NMR as a probe of nanostructured domains in ionic liquids: Does domain segregation explain increased performance of free radical polymerisation?

Abstract

Rotating frame nuclear Overhauser effect spectroscopy (ROESY) has been used to probe the chemical environment in dialkyl-imidazolium ionic liquids. A qualitative use of the distance dependence of the rotating frame Overhauser enhancement (ROE) has shown that reactants and intermediates have variable affinities for the distinct domains that are proposed within ionic liquids. A model system based on the free radical polymerisation of methyl methacrylate (MMA) has been developed to investigate any differing affinities, and to investigate the hypothesis that segregation of species between domains within the ionic liquid structure is contributory towards the generation of unexpectedly high rates of polymerisation and final polymer molecular weights.

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Introduction

Over the past decade interest in the use of ionic liquids has drastically increased leading to an ever expanding number of applications for these very unique materials.¹ As the field has progressed, it has become apparent that a solvent environment very different to that in conventional molecular solvents exists within room temperature ionic liquids. Recent theoretical^2–4 and experimental^5,6 data suggests that a nanostructured environment exists in the liquid state conferring elements of order upon the overall solvent matrix. The formation of distinct nano-scaled domains, the size of which is tuneable by judicious choice of the anion/cation structures, is generally accepted to be dynamic in nature and is believed to impart differential solvent character within the bulk liquid itself. Previous spectroscopic and theoretical studies, that are largely focussed on small molecule-based probes, have indicated preferential partitioning into different solvation environments within the ionic liquid based solvent. For example, theoretical studies⁴ show that the solutes n-hexane, acetonitrile, methanol and water each exhibit preferential solubility into discrete solvation environments, based upon variation of the specific intermolecular interactions. Optical Kerr effect spectroscopy,^7–9fluorescence spectroscopy¹⁰ and the direct measurement of Kamlett Taft parameters,^11,12 have been used to investigate solvation processes, and assess the impact of variations in functionality. In some cases the role of solvent–solute interactions have even been described as influencing reaction outcomes, i.e., via the variation of apparent nucleophilicity^13,14 and protection of reactive intermediates.¹⁵ These studies highlight the degree of complexity associated with solvent–solute interactions in ionic liquids, furthermore the effects of distinct local solvent environments upon reaction chemistries in ionic liquids are still largely unexplored.

It is well known that the nuclear Overhauser effect, and particularly nuclear Overhauser effect spectroscopy (NOESY) can be used to accurately determine intermolecular distances in the liquid or solution state via the through-space interaction of neighbouring spins.^16,17 It has also been shown that NOESY, rotating frame Overhauser spectroscopy (ROESY) and heteronuclear Overhauser spectroscopy (HOESY) can been applied to the mapping of intermolecular interactions within ionic liquids.^18–25 The direct application of the measured relationship between the volumetric integral of ROE cross peaks and resonance distance, to infer structural information about ionic liquid-based systems, was introduced by Osteryoung and co-workers in 1995 where internuclear distances between the protons of imidazolium cations were assessed on a number of [C₂C₁Im][Cl]:[AlCl₃] melts.²¹ This approach has been further pioneered by Mele and co-workers to elucidate the liquid structure of imidazolium ionic liquids²² and the perturbation of that structure with the introduction of solute molecules, specifically water.^18,26 In order to investigate the effect that nano-scale phase separation may have on complex systems in ionic liquids, it is necessary to develop probe systems to model the dynamic behaviour of solutes in the absence of other species that may well disrupt, or initiate, solvent–solute interactions. NMR based experiments, if carried out using externally doped deuterated material, i.e., using co-axial insert tubes to contain a suitable lock solvent, provide an ideal opportunity.

