Radical Mediated Thiol-Ene Emulsion Polymerizations

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Radical Mediated Thiol-Ene Emulsion Polymerizations Olivia Z. Durham,† Dana V. Chapman,† Sitaraman Krishnan,*,‡,§ and Devon A. Shipp*,†,§ †

Department of Chemistry & Biomolecular Science, ‡Department of Chemical & Biomolecular Engineering, and §Center for Advanced Materials Processing, Clarkson University, Potsdam, New York 13699, United States S Supporting Information *

ABSTRACT: Radical mediated thiol−ene emulsion polymerizations for the synthesis of aqueous dispersion of polymer particles with narrow particle size distributions are reported. Submicrometer polymer particles comprising of cross-linked or linear chains were obtained using a water-soluble thermal initiator, without the need for a high-intensity emulsification step. Particles were prepared using 1,3,5-triallyl-1,3,5-triazine2,4,6(1H,3H,5H)-trione, pentaerythritol tetrakis(3-mercaptopropionate), trimethylolpropane diallyl ether, 1,6-hexanedithiol, and 3,6-dioxa-1,8-dithiooctane monomers. Potassium persulfate was used as the water-soluble initiator, and sodium dodecyl sulfate was used as the surfactant. The effects of surfactant, initiator, and comonomer concentrations on the particle diameters in the emulsion polymers are reported. The order of dependence of particle diameter on the surfactant concentration was −(0.44 ± 0.03) and on the initiator concentration was −(0.53 ± 0.04). The particle size distributions in the cross-linked latexes were generally broad. A seeded emulsion polymerization reaction, consisting of delayed addition of the cross-linker, was found to result in cross-linked thiol−ene latexes with narrow particle size distribution. borne suspension thiol−ene/yne “click” polymerizations represent one of the first accounts of such syntheses.11−13 We have shown this highly efficient approach yields polymer particles with diameters ranging from just a few hundred nanometers up to hundreds of micrometers. Other researchers have built upon our early work by expanding to thiol−ene/yne miniemulsions, porous thiol−ene particles, heterogeneous thiol−Michael “click” polymerizations, fast polymerization rates in thiol−ene particles, and using other thiol-X chemistries for particle formation.14−24 There are several significant “built-in” benefits within heterogeneous “click” thiol−ene/yne polymerizations in addition to the “click” chemistry tenets of high yields high selectivity, atom efficiency, orthogonality, and ease of use.25−28 First, the step-growth nature of thiol−ene/yne polymerizations provides an excellent handle to finely tune chemical functionality within the particles, to an extent far greater than what can be achieved using traditional chain-growth polymerizations.26,27,29−32 Second, the shift from solvent-based polymer production and use to water-based systems is critical in producing environmentally friendly and sustainable polymerbased technologies, such as paints and industrial coatings. Third, such materials address critical needs in various nanotechnologies like nanomedicine since such particles underpin many key developments in this field over the past

1. INTRODUCTION Numerous polymer-based colloidal systems, with a variety of shapes, sizes, and chemical properties, have been developed for a wide range of technologies.1,2 The widespread use of polymer colloids stems from several factors, including the many chemical and mechanical properties that can be introduced via appropriate selection of monomers and the ability to tune particle size, which is especially important in the sub-100 nm scale, and therefore manipulate parameters such as surface area, light−matter interactions (e.g., scattering), and the stability of multicomponent systems. Polymer nanoparticles and composites are now being considered for novel biomedical, film formation, and other advanced technological applications because of the specific advantages offered by this size range in tuning biological, mechanical, and optical responses.3−7 Furthermore, ease of production is a key factor in the extensive use of polymer colloids. However, the polymerizations used to make such colloids have nearly always been radical-mediated chain-growth reactions. This is undoubtedly because of the great industrial importance of this type of polymer synthesis. But nonradical chain-growth polymerization or radical-mediated step-growth polymerization mechanisms are far less common in water-borne systems. An examination of the literature reveals that nonradical chain-growth polymerization have indeed been mildly successful, mostly in coordination polymerizations such as acyclic diene metathesis or ringopening metathesis polymerization.8−10 Radical-mediated stepgrowth polymerizations have barely been touched; in fact, our recent publications of photo and thermally initiated water© XXXX American Chemical Society

