A Novel Method for Preparing Click-Ready Latex and Latex with

Article pubs.acs.org/IECR

A Novel Method for Preparing Click-Ready Latex and Latex with Stability against High Electrolyte Concentrations Lei Yang,*,†,‡ Jianqing Xu,† Jianli Han,†,‡ Yifeng Shen,†,‡ and Yingwu Luo§ †

Engineering Research Center for Eco-Dyeing and Finishing of Textiles, and ‡Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, College of Materials and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018, Zhejiang, China § The State Key Laboratory of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, China ABSTRACT: A novel method to synthesize click-ready latex based on reversible addition−fragmentation chain transfer (RAFT) miniemulsion polymerization is proposed. It involves the use of poly(acrylic acid)30-b-poly((butyl acrylate)2-co-(cyclohex-3enylmethyl acrylate)3) amphiphilic macro-RAFT agent as both the surfactant and polymerization mediator. It is demonstrated that the cyclohexenyl groups incorporated onto polymer chains are located on the surface of polymeric particles and remained unreacted during miniemulsion polymerization. These surface-rich cyclohexenyl groups allow highly efficient radical-mediated thiol−ene click (TEC) reactions with the water-soluble thiol compound dithiothreitol. The TEC reaction with thiol-terminated poly(N,N-dimethylacrylamide) (SH-PDMAAm) yielded particles with over one PDMAAm brushes per square nanometer. Such densely grafted brushes increased the particles’ critical coagulation concentration of sodium chloride by 1 order of magnitude, compared with particles without covalent-bonded PDMAAm.

1. INTRODUCTION Colloidal dispersions of polymers produced from emulsion polymerization have been used in a wide variety of applications, particularly in the fields of paints, coatings, adhesives and textiles.1−3 In most cases, the polymeric particles must remain well-dispersed within a relatively large range of electrolyte concentrations. Nonionic surfactants are frequently used to increase the colloidal stability against relatively high salt concentrations, which are physically adsorbed onto colloidal particles and stabilize particles primarily by steric repulsion. However, the presence of these physically adsorbed surfactants has some disadvantages, such as forming some surfactant-rich domains in the coatings or leading to latex destabilization via desorption during formulations. A promising solution to these problems is to use reactive nonionic surfactants, such as a surfmer. Surfmers are a class of well-developed reactive surfactants that are characteristic of vinylic surface-active materials. In principle, they could be incorporated onto the polymer chains and improve the latex stability against electrolyte without surfactant migration. However, extensive research has suggested that the colloidal stability of the surfmerstabilized latex was quite complex and was highly dependent on the copolymerization reactivity ratios of surfmers and monomers.4 Click chemistry is a simple and robust method for the postfunctionalization and preparation of complex polymer materials.5−8 One of the most well-known click reactions involves a CuI-catalyzed azide−alkyne cycloaddition.9 Because of its high yields under mild conditions in aqueous and organic media, this technique has rapidly found its use within applications such as polymer microsphere surface modifications.10−15 Quite recently, alkyne−azide cycloadditions were also extended to modify the interior of the particles, which was © 2015 American Chemical Society

demonstrated to be a versatile platform to prepare uniform microgels.16 Radical thiol−ene chemistry (TEC) is another type of “click” reaction.17−19 Compared with azide−alkyne cycloaddition reactions, TEC offers several significant advantages, especially by eliminating the use of any potentially toxic metal catalyst. Pierrer et al. produced soft nanoparticles with a thiocarbonyl thiol group-functionalized surface. After reduction, the thiocarbonyl thiol moieties were converted into thiols that were capable of thiol−ene chemistry.20 A simplified approach to prepare “thiol−ene click chemistry-ready” latex was proposed by Zhang et al.,21 Barner-Kowollik et al.22, and Mecking et al.,23 where some residual vinylic groups were introduced on the particle surface through (co)polymerization with divinylic monomers. TEC reactions on these particles demonstrated good results in terms of high efficiency and easy operation. However, either the cross-linked structure or the residual catalyst limited the latex applications as film-forming materials, such as coatings and textile binders. Cyclohexene is viewed as a special class of alkenes. Its double bond has low reactivity in free radical polymerization due to steric hindrance.24 To date, no free radical homopolymerization of cyclohexene has been reported. In contrast, for an electronrich alkene, the thiol−ene reaction of nucleophilic thiyl radical and cyclohexene was shown to proceed with high efficiency.25,26 This feature makes the use of cyclohexene as a functional group very attractive for the synthesis of “clickready” polymer materials with linear chain structures. Received: Revised: Accepted: Published: 5536

