Synthesis and Characterization of Novel Polyacid-Stabilized Latexes

Aug 14, 2012 - Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, ... excellent freeze–thaw stability and remained colloidally...
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Synthesis and Characterization of Novel Polyacid-Stabilized Latexes Pengcheng Yang and S. P. Armes* Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, United Kingdom S Supporting Information *

ABSTRACT: A series of novel polyacid macromonomers based on 2-hydroxypropyl methacrylate (HPMA) were prepared by atom transfer radical polymerization (ATRP) via a two-step route. First, a range of well-defined PHPMA homopolymer precursors were synthesized by ATRP using a tertiary amine-functionalized initiator, 2-(dimethylamino)ethyl-2-bromoisobutyrylamide, and a CuCl/2, 2′-bipyridine (bpy) catalyst in alcoholic media at 50 °C. ATRP polymerizations were relatively slow and poorly controlled in pure isopropanol (IPA), especially when targeting higher degrees of polymerization (DP > 30). Improved control was achieved by addition of water: low polydispersity (Mw/Mn < 1.25) PHPMA homopolymers of DP = 30, 40, 50, 60, or 70 were successfully prepared using a 9:1 w/w % IPA/water mixture at 50 °C. These PHPMA homopolymer precursors were then derivatized to produce the corresponding poly(2-(succinyloxy)propyl methacrylate) (PSPMA) macromonomers by quaternizing the tertiary amine end-group with excess 4-vinylbenzyl chloride, followed by esterification of the pendent hydroxyl groups using excess succinic anhydride at 20 °C. These polyacid macromonomers were evaluated as reactive steric stabilizers for polystyrene latex synthesis under either aqueous emulsion polymerization or alcoholic dispersion polymerization conditions. Near-monodisperse polystyrene latexes were obtained via aqueous emulsion polymerization using 10 wt % PSPMA macromonomer (with respect to styrene monomer) with various initiators as evidenced by scanning electron microscopy, disk centrifuge photosedimentometry and light scattering studies. PSPMA macromomer concentrations as low as 1.0 wt % also produced near-monodisperse latexes, suggesting that these PSPMA macromonomers are highly effective stabilizers. Alcoholic dispersion polymerization of styrene conducted in various ethanol/water mixtures with 10 wt % PSPMA50 macromonomer produced relatively large near-monodisperse latexes. Increasing the water content in such formulations led to smaller latexes, as expected. Control experiments conducted with 10 wt % PSPMA50 homopolymer produced relatively large polydisperse latexes via emulsion polymerization and only macroscopic precipitates via alcoholic dispersion polymerization. Thus the terminal styrene group on the macromonomer chains is essential for the formation of well-defined latexes. FT-IR spectroscopy indicated that these latexes contained PSPMA macromonomer, whereas 1H NMR spectroscopy studies of dissolved latexes allowed stabilizer contents to be determined. Aqueous electrophoresis and X-ray photoelectron spectroscopy studies confirmed that the PSPMA macromonomer chains were located at the latex surface, as expected. Finally, these polyacid-stabilized polystyrene latexes exhibited excellent freeze−thaw stability and remained colloidally stable in the presence of electrolyte.

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INTRODUCTION Macromonomers are polymers that contain at least one polymerizable group that is typically located at the chain end. They have been widely used as building blocks for the synthesis of well-defined graft copolymers,1 star copolymers,2 and as reactive stabilizers for preparing sterically stabilized latexes.3 Conventional surfactants stabilize latex particles merely by physical absorption and hence can be easily displaced from the latex surface under shear, with subsequent loss of colloidal stability. In contrast, macromonomers are covalently grafted via their polymerizable group(s), so they cannot be displaced from the latex surface. Moreover, macromonomers allow surfactantfree formulations to be developed, which can eliminate the problem of surfactant diffusion and segregation during film formation. Surface-active macromonomers, or surfmers, have also been extensively studied in the context of latex syntheses.4 © 2012 American Chemical Society

In principle, sterically stabilized latexes prepared using macromonomers exhibit enhanced colloidal stability at high shear and in the presence of electrolyte, as well as much better freeze−thaw stability.5 Various routes have been explored to prepare well-defined macromonomers. For example, Haddleton and co-workers investigated the synthesis of various relatively polydisperse macromonomers via catalytic chain transfer polymerization (CCTP).6 Furthermore using a 4-vinylbenzyl alcohol-functionalized initiator, both Nagasaki et al.7 and Lascelles et al.8 reported the direct synthesis of styrene-functionalized macromonomers using either 2-(diethylamino)ethyl methacrylate Received: July 3, 2012 Revised: August 14, 2012 Published: August 14, 2012 13189

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utilized to prepare pH-responsive liquid marbles.18 In the present paper, we report the optimized ATRP synthesis of a series of PHPMA homopolymers of varying degrees of polymerization using the tertiary amine-functionalized initiator shown in Figure 1. These homopolymers were then converted