Received: October 12, 2016 Revised: January 6, 2017

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Macromolecules decade. Importantly, the potential use of photoinitiation in these polymerizations is of significant interest as these are typically more energy efficient and can provide spatial and temporal resolution. Because of these multiple and unique features, heterogeneous thiol−ene/yne “click” polymerizations have the potential to produce materials that may find use in many applications, ranging from protective materials, binders in pigments, fibers, and drug or protein delivery to cosmetics. However, notably missing from the list of successful heterogeneous radical thiol−ene/yne polymerizations are emulsion polymerizations (EPs). In conventional EPs, which typically use vinyl monomers that are sparingly water-soluble, along with surfactant and water-soluble initiators, the polymerization occurs either within micelles and/or polymer particles, depending on the type of particle nucleation mechanism involved. The nucleation process involves oligomeric radicals that enter micelles or precipitate to form the growing polymer particle.1,33 The average molecular weight (in interval II of the EP reaction)34 is generally high, and there are only a few growing chain ends in each particle. In fact, in the case of particles that are relatively small (less than about 100 nm in diameter), there is at most one propagating free radical per particle during most of the reaction (except in interval III when gel effect occurs because of a diffusion-limited termination rate in the highly viscous medium that is predominantly polymeric).35,36 It is these kinetic factors that are envisaged to be significantly different when considering step growth emulsion polymerization because of the low molecular weights of species formed throughout much of the polymerization, as opposed to chain-growth polymerization of vinyl monomers. It has been suggested that emulsion polymerizations may not be possible in step-growth polymerizations because of the low molecular weight polymers that are initially developed.1,33,37 Nevertheless, we demonstrate herein that radical mediated emulsion polymerization of thiol−ene monomers can indeed be achieved using characteristic emulsion polymerization reaction conditions. This approach provides an avenue for the production of sub-100 nm particles without the need of significant surfactant concentrations or high-energy dispersions, as required by miniemulsion polymerizations. This synthetic approach demonstrates applicability in synthesis of both crosslinked and linear polymer particles. Furthermore, we have discovered that nanoparticles with relatively narrow size distributions can be made by judicious choice of monomers and stabilizers.

Chart 1. Monomer Species Used in Radical Thiol−Ene Emulsion Polymerizations for the Synthesis of Linear and Crosslinked Polymer Particles

HDT and TMPDAE/TEGDT, and (iii) lightly cross-linked particles of TMPDAE/TEGDT/PETMP terpolymer were synthesized. 2.2.1. Batch Emulsion Copolymerization of TTT and PETMP. Stock solutions for the SDS surfactant of 10 and 100 mM concentrations and the KPS initiator of 50 mM concentration were prepared using deionized water (Millipore, 18.2 MΩ cm resistivity). In a 20 mL scintillation vial, 0.406 g (∼0.35 mL) of TTT and 0.602 g (∼0.47 mL) of PETMP were added (using 1 mL syringes with 20G 1.5 in. needles) and weighed using a microbalance. Next, deionized water and an aliquot of SDS stock solution were added to the monomers according to the desired surfactant concentration. The reaction vial was placed in an oil bath at ambient temperature. The reaction mixture was stirred at 750 rpm via magnetic stirring as the oil bath heated to 60 °C (10−15 min). During the heating, ultrahigh purity nitrogen gas blanket was placed into the vial (by purging the headspace for about 2 min). The vials were stoppered with a rubber septum. Once the temperature reached 60 °C, an aliquot of the 50 mM aqueous KPS solution was quickly added. A 2 mM SDS and 5 mM KPS reaction involved emulsifying the TTT and PETMP monomer mixture in an aqueous phase comprising of 7 mL of deionized water, 2 mL of the 10 mM aqueous SDS solution, and 1 mL of 50 mM aqueous KPS solution. The emulsion was then allowed to react for 4 h. Additional reactions were conducted with variation in surfactant concentration in the range of 2−80 mM (see Table 1) and initiator concentration in the range of 2.5−10 mM.

2. MATERIALS AND METHODS

Table 1. Polymerization Recipes To Study the Effect of Surfactant Concentration

2.1. Materials. The following materials were purchased from Sigma-Aldrich unless otherwise noted and were used as received: 1,3,5triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione (TTT, CAS no. 1025-156, FW 249.27 g/mol), trimethylolpropane diallyl ether (TMPDAE, CAS no. 682-09-7, FW 214.3 g/mol), pentaerythritol tetrakis(3mercaptopropionate) (PETMP, CAS no. 7575-23-7, FW 488.66 g/ mol), 1,6-hexanedithiol (HDT, CAS no. 1191-43-1, FW 150.31 g/ mol), 3,6-dioxa-1,8-dithiooctane (tri(ethylene glycol)dithiol, TEGDT, CAS no. 14970-87-7, FW 182.3 g/mol), sodium dodecyl sulfate (SDS, CAS no. 151-21-3, FW 288.38 g/mol, J.T. Baker), and potassium persulfate (KPS, CAS no. 7727-21-1, FW 270.32 g/mol). Distilled deionized water (Millipore Type 1, ∼18.2 MΩ cm) was used in latex synthesis. Chart 1 shows the chemical structures of the monomers used in the present study. 2.2. Latex Synthesis. Three different sets of latexes comprising of (i) cross-linked particles of TTT and PETMP monomers, (ii) polymer particles with linear (un-cross-linked) copolymer chains of TMPDAE/

DI water TTT PETMP SDS KPS a

10 mL 406 mg (1.63 mmol) 602 mg (1.23 mmol) 5.8−230.7 mg (2−80 mM)a 13.5 mg (5 mM)a

Based on water.

