Polymerization of Methyl Methacrylate with Lithium Triflate. A Kinetic

Jul 11, 2013 - A Kinetic and Structural Study ... IR spectroscopy experiments and theoretical studies confirm the formation of a complex between the l...
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Polymerization of Methyl Methacrylate with Lithium Triflate. A Kinetic and Structural Study Laura Hermosilla,† Paloma Calle,*,† Pilar Tiemblo,‡ Nuria García,‡ Leoncio Garrido,‡ and Julio Guzmán‡ †

Departamento de Química Física, Universidad Autónoma de Madrid, 28049 Madrid, Spain Departamento de Química Física, Instituto de Ciencia y Tecnología de Polímeros, Consejo Superior de Investigaciones Científicas (ICTP-CSIC), Juan de la Cierva, 3, 28006 Madrid, Spain

‡

S Supporting Information *

ABSTRACT: The radical photopolymerization of methyl methacrylate in the presence of lithium triflate is investigated by EPR and NMR spectroscopies in order to assess the effect of the ionic medium on the polymerization kinetics. The EPR spectra show a notorious increase in the concentration of propagating radical as result of a dramatic decrease in the average termination rate coefficient. IR spectroscopy experiments and theoretical studies confirm the formation of a complex between the lithium cation and the oxygen atom at the carbonyl position of the ester in the methyl methacrylate. The electrostatic repulsion between the complexed propagating radicals, specifically at the end of the chains, allows explaining the observed low termination rate. This strong interaction would lead to “quasi-living” polymer chains from the first steps of the polymerization. properties in regard to conductivity and molecular weights.19,20 However, only a few investigations on the kinetics of radical polymerization in the presence of solid salts have been performed.21,22 Recently, a study by Pedrón et al.22 has analyzed the effects of the presence of lithium trifluoromethanesulfonate (lithium triflate, LiTf) on the propagation and termination rate coefficients of the polymerization of methacrylic monomers with oxyethylene units in the side chain, concluding that the same trend is observed than in the case of ILs. However, the specific interactions taking place between monomers and ionic salts are not the same than those observed in ILs.22 Therefore, specific studies are required in order to establish correctly the features of the polymerization process for a further optimization of the polymer properties. On these grounds, we have centered our interest in the study of the influence of lithium salts on the mechanistic and kinetic features of the radical polymerization of methacrylic monomers. In particular, the goal of the present work is the study of the photochemical polymerization of methyl methacrylate in ethylene carbonate (EC) solution, in the presence and absence of lithium triflate, to determine the form in which this salt affects the propagation and termination rates. EC was used as solvent since it highly increases the solubility of LiTf. Moreover, EC is one of the most used plasticizers since its presence increases the flexibility of the polymer chain and enhances the ionic conductivity, thanks to its high dielectric

1. INTRODUCTION Conventional and mediated radical polymerizations carried out in the presence of ionic compounds have been widely studied in the past few years chiefly centered on reactions in ionic liquids (ILs).1−18 Apart from its indubitable academic interest, as well as the environmental benefits associated with the use of ILs as solvents, these investigations also point out the important properties of the polymers so obtained, for instance conductivity5,15 and high molecular weights.9 The investigations have focused mainly on the effects of a great variety of ILs (based on imidazolium, pyridinium, and alkylammonium salts, with different alkyl substituent in the cation and with different anions) on the main characteristics of the obtained materials, such as molecular weight and polymer tacticity. Some of them have also dealt with the influence of ILs on the mechanistic and kinetic features of the radical polymerization.8,11,12,17,18 As a common trend, these works have reported a moderate increase in the propagation rate constant, kp, and a great decrease in the average termination rate coefficient, ⟨kt⟩, as well as a remarkable acceleration of the reactions in the presence of ILs compared to polymerization in conventional solvents, leading to polymers with very high molecular weights. The reduction of ⟨kt⟩ has been attributed to the high viscosity of these solvents, but the explanation of the increase in kp is not entirely satisfactory, although the polarity of the medium is proposed to play an important role.8,13 Not so profusely, studies on polymerization in the presence of solid ionic salts have also been carried out, providing comparable conditions of viscosity and polarity than those obtained with ILs, thus giving rise to polymers with similar © 2013 American Chemical Society