(DEA) or 2-(dimethylamino)ethyl methacrylate (DMA) via oxyanionic polymerization. However, these polymerizations require rigorously dried THF and appear to be restricted to various tertiary amine methacrylates. Also, such living anionic polymerizations are incompatible with functional monomers containing hydroxyl or carboxylic acid groups, unless protecting group chemistry is employed. Direct syntheses of macromonomers have also been explored by using vinyl-functionalized initiators9 in combination with controlled radical polymerization techniques such as atom transfer radical polymerization (ATRP).10 However, appropriate selection of the initiator and ligand was required for the production of welldefined macromonomers. For example, Zeng et al.13 investigated the ATRP synthesis of various DMA-based macromonomers using a CuBr-based ATRP formulation, but only an allyl 2-bromoisobutyrate initiator used in conjunction with a PMDETA ligand produced the desired low polydispersity macromonomers. Other initiator/ligand combinations had either no activity for the ATRP of DMA or the initiator vinyl group copolymerized with the monomer to yield an ill-defined polydisperse polymer. Alternatively, macromonomers can be prepared using a functional initiator that enables post-polymerization modification.11−19 In principle, this route is much more versatile, with many different functional macromonomers being readily prepared. Müller et al. used 2-hydroxyethyl 2-bromoisobutyrate as a functionalized initiator to produce well-defined n-butyl and t-butyl acrylate homopolymers.12 These hydroxy-functional homopolymer precursors were then esterified using methacryloyl chloride to yield near-monodisperse acrylic macromonomers. Similarly, the combination of ATRP and click chemistry has also been employed to prepare well-defined macromonomers.13 For example, Topham and co-workers13c prepared macromonomers using an azido α-functionalized ATRP initiator to polymerize various hydrophilic methacrylates, followed by a click reaction with propargyl (meth)acrylate. Well-defined polyacid macromonomers such as poly(methacrylic acid) (PMA)14 and poly(acrylic acid) (PAA)15 have been prepared by living anionic polymerization via protecting group chemistry and subsequently used to prepare poly(methyl methacrylate) latexes. These polyacid macromonomers confer so-called electrosteric stabilization due to the anionic charge on the grafted macromonomer chains. Recently, our group reported the synthesis of well-defined macromonomers by ATRP via a two-step post-polymerization modification route. First, homopolymer precursors based on either 2-(methacryloyloxy)ethyl phosphorylcholine (MPC)5a or glycerol monomethacrylate (GMA)5b were synthesized using a tertiary amine-functionalized ATRP initiator in methanol at 20 °C. The corresponding near-monodisperse macromonomers were then produced by quaternizing the tertiary amine end groups with 4-vinylbenzyl chloride (4-VBC). In the present work, we extend this synthetic route to produce a series of welldefined macromonomers based on a third monomer, 2hydroxypropyl methacrylate (HPMA). Previously, HPMA has been polymerized to high conversion with good control by ATRP.16 Furthermore, Bories-Azeau et al. have shown that the pendent hydroxyl groups on PHPMA homopolymers can be fully esterified using excess succinic anhydride to produce lowpolydispersity polyacids.17 We have recently reported a single example of such a polyacid macromonomer based on fully esterified PHPMA macromonomer, which was subsequently

Figure 1. ATRP synthesis of PHPMA homopolymer in a 9:1 w/w % IPA/water mixture at 50 °C.

into the corresponding well-defined polyacid macromonomers and subsequently used to prepare a range of electrosterically stabilized polystyrene latexes using either aqueous emulsion or alcohol dispersion polymerization.

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EXPERIMENTAL SECTION

Materials. 2-Hydroxypropyl methacrylate (HPMA) was kindly donated by Cognis Performance Chemicals (Hythe, UK) and used without further purification. HPMA monomer actually comprises an isomeric mixture of 75% HPMA and 25% 2-hydroxyisopropyl methacrylate,16 but for clarity only the chemical structure of the major HPMA isomer is shown in Figure 1. Styrene (Aldrich) was passed through a column of basic alumina to remove inhibitors and then stored at −20 °C prior to use. Silica gel 60 (0.063−0.2 nm particle diameter) was supplied by Merck (Darmstadt, Germany). 4Vinylbenzyl chloride (4-VBC), Cu(I)Cl (99.995%), 2,2′-bipyridine (bpy, 99%), and succinic anhydride (SA) were purchased from Aldrich and were used as received. 2,2′-Azobisisobutyronitrule (AIBN; BDH), 2,2′-azobis(isobutyramidine) dihydrochloride (AIBA, 97%; Aldrich, UK), and ammonium persulfate (APS, Aldrich, UK) were used as received. Isopropyl alcohol (IPA) and other solvents were supplied by Fisher and were used as received. Deionized water (obtained from an Elgastat Option 3A water purification unit) was used for all experiments. A regenerated cellulose dialysis membrane (Spectra/ Por 6, molecular weight cut-off 1000 Da) was also purchased from Fisher. Synthesis of 2-(Dimethylamino)ethyl-2-bromoisobutyrylamide ATRP Initiator. The synthesis of this ATRP initiator was conducted as reported previously.5a Briefly, 2-dimethylethylenediamine (8.815 g, 0.10 mol) and triethylamine (40.91 g, 0.40 mol) were dissolved in dichloromethane (250 mL) in a 500 mL round-bottomed flask. The solution was cooled using an ice bath and purged with dry nitrogen for 20 min before dropwise addition of 2-bromoisobutyryl bromide (25.29 g, 0.11 mol). This reaction mixture was then stirred at 20 °C for 48 h and subsequently washed with an aqueous solution of 0.10 M NaHCO3 until all the triethylammonium bromide salts were removed. The organic layer was separated and then further washed five times with 200 mL portions of deionized water. The combined organic phases were dried over MgSO4, and the solvent was removed under reduced pressure at 20 °C to afford a pale-brown liquid (17.24 g, 72% yield). 1 H NMR (400 MHz, CD3OD): δ 1.88 (6H, s, 2CH3), 2.26 (6H, s, N(CH3)2), 2.45 (2H, t, J = 7.0 Hz), (CH3)2NCH2), 3.31 (2H, t, J = 7.0 Hz, CH2NHCOO(CH3)2Br). Elemental analysis % (calculated %): C 39.21 (40.52), H 7.24 (7.23), N 11.09 (11.81), Br 33.50 (33.70). 13 C NMR and IR spectrum and ES-MS analysis were all consistent with the target structure (see the Supporting Information, Figure S1− S3). ATRP Synthesis of PHPMA Homopolymer Precursor at 50 °C. In a 100 mL two-neck round-bottomed flask, 2-(dimethylamino)ethyl-2-bromoisobutyrylamide initiator (94.8 mg, 0.4 mmol), 213190

