Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Ring-Opening Metathesis Polymerization in Miniemulsion Using a TEGylated Ruthenium-Based Metathesis Catalyst Chunyang Zhu,† Xiaowei Wu,‡ Olena Zenkina,‡ Matthew T. Zamora,‡ Karen Moffat,§ Cathleen M. Crudden,*,‡ and Michael F. Cunningham*,†,‡ †
Department of Chemical Engineering, Queen’s University, 19 Division St., Kingston, Ontario, Canada K7L 3N6 Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6 § Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1 Macromolecules Downloaded from pubs.acs.org by UNIV OF TEXAS AT EL PASO on 11/05/18. For personal use only.
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ABSTRACT: Ring-opening metathesis polymerization (ROMP) of 1,5-cyclooctadiene (COD) in miniemulsion was conducted utilizing a water-soluble TEGylated ruthenium alkylidene catalyst that was designed to undergo phase transfer from the aqueous phase to the monomer droplets or polymer particles following activation. The catalyst yielded colloidally stable latexes with ∼100% conversion, often in less than 1 h. Kinetic studies revealed first-order kinetics with good livingness as confirmed by the shift of gel permeation chromatography (GPC) traces. Depending on the surfactants used, the particle sizes ranged from 100 to 300 nm with monomodal distributions. The more strained cyclic olefin norbornene (NB) could also be efficiently polymerized in miniemulsion with full conversion and without coagulum formation.
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INTRODUCTION Ruthenium-based metathesis catalysts are employed extensively in ring-opening metathesis polymerization (ROMP) due to their remarkable tolerance to air, moisture, and various functional groups.1 These features have propelled ROMPbased polymers into many fields such as biomaterials,2 liquid crystalline polymers,3 self-healing materials,4 degradable plastics,5 and nanocomposites.6 Unlike many other commercial polymerization processes that have been adapted to waterbased systems for both environmental and economic reasons, comparatively little attention has been given to extend ROMP into aqueous dispersed systems. Polymerization in aqueous dispersed systems offers numerous advantages.7 Introducing water as the continuous phase largely reduces the environmental impact and health concerns associated with using organic solvents, specifically with regard to the release of volatile organic compounds (VOCs) during large-scale syntheses. A dispersed system also improves heat transfer, decreases viscosity, and minimizes the cost of postreaction separations.8 Aqueous dispersions are most commonly used in free radical polymerizations,9 including reversible deactivation radical polymerization (RDRP).10−12 Emulsion polymerization, the most commonly used process to © XXXX American Chemical Society
prepare aqueous dispersions, accounts for approximately onehalf of all industrial polymers made by free radical polymerization. However, reports related to ROMP in dispersed systems are far less common.13−23 In the limited papers published to date, improvements are needed in terms of polymer yields, control of molecular weights, colloidal stability, and in some cases prevention of coagulum. In general, proofof-concept experiments conducted with low monomer-tocatalyst ratios (M:C) have limited industrial relevance. In early seminal work, Claverie et al. performed emulsion ROMP employing a hydrophilic Ru alkylidene 2 (Scheme 1) that was similar to Grubbs’ first-generation catalyst (1) but with water-soluble phosphines.21 Using catalyst 2, polynorbornene (PNB) nanoparticles (50−100 nm) were obtained by emulsion polymerization with high yield, but the resulting latex was prone to flocculation. Alternatively, by modifying the benzylidene moiety of 1 to yield catalyst 3 (Scheme 1), Quemener et al. synthesized PNB in aqueous media via miniemulsion ROMP.20 Miniemulsion polymerization is a variant of emulsion polymerization in which the monomer is Received: October 19, 2018
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DOI: 10.1021/acs.macromol.8b02240 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
module combined with a Waters 410 differential refractometer. THF was used as the eluent, and molecular weights were corrected according to Mark−Houwink parameters.37 Mark−Houwink parameters were used for polybutadiene (a = 0.74 and K = 0.000256 dL/ g).38 Particle size distributions were determined by dynamic light scattering (DLS) on a Zetasizer Nano ZS from Malvern Instruments at 25 °C with a 173° scattering angle. Samples were diluted in water by a factor of 10 prior to DLS measurement at room temperature to minimize multiple scatterings caused by high concentration. Synthesis of TEGylated Ru-Based Metathesis Catalyst. 2-(2(2-Methoxyethoxy)ethoxy)ethyl 4-Methylbenzenesulfonate (7). Triethylene glycol monomethyl ether (12.80 mL, 80 mmol) and ptoluenesulfonyl chloride (16.0 g, 84 mmol) were dissolved in dichloromethane (DCM), followed by the addition of powdered KOH (18.0 g, 320 mmol) slowly at 0 °C. After stirring for 5 h (below 5 °C), the reaction mixture was quenched with water. The product was extracted into DCM three times, washed with brine, dried over MgSO4, and used without further purification. Yield: 25.2 g, 99%. 