March 19, 2015 April 29, 2015 May 6, 2015 May 7, 2015 DOI: 10.1021/acs.iecr.5b01037 Ind. Eng. Chem. Res. 2015, 54, 5536−5542

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Industrial & Engineering Chemistry Research

cyclohexenyl group), c 12.21 (31H, −COOH × 31), d 1.1 (3H, >CH−CH3). Molecular weight is estimated to be 3153 g mol−1 according to NMR spectrum in Figure 1.

Recently, using the well-designed amphiphilic macromolecular RAFT (macro-RAFT) agents as both reactive surfactant and polymerization mediator in RAFT (mini)emulsion polymerization has achieved great success in terms of good colloidal stability, controlled molecular weight, and narrow molecular weight distribution.26−30 Because RAFT agents can act as both the surfactant and polymerization mediator, polymerization would start on the interface of minidroplets and then polymer chains would grow inward.26−30 Not only chain microstructures, such as block copolymer31−33 and gradient copolymer,34 but also nanostructured objects, such as nanocapsules35−37, nanoworms and nanorods,38 nanoobjects with thermoresponsive shapes,39,40 nanoparticles either consisting of star polymers41 or with core−shell32,42 and onionic structures43 have been prepared by such a unique interfacial polymerization. In comparison with surfmers, the molecular architecture, composition, and molecular weight of the macro-RAFT agent can be easily adjusted owing to the livingness of RAFT polymerization in the synthesis of the macro-RAFT agent. As a result, it is possible to tune the particle surface functionalities via delicately designed amphiphilic macro-RAFT agents.32 In this work, miniemulsion polymerizations are mediated by a cyclohexenyl (CHE)-functionalized amphiphilic macro-RAFT agent. It is expected that polymer latexes with CHE-enriched surfaces and preset polymer chain structures would be obtained. Such latex particles were further postfunctionalized through thiol−ene click coupling reaction.

Figure 1. 1H NMR spectra of CHE-functionalized trithiocarbonate macro-RAFT agent.

2.3. Synthesis of Thiol End-Functionalized PDMAAm. Thiol end-functionalized PDMAAm was prepared by cleavage of −SC(S)SC4H9 group from PDMAAm trithiocarbonate. Step1. Synthesis of PDMAAm trithiocarbonate. TTCA-4 (0.24 g, 1 mmol), DMAAm (14.85 g, 50 mmol), and AIBN (0.016 g, 0.1 mmol) were dissolved in 1,4-dioxane (18 g, 0.204 mol). The solution was transferred into a flask and stirred at 70 °C for 4 h under nitrogen atmosphere. GC analysis indicated that 99% conversion was achieved. The product was collected by precipitation of the mixture in large amount of cyclohexane and dried in vacuum oven under reduced pressure at 50 °C. The structure of the product was obtained from 1H NMR spectrum in Figure 2.