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hydroxypropyl methacrylate monomer (HPMA, 2.88 g, 20 mmol, target degree of polymerization, DP = 50) were dissolved in a 9:1 w/w % IPA/water mixture to obtain a 50% w/w solution of HPMA. This mixture was degassed using a stream of dry nitrogen gas for 40 min before being heated to 50 °C. Cu(I)Cl catalyst and 2,2′-bipyridine (relative molar ratios of ME-Br: Cu(I)Cl: bpy = 1:1:2) were then added quickly under a nitrogen blanket to start the polymerization. The reaction mixture turned dark-brown and became viscous as the reaction progressed. After 18 h, the reaction mixture was quenched by exposing to air, followed by dilution with methanol (80 mL). The spent ATRP catalyst was removed by passing the green solution through a silica gel column. A white solid product was obtained after drying under vacuum (2.45 g, 85% yield). 1H NMR spectroscopy (see the Supporting Information, Figure S4) was used to determine the mean DP by comparing the integrated signal at δ 2.3 (assigned to the six equivalent dimethylamino protons of the initiator) to that at δ 0.5− 1.5 (assigned to the methyl backbone protons and pendent methyl protons of the polymerized HPMA unit). Quaternization of PHPMA Homopolymer Precursor. The PHPMA50 homopolymer (7.58 g, 1.00 mmol) was dissolved in methanol (20 mL). Excess 4-vinylbenzyl chloride (0.51 g, 3.00 mmol; 4-VBC/amine molar ratio = 3:1) was then added to this solution and the reaction was stirred at 20 °C for 48 h. Solvent was partially removed under vacuum and the crude PHPMA macromonomer was precipitated into cyclohexane (300 mL). The recovered off-white copolymer was redissolved in methanol and reprecipitated into cyclohexane to remove all traces of unreacted 4-VBC. A white solid was obtained after drying under vacuum. The overall yield of isolated, purified PHPMA macromonomer was 7.05 g (91% yield). Esterification of PHPMA Macromonomer Using Succinic Anhydride. In a 100 mL round-bottomed flask, PHPMA50 macromonomer (2.00 g) was dissolved in THF (25 mL). Two molar equivalents of succinic anhydride and triethylamine (TEA) relative to the hydroxyl groups of the macromonomer were added under nitrogen. Esterification was allowed to proceed at 20 °C for 48 h. THF was then removed under vacuum, and the crude product was dissolved in deionized water at around pH 8 (adjusted by addition of NaOH solution). The macromonomer was purified by dialysis against 0.1 M NaHCO3 for at least two days to ensure complete removal of impurities, followed by dialysis against pure water for three days and then freeze-dried from water overnight. The final esterified poly(2(succinyloxy)propyl methacrylate) macromonomer was obtained as a white solid and is hitherto denoted as “PSPMA50” for brevity. Aqueous Emulsion Polymerization of Styrene. The PSPMA50 macromonomer (0.25 g) was dissolved in water (20.0 g) and mixed with styrene (2.50 g) in a 50 mL round-bottomed flask. The reaction mixture was purged with nitrogen for 30 min before being heated to 70 °C under a nitrogen blanket (or 60 °C for AIBA). Separately, either APS or AIBA initiator (0.025 g; 1.0 wt % based on styrene) was dissolved in water (2.50 g) and degassed with nitrogen. This initiator solution was injected into the flask to start the polymerization. The polymerizing solution turned milky-white within 30 min, and stirring was continued for 24 h at 70 °C. The resulting latex was purified by repeated centrifugation-redispersion cycles to remove any residual styrene monomer and unreacted macromonomer, with each successive supernatant being carefully decanted and replaced with deionized water. A similar protocol was used in attempts to prepare latex using an AIBN initiator. In a 50 mL round-bottomed flask, PSPMA50 macromonomer (2.50 g) was dissolved water and degassed by nitrogen before being heated to 70 °C. The AIBN initiator (0.025 g; 1.0 wt % based on styrene) was dissolved in styrene monomer (2.50 g) and purged with nitrogen and subsequently injected into the reaction flask. The polymerization was allowed to continue for 24 h at 70 °C and the resulting dispersion was purified by repeated centrifugation− redispersion cycles. Alcoholic Dispersion Polymerization of Styrene in Various Ethanol/Water Mixtures. The PSPMA50 macromonomer (0.25 g) was dissolved in an ethanol/water mixture in a 50 mL round-bottomed flask. The reaction mixture was purged with nitrogen for 30 min before being heated to 70 °C under a nitrogen blanket. Separately, the AIBN

initiator (0.025 g; 1.0 wt % based on styrene) was dissolved in styrene monomer (2.50 g) and degassed with nitrogen. The monomer and initiator were injected to the flask to start the polymerization. The polymerizing solution turned milky-white within 30 min, and stirring was continued for 24 h at 70 °C. The resulting latex was purified by repeated centrifugation-redispersion cycles to remove any residual styrene monomer and unreacted macromonomer, with each successive supernatant being carefully decanted and replaced with deionized water. 1 H NMR Spectroscopy. All 1H NMR spectra were recorded in either d4-methanol or d5-pyridine using a 400 MHz Bruker Avance-400 spectrometer. Gel Permeation Chromatography (GPC). Analysis was performed using a refractive index detector with HPLC grade THF eluent containing 2.0% v/v TEA at a flow rate of 1.0 mL min−1. BHT stabilizer was used as an internal standard and the GPC column temperature was set at 30 °C. Calibration was carried out using a series of near-monodisperse poly(methyl methacrylate) (PMMA) standards. Data were analyzed using PL Cirrus GPC software (version 2.0) supplied by Polymer Laboratories. Molar mass distributions were also assessed using a GPC set-up comprising two Polymer Laboratories PL gel 5 μm Mixed-C columns and one Phenogel 5 μm linear/mixed guard column maintained at 60 °C in series with a Varian 390 LC refractive index detector. The mobile phase was N,N′-dimethyl formamide (DMF) containing 10 mM LiBr operating at a flow rate of 1.0 mL min−1. Ten near-monodisperse PMMA standards (Mp = 625−618 000 g mol−1) were used for calibration (K = 2.094 × 10−3, α = 0.642). FT-IR Spectroscopy. Each sample (1.0 mg) was ground up with 150 mg KBr to afford a fine powder and compressed into a pellet by applying a pelletization pressure of 8 tonnes for 10 min. FTIR spectra were recorded using a Thermo Nicolet iS10 spectrometer at 4.0 cm−1 resolution, with 64 scans being recorded per spectrum. Dynamic light scattering (DLS). Hydrodynamic diameters were measured at 25 °C using a Malvern Zetasizer NanoZS Instrument equipped with a 4 mW He−Ne solid-state laser operating at 633 nm. Backscattered light was detected at 173° and the mean particle diameter was calculated from the quadratic fitting of the correlation function over thirty runs each of ten seconds duration. All measurements were performed in triplicate on 0.01 w/v % aqueous latex and determined as a function of pH, which was adjusted using NaOH or HCl as required. Aqueous Electrophoresis. Zeta potentials were calculated from electrophoretic mobilities using the same Malvern Instruments Zetasizer NanoZS instrument equipped with an autotitrator (MPT-2 multipurpose titrator, Malvern Instruments). The solution pH was varied from 2 to 9 in the presence of 1 mM NaCl background electrolyte using either dilute NaOH or HCl as required. Disk Centrifuge Photosedimentometry (DCP). Latex diameters were determined using a CPS disk centrifuge photosedimentometer (model number DC 24000). Samples (0.10 mL) were prepared by addition of one drop of latex dispersion to deionized water (10 mL) and then were injected into an aqueous spin fluid consisting of a 2−8% sucrose gradient. The density of the polystyrene latex particles was taken to be 1.05 g cm−3. This is a reasonable assumption for micrometer-sized latexes, where the thickness of the steric stabilizer layer is negligible compared to the particle diameter. For smaller latexes (particularly for diameters of less than 200 nm), the stabilizer layer thickness becomes significant, resulting in a reduction in the effective particle density. The DCP technique requires an accurate knowledge of the density difference (buoyancy factor) between the particles and the spin fluid, Thus when calculating the weight-average particle diameter Dw, any uncertainty in density produces an associated systematic error in Dw. Scanning Electron Microscopy (SEM). Images were obtained using a FEI Sirion field-emission scanning electron microscope. All samples were dried onto adhesive carbon disks and sputter-coated with a thin layer of gold prior to examination to prevent sample charging problems. 13191