Received: April 22, 2013 Revised: June 25, 2013 Published: July 11, 2013 5445

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2. EXPERIMENTAL SECTION

were variable between 2.5 × 104−2.5 × 105 and 1−5, respectively, depending on the spectral intensity. The concentration of the propagating radical species at different polymerization reaction times was determined by double integration of the EPR signal and its comparison to that corresponding to the EPR spectrum of well-known concentration of TEMPOL solutions, the stable radical used as standard. 2.4. NMR Measurements. The NMR experiments determined the unreacted monomer concentration for a given photopolymerization time. The samples, previously monitored by EPR, were immediately placed in the NMR spectrometer, in the same quartz tube, to avoid losses associated with sample transference. The used NMR signals were those corresponding to the unsaturated protons of the monomer and the protons associated with the ethylene carbonate solvent. The NMR spectra were registered in a Bruker Avance 400 spectrometer equipped with a 89 mm wide bore, 9.4 T superconducting magnet (proton Larmor frequency at 400.14 MHz). The reported data were acquired at room temperature with a 5 mm diameter proton probe head, using 40° flip angle radio-frequency pulses. The spectral width was 10 kHz, and the repetition rate 20 s. The spectra were referenced to ethylene carbonate (1H δ: 4.54 ppm)35 secondary to tetramethylsilane. 2.5. FT-IR Spectroscopy. Measurements of the IR absorption of the monomer and the lithium salt in different mixtures were carried out by using a PerkinElmer Spectrum-One FT-IR spectrometer. The infrared spectra were recorded on droplets of bulk MMA, MMA + LiTf solutions with [LiTf]/[MMA] = 1/12 and 1/24, and solutions of MMA + EC + LiTf and PMMA + EC + LiTf both with 50/50 w/w MMA/EC and [LiTf]/[MMA] ratios of 1/3 and 1/6. Four scans at 2 cm−1 resolution were acquired using an attenuated total reflection (ATR) device. Special attention was devoted to the absorption bands corresponding to the carbonyl group of the ester group of the monomer and to the CF3SO3− anion of the salt as well as to the changes in the whole spectrum. 2.6. Viscosity Measurements. The viscosity of the mixtures before polymerization reactions were determined with an Anton Paar SVM 3000 viscosimeter at 293 K.

2.1. Materials. Commercial MMA (Aldrich, 99%) was purified by distillation under high vacuum. 2,2-Dimethoxy-2-phenylacetophenone, DMPA (Ciba), was crystallized from methanol and dried under high vacuum at room temperature. Lithium trifluoromethanesulfonate (Aldrich, 99%) was dried in high vacuum for 24 h. Ethylene carbonate (Fluka, ≥99.0%) and 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxyl, TEMPOL (Aldrich, 98%), were used as received. 2.2. Polymerization Reactions. Photopolymerization of MMA was carried out in solution of ethylene carbonate (50/50 w/w) at 260 K in the absence and presence of lithium triflate salt and using DMPA as photoinitiator. The concentration of MMA was 4.9 mol L−1 in all experiments, and those of DMPA were 0.08 and 0.40 mol L−1 for reactions with and without LiTf, respectively. In order to achieve a radical concentration detectable by EPR spectroscopy in the experiments without LiTf, the concentration of DMPA was 5-fold higher than that used in the presence of salt. Reactions with LiTf were performed using two different concentrations, 0.817 and 1.63 mol L−1, which correspond to molar ratios [LiTf]/[MMA] of 1/6 and 1/3, respectively. The reaction mixture was continuously irradiated during the polymerization reactions by using a Bruker ER 203UV 100 W Hg lamp at 200−2000 nm wavelength range. 2.3. EPR Measurements. The evolution of the photopolymerization reactions was monitored by in situ EPR spectroscopy. The reaction mixtures were placed in a 3 mm diameter quartz tube and subjected to several freeze−pump out−thaw cycles to remove dissolved oxygen, previously to the photopolymerization and EPR monitoring. Spectra were recorded at different times of the polymerization using a Bruker ESP 300 spectrometer. Temperature control (at 260 K) was achieved by a Bruker BVT2000 nitrogen-flow system. The conditions to register the spectra were: microwave frequency, 9.5 GHz; modulation frequency, 100 kHz; modulation amplitude, 5 G; conversion time, 40 ms; time constant, 655 ms; sweep time, 42 s; power, 6.32 mW; the receiver gain and the scan number

3. COMPUTATIONAL DETAILS 3.1. Li+−Monomer Interaction Modeling. The potential energy surfaces (PES) of the Li+−monomer systems, for methyl, ethyl, and butyl methacrylic monomers, were obtained by computing their energies at different relative positions of the cation. These calculations were carried out using a model consisting of a three-dimensional grid defined by the Cartesian axes of the monomer and with a grating of 1 Å, inside of which the monomer is centered and the Li+ ion is placed in each point of the grid (excluding atoms’ overlaps). This strategy is equivalent to consider an approaching of the cation toward the monomer in all possible directions, so that a complete description of the PES of these models was obtained. The B3LYP/6-31+G(d,p) level of theory was used for geometry optimizations of the monomers.36−40 Subsequently, an energy calculation of the overall systems Li+−monomer was carried out on the previous optimized structures for each point of the grid using two different theoretical methodologies for the sake of comparison: B3LYP and MP2,41−46 both combined with the 6-31+G(d,p) basis set. Finally, the structure of minimum energy of the Li+−monomer systems was obtained from a full geometry optimization with B3LYP/6-31+G(d,p), taking as input the point corresponding to the minimum energy from the previously calculated PES. 3.2. Propagation Rate Coefficient Calculation. A theoretical evaluation of the propagation rate constant for the polymerization of MMA in the absence and presence of the lithium salt was performed. As in the calculations described