2. EXPRIMENTAL SECTION 2.1. Materials. Acrylic acid (AA), butyl acrylate (BA), styrene (St), and N,N-dimethylacrylamide (DMAAm) were purified by vacuum distillation. Deionized water (conductivity 99%), sodium dodecyl sulfate (SDS), n-hexadecane (HD, costabilizer), dithiothreitol (DTT), 1,4-dioxane and Darocur 2959 were used without further purification. Cyclohex-3-enylmethyl acrylate25 and RAFT agent S-1-butyl-S′-(αmethyl-α′-acetic acid) trithiocarbonate (TTCA-4)44 were synthesized and purified according to the literature. 2.2. Synthesis of CHE-Functionalized Trithiocarbonate Macro-RAFT Agent: Poly(acrylic acid)30-b-poly((butyl acrylate)2-co-(cyclohex-3-enylmethyl acrylate)3) Amphiphilic Macro-RAFT Agent. The macro-RAFT agent was synthesized by a two-step solution polymerization under nitrogen atmosphere at 80 °C. During the first step, a solution containing AA (4.61 g, 6.4 × 10−2 mol), TTCA-4 (0.51 g, 2.1 × 10−3 mol), V-501 (0.06 g, 0.2 × 10−3 mol), and 1,4dioxane(13.0 g, 0.147 mol) was transferred into a flask and left to react for 3 h. Deoxygenated solution containing BA (1.64 g, 1.28 × 10−2 mol), CEA (2.12 g, 1.28 × 10−2 mol), V-501 (0.06 g, 0.2 × 10−3 mol), and 1,4-dioxane (5.0 g, 5.7 × 10−2 mol) was then introduced to the reactor. This second step was left to react for 5 h. The macro-RAFT agent was collected by precipitation of the mixture in cyclohexane and was dried in a vacuum oven under reduced pressure at 50 °C. Its chemical structure was elucidated using gas chromatography (GC) data (AA conversion: 99%; BA conversion: 34%; CEA conversion: 54%), and confirmed by 1H NMR spectrum. 1 H NMR (400 MHz, DMSO-d6, δ, ppm): a 0.88 (9H, −CH2−CH3 × 3), b 5.60 (6H, −CHCH− × 3 in

Figure 2. 1H NMR spectra of PDMAAm trithiocarbonate.

H NMR (400 MHz, DMSO-d6, δ, ppm): a 0.88 (3H, −CH2−CH3 × 1), b 3.1−2.71 (294H, −N(CH3)2 × 49). Molecular weight is estimated to be 5077 g mol−1 according to 1 H NMR. Step 2. Cleavage of −SC(S)SC4H9 group from PDMAAm trithiocarbonate. A solution of PDMAAm trithiocarbonate (1g, Mn, NMR = 5098 g mol−1, 1.96 × 10−4 mol) and ethylamine (1 g, 0.022 mol) in 3 mL tetrahydrofuran (THF) was stirred at 20 °C for 24 h under nitrogen atmosphere. The mixture was then added dropwise into cyclohexane, resulting in the precipitation of a white solid. The solid was dissolved in 3 mL of THF, 1

5537

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Industrial & Engineering Chemistry Research followed by stirring in the presence of acetic acid (2 mL) and zinc power (1 g) at 40 °C for 4 h. After filtration and after precipitation from cyclohexane, a white solid was obtained and dried in a vacuum at 50 °C. More than 95% of the trithiocarbonate RAFT groups had been removed, as calculated from the UV−vis spectra in Figure 3 at 311 nm. Final yield: 63%, Mn,NMR = 4957 g mol−1.

Table 1. Experimental Conditions and Results of TEC Reactions sample

thiol (g)a

latex (g)b

ini (g)c

thiol/CHE/ini (mmol)

UV irradiation (h)

CCHE (%)d

NP-1 NP-2 NP-3 NP-4 NP-5 NP-6 NP-7

0 0.13 0 4.11 1.39 0.69 4.11

6 6 6 6 6 6 6

0 0.09 0.09 0.09 0.09 0.09 0.09

0/0.28/0 0.83/0.28/0.42 0/0.28/0.42 0.83/0.28/0.42 0.28/0.28/0.42 0.14/0.28/0.42 0.83/0.28/0.42

0 5 5 5 5 5 0

0 94.6 0 65.9 59.7 44.0 0

a

DTT was applied in NP-2. SH-PDMAAm49 was applied in the rest samples if thiol compound was required. bFinal latex in experiment 1. c Daricure 2959. dConversion of CHE group.

min−1. The measurement was calibrated using narrow polystyrene standards (Polymer Laboratory) with molecular weight ranging from 1200 to 666 000 g mol−1. The theoretical number-average molecular weights (Mn,the) were calculated by eq 1.