2.2.2. Synthesis of Linear Thiol−Ene Latexes Using Emulsion Copolymerization of Difunctional Monomers. Table 2 gives the recipes for the synthesis of thiol−ene latexes comprising of linear polymer chains (Chart 2). TMPDAE was used as the difunctional ene, and the water-soluble TEGDT and the relatively water-insoluble HDT were used as the difunctional thiols. The polymerization reactions were carried out by adding the required masses (Table 2) of B

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Table 2. Polymerization Recipes To Prepare Linear Copolymer Latexes Incorporating the Difunctional Monomers, TMPDAE, TEGDT, and HDT (Reaction Temperature 60 °C and Reaction Time 3 h) DI water SDS KPS TMPDAE TEGDT

463 mg (2.54 mmol)

HDT TMPDAE/TEGDT/HDT (mol %) a

50/50/0

347 mg (1.91 mmol) 95 mg (0.63 mmol) 50/37.5/12.5

10 mL 23.1 mg (8 mM)a 13.5 mg (5 mM)a 544 mg (2.54 mmol) 232 mg (1.27 mmol) 191 mg (1.27 mmol) 50/25/25

116 mg (0.64 mmol) 286 mg (1.91 mmol) 50/12.5/37.5

382 mg (2.54 mmol) 50/0/50

Based on water. the recipe table (after the start of the polymerization). The total reaction time was 4 h. For example, in the reaction with x = 0.05, 0.544 g of TMPDAE and 0.440 g of TEGDT were added to the reaction vial, followed by the addition of 8 mL of 10 mM aqueous SDS stock solution and 1 mL of DI water. The vial was sealed with a rubber septum and immersed in an oil bath at 60 °C. The vial was purged with nitrogen gas and the contents were mixed using a magnetic stirrer, after which 1 mL of 50 mM aqueous KPS solution was injected into it. Three hours later, 31 mg of PETMP was added to the vial (using a micropipet weighed before and after dispensing the monomer), and the reaction was allowed to continue for an additional 1 h. 2.3. Characterization Methods. Particle size and size distributions were determined using a Brookhaven 90Plus nanoparticle size analyzer. Dynamic light scattering (DLS) measurements were conducted at room temperature in water as the medium. A scattering angle of 90° was used. The method of cumulants was used to determine the Z-average particle diameter, dz, and the polydispersity index (PDI). In this method, the normalized intensity autocorrelation function, g2(td) = ⟨I(t)I(t − td)⟩/⟨I2⟩, where I is the measured intensity, the angle brackets indicate a time average, and td is the time delay, is correlated to td using ln[g2(td) − 1]1/2 = a0 − ⟨Γ⟩td + (μ2/2) td2 − +... The values of ⟨Γ⟩ and μ2, determined by curve fitting, are used to calculate the Z-average diffusion coefficient, Dz, and the polydispersity index: Dz = ⟨Γ⟩q2 and PDI = μ2/⟨Γ⟩2. (q is the magnitude of the scattering wavevector.) The Stokes−Einstein equation is used to calculate dz from the measured diffusion coefficient. The Mie scattering theory can be used to obtain the volume-weighted particle size distribution, and the volume-average particle diameter, dv, which is related to the number concentration of polymer particles in the dispersion. However, because the refractive indices and extinction coefficients of the thiol−ene copolymer particles, of different chemical compositions, were unknown, only intensity-weighted distributions are reported and discussed. Gel permeation chromatography (GPC) was performed using Waters 515 HPLC pump operating at 30 °C, with tetrahydrofuran

Chart 2. TMPDAE/HDT and TMPDAE/TEGDT Copolymers

TMPDAE, TEGDT, and HDT to the reaction vial, followed by the addition of 8 mL of 10 mM aqueous SDS stock solution and 1 mL of DI water. The vial was sealed with a rubber septum and immersed in an oil bath at 60 °C, and the contents were mixed using a magnetic stirrer. After equilibration at 60 °C and purging the headspace with nitrogen gas for about 2 min, 1 mL of 50 mM aqueous KPS solution was injected into the vial. The reaction was carried out for 3 h, following which the vial was removed from the oil bath and cooled to room temperature. 2.2.3. Semibatch Emulsion Copolymerization for Synthesis of Cross-Linked Thiol−Ene Latexes with Narrow Particle Size Distribution. The standard reaction recipe, consisting of 10 mL of water, 8 mM SDS, and 5 mM KPS, was used. The ene to total thiol mole ratio was kept fixed at 1:1. The moles of PETMP and TEGDT, nPETMP and nTEGDT, respectively, were calculated using the equations nPETMP = xnTMPDAE/2 and nTEGDT = (1 − x)nTMPDAE, respectively, where nTMPDAE is the moles of TMPDAE and x is the mole fraction of thiol groups from the PETMP cross-linker. Four different values of x were investigated: 0, 0.05, 0.5, and 0.9 (cf. Table 3). The PETMP cross-linker was added to the reaction mixture at the time specified in