constant (89.1), donor number (16.4), and boiling temperature (521 K).23,24 Therefore, the gel polymer electrolyte obtained is expected to present high ionic conductivity and, thus, potential applications in various electronic devices such as rechargeable lithium batteries. Experimental quantitative determination of both radical concentration and monomer consumption during the polymerization reactions, by electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopies, respectively, was performed. EPR spectroscopy has shown to be a suitable technique to obtain reliable values of the polymerization rate coefficients in both bulk and conventional solvents25−34 and also in the presence of ionic liquids.17 This spectroscopy is a unique tool allowing to connect the increase in propagating radical concentration (increase in the signal intensity) with changes in the medium viscosity (dramatic change in the signal profile). Moreover, the determination of monomer conversion by NMR spectroscopy allows a close link with the kinetic determination by EPR since the measurements can be done with the same sample observed by EPR and in the same tube, avoiding handling errors. For a correct interpretation of the kinetic mechanism, a systematic study of the interactions taking place in lithium salt/ monomer solutions was carried out by means of Fourier transform infrared spectroscopy (FT-IR) complemented with a theoretical analysis devoted to the establishment of the potential energy surface of the Li+−monomer system. In addition, a theoretical study to analyze the effect of the presence of the ionic compound in the propagation rate constant was undertaken, and the results were compared to those experimentally obtained.

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above, the B3LYP/6-31+G(d,p) level of theory was used for geometry optimizations, and energy refinement and calculation of the reaction rate constants were subsequently carried out at the MPW1K/6-31+G(d,p) level. The B3LYP/6-31+G(d,p) combination has been extensively proven to be suitable for obtaining good geometries for stationary points but is known to exhibit a general tendency to underestimate reaction barriers.47 Therefore, additional calculations with MPW1K/6-31+G(d,p) were carried out on the optimized structures to obtain more accurate values of the reaction rate coefficients, since this functional was especially developed for kinetic applications48,49 and has shown to provide reliable results on barrier height calculations.50,51 First, geometry optimization of the transition states (TSs) of the reactions was computed at the B3LYP/631+G(d,p) level of theory. Second, internal reaction coordinate (IRC)52,53 calculations were carried out in order to find the structures of reactants and products that are connected by each transition state. Reaction rate coefficients were predicted in the temperature range 150−350 K on the basis of the transition state theory (TST),54−57 and kinetic parameters were estimated by fitting the data to the Arrhenius equation. All geometry optimizations were followed by a normal-mode analysis to confirm the nature of the stationary points (local minimum or transition state). Harmonic vibrational frequencies were computed at the same level of theory as the geometry optimization and used to provide zero-point vibrational energy (ZPVE). Electronic energies were corrected with the corresponding ZPVE scaled by a factor of 0.9806 or 0.9608 for B3LYP or MP2, respectively.58 Basis set superposition errors (BSSE) were estimated by use of the counterpoise method.59,60 The Gaussian 09 software package61 was employed for all the computations.

Figure 1. EPR spectra of the photoinitiated polymerization of MMA/ EC solution at 260 K registered at different reaction times, with values of [LiTf]/[MMA] = 0, 1/6, and 1/3 for (a), (b), and (c), respectively.

spectrum, explained in terms of hindering of the free rotation of the propagating radical end. 62,63 The higher the salt concentration, the more significant the increase in the EPR signal was, and at earlier reaction time the spectral pattern change was observed. For instance, the 13- to 9-line transition occurred at around 120 and 540 s for polymerizations with ratio [LiTf]/[MMA] of 1/3 and 1/6, respectively. In contrast, the spectral transition was never observed in the polymerization without the ionic compound, not even at a polymerization time of 1140 s. The assignation of the spectra shown in Figure 1 to propagating polymethacrylate radicals with different conformational mobility is well documented;62,63 therefore, we do not extend on it. The variation of the radical concentration, cR, during the photopolymerization of MMA at 260 K in the presence and absence of LiTf is shown in Figure 2. In the polymerization

4. RESULTS AND DISCUSSION 4.1. Radical Photopolymerization Kinetics. The comparative study of the kinetics of the radical photopolymerization of MMA in ethylene carbonate at 260 K, with and without lithium triflate, was carried out by means of EPR and NMR spectroscopies. A thorough analysis of the evolution of the radical concentration with time and determination of the monomer conversion were performed. The reactions were done in solution of EC since the mixture MMA/EC allows much higher solubility of LiTf than bulk MMA (the maximum molar ratio of LiTf in MMA is around 1/12, whereas it reaches 1/3 in MMA/EC (50/50 w/w) solution). At temperatures over 260 K, the high dielectric constant of the monomer/solvent mixture prevented to detect the EPR signal. Dielectric losses drastically decreased when temperature went below 260 K, being possible to get the EPR spectrum. Figure 1 shows the EPR spectra registered at different reaction times for the photopolymerization of MMA/EC without LiTf and with the two analyzed concentrations of salt. The spectrum recorded immediately after the initiation of the photopolymerization consisted in a 13-line spectrum, characteristic of the polymerization of methacrylic monomers in a highly mobile environment. This circumstance was previously observed at low conversion in bulk polymerization or when the reactions were carried out in solution.62 With the progress of the reaction, a great increase in the intensity of the propagating radical signal for the two samples containing lithium salt was observed, simultaneously to a change in the spectral pattern. The signal evolved from the 13-line to a 9-line

