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Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor
European Polymer Journal xxx (2016) xxx–xxx
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Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor James Gardiner a,⇑, Christian H. Hornung a, John Tsanaktsidis a, Duncan Guthrie b a b
CSIRO Manufacturing, Bag 10, Clayton South, Victoria 3169, Australia Vapourtec Ltd, Park Farm Business Centre, Bury St Edmunds IP28 6TS, United Kingdom
a r t i c l e
i n f o
Article history: Received 18 December 2015 Received in revised form 19 January 2016 Accepted 20 January 2016 Available online xxxx Keywords: RAFT polymerisation Photo-initiated polymerisation Continuous flow Photochemical reactor
a b s t r a c t The RAFT (Reversible Addition–Fragmentation Chain Transfer) approach allows for greatly enhanced control over radical polymerisation processes, resulting in polymers with low dispersity. Classically, RAFT polymerisations are conducted in batch using thermal initiators. Herein, we describe a novel photo-initiated RAFT polymerisation procedure, using a tubular continuous flow reactor for the polymerisation of (meth)acrylates and acrylamides at close to ambient temperatures. This approach makes use of the excellent light penetration properties of millimetre-size fluoropolymer tubing, enabling the synthesis of multigrams/kgs of RAFT polymer per day. A tubular photochemical reactor presents a very practical and compact design for the continuous manufacture of RAFT polymers in quantities that would otherwise be difficult to achieve in a batch system using photo-initiation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The Reversible Addition–Fragmentation Chain Transfer (RAFT) method is a controlled radical polymerisation technique which enables the synthesis of polymers with well-defined architecture and narrow molar weight distribution. RAFT exhibits great tolerance to reaction conditions, it is compatible with most monomers, and can provide polymers with defined structures and end-groups [1–7]. In recent years ambient temperature processes for RAFT polymerisation, using photo-initiators instead of the conventional thermal Vazo initiators, were conducted by several researchers [8–13]. This method presents an interesting alternative for systems containing temperature sensitive components or where initiation by visible or UV light is preferred over thermal initiation. Continuous flow processing is becoming an increasingly disruptive technology in a range of chemical manufacturing sectors and a series of different innovative flow reactor designs for laboratory and pilot or production scale have been developed over the past few years. These new continuous flow reactors consist of either tubular or plate-type modules in which the chemical reactions take place under well-defined flow conditions. Flow reactors offer many advantages over conventional batch reactors, including enhanced process control, higher yields & purities, shorter reaction times, better scalability, smaller physical footprint and superior inherent safety [14–25]. Recently, the use of continuous flow reactors for photo-initiated controlled radical polymerisations has been reported in the literature by several different research groups. Johnson and co-workers have performed continuous flow living radical polymerisations of acrylates and acrylamides using trithiocarbonate iniferters, an organic photo-redox catalyst and visible
⇑ Corresponding author. E-mail address: [email protected] (J. Gardiner). http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033 0014-3057/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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light. Their experimental set-up consisted of fluoropolymer tubing, syringe pumps and a compact fluorescent lamp, with which it was possible to synthesise homo-polymers and block copolymers with low dispersities between <1.1 and 1.4 [26,27]. Junkers and co-workers demonstrated the use of microfluidic photochemical reactors for copper-mediated, and later cobalt-mediated, radical polymerisations in continuous flow. Fast polymerisations of vinyl acetate and methyl acrylate under UV irradiation were achieved without loss of polymerisation control [28,29]. Most recently Hawker and co-workers conducted continuous flow experiments for the synthesis of poly(methyl methacrylate) with dispersities around 1.2 via a light-mediated controlled radical polymerisation. This study compared different widely available fluoropolymer tubing materials for use in a photo-induced polymerisation [30]. The major benefit of using continuous flow tubular reactors for photo-initiated chemistry is the ability to efficiently capture the maximum number of photons by constructing the reactor around the lamp and providing a fluidic chamber with a short light penetration pathway. The scaling up of traditional photoinduced batch reactions suffer significantly from the so-called ‘Beer–Lambert penalty’ where a strong light gradient exists such that light is absorbed only at the exposed surface layers of the solvent. Continuous flow reactors overcome this by utilising narrow optically clear tubing that allows for significantly enhanced light efficiency and mixing. The feasibility as well as benefits and disadvantages of continuous flow processing for photochemical reactions have been demonstrated previously by Booker-Milburn, Oelgemöller and others [31–36]. In this paper, we present work conducted on a novel, commercially available photochemical reactor, the Vapourtec UV-150 [37,38], for the UV-initiated continuous flow synthesis of RAFT polymers. The tubular photochemical reactor can be operated with one of two light sources, either a high intensity medium pressure metal halide lamp or an LED array. This study is the first to investigate the effect of different wavelength UV radiation in combination with different photo-initiators on the performance of the continuous flow RAFT polymerisation reaction and the quality of the polymer product. A series of different monomers were successfully applied to this procedure, using different reagent ratios and process conditions. By tuning the wavelength so the thiocarbonylthio core of the RAFT agent was excited, it was also possible to conduct initiator free polymerisations, whereby the chain transfer agent itself acted as the source of initiating radicals starting the polymer chain growth, i.e. as an iniferter. The continuous flow results were then compared to conventional batch procedures using thermal initiators.