Table 3. Recipes for the Synthesis of Cross-Linked TMPDAE/TEGDT/PETMP Thiol−Ene Latexes, with Delayed Addition of the PETMP Cross-Linker (Reaction Temperature = 60 °C) DI water SDS KPS TMPDAE TEGDT

463 mg (2.54 mmol)

PETMP total monomer mass (mg) x

1007 mg 0

440 mg (2.41 mmol) 31 mga (0.06 mmol) 1015 mg 0.05

10 mL 23.1 mg (8 mM)a 13.5 mg (5 mM)a 544 mg (2.54 mmol) 231 mg (1.27 mmol) 310 mgb (0.63 mmol) 1085 mg 0.50

46 mg (0.25 mmol) 559 mgc (1.14 mmol) 1149 mg 0.90

620 mg 1164 mg 1.00

a Added 3 h after initiation of TMPDAE/TEGDT copolymerization. bAdded 2 h after initiation of TMPDAE/TEGDT copolymerization. cAdded 1 h after initiation of TMPDAE/TEGDT copolymerization.

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Macromolecules (THF) as the eluent, two Polymer Laboratories columns (PLgel Mixed C), and a Viscotek LR40 refractometer. Monodisperse polystyrene standards were used for molecular weight calibration. Polymer particles were analyzed using field emission scanning electron microscopy (SEM) using a JEOL JSM 6300 instrument. A drop of the latex, diluted with Millipore water, was placed on an aluminum stub and dried in a vacuum oven. Prior to analysis, samples were sputtercoated with a thin Au/Pd layer. 1H (400 MHz) nuclear magnetic resonance (NMR) spectroscopy was performed on Bruker Avance DMX-400 instrument with a BBO probe using CDCl3 as solvent. Differential scanning calorimetry (DSC) was performed using a TA Instruments Q100 calorimeter.

particle size. Table 1 gives the polymerization reaction recipes. All reactions were carried out in sealed glass vials, immersed in an oil bath at 60 °C, and magnetically stirred. The cmc of SDS in water depends on factors such as ionic strength and water solubility of monomers and is expected to be in the range of 5− 10 mM under the reaction conditions used herein.36 Therefore, the range of 2−80 mM spans both the homogeneous and micellar solutions of the surfactant. Table 4 gives the properties of these dispersions. Table 4. pH, Effective Particle Diameter, Polydispersity, and ζ-Potential of Thiol−Ene Latexes Synthesized Using the Recipes Given in Table 1

3. RESULTS AND DISCUSSION 3.1. Cross-Linked TTT-PETMP Latex Synthesis. To initially demonstrate that radical mediated thiol−ene emulsion polymerizations can be successfully performed, we polymerized the monomers TTT and PETMP, which are widely examined and known to produce a cross-linked polymer. Reactions used 10 wt % monomers relative to the aqueous phase with a 1:1 mol ratio of ene to thiol functionality. The 2 mM SDS reaction began as a gray, opaque solution that turned milky-white about 30 min after the addition of the KPS, indicating the formation of polymer particles. 1H NMR spectroscopy (Supporting Information Figure S1) established that high conversions (>96%) were achieved within 2 h of polymerization. Dynamic light scattering analysis of the polymerized latex showed a unimodal polydisperse peak with a Z-average particle diameter of about 355 nm. The PDI was 0.03. Thus, a significantly smaller particle size, compared with those in dispersions prepared by suspension polymerization that we previously reported,11 was obtained. Because of the use of a water-soluble initiator in the present study, and our observation that the particles are significantly smaller in size than the monomer droplets (average size ∼10 μm), a particle formation mechanism similar to conventional EP is likely. In conventional EP, the polymerization is initiated in the aqueous phase, but propagation occurs in the monomer swollen “primary” polymer particles that are formed by precipitation of oligomers that have lower water solubility than the monomers.36 When the surfactant concentration is above the critical micelle concentration (cmc), micellar nucleation is also possible in conventional EP, in which case, the particle formation is attributed to monomer swollen surfactant micelles. The mechanism of suspension polymerization, employing oil-soluble initiators, is not expected to result in the small particle sizes observed in the present study. In order to evaluate this novel polymerization system more fully, we undertook a systematic investigation of the effects of surfactant and initiator concentrations on the particle size of the thiol−ene latexes synthesized using EP-like recipes. Such studies have been commonly done for standard vinyl EP systems and provide valuable information regarding the behavior and mechanism of these complex reactions. Furthermore, we also report in the following sections a novel strategy for the formation of cross-linked thiol−ene latexes, with low polydispersity of particle size distribution, using uncross-linked seed particles. 3.2. Effects of Surfactant and Initiator Concentration on Particle Size. Cross-linked thiol−ene latexes, comprising of copolymers of TTT and PETMP, were synthesized using KPS as the thermal initiator, and surfactant concentration ranging from 2 to 80 mM in order to examine the effect of the molar concentration of SDS, [SDS], on particle nucleation and