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X-ray Photoelectron Spectra (XPS). Spectra were acquired using a Kratos Axis Ultra DLD X-ray photoelectron spectrometer equipped with a monochromatic Al KR X-ray source (hν = 1486.6 eV) and operating at a base pressure of 1 × 10−8 to 1 × 10−10 mbar. The step sizes used were 1.0 eV for the survey spectra (pass energy = 160 eV) and 0.1 eV for the high resolution spectra (pass energy = 80 eV). An aqueous dispersion of latex was dried onto silicon wafers and evacuated to ultrahigh vacuum prior to XPS measurements. Carbonyl peak deconvolution was conducted using Casa XPS software version 2.3.15 provided by the instrument manufacturer (Kratos).

of water has been shown to improve control during the polymerization of methacrylates.23 For a 9:1 w/w % IPA/water mixture, the HPMA polymerization followed first-order kinetics as shown in Figure 2, suggesting a constant number of polymer

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RESULTS AND DISCUSSION Initially, the ATRP synthesis of HPMA was carried out in 100% isopropanol (IPA) at 50 °C. This secondary alcohol was chosen as a solvent in order to minimize the possibility of transesterification, which is known to occur between methacrylic monomers and primary alcohols.19 The HPMA polymerization was well controlled in terms of its target degree of polymerization (DP) and final polydispersity for DP values below 50. For higher DPs, poor “living” character and reduced initiator efficiencies were obtained (see Supporting Information, Table S1). For example, 1H NMR studies indicated that the actual DP of PHPMA60 prepared using 100% IPA at 50 °C was 73, although the final polydispersity remained relatively low at 1.25. A similar problem of loss of control was encountered by Thompson and co-workers when investigating the ATRP synthesis of 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) in methanol at 20 °C with the same tertiary aminefunctionalized initiator.5a In this earlier study, the polymerization of MPC was only well-controlled if the target DP was relatively low. Above a target DP of 35, rather broad molecular weight distributions (Mw/Mn > 2.0) were obtained and the actual DP determined by 1H NMR was significantly higher than that targeted. Moreover, in the present study the HPMA polymerization only proceeds rather slowly when conducted in 100% IPA; for example, only 92% conversion was achieved after 72 h when targeting a DP of 70. To further evaluate the ‘living’ character of the current protocol, a kinetic study was conducted for a PHPMA70 synthesis. Figure S5a (see Supporting Information) shows the evolution of molecular weight with conversion. Strongly non-linear behavior is observed: relatively high polydispersities are obtained in the early stages of the polymerization, suggesting poor living character for this 100% IPA formulation. Furthermore, GPC analysis (see the Supporting Information, Figure S5b) indicated that bimodal molecular weight distributions are obtained below 30% conversion. This suggests that either slow initiation (hence all the chains do not start to grow simultaneously) or some termination occurs at an early stage of the polymerization. However, the low molecular weight peak disappears in the later GPC traces. Therefore it is unlikely that the bimodal molecular weight distribution is caused by termination. It is well-known in the literature that amide-based ATRP initiators have poor initiation efficiencies. Tang and Matyjaszewski20 recently reported activation rate constants for various initiators used for Cu-mediated ATRP. It was found that amide-based ATRP initiators are up to an order of magnitude less active than the corresponding ester-based initiators when used under the same conditions. Also, a number of papers has reported similar difficulties with amide-based initiators in ATRP syntheses.21 To overcome this problem, a small amount of water was added to the reaction mixture. It is well-known that ATRP kinetics can be much faster in the presence of highly polar solvents such as water.22 Moreover, adding just a small amount

Figure 2. (a) Conversion vs time and semilogarithmic plots and (b) evolution of number-average molecular weight vs conversion for the ATRP synthesis of PHPMA70 at 50 °C in a 9:1 w/w % IPA/water mixture. Relative molar ratios for initiator: Cu(I)Cl: bpy were 1:1:2. The dotted line represents the theoretical Mn.

radicals during the polymerization. In addition, the evolution of molecular weight with conversion was linear and low polydispersities were maintained throughout the polymerization. These results suggest much improved initiator efficiency and living character.24 The GPC curves (see Figure 3) were relatively narrow and unimodal, with no evidence of any tailing or low molecular weight shoulders. Table 1 summarizes the ATRP synthesis of PHPMA homopolymer in a 9:1 w/w % IPA/water mixture at 50 °C. Clearly, the rate of polymerization is significantly faster on addition of water and almost 100% conversion was achieved within 24 h. Good

Figure 3. THF GPC traces obtained for the ATRP synthesis of PHPMA70 in 9:1 wt % IPA/water at 50 °C. 13192

