Article pubs.acs.org/Macromolecules
Mechanistic Investigation into the Accelerated Synthesis of Methacrylate Oligomers via the Application of Catalytic Chain Transfer Polymerization and Selective Microwave Heating Kevin Adlington,†,‡ G. Joe Jones,†,‡ Jaouad El Harfi,†,‡ Georgios Dimitrakis,† Alastair Smith,† Sam W. Kingman,† John P. Robinson,† and Derek J. Irvine†,‡,* †
National Centre for Industrial Microwave Processing, Process and Environmental Research Division, Faculty of Engineering, University of Nottingham, Nottingham, NG7 2RD, U.K. ‡ School of Chemistry, University of Nottingham, Nottingham, NG7 2RD, U.K. S Supporting Information *
ABSTRACT: The synthesis of methyl methacrylate (MMA) oligomers by catalytic chain transfer polymerization (CCTP) is demonstrated to be significantly accelerated by the use of microwave heating. The CCTP reactions, which use a cobalt-based catalyst to very efficiently control the molecular weight of the final polymer, were conducted in both a conventional oil bath and a CEM Discover microwave reactor with a target set point of 80 °C. The required reaction time was shown to be reduced from 300 to 3 min, while also retaining control over the polymerization. Additionally, for the first time the bulk temperature of these catalyzed polymerizations was monitored in both heating methods by the use of internal optical fiber sensors. It was demonstrated that, to monitor the temperature of the reaction correctly, it is essential to use an optical fiber sensor rather than the external IR sensor supplied with the reactor. The acceleration in the synthesis during microwave heating was attributed to selective heating of the radical and oligomeric species within the reaction, which lead to both rapid heating of the reaction bulk to reaction temperature and average reaction temperatures that were higher than the chosen set point. However, comparative reactions carried out under conventional heating (CH) conditions at the true reaction temperature of the microwave experiments (MWH) showed that MWH was able to produce significantly greater yields than the CH experiments after only 3 min, indicating the existence of a real selective heating effect during the reaction. Three methods have been investigated to optimize the acceleration achieved in the MWH experiments while retaining control and yield levels within the MWH experiments. These were varying the; solvent concentration, initiator concentration and chain transfer agent concentration. It was demonstrated that by understanding the influence of the microwave heating that it was possible to retain control over the molecular structure of the product polymer at the accelerated rate.
■
radical processes (SFRP),1 a number of studies have investigated the effect of MWH on CRP systems. For example, MWH has been applied to both reversible addition− fragmentation chain transfer (RAFT),2−4 and atom transfer radical polymerization (ATRP).5−7 The focus of these studies was to reduce the time required to make polymers with 3dimensional architectures, while retaining control of the
INTRODUCTION
The application of microwave heating (MWH) methods in polymer synthesis has been a growing area of research over recent years. Much of this interest has been created by numerous reports of microwave processes demonstrating increased rates of polymerization and higher final product yields when compared to conventional heat (CH) reactions conducted at, what was believed to be, the same temperature. Consequently, because the application of controlled radical polymerization (CRP) methodologies typically leads to extended reaction times, when compared to standard free © XXXX American Chemical Society
Received: January 4, 2013 Revised: April 16, 2013
A
dx.doi.org/10.1021/ma400022y | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
the use of MWH is imparting to the reaction. The aim was to explain the root causes behind in the literature reported rate accelerations. This work combines a study of the dielectric properties of the reaction precursors and products, with a systematic study of the effect that concentration and catalyst level have upon the reaction rate and the final product Mwt. Consequently, the MWH polymerization results were compared to CH equivalents conducted at the same bulk temperature, where the actual reaction temperature was rigorously determined via a direct measurement of the bulk mixture using an optical fiber probe. This comparison has allowed specific conclusions to be drawn about selective heating and the molecular species most likely to be delivering such an effect.