[SDS] (mM)

pH

ζ-potential (mV)

2 4 6 8 20 40 60 80

7.60 7.90 7.77 7.11 7.87 7.97 7.93 7.97

−34 −34 −44 −45 −42 −45 −43 −52

Z-average particle diameter (nm) 355 235 215 182 127 98 87 73

± ± ± ± ± ± ± ±

14 12 44 16 21 2 12 5

polydispersity 0.03 0.06 0.13 0.16 0.22 0.25 0.10 0.12

± ± ± ± ± ± ± ±

0.01 0.03 0.11 0.11 0.04 0.01 0.08 0.10

The ζ-potential measurements showed that the particle surfaces were negatively charged, as expected, since both the surfactant and the initiator fragments are anionic. The magnitude of the ζ-potential increased from about 34 mV to about 52 mM when the SDS concentration was increased from 2 to 80 mM. The Z-average particle diameters ranged from about 355 nm to about 73 nm, with a clear trend of decreasing particle size as the SDS concentration was increased. Figure 1 shows an SEM

Figure 1. SEM image of PETMP-TTT particles and the dynamic light scattering particle size distribution data (inset); particles were made using the recipe in Table 1, [SDS] = 8 mM, [KPS] = 5 mM, T = 60 °C; dz ≅ 182 nm and PDI ≅ 0.16.

image of the polymer particles in a latex prepared using 8 mM SDS. A relatively broad distribution is observed, consistent with the PDI of approximately 0.16, measured using DLS. Size distributions are classified monodisperse or broad depending on whether the polydispersity was below 0.02 or above 0.08, respectively. The intermediate range of 0.02 to 0.08 is considered narrow.33 On the basis of this classification, the D

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Macromolecules size distributions of the latexes reported in Table 2 can be inferred to be broad in general. The PDI showed a nonmonotonic trend in its dependence on SDS concentration. It increased with an increase in [SDS] up to 40 mM, and then decreased when [SDS] was further increased. Figure 2 shows a plot of the Z-average particle diameter versus SDS concentration. A smooth trend, as shown by the

Figure 3. Dependence of Z-average particle diameter in the PETMP/ TTT thiol−ene latexes on initiator concentration ([SDS] = 2 mM, T = 60 °C).

Figure 4 shows a parity plot for the measured Z-average particle diameters and the particle diameters predicted by eq 1.

Figure 2. Dependence of Z-average particle diameter in the PETMP/ TTT thiol−ene latexes on surfactant concentration ([KPS] = 5 mM, T = 60 °C).

dashed curve, is observed. Within the limits of uncertainties of measurements, there is no difference in the order of dependence of the particle diameter on SDS concentration, below and above the cmc. The order of dependence of the particle diameter on [SDS] was −0.44. In order to study the effect of initiator concentration, reactions were performed at different concentrations of KPS, at a fixed concentration of SDS. Table 5 summarizes the results of Figure 4. Parity plot for the reported correlation. Adjust R2 value for the fit was 0.96, and the root-square mean error in prediction was about 20 nm.

Table 5. Effect of KPS Concentration on Particle Diameter in PETMP/TTT Thiol−Ene Latexesa [KPS] (mM)

[SDS] (mM)

