Getting faster: low temperature copper-mediated SARA ATRP of methacrylates, acrylates, styrene and vinyl chloride in polar media using sulfolane/water mixtures

Abstract

Supplemental activator and reducing agent atom transfer radical polymerization (SARA ATRP) of acrylates, methacrylates, styrene and vinyl chloride was successfully performed in sulfolane/water mixtures using ppm amounts of soluble copper. The catalytic effect of the presence of water in the reaction mixtures resulted in a notable acceleration of the polymerization of the different monomers studied. The first-order kinetics with monomer conversion and the low dispersity values (Đ) of the polymers revealed the controlled features of the polymerization. As a proof-of-concept, an ABA block copolymer of poly(methyl acrylate)-b-poly(vinyl chloride)-b-poly(methyl acrylate) was prepared, confirming also the “living” character of the polymers. The results presented in this contribution extend the importance of sulfolane as an universal industrial solvent for the SARA ATRP of a broad range of monomer families by significantly enhancing the polymerization rate, due to the selective addition of water to the solvent mixture. The incorporation of small amounts of water in the solvent mixture has also allowed the use of FDA-approved sulfites as the SARA agent, which was not possible using pure sulfolane as the polymerization solvent.

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Introduction

Reversible deactivation radical polymerization (RDRP) methods have witnessed a remarkable development over the last two decades, since they allow the preparation of tailor-made polymers. Atom transfer radical polymerization (ATRP), which is a metal-catalyzed RDRP, is one of the most studied methods due to its robustness, versatility, monomer tolerance and mild reactions conditions.¹ Important research efforts have been devoted to the reduction of the amount of metal complexes required to perform ATRP reactions. On this matter, different ATRP variations have been developed, lowering the required amount of catalyst to ppm levels to afford fast and controlled polymerizations. These methods include: activators regenerated by electron transfer (ARGET) ATRP,² initiators for continuous activator regeneration (ICAR) ATRP,³ electrochemically mediated ATRP (e-ATRP)⁴ and supplemental activator and reducing agent (SARA) ATRP.^5,6 From both environmental and industrial standpoints, the SARA ATRP is a very attractive technique since it can be performed using heterogeneous zerovalent metals^6–8 (easily removed from the reaction medium) and Food and Drug Administration (FDA)-approved inorganic sulfites as SARA agents.^9–12 These catalysts will then be used for the continuous regeneration of the activator species (e.g., CuX; X: halide) during the polymerization and will also participate on the direct activation of alkyl halides, to compensate for some termination reactions that might occur (Scheme 1).¹³ Nevertheless, Cu(I) is the main activator as it is in normal ATRP.

Scheme 1 General mechanism of the Cu(0)/CuX₂/L-catalyzed SARA ATRP (L: ligand and X: halide).

Several well-defined polymers and block copolymers have been synthesized using SARA ATRP in different solvents, such as water,¹⁴ alcohol/water mixtures^7,10,12,15 and anisole.¹¹

Recently, our research group has reported the use of a dipolar aprotic industrial solvent, sulfolane, as an universal solvent for the SARA ATRP of acrylates, methacrylates, styrene and vinyl chloride.¹⁶ The robustness of the system was clearly demonstrated by the straightforward synthesis of PMA-b-PVC-b-PMA and PS-b-PVC-b-PS triblock copolymers. The use of an universal industrial solvent for the controlled synthesis of macrostructures based on a wide range of monomer families by SARA ATRP provides an effective and easy route to access a vast portfolio of different block copolymers at a potential large scale. The presence of water in solvent mixtures has been shown to significantly increase the rate of polymerization for the SARA ATRP of different monomers using either zero-valent metals¹⁷ or sulfites,^10–12 while maintaining the controlled features of the polymerizations. This is due to the increase of the activity of Cu(I) complexes in more polar medium. The maximum water content is typically determined by the solubility of the polymer in the solvent mixtures (e.g., ≈35% for poly(methyl acrylate) (PMA) with DP = 222 in ethanol/water mixtures at 30 °C;¹⁷ ≈5% for poly(2-diisopropylamino)ethyl methacrylate with DP = 50 in isopropanol/water mixtures at 40 °C).¹¹ In this contribution, we discuss the influence of water in sulfolane/water mixtures as the solvent for faster SARA ATRP reactions of different monomer families, which can be advantageous in the case of a future industrial scale implementation of the method.

