Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 4203−4213
pubs.acs.org/IECR
How Do Reaction and Reactor Conditions Affect Photoinduced Electron/Energy Transfer Reversible Addition−Fragmentation Transfer Polymerization? Pierce N. Kurek,† Alex J. Kloster,† Kyle A. Weaver,† Rodrigo Manahan,‡ Michael L. Allegrezza,† Nethmi De Alwis Watuthanthrige,† Cyrille Boyer,*,‡ Jennifer A. Reeves,*,† and Dominik Konkolewicz*,† †
Department of Chemistry and Biochemistry, Miami University, 651 E High St., Oxford, Ohio 45056, United States Centre for Advanced Macromolecular Design and Australian Centre for NanoMedicine, School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia
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ABSTRACT: The impact of conditions was investigated on a model photoinduced electron/energy transfer reversible addition−fragmentation chain transfer (PET-RAFT) polymerization. Within the cylindrical geometries studied, with relatively small changes in path length, the impact of reaction vessel dimensions and dilution was relatively small on the polymerization kinetics and control of the polymerization. This suggests that PET-RAFT can be relatively insensitive to small changes in reactor geometry and reaction volume when cylindrical systems are used. The intensity of the photoreactor was a key factor in determining reaction rate, with an approximate 1/2 order scaling of the apparent rate with intensity. Reactant concentration ratios were also important, with an approximate 1/2 order of the apparent rate with the photocatalyst loading and an approximate −1/2 order scaling apparent polymerization rate coefficient with the RAFT agent concentration. However, there is a limit to rate increases with higher Ir catalyst loadings due to the optical density.
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INTRODUCTION Polymerization reactions are important industrial processes with industrialized nations consuming close to 100 kg/person/ year of polymer-based materials.1 With this high demand for plastics and polymers it is important to have efficient and optimized reactions for the synthesis of polymers. Mechanistic studies and investigations of the reaction kinetics can provide models that enable the outcomes of reactions to be predicted for a targeted polymer property.2,3 In particular, investigating how the reactor and reaction conditions impact material properties is important for efficient polymerization.2,4,5 Radical polymerization is among the most commonly used processes for creating high molecular weight polymers due to the simplicity of the process, compatibility with monomer functionality, and tolerance to trace impurities.5 However, poor control over polymer microstructure and broad molecular weight distributions have been a traditional limitation of conventional, free radical polymerization.3 To overcome the limitations on polymer structure present in conventional radical polymerization, reversible deactivation radical polymerization (RDRP) methods have been developed.4,6,7 These RDRP methods include nitroxide-mediated polymerization,8,9 atom-transfer radical polymerization,10−12 and reversible addition−fragmentation chain transfer polymerization (RAFT).13−16 In all cases, a dynamic equilibrium © 2018 American Chemical Society
between propagating radicals and dormant organic or polymeric compounds is maintained throughout a RDRP process, enabling control over the polymer and near uniform growth of each chain.17 Of the RDRP processes, RAFT has received significant interest due to its excellent compatibility with a wide range of functional monomers and its ability to be implemented under simple and sometimes near ambient conditions.15 A key feature of RAFT is that an external and continuous source of radicals is necessary to maintain polymerization and offset unavoidable radical termination. Typically, this is achieved using a thermal radical initiator, although recent efforts have focused on photochemical radical generation approaches, since this can lead to well-defined polymers created under mild near ambient conditions.18−20 Of the photochemical approaches to RAFT polymerization, the photoinduced electron/energy transfer (PET)-RAFT process has gained substantial attention.21 In one particular formulation, a photoredox catalyst, tris[2-phenylpyridinatoC2,N]iridium(III) (Ir(ppy)3), is used to generate radicals form the excited state IrIII complex interacting with RAFT chain Received: Revised: Accepted: Published: 4203
December 29, 2017 February 23, 2018 March 1, 2018 March 13, 2018 DOI: 10.1021/acs.iecr.7b05397 Ind. Eng. Chem. Res. 2018, 57, 4203−4213
Article
Industrial & Engineering Chemistry Research
Scheme 1. Proposed Mechanisms of PET-RAFT: (A) Gives a Proposed Energy Transfer Mechanism, (B) Gives the Proposed Electron Transfer Mechanism, (C) Shows RAFT Degenerative Exchange, and (D) Shows Radical Loss Pathwaysa
a
Note that the X radical in (D) is a carbon-centered radical.