2. Experimental 2.1. Materials and analysis The RAFT agent 1 (see Fig. 1) was obtained from Boron Molecular. The polymerisation initiators azobis-(cyclohexanenitrile), 2 (supplied by DuPont as Vazo88), 2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone, 3 (supplied by Ciba as Irgacure 369), and phenyl bis (2,4,6-trimethyl benzoyl) phosphine oxide, 4 (supplied by Ciba as Irgacure 819) were used without further treatment (see Fig. 1). The monomers methyl acrylate (MA), methyl methacrylate (MMA), N,N-dimethyl acrylamide (DMA) and N-isopropyl acrylamide (NIPAM) were pre-treated using polymer resin (for removal of hydroquinone and monomethyl ether hydroquinone, Sigma Aldrich, Cat. No: 31,133-2) in order to remove the polymerisation inhibitor. The solvent acetonitrile (MeCN) was obtained from Merck KGaA and used without further purification. Reaction conversions were calculated from 1H NMR spectra. For calculating the conversion of polymerisation, 1,3,5trioxane was used as an internal standard. 1H NMR spectra were recorded on a Bruker AC-400 spectrometer in deuterated chloroform (solvent residual as internal reference: 7.26 ppm). Average molecular weight of the polymer, Mn, and its dispersity, Ð, were measured using size exclusion chromatography (SEC). SEC (DMAc) was performed on a Shimadzu instrument
Fig. 1. RAFT agent and initiators.
Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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equipped with a CMB-20A controller system, a SIL–20A HT autosampler, a LC-20AT tandem pump system, a DGU-20A degasser unit, a CTO-20AC column oven, a RDI-10A refractive index (RI) detector, and 4 Styragel (Waters) columns (HT2, HT3, HT4 and HT5). N,N-Dimethylacetamide (DMAc) (containing 4.3 g/L LiBr) was used as eluent at a flow rate of 1 mL/min. The column temperature was set to 80 °C and the RI detector temperature was set to 40 °C. The SEC was calibrated with low dispersity poly(MMA) standards, and molar mass are reported as poly(MMA) equivalents. Mn and Ð were evaluated using Shimadzu software (LabSolutions version 5.63). A 3rd order polynomial was used to fit the log M vs. time calibration curve, which was approximately linear across the molar mass range of interest. 2.2. Batch RAFT polymerisation using thermal initiation A starting material solution of 595 mg monomer, 5 (DMA), 2.93 mg initiator 2, 24.22 mg RAFT agent 1, in 1.382 mL MeCN (total volume = 2 mL), was premixed and degassed using nitrogen purging. The polymerisation was conducted on a laboratory microwave reactor (Biotage Initiator) at 100 °C with a reaction time of 1 h. A yellow viscous polymer solution was obtained after reaction, from which conversion was determined by 1H NMR and Mn and Ð were obtained by SEC (see Scheme 1). 2.3. Continuous flow RAFT polymerisation using photo-initiation A Vapourtec UV-150 photochemical reactor was used to carry out photo-initiated RAFT polymerisation reactions under continuous flow conditions. Transmission spectra of the UV filters used to control the wavelength of light exposed to the reactor coil are shown in Fig. 2. Reaction temperatures were selected based on the operating parameters of the lamp and filter combinations and the achievable temperatures of the cooling system. To cool the reactor coil to the required temperature and to also overcome excessive oxygen permeability of the tubing, the UV-150 reactor coil was continually flushed with pre-cooled nitrogen (passed over dry ice) via a regulator valve. As RAFT polymerisations are traditionally oxygen sensitive, an insufficient flow of nitrogen around the reactor coil was found to result in little or no polymerisation occurring. MeCN was used as solvent due to its low UV cut-off wavelength (200 nm) and high auto-ignition temperature (524 °C) under UV conditions. The polymerisation was conducted on a Vapourtec R2/R4 flow reactor system using the UV-150 photochemical reactor module [37,38]. The reactor module can be operated with different light sources, either a 150 W mercury lamp or a range of LED arrays. For all herein presented experiments, the mercury lamp was used in combination with one of four interchangeable UV filters. Filter 2 is a band pass filter with transmission between 270 nm and 400 nm. Filter 3 is a long pass filter with transmission above 310 nm. Filter 4 is a band pass filter with transmission in the range 330–380 nm. Filter 6 is a band pass filter with transmission in the range 350–590 nm. Total area of the reactor coil exposed to the light source was 129 cm2. The use of a 365 nm LED with the UV-150 photochemical reactor was also investigated at first, however the mercury lamp produced better initial results; hence all further work was focused on this light source. The 365 nm LED has 1.7 times the photo flux that was available from the 150 W mercury lamp in combination with Filter 4, and the use of this new narrow band 365 nm LED light source appears worthy of further investigations in the future. The Vapourtec UV-150 photochemical reactor module uses reactor coils manufactured from PFA tubing with an ID of 1.3 mm (reactor volume: 10 mL). PFA is a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether, which exhibits acceptable transmittion in the UV and visible wavelengths when the wall thickness is kept below 0.3 mm. PFA was selected in preference to FEP (Fluorinated ethylene propylene) tubing solely for its increased durability. PFA and FEP have similar properties with high transparency and oxygen permeability values of 881 and 748 respectively (cm3/100 in.2 24 h atm mil 1). Other types of tubing such as Tefzel or Halar are less transparent but offer superior oxygen permeability properties with values as low as 100 and 25 respectively (cm3/100 in.2 24 h atm mil 1) [30]. The mercury lamp light source of the photochemical reactor can be variably tuned between 100% and 50% intensity. The photochemical reactor module was operated with a dry ice dewar to regulate the temperature between 30 and 40 °C. The flow rate was set to 2 mL/min resulting in a reaction time of 5 min. A 40 psi backpressure regulator was positioned inline after the reactor coil in order to prevent solvent from boiling. The following procedure is typical; it describes experiment 1.12 (Table 1, vide infra); all other continuous flow experiments in Tables 1 and 2 were performed analogously. A starting material solution of 1.487 g monomer, 5 (DMA), 11 mg initiator 3, 60.55 mg RAFT agent 1 and 25 mg 1,3,5-trioxane, in 3.454 mL MeCN (total volume = 5 mL), was premixed and degassed using nitrogen purging. A 2 mL sample was injected into the reactor via a sample loop, which was flushed with a constant stream of MeCN. These conditions were developed from previous work, which describes procedures for the thermally initiated continuous flow synthesis of RAFT polymers on Vapourtec R2/R4 reactors [39–41]. A yellow polymer solution was obtained after reaction, from which the conversion was determined by 1H NMR and Mn and Ð were obtained by SEC. 3. Results and discussion UV polymer curing has become a preferred method of curing in many industry processes [42]. As such we selected two industrially used photo-initiators, Irgacure 369, 3, and Irgacure 819, 4, (Fig. 1) for the polymerisation of N,N–dimethylacrylamide (DMA). Irgacure 369, 3, is commonly used in industry (e.g. graphic arts, electronics) for UV-curing of pigmented Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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Scheme 1. Solution-phase RAFT polymerisation using different monomers, initiators and initiation mechanisms.