Figure 2. Concentration of radicals vs time in the photopolymerization of MMA in EC solution at 260 K with values of [LiTf]/[MMA] = 0, 1/6, and 1/3 for (a), (b), and (c), respectively.

without lithium triflate (blue triangles), the radical concentration remained almost unchanged with time up to the long reaction times (∼1620 s), as expected according to the stationary state attained in conventional radical polymerizations. Conversely, as shown clearly in this figure, in the presence of lithium triflate a stationary state was not reached, and a great increase in the radical concentration occurred from short polymerization times, being this increase much more 5447

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relevant and earlier observed for the highest concentration of the ionic salt. For instance, at a reaction time of about 540 s, the concentration of radical was 4.9 × 10−7 mol L−1 in the absence of salt, while it was 4-fold for polymerization experiments with the low LiTf concentration and more than 2 orders of magnitude larger for the sample with the highest concentration of salt. The monomer conversion, α, achieved in the different polymerization reactions was determined by measuring the remaining monomer concentration by NMR spectroscopy, immediately after the EPR experiments in order to avoid postpolymerization reactions. The resonance signals corresponding to the protons associated with the ethylene carbonate were used as reference. As an example, Figure 3 depicts the 1H

Figure 4. Variation of the time derivative of the radical concentration vs the square of the radical concentration, and variation of ⟨kt⟩ vs time (inset) for the photoinitiated polymerization of MMA/EC solution at 260 K, with values of [LiTf]/[MMA] = 0, 1/6, and 1/3 for (a), (b), and (c), respectively.

is no variation of the radical concentration with time in such experiment, as shown above. The polymerization with the highest LiTf concentration reached a very small value of ⟨kt⟩, which reflects in an almost zero asymptotic slope of the dcR/dt versus cR2 curve. Therefore, according to eq 1, the value of the initiation rate for this polymerization reaction, Rinit, could be estimated by the asymptotic value of that curve, that is, ∼3.4 × 10−7 mol L−1 s−1. This value can be considered also an estimation of the initiation rate for polymerization with [LiTf]/ [MMA] = 1/6, but that of the reaction in the absence of salt has to be considered 5-fold higher, that is, ∼1.7 × 10−6 mol L−1 s−1, due to the difference in the photoinitiator concentration. The average termination rate coefficients for the polymerization reactions in the presence of lithium salt, ⟨kt⟩non‑stat, at the different reaction times can be obtained by applying again eq 1 to each point, i.e.

Figure 3. 1H NMR spectrum corresponding to the photoinitiated polymerization of MMA/EC solution in the absence of LiTf.

⟨k t⟩non‐stat =

NMR spectrum corresponding to the MMA/EC solution without LiTf. The observed peaks corresponded to the protons of MMA, PMMA, and EC, whose assignments are also shown in Figure 3. The low-intensity peaks observed at 3.30 ppm and in the spectral region between 7 and 8 ppm were associated with the methyl (−OCH3) and aromatic protons of the initiator, respectively. The monomer conversions thus obtained were 38, 95, and 17% for the polymerizations with [LiTf]/ [MMA] = 0, 1/6, and 1/3, respectively, shown in Figure 2. As previously explained, the steady state condition was not fulfilled in the polymerizations with LiTf. The rate of radical formation can be described by the equation34 dc R = R init − 2⟨k t⟩c R 2 dt

R init − 2c R 2

dc R dt

(2)

In the polymerization of MMA without LiTf, the stationary average termination rate constant, ⟨kt⟩stat, can be estimated by applying stationary state condition to eq 1, i.e. R ⟨k t⟩stat = 2init 2c R,stat (3) where cR,stat is the radical concentration in the stationary state, that is, around 4.9 × 10−7 mol L−1 according to the experimental data (blue triangles in Figure 2). The value of ⟨kt⟩stat obtained is found to be about 3.5 × 106 L mol−1 s−1, which seems a likely coefficient for the polymerization of MMA at 260 K.64 The termination rate coefficients for the three polymerization reactions are shown in the inset of Figure 4. As can be clearly seen, in contrast to the stationary state experiment, this parameter varies with reaction time for polymerizations in the presence of lithium salt, becoming smaller as the reaction proceeds. The decrease is sharper for the highest concentration of the ionic compound. For instance, in the polymerization with the highest amount of LiTf, ⟨kt⟩ ranges

