfer radical dispersion polymerization of styrene.(50) Polystyrene particles with PEO chains covalently anchored on the surfaces are obtained. Control over both the particle size and polymer chain growth is achieved using a two-stage technique consisting of an initial free radical polymerization for the nucleation step followed by a reverse ATRP for the controlled polymerization process. The limited monomer conversion at the FRP step ensures that the overall process is mainly controlled by the reverse ATRP step, resulting in the formation of particles containing well-defined polymer chains.
Another extension of heterogeneous controlled polymerization is the preparation of hydrophylic nano-particles in an inverse miniemulsion ATRP.(51) Stable colloidal nanoparticles of well-controlled water-soluble poly(oligo(ethylene glycol) monomethyl ether methacrylate) (P(OEOMA)) homo- and copolymers were successfully synthesized by inverse miniemulsion atom transfer radical polymerization using activators generated by electron transfer (AGET ATRP) at ambient temperature, (30 0C). An oil soluble surfactant (sorbitan monooleate (Span 80)) was employed in conjunction with ascorbic acid as a reducing agent. An oxidatively stable CuBr2/tris[(2-pyridyl)methyl]amine (CuBr2/TPMA) catalyst complex at a ratio of [CuBr2/TPMA]0/[initiator]0 = 0.5 /1 was selected as the catalyst precursor/initiator for the controlled AGET ATRP inverse miniemulsion.
The effect of reaction parameters on the level of control attained in an inverse miniemulsion utilizing AGET ATRP and colloidal stability were explored. It was found that the use of water-soluble poly(ethylene oxide)-bromoisobutyrate macroinitiators (PEO-Br) with long chain EO units, and addition of up to 90% ascorbic acid compared to the moles of Cu(II) complex added to the reaction medium, and appropriate amount of water resulted in the formation of stable colloidal particles with diameter less than 200 nm and well-controlled P(OEOMA) with Mw/Mn < 1.3.(52) The addition of a long chain poly(ethylene glycol) monomethyl ether (PEO-OMe) as a costabilizer improved colloidal stability without interfering with the polymerization.
A water-soluble CuBr2/bipyridine catalyst complex was also suitable for AGET ATRP of OEOMA in inverse miniemulsion. Finally, colloidal particles of well-controlled block copolymers of OEOMA with different sizes of OEO side chains (DP of EO = 5 and 9) were produced with relatively low polydispersity, 1.3, at 85% conversion.
A review of this earlier work(53) described the development of microgel/nanogel particles as drug delivery carriers for biological and biomedical applications.(54) Microgels/nanogels are crosslinked polymeric particles, which can be considered as hydrogels if they are composed of water soluble/swellable polymer chains. They possess high water content, biocompatibility, and desirable mechanical properties. They offer unique advantages for polymer-based drug delivery systems (DDS): a tunable size from nanometers to micrometers, a large surface area for multivalent bioconjugation, and an interior network for the incorporation of biomolecules(55) all present in a biodegradable particle generating polymer fragments below the renal threshold.
A recent paper(56) described how colloidal particles with a readily tunable size, ranging from ca. 35 to 200 nm, were obtained by adjusting the amount and the type of reducing agent used for the generation of activators in the AGET ATRP and by adjusting the aqueous phase fraction in the inverse microemulsion system. The formed particles contained well-defined water-soluble polymers with relatively narrow Mw/Mn < 1.5. The introduction of a small amount of divinyl cross-linkers into the system allowed the synthesis of cross-linked hydrogel nanoparticles. The cross-linked particles retained their morpholology when they were redispersed in methanol or water, as evidenced by the constant particle size determined by dynamic light scattering
Miniemulsion ATRP has been demonstrated to be an efficient procedure for the preparation of block and graft copolymers from multifunctional macroinitiators.(5,44) As detailed in the referenced papers, this is partially a result of compartmentalization of the multifunctional initiators in the well dispersed stable “oil” droplets.(57-58) This is shown schematically below. Because there are fewer active radicals in each miniemulsion droplet than in a bulk polymerization there is less inter-particle termination and significant levels of inter-particle crosslinking are avoided, allowing the reaction to be driven to higher conversion.
The AFM images on the right show the lack of inter-particle crosslinking in the nanocomposite structure formed in a “grafting from” surface active silica particles in a miniemulsion polymerization. One reaction was conducted using SR&NI initiation and the other AGET ATRP. The second image shows significantly less unattached polymer chains in the AGET system and a total absence of linked particles due to the added ability to control the concentration of the deactivator in the dispersed droplets.
The same principle holds true for an ATRP “grafting from” a soluble linear multifunctional macroinitiator. The following images show that bottle-brush copolymers can also be prepared in an AGET ATRP miniemulsion polymerization and that brush-brush crosslinking reactions are minimized in such a system.(59)
The images shown above are atomic force microscopy images of a bottle brush copolymer prepared by ARGET ATRP in miniemulsion with ascorbic acid as the reducing agent at an ascorbic acid to Cu(II) ratio of 1:4. They images clearly show that almost no crosslinking occurred and that no homopolymer was formed in the reaction.
Efficient synthesis of polymeric nanostructures with improved stability and various site-specific functionalities was improved and further expanded by using the reactive surfactant concept in biphasic ATRP systems. Reactive surfactants involve the incorporation of surface-active components containing reactive α- and -ω end groups into a controlled radical polymerization process in dispersed media. A recent advance in AGET ATRP describes the use of an amphiphilic block copolymer that functions as both a stabilizer and a macroinitiator.(36,60) The following schematic shows the topology of a reactive surfactant for a standard emulsion system.