Fig. 2. Spectra of the wavelengths of light emitted by the mercury lamp and that transmitted through Vapourtec UV filters 2, 3, 4 and 6.
Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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Table 1 Experimental conditions and results for the RAFT polymerisation of DMA in batch and continuous flow, optimisation of the photo-initiated flow process using different initiators and process conditions.
a b c
100% lamp intensity without filter corresponds to 150 W. 1Molar ratio of monomer to RAFT agent to initiator (M/R/I). 1MWs are given against pMMA standards.
Table 2 Experimental conditions and results for the photo-initiated RAFT polymerisation of MA, MMA and NIPAM in continuous flow; all experiments were performed using UV filter 4 at 100% lamp intensity (irradiance = 38 mW/cm2) and a reaction temperature of 40 °C.
a
Molar ratio of monomer to RAFT agent to initiator (M/R/I).
systems such as inks, coatings and varnishes, while Irgacure 819, 4, is especially suited for white pigmented formulations, glass fibre reinforced polyester/styrene systems, and clearcoats. Both photo-initiators are efficient sources of radicals under UV conditions with UV absorption spectra in acetonitrile indicating local absorption maxima around 300–350 nm [43–45]. Results for the continuous flow photo-polymerisation of DMA in the presence of RAFT agent 1 are shown in Table 1. A comparison of the different UV filters and initiators used over a 30 min residence time is given in Fig. 3. In all cases polymerisation proceeded with varying degrees of control over conversion, molecular weight and polydispersity. Polymerisations carried out with photo-initiator 3 and Filter 3 gave high conversions (>90%) at 100% lamp intensity, with control of MW and polydispersities <1.5. Increasing the residence time led to higher conversions however some loss of control over molecular weight and polydispersity were also observed. An absorbance spectrum of RAFT agent 1 (not shown) indicates a local maxima at around 310 nm (see for example previous work [46]) suggesting that on exposure to UV light the RAFT agent itself has the potential to fragment via a radical mediated process. This is illustrated in a control experiment whereby DMA can be polymerised with 1 in the absence of any photo-initiator (entries 1.21 and 1.22, Table 1). Here, the RAFT agent acts as an iniferter, and is solely responsible for initiating, controlling and terminating the reaction. This suggests that loss of control Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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(a)
(d)
(b)
(e)
(c)
(f)
Fig. 3. Diagrams a–c: comparison of different UV filter and initiator combinations at 30 min reaction time, 40 °C, M:R:I = 100/1.0/0.2 and 100% lamp intensity (see also entries 1.3, 1.11, 1.13 and 1.15); diagrams d–f: effect of lamp intensity reaction time for polymerisations with filter 3, initiator 3, at 40 °C and M:R:I = 100/1.0/0.2 (see also entries 1.1–1.9).