Figure 3. UV absorption spectra of PDMAAm trithiocarbonate and the prepared thiol-terminated PDMAAm.

M n,the = MRAFT,GPC +

2.4. Miniemulsion Polymerization Mediated by CHEFunctionalized Macro-RAFT Agent. St (10 g, 96 mmol) was first mixed with hexadecane (0.5 g, 2.23 mmol) comprising the preliminary organic phase. This phase was thoroughly mixed by magnetic stirring until homogeneous. Under constant agitation, this mixture was added to the continuous phase, a solution of SDS (0.5 g, 1.78 mmol) and macro-RAFT agent (3.15 g, 1 mmol) in water (35 g, 1.94 mol). Pure water (5 g, 0.27 mol) was held aside to dissolve KPS (0.05 g, 0.2 mmol). The emulsion was then left under rapid agitation for a period of 10 min to homogenize. To achieve characteristic miniemulsion monomer droplet size, the coarse emulsion mixture was ultrasonified by using a Scientz JY92-II sonifier (amplitude 70%, 650 W) for 99 cycles in a cooled water bath. Each cycle lasted 10 s with an interval of 5 s. The obtained miniemulsion was then transferred into a four-necked 250 mL flask equipped with a condenser, a nitrogen inlet, a sampling port, and a mechanical stirrer. Reactor contents were further purged and allowed to come to reaction temperature (70 °C), at which point the KPS solution was injected to start the polymerization. Samples were taken at regular time intervals for analysis. 2.5. Thiol−ene Click Reaction of Thiol Compounds to PS Nanoparticles. The click reaction between either DTT or SH-PDMAAm49 and PS nanoparticles was performed in the following steps: DTT or SH-PDMAAm49 and Darocur 2959 (photoinitiator) were dissolved in 6 g of PS latex from miniemulsion polymerization experiment 1, NP-1. The mixture was transferred to a 10 mL quartz beaker with septa, and irradiated by UV light for 5 h at ambient temperature under magnetic stirring. The radiation source is a YINGFENG YZ T8 40W × 4 high-pressure mercury arc lamp. The recipes are shown in Table 1. 2.6. Characterization. 2.6.1. GPC analysis. Molecular weights and dispersities (Đ) of polymers were determined at 30 °C by GPC (Waters 2707/1525/2414) with three Waters Styragel columns (Styragel HR3, HR4, and HR1 (or HR5); the effective molecular weight range covers 100−600,000 g mol−1 for polystyrene). The eluent was THF with a flow rate of 1 mL

x[M ]0 MM [RAFT]0 + f [I ]0 (1 − e−kdt )

(1)

