Methyl acrylate polymerizations in the presence of a copper/N₃S₃ macrobicyclic cage in DMSO at 25 °C

Abstract

Macrobicyclic hexa-amine cage ligands are known to completely encapsulate transition metals inside the ligand cavity. Recent work has shown that a five-coordinate bromido complex [Cu(AMME-N₃S₃sar)Br]⁺ was observed, featuring a novel tetradentate (N₂S₂) coordinated form of the cage ligand in DMSO. Reduction to its monovalent state results in no change in the geometry of the complex. The electrochemistry of this complex in the presence of an alkyl halide showed that this complex has a low activation to convert these alkyl halides to their corresponding incipient radicals. Further polymerization kinetics with methyl acrylate showed that the molecular weight was uncontrolled, implying that deactivation was negligible. Therefore, this copper/ligand complex with an alkyl halide initiator only acts as an initiation source. This led us to use this Cu/AMME-N₃S₃sar complex to initiate MA polymerizations at room temperature in the presence of a RAFT agent. The results showed that the molecular weight distribution was controlled and followed ideal ‘living’ radical behavior, in which the molecular weight polydispersity for a range of different molecular weight targets was less than 1.1.

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Introduction

Copper complexes of multidentate polyamine ligands are very effective catalysts in ‘living’ radical polymerizations.^1–3 The monovalent form of the catalyst (Cu(I)L) activates an alkyl halide (RX) through a partial oxidative addition releasing the alkyl radical (Rʹ) while the halide ion (X⁻) is coordinated to Cu(II). The reverse (deactivation) reaction is also important in controlling the concentration of free-radicals (eqn (1)).

In general, a more negative Cu(II/I) redox potential (E′) of the complex has been correlated with higher activity in atom radical transfer polymerizations (ATRP). Most nitrogen based macrocyclic ligands are highly active compared to their acyclic bi-, tri- or tetradentate analogues, with the highest known catalytic activity found for a bicyclic cyclam macrocycle bearing a strap adjoining opposite N-donors.⁴ On the other hand, macrocyclic ligands with mixed donor atoms (N and S) are less studied,⁵ and the ability of S-donors to stabilize Cu(I) would be expected to lower catalytic activity according to current models.²

Macrobicyclic hexa-amine cage ligands⁶ are known to completely encapsulate transition metals inside the ligand cavity, and once inside, the metal is extremely difficult to remove. In previous work, the macrobicyclic mixed donor (N₃S₃) encapsulating ligand (see Scheme 1), 1-methyl-8-amino-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]eicosane (AMME-N₃S₃sar) complexed with CuBr in toluene did not effectively control the polymerization of styrene (STY) at 60 or 100 °C, but acted only as an initiation source while deactivation by CuBr₂/AMME-N₃S₃sar was found to be negligible.⁵ The coupling reaction between α,ω-PSTY (i.e. Br-PSTY-Br) using CuBr/AMME-N₃S₃sar and excess Cu(0) in toluene at 100 °C gave no loss of the starting Br-PSTY-Br.⁵ However, changing the solvent to DMSO and decreasing the temperature to 60 °C led to 87% consumption of starting polymer in only 10 min. with the concomitant formation of high molecular weight multiblock copolymer. Since the complex showed no disproportionation of Cu(I) to Cu(0) and Cu(II) in DMSO, we do not believe that nascent Cu(0) plays a role in this reaction, as shown to be the case in single electron transfer-living radical polymerization (SET-LRP).⁷

Scheme 1 Structures of high activating ligands and minimized geometries of AMME-N₃S₃sar ligand calculated by MP2.

We therefore, carried out an in-depth study⁸ into the nature of this phenomenon by examining the coordination chemistry of the [Cu(AMME-N₃S₃sar)Br]⁺ complex in DMSO. A five-coordinate bromido complex [Cu(AMME-N₃S₃sar)Br]⁺ was observed featuring a novel tetradentate (N₂S₂) coordinated form of the cage ligand. This copper(II) complex was characterized by X-ray crystallography and with solution spectroscopy. The kinetics of the interconversion between these six- and five-coordinate Cu(II) complexes were resolved (Scheme 2). These processes were reversible. When bromide concentrations were very low, [Cu(AMME-N₃S₃sar)Br]⁺ reverted to the six-coordinate complex [Cu(AMME-N₃S₃sar)]²⁺ while [Cu(AMME-N₃S₃sar)]²⁺ reacted cleanly with excess bromide ions in DMSO to generate five-coordinate [Cu(AMME-N₃S₃sar)Br]⁺. Electrochemistry of [Cu(AMME-N₃S₃sar)Br]⁺ in DMSO shows that upon reduction to the monovalent state, a five-coordinate tetradentate conformation formed exclusively regardless of whether a six- or five-coordinate copper(II) complex was the precursor. This shows that in DMSO the bromide ion can be removed resulting in the hexadentate coordination of Cu(II) within the cavity of the cage ligand. We also find that Cu(I) resided on the face of the ligand (Scheme 2), and even if encapsulated initially as Cu(II) will rapidly rearrange to be located on the face upon reduction.