2.5 4 5 6 8 10

2 2 2 2 2 2

dz (nm) 509 399 355 293 280 237

± ± ± ± ± ±

53 7 14 24 22 11

PDI 0.09 0.07 0.03 0.07 0.06 0.11

± ± ± ± ± ±

A good agreement, with a coefficient of determination, R2 = 0.966, was obtained. The observed orders of dependence of the particle diameter on surfactant and initiator concentrations are higher than those predicted by the Smith−Ewart kinetics of conventional EP, which predicts a value of −0.2 for the surfactant concentration and −0.133 for the initiator concentration. (Note that the Smith−Ewart kinetics predicts that the number concentration of polymer particles, Np, is proportional to surfactant concentration raised to a power of 0.6 and initiator concentration raised to a power of 0.4. The fact that the final diameter of the polymer particles is inversely proportional to Np1/3 leads to the −0.2 and −0.133 order dependences stated above.) When surface charge density of the polymer particles, determined by the surface concentration of the adsorbed surfactant molecules, is the factor determining the final particle concentration in the latex, the particle diameter is expected to be proportional to surfactant concentration raised to a power of −1.36 The value of −0.44 observed in the present work is lower than this value. The relatively high order of dependence of the particle diameter on the initiator concentration is noteworthy and could be due to side reactions of the initiator with the thiol monomer and ionic strength effects that influences particle nucleation in the aqueous phase. For example, H-abstraction from thiol groups by the sulfate radicals may be a primary

0.01 0.03 0.01 0.03 0.04 0.04

a

See Table 1 for the amounts of monomers and water in the reaction recipes.

these experiments for [SDS] = 2 mM. The order of dependence of the particle diameter on [KPS] was −0.53 (see Figure 3). No clear trend in the variation of PDI with KPS concentration was observed. More reactions were performed by varying [KPS] while keeping [SDS] fixed at 4 and 8 mM. On the basis of Z-average particle diameter data from 25 different reactions, the following correlation was obtained. dZ = (1090 ± 80)[KPS]−(0.53 ± 0.04) [SDS]−(0.44 ± 0.03)

(1)

In this equation, dZ is in nanometers, and [KPS] and [SDS] are the initiator and surfactant concentrations, respectively (in mM). The values of the fit parameters are given along with the standard uncertainties. E

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Table 6. pH, ζ-Potential, Effective Particle Diameter, and Polydispersity of the Linear Thiol−Ene Latexes Synthesized Using the Recipes Given in Table 2 TMPDAE/TEGDT/HDT (mol %) pH ζ-potential (mV)a dZ (nm)a PDIa Mn (g/mol)b Đb Tg (°C)c

50/50/0 7.4 ± 0.5 −37 ± 8 950 ± 25 0.005 ± 0.001 3600 2.6 ± 0.1 −55 ± 2

50/37.5/12.5 8.1 ± 0.5 −35 ± 5 570 ± 50 0.005 ± 0.001 4500 3.1 ± 0.1 −57 ± 2

50/25/25 7.9 ± 0.5 −55 ± 5 410 ± 60 0.005 ± 0.001 5900 3.3 ± 0.1 −60 ± 2

50/12.5/37.5 7.5 ± 0.5 −58 ± 5 360 ± 60 0.005 ± 0.001 5800 3.8 ± 0.1 −59 ± 2

50/0/50 8.0 ± 0.5 −49 ± 4 330 ± 40 variabled 5000 3.7 ± 0.1 −37 ± 2

Measurements were typically made after 1 day of synthesis. bRelative to polystyrene standards in THF solvent at 30 °C. cHeating rate of 10 °C/ min. dSee text for discussion.

a

particles aggregated during storage, with the PDI ranging from 0.005 on the day of synthesis to up to about 0.25 after a week. The aggregation is evidently due to the hydrophobic effect, arising from the hydrophobic nature of the copolymer. In contrast, the PDI of the hydrophilic TMPDAE/TEGDT copolymer latex remained relatively stable over this period. An analysis of the monomer droplet size in the reaction mixture, before the start of polymerization, showed that the average droplet diameter was significantly lower in the TMPDAE/TEGDT emulsion (about 500 nm) than in the TMP/HDT emulsion (about 20 μm). The smaller droplet size in the TMPDAE/TEGDT system is because of the watersoluble nature of the TEGDT comonomer. The bulk of this monomer would preferentially be present in the aqueous phase, leaving mostly TMPDAE in the monomer droplet phase (and, perhaps, cosolubilizing TMPDAE in the aqueous phase). Clearly, the presence of the two monomers in different phases would have a strong effect on copolymer formation and particle nucleation and growth. In the case of the less water-soluble HDT monomer, both monomers would be present predominantly in the monomer droplet phase, yielding larger monomer droplets. Gel permeation chromatography was used to characterize the molecular weights of the linear polymers. The number-average molecular weights reported in Table 6 are significantly lower than those observed in conventional EP but are similar to the values seen in step-growth polymerization. The dispersity of the molecular weight distribution is, however, higher compared with the theoretical asymptotic value of 2 for a polycondensation reaction. All of the linear copolymers exhibited low glass transition temperature values, in the range of −62 to −35 °C. Because of the low Tg and liquid-flow behavior of the polymers at room temperature, we were unable to use the SEM technique for characterization of particle size and distribution (see Figure S2). However, the trends in particle size distributions, discussed above, were confirmed by measurements using another dynamic light scattering instrument, Malvern ZetaSizer ZS, that uses a 633 nm laser and 173° scattering angle. 3.4. Cross-Linked Latexes with Narrow Size Distribution. Cross-linked thiol−ene latex particles comprising of TMPDAE and TEGDT could be synthesized by incorporating the tetrafunctional thiol, PETMP, in the TMPDAE/TEGDT emulsion polymerization reaction. In initial experiments, all three monomers, TMPDAE, TEGDT, and PETMP, were mixed together at the start of the polymerization. The particle size distributions were relatively broad (PDI > 0.10). To lower the polydispersity of the TMPDAE/TEGDT/PETMP crosslinked latexes, the strategy of delayed addition of the PETMP