where [I]0, [RAFT]0, kd, and f are initial molar concentrations of initiator and RAFT agent, dissociation rate constant of initiator, and initiation efficiency, respectively; kd is 2.25 × 10−5 s−1, f was estimated to be 0.6 with the kinetic parameter values45 under the current reaction conditions. MRAFT,GPC, MM, [M]0, x, and t are the molecular weight of the RAFT agent measured by GPC, molecular weight of monomer, initial monomer concentration, monomer conversion, and polymerization time, respectively. 2.6.2. NMR Analysis. 1H NMR spectra were measured on Bruker Avance AV 400 MHz Digital FT-NMR Spectrometer at room temperature (Internal reference: TMS (tetramethylsilane); 1% solution in DMSO-d6.). Macro-RAFT agent was measured on NMR directly. Polymeric samples from miniemulsion run and TEC reactions were purified before NMR analysis as follows: The latex polymers were milled into small pieces and washed five times with hot water under ultrasound treatment to remove SDS or the excess of DTT or SH-PDMAAm49. HD was then driven off from the polymers at 115 °C for 8 h under vacuum. 2.6.3. Fourier Transform Infrared Spectroscopy (FT-IR) Measurement. FT-IR spectra were recorded on Thermo Nicolet Avatar 370 FT-IR at room temperature using KBr pellet method. A sample from the miniemulsion run was purified as described in the previous section. Other samples were measured directly. 2.6.4. Dynamic Light Scattering (DLS) Analysis. The samples for DLS analysis were diluted with 50 parts of deionized water or NaCl solution. The particle size was measured by DLS (Malvern Zetasizer Nano S). 2.6.5. Transmission Electron Microscopy (TEM) Measurement. TEM measurements were performed on JEM-1230 (JEOL), and the samples were prepared as follow: 0.03 g of latex was diluted by 20 g of deionized water. One drop of the dilution was placed onto the carbon-coated copper mesh and dried at room temperature. 5538

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3. RESULTS AND DISCUSSION 3.1. Miniemulsion Polymerization Mediated by the CHE-Functionalized Macro-RAFT Agent. During RAFT miniemulsion polymerization (experiment 1), the apparent colloidal stability was excellent with high concentrations of HD and SDS as 5 wt % (based on the weight of St). As shown in Table 2, experiment 1 proceeded rapidly and achieved full

latex polymer chains and macro-RAFT agent had the same N value of 3, indicating that CHE groups were completely retained and only St monomers were polymerized during RAFT miniemulsion polymerization. After TEC reaction, the NMR data in Figure 4c indicate that N significantly decreased to 0.16, and the conversion of CHE (CCHE) groups was 94.6%. A control (NP-3) run was carried out without the addition of any thiol compounds. 1H NMR data in Figure 4d suggests that photoinitiator radicals alone should not consume CHE groups. In addition, an earlier study demonstrated that CHE groups with CC double bonds could not undergo homopolymerization.26 Therefore, the conversion of CHE groups in NP-2 was exclusively a result of the TEC reaction. Because DTT is watersoluble and immiscible with the latex polymer, the TEC reaction should be confined at the water−particle interfaces at room temperature (Note: much lower than the Tg of PS.). The high reaction efficiency indicated that at least 94.6% CHE groups were located at the particle surfaces. The surface density σCHE of CHE groups at NP-1 particle surface was then calculated using eq 2. The calculated σCHE at NP-1 particle surface was approximately 1.73 units nm−2.

Table 2. Kinetic Data Collected during RAFT Miniemulsion Polymerization

expt 1

time (min)

conversion (%)

Mn,the (g mol−1)

Mn,exp (g mol−1)

Đ

30 45 75 120

28 41 87 >99

5980 7252 11908 13052

5537 7187 11558 12795

1.14 1.15 1.16 1.15

conversion within 2 h. The obtained polymers have theoretically predictable molecular weights and narrow molecular weight distributions, suggesting the full conversion of macro-RAFT agents and, thus, the incorporation of CHE groups into the latex polymer chains. Additionally, the CHE groups are expected to be localized at the surfaces of nanoparticles in the final latex, due to their direct connections with the hydrophilic PAA blocks. This distribution profile is verified by interfacial thiol−ene click chemistry in combination with NMR analysis presented in the following section. 3.2. Quantification of CHE Groups on Latex Particle Surface. A water-soluble small thiol compound DTT was engaged during the interfacial TEC reaction. The experimental recipe is summarized in Table 1. The NMR spectra for the latex polymers before (NP-1) and after (NP-2) TEC reactions are compared in Figure 4. In the