molecular structure of the product. This work shows an increase in the rate of polymerization for both RAFT and ATRP when polymerized using MWH. However, the authors gave no explanation for this rate increase. To-date, there have also been no studies into the effect of applying MWH to catalytic chain transfer polymerization (CCTP). CCTP involves the introduction of low spin Co(II) complexes to SFRP and results in both a significant reduction in the isolate polymer’s molecular weight (Mwt) and the production of vinyl terminated polymer structures.8−11 Further work by Gridnev,12,13 DuPont,14,15 and ICI/Zeneca16,17 has led to a greater insight into the mechanism of the catalytic process and cobaloxime catalysts becoming one of the most favored families of CCTP agents. Of these, the BF2-bridged catalyst is most commonly used because it is less air sensitive than the hydrogen bridged versions. Additionally, the R group can be varied to affect the solubility and activity of the catalyst, but is most commonly methyl or phenyl. These complexes are reported as MeCoBF and PhCoBF respectively. A wide range of monomers can be efficiently controlled by CCTP but methacrylates are most widely used because they have a αmethyl substituent which makes the monomer more susceptible. The accepted mechanism of CCTP takes place via a twostep process. In step 1 (Rn + Co(II) → Pn+ Co(III)−H) a growing polymer chain (Rn) interacts with the Co(II) complex which results in a dead polymer chain (Pn) and formation of a Co(III) hydride species which is thought to be the rate limiting step. During step 2 (Co(III)−H + monomer → R1 + Co(II)), this Co(III)H intermediate reacts with a monomer to form a new monomeric radical (R 1 ) and reform the Co(II) catalyst.12,18,19 In CH, energy is delivered via conductive, convective or radiative heat transfer. In MWH, or dielectric heating, energy is transferred volumetrically via direct coupling of the material with an alternating electric field. Thus, the heating characteristic of a material depends on the dielectric properties. There are numerous literature sources which discuss the fundamentals of microwave heating and so this will not be discussed here.20 However, the dielectric properties of a particular molecular entity can be expressed in a number of ways to rationalize/ predict how it will respond to an incident electromagnetic field (EF).21 The real part of its complex permittivity (dielectric constant, ε′) defines the extent to which it will store energy through forms of polarization via interaction with the EF.21 Meanwhile, the imaginary part of complex permittivity (dielectric loss factor, ε″) expresses its ability to dissipate this stored energy into heat.21 A third parameter termed loss tangent (tan δ = ε″/ε′) can then be defined, which indicates the potential for the material to heat under the influence of an applied EF of a certain frequency. Clearly, if an alternating EF of high frequency is applied to a complex mixture, such as a chemical reaction medium, then the overall measured dielectric response will depend on/be a function of the respective dielectric properties of all the molecular species present and the interactions that occur between them.22 CCTP has been shown to be very effective in the polymerization of methyl methacrylate (MMA), with PhCoBF exhibiting a very high chain transfer constant (Cs) (15000− 30000) when compared to other CTAs such as thiols (∼1).23 Because of the high Cs value, low quantities of catalyst are needed to form the very low Mwt polymers. In this paper the effect of selective heating of specific molecular entities within a CCTP polymerization is investigated to define what difference
■
EXPERIMENTAL SECTION
Materials. PhCoBF (DuPont) and methyl isobutyrate (99%, Aldrich) were used as supplied without further purification. MMA (99%, Aldrich) was passed through a column of basic alumina to remove the inhibitors prior to use, and 2,2-azobis(isobutyronitrile) (AIBN, 98%, Aldrich) was purified by recrystallization with methanol. Toluene was molecularly distilled and then dried with molecular sieves. Reactor Geometries. All MWH reactions were conducted in a CEM Discover (maximum power 300 W) microwave reactor equipped with an IR temperature sensor. An additional optical fiber temperature sensor was introduced directly into the reaction bulk via a septum using a cylindrical choke fitted to the reactor to arrest any microwave leakage. This allowed access to the vessel while keeping the microwave energy contained within the reactor to a level such that legislation was not exceeded. Comparative CH experiments were conducted using a standard oil bath method where the heating fluid temperature was controlled by a thermocouple in the oil bath which was cross referenced to an internal bulk temperature measurement using an OF probe. As the measurement of dielectric properties is outside the scope of the polymerization work described in this paper, it has not been included here. Instead, a full description of the techniques used can be found in prior publications on dielectric assessment by the authors.24−27 Characterization. Gel Permeation Chromatography. GPC was performed on a Polymer Laboratories GPC-120 instrument at 40 °C equipped with a PLgel 5 μm guard column and two 30 cm PolarGel-M Columns in series coupled with a refractive index detector using HPLC grade THF as the mobile phase at a flow of 1.0 cm3.min−1. The GPC was calibrated with poly(methyl methacrylate) narrow PDI standards ranging from 690−1,944,00 g·mol−1. All GPC equipment and standards were supplied by Polymer Laboratories (Varian) GPC data were analyzed using the Cirrus GPC Offline software package. For the low Mwt materials produced. The standard deviations of the GPC measurements were calculated from over 10 measurements and were defined to be ±11 g·mol−1. 