(MA) catalyzed by Ir(ppy)3 using trithiocarbonate CTAs is used in this investigation of how external and intrinsic conditions impact photochemical polymerizations.
transfer agents (CTAs). Two potential mechanisms are the energy transfer mechanism in Scheme 1A and the electron transfer pathway in Scheme 1B. The mechanism in Scheme 1A is an energy transfer pathway, where the excited state Ir complex transfers energy to the RAFT chain transfer agent, facilitating homolytic cleavage of the carbon sulfur bond. The mechanism in Scheme 1B is an electron transfer process, where the excited state Ir complex transfers an electron to the CTA, facilitating radical generation from the RAFT chain transfer agent, similar to an atom transfer radical polymerization activation of an alkyl pseudo-halide. Once the radicals are generated, they can propagate or undergo degenerative exchange through the RAFT intermediate radical as shown in Scheme 1C, and finally they can participate in termination reactions either with other propagating radicals or through cross-termination reactions of the RAFT intermediate radical and other propagating radicals.22−25 In addition to Ir(ppy)3 other chromophores have been used in RAFT polymerization, including tris(2,2′-bipyridyl)ruthenium(II) chloride,26−28 porphyrin complexes,29−31 organic chromophores,32,33 and direct excitation of the RAFT end group34−40 or monomer.41 One key difference between photochemical processes and thermally driven reactions is the potential effect of optical density, which can impact the overall reaction rate when the scale of the reaction is altered. Optical density arises due to the absorbance of photons by molecules closer to the light source, decreasing the available light for molecules in the reaction vessel further from the light source.42−44 due to the widespread use of PET-RAFT and other photochemical RAFT processes, it is important to determine how the kinetics of the PET-RAFT process are affected by reaction conditions as well as reactor intensity and geometry, due to the optical density mentioned. Although flow approaches have been used to mitigate the potential variability due to the aforementioned dependence of the reactions on external parameters and reaction concentrations,45−52 these flow systems also involve a multiparameter optimization that involves light intensity, reactor geometry, and complex flow rates and regimes especially for non-Newtonian fluids.53 To study the effects of system parameters on PETRAFT, a systematic study with variations in both reactant concentrations and reactor parameters was conducted. A model batch system of PET-RAFT polymerization of methyl acrylate
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EXPERIMENTAL METHODS Materials. All reagents were obtained from commercial suppliers unless otherwise stated. All reagents were used as received unless otherwise stated. Tris[2-phenulpyridinatoC2,N]iridium(III) (Ir(ppy)3) was dissolved into a stock solution in N,N-dimethylformamide (DMF). 2-(Propionic acid)ylbutyl trithiocarbonate (PABTC) and 2-(propionic acid)yldodecyl trithiocarbonate (PADTC) were synthesized as outlined in the literature.54 Typical Polymerization of Methyl Acrylate under Blue Light in a Cylindrical Reactor. In a 20 mL glass vial equipped with a magnetic stirrer bar, methyl acrylate (2.50 g, 29 mmol), PADTC (0.051 g, 0.15 mmol), and tris[2-phenulpyridinato-C2,N]iridium(III) (0.095 mg, 0.00015 mmol) added from a stock solution of concentration 0.1 mg/mL in DMF (0.95 mL) were added and dissolved in 2.5 g (2.25 mL) DMSO. The reaction mixture was purged with argon for 15 min and placed inside the blue reactor with 440 nm wavelength and 11.6 ± 0.3 intensity and stirred for 2 h. Samples were taken out from the solution at allotted times in order to obtain kinetic data through NMR and SEC. Other experiments were performed with a similar approach but with different amounts of monomer, solvent, PADTC, or tris[2-phenulpyridinatoC2,N]iridium(III). General Procedures for Kinetic Studies of Polymerization of Methyl Acrylate with Ir(PPy)3. For reaction in the presence of [MA]:[PABTC]:[Ir(PPy)3] of 200:1:0.0002 (1 ppm Ir as a molar ratio to the MA monomer), a reaction stock solution consisting of MA (300 μL, 0.285 g, 3.31 mmol), PABTC (3.9 mg, MW: 238.39 g/mol, 0.0166 mmol), and Ir(PPy)3 (0.0022 mg, 3.32 × 10−6 mmol) dispersed in DMSO (300 μL) was prepared in a 2 mL glass vial covered in aluminum foil. The solution was placed in FTNIR quartz cuvette (1 cm × 2 mm). The FTNIR quartz cuvette was sealed with a septum before purging under nitrogen for 20 min. The sample was polymerized under blue (λmax = 460 nm) LED light irradiation, intensity 0.77 mW/cm2. Every 5 min, the sample was put into the holder manually, and each spectrum in the 4204