(1)

where cR is the radical concentration, Rinit is the photoinitiation rate, and ⟨kt⟩ is the average termination rate coefficient. Figure 4 represents the data corresponding to dcR/dt versus cR2 for the two polymerizations carried out in the presence of lithium salt (calculated from values in Figure 2). Data corresponding to the reaction without LiTf are not included in that graph since there 5448

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from 3.5 × 105 L mol−1 s−1 at the beginning of the reaction to 1.0 × 10−3 L mol−1 s−1 when it was stopped; i.e., a reduction of 8 orders of magnitude took place for a reaction time of 540 s, which corresponds to a monomer conversion of only about 17%. The values of the propagation rate constant, kp, in the absence and presence of LiTf can be obtained by means of eq 4, assuming that the coefficient remains unchanged during the reaction time ln

1 = kp 1−α

∫0

t

c R dt

(4)

The experimental values of kp thus calculated were 460, 263, and 62 L mol−1 s−1 for polymerizations with [LiTf]/[MMA] = 0, 1/6, and 1/3, respectively. These results indicate that the addition of LiTf leads to a decrease in the kp value that is larger as the LiTf concentration increases. According to the experimental results obtained in this work, the presence of the lithium salt in the MMA polymerization leads to a dramatic decrease in ⟨kt⟩ and also to a moderate reduction of the kp values. As pointed out earlier, the significant diminution in ⟨kt⟩ is in agreement with prior investigations on the polymerization of ethylene oxide methacrylates with LiTf,22 but the reduction in kp is contrary to the precedent study, where an enhancement in this parameter was reported. 4.2. Propagation Reaction Modeling. Computational methods, in particular those based on the density functional theory (DFT), have previously shown their ability to provide reliable values of the polymerization reaction rate coefficients.65−75 These precedents prompted us to study also theoretically the effect of the addition of lithium salt in the reaction medium on the propagating rate constant, by modeling the propagation reaction of MMA in both the presence and absence of LiTf. The ability of lithium cation for an efficient coordination with oxygen atoms in crown ethers and other polyethers has been previously studied,76 being this strong interaction responsible for the solubility of alkaline salts in the organic medium. So, it is expected to observe such an interaction in the MMA−LiTf system, causing modifications on the propagating and termination rate coefficients. Therefore, a previous study of this interaction was performed for a correct description of the species involved in the propagation reaction. 4.2.1. MMA−Li+ Interaction. A computational investigation on the energy and geometry of the interaction between the lithium cation and the MMA monomer was undertaken by computing the potential energy surface corresponding to the Li+−monomer system. The study was focused on methyl methacrylate, but it was extended also to ethyl (EMA) and butyl (BMA) methacrylates to establish a possible effect of the alkyl side chain in such an interaction. Figure 5 shows the isopotential energy curves of MMA interacting with lithium cation, Li+−MMA system, at the molecular plane of the monomer (XY plane, Z = 0), computed at the MP2/6-31+G(d,p) level of theory. The minimum energy was located at Z = 0, that is, when the Li+ was placed in the same plane that the molecular backbone. There are two fair potential wells localized at the oxygen-containing sites: one corresponding to the coordination of the lithium ion with the ether-like oxygen and another, much deeper, for the interaction of the cation with the carbonyl-like oxygen (relative energies of −27.6 and −47.0 kcal mol−1, respectively). These high values

Figure 5. Isopotential energy curves of the Li+−MMA system at the molecular backbone plane of the monomer (Z = 0), computed at the MP2/6-31+G(d,p) level of theory (X, Y, and Z stand for Cartesian axes; distances in angstroms, energies in kcal mol−1).

evidence the strong favorable interaction for the complexation of the lithium cation with the ester group. The potential energy surfaces corresponding to Li+−EMA and Li+−BMA systems (provided in the Supporting Information) were qualitatively similar to that described for MMA, which indicates that differences in the length of the alkyl side chain do not affect the interaction between the cation and the methacrylic monomer. PES were computed also at the B3LYP/6-31+G(d,p) level of theory for the sake of comparison. Both calculations schemes, B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p) gave rise to practically the same potential energy surfaces and, therefore, to the same conclusions. Table 1 reports the relative energies (ΔE) of the two wells for the three analyzed systems at the MP2/6-31+G(d,p) level Table 1. Relative Energies of Li+−Monomer Complexes at the Two Potential Wells Computed at the MP2/631+G(d,p) Level of Theory

of theory. From these data it is deduced that the interaction energies are similar in the three methacrylates, around −30 and −50 kcal mol−1 for the coordination of the cation with the ether-like and carbonyl-like oxygens, respectively. Figure 6 represents a schematic structure of the coordination complex in its minimum energy (full optimization) at B3LYP/ 6-31+G(d,p). The lithium cation is located quite close to the carbonyl oxygen, and no interaction of this cation with the ether-like oxygen is expected. The position of the Li+ is similar in methyl, ethyl and butyl methacrylic complexes, being the Li+−O distance and the Li+−O−C angle close to 1.75 Å and 160°, respectively, values close to those reported for other compounds with similar interactions.77 In short, according to these theoretical calculations, the preferred position for the complexation of the lithium cation 5449