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to the initiator end-group at δ 3.1 with that of the aromatic signals suggested that essentially 100% quaternization was achieved for each PHPMA macromonomer. The hydroxyl groups on the HPMA residues of the macromonomer chains were then esterified using SA, producing a new signal at δ 2.5− 2.6 due to the four protons adjacent to the carboxylic acid. The degree of esterification was assessed by comparing the four protons due to these succinic acid groups (j, j′) at δ 2.5−2.6 to the six methyl protons (i, i′, f) of the PHPMA macromonomer at δ 0.9−1.3. In all cases, more than 99% esterification was obtained within 24 h at 20 °C. DMF GPC analysis (see Figure 4b) of the PHPMA homopolymer precursor and corresponding macromonomer confirmed that quaternization had no significant effect on the molecular weight distribution of the macromonomer. For GPC characterization of the PSPMA macromonomer, the pendent acid groups were fully esterified using an excess of trimethylsilyldiazomethane.25 The polydispersities of the methylated polyacid macromonomers are generally similar to those obtained for corresponding PHPMA homopolymer precursors (see Figure 6). This is reasonable, since esterification was conducted under relatively mild conditions, thus the final polyacid macromonomer should exhibit the narrow molecular weight distribution of the corresponding PHPMA precursor.26The high molecular weight shoulder observed in the DMF GPC trace of PHPMA homopolymer is due to a low level of dimethacrylate impurity (0.26 mol % as judged by HPLC) in the HPMA monomer. This impurity is known to cause either cross-linking or light branching, depending on the mean degree of polymerization of the PHPMA chains.27,28 In this case, because the target degree of polymerization is relatively low, the overall polydispersity remains below 1.25 in all syntheses. PSPMA macromonomers with target DPs of 30, 50, and 70 were then evaluated as a reactive electrosteric stabilizer for latex syntheses (see Figure 7). Table 2 summarizes the results obtained for various PSPMA-stabilized polystyrene latexes prepared via aqueous emulsion polymerization. Using an anionic initiator (APS), coagulum-free polystyrene latexes with narrow particle size distributions could be prepared

Table 1. Mean Degrees of Polymerization, Monomer Conversions, and GPC Molecular Weight Data for the ATRP Homopolymerization of 2-Hydroxypropyl Methacrylate (HPMA) in 9:1 IPA/Water at 50 °C (THF GPC calibrated with poly(methyl methacrylate) standards) entry

target DP

DP (1H NMR)

conv (%)

reaction time (h)

Mn

Mw/Mn

1 2 3 4 5

30 40 50 60 70

29 43 51 60 72

100 100 99 100 98

12 12 18 18 24

10 200 12 400 13 500 16 000 19 400

1.24 1.23 1.21 1.21 1.21

control was also observed, since polydispersities remained below 1.25 throughout the polymerization. The actual DP calculated from 1H NMR was in good agreement with the target DP range of 30−70. Therefore, it is clear that the ATRP of HPMA using a tertiary amine-functionalized initiator in a 9:1 w/w % IPA/water mixture has much better living character than the same polymerization conducted in pure IPA. Polyacid macromonomers were then synthesized from these PHPMA precursors, as depicted in Figure 4a. First, PHPMA homopolymers were fully quaternized using excess 4-VBC in methanol at room temperature to produce the corresponding PHPMA macromonomer. After purification, these PHPMA macromonomers were fully esterified using excess succinic anhydride to produce polyacid (PSPMA) macromonomers. Assigned 1H NMR spectra of the tertiary amine-functionalized initiator, PHPMA30 homopolymer, PHPMA30 macromonomer, and the corresponding PSPMA30 macromonomer are shown in Figure 5. The tertiary amine-functionalized initiator has a strong signal due to six equivalent methyl protons at δ 2.3. After polymerization, in addition to the expected PHPMA signals, these initiator end-group signals remained visible. Complete quaternization of the PHPMA precursor resulted in a shift of the initiator end-group signal from δ 2.3 to δ 3.1; in addition, new aromatic and vinyl signals appeared at δ 5.2−7.5 due to the terminal styrene groups. Comparing the integrated signal due

Figure 4. (a) Synthesis of PSPMA polyacid macromonomer: quaternization of tertiary amine-capped PHPMA homopolymer with excess 4-VBC in methanol at 20 °C for 48 h, following by complete esterification of purified PHPMA macromonomer with excess succinic anhydride at 20 °C. (b) DMF GPC traces obtained for PHPMA50 homopolymer and PHPMA50 macromonomer, respectively (vs poly(methyl methacrylate) calibration standards). 13193

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Figure 5. 1H NMR spectra recorded in d4-methanol for (a) tertiary amine-functionalized ATRP initiator, (b) PHPMA30 homopolymer, (c) fully quaternized PHPMA30 macromonomer, (d) fully esterified PSPMA30 macromonomer.

using each of the three macromonomers (see Figure 8). The macromonomer DP did not significant affect the particle diameter, with latexes of around 100 nm diameter being obtained in all cases (entries 1−3, Table 2). However, 1H NMR analysis (see the Supporting Information, Figure S7) of dried latexes indicated that approximately 7.3 wt % PSPMA30 macromonomer was incorporated compared with only 3.4 wt % PSPMA70 macromomer. This suggests that longer macromomer chains have lower grafting efficiencies. This lower stabilizer grafting density affects the colloidal stability of the latex when challenged with added electrolyte. Thus latexes prepared using the PSPMA30 macromonomer remained colloidally stable up to 0.30 M CaCl2 whereas latexes prepared using longer PSPMA macromonomers (DP = 50 or 70) begin to flocculate under such conditions (see later). The PSPMA50 macromonomer proved to be a rather efficient stabilizer:

Figure 6. DMF GPC traces obtained for selected PSPMA n macromonomers (after exhaustive methylation using an excess of trimethylsilyl diazomethane).

Figure 7. Reaction scheme for the synthesis of PSPMA-stabilized polystyrene latex via either aqueous emulsion or alcoholic dispersion polymerization of styrene. 13194

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Table 2. Summary of Yields, Particle Sizes, and Stabilizer Contents Obtained for Polystyrene Latexes Prepared with and without PSPMA Macromonomer via Aqueous Emulsion Polymerization particle diameter (nm) entry

stabilizer type

1 2 3 4 5 6 7 8

PSPMA30 PSPMA50 PSPMA70 PSPMA50 PSPMA50 PSPMA50 PSPMA50 PSPMA50 homopolymer no stabilizer no stabilizer

9 10

stabilizer concentration (wt %)

initiatior type

latex yield (%)

DLS (PDI)

10 10 10 10 10 5 1 10

APS APS APS AIBN AIBA APS APS AIBN

100 95 100 97 100 83 76 55c

131 (0.06) 125 (0.03) 109 (0.09) 110 (0.08) 66 (0.08) 134 (0.03) 184 (0.04) 179 (0.05)

131 123 124 90 65 122 149 741

0 0

APS AIBA

59 61

613 (0.03) 914 (0.28)

n.d. n.d.