The reactive surfactant can be an initiator for an ATRP (an inisurf) or contain an unsaturated group that will be incorporated into the growing copolymer chain (a surfmer).
Amphiphilic poly(ethylene oxide)-b-polystyrene (PEO-PS-Br) block copolymers with various molecular weights were synthesized and used as macroinitiators/stabilizers, i.e. surfactants with reactive ω-ATRP initiating functionality, for AGET ATRP of butyl acrylate in miniemulsion that were used either with or without the addition of ethyl 2-bromoisobutyrate (EBiB) as co-initiator.(36) The addition of a low molecular weight coinitiator allows one to independently control the final particle size and molecular weight of the (co)polymers formed in the polymerization. Under both conditions, the reactions were well controlled and stable latexes were formed. The presence of an added low molecular weight initiator, EBiB, allowed the amount of surfactant used in the reaction to be reduced to 1.7-4 wt% vs monomer, and because of the covalent linking of the surfactants to polymer chains, no free surfactant was left in the reaction system and since the surfactant was tethered to the surface of the latex particle the possibility of migration during film formation was also avoided.
A further example providing surface functionality to the formed latex particle was the preparation and use of a dual-functional amphiphilic block copolymer, α-azido-ω-2-chloroisobutyrate-poly(oligo(ethylene oxide) monomethyl ether methacrylate)-b-poly(n-butyl methacrylate), prepared by atom-transfer radical polymerization as a dual-reactive surfactant,(37) The amphiphilic nature of the block copolymers allowed them to be used as surfactant/initiators and the polymerization was initiated at the oil-water interface, with polymer chains slowly growing inward in a controlled manner after activation of the catalyst. Polymeric nanocapsules with cross-linked shells and the latent azido functionality on the particle surface were obtained. Introduction of cross-linking agents (X in the following schematic) with various degradable linkages into the system resulted in the formation of nanocapsules that could be cleaved under specific conditions.
The preserved latent α-terminal azido groups in the dual-reactive surfactant were utilized to attach a fluorescent dansyl probe (exemplifying a targeting agent for bio-applications) and/or atom-transfer radical polymerization initiators to grow linear polymer chains forming an additional shell covalently connected to the nanocapsules.
The other approach that uses a reactive surfactant monomer was exemplified by the use of 11'-(methacryloyloxy)-undecyl(trimethyl)ammonium bromide (MUTAB) which was synthesized and used as a stabilizer and comonomer for an AGET ATRP of Bu methacrylate in emulsion.(35) Stable polymer latexes with size ranging from 40 to 200 nm were obtained by adjusting the amount of reactive surfactant introduced to the system. The concentration of the surfactant could be decreased to as low as 1.3 wt % in the final emulsion (5.9 wt % vs monomer), while still maintaining good colloidal stability. A positive zeta potential measured for the latex particles indicated that the cationic reactive surfactants were covalently anchored on the surfaces of the formed particles. A well-controlled ATRP provided a smooth growth in the MW of the polymer chains as the polymerization progressed and by the formation of polymers with preserved chain-end functionalities.
The scalability of this synthetic approach for the production of functional polymer particles can potentially be used for the commercial preparation of particles for many applications due to their stable and multifunctional structure. For example, cationic polymer particles prepared by emulsion ATRP using cationic surface active monomers as reactive stabilizer were evaluated for antifungal coating applications.(35) Preliminary results showed that the coating could prevent Candida albicans biofilm formation. The cationic charges on the particles surfaces lead to the antibacterial behavior, while the covalent linking the surfactant to particle surfaces provided a coating stability that was superior to materials prepared by normal emulsion techniques.
Pickering emulsions tend to provide excellent long-term stability against coalescence and coarsening despite having very large discontinuous phase volume fractions. Polymer-grafted nanoparticles can be highly efficient Pickering emulsifiers, capable of stabilizing emulsions for several months to well over a year at extremely low particle concentrations. Silica nanoparticles with poly(styrene sulfonate) (PSS) brushes grafted from their surfaces stabilized trichloroethylene-in-water emulsions at particle concentrations as low as 0.04 wt% in the aqueous phase(61)while silica nanoparticles with poly(dimethylamino-ethyl methacrylate) (PDMAEMA) brushes grafted from their surfaces stabilized xylene-in-water and cyclohexane-in-water emulsions for many months at 0.05 wt% concentrations in the aqueous phase.(62-63) These studies indicate that tethering of multiple polymer chains to a single entity dramatically improves emulsification performance since colloidal particles without polymer decoration typically require higher concentrations, on the order of 1 wt%, to provide stable emulsions.(64)
Uniform Star macromolecules(65) prepared by chain extending/crosslinking macromonomers with a divinyl crosslinker(66) are also being evaluated as Pickering emulsifiers.
Preliminary results indicate that tuning the composition of the arms allows these star polymers to stabilize either oil-in-water or water-in-oil emulsions at extremely low surfactant concentration, <0.01 wt% vs. total weight of water and xylene used for emulsion formation.
The successful generation of stable water-in-oil emulsion using limited amounts of star polymers leads to a potential utilization of the star polymers as stabilizers for diesel fuel emulsion applications, aiming to reduce the emission of pollutants.
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