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Figure 6. Schematic structure of the coordination complexes in their minimum energy at the B3LYP/6-31+G(d,p) level of theory. Li+−O distances and Li+−O−C angles are listed. R = −CH3, −CH2CH3, and −(CH2)3CH3 for methyl, ethyl, and butyl methacrylates, respectively.

with the methacrylic monomers is the carbonyl oxygen of the ester. Taking in account that the magnitude of the interaction energy of Li+ with the ether-like oxygen is 20 kcal mol−1 lower, coordination to carbonyl-like oxygen is expected to be the dominant one. 4.2.2. Propagating Rate Coefficient Computation. On the basis of the results provided by the PES for the MMA−Li+ interaction, we proceeded to determine the propagation rate constant of the propagation reaction of MMA in the presence of LiTf and to compare it to that obtained in the absence of salt. Because of the large size of the polymeric methacrylic radicals involved in the reactions, which would make the calculations unaffordable, a simple model consisting of two monomeric units was considered to represent the propagating radical. The head-to-tail attack was assumed to be the most favorable mode. The bidentate structure of the LiTf ion pair was considered since it was the structure with the minimum energy according to our present calculations and also previously reported.78,79 Three propagation reaction systems were analyzed for the sake of comparison, namely (i) bulk polymerization, that is, both radical (R) and monomer (M) without LiTf (labeled R + M reaction henceforth); (ii) radical uncomplexed and monomer complexed with LiTf (R + MLi reaction); (iii) both radical and monomer interacting with one LiTf molecule (RLi + MLi reaction). As an example, the optimized geometry of the transition state corresponding to the R + MLi propagation reaction is depicted in Figure 7. The kp values for the three reaction systems were calculated according to the TST between 150 and 350 K (including 260 K, experimental temperature). Thereafter, the kinetic parameters were estimated by fitting these data to the Arrhenius equation. The values obtained at 260 K are given in Table 2, where all the kinetic parameters are tabulated, i.e., preexponential factor (A), activation energy (Ea), and propagation rate coefficient at 260 K (kp,260 K). As it is very well-known, the quantitative absolute rate coefficients predicted by DFT calculations not always match correctly the experimental one, especially when the reactive systems are represented by very simple models like in this case.80 Our main objective in this study is the prediction of the qualitative trend followed by the propagation rate constants in the presence of LiTf. Nevertheless, even in that case, it is necessary to check whether the computational method is suitable to describe the system at hand. Thus, the computed coefficients of the propagating reaction of MMA in absence of lithium salt (R + M reaction) were compared to those corresponding to MMA bulk polymerization determined experimentally by pulsed laser polymerization−size exclusion chromatography (PLP-SEC).81

Figure 7. Schematic representation of the transition state corresponding to the reaction between the propagating radical without LiTf and the monomer complexed with LiTf (R + MLi reaction).

Table 2. Frequency Factor (A), Activation Energy (Ea), and Propagation Rate Coefficient at 260 K (kp,260 K), Corresponding to the Three Studied Propagation Reactions Calculated at the MPW1K/6-31+G(d,p)//B3LYP/631+G(d,p) Level R+M R + MLi RLi + MLi

A (L mol−1 s−1)

Ea (kJ mol−1)

kp,260 K (L mol−1 s−1)

3.5 × 10 4.8 × 107 2.2 × 108

22.5 30.9 35.9

105 30 13

6

Figure 8 depicts the variation with temperature of both theoretical and experimental data in the range 150−350 K. The

Figure 8. Theoretical and experimental81 dependence of kp with temperature for MMA bulk polymerization in the range 150−350 K.

agreement between both set of data is excellent for temperatures below ∼250 K. For higher temperatures, the calculations overestimate kp, most likely due to the simplification made in the calculations. Presumably, the differences can be ascribed mainly to the drastic shortening of the length of the polymeric propagating radical and to the lack of a proper molecular environment in gas-phase calculations. As a consequence, the performed calculations provide higher kp values than those 5450

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Figure 9. (a) ATR-FTIR spectra of the carbonyl stretching region in MMA, MMA + LiTf solutions with [LiTf]/[MMA] = 1/24 and 1/12, and MMA + EC + LiTf solutions with [LiTf]/[MMA] = 1/6 and 1/3. (b) Complexation ratio ([CO]complex/[MMA]) as a function of the [LiTf]/ [MMA] ratio.