Experimental section

Materials

Methyl acrylate (MA) (99% stabilized; Acros), methyl methacrylate (MMA) (99% stabilized; Acros) and styrene (Sty) (+99%; Sigma-Aldrich), were passed over a sand/alumina column before use to remove the radical inhibitors.

Vinyl chloride (VC) (99%) was kindly supplied by CIRES Lda, Portugal.

Copper(II) bromide (CuBr₂) (+99% extra pure, anhydrous; Acros), copper(II) chloride (CuCl₂) (max. 0.0008% AS; Merck), zerovalent iron powder (Fe(0)) (99%, ≈70 mesh, Acros), deuterated chloroform (CDCl₃) (+1% tetramethylsilane (TMS); Euriso-top), deuterated tetrahydrofuran (THF-d₈) (99.5%; Euriso-top), sulfolane (+99%; Acros), ethyl 2-bromoisobutyrate (EBiB) (98%; Aldrich), p-toluenesulfonyl chloride (TsCl) (98%; Merck), ethyl α-bromophenylacetate (EBPA) (97%; Aldrich), bromoform (CHBr₃) (+99% stabilized; Acros), tris(2-aminoethyl)amine (TREN) (96%; Sigma-Aldrich), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) (99%; Aldrich), 2,2′-bipyridine (bpy) (>99%, Acros) polystyrene (PS) standards (Polymer Laboratories) were used as received.

High-performance liquid chromatography (HPLC) grade THF (Panreac) was filtered (0.2 μm filter) under reduced pressure before use. Tris[2-(dimethylamino) ethyl] amine (Me₆TREN) was synthesized according to the procedure described in the literature.¹⁸ Metallic copper (Cu(0), d = 1 mm, Sigma Aldrich) was washed with HCl in methanol and subsequently rinsed with acetone and dried under a stream of nitrogen following the literature procedures.⁶ Purified water (Milli-Q®, Millipore, resistivity > 18 MΩ cm) was obtained by reverse osmosis.

Instrumentation

The chromatographic parameters of the samples were determined using high performance size exclusion chromatography HPSEC; Viscotek (Viscotek TDAmax) with a differential viscometer (DV); right-angle laser-light scattering (RALLS, Viscotek); low-angle laser-light scattering (LALLS, Viscotek) and refractive index (RI) detectors. The column set consisted of a PL 10 mm guard column (50 × 7.5 mm²) followed by one Viscotek Tguard column (8 μm), one Viscotek T2000 column (6 μm), one Viscotek T3000 column (6 μm) and one Viscotek LT4000L column (7 μm). HPLC dual piston pump was set with a flow rate of 1 mL min⁻¹. The eluent (THF) was previously filtered through a 0.2 μm filter. The system was also equipped with an on-line degasser. The tests were done at 30 °C using an Elder CH-150 heater. Before the injection (100 μL), the samples were filtered through a polytetrafluoroethylene (PTFE) membrane with 0.2 μm pore. The system was calibrated with narrow PS standards. The dn/dc was determined as 0.063 for PMA, 0.068 for poly(methyl methacrylate), 0.105 for poly(vinyl chloride) (PVC) and 0.185 for polystyrene (PS). Molecular weight (M_n^SEC) and Đ of the synthesized polymers were determined by multidetectors calibration using the OmniSEC software version: 4.6.1.354.

400 MHz ¹H NMR spectra of reaction mixture samples were recorded on a Bruker Avance III 400 MHz spectrometer, with a 5 mm TIX triple resonance detection probe, in CDCl₃ with tetramethylsilane (TMS) as an internal standard or THF-d₈. The monomer conversion was determined by integration of monomer and polymer peaks using MestRenova software version: 6.0.2-5475.

Atomic Absorption Spectroscopy (AAS) was performed on a Perkin Elmer (Model 3300, USA) to evaluate the content of residual copper catalyst in the purified polymers.