Scheme 2 Interconversion of Cu(II)/AMME-N₃S₃sar complexes in DMSO.

In this paper, we provide further evidence using cyclic voltammetry on the activity of [Cu(AMME-N₃S₃sar)Br]⁺ in DMSO as a polymerization catalyst. In addition, the kinetics for the polymerization of methyl acrylate (MA) in DMSO using CuBr/AMME-N₃S₃sar and ethyl 2-bromoisobutyrate (EBiB) at 25 °C were evaluated, and the external orders of reaction determined for Cu(I), Cu(II) and ligand. This unique redox system was then used to initiate reversible addition-fragmentation chain transfer (RAFT)⁹ polymerization of MA at 25 °C using a highly reactive chain transfer agent (CTA), resulting in the production of polymer with well controlled molecular weight and narrow molecular weight distributions (MWDs).

Experimental

Materials

Ethanol (Ajax, 99.8%) was dried with magnesium (Reidel-de Haën, 99.5%) and iodine (Univar, 99.8%), and distilled. Sodium metal (Riedel-de Haën, 99%), cysteamine hydrochloride (Fluka, 97%), cobalt(II) nitrate hydrate (UniLab, 96%), sodium carbonate anhydrous (Pronalys, 99.8%), formaldehyde (Univar, 37% in H₂O), nitromethane (Aldrich, 96%), zinc powder (Analar, 90%), hydrogen peroxide (Univar, 30% in H₂O), hydrochloric acid (Ajax, 32% in H₂O), methanol (Univar, 98%), sodium cyanide (Sigma, 99.8%), potassium hydroxide (Chem Supply, 85%), sodium sulfate (Univar, 99%), chloroform (CHCl₃, Pronalys, 99%), dichloromethane (CH₂Cl₂, Labscan, AR grade), diethyl ether (Et₂O, Pronalys, AR grade), tetrahydrofuran (THF, HPLC grade, LABSCAN, 99.8%), toluene (HPLC, LABSCAN, 99.8%), dimethyl sulfoxide (DMSO, LABSCAN, 99.8%), formic acid (Univar, 90%), butanethiol (Aldrich, 99%), triethylamine (Et₃N, Fluka, purum), carbon disulfide (CS₂, Riedel-de Haën, 99%), methyl 2-bromopropionate (MBP, Aldrich, 98%), ethyl 2-bromoisobutyrate (EBiB, Aldrich, 98%), cuprous bromide (CuBr, Aldrich, 99.999%), and cupric bromide (CuBr₂, Aldrich, 99%) were all used as received. Methyl acrylate (MA, Aldrich, 99%, 100 ppm monomethyl ether hydroquinone inhibitor) was purified by passage through a column of activated basic alumina (Aldrich, Brockmann I, standard grade, ∼150 mesh, 58 Å).

Synthesis

1-methyl-8-amine-3,13,16-trithia-6,10,19-triazabicyclo[6.6.6]icosane (AMME-N₃S₃sar). AMME-N₃S₃sar shown in Scheme 1 was synthesized following the previously described method by Bell et al.⁸

Methyl 2-(butylthiocarbonothioylthio)propanoate (methoxycarboxyethylbutyl trithiocarbonate – MCEBTTC)¹⁰. To a stirred solution of 1-butanethiol (10 mL, 0.093 mol) and Et₃N (14.3 mL, 0.103 mol) in DCM (100 mL) under nitrogen was added CS₂ (6.18 mL, 0.103 mol) in CH₂Cl₂ (50 mL) via a pressure equalising funnel over a period of 30 min at 0 °C. The solution gradually turned yellow during addition. Upon complete addition, the reaction mixture was allowed to stir at room temperature for 1 h before MBP (11.46 mL, 0.103 mol) in CH₂Cl₂ (50 mL) was added over 30 min. This was then allowed to stir for a further 2 h. The solution was then taken to dryness and redissolved in Et₂O and filtered to remove the Et₃N -HBr salt. The solution was then washed with cold 10% HCl solution (3 × 50 mL) and Milli-Q water (3 × 50 mL) and then dried over anhydrous MgSO₄. The ether was then removed and a yellow, oily substance remained. Purification by column chromatography (19 : 1 pentane–ethyl acetate, second band) gave the desired product.