lithium salt did not significantly shift the noncomplexed band. This means that part of the monomer molecules remained free of the lithium interaction coexisting with the salt−monomer complexes. Something similar happened in the presence of EC, where there was a band shift of about 3 cm−1 for the monomer carbonyl band and the lithium complexes peak roughly at the same position as in the case of the bulk monomer. The complexation ratio ([CO]complex/[MMA]) was calculated from the areas obtained after decovolution of the carbonyl band into two components. The results for the different systems are depicted as a function of the lithium to monomer ratio in Figure 9b. It can be seen that the complexation ratio increases as increases the salt concentration, both in solution with and without EC. The addition of EC reduced the complexation efficiency of the lithium (the ratio of monomer complexed to lithium salt). The origin of this loss of efficiency could be the high solubility of the salt in EC. The EC dissolves the lithium salt and allows increasing the ratio salt to monomer. Comparison of the FT-IR spectra of EC with and without LiTf (not shown) strongly suggests a solvation of the lithium salt by the EC molecules: there are not new bands or shoulders in CO spectral region. The inspection of the spectral region assigned to the symmetric stretching corresponding to the SO3 end of the CF3SO3− anion of the salt points out the ionic association in the different systems, as can be observed in Figure 10. In this region, free of monomer and EC contributions, there were three main components at 1031, 1041, and 1052 cm−1 assigned to free ions, ions pairs, and ion aggregates, respectively.78 The mixtures of monomer and salt presented a broad band in between the aggregates and ion pairs locations, indicating a fair contribution of both species. However, the addition of EC to the systems changed deeply the structure of the spectra. For the systems containing EC, the three components for the ionic associations were clearly detected. Both mixtures, [LiTf]/ [MMA] = 1/6 and 1/3, depicted the maximum contribution to the composed band at the ion pair location. It is supposed that the ion aggregation slows down the mobility of the ions. Then, it is expected that higher ionic mobility will occur in the system in the presence of EC. The polymerization reaction of the trisystem gave rise again to changes in the ionic association. Figure 10 includes the spectrum of the product obtained after polymerization of the mixture MMA + EC + LiTf with [LiTf]/[MMA] = 1/3

expected in a more hindered reactive system as would be the case of the experimental state. Notwithstanding, despite the simplicity of the model considered, there is a very good correlation between experimental and computed data at the temperature at which the experiments have been carried out in the present work, indicating that the methodology is appropriate for extracting at least qualitative conclusions. As can be confirmed from data given in Table 2, both A and Ea parameters increased when the reaction took place in the presence of LiTf, being higher for the case when both reactants were complexed. However, the values of kp followed the opposite trend, which indicates that the activation energy term dominates over the pre-exponential factor; that is, the increment in barrier height was not compensated by the increase in entropy of the transition state. So, the values of kp for polymerizations with and without LiTf, theoretically calculated, followed the same trend than that experimentally determined; i.e., the kp values were lower for the reactions carried out in the presence of the ionic compound compared to the reference system. 4.3. FT-IR Study of the Monomer/Salt System. As concluded from the preceding section, the molecular interactions of MMA and the lithium salts seem to be responsible of the sharp changes in polymerization kinetics. To go deeper in the nature of these interactions, the system was studied by FT-IR spectroscopy as well. This technique is a powerful tool to follow the structural modifications which accompany the solubilization of lithium salts in oxygencontaining organic compounds.82,83 The modifications in the stretch vibrations of the MMA as a result of the lithium cation addition were studied, with special attention to the bond stretching regions for the CO in the monomer, and also those of the SO3 in the lithium triflate salt. Figure 9a shows the carbonyl stretching band in the FT-IR spectra for bulk MMA, and the trisystems MMA + EC + LiTf with [LiTf]/[MMA] values of 1/6 and 1/3, before the polymerization reactions. The figure also includes the spectra for the solutions of MMA with LiTf in two molar ratios, [LiTf]/[MMA] = 1/24 and 1/12, the latter being the saturation point of LiTf in the bulk monomer. The coordination complex described above is nicely detected in these spectra. The addition of LiTf provoked the occurrence of a band shoulder at lower wavenumbers associated with the complexed monomer molecules. However, the presence of the 5451

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reported also very low values of ⟨kt⟩ but in steady state, explaining them by the retardation of the bimolecular termination due to the decrease in the flexibility of the propagating polymer chain. Moreover, the electrostatic repulsion between the complexed propagating chains, specifically at the active radical center, would also contribute to the observed low termination rate. Electrostatic effects would imply a chemical control of the termination reaction instead of a diffusion control, which would reflect in turn in the viscosity of the samples. The viscosity of the system with [LiTf]/[MMA] = 1/3 before polymerization was found to be 4 times higher than that without LiTf (5.3 and 1.3 mPa s, respectively). In a diffusion-controlled termination process, this increase in viscosity would give rise to a 4-fold diminution of ⟨kt⟩.64 However, a 10-fold decrease is found in our system from the beginning of the reaction (3.5 × 106 compared to 3.5 × 105 L mol−1 s−1). The extremely low value of ⟨kt⟩ reached at a conversion of only 17% in the case of polymerization with the higher concentration of salt (∼1.0 × 10−3 L mol−1 s−1) also indicates that diffusion is not the only contribution to the control of the termination reaction. It is evident that there is something more than viscosity playing a role on this process, although a diffusion controlled contribution to the decrease in ⟨kt⟩ cannot be excluded. As shown by the above FT-IR spectra, the lithium salt is present mainly in the form of ion pairs and ion aggregates; that is, each single cation is accompanied by one, two, three, or more bulky triflate anions. Moreover, the solvent EC molecules are, on their turn, likely located as a solvation sphere around the ions. Therefore, it goes without saying that the diffusion of the growing lithium-complexed radicals will be impeded, not only the translational but also the segmental, thus contributing to the control of the termination reaction. Although it is not easy to establish the mechanism taking place, it is possible to state that these two factors, chemically hindered and low diffusion coefficient, make the charged radicals propagate in non-steady-state conditions and terminate with a very low ⟨kt⟩, even at low conversions.