Procedures

Typical procedure for the [Cu(0)]₀/[CuBr₂]₀/[Me₆TREN]₀ = Cu(0) wire/0.1/1.1 catalyzed SARA ATRP of MA in sulfolane/water = 90/10 (v/v). Cu(0) wire (l = 5 cm) and a solution of CuBr₂ (3.506 mg, 0.016 mmol) and Me₆TREN (39.76 mg, 0.173 mmol) in sulfolane (1.42 mL) and water (157.9 μL) were placed in a Schlenk tube reactor. A mixture of MA (3.16 mL, 34.85 mmol) and EBiB (30.62 mg, 0.157 mmol) was added to the reactor that was sealed, by using a glass stopper, and frozen in liquid nitrogen. The Schlenk tube reactor containing the reaction mixture was deoxygenated with four freeze–vacuum–thaw cycles and purged with nitrogen. The reactor was placed in a water bath at 30 °C with stirring (700 rpm). During the polymerization, different reaction mixture samples were collected by using an airtight syringe and purging the side arm of the Schlenk tube reactor with nitrogen. The samples were analyzed by ¹H NMR spectroscopy to determine the monomer conversion and by SEC, to determine M_n^SEC and Đ of the polymers.

The final polymer was precipitated in water and then dissolved in acetone and passed over a sand/alumina column to remove the copper catalyst. The solution was concentrated and the product was recovered by precipitation in water. The content of residual copper catalyst in the PMA was determined by AAS (100 mg of PMA in 10 mL of ethanol/water = 80/20 (v/v)).

The polymerizations of MMA and Sty were conducted following the same procedure and adjusting the reaction conditions (e.g., temperature, catalyst nature, etc.).

Typical procedure for the [Cu(0)]₀/[TREN]₀ = Cu(0) wire/1 catalyzed SARA ATRP of VC. A 50 mL Ace glass 8645#15 pressure tube, equipped with bushing and plunger valve, was charged with a mixture of CHBr₃ (82.9 mg, 0.33 mmol), TREN (48.0 mg, 0.33 mmol), Cu(0) wire (l = 5), sulfolane (4.50 mL) and water (0.50 mL). The precondensed VC (5 mL, 72.8 mmol) was added to the tube. The exact amount of VC was determined gravimetrically. The tube was closed, placed in liquid nitrogen and degassed through the plunger valve by applying reduced pressure and filling the tube with N₂ about 20 times. The valve was closed, and the tube reactor was placed in a water bath at 42 °C with stirring (700 rpm). The reaction was stopped by plunging the tube into ice water. The tube was slowly opened, the excess of VC was evaporated inside a fume hood, and the mixture was precipitated into methanol. The polymer was separated by filtration and dried in a vacuum oven until constant weight to produce. The monomer conversion was determined gravimetrically. SEC was used for the determination of PVC's M_n^SEC and Đ.

Typical procedure for the synthesis of PMA-b-PVC-b-PMA block copolymers by “one-pot” [Cu(0)]₀/[TREN]₀ = Cu(0) wire/1 catalyzed SARA ATRP. A 50 mL Ace glass 8645#15 pressure tube, equipped with bushing and plunger valve, was charged with a mixture of CHBr₃ (110.5 mg, 0.44 mmol), TREN (63.9 mg, 0.44 mmol), Cu(0) wire (l = 5), sulfolane (2.7 mL) and water (0.3 mL). The precondensed VC (3.0 mL, 43.7 mmol) was added to the tube. The exact amount of VC was determined gravimetrically. The tube was closed, submerged in liquid nitrogen and degassed through the plunger valve by applying reduced pressure and filling the tube with nitrogen about 20 times. The valve was closed, and the tube reactor was placed in a water bath at 42 °C with stirring (700 rpm). After 1.25 h, the reaction was stopped by plunging the tube into ice water. The tube was slowly opened and the excess VC was evaporated inside a fume hood. The monomer conversion was determined gravimetrically (47.3%), and the M_n^SEC = 7.8 × 10³ and Đ = 1.69 were determined by SEC analysis. A mixture of sulfolane (10.8 mL), water (1.2 mL) and MA (12 mL, 132.4 mmol) was added to the same 50 mL Ace glass 8645#15 pressure tube (without any purification of the α,ω-di(bromo) PVC macroinitiator). The tube was closed, submerged in liquid nitrogen and degassed through the plunger valve by applying reduced pressure and filling the tube with nitrogen about 20 times. The valve was closed, and the tube reactor was placed in a water bath at 42 °C with stirring (700 rpm). The reaction was stopped after 0.5 h and the mixture was analyzed by ¹H NMR spectroscopy in order to determine the MA conversion and by SEC, to determine the M_n^SEC and Đ of the PMA-b-PVC-b-PMA triblock copolymer.