1 H (δH) NMR Spectra. These were recorded at 25 °C using a Bruker DPX-300 spectrometers (300 MHz). Chemical shifts were recorded in δH (ppm). Samples are prepared as solutions in CDCl3 to which chemical shifts are referenced (residual chloroform at 7.26 ppm). Analysis of the spectra was carried out using ACDLABS 12 software The 1H NMR (300 MHz, CDCl3, 25 °C) for MMA and CCTP derived PMMA are as follows: MMA. δH 3.70 (3H, CH3−O), 6.07 (1H, CCH2), 5.57 (1H, CCH2), 1.90 (3H, CH3). CCTP oligomers of PMMA δH 3.64 (6H, CH3−O), 6.29 (1H, CCH2), 5.79 (1H, CCH2), 2.63 (2H, CH2CCH2), 1.17 (6H, 2CH3). UV−Vis Analysis. This was conducted on neat samples of the experimental solution taken every 30 min without further dilution, where the reaction was quenched by being directly cooled upon introduction of the sample into the analysis cuvette. The background corrected data was collected on a Lambda 25 UV/vis spectrometer and program Lambda 25 spectra at a scan speed 480 nm/min from 500 to B
dx.doi.org/10.1021/ma400022y | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
275 nm where the key analysis extinction coefficient for AIBN is at 350 nm. General Polymerization Procedures. Typical CH experiment. PhCoBF (1.6 mg, 2.53 × 10−3 mmol) and the relevant solvent (toluene, 15 mL) were added to a Schlenk tube and sonicated for 30 s to aid dissolution of the catalyst. The solution was then degassed using argon prior to the addition of azobis(isobutyronitrile) (AIBN, 0.282 g, 1.72 mmol) and a magnetic stirrer to the Schlenk tube under an argon atmosphere. The mixture was then stirred until homogeneous, after which the methyl methacrylate monomer (MMA) (15 mL, 140 mmol), which had already been degassed using argon, was then transferred under an argon atmosphere to the reaction flask and heated in a thermostated oil bath at 80 °C and stirred for the prescribed reaction time. Termination of the reaction involved rapid cooling of the vessel upon removing it from the oil bath to ensure that reaction mixture remained representative of the reaction progress at that time. The solution was then precipitated into an excess of hexane (∼1 L) and the product retrieved by filtration. If no precipitate was retrieved, the solvents were removed by use of a rotary evaporator to isolate the product MMA oligomers as oils. The product oligomers were then dried for 7 days in a vacuum oven (25 °C, 10−1 mbar) and then analyzed by GPC and 1H NMR as described above. Typical MWH Polymerization Experiment. The reagent preparation procedure was as described above for the CH reactions. However, the reaction was conducted in a CEM Discover microwave reactor capable of delivering a maximum power of 300 W at a frequency of 2.45 GHz and fitted with a cylindrical choke to contain the input energy within the reactor while allowing access to the reaction vessel. The internal bulk temperatures were continually measured using both an optical fiber (OF) probe inserted directly into the reaction mixture and the CEM’s external infrared (IR) sensor. In these microwave experiments, the IR measurement was used to control the power input required to keep the bulk temperature constant at the target set point of 80 °C. Initiator Decomposition Study. AIBN (0.5 g, 1.28 mmol) was added to a volumetric flask and a 50 mmol solution produced with either methyl isobutyrate or toluene which had been degassed with argon for >45 min prior to use. This preprepared solution was placed in a quartz reaction vessel equipped with a magnetic stirrer, while under an argon atmosphere. This vessel was then introduced either to an oil bath preheated to a set temperature or a Discover CEM reactor programmed for a 60 min heating ramp and fitted with a 20 cm cylindrical choke to contain the input energy within the reactor vessel. The target set temperature range was defined by the initiator’s 10 and 1 h half-life temperatures (t1/2), for AIBN the t1/2 10 h is 66 °C and t1/2 1 h is 84 °C.25 Internal bulk temperatures were continually measured using both an OF probe inserted directly into the reaction mixture and the CEM’s IR sensor. In MWH experiments the OF measurement was used to gauge the power input required to keep the bulk temperature constant. Once the bulk temperature had stabilized, the progress of the reaction was then followed with time by removing samples at set time periods. These samples were taken from the bulk using an argon flushed syringe and placed directly into a cooled cuvette. The cooled samples were immediately introduced to the UV−vis spectrometer for analysis.