Figure 10. ATR-FTIR spectra of the symmetric SO3 stretching region in MMA, MMA + LiTf solutions with [LiTf]/[MMA] = 1/24 and 1/ 12, MMA + EC + LiTf solutions with [LiTf]/[MMA] = 1/6 and 1/3, and PMMA + EC + LiTf solution with [LiTf]/[MMA] = 1/3.

(spectrum PMMA + EC + LiTf (3:1) in pink). As can be seen, the IR band shifted to the free ion location during the polymerization reaction compared to the spectra before the reaction. This fact has two main consequences: first, the polymerized system should show ion conductivity since the ions are predominantly free, and second, the species in the polymerization media might be varying along the reaction time. It is suspected that the complexation efficiency of the lithium cation and the monomer complexation ratio are changing as the polymer is synthesized, and this likely has an influence on the kinetic parameters. This means that the kp determined for these trisystems would be probably an average value representing the possibilities of reacting couples (complexed monomer and radical, monomer reacting with complexed radical, complexed monomer with uncomplexed radical or both, monomer and radical, uncomplexed). The results obtained in this work show that the presence of the lithium salt leads to a moderate reduction of the kp value for MMA polymerization. As pointed out earlier, this diminution is contrary to a previous study on ethoxy methacrylates.22 The different result can be explained according to the chemical structures of the monomers. Indeed, it was observed that at the same molar concentration of LiTf, the carbonyl complexation is much larger in MMA compared to the simplest monomer of the ethoxy methacrylates series, which is easily explained by the complexation with the ether oxygen atom(s) in the side chain in ethylene oxide methacrylates. This situation would lead to a completely different kinetics for both monomers. On the other hand, the presence of EC molecules could be also responsible of the different kinetic trend. The EC molecules solvating the lithium salt in MMA could hinder the monomer reaction site inducing the decrease observed in kp: the higher salt concentration, the larger complex formation, the more hindered the MMA reaction site. On the other hand, the values of the termination rate coefficient obtained for polymerization of complexed MMA are very low compared to that in the absence of salt, even at the very beginning of the reaction. The formation of a complex between the lithium cation and the oxygen atom at the carbonyl position of the ester in the MMA monomer could explain the low values of ⟨kt⟩ by steric hindrance. Preceding works on sterically very hindered monomers, such as fumarates,84

5. CONCLUSIONS The radical photopolymerization of methyl methacrylate in the presence of lithium triflate has been thoroughly investigated with EPR, NMR, and IR spectroscopies, in conjunction with theoretical analysis. The values of the kinetic polymerization rate coefficients, kp and ⟨kt⟩, obtained by EPR and NMR experiments, resulted to be influenced by the presence of the lithium triflate, being their values both lower than those obtained in the absence of the lithium salt. The effect on the propagation rate coefficient was much less pronounced than on the termination one (decrease of 1 versus 9 orders of magnitude approximately for the higher concentration of LiTf). As a consequence, the overall reaction rate, that is proportional to the kp/⟨kt⟩1/2 ratio, is much greater in polymerizations with lithium triflate. The decrease in the rate parameters can be interpreted taking into account a quite strong complexation between the lithium cation and the ester group of monomer and radical, conclusions obtained from theoretical study as well as from IR absorption experiments. The computed values of the propagation rate coefficients, considering such a complexation, also predicted the decrease in the kp values, due to the increase of the activation barriers for the repulsive interactions between the charged reacting species, the monomer and the polymeric radical, both Li-coordinated. This effect should be much more pronounced 5452

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in the case of the reaction between two growing radicals in the termination process where the Li−Li distance is one bond shorter than in the propagation reaction, which would explain the observed much larger reduction of ⟨kt⟩ compared to kp.

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ASSOCIATED CONTENT

S Supporting Information *

Isopotential energy curves at Z = 0 (molecular plane of the monomer) of Li+−EMA and Li+−BMA systems at the MP2/631+G(d,p) level of theory; imaginary and low frequency modes of the B3LYP/6-31+G(d,p) optimized geometries of all transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (P.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dedicated to Professor Carlos Sieiro on the occasion of his 70th birthday. The authors thank reviewers for their wise and fruitful comments. The Spanish Ministry is gratefully acknowledged for financial support (MAT2011-29174-C02-02).

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REFERENCES

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