¹H NMR (CDCl₃) δ 0.92 (tr, J = 7.5 Hz, 3H, CH₃), 1.43 (mult, J = 7.5 Hz, 2H, CH₂), 1.62 (d, J = 7.5 Hz, 3H, CH₃), 1.65 (quin, J = 7.5 Hz, 2H, CH₂), 3.36 (tr, J = 7.5 Hz, 2H, CH₂), 3.73 (s, 3H, CH₃), 4.84 (quad, J = 7.5 Hz, 1H, CH); ¹³C NMR (CDCl₃) δ 13.55, 16.91, 22.02, 29.89, 36.94, 47.68, 52.82, 171.63 (CH–C(= O)–O), 221.99 (S–C(= S)–S)

Techniques

Size exclusion chromatography (SEC). All polymer samples were dried in a vacuum oven for two days at 25 °C, dissolved in THF to a concentration of 1 mg mL⁻¹ and then filtered over a 0.45 μm PTFE syringe filter. Analysis of the molecular weight distributions of the polymers was accomplished by using a Waters 2695 Separations Module, fitted with a Waters 410 refractive index detector held at a constant temperature of 35 °C, a Waters 996 Photodiode Array detector, two Ultrastyragel linear columns (7.8 × 300 mm) and one Styragel linear column kept in series. These columns were held at a constant temperature of 40 °C for all analyses. The columns used separate polymers in the molecular weight range of 500–6 × 10⁶ g mol⁻¹ with high resolution. THF was the eluent used at a flow rate of 1.0 mL min⁻¹. Calibration was carried out using narrow molecular weight PSTY standards (PDI ≤ 1.1) ranging from 500 to 2 × 10⁶ g mol⁻¹. All samples were converted to absolute molecular weights using Mark–Houwink parameters for styrene (K = 1.62 × 10⁻² cm³ g⁻¹, α = 0.71) and methyl acrylate (K = 7.88 × 10⁻³ cm³ g⁻¹, α = 0.788). Data acquisition was performed using Empower software and molecular weights were calculated relative to polystyrene standards.

¹H and ¹³C nuclear magnetic resonance (NMR). All NMR spectra were recorded on a Bruker DRX 300 MHz spectrometer using an external lock (D₂O, CDCl₃) and utilizing a standard internal reference (1,4-dioxane, solvent reference). ¹³C NMR spectra were recorded by decoupling the protons and all chemical shifts are given as positive downfield relative to these internal references.

Cyclic voltammetry. Cyclic voltammetry was performed with a BAS100B/W potentiostat employing a glassy carbon working electrode, platinum auxiliary electrode and nonaqueous Ag/AgNO₃ reference electrode containing 0.1 M Et₄NClO₄ (in DMSO or acetonitrile) which was also the supporting electrolyte in all experiments. All solutions contained 1 mM concentrations of copper complex and were purged with nitrogen before measurement. Scanning rates of 50 mV s⁻¹ to 2000 mV s⁻¹ were used and potentials are cited relative to an external ferrocenium/ferrocene (Fc^+/0) reference potential measured under the same conditions.

Polymerisation of methyl acrylate in DMSO with CuBr and AMME-N₃S₃sar at 25 °C. A typical ATRP synthesis for MA in DMSO is described. MA (18.18 g, 4.33 M), DMSO (17.45 g, 4.69 M), EBiB (0.1622 g, 0.882 mmol) and AMME-N₃S₃sar was added to a round bottom Schlenk flask and degassed by three freeze-pump-thaw cycles. An argon line attached to a bubbler was fitted to the vessel and positive pressure of argon was permitted to flow through the system. CuBr (0.1255 g, 0.875 mmol) was added and a glass stopper was immediately fitted to the flask. This was then placed in a thermostated water bath at 25 °C and the reaction mixture stirred with a magnetic stirrer. Samples were taken through the side arm at regular intervals via a syringe. Conversion was monitored by proton NMR (in CDCl₃) and calculated by integration of the vinyl peaks (5.7–6.2 ppm) and the methyl ester protons associated with the side chain (3.5–3.7 ppm) and normalized for number of protons. The molecular weight distribution was measured by size exclusion chromatography (SEC).