during the RAFT mediated polymerisation reaction may be due to photo-degradation of the RAFT agent and subsequent scission or termination side reactions occurring during the polymerisation reaction. The ongoing photo-activation of the thiocarbonylthio core of the RAFT agent, leading to decomposition or partial removal of the RAFT end group during the polymerisation reaction, would explain an increase in the polydispersities of the resulting polymers. This effect has previously been documented when molar ratios of photo-initiators were used as a fast method for the removal of RAFT-polymer disulphide ester end groups [47]. Use of filter 4 (330–380 nm) to minimise potential absorption of RAFT agent around 310 nm resulted in slower monomer conversion with a 60 min residence time giving a similar result to that after 10 min using filter 3. Notably, the use of filter 4 with longer residence times gave lower polydispersities than with filter 3 for a given conversion, suggesting that the use of a narrower wavelength band was beneficial in limiting degradation of RAFT agent. Polymerisations carried out with photoinitiator 4 also gave good conversions however a notable increase in polydispersity was observed. Use of wavelengths above 350 nm (Filter 6, 360–390 nm) resulted in low conversions even at longer residence times, indicating inefficient activation of the photo-initiator (absorbance maxima between 300 and 350 nm). To speed up reaction times of the DMA polymerisation we explored the use of high RAFT:photo-initiator ratios. Traditionally, low initiator concentrations are used to elicit control over polymerisation reactions however for many processes it is the
Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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rate of reaction that is of interest so as to obtain the product in the shortest possible timeframe. In these cases low polydispersities are not necessarily paramount. To achieve these fast polymerisation times, we investigated the use of a 1:1 ratio of RAFT agent to photo-initiator (Table 1, entries 1.17 and 1.18). This resulted in high conversion, >99%, in as little as 5 min using filter 3 with either 3 or 4, with polydispersity of around 1.6. These higher values are presumably caused by increased chain scission/termination under the forcing conditions applied. Polymerisation of DMA was also carried out with photo-initiator 3 and 4, in the absence of RAFT agent (Table 1, entries 1.19 and 1.20). Complete conversion was observed within 5 min, however SEC analysis revealed polymers with bimodal distributions, high polydispersities, and a lack of molecular weight control, thereby illustrating the importance of the RAFT agent in controlling the polymerisation process. It is of note that the choice of filter and initiator and the length of residence time are key factors in photo-polymerisations of this type. For example, the use of filter 4 gave high conversion (91%) and a polydispersity of 1.28 after 60 min (entry 1.12), while when filter 3 was used a similar result could be obtained in only 10 min (entry 1.1), though at the price of slightly higher polydispersity. However, if a longer residence time was then used to push the conversion above 97%, polydispersity was seen to increase considerably (entries 1.2 and 1.3). This suggests a payoff between the wavelength of light (i.e. choice of filter) used to initiate the polymerisation and the length of time the RAFT group is exposed to light. While the use of broader wavelength spectrum of filter 3 (wavelengths above 310 nm) enhances the polymerisation rate, it also appears to enhance side reactions such as scission and termination by interacting with the RAFT end group of the polymer as it is growing. This possible fragmentation of the RAFT group during polymerisation is further brought out by the fact that polymerisation occurs in the absence of a photo-initiator (entries 1.21 and 1.22). The use of filter 4 to provide a much narrower range of wavelengths (330–380 nm) appears to result in a more controlled polymerisation process involving fewer side reactions, but in return requires a longer residence time. Photo-initiated polymerisation reactions performed in continuous flow utilising photo-initiators 3 and 4 were compared with traditional thermally initiated batch reactions in order to assess the effectiveness of photo-mediated radical polymerisations. Batch reactions were carried out on a 2 mL scale in microwave vials and heated at the specified temperatures in a microwave reactor. Polymerisation of DMA under thermal batch conditions (entry 1.23) resulted in complete conversion in 60 min at 100 °C to give pDMA with the expected molecular weight and low polydispersity. A 10-fold increase in the initiator concentration (entries 1.23 and 1.24) at higher temperature resulted in an increase in reaction rate to give complete conversion to pDMA within 5 min with a small increase in polydispersity. An additional control experiment was also performed, where the thermal initiator (Vazo-88) was left out, resulted in no polymerisation illustrating the requirement of a radical source to initiate the reaction under thermal conditions. This is in contrast to the equivalent experiments in the UV-150 photochemical reactor, where even in the absence of a photo-initiator (entries 1.21 and 1.22) polymerisation still readily took place, albeit with diminished control over molecular weight and polydispersity. This demonstrates that the RAFT agent could be used as an iniferter under the given conditions inside the photochemical reactor. Having investigated the RAFT-mediated photo-polymerisation of DMA using continuous flow, we next explored the use of these conditions for the polymerisation of other activated monomers, namely methyl acrylate (MA), methyl methacrylate (MMA), and N-isopropylacrylamide NIPAM (Table 2). Filter 4 was used with a focus on achieving high conversion at low residence times. The results indicate that the method can be applied to a range of acrylates and acrylamides, as well as methacrylates. For MA and NIPAM using initiator 3, conversions of 83% and 92% (entries 2.3 and 2.8) were achieved with 60 min residence times giving polymers with polydispersities around 1.7. As the tertiary RAFT agent 1 is also suitable for use with methacrylates, polymerisation of MMA also proceeded albeit requiring a longer residence time. The use of initiator 4 gave similar conversions and polydipersities to that of initiator 3.