σCHE = NAN

ρD 6M n

(2)

with the Avogadro constant, NA; the number of CHE groups per polymer chain, N = 3; the density of the PS, ρ = 1.06 g cm−1; the molecular weight of NP-1 polymer measured by GPC, and Mn = 13052 g mol−1; the average hydrodynamic diameter of the PS particle estimated by TEM statistics, D = 72 nm. 3.3. Click Reaction of Thiol-Terminated PDMAAm (SHPDMAAm49) to CHE-Functionalized Nanoparticles. In the second series of TEC reaction, latex NP-1 was reacted with a macromolecular thiol compound, SH-PDMAAm49. In comparison with the reaction employing a small molecular thiol compound DTT, TEC reactions (NP-4, NP-5 and NP-6) with SH-PDMAAm49 yielded much lower CCHEs as a result of steric effects.46 1H NMR spectra in Figure 5b shows that CCHE only reached 65.9% (Note: It equals to 1−2.03/5.93). When a ratio of 3 equiv of thiol with respect to CHE groups was employed in NP-4. Further reducing the ratio of thiol:CHE group resulted in a slight decrease of CCHE. The CCHE values are 59.7 and 44.0% for cases with ratios of 1:1 (NP-5, Figure 5c) and 0.5:1 (NP-6, Figure 5d), respectively. The average surface density of

Figure 4. 1H NMR spectra of (a) macro-RAFT agent; (b) PS polymers from miniemulsion polymerization experiment 1, NP-1; (c) polymers from TEC reaction between NP-1 and DTT in the presence of Darocur 2959, NP-2; (d) polymers from the reaction between NP-1 and Darocur 2959 without any thiol compounds, NP-3.

spectrum, the signals at 5.6 ppm and at 0.9 ppm are assigned to the vinylic protons (ACH) in the CHE groups, and methyl protons (A−CH3) at chain ends, respectively. Based on the comparison of resonance intensities of these two types of protons, the average number of CHE groups (N) per chain is calculated by N = 4.5 × ACH/A−CH3. Before TEC reaction,

Figure 5. 1H NMR spectra of (a) PS polymer from miniemulsion polymerization experiment 1, NP-1; polymer from TEC reaction obtained at a molar ratio of SH/CHE/Ini.: (b) 6/2/3, NP-4; (c) 2/2/ 3, NP-5; (d) 1/2/3, NP-6. 5539

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attributed to the PS segment. Taking into account that THF is a poor solvent for PDMAAm, it was concluded that these THFinsoluble polymers should have a higher mass fraction of PDMAAm than the THF-soluble ones. Furthermore, it was observed that these covalently attached PDMAAm chains significantly improved dispersion stability against high electrolyte concentrations, which will be shown later. The morphologies of nanoparticles before and after TEC reactions are compared. The TEM image in Figure 8a shows that the average particle size of particles before TEC reaction is about 72 nm in diameter. Figure 8b,c indicates that the TEC reactions did not affect either the morphologies or the diameters of nanoparticles. 3.4. Colloidal Stability in the Presence of NaCl. To determine the colloidal stability of PDMAAm-functionalized PS latexes with respect to the degree of functionalization, the average diameters of a series of PDMAAm-functionalized nanoparticles were examined in water at various NaCl concentrations (ranging from 9 × 10−3 to 4 mol L−1). As shown in Figure 9, the average particle size of NP-1 is about 86 nm as estimated by DLS, which is somewhat bigger than that observed in the TEM. The possible reason for this difference is that DLS measurement gives the hydrodynamic diameter, which accounts for the contribution of surface anchored PAA blocks, while TEM picture gives the diameter of the dried samples.48 After the addition of NaCl, a collapse of the PAA blocks is accordingly assigned to a decreasing solvation power of the water. As a result, the particle size of nanoparticles (NP-1) decreased to 76 nm, which was very close to the TEM result. An increase of NaCl concentration beyond the critical coagulation concentration (Ccrit) of 0.1 M caused NP-1 dispersion to coagulate, as indicated by the steep increase in particle size. NP-7 was prepared by physical mixing of NP-1 with SH-PDMAAm49 and photoinitiator. The curve shows that NP-7 has a larger particle size than NP-1 has, even at very low concentration of NaCl. This may suggest the adsorption of some SH-PDMAAm49 chains onto the particle surface, as a result of hydrophobic interactions between thiol groups and particles. However, these physically attached PDMAAm chains do not seem to enhance NP-7 nanoparticles stability against salt-induced aggregation because the aggregation of NP-7 appears at nearly the same Ccrit as NP-1 does. The other three samples, NP-4, NP-5, and NP-6, underwent TEC reactions with varying σPDMAAms from 1.14 to 0.76 chains nm−2. Those grafted PDMAAm49 chains sterically stabilized latex and improved the latex stability against NaCl. A closer inspection reveals that Ccrit for NP-6 with σPDMAAm = 0.76 chains nm−2 increases to 0.4 M. Ccrit reaches an even higher value of 1 M for NP-5 with σPDMAAm = 1.03 chains nm−2, which is 10 times that of NP-1. A further increase of σPDMAAm of NP-4 to 1.14 chains nm−2 does not increase the colloidal stability further.

