Letter pubs.acs.org/macroletters
Observed Photoenhancement of RAFT Polymerizations under Fume Hood Lighting Laura P. da M. Costa,†,‡ Thomas G. McKenzie,†,‡ Kyra N. Schwarz,‡ Qiang Fu,† and Greg G. Qiao*,† †
Polymer Science Group, Department of Chemical and Biomolecular Engineering and ‡Ultrafast and Microspectroscopy Laboratories, School of Chemistry, The University of Melbourne, Parkville, VIC 3010, Australia S Supporting Information *
ABSTRACT: Given the recent findings of exogenous radical initiator/catalyst-free reversible addition−fragmentation chain transfer (RAFT) radical polymerization under both UV and visible light irradiation, the effect of standard laboratory lighting conditions (fluorescent tube lights) on traditional RAFT reactions, that is, those conducted in the presence of a thermally activated radical initiator, remains unknown. This is investigated in the current study, where a significant “photoenhancement” is observed for most cases under typical RAFT reaction conditions, indicating that fume hood lights can contribute to the generation of radicals in RAFT reactions. Given the observed emission spectrum of a typical fluorescent light source, the photoenhancement is proposed to occur through a visible light activation pathway. These findings are crucial for ensuring maximum reproducibility of controlled polymerizations conducted in the presence of typical sources of irradiation encountered in a standard chemical laboratory.
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photochemical iniferter/RAFT process.13−18 Our group19,20 (and others21,22) have recently demonstrated that most RAFT agents, and particularly trithiocarbonates, can also be photoactivated under visible light (λ > 400 nm) to induce a radical polymerization process. This allows for a simple and convenient method of photocontrolled radical polymerization with high chemical fidelity, and supplements other techniques that include derivatives of the main RDRP protocols, namely, photo-RAFT/iniferter,13−18,23−41 photo-ATRP, 42−53 and photo-NMP (or “NMP2”),54−58 as well as other means of activating thiocarbonylthio compounds toward polymerization.59 Given the demonstrated photoactivation of various RAFT agents under visible and UV light in the absence of added thermal or redox radical initiators, we aimed to assess the contribution of these photochemical pathways in a traditional RAFT system (i.e., employing an added radical initiator) under typical RAFT reaction conditions. The aim of the investigation was therefore to determine the effect of laboratory lights on the kinetics of a RAFT polymerization, which, to the best of our knowledge, represents the first such study for RAFT reactions. The combination of monomer, RAFT agent, and a thermally activated radical initiator were judiciously selected. Methyl methacrylate (MMA) and methyl acrylate (MA) were employed as model monomers together with a 4-cyano-4(((dodecylthio)carbonothio)pentanoic acid (CDPA) RAFT agent, as the latter is considered a good chain transfer agent
number of recently developed photocontrolled reversible deactivation radical polymerization (photo-RDRP) methods1−3 have strikingly similar reagents to traditional (i.e., nonphoto) approaches, thus, raising the following question: what is the contribution of photochemical pathways in these traditional systems when no attempt is made to protect the reaction mixture from environmental light sources? Matyjaszewski et al. have investigated the significance of ambient light on an “initiators for continuous activator regeneration” (ICAR) ATRP system and concluded that it has a negligible influence on the rate of polymerization unless atypically low concentrations of the Cu(I)-regenerating agent are employed.4 A more detailed investigation was carried out by the Jordan group, where various (yet typical) ATRP-type reactions were assessed, including standard Cu(I) ATRP, ATRP with a Cu(I)/Cu(II) catalyst system, and systems of activators (re)generated by electron transfer (A(R)GET-ATRP).5 They found that under standard laboratory lighting a photoenhancement effect can be clearly observed under certain reaction conditions that are not atypical for the given type of ATRP reaction. Hence, the effect of photochemistry in these reactions can be thought of as an example of competitive equilibria6 that cannot be neglected. Since its discovery in 1998,7 reversible addition−fragmentation chain transfer (RAFT) polymerization8 has been steadily growing in popularity due to its ease of implementation and versatility. Essential to ensuring good control in a RAFT polymerization is the use of a suitable thiocarbonylthiocontaining compound that acts as an efficient chain transfer agent (or “RAFT agent”).9 It has long been established that, similar to the pioneering work of Otsu10−12 with dithiocarbamate “iniferters” (initiator-transfer agent-terminator), RAFT agents can often be activated via UV irradiation to induce a © XXXX American Chemical Society
Received: October 31, 2016 Accepted: November 1, 2016
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DOI: 10.1021/acsmacrolett.6b00828 ACS Macro Lett. 2016, 5, 1287−1292
Letter
ACS Macro Letters for both monomer types. The initiator 2,2′-azobis-isobutyronitrile (AIBN), a ubiquitous initiator for RAFT reactions, was used for all cases. A reaction temperature of 70 °C was chosen, with N,N′-dimethylformamide (DMF) as the reaction solvent, together representing widely used, or “standard”, RAFT reaction conditions. Anticipating that any photoenhancement would be more pronounced at slower reaction rates,5 we initially studied RAFT polymerizations containing a low amount of radical initiator. Although AIBN (and some products of decomposition) can absorb light in the ultraviolet (UV) wavelength range,60 the extinction coefficient is low in the visible region (Figure S3). Therefore, under the current experimental conditions photolysis of the RAFT agent is assumed to be the primary pathway for a photochemical contribution to the reaction. The generation of additional carbon-centered radicals in solution from photolysis would increase the instantaneous radical concentration, manifesting as an increase in the observed rate of polymerization. The initiation pathways via both thermal and photochemical radical formation are illustrated in Scheme 1. Other fixed conditions Scheme 1. Possible Initiation Pathways for Traditional RAFT Reaction without Shielding from Light
Figure 1. Different lighting scenarios employed in the current study: (a) fume hood light (FHL) switched on, together with standard lab lighting (i.e., ceiling lights) (H); (b) reaction flask contained in the dark by covering with foil (D); and (c) no attempt made to exclude ambient lab light (from ceiling lights, etc.), but fume hood light kept off for duration (A). (d) RAFT agent, monomers, and radical initiator employed for all experiments.
approximately 1.2 m from the fume hood light source. It should be noted that no direct sunlight enters the workspace at any time during the day, and blinds typically cover all external windows. The illumination at the working distance was measured using a digital lux meter to give a value of 0.327 lm/cm2 when the fume hood light was on, compared with a value of 0.006 lm/cm2 when the fume hood light was off (i.e., for the “ambient lab light (A)” condition). An emission spectrum of the fluorescent tube lights was obtained using an optical fiber with a working range of 200−900 nm coupled to a spectrometer (MayaPro2000), processed using the Ocean Optics SpectraSuite software package. As seen in Figure S1, minimal emission is detected at wavelengths in the UV region (λ < 400 nm), while a significant broad emission centered at 450 nm is clearly observable, along with multiple sharp peaks throughout the visible wavelength region (400−700 nm). Using a hand-held visible light meter (Solarmeter, Model 9.4) with a maximum sensitivity (λmax) at 450 nm the intensity at the working distance was below the detection limit, that is,