DCP ± ± ± ± ± ± ± ±

17 19 23 8 11 18 39 215; 167 ± 16

SEM

stabilizer content (wt %)b

Γ (mg m−2)d

114 106 102 100 60a 118 141 952; 143

7.3 5.9 3.3 3.4 7.6 4.6 0.6 2.6

1.5 1.2 0.6 0.9 0.9 0.6 0.2 n.d.

560 250, 750

n.d. n.d.

n.d. n.d.

a Determined by transmission electron microscopy (TEM). bDetermined by 1H NMR spectroscopy. cSome coagulum was observed for this synthesis. dCalculated using the SEM diameter (see the Supporting Information1 for an example of calculation). nd = not determined.

Figure 8. Particle size and morphology of selected polystyrene latexes: (a) SEM image of a PSPMA50−PS latex prepared using 10 wt % macromonomer and an anionic APS initiator; (b) SEM image of a PSPMA50−PS latex prepared using 10 wt % macromonomer and a non-ionic AIBN initiator; (c) TEM image of a PSPMA50−PS latex prepared using 10 wt % macromonomer and a cationic AIBA initiator; (d, f) SEM images of PSPMA50−PS latexes prepared using either 5.0 or 1.0 wt % macromonomer respectively using an anionic APS initiator in each case; (f) SEM image of a PSPMA50−PS latex prepared using 10 wt % PSPMA50 homopolymer with AIBN initiator; (g, h) SEM images of charge-stabilized polystyrene latexes prepared in the absence of any macromonomer using either an APS or an AIBA initiator, respectively.

initiated latexes in terms of their size distributions. Moreover, significantly smaller particles of 60 nm diameter were produced when using the cationic AIBA initiator. The unusually small size of this latex indicated by DLS studies was confirmed by TEM and DCP. A possible explanation may be electrostatic interaction between the anionic PSPMA macromonomer and the cationic initiator during the emulsion polymerization of styrene.29 It has been reported30 that the rate of emulsion polymerization is particularly high for formulations in which the ionic initiator and polyelectrolyte corona are both anionic (or both cationic), since electrostatic repulsion apparently leads to enhanced radical mobility. In contrast, formulations comprising an ionic initiator and an oppositely charged polyelectrolyte are subject to counterion interactions and the subsequent loss of

reducing its concentration in the latex formulation to as little as 1.0 wt % (entry 7, Table 2) still produced colloidally stable particles. Recently, Thompson et al. reported using a non-ionic poly(glycerol monomethacrylate)-based macromonomer as a steric stabilizer for latex synthesis.5b One limitation of this particular stabilizer was that an oil-soluble AIBN initiator was found to be essential for a successful aqueous emulsion polymerization formulation; only coagulum was obtained when using ionic initiators such as APS or AIBA. Thus, AIBN and AIBA initiators were also evaluated in latex syntheses conducted with the anionic PSPMA50 macromonomer in the present work (see entries 4 and 5, Table 2). Surprisingly, AIBN-initiated latexes proved to be rather similar to APS13195

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Table 3. Summary of Latex yields, Particle Sizes, And Stabilizer Contents Obtained Using 10 wt % PSPMA50 Macromonomer and AIBN Initiator in an Alcoholic Dispersion Polymerization Formulation using Various Ethanol/Water Mixtures at 70 °C particle diameter (nm) entry

ethanol/water (w/w %)

latex yield %

1 2 3 4 5 6 7 8a

20/80 30/70 40/60 50/50 60/40 70/30 80/20 80/20

96 97 100 100 98 100 100 0

DLS (PDI) 179 282 316 405 593 645 799

(0.05) (0.01) (0.02) (0.21) (0.13) (0.05) (0.15)

DCP

SEM

stabilizer content (wt %)b

Γ (mg m−2)c

± ± ± ± ± ± ±

166 216 271 783, 265 379 476 545

2.77 1.84 1.41 0.72 1.36 0.89 0.81

0.8 0.7 0.7 n.d. 0.9 0.7 0.7

173 227 297 562 421 561 731

23 7 43 260 38 28 128

a Control experiment conducted with 10 wt % PSPMA50 homopolymers (only a macroscopic precipitate was obtained). bDetermined by 1H NMR spectroscopy. cCalculated using the SEM diameter. nd = not determined.

Figure 9. Scanning electron microscopy images obtained for polystyrene latexes prepared with 10 wt % PSPMA50 macromonomer using an AIBN initiator via alcoholic dispersion polymerization in various ethanol/water mixtures: (a) ethanol/water = 20/80 w/w %; (b) ethanol/water = 30/70 w/w %; (c) ethanol/water = 40/60 w/w %; (d) ethanol/water = 50/50 w/w %; (e) ethanol/water = 60/40 w/w %. (f) ethanol/water = 70/30 w/w %; (g) ethanol/water = 80/20 w/w %; (h) amorphous polystyrene precipitate obtained in an attempt to prepare polystyrene latex with PSPMA50 homopolymer using AIBN initiator via alcoholic dispersion polymerization in an 80/20 w/w % ethanol/water mixture.

mobility, and hence exhibit strongly retarded rates of polymerization. In the present study, entry of the cationic radicals derived from AIBA initiator into the growing polystyrene particles is likely to be inhibited by the grafted anionic PSPMA macromonomer chains. Hence homogeneous nucleation is favored, which generates a large number of relatively small latex particles. Nevertheless, the PSPMA macromonomer seems to be more versatile than the previously reported PGMA macromonomer since the former can be utilized with all three types of initiator (anionic, cationic or nonionic) under emulsion polymerization conditions (see Figure 8). If it is assumed that all the macromonomer chains are located at the latex surface, the absorbed amount, Γ, can be calculated (see the Supporting Information). Such Γ values are approximately constant (0.9−1.2 mg m−2) for all the latexes prepared using the 10 wt% PSPMA50 macromonomer as expected (see entries 2, 4, and 5, Table 2). Moreover, lower Γ values were obtained for latexes prepared at lower PSPMA50

macromonomer concentrations, as expected (see entries 6 and 7, Table 2). Control latex experiments were conducted using either APS or AIBA initiator in the absence of any macromonomer (see Figure 8). However, these emulsion polymerizations suffered from poor styrene conversions and produced much larger latexes (entries 9 and 10, Table 2). Another control experiment was conducted using PSPMA50 homopolymer (synthesized using the same tertiary amine-functionalized initiator and then esterified using excess succinic anhydride under the same conditions used for the PSPMA macromonomers), which produced a large amount of coagulum. The latex fraction isolated from this reaction exhibited a bimodal size distribution (see Figure 8f). Surprisingly, 1H NMR studies confirmed that a relatively large amount of PSPMA50 was incorporated into this latex (see entry 8, Table 2). However, XPS studies indicated only a very low coverage of PSPMA50 homopolymer at the latex surface, as judged by the relatively weak C−O and CO signals due to the PSPMA homopolymer at around 285−288 13196