4. Conclusions Herein we describe the use of a novel tubular continuous flow reactor for photo-initiated RAFT polymerisation. We have investigated the effect of different wavelengths and two different photo-initiators on the performance of the polymerisation and the quality of the obtained polymer product. The use of wavelengths between 310 and 380 nm (Filter 4) resulted in photo-initiated polymerisation of DMA with good conversion, control of molecular weight and low polydispersity (<1.3). It was found that a careful interplay exists between the range of wavelengths used and residences time, with broader wavelength ranges resulting in faster conversion but an increase in polydispersity due to scission and termination side reactions caused by further activation of the RAFT end groups. We further compared different process conditions and reagent ratios, and also conducted initiator free polymerisations. In the latter case, the RAFT agent itself acted as a source of initiator radicals starting the polymer chain growth. Excitation of the thiocarbonylthio core of the RAFT agent led to the formation of sufficient amounts of initiating radicals for the polymerisation to progress in a matter of several minutes to an hour. Under these reaction conditions the RAFT agent acted as an iniferter, and was solely responsible for initiating, controlling and terminating the reaction. Alternatively, using high initiator and RAFT agent loadings, we also demonstrated very fast polymerisation with close to full conversion after only 5 min at 30 °C, compared to standard thermal RAFT process conditions which usually require long reaction times or forcing conditions at elevated temperatures (>100 °C), or both. This work indicates that photo-initiated RAFT polymerisation under continuous flow conditions has potential for wider application within polymer research and industry. Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor James Gardiner a,⇑, Christian H. Hornung a, John Tsanaktsidis a, Duncan Guthrie b a b
CSIRO Manufacturing, Bag 10, Clayton South, Victoria 3169, Australia Vapourtec Ltd, Park Farm Business Centre, Bury St Edmunds IP28 6TS, United Kingdom
a r t i c l e
i n f o
Article history: Received 18 December 2015 Received in revised form 19 January 2016 Accepted 20 January 2016 Available online xxxx Keywords: RAFT polymerisation Photo-initiated polymerisation Continuous flow Photochemical reactor
a b s t r a c t The RAFT (Reversible Addition–Fragmentation Chain Transfer) approach allows for greatly enhanced control over radical polymerisation processes, resulting in polymers with low dispersity. Classically, RAFT polymerisations are conducted in batch using thermal initiators. Herein, we describe a novel photo-initiated RAFT polymerisation procedure, using a tubular continuous flow reactor for the polymerisation of (meth)acrylates and acrylamides at close to ambient temperatures. This approach makes use of the excellent light penetration properties of millimetre-size fluoropolymer tubing, enabling the synthesis of multigrams/kgs of RAFT polymer per day. A tubular photochemical reactor presents a very practical and compact design for the continuous manufacture of RAFT polymers in quantities that would otherwise be difficult to achieve in a batch system using photo-initiation. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction The Reversible Addition–Fragmentation Chain Transfer (RAFT) method is a controlled radical polymerisation technique which enables the synthesis of polymers with well-defined architecture and narrow molar weight distribution. RAFT exhibits great tolerance to reaction conditions, it is compatible with most monomers, and can provide polymers with defined structures and end-groups [1–7]. In recent years ambient temperature processes for RAFT polymerisation, using photo-initiators instead of the conventional thermal Vazo initiators, were conducted by several researchers [8–13]. This method presents an interesting alternative for systems containing temperature sensitive components or where initiation by visible or UV light is preferred over thermal initiation. Continuous flow processing is becoming an increasingly disruptive technology in a range of chemical manufacturing sectors and a series of different innovative flow reactor designs for laboratory and pilot or production scale have been developed over the past few years. These new continuous flow reactors consist of either tubular or plate-type modules in which the chemical reactions take place under well-defined flow conditions. Flow reactors offer many advantages over conventional batch reactors, including enhanced process control, higher yields & purities, shorter reaction times, better scalability, smaller physical footprint and superior inherent safety [14–25]. Recently, the use of continuous flow reactors for photo-initiated controlled radical polymerisations has been reported in the literature by several different research groups. Johnson and co-workers have performed continuous flow living radical polymerisations of acrylates and acrylamides using trithiocarbonate iniferters, an organic photo-redox catalyst and visible
⇑ Corresponding author. E-mail address: [email protected] (J. Gardiner). http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033 0014-3057/Ó 2016 Elsevier Ltd. All rights reserved.
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light. Their experimental set-up consisted of fluoropolymer tubing, syringe pumps and a compact fluorescent lamp, with which it was possible to synthesise homo-polymers and block copolymers with low dispersities between <1.1 and 1.4 [26,27]. Junkers and co-workers demonstrated the use of microfluidic photochemical reactors for copper-mediated, and later cobalt-mediated, radical polymerisations in continuous flow. Fast polymerisations of vinyl acetate and methyl acrylate under UV irradiation were achieved without loss of polymerisation control [28,29]. Most recently Hawker and co-workers conducted continuous flow experiments for the synthesis of poly(methyl methacrylate) with dispersities around 1.2 via a light-mediated controlled radical polymerisation. This study compared different widely available fluoropolymer tubing materials for use in a photo-induced polymerisation [30]. The major benefit of using continuous flow tubular reactors for photo-initiated chemistry is the ability to efficiently capture the maximum number of photons by constructing the reactor around the lamp and providing a fluidic chamber with a short light penetration pathway. The scaling up of traditional photoinduced batch reactions suffer significantly from the so-called ‘Beer–Lambert penalty’ where a strong light gradient exists such that light is absorbed only at the exposed surface layers of the solvent. Continuous flow reactors overcome this by utilising narrow optically clear tubing that allows for significantly enhanced light efficiency and mixing. The feasibility as well as benefits and disadvantages of continuous flow processing for photochemical reactions have been demonstrated previously by Booker-Milburn, Oelgemöller and others [31–36]. In this paper, we present work conducted on a novel, commercially available photochemical reactor, the Vapourtec UV-150 [37,38], for the UV-initiated continuous flow synthesis of RAFT polymers. The tubular photochemical reactor can be operated with one of two light sources, either a high intensity medium pressure metal halide lamp or an LED array. This study is the first to investigate the effect of different wavelength UV radiation in combination with different photo-initiators on the performance of the continuous flow RAFT polymerisation reaction and the quality of the polymer product. A series of different monomers were successfully applied to this procedure, using different reagent ratios and process conditions. By tuning the wavelength so the thiocarbonylthio core of the RAFT agent was excited, it was also possible to conduct initiator free polymerisations, whereby the chain transfer agent itself acted as the source of initiating radicals starting the polymer chain growth, i.e. as an iniferter. The continuous flow results were then compared to conventional batch procedures using thermal initiators.