Results and discussion

Our research group¹⁶ has reported for the first time the use of sulfolane as an universal solvent for the Cu(0)-mediated SARA ATRP of acrylates, methacrylates, Sty and VC. The main goal of the work was the substitution of dimethyl sulfoxide (DMSO), which was a common solvent for the polymerization of several monomer families, by an industrially viable solvent such as sulfolane. The results showed fast and controlled polymerizations, using low amounts of soluble copper for the SARA ATRP of MA, MMA, St and VC. It is accepted that water is the most suitable solvent for an industrial scale polymerization due to its low cost and non-toxic character, when compared to organic solvents. In addition, it is known that the addition of water to a SARA ATRP solvent system leads to an increase of the reaction rate, which can also contribute to reduce the cost of production in large scale.

The effect of water on the SARA ATRP of MA was firstly evaluated using different ratios sulfolane/water in order to find the optimal value that could afford very fast yet controlled polymerizations. Table 1 shows that the presence of water in the reaction mixture allowed the increase of the reaction rate in comparison to pure sulfolane (e.g., 1.5 times faster for 10% water content), as it was previously observed for other SARA ATRP systems.¹⁶

Table 1 Kinetic data for the Cu(0)-mediated SARA ATRP of MA in sulfolane/water mixtures. Conditions: [MA]₀/[solvent] = 2/1 (v/v); [MA]₀/[EBiB]₀/[Cu(0) wire]₀/[CuBr₂]₀/[Me₆TREN]₀ = 222/1/1/0/1.1; T = 30 °C

Entry	S/W^a (v/v)	kp^app (h^−1)	Time (h)	Conv.^b (%)	Mn^th × 10^−3	Mn^SEC × 10^−3	Đ
a Sulfolane/water.b Maximum monomer conversion obtained in the reaction.
1	100/0	1.426	1.3	83	16.0	17.4	1.05
2	95/5	1.527	1.2	86	16.6	21.9	1.11
3	90/10	2.266	1.2	93	17.9	23.8	1.06
4	80/20	2.240	1.0	81	15.6	24.3	1.25
5	70/30	1.100	1.0	69	13.3	21.8	2.03
6	65/35	—	2.6	69	13.8	33.1	1.37

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Up to 20% of water content in the solvent mixture, the rate of reaction was first-order with respect to monomer conversion and the molecular weights determined by SEC were in close agreement with the theoretical values (Fig. S1†). In addition, the resulting PMA presented very low dispersity (Đ ≤ 1.1) throughout the reaction. Water contents of 30% or higher were found to be not suitable for this particular sulfolane-based solvent mixture, since there was precipitation of the PMA (DP = 222), which ultimately hampered any control over the polymerization (Table 1, entries 5 and 6, Fig. S1†).

The kinetic data suggests that 10% of water represents an interesting compromise between the solubility of the polymer, the rate of reaction and the control over the molecular weight of the polymers. Therefore, this value was selected for the following studies.

SARA ATRP of (meth)acrylates, styrene and vinyl chloride in sulfolane/water = 90/10 (v/v)

The use of the solvent mixture sulfolane/water = 90/10 (v/v) was investigated for the Cu(0)-catalyzed SARA ATRP of MA, MMA, Sty and VC (non-activated monomer) (Fig. 1). The catalytic complexes and initiators were selected according to the structure of the monomers, in order to provide well-controlled polymerizations.¹⁶ Preliminary experiments were carried out for the SARA ATRP of MA catalyzed by Cu(0) in presence of Me₆TREN. The CuBr₂/Me₆TREN complex is known to be one of the most powerful deactivators for the ATRP of MA in polar media, providing excellent control during the polymerization.¹⁰ In the case of the MMA polymerization, the induction period was previously eliminated by adding CuBr₂ in the beginning of the polymerization and the use of bpy as the ligand provided PMMA with relatively narrow Đ.⁵ Regarding the St polymerization, it is known that the ligand Me₆TREN increases the apparent rate constant of polymerization when compared with PMDETA. In addition, the polymerization is usually performed at high temperatures (e.g., 60 °C) to afford a reasonable rate of polymerization.⁸ Finally, the SARA ATRP of VC requires the use of TREN as the ligand, to efficiently mediate the ATRP equilibrium, and CHBr₃ as the initiator, to obtained PVC with a well-defined structure.¹⁹