DOI: 10.1021/acs.iecr.7b05397 Ind. Eng. Chem. Res. 2018, 57, 4203−4213
Article
Industrial & Engineering Chemistry Research spectral region of 7000−4000 cm−1 was constructed from 10 scans with a resolution of 4 cm−1. The total collection time per spectrum was about 5 s. Ultraviolet−Visible Spectroscopy (UV−Vis). All UV−vis spectra were recorded on a CARY 5000 UV−visible−NIR spectrometer. Nuclear Magnetic Spectroscopy. Monomer conversion was determined by nuclear magnetic resonance (NMR). All NMR was performed on a Bruker 300 or 500 MHz spectrometer. Size Exclusion Chromatography Using Tetrahydrofuran (THF) as the Eluent. All size exclusion chromatography (SEC) was performed on an Agilent 1260 gel permeation chromatography system (GPC) equipped with an isocratic pump, a degasser, an autosampler, a guard, 2 × PL Gel Mixed B columns, and a refractive index detector. The eluent was tetrahydrofuran running at 1 mL/min at 30 °C. The system was calibrated with poly(methyl methacrylate) (PMMA) standards in the range of 617 000 to 1010. Molecular weights reported are relative molecular weights compared to the PMMA standards with no further correction. Fourier Transform near Infrared Spectroscopy. Online Fourier transform near-infrared (FTNIR) spectroscopy was used to measure the monomer conversions by following the decrement of the vinylic C−H stretching overtone of the monomer at ∼6200 cm−1. A Bruker IFS 66/S Fourier transform spectrometer equipped with a tungsten halogen lamp, a CaF2 beam splitter, and a liquid nitrogen cooled InSb detector was used. Spectra were analyzed with OPUS software. Construction and Characterization of Photoreactors. Custom made photoreactors were built by wrapping and attaching a 5, 2.5, or 1 m blue LED strip, in a helical pattern inside a metal cylinder of diameter 15 cm for the systematic study of reaction and reactor conditions. The emission spectrum of the blue LED photoreactor was characterized using a Princeton Instruments SP2300 300 mm spectrograph equipped with a 300 groove/mm grating blazed at 500 nm and a SpectraPro 512 × 512 BUV electron-multiplying CCD.35 The power intensity of each photoreactor was determined using a Thorlabs PM100A light power meter, equipped with a S120VC silicon photodiode.35 For the online FTNIR study, the polymerization was performed in quartz cell with a path of 0.2 cm. Blue LED (λ = 460 nm, with variable light intensities) was employed in these experiments. The cuvette was placed to receive light on only one side.
Scheme 2. PET-RAFT n of Methyl Acrylate Using Ir(ppy)3 and Trithiocarbonate CTA
Figure 1. (A) Photograph of typical photoreactor system used in this kinetic study with the two distinct reaction vessels. (B) UV−vis spectra of Ir(ppy)3 and the PADTC chain transfer agent.
well as reactor conditionsintensity, reaction vessel volume, and geometryon the outcomes of PET-RAFT. The ultraviolet−visible spectra of the PADTC RAFT agent and the Ir(ppy)3 photocatalyst are given in Figure 1B. Ir(ppy)3 has strong absorption across the UV, violet, and blue and even into the green region of the UV−vis spectrum, while PADTC has strong absorption in the UV, as well as a second peak near 440 nm. In any photochemical process, especially one with a photocatalyst, the impact of the catalyst loading must be evaluated. Initially, the impact of Ir(ppy)3 catalyst loading was investigated on the polymerization. Figure 2A demonstrates that as expected increases of the photocatalyst loading lead to an increase in reaction rate. In all cases acceptable agreement was observed between a linear semilogarithmic fit and the experimental data. At low catalyst loadings (