2. Experimental 2.1. Materials and analysis The RAFT agent 1 (see Fig. 1) was obtained from Boron Molecular. The polymerisation initiators azobis-(cyclohexanenitrile), 2 (supplied by DuPont as Vazo88), 2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone, 3 (supplied by Ciba as Irgacure 369), and phenyl bis (2,4,6-trimethyl benzoyl) phosphine oxide, 4 (supplied by Ciba as Irgacure 819) were used without further treatment (see Fig. 1). The monomers methyl acrylate (MA), methyl methacrylate (MMA), N,N-dimethyl acrylamide (DMA) and N-isopropyl acrylamide (NIPAM) were pre-treated using polymer resin (for removal of hydroquinone and monomethyl ether hydroquinone, Sigma Aldrich, Cat. No: 31,133-2) in order to remove the polymerisation inhibitor. The solvent acetonitrile (MeCN) was obtained from Merck KGaA and used without further purification. Reaction conversions were calculated from 1H NMR spectra. For calculating the conversion of polymerisation, 1,3,5trioxane was used as an internal standard. 1H NMR spectra were recorded on a Bruker AC-400 spectrometer in deuterated chloroform (solvent residual as internal reference: 7.26 ppm). Average molecular weight of the polymer, Mn, and its dispersity, Ð, were measured using size exclusion chromatography (SEC). SEC (DMAc) was performed on a Shimadzu instrument
Fig. 1. RAFT agent and initiators.
Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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equipped with a CMB-20A controller system, a SIL–20A HT autosampler, a LC-20AT tandem pump system, a DGU-20A degasser unit, a CTO-20AC column oven, a RDI-10A refractive index (RI) detector, and 4 Styragel (Waters) columns (HT2, HT3, HT4 and HT5). N,N-Dimethylacetamide (DMAc) (containing 4.3 g/L LiBr) was used as eluent at a flow rate of 1 mL/min. The column temperature was set to 80 °C and the RI detector temperature was set to 40 °C. The SEC was calibrated with low dispersity poly(MMA) standards, and molar mass are reported as poly(MMA) equivalents. Mn and Ð were evaluated using Shimadzu software (LabSolutions version 5.63). A 3rd order polynomial was used to fit the log M vs. time calibration curve, which was approximately linear across the molar mass range of interest. 2.2. Batch RAFT polymerisation using thermal initiation A starting material solution of 595 mg monomer, 5 (DMA), 2.93 mg initiator 2, 24.22 mg RAFT agent 1, in 1.382 mL MeCN (total volume = 2 mL), was premixed and degassed using nitrogen purging. The polymerisation was conducted on a laboratory microwave reactor (Biotage Initiator) at 100 °C with a reaction time of 1 h. A yellow viscous polymer solution was obtained after reaction, from which conversion was determined by 1H NMR and Mn and Ð were obtained by SEC (see Scheme 1). 2.3. Continuous flow RAFT polymerisation using photo-initiation A Vapourtec UV-150 photochemical reactor was used to carry out photo-initiated RAFT polymerisation reactions under continuous flow conditions. Transmission spectra of the UV filters used to control the wavelength of light exposed to the reactor coil are shown in Fig. 2. Reaction temperatures were selected based on the operating parameters of the lamp and filter combinations and the achievable temperatures of the cooling system. To cool the reactor coil to the required temperature and to also overcome excessive oxygen permeability of the tubing, the UV-150 reactor coil was continually flushed with pre-cooled nitrogen (passed over dry ice) via a regulator valve. As RAFT polymerisations are traditionally oxygen sensitive, an insufficient flow of nitrogen around the reactor coil was found to result in little or no polymerisation occurring. MeCN was used as solvent due to its low UV cut-off wavelength (200 nm) and high auto-ignition temperature (524 °C) under UV conditions. The polymerisation was conducted on a Vapourtec R2/R4 flow reactor system using the UV-150 photochemical reactor module [37,38]. The reactor module can be operated with different light sources, either a 150 W mercury lamp or a range of LED arrays. For all herein presented experiments, the mercury lamp was used in combination with one of four interchangeable UV filters. Filter 2 is a band pass filter with transmission between 270 nm and 400 nm. Filter 3 is a long pass filter with transmission above 310 nm. Filter 4 is a band pass filter with transmission in the range 330–380 nm. Filter 6 is a band pass filter with transmission in the range 350–590 nm. Total area of the reactor coil exposed to the light source was 129 cm2. The use of a 365 nm LED with the UV-150 photochemical reactor was also investigated at first, however the mercury lamp produced better initial results; hence all further work was focused on this light source. The 365 nm LED has 1.7 times the photo flux that was available from the 150 W mercury lamp in combination with Filter 4, and the use of this new narrow band 365 nm LED light source appears worthy of further investigations in the future. The Vapourtec UV-150 photochemical reactor module uses reactor coils manufactured from PFA tubing with an ID of 1.3 mm (reactor volume: 10 mL). PFA is a copolymer of tetrafluoroethylene and perfluoroalkyl vinyl ether, which exhibits acceptable transmittion in the UV and visible wavelengths when the wall thickness is kept below 0.3 mm. PFA was selected in preference to FEP (Fluorinated ethylene propylene) tubing solely for its increased durability. PFA and FEP have similar properties with high transparency and oxygen permeability values of 881 and 748 respectively (cm3/100 in.2 24 h atm mil 1). Other types of tubing such as Tefzel or Halar are less transparent but offer superior oxygen permeability properties with values as low as 100 and 25 respectively (cm3/100 in.2 24 h atm mil 1) [30]. The mercury lamp light source of the photochemical reactor can be variably tuned between 100% and 50% intensity. The photochemical reactor module was operated with a dry ice dewar to regulate the temperature between 30 and 40 °C. The flow rate was set to 2 mL/min resulting in a reaction time of 5 min. A 40 psi backpressure regulator was positioned inline after the reactor coil in order to prevent solvent from boiling. The following procedure is typical; it describes experiment 1.12 (Table 1, vide infra); all other continuous flow experiments in Tables 1 and 2 were performed analogously. A starting material solution of 1.487 g monomer, 5 (DMA), 11 mg initiator 3, 60.55 mg RAFT agent 1 and 25 mg 1,3,5-trioxane, in 3.454 mL MeCN (total volume = 5 mL), was premixed and degassed using nitrogen purging. A 2 mL sample was injected into the reactor via a sample loop, which was flushed with a constant stream of MeCN. These conditions were developed from previous work, which describes procedures for the thermally initiated continuous flow synthesis of RAFT polymers on Vapourtec R2/R4 reactors [39–41]. A yellow polymer solution was obtained after reaction, from which the conversion was determined by 1H NMR and Mn and Ð were obtained by SEC. 3. Results and discussion UV polymer curing has become a preferred method of curing in many industry processes [42]. As such we selected two industrially used photo-initiators, Irgacure 369, 3, and Irgacure 819, 4, (Fig. 1) for the polymerisation of N,N–dimethylacrylamide (DMA). Irgacure 369, 3, is commonly used in industry (e.g. graphic arts, electronics) for UV-curing of pigmented Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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Scheme 1. Solution-phase RAFT polymerisation using different monomers, initiators and initiation mechanisms.