conjugated PDMAAm49 chains at particle surfaces (σPDMAAm) were calculated by equation σPDMAAm = σCHE × CCHE. As CCHE decreases from 65.9 to 59.7% and then to 44.0%, σPDMAAm is calculated accordingly to be 1.14, 1.03, and 0.76 chains nm−2. All these values are higher than the figure of 0.7 chains nm−2 ordinarily for high-density grafting,47 suggesting that all samples were densely grafted with PDMAAm chains. The successful grafting of PDMAAm chains was further confirmed by GPC results. Taking NP-4 with σPDMAAm = 1.14 chains nm−2 as an example, its GPC chromatogram in Figure 6

Figure 6. GPC curves of polymers from (a) miniemulsion polymerization experiment 1, NP-1; and TEC reaction with (b) DTT, NP-2 and (c) SH-PDMAAm49, NP-4.

confirmed the presence of a higher molecular weight species with bonded SH-PDMAAm49. Meanwhile, a negligible increase in molecular weight was observed for NP-2 bonded with DTT. It should be noted that a portion of the samples from click reactions employing SH-PDMAAm49 were insoluble in THF. Furthermore, the insoluble polymer fractions greatly depend on the σPDMAAms, which decreased from 57 to 42% and then to 28% as σPDMAAm decreased from 1.14, 1.03, and down to 0.76 chains nm−2. In contrast, latex polymers that did not undergo TEC reaction could be easily dissolved in THF to form a transparent and stable solution. FT-IR spectrum of the insoluble portion is compared to those of SH-PDMAAm49 and NP-1 polymer in Figure 7. The carbonyl band at around 1647 cm−1 is attributed to the PDMAAm segment, and the benzene ring band vibrations at 1602, 1493, and 1452 cm−1 are

4. CONCLUSIONS In this work, it was demonstrated that a given amount of CHE groups could be introduced on the surfaces of PS nanoparticles via RAFT miniemulsion polymerization mediated by CHE functionalized amphiphilic macromolecular RAFT agent. The CHE groups remained unreacted during miniemulsion polymerization but easily reacted with water-soluble thiols via thiol−ene click (TEC) reactions. The TEC reaction with a thiol-terminated poly(N,N-dimethylacrylamide) (SHPDMAAm) yielded particles with over one PDMAAm brushes

Figure 7. FT-IR spectra of (a) SH-PDMAAm49; (b) PS polymers from miniemulsion polymerization experiment 1, NP-1; (c) THF-insoluble portion of polymers from TEC reaction with SH-PDMAAm49, NP-4. 5540

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Figure 8. TEM images of PS nanoparticles from (a) miniemulsion polymerization, NP-1; (b) TEC reaction employing DTT, NP-2; (c) TEC reaction employing SH-PDMAAm49, NP-4.