Fig. 1 Kinetic plots of conversion and ln[M]₀/[M] vs. time and plot of number-average molecular weights (M_n^SEC) and Đ (M_w/M_n) vs. conversion for the SARA ATRP of (a) MA, (b) MMA, (c) St and (d) VC using Cu(0) wire as the SARA agent in solvent sulfolane/water = 90/10 (v/v). Conditions: (a) [MA]₀/[EBiB]₀/Cu(0) wire/[Me₆TREN]₀ = 222/1/Cu(0) wire/1.1 (molar), [MA]₀/[solvent] = 2/1 (v/v) and T = 30 °C; (b) [MMA]₀/[EBPA]₀/Cu(0) wire/[Cu(II)Br₂]₀/[bpy]₀ = 222/1/Cu(0) wire/0.1/2.2 (molar), [MMA]₀/[solvent] = 1/1 (v/v) and T = 40 °C; (c) [St]₀/[EBiB]₀/Cu(0) wire/[PMDETA]₀ = 222/1/Cu(0) wire/1.1 (molar), [St]₀/[solvent] = 2/1 (v/v) and T = 60 °C; (d) [VC]₀/[CHBr₃]₀/[Cu(0) wire]₀/[TREN]₀ = 222/1/Cu(0) wire/1, [VC]₀/[solvent] = 1/1 and T = 42 °C; Cu(0): d = 1 mm, l = 5 cm.

All the polymerizations were first-order with respect to monomer conversion and all the kinetic data obtained were with agreement with the ones previously reported for the polymerizations in anhydrous sulfolane.¹⁶

Additionally, the rate of polymerization was considerably higher than the previously reported for MA, MMA and VC (≈59% for MA, ≈43% for MMA and ≈88% for VC). However, it is interesting to notice that for Sty, the addition of 10% of water in the reaction mixture did not influence the rate of polymerization. Nevertheless, the control over the PS molecular weight was good. Even though, these are encouraging results which suggest that the SARA ATRP method developed could be potentially used at an industrial scale. Faster reactions using industrial solvents will turn the large production much more affordable.

Moreover, due to the low concentration of metal catalyst used, the pure polymers presented a residual copper content (e.g., 20 ppm of copper in a PMA sample derived from a [MA]₀/[EBiB]₀/[Cu(0) wire]₀/[Me₆TREN]₀ = 222/1/1/1.1 SARA ATRP or 122 ppm of copper in a PMA sample derived from a [MA]₀/[EBiB]₀/[Cu(0) wire]₀/[CuBr₂]/[Me₆TREN]₀ = 222/1/1/0.1/1.1 SARA ATRP). Therefore, we strongly believe that the creation of industrially feasible ATRP methods for the production of common polymers will allow a wider use of the technique.

Influence of the catalytic system

In our previous report on the SARA ATRP in sulfolane,¹⁶ it was demonstrated that the polymerization could not be performed using the FDA-approved Na₂S₂O₄ as the SARA agent, due to the insolubility of this compound in the solvent. However, the use of Na₂S₂O₄ as a co-catalyst is a very interesting and safe alternative to the use of copper, since it is not a metal-compound and it is commonly used in some beverages (e.g., wine). Previous results have shown that the addition of water to organic-based solvent mixtures provides a slow dissolution of Na₂S₂O₄ during the SARA ATRP, affording the continuous regeneration of Cu(I) activator species and a controlled polymerization.^10,12

Table 2 (entry 4) shows that well-defined PMA could be obtained at high monomer conversion (DP = 222), in less than 2 h, when Na₂S₂O₄ was used as the SARA agent in a sulfolane/water = 90/10 (v/v) mixture. In addition, the polymerization rate was similar to the one obtained using Cu(0) as the SARA agent (Table 2, entry 2). These are very encouraging results, which clearly demonstrate the versatility of the SARA ATRP method developed.