Fig. 2. Spectra of the wavelengths of light emitted by the mercury lamp and that transmitted through Vapourtec UV filters 2, 3, 4 and 6.
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Table 1 Experimental conditions and results for the RAFT polymerisation of DMA in batch and continuous flow, optimisation of the photo-initiated flow process using different initiators and process conditions.
a b c
100% lamp intensity without filter corresponds to 150 W. 1Molar ratio of monomer to RAFT agent to initiator (M/R/I). 1MWs are given against pMMA standards.
Table 2 Experimental conditions and results for the photo-initiated RAFT polymerisation of MA, MMA and NIPAM in continuous flow; all experiments were performed using UV filter 4 at 100% lamp intensity (irradiance = 38 mW/cm2) and a reaction temperature of 40 °C.
a
Molar ratio of monomer to RAFT agent to initiator (M/R/I).
systems such as inks, coatings and varnishes, while Irgacure 819, 4, is especially suited for white pigmented formulations, glass fibre reinforced polyester/styrene systems, and clearcoats. Both photo-initiators are efficient sources of radicals under UV conditions with UV absorption spectra in acetonitrile indicating local absorption maxima around 300–350 nm [43–45]. Results for the continuous flow photo-polymerisation of DMA in the presence of RAFT agent 1 are shown in Table 1. A comparison of the different UV filters and initiators used over a 30 min residence time is given in Fig. 3. In all cases polymerisation proceeded with varying degrees of control over conversion, molecular weight and polydispersity. Polymerisations carried out with photo-initiator 3 and Filter 3 gave high conversions (>90%) at 100% lamp intensity, with control of MW and polydispersities <1.5. Increasing the residence time led to higher conversions however some loss of control over molecular weight and polydispersity were also observed. An absorbance spectrum of RAFT agent 1 (not shown) indicates a local maxima at around 310 nm (see for example previous work [46]) suggesting that on exposure to UV light the RAFT agent itself has the potential to fragment via a radical mediated process. This is illustrated in a control experiment whereby DMA can be polymerised with 1 in the absence of any photo-initiator (entries 1.21 and 1.22, Table 1). Here, the RAFT agent acts as an iniferter, and is solely responsible for initiating, controlling and terminating the reaction. This suggests that loss of control Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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(a)
(d)
(b)
(e)
(c)
(f)
Fig. 3. Diagrams a–c: comparison of different UV filter and initiator combinations at 30 min reaction time, 40 °C, M:R:I = 100/1.0/0.2 and 100% lamp intensity (see also entries 1.3, 1.11, 1.13 and 1.15); diagrams d–f: effect of lamp intensity reaction time for polymerisations with filter 3, initiator 3, at 40 °C and M:R:I = 100/1.0/0.2 (see also entries 1.1–1.9).