initiation mechanism in these polymerizations; by comparison, sulfate radicals add to chain-growth vinyl monomers to initiate conventional EP reactions. Unlike the relatively water-insoluble monomers such as styrene and n-butyl methacrylate that are common in conventional EP, the monomers of the present study are soluble in water to significant concentrations. Hence, water solubility of the comonomers is expected to play an important role in determining the number of particles nucleated and consequently the diameters of the particles in the final latex.38 The aqueous phase is expected to be the predominant site of particle nucleation (compared to the micellar phase) in the case of relatively water-soluble monomers, even when the surfactant concentration is significantly above the cmc. Indeed, the incorporation of a water-soluble comonomer, tri(ethylene glycol) dithiol (TEGDT), in the reaction recipe was found to have a dramatic effect on the particle size distribution in the final latex (vide inf ra). 3.3. Effect of Copolymer Composition on Particle Size in TMPDAE/HDT and TMPDAE/TEGDT Linear Copolymer Latexes. Latexes of linear (un-cross-linked copolymers) were obtained by EP of TMPDAE, HDT, and TEGDT, using the recipes shown in Table 2. A surfactant concentration of 8 mM and an initiator concentration of 5 mM were used. Five different polymer compositions, with TMPDAE/TEGDT/ HDT molar ratios of 50/50/0, 50/37.5/12.5, 50/25/25, 50/ 12.5/37.5, and 50/0/50, were investigated. The total monomer concentration was approximately the same in all the latexes, ranging from about 10 to 9.3 wt % based on water. Table 6 gives the properties of the latex, namely, pH, ζpotential, dZ, and PDI, and the properties of the linear polymer in the latex (obtained by drying the latex), namely, the numberaverage molecular weight, Mn, and the molecular weight dispersity index, Đ, determined by GPC analysis of the tetrahydrofuran solutions of the linear polymers at 30 °C, and glass transition temperature, Tg, measured using DSC. The Z-average particle diameter was higher in the case of particles with higher TEGDT concentration because of an increase in the hydrophilicity of the copolymer and, therefore, a greater degree of swelling in water. However, unexpectedly, the addition of TEGDT to the recipe resulted in a significant decrease in the polydispersity. The TMPDAE/TEGDT copolymer particles formed a relatively stable latex with low PDI (∼0.005). A similar result could not be confirmed for the second linear copolymer system, TMPDAE/HDT, that did not contain any TEGDT. The PDI of the TMPDAE/HDT latex showed significant batch-to-batch variations, indicating colloidally unstable latex. In spite of the relatively high value of the magnitude of ζ-potential, the F

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Table 7. Properties of the TMPDAE/TEGDT/PETMP Thiol−Ene Latexes of Different Compositions Synthesized Using the Recipes Given in Table 3 xa dZ (nm PDI Tg (°C) a

0 950 ± 25 0.005 ± 0.001 −55 ± 2

0.05 650 ± 100 0.005 ± 0.001 −54 ± 2

0.50 880 ± 100 0.12 ± 0.04 n.d.b

0.90 670 ± 100 0.12 ± 0.05 n.d.

1.00 320 ± 20 0.16 ± 0.05 n.d.

x is the mole fraction of thiol groups from the PETMP cross-linker. bn.d. = not determined.