1H NMR (CDCl3): δ 7.80−7.82 (dd, 2H), 7.34−7.37 (dd, 2H), 4.16− 4.19 (t, 2H), 3.69−3.71 (t, 2H), 3.60−3.63 (m, 6H), 3.53−3.55 (t, 2H), 3.38 (s, 3H), 2.56 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 144.77, 133.09, 129.81, 127.97, 71.91, 70.75, 70.56, 70.55 69.22, 68.67, 59.01. 21.62. HRMS (ESI) m/z: calcd for C14H22SO6, 318.1137; found, 318.1129. 4-(2-(2-(2-Methoxyethoxy)ethoxy)ethoxy)pyridine (8). 2-(2-(2Methoxyethoxy)ethoxy)ethyl 4-methylbenzenesulfonate (9.5 g, 30 mmol), CsCO3 (19.5 g, 60 mmol), and 4-hydroxypyridine (6.0 g, 60 mmol) were dissolved in dry dimethylformamide (DMF). The reaction mixture was refluxed for 48 h under argon, and DMF was removed under vacuum. Water and DCM were then added to the crude reaction mixture. The product was extracted into DCM three times, washed with brine, dried over MgSO4, filtered, concentrated, and further purified by silica gel column chromatography using 9% MeOH in DCM as eluent. Yield: 3.1 g, 43%. 1H NMR (CDCl3): δ 8.41 (dd 2H), 6.81 (dd 2H), 4.15 (t, 2H), 3.86 (t, 2H), 3.63−3.73 (m, 6H), 3.52−3.55 (tr, 2H), 3.36 (s, 3H). 13C{1H} NMR (100 MHz, CDCl3): δ 164.78, 151.07, 110.31, 71.91, 70.88, 70.64, 70.57, 69.31, 67.19, 58.99. HRMS (ESI) m/z: calcd for [M + 1] C12H20NO4, 242.13868; found, 242.13812. TEGylated Ruthenium-Based Catalyst (5). Grubbs’ secondgeneration catalyst (G2) (9) (40 mg, 0.05 mmol) was weighed into a 20 mL vial in a nitrogen-filled glovebox. Dry DCM (1 mL) was added to the vial, and the mixture was vigorously shaken to dissolve the catalyst. Compound 8 (180 mg, 0.75 mmol) was then added to the vial. A color change from brown to green was observed immediately. Pentane was added to the vial, and a green precipitate was observed. The vial was kept refrigerated in the glovebox for 12 h at −26 °C, and the pentane was then slowly decanted. The product was washed with pentane, dried under vacuum to give a green viscous oil, and further purified by silica gel column chromatography using 9% MeOH in DCM as eluent. Yield: 23 mg, 45%. 1H NMR (CD2Cl2): δ 19.16 (s, 1H, RuH); 8.45 (d, 4H, 2,6-pyrCH); 6.85 (d, 4H, 3,5-pyrCH); 7.72 (br s, 2H), 6.53 (br s, 2H, 3,5-MesCH); 7.65 (d, 2H,), 7.51 (t, 1H,), 7.09 (t, 2H, PhCH); 4.20 (br s, 4H, pyr−O−CH2); 3.87 (br s, 4H, pyr−O−CH2CH2); 3.53 (s, 6H), 2.34 (s, 6H, 2,6-MesCH3); 3.34 (s, 16H, O−C2H4−O), 3.75−3.50 (m, 16H). FTMS + p ESI Full ms; m/ z calcd for C52H70O8N4ClRu, 1051.3661; found, 1051.3687. Influence of Air and Moisture on Catalytic Performance of 5. The influence of air and moisture on catalytic performance was monitored by monomer conversion with 1H NMR under various conditions. A J-Young NMR tube with Teflon screw cap was charged with 0.5 mg of TEGylated catalyst (5) and 0.5 mL of anhydrous degassed CD2Cl2 in a glovebox to form a catalyst solution (9.5 × 10−4 mol/L). To this, 0.06 mL of 1,5-cyclooctadiene (COD) was added under argon to initiate the polymerization. The monomer conversion was monitored by comparing the vinyl peaks of COD (7.50 ppm) and 1,4-polybutadiene (poly-COD) (7.34 ppm). The polymerization was terminated by addition of ethyl vinyl ether after 20 min. A series of parallel experiments with alternative solvents such as degassed CD2Cl2
Scheme 1. Ruthenium-Based Metathesis Catalysts
dispersed in submicrometer droplets by a high-shear emulsification procedure that are directly nucleated to give particles, whereas in emulsion polymerization particles are created by a complex nucleation process.24−27 Quemener et al. observed a large amount of coagulum, and the polymer properties were irreversibly modified by the incorporation of functional groups into the resulting polymer. Because these early studies were conducted with variants of first-generation metathesis catalysts, we sought to take advantage of the significant improvements in catalyst activity for the Grubbs family of catalysts that have taken place over the past 20 years.28−36 Because of its high activity, Grubbs’ third-generation catalyst (4) was chosen as the catalyst framework, which was also explored by Emrick et al., although their multistep synthetic strategy was complicated by the fact that the catalyst could not be purified from excess ligand.32,33 To accomplish this, we synthesized a TEGylated catalyst that was readily dissolved in the aqueous phase of a miniemulsion system, becoming hydrophobic upon the dissociation of TEGtagged pyridines after initiation of polymerization. The activated catalyst subsequently diffuses to the submicrometer monomer droplets, where the remainder of the polymerization occurs. Latex particles ranging from 100 to 300 nm were produced with ∼100% conversion. The reaction kinetics, evolution of molecular weight, and the colloidal characteristics of the miniemulsion polymerization are described in detail. The effects of various surfactants, catalyst loadings, and monomer content will also be discussed.
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EXPERIMENTAL SECTION
Materials. All reagents were purchased from Sigma-Aldrich and used without further purification unless otherwise specified. Grubbs’ second-generation catalyst was received from Materia Inc. and was used without purification as the starting material for the preparation of our version of the third-generation catalyst. All solvents and monomers used in this study were dried, distilled using standard procedures, and degassed with three freeze−pump−thaw cycles. They were then stored in a glovebox filled with nitrogen (