The use of ionic liquids as solvents for free radical polymerisation (FRP) is growing rapidly²⁷ and there are now many examples of ionic liquids being used as a reaction medium for both conventional FRP²⁸ and many of the more recently developed controlled radical techniques (CRP), such as atom transfer radical polymerisation (ATRP),^29–31nitroxide mediated polymerisation (NMP)³² and reversible addition fragmentation chain transfer (RAFT) polymerisation.^33–35 Cumulatively these studies have investigated a range of different monomer systems involving a wide variety of reaction conditions. In summary, two common observations may be distilled from these complementary studies. Firstly, the reaction times required to achieve high conversions are drastically reduced for FRP type reactions conducted in ionic liquids,³⁶ when compared to analogous systems involving conventional solvents. Secondly, and potentially much more interesting on a scientific level, is the common observation that isolated polymers are typically found to have molecular weights which are at least an order of magnitude larger than those materials produced in organic solvents. Furthermore, the relative concentration of monomer in the ionic liquid has also been shown to have a significant effect on the outcome of the polymerisations.²⁷ If reactions are run in a dilute monomer regime, unexpectedly high levels of mechanistic control can be exerted over polymerisation which cannot be achieved in molecular solvents, i.e., high molecular weight block copolymers can be successfully produced by sequential addition of a second monomer unit to a “resting” polymer in the absence of any CRP control agent.^28,37 This single observation poses some significant mechanistic questions. Do radical mediated processes in ionic liquids follow the same mechanisms as in organic solvents? Does the ionic liquid in some way interact with the propagating, or “resting”, radical? This consequently poses fundamental questions regarding the role of the ionic liquid solvent in the mechanism of radical reactions: is it a passive reaction medium? Or does it participate in the stabilisation, or transient protection of the solvated radical? Indeed, it has already been postulated that the formation of charge transfer complexes to radical species may occur in ionic liquids.³⁶ Furthermore, it has been postulated that the encapsulation of reactant species into nanostructured domains in the ionic liquid (Fig. 1) could be one of the responsible contributory factors for the observed phemomena. As such, the reactive species involved in FRP may offer a very suitable probe of the nanoscale structure in ionic liquids.

Fig. 1 Adapted from ref. 2. Snapshot from a molecular dynamics simulation on [C₄C₁Im][PF₆] and inferred relevance on the ionic liquid used in this investigation, [C₄C₁Im][Tf₂N]. The different colours represent the different domains in the ionic liquid: green = non-polar domain and red = polar domain. These colours will be used throughout this investigation to represent the different domains. Reprinted with permission from ref. 2. Copyright 2006 American Chemical Society.

The aim of this investigation was to use oligomeric species of increasing molecular mass, isolated from the catalytic chain transfer polymerisation (CCTP) of MMA, as probe molecules to investigate the affinity of reactant species for the different domains in an ionic liquid. This reaction system was chosen for several reasons: firstly, the transition from monomer to pentamer involves little change in chemical structure, no additional functional groups are either added or removed as we traverse the homologous series, and consequently the resonance frequencies of the protons investigated in NMR correlations remain relatively constant. Secondly, the proton resonances chosen as probes of each domain show significant chemical shift separation from the proton resonances of the ionic liquid itself. Investigations were conducted in the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C₄C₁Im][Tf₂N], for two reasons: both MMA and PMMA (MMAn) are soluble in the ionic liquid and probably more significantly, the composition of the ionic liquid ensures that the ¹H NMR is as simple as possible to minimise the complexity in 2D NMR spectra. The perfluorinated nature of the anion ensures that the ¹H NMR contains only resonances originating from the cation which has signals characteristic of both the polar, or charged, domain (coloured red in Fig. 1), and the non polar, van der Waals dominated domain (represented by the green region in Fig. 1) within the bulk liquid. Here we demonstrate a general NMR based method that has been designed to investigate selective solubility of species between the proposed dynamic domains in ionic liquids.

Discussion of methodology and results

Materials

Unless otherwise stated, all chemicals were obtained from Sigma-Aldrich or Alfa Aesar and were used as received except for 1-methylimidazole, which was distilled over calcium hydride prior to use. Lithium bis(trifluoromethanesulfonyl)imide was obtained from 3M and used as received. The ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, was prepared according to the literature.³⁸ The purity of the ionic liquid was assessed by ¹H and ¹³C NMR spectroscopy, the level of ionic (halide and lithium) contamination was assessed by ion chromatography and the water content by coulometric Karl Fisher titration. Chloride concentration was found to be < 100 ppm, Li⁺ was also found to be < 100 ppm and water was < 20 ppm.