during the RAFT mediated polymerisation reaction may be due to photo-degradation of the RAFT agent and subsequent scission or termination side reactions occurring during the polymerisation reaction. The ongoing photo-activation of the thiocarbonylthio core of the RAFT agent, leading to decomposition or partial removal of the RAFT end group during the polymerisation reaction, would explain an increase in the polydispersities of the resulting polymers. This effect has previously been documented when molar ratios of photo-initiators were used as a fast method for the removal of RAFT-polymer disulphide ester end groups [47]. Use of filter 4 (330–380 nm) to minimise potential absorption of RAFT agent around 310 nm resulted in slower monomer conversion with a 60 min residence time giving a similar result to that after 10 min using filter 3. Notably, the use of filter 4 with longer residence times gave lower polydispersities than with filter 3 for a given conversion, suggesting that the use of a narrower wavelength band was beneficial in limiting degradation of RAFT agent. Polymerisations carried out with photoinitiator 4 also gave good conversions however a notable increase in polydispersity was observed. Use of wavelengths above 350 nm (Filter 6, 360–390 nm) resulted in low conversions even at longer residence times, indicating inefficient activation of the photo-initiator (absorbance maxima between 300 and 350 nm). To speed up reaction times of the DMA polymerisation we explored the use of high RAFT:photo-initiator ratios. Traditionally, low initiator concentrations are used to elicit control over polymerisation reactions however for many processes it is the
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rate of reaction that is of interest so as to obtain the product in the shortest possible timeframe. In these cases low polydispersities are not necessarily paramount. To achieve these fast polymerisation times, we investigated the use of a 1:1 ratio of RAFT agent to photo-initiator (Table 1, entries 1.17 and 1.18). This resulted in high conversion, >99%, in as little as 5 min using filter 3 with either 3 or 4, with polydispersity of around 1.6. These higher values are presumably caused by increased chain scission/termination under the forcing conditions applied. Polymerisation of DMA was also carried out with photo-initiator 3 and 4, in the absence of RAFT agent (Table 1, entries 1.19 and 1.20). Complete conversion was observed within 5 min, however SEC analysis revealed polymers with bimodal distributions, high polydispersities, and a lack of molecular weight control, thereby illustrating the importance of the RAFT agent in controlling the polymerisation process. It is of note that the choice of filter and initiator and the length of residence time are key factors in photo-polymerisations of this type. For example, the use of filter 4 gave high conversion (91%) and a polydispersity of 1.28 after 60 min (entry 1.12), while when filter 3 was used a similar result could be obtained in only 10 min (entry 1.1), though at the price of slightly higher polydispersity. However, if a longer residence time was then used to push the conversion above 97%, polydispersity was seen to increase considerably (entries 1.2 and 1.3). This suggests a payoff between the wavelength of light (i.e. choice of filter) used to initiate the polymerisation and the length of time the RAFT group is exposed to light. While the use of broader wavelength spectrum of filter 3 (wavelengths above 310 nm) enhances the polymerisation rate, it also appears to enhance side reactions such as scission and termination by interacting with the RAFT end group of the polymer as it is growing. This possible fragmentation of the RAFT group during polymerisation is further brought out by the fact that polymerisation occurs in the absence of a photo-initiator (entries 1.21 and 1.22). The use of filter 4 to provide a much narrower range of wavelengths (330–380 nm) appears to result in a more controlled polymerisation process involving fewer side reactions, but in return requires a longer residence time. Photo-initiated polymerisation reactions performed in continuous flow utilising photo-initiators 3 and 4 were compared with traditional thermally initiated batch reactions in order to assess the effectiveness of photo-mediated radical polymerisations. Batch reactions were carried out on a 2 mL scale in microwave vials and heated at the specified temperatures in a microwave reactor. Polymerisation of DMA under thermal batch conditions (entry 1.23) resulted in complete conversion in 60 min at 100 °C to give pDMA with the expected molecular weight and low polydispersity. A 10-fold increase in the initiator concentration (entries 1.23 and 1.24) at higher temperature resulted in an increase in reaction rate to give complete conversion to pDMA within 5 min with a small increase in polydispersity. An additional control experiment was also performed, where the thermal initiator (Vazo-88) was left out, resulted in no polymerisation illustrating the requirement of a radical source to initiate the reaction under thermal conditions. This is in contrast to the equivalent experiments in the UV-150 photochemical reactor, where even in the absence of a photo-initiator (entries 1.21 and 1.22) polymerisation still readily took place, albeit with diminished control over molecular weight and polydispersity. This demonstrates that the RAFT agent could be used as an iniferter under the given conditions inside the photochemical reactor. Having investigated the RAFT-mediated photo-polymerisation of DMA using continuous flow, we next explored the use of these conditions for the polymerisation of other activated monomers, namely methyl acrylate (MA), methyl methacrylate (MMA), and N-isopropylacrylamide NIPAM (Table 2). Filter 4 was used with a focus on achieving high conversion at low residence times. The results indicate that the method can be applied to a range of acrylates and acrylamides, as well as methacrylates. For MA and NIPAM using initiator 3, conversions of 83% and 92% (entries 2.3 and 2.8) were achieved with 60 min residence times giving polymers with polydispersities around 1.7. As the tertiary RAFT agent 1 is also suitable for use with methacrylates, polymerisation of MMA also proceeded albeit requiring a longer residence time. The use of initiator 4 gave similar conversions and polydipersities to that of initiator 3.
4. Conclusions Herein we describe the use of a novel tubular continuous flow reactor for photo-initiated RAFT polymerisation. We have investigated the effect of different wavelengths and two different photo-initiators on the performance of the polymerisation and the quality of the obtained polymer product. The use of wavelengths between 310 and 380 nm (Filter 4) resulted in photo-initiated polymerisation of DMA with good conversion, control of molecular weight and low polydispersity (<1.3). It was found that a careful interplay exists between the range of wavelengths used and residences time, with broader wavelength ranges resulting in faster conversion but an increase in polydispersity due to scission and termination side reactions caused by further activation of the RAFT end groups. We further compared different process conditions and reagent ratios, and also conducted initiator free polymerisations. In the latter case, the RAFT agent itself acted as a source of initiator radicals starting the polymer chain growth. Excitation of the thiocarbonylthio core of the RAFT agent led to the formation of sufficient amounts of initiating radicals for the polymerisation to progress in a matter of several minutes to an hour. Under these reaction conditions the RAFT agent acted as an iniferter, and was solely responsible for initiating, controlling and terminating the reaction. Alternatively, using high initiator and RAFT agent loadings, we also demonstrated very fast polymerisation with close to full conversion after only 5 min at 30 °C, compared to standard thermal RAFT process conditions which usually require long reaction times or forcing conditions at elevated temperatures (>100 °C), or both. This work indicates that photo-initiated RAFT polymerisation under continuous flow conditions has potential for wider application within polymer research and industry. Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033
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Please cite this article in press as: J. Gardiner et al., Continuous flow photo-initiated RAFT polymerisation using a tubular photochemical reactor, Eur. Polym. J. (2016), http://dx.doi.org/10.1016/j.eurpolymj.2016.01.033