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eV (see later). Given the highly surface-specific nature of XPS, this suggests that most of the PSPMA50 homopolymer must be located within the latex, rather than on its surface. These control experiments also confirm the importance of having a terminal vinyl group on the PSPMA chains to ensure its efficient copolymerization with styrene to produce well-defined electrosterically stabilized latexes. The dispersion polymerization of styrene in various ethanol/ water mixtures was also investigated using PSPMA50 macromonomer in order to target somewhat larger latex particles. Water was added to adjust the solvency and thus manipulate the particle size distribution. The mean latex diameter could be varied between 179 and 799 nm, with smaller particles being obtained at higher water contents (see Table 3). This is because nucleation of insoluble polystyrene chains occurs more readily in more polar media. Therefore, a much larger number of nuclei are formed and the final particle diameter is reduced.31 All latexes had relatively narrow particle size distributions (see Figure 9), except for one synthesis conducted in a 50/50 w/w % ethanol/water (entry 4, Table 3) mixture that produced particles with a bimodal size distribution. In view of the bimodal size distribution observed for this entry, this latex formulation was repeated, but the same result was obtained. The reason for the lack of control over the latex diameter obtained at this 50/50 w/w % ethanol/water composition is not known. The narrowest particle size distributions were obtained when using either 30 wt % ethanol or 30 wt % water, as judged by DLS, DCP and SEM analysis. The amount of PSPMA50 macromonomer grafted onto the latex surface varied between 0.81 and 2.77 wt %, which corresponds to Γ values of 0.7−0.9 mg m−2. Thus these latexes exhibit similar surface chemistry to those prepared via aqueous emulsion polymerization; the ability to systematically vary the latex dimensions over such a wide range is expected to be particularly interesting in the context of calcium carbonate occlusion experiments.32 A control experiment using PSPMA50 homopolymer conducted in an 80/20 ethanol/water mixture produced only a macroscopic precipitate (see Figure 9h). This is not particularly surprising: the lack of a polymerizable group leads to a relatively low level of PSPMA surface grafting, which is clearly insufficient to stabilize the latex particles. FT-IR spectra were recorded for the PHPMA50 homopolymer, PSPMA50 macromonomer and selected polystyrene latexes (see Figure 10). A characteristic carbonyl stretch at 1730 cm−1 was observed for the PSPMA50 homopolymer, PSPMA50 macromonomer and both the 66 and 125 nm PSPMA50-PS latexes prepared by aqueous emulsion polymerization. Thus the presence of macromonomer in these two latexes is confirmed. In contrast, no such band was observed in the IR spectrum of the charge-stabilized latex, as expected. It is also perhaps noteworthy that this characteristic carbonyl band was barely visible in the IR spectrum recorded for the 799 nm PSPMA50-PS latex prepared by alcoholic dispersion polymerization. This is because this much larger latex has a rather lower specific surface area and hence a lower stabilizer content per unit mass. The FT-IR and 1H NMR studies confirm the presence of PSPMA macromomer in the latex particles, but of course these techniques provide no information regarding its spatial location. Hence XPS and aqueous electrophoresis were used to further characterize the PSPMA-stabilized latexes. Using the former technique, C1s core-line spectra were recorded for the bare charge-stabilized polystyrene latex, PSPMA50 macromonomer

Figure 10. FT-IR spectra recorded for PHPMA50 homopolymer, PSPMA50 macromonomer, a charge-stabilized polystyrene latex prepared by aqueous emulsion polymerization in the absence of macromonomer, a 125 nm polystyrene latex synthesized by aqueous emulsion polymerization using PSPMA50 macromonomer and an APS initiator, a 66 nm polystyrene latex synthesized by aqueous emulsion polymerization using PSPMA50 macromonomer and an AIBA initiator, and a 799 nm polystyrene latex synthesized by alcoholic dispersion polymerization using PSPMA50 macromonomer and an AIBN initiator.

and PSPMA-stabilized polystyrene latexes prepared by either aqueous emulsion or alcoholic dispersion polymerization (see Figure 11). Prominent signals due to C−O and CO are discernible at around 285−288 eV in the spectra obtained for the latter three samples due to the ester group present in PSPMA macromonomer, but no such signal was observed for the charge-stabilized polystyrene. Given that the typical XPS sampling depth is 2 to 8 nm,33 this confirms the presence of the PSPMA macromonomer chains at the latex surface. It was also possible to estimate the surface coverage of the PSPMA50 macromonomer chains over the latex particles using the same C1s region. Thus the integrated area of the deconvoluted C O signal at 287 eV obtained for each PSPMA-PS latex was compared to that observed for the PSPMA macromonomer reference (See Figure11c). PSPMA stabilizer surface coverages were calculated to be 48, 31, and 27% for the 66, 125, and 799 nm diameter PSPMA50-stabilized polystyrene latexes, respectively. Figure 12 shows the aqueous electrophoresis curves obtained for the charge-stabilized polystyrene latex and selected PSPMA50-stabilized latexes. As expected, the bare polystyrene latex prepared with the AIBA initiator exhibited positive zeta potentials over a wide pH range.34 In contrast, PSPMA50stabilized latexes prepared using the same initiator exhibited negative zeta potentials, since the PSPMA50 chains become highly ionized (i.e., anionic) at high pH and remain grafted at the latex surface. Thus the anionic character of the macromonomer completely masks the underlying cationic surface charge due to the initiator fragments. Similarly the PSPMA50stabilized latex prepared with the non-ionic AIBN initiator or 13197

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(see Figure 12). DLS studies indicated that all PSPMA50stabilized latexes become highly flocculated below pH 3.5 (see Figure 13). This is because the PSPMA chains are almost fully