Table 1. Microwave-Mediated CCTP of MMA Using AIBN (0.282g) and PhCoBF in toluenea CH
MWH
entry
MMA/tol (v/v)
time (min)
conversionb (%)
time (min)
conversionb (%)
1 2 3 4 5 6
50:50 60:40 65:35 70:30 75:25 100:0
300 300 300 300 300 300
65 66 76 77 85 69
300 300 3 3 3 3
76 84 15 19 20 40
a
Polymerization conducted in a CEM microwave reactor using 300W maximum power. bConversion determined by NMR.
Figure 1. Overlay of GPC traces of the 50:50 v/v MMA: toluene CCTP oligomerization of MMA when conducted with CH and MH conducted with a temperature set point of 80 °C.
two reactions were significantly different, with an additional 11% relative conversion achieved in the MWH case. Additionally, no significant exotherm was observed in either experiment at this monomer:solvent ([M]:[S]) ratio via either the IR or OF probes. Consequently, a series of reactions were conducted at lower [M]:[S] concentrations to ascertain if the conversion to oligomer could be increased. This change will also reduce the VOC footprint of any commercial scale process developed from these laboratory results and so will reduce its environmental impact significantly. Table 1, Entries 2−6 compare the data from reactions conducted with [M]:[S] ratios of 60:40 through to 100:0, where in the latter bulk reaction the monomer is acting as both solvent and reactant. In the CH experiments, reducing the solvent concentration from 50:50 to 75:25 resulted in an increase in the yield of polymer obtained from a 300 min reaction. This increase in conversion has been attributed to a small but measurable temperature increase of the reaction medium, i.e., ∼80 °C at [M]:[S] 50:50 to ∼85 °C at 75:25. However, when the reaction was conducted in the bulk the yield was observed to exhibit a decrease from that of the 75:25 experiments. This has been attributed to increased levels of initiator “burn out”, i.e., direct initiator radical coupling to form decomposition byproduct species. This process has been promoted over initiation of monomer by an increase in radical local concentration at the higher reaction temperature being coupled with a decreased radical diffusion due to the higher medium viscosity caused by the lack of additional solvent. In comparison, the MWH experiments are observed to have achieved greater yields than the CH equivalents at [M]:[S] ratios of 50:50 and 60:40. Furthermore, reducing the solvent concentration by this 10% resulted in an additional 18% increase in conversion. Furthermore, when the MWH reactions were carried out with solvent concentration below 40%, a very significant exotherm was observed, such that the reactions were
■
RESULTS AND DISCUSSION Initial reactions on the CCTP-controlled oligomerization of MMA were conducted in a 50:50 v/v MMA:toluene solution. This system was applied to limit any potential exotherm within the reaction medium by ensuring that the viscosity was kept low and hence good heat transfer was retained. The reaction conditions and yield data related to both these CH and MWH polymerizations are contained in Table 1, entry 1, while the GPC data are shown in Figure 1. The GPC profiles of the polymer products from both heating methods were identical, within the error of the measurement technique used. However, the product yields obtained from the C
dx.doi.org/10.1021/ma400022y | Macromolecules XXXX, XXX, XXX−XXX
Macromolecules
Article
exothermic polymerization reactions, the external IR measurement does not truly reflect the internal bulk temperature. This confirmed the need to use both measurements throughout the rest of the study, to ensure that the reaction conditions were being followed accurately. In subsequent experiments, the OF readings were always found to be higher than the IR, as would be expected in both volumetric heating and/or an exothermic reaction. A series of kinetic experiments were conducted with both heating methods to determine the conversion achieved with time. Figure 4 demonstrates the typical results from two such kinetic experiments, one of which contains a solvent level of >40% and the second