Table 2 Molecular weight parameters of the PMA-Br prepared by SARA ATRP in sulfolane and sulfolane/water = 90/10 (v/v) at 30 °C, using different SARA agents. Reaction conditions: [MA]₀/[EBiB]₀ = 222; [MA]₀/[solvent] = 2/1 (v/v); [Cu(0)]₀/[CuBr₂]₀/[Me₆TREN]₀ = Cu(0) wire/0.1/1.1; [Na₂S₂O₄]₀/[CuBr₂]₀/[Me₆TREN]₀ = 1/0.1/0.2

Entry	S/W^b (v/v)	SARA agent	kp^app (h^−1)	Time (h)	Conv. (%)	Mn^th × 10^−3	Mn^SEC × 10^−3	Đ
a No polymerization occurred.b Sulfolane/water.
1	100/0	Cu(0)	1.002	1.3	72	13.8	17.0	1.1
2	90/10	Cu(0)	1.642	1.0	78	15.1	22.2	1.04
3^a	100/0	Na2S2O4	—	5.0	0	—	—	—
4	90/10	Na2S2O4	1.348	1.0	77	14.8	20.4	1.12

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Synthesis of a PMA-b-PVC-b-PMA triblock copolymer by “one-pot” SARA ATRP

One of the advantages of using RDRP methods is the possibility of designing complex polymeric structures, due to the high chain-end functionality of the resulting polymers. In this work, a α,ω-di(bromo)PVC (conv._VC = 47.3%, M_n^th = 3.8 × 10³, M_n^SEC = 7.8 × 10³, Đ = 1.69) obtained by bromoform-initiated SARA ATRP was used as a macroinitiator for the preparation of a PMA-b-PVC-b-PMA (conv._MA = 84.3%, M_n^th = 56.4 × 10³, M_n^GPC = 36.3 × 10³, Đ = 1.48) triblock copolymer by “one-pot” SARA ATRP in sulfolane/water = 90/10 (v/v). The chromatograms shown in Fig. 2 demonstrate a shift of the macroinitiator's molecular weight distribution towards high molecular weight values (lower retention volume), with no increase of the dispersity of the block copolymer, confirming the retention of the chain-end functionality of the PVC prepared by SARA ATRP. Additionally, the structure of the well-defined PVC macroinitiator was confirmed by ¹H NMR spectroscopy (Fig. S2†).

Fig. 2 SEC chromatograms of a α,ω-di(bromo)PVC (conv._VC = 47.3%, M_n^th = 3.8 × 10³, M_n^SEC = 7.8 × 10³, Đ = 1.69) macroinitiator (black line) and PMA-b-PVC-b-PMA triblock copolymer (conv._MA = 84.3%, M_n^th = 56.4 × 103, M_n^SEC = 36.3 × 10³, Đ = 1.48) (blue line), after “one-pot” chain extension by SARA ATRP in sulfolane/water = 90/10 (v/v).

These results suggest that the SARA ATRP in sulfolane/water mixtures could be a very useful tool for the macromolecular engineering field. The structure of the block copolymer was confirmed by ¹H NMR analysis (Fig. 3).

Fig. 3 ¹H NMR spectrum (solvent: THF-d₈) of a purified PMA-b-PVC-b-PMA (M_n^SEC = 36.3 × 10³, Đ = 1.48) triblock copolymer obtained by “one-pot” SARA ATRP in sulfolane/water = 90/10 (v/v).

Conclusions

Well-defined (co)polymers derived from acrylates, methacrylates, styrene and vinyl chloride were synthesized through SARA ATRP in sulfolane/water mixtures using a low concentration of soluble copper. The addition of water to the reaction mixture was found to increase the polymerization rate compared to data published by our research group using pure sulfolane as the polymerization solvent.¹⁶ The reaction conditions used can be considered very attractive for a future industrial implementation of the technique, providing a convenient and robust method to afford a wide range of macromolecules.

Acknowledgements

Patrícia V. Mendonça acknowledges FCT-MCTES for her PhD scholarship (SFRH/BD/69152/2010). Carlos M. R. Abreu acknowledges FCT-MCTES for his Ph.D. scholarship (SFRH/BD/88528/2012). The authors acknowledge FCT-MCTES for funding (PTDC/EQU-EPR/098662/2008 and PTDC/EQU-EPR/114354/2009). The ¹H NMR data were obtained at the Nuclear Magnetic Resonance Laboratory of the Coimbra Chemistry Centre (http://www.nmrccc.uc.pt), University of Coimbra, supported in part by the Grant REEQ/481/QUI/2006 from FCT, POCI-2010 and FEDER, Portugal.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra20872f

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