Oligomer synthesis

MMA_n oligomers were prepared using a modified literature procedure³⁹ in which catalytic chain transfer polymerisation (CCTP) of MMA (1 L, 9.35 mol) was carried out in refluxing acetone (1 L) using 4,4′-azobis(4-cyanovaleric acid) (1.50 g, 5.4 mmol) as initiator and PhCoBF (0.50 g, 7.9 mmol) as the chain transfer catalyst. The reaction was conducted under an atmosphere of dry nitrogen gas and the reaction time was 3 h, at which point the conversion of monomer reached the desired conversion of ∼10%. Post-polymerisation, a brown slurry was recovered after the removal of residual monomer and solventviaevaporation. The crude, viscous product was diluted by the addition of CH₂Cl₂ (200 ml), the resultant solution was then passed through a column of neutral alumina to remove any components containing the initiator moiety. Subsequent fractionation by distillation afforded MMA dimer (52 g, 44–46 °C/0.37 mbar) and trimer (5 g, 90–92 °C/0.15–0.17 mbar) as colourless liquids. Column chromatography (silica gel; CH₂Cl₂/EtOAc = 96/4) of the crude product yielded MMA tetramer (1.30 g) and pentamer (0.53 g) as colourless oils which solidified over a period of 2–3 weeks. Spectroscopic data of each fraction was consistent with that published in the literature.^39,40

Instrumentation

Ion chromatographs were recorded on a Dionex ICS-3000 fitted with an RFIC IonPac AS-20 column, using an isocratic eluent stream of H₂O:MeCN:NaOH (60 : 25 : 15) for anion analysis and an RFIC IonPac CS-17 column with an isocratic eluent stream of H₂O:MeCN:MSA (60 : 25 : 15) for analysis of cations. Coulometric Karl Fischer titration was recorded on a Mitsubishi CA-100 moisture meter. Unless otherwise stated, NMR experiments were performed at 298 K. ¹H and ¹³C NMR spectra were acquired on a Bruker DPX300 instrument fitted with a 5 mm autotunable broad-band (BBFO) probe. 2D ROESY spectra and ROE build up curves were recorded on a Bruker AV(III)500 instrument fitted with a 5 mm autotunable dual ¹H/¹³C (DCH) cryoprobe.

ROESY experiments

The ionic liquid was degassed under vacuum (2 × 10⁻² mbar) for 12 h prior to each experiment to remove any paramagnetic oxygen from the sample. The sample was placed in a 2 mm external diameter insert and surrounded by DMSO-d₆ solvent to provide the field-frequency lock (this configuration gives shorter NMR pulsewidths than placing the ionic liquid around the insert⁴¹). All 2D ROESY experiments were obtained using a phase alternated spin-lock⁴² of 200 ms duration at a field strength of 3 kHz unless otherwise stated. 256 increments of 2k data points were recorded covering a spectral width of 13 ppm in each dimension with a relaxation delay of 3 s. Data matrices were zero filled and multiplied by a cosine-squared window in both dimensions prior to Fourier transformation. Small regions containing the cross peaks of interest were strip transformed, baseplane corrected and processed independently as described in the ESI†.

Data analysis and general methodology

Each species was dissolved in the ionic liquid and investigated using a 2D ROESY pulse sequence.⁴² Initial attempts to use a NOESY sequence for the investigation failed, presumably due to the molecular tumbling rate falling into the ‘intermediate regime’ between fast and slow tumbling (positive and negative NOE, respectively) resulting in little or no observable signal: hence the choice of the ROESY pulse sequence for the investigation.

The concentration of each species in the ionic liquid is shown in Table 1. In order to model the growing polymer chain, the concentration of MMA units was kept constant in all experiments.

Table 1 Concentration of domain probe in each sample

Probe	Concentration/mM	Equivalence of MMA units^a
a Equivalence of MMA = 1 by definition: equivalence of subsequent MMA oligomers defined as [MMA]/([Oligomer] × (no. MMA units)).
MMA	46.4	1
MMA2	23.6	0.983
MMA3	15.5	0.998
MMA4	11.9	0.975
MMA5	9.3	0.998

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Proton NMR spectra were assigned (Table 2) and the resonances of interest labelled according to the Schemes given in Fig. 2 and Fig. 3 (imidazolium protons are denoted H1-C to H8-C; oligomeric protons are H1-O to H6-O). The acquisition of 2D ROESY data was optimised by the measurement of 1D ROE build up curves for each sample, in order to determine a suitable spin lock time. A spin lock time of 200 ms was chosen, since this represented a section of the build up curve near the top of the linear build up regime maximising signal-to-noise whilst maintaining the distance dependence of the ROE. The spin lock time of 200 ms was less than half the value of the shortest T₁ in the sample.

Table 2 Chemical shifts of the resonances investigated

Resonance	δ (ppm)^a
a δ relative to external d6-DMSO at 2.5 ppm.
H2-C	8.04
H8-C	0.35
H1-O	5.61
H2-O	4.99
H3-O	3.13
H4-O	3.06–3.02
H5-O	2.09–1.90
H6-O	1.75–1.45

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Fig. 2 The ionic liquid used in this investigation; [C₄C₁Im][Tf₂N] with numbered assignment of the imidazolium protons (H1-C to H8-C, where C denotes cation in each case).