cross-linker was considered. Seed polymer particles were first synthesized by polymerizing the comonomers TMPDAE and TEGDT to obtain monodisperse latex. The molar concentration of TEGDT was kept lower than that of TMPDAE so that the polymer chains would be terminated by ene functionalities, for further cross-linking with PETMP. Table 3 gives the recipes for the synthesis of TMPDAE/ TEGDT/PETMP latexes with delayed addition of PETMP. Three different delayed addition (semibatch) reactions were examined, wherein 95, 50, and 10% of the thiol groups were from TEGDT and 5, 50, and 90% of the thiol groups were introduced by late addition of PETMP. These reactions correspond to the values of x equal to 0.05, 0.50, and 0.90, respectively, in the overall formulation. The total mass of the three comonomers was about 1 g and varied over a narrow range (1.007−1.149 g) for these values of x. Two reference latexesone not containing any PETMP cross-linker (x = 0) and the other not containing any TEGDT and in which all the PETMP was added at the start of the polymerization (x = 1) were also synthesized for comparison. The properties of these five latexes are given in Table 7. The cross-linked particles containing the lowest concentration of PETMP (x = 0.05) showed the smallest polydispersity. The effect of cross-linking is clearly evident in the hydrodynamic size of the particles. Without the cross-linker, the Z-average particle diameter was about 950 nm, and with the incorporation of a small amount of cross-linker (x = 0.05), the swelling of the particles by water was significantly lower, and the Z-average particle diameter was also lower (about 650 nm). There was no significant difference in the PDI values for both the un-cross-linked particles and the lightly cross-linked particles (in both cases, PDI ≅ 0.005). The Z-average particle diameter decreased and the polydispersity increased as more PETMP was added earlier during the course of polymerization. When the particles were prepared without using any TEGDT during the seed stage (that is, when all the thiol groups were from PETMP), dZ was about 320 nm and the PDI was about 0.16. When the particles were prepared with 95 mol % of thiol groups from TEGDT during the seed stage polymerization and 5 mol % of the thiol groups from the delayed addition of PETMP (x = 0.05), dZ was about 650 nm and the PDI was quite low (≅0.005). If instead of the delayed addition of PETMP all three monomers were mixed together at the start of the reaction (keeping the overall composition the same, that is, x = 0.05), dZ was 645 ± 100 nm and the PDI was 0.12 ± 0.06. Thus, although the average particle diameter remained approximately the same, the distribution was significantly broader (0.12 vs 0.005). Figure 5 summarizes the intensity-weighted particle size distributions in latexes reported in Table 7. It is evident from these plots that the distribution was narrower when the crosslinker (PETMP) concentration was lower and when the crosslinker was added late during the reaction, as already discussed.

Figure 5. Intensity-weighted particle size distribution in linear TMPDAE/TEGDT copolymer latex (a), the latexes of TMPDAE/ TEGDT copolymer cross-linked with 5% (b), 50% (c), and 90% (d) thiol groups from PETMP, and the cross-linked TMPDAE/PETMP copolymer latex (e).

4. CONCLUSIONS In this article, we have demonstrated that multiphase thiol−ene reactions using a water-soluble thermal initiator can be used to obtain polymer latexes with submicrometer particle sizes, similar to conventional emulsion polymerization. When a trifunctional ene monomer (TTT) was copolymerized with a tetrafunctional thiol (PETMP) in an aqueous emulsion, using potassium persulfate as the thermal initiator, the order of dependence of the Z-average particle diameter in the final latex on the initiator concentration was −0.53 and that on the concentration of the sodium dodecyl sulfate surfactant was −0.44. Such high dependence of particle diameters on the initiator concentration is unusual in conventional EP. The cross-linked thiol−ene particles had a relatively broad particle size distribution. Molecular weights of the polymers obtained by the reaction of difunctional ene and thiol monomers using the KPS and SDS system were characteristic of a step-growth polymerization mechanism, i.e., relatively low molecular weights. The particle size distributions in the linear thiol−ene copolymer latexes were relatively narrow compared with the cross-linked system. The Z-average particle sizes in the linear copolymer latexes were also, in general, larger because of higher swelling of the polar copolymers in water. Copolymer particles containing the hydrophilic tri(ethylene glycol)dithiol monomer were particularly large in size (and had better colloidal stability) compared with the particles containing the hydrophobic 1,6-hexanedithiol comonomer. Cross-linked thiol−ene terpolymer latexes with narrowly dispersed particle sizes could be obtained by delayed addition of the tetrafunctional thiol cross-linker. Copolymer latexes containing ene-terminated linear polymer chains were crosslinked by postaddition of PETMP. One of these latexes retained the narrow particle size distribution of the linear copolymer system, but the Z-average particle diameter was G

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significantly lower due to lesser water-swelling (limited by the cross-link points). The thiol−ene emulsion polymerization reported herein demonstrates the versatility of this click reaction in heterogeneous polymerization processes. The shift toward an EP-type of reaction has led to the ability to produce smaller polymer particles, with much less surfactant and agitation intensities used by other heterogeneous thiol−ene and thiol− yne polymerizations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02228. NMR spectra; SEM images (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected], Tel +1-315-268-6661, Fax +1315-268-6654. *E-mail [email protected], Tel +1-315-268-2393, Fax +1315-268-6610. ORCID

Devon A. Shipp: 0000-0002-8709-1667 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ngoc-Tram Le for assistance in dynamic light scattering measurements. We also thank the Center for Advanced Materials Processing, a New York State Center for Advanced Technology, and the Department of Chemistry and Biomolecular Science at Clarkson University, for support.

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REFERENCES

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