Figure 13. Variation in zeta potential and mean hydrodynamic diameter with solution pH for a 0.01 w/v % aqueous dispersion of PSPMA50-PS latex (prepared using an APS initiator; entry 2, Table 2) in the presence of 10 mM NaCl.

protonated at this pH and hence become hydrophobic (N. B. the pKa of PSPMA homopolymer is approximately 5.535). Above pH 3.5, PSPMA chains acquire substantial anionic character due to ionization, and consequently the PSPMAstabilized particles become well dispersed in the aqueous solution. In contrast, a control experiment conducted with the charge-stabilized APS-initiated PS latex confirmed that no flocculation occurred below pH 3.5. Thus the grafted macromonomer chains confer pH-responsive behavior on the PSPMA-stabilized latexes. Finally, the stability of selected latexes was also evaluated under relatively stringent conditions (see Table 4). A single freeze/thaw cycle was sufficient to cause complete precipitation of the charge-stabilized latex. In contrast, PSPMA50-stabilized latexes were readily redispersed after thawing. The effect of electrolyte was also studied by adding increasing amounts of CaCl2. Again, the charge-stabilized latex began to flocculate in the presence of just 0.01 M CaCl2. In contrast, PSPMA50stabilized latexes exhibited substantially better colloidal stability, tolerating the presence of up to 0.30−0.50 M CaCl2 without any signs of flocculation, as judged by DLS studies. This is consistent with their suggested electrosteric stabilization mechanism, which profoundly enhances the latex colloid stability.

Figure 11. XPS core-line C1s spectra obtained for: (a) a chargestabilized polystyrene latex prepared with an anionic APS initiator (entry 9, Table 2); (b) PSPMA50 homopolymer-stabilized polystyrene latexes (entry 8, Table 2); (c) PSPMA50 macromonomer; (d) 66 nm PSPMA50-PS latex prepared by aqueous emulsion polymerization (entry 5, Table 2); (e) 125 nm PSPMA50-PS latex prepared by aqueous emulsion polymerization (entry 2, Table 2); (f) 799 nm PSPMA50-PS latex prepared by alcoholic dispersion polymerization (entry 7, Table 3).

■

CONCLUSIONS A two-step synthesis route has been used to prepare novel welldefined polyacid macromonomers. First, a series of nearmonodisperse PHPMA homopolymer precursors were synthesized using a tertiary amine-functionalized ATRP initiator in alcoholic media at 20 °C. The polymerization conditions were optimized, and PHPMA homopolymer precursors with mean degrees of polymerization ranging from 20 to 70 were obtained with good molecular weight control and low polydispersities (Mw/Mn < 1.25) using a 9:1 w/w % isopropanol/water mixture at 50 °C. These well-defined PHPMA homopolymer precursors were then fully quaternized using 4-vinylbenzyl chloride and subsequently esterified with excess succinic anhydride to afford the corresponding near-monodisperse PSPMA macromonomers under mild conditions. These macromonomers were evaluated as reactive steric stabilizers for polystyrene latex

Figure 12. Aqueous electrophoresis curves obtained for a chargestabilized polystyrene latex (prepared using an AIBA initiator; entry 10, Table 2) and various PSPMA50-stabilized polystyrene latexes (entries 2, 4, and 5, Table 2; entry 7, Table 3).

the APS initiator also exhibited strongly negative zeta potentials from pH 2 to pH 9. These two latexes are slightly more anionic than the corresponding latex prepared using the AIBA initiator, which suggests that the surface initiator groups exert some weak influence on the overall zeta potential, at least in the latter case 13198

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Table 4. Summary of the Colloid Stabilities of Various Latexes: (A) in the Presence of Varying Amounts of Added CaCl2 and (B) after a Single Freeze−Thaw Cycle at −20 °Ca CaCl2 concentration (M) latex type

DLS diameter (nm)

0.05

0.10

0.15

0.20

0.30

0.50

freeze−thaw stability (DLS diameter, nm)

APS-PS AIBA-PS PSPMA30-PS PSPMA50-PS PSPMA70-PS PSPMA50-PS (AIBA initiator) PSPMA50-PS (AIBN initiator) PSPMA50-PS (Dispersion)

613 914 131 125 109 66 110 799

X X √ √ √ √ √ √

X X √ √ √ √ √ √

X X √ √ √ X √ √

X X √ √ X X X √

X X √ X X X X √

X X X X X X X √

aggregation aggregation 131 127 110 68 110 812

a √ Indicates no change in DLS diameter within experimental error; X Indicates substantial particle flocculation, as judged by visual inspection and DLS.

■

syntheses by both aqueous emulsion and alcoholic dispersion polymerization. Near-monodisperse sterically stabilized polystyrene latexes of ∼120 nm diameter were prepared using 10 wt % PSPMA macromonomer and either APS or AIBN initiators via aqueous emulsion polymerization. Using a cationic AIBA initiator resulted in a relatively small latex diameter of 66 nm. Reducing the PSPMA macromonomer concentration did not greatly affect the latex size distribution; in particular, a 180 nm diameter latex could be obtained using only 1.0 wt % PSPMA50 macromonomer with the APS initiator. Alcoholic dispersion polymerization was conducted using 10 wt % PSPMA50 in various ethanol/water mixtures. These latexes were also relatively uniform in size: mean diameters could be varied from 179 to 799 nm, with smaller latexes being obtained at higher water contents as expected. 1H NMR and FT-IR spectroscopy confirmed the presence of PSPMA macromonomer within the latexes. These grafted macromonomer chains are located at the latex surface as evidenced by aqueous electrophoresis and XPS. Compared to a charge-stabilized polystyrene latex reference prepared in the absence of any macromonomer, these electrosterically stabilized latexes exhibited excellent resistance to both freeze−thaw cycles and electrolyte-induced flocculation up to 0.50 M CaCl2.

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ASSOCIATED CONTENT

S Supporting Information *

13

C NMR, FT-IR spectrum, and ES-MS analysis of 2(dimethylamino)ethyl-2-bromoisobutyrylamide ATRP initiator, synthesis and characterization data of ATRP homopolymerization of HPMA in pure IPA, additional GPC and 1H NMR data for PHPMA homopolymer, typical 1H NMR spectrum of PSPMA macromonomer and PSPMA-stabilized PS latex, sample calculations of PSPMA-stabilized content and absorption amount. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.C.Y. thanks his family for their generous financial support of his PhD studies. 13199

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