Fig. 3 Structures of a) MMA, b) MMA₂ and c) MMA₃, MMA₄ and MMA₅ the oligomeric species used in this investigation showing coloured assignment of the protons (Black = H1-O, Red = H2-O, Green = H3-O, Blue = H4-O, Orange = H5-O, Pink = H6-O: O denotes oligomer).

In order to study the individual environments of the MMA_n (n = 1–5) oligomers in the ionic liquid solvent, it was necessary to measure volume integrals of the 2D ROESY cross peaks between the protons of the oligomer and those of the imidazolium cation. Since the intensity of a given ROESY cross peak is directly related to the distance between the spins,⁴³ any preference of the MMA_n molecule for a particular ionic liquid domain should be manifested as a departure from the anticipated or “ideal” intermolecular ROESY cross peak intensities expected in a completely homogeneous sample. The imidazolium cation resonances H2-C and H-8C are well resolved in the proton spectrum and are ideal reporters for the polar and non-polar domains of the ionic liquid respectively: therefore, the ROESY cross peaks of these protons were the target of the current work.

Unfortunately, measurement of the volume integrals of the oligomer-cation ROESY cross peaks was complicated by the low intensity of these peaks relative to the diagonal (leading to minor baseplane distortions). Consequently, it was desirable for the peaks of interest to be processed independently to the rest of the spectrum. To obtain a full volumetric integral a 1D projection was produced by summing the data in the F2 direction, separating positive and negative signals. Consequently minor artefacts, which caused distortion of the 1D peak shape, could then be processed separately and recombined after peak fitting to minimise error in the fitting routine.

The 1D summations were then subjected to a peak fitting operation using fixed peak positions, obtained from the relevant 1D spectra (Fig. 4). The peak widths at half maximum (FWHM) were fixed and based upon peak widths of the isolated signals of relevant peaks. A Voigt type GL (30% Lorentzian) peak shape was used for the fittings.

Fig. 4 Top: Surface and contour plots of the ROE cross peak from (left) H1-O (MMA₃) to H2-C and (right) H2-O (MMA₂) to H8-C. Bottom: Positive 1D summations and fit peaks for the above cross peaks. Note: the double-headed arrows on the structures indicate the through space interactions considered in each case.

As stated above, any preference of an MMA molecule for a particular domain within the ionic liquid should be manifest by variance from a theoretical or “ideal” intensity of the relevant intermolecular ROE peaks. Since there are 1 and 3 protons respectively contributing to the ionic liquid H2-C and H8-C proton resonances, it might be expected that the ratio of the relative volumes of the cross peaks between any given oligomer resonance and these protons would be 1:3. However, this would only be the case if the oligomer molecule investigated was, on average across the whole sample for the duration of the experiment, equidistant from the imidazolium ring and the end of the alkyl chain of the cation, i.e. if the sample was homogenous and there was no domain segregation of the MMA_n species. Clearly if the solute oligomer molecule in question was typically closer to the imidazolium ring (selectively solubilised in the polar domain), the volume of the oligomer/H8-C ROE cross peak would be selectively reduced compared to that of the oligomer/H2-C cross peak. On the other hand, if the solute molecule in question is closer to the alkyl chain, i.e. selectively solubilised in the non-polar domain, then an increase would be expected.

To gain a qualitative view of the selective solubility of the different MMA_n (n = 1–5) species in each ionic liquid domain, the volume integral of the intermolecular ROE cross peak between H2-C (polar domain) or H8-C (non-polar domain) and each oligomeric resonance (H1-O to H6-O) was calculated and compared [eqn (1)]. We have termed this value the Domain Preference Integral Ratio (DPIR).

The preference of an MMA_n molecule (or fragment) for either domain can be seen as a deviation of its DPIR from the ideal value of 3. When DPIR < 3 the probe is selectively soluble in the polar domain, when DPIR > 3 the non polar domain is favoured.

Note that a common problem associated with ROESY spectroscopy is attenuation of the cross peak intensity as a function of distance of the proton signal from the offset frequency. This so-called off-resonance effect has previously been shown to be an ever present hindrance to the measurement of exact distances in ROESY spectroscopy.⁴⁴ However, this problem has been avoided in the current work since we are looking at sample dependent changes in the relative volumes of the cross peaks occurring in the same spectrum and, importantly, these peaks have very nearly the same chemical shift from sample to sample. Consequently off-resonance effects are a negligible source of error in this work and the use of the DPIR has the advantage that we can disregard the usual problems with using distance information from ROESY spectra.