Figure 9. Particle size of various PS latexes in NaCl solution. PS particles from miniemulsion polymerization (without thiol compound), experiment 1, NP-1(△); Product from TEC reaction with SH-PDMAAm49: PDMAAm49 chains per PS chain σPDMAAm = 1.14 chains nm−2, NP-4 (★); σPDMAAm = 1.03 chains nm−2, NP-5(■); σPDMAAm = 0.76 chains nm−2, NP-6 (▲); Product from physical mixing PS particles with SH-PDMAAm49 and photoinitiator but without UV initiation, NP-7 (□). (2) Kendall, K.; Padget, J. C. Latex coalescence. Int. J. Adhes. Adhes. 1982, 2, 149−154. (3) Huang, P.; Chao, Y.; Liao, Y. T. Preparation of fluoroacrylate nanocopolymer by miniemulsion polymerization used in textile finishing. J. Appl. Polym. Sci. 2004, 94, 1466−1472. (4) Guyot, A. Recent progress in reactive surfactants in emulsion polymerization. Macromol. Symp. 2002, 179, 105−132. (5) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click chemistry: Diverse chemical function from a few good reactions. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (6) Iha, R. K.; Wooley, K. L.; Nystrom, A. M.; Burke, D. J.; Kade, M. J.; Hawker, C. J. Applications of orthogonal “click” chemistries in the synthesis of functional soft materials. Chem. Rev. 2009, 109, 5620− 5686. (7) Sumerlin, B. S.; Vogt, A. P. Macromolecular engineering through click chemistry and other efficient transformations. Macromolecules 2009, 43, 1−13. (8) Nandivada, H.; Jiang, X.; Lahann, J. Click chemistry: versatility and control in the hands of materials scientists. Adv. Mater. 2007, 19, 2197−2208. (9) Meldal, M.; Tornøe, C. W. Cu-catalyzed azide−alkyne cycloaddition. Chem. Rev. 2008, 108, 2952−3015. (10) Li, N.; Binder, W. H. Click-chemistry for nanoparticle modification. J. Mater. Chem. 2011, 21, 16717−16734. (11) Staff, R. H.; Willersinn, J.; Musyanovych, A.; Landfester, K.; Crespy, D. Janus nanoparticles with both faces selectively functionalized for click chemistry. Polym. Chem. 2014, 5, 4097−4104. (12) Baier, G.; Siebert, J. M.; Landfester, K.; Musyanovych, A. Surface click reactions on polymeric nanocapsules for versatile functionalization. Macromolecules 2012, 45, 3419−3427. (13) Speyerer, C.; Borchers, K.; Hirth, T.; Tovar, G. E.; Weber, A. Surface etching of methacrylic microparticles via basic hydrolysis and

per square nanometer. Such brush modified particles increased the critical coagulation concentration of sodium chloride by 1 order of magnitude, compared with that for particles without covalently bonded PDMAAm. As CHE groups can be further functionalized via click chemistry, it is proposed that RAFT miniemulsion polymerization mediated by cyclohexenyl-functionalized amphiphilic macro-RAFT agent could provide a basis upon which functional and high performance latexes may be built.

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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 This project is financially supported by Zhejiang Provincial Natural Science Foundation of China (LY15E030013 and LY12E03008), Doctoral Discipline Foundation for Young Teachers in the Higher Education Institutions of Ministry of Education (20103318120002), Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology of Zhejiang Sci-Tech University, National Natural Science Funds for Distinguished Young Scholar (Grant No. 21125626).

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REFERENCES

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DOI: 10.1021/acs.iecr.5b01037 Ind. Eng. Chem. Res. 2015, 54, 5536−5542

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DOI: 10.1021/acs.iecr.5b01037 Ind. Eng. Chem. Res. 2015, 54, 5536−5542