The DPIR (Domain Preference Integral Ratio) for each oligomeric resonance of the MMA_n species studied is shown in Fig. 5.

Fig. 5 DPIR for each oligomeric resonance showing segregation into the polar (red) or non-polar (green) domain of the ionic liquid. The ROE cross peak from H6-O to H8-C for MMA₅ lay too close to the diagonal for any reliable data to be collected.

Upon inspection, if one considers the data presented in Fig. 5, it is immediately apparent that an increase in DPIR (indicative of an increase in selective solubility in the non-polar domain) can be observed with increasing oligomeric size. Furthermore, the DPIR measured for a given oligomeric proton is influenced by the functionality of its environment; with the alkyl groups having a greater selective solubility in the non-polar domain than the vinyl functionalities. Both of these observations have significant implications for FRP processes in ionic liquids, allowing mechanistic explanations of previously reported experimental data as described below.

Primarily, the observation of significantly increased DPIR with increased molecular size is highly suggestive that oligomeric species will shift preferentially from the polar domain into the non-polar domain of the ionic liquid as the polymerisation progresses. Partitioning of the growing polymer chain in this way may result in the system adopting a pseudo emulsion-type polymerisation mechanism. This would be expected to lead to an increased rate of reaction and higher molecular weight polymer products when compared to systems carried out in well solubilised homogeneous systems. Consequently, the variation in DPIR observed for MMA_n oligomers in the current work is consistent with data and hypotheses reported in recent studies on the outcomes of FRP reactions.²⁸ The partitioning of the growing polymer chain also supports recent observations of the efficacy of different RAFT agents in controlling the FRP of MMA.³⁴

The variation of DPIR with respect to the environment surrounding the oligomer-bound functional group is strong evidence that the growing polymer species are susceptible to steric control by the domain structure within the ionic liquid. The vinyl protons (H1-O and H2-O) are situated near the interface between the two domains whilst the less polar alkyl backbone (H5-O and H6-O) is situated fully in the non-polar domain. The strong partitioning that is observed for the alkyl backbone protons (DPIRs of > 20) is highly significant as it indicates that an increase in the number of these protons (a growing polymer chain) would facilitate further partitioning into the non-polar domain.

Conclusions

The use of a dynamic system of probe molecules has shown that segregation of reactive species into nanoscale domains in room temperature ionic liquids can be observed by the use of ROESY spectroscopy. Furthermore, a qualitative method, which is universally applicable, for distinction between polar and non-polar domains in imidazolium-based ionic liquids has been introduced.

More specifically, by employing ROESY NMR spectroscopic techniques, it has been shown that oligomeric forms of MMA show a time averaged preferential concentration within one of the nanoscale domains of the ionic liquid [C₄C₁Im][Tf₂N]. We proposed that this is the non-polar domain, and that the confinement of these species within this particular region provides a plausible explanation for recent kinetic observations on the polymerisation of MMA in ionic liquids. It has also been shown that the growing polymer chain migrates further into the non-polar domain as the molecular weight is extended and that polar groups within the polymer, such as vinyl groups, can affect the conformation of the backbone chain within the ionic liquid, orienting themselves towards the polar region.

This study has demonstrated that a well implemented and widely accessible spectroscopic technique can yield a wealth of mechanistic and structural information, producing evidence for domain segregation and preferential orientation of reactive species in ionic liquids. Our results support the hypothesis that these mechanistic differences are being introduced as a result of the segregated solvent environments within the ionic liquid itself. Furthermore, these results suggest that these changes can be taken through to high molecular weight/conversion by a combination of the functional group affinity for domain interface and associated nano-scale segregation into these domains. While we are not able to comment upon the actual details of radical and/or control agent–ionic liquid interactions, our data strongly suggests propagation occurs at the ionic liquid domain interface. These experiments provide insight into ionic liquid-based chemistries far beyond the scope of polymerization reactions and are thus expected to be of significant interest and utility in the rapidly expanding field of ionic liquid research.

Acknowledgements

Financial support from the Australian research council (DP0880032) and the EPSRC under the Science and Innovation Award (DICE) is gratefully acknowledged (EP/D501229/1). P.L. is holder of an EPSRC Advanced Research Fellowship (EP/D073014/1).The authors thank Dr Richard Bourne for helpful discussions and assistance in the generation of figures.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c1sc00207d

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