Results and discussion

Cyclic voltammetry of AMME-N₃S₃sar/CuBr

The objective of the electrochemical measurements was to provide insight into the redox properties of Cu complexes of AMME-N₃S₃sar that form upon mixing each component together in solution as in a typical polymerization experiment, and correlating this to other copper/ligand complexes know to undergo ATRP. Fig. 1 shows the voltammograms starting with a mixture of CuBr₂ and AMME-N₃S₃sar in DMSO at ambient temperature. On the cathodic sweep two peaks were found associated with reduction of the mixture of five-coordinate [Cu(AMME-N₃S₃sar)Br]⁺ (higher potential) and six-coordinate [Cu(AMME-N₃S₃sar)]²⁺ (lower potential) each to their monovalent forms. It should be noted that these assignments are based on our previous studies with a crystalline (and crystallographically characterized) sample of [Cu(AMME-N₃S₃sar)Br]Br; the results are indistinguishable.⁸ The reverse sweep shows a single anodic peak at −620 mV, representing a common Cu(I) species bound in a tetradentate fashion on the face of the ligand. This illustrates that reduction of either six-coordinate Cu(II) (inside the cage) or five coordinate Cu(II) (partially coordinated) leads to the same tetradentate coordinated Cu(I) complex through rapid equilibration after reduction.

Fig. 1 Cyclic voltammograms of [Cu(AMME-N₃S₃sar)]^2+/+ in DMSO. Complex concentrations are 1 mM and all voltammograms are recorded at 100 mV s⁻¹ scanning rate in 0.1 M Et₄NClO₄ and are referenced to Ag/AgNO₃.

The redox potentials, ΔE_p, of CuBr/AMME-N₃S₃sar complex in DMSO relative to the ferrocene/ferrocenium external standard were, in the case of a mixture of Cu(I/II)/AMME-N₃S₃sar, slightly higher than 60 mV but in line with what was previously observed for a wide range of copper/ligand complexes in MeCN.^3,11 The non-aqueous solution resistance is most likely responsible for this feature as no compensation for iR drop was made. The average peak potentials (of the anodic and higher potential cathodic peaks) provide insight into the reducing power of the tetradentate coordinated Cu(I) species. A linear correlation between the Cu(II/I) redox potential and the equilibrium constant (where K_eq^app = k_act/k_deact, see eqn (1)) determined from the kinetics of polymerization was found from previous literature.^3,11

The redox potential for the copper/AMME-N₃S₃sar complex was similar to the values found in the literature^3,11 for other copper/ligand complexes (e.g. CuBr/Me₆TREN in MeCN), suggesting that the AMME-N₃S₃sar complex should also have a similar activation capability of reducing the dormant initiating and polymeric halide species R-X or P-X, respectively, to their corresponding radicals. Voltammograms of the CuBr₂/AMME-N₃S₃sar complex mixture formed in situ in the presence of increasing concentrations of a conventional ATRP initiator, ethyl 2-bromoisobutyrate (EBiB), allowed a qualitative investigation of the catalytic activity of the Cu(I) complex toward radical activation (Fig. 2 and Fig. S2 in the ESI†). As the potential was swept in the negative direction, the Cu(I) form is present exclusively within the diffusion layer at the electrode surface and should be able to react with EBiB to generate an incipient radical. If activation is fast, the Cu(I) complex is consumed in this chemical reaction before it can be reoxidised electrochemically and a small (or no) anodic peak will be observed on the reverse sweep (a so called EC mechanism). Furthermore, if this coupled chemical reaction is fast, there will be an amplification of the cathodic current as the system becomes catalytic and the Cu(II) product of the activation is regenerated at the electrode (a so called EC_cat mechanism). The concentration of radicals will depend on the activation by the CuBr/AMME-N₃S₃sar complex. A greater activation rate coefficient will result in a higher radical concentration close to the electrode with a greater rate of bimolecular termination, and thus a greater build up of Cu(II) species.

Fig. 2 Cyclic voltammograms of [Cu(AMME-N₃S₃sar)]^2+/+ (a) without EBiB and (b) with EBiB (100 mM) in DMSO. Complex concentrations are 1 mM and all voltammograms are recorded at 100 mV s⁻¹ scanning rate in 0.1 M Et₄NClO₄ and are referenced to Ag/AgNO₃.

An EC_cat mechanism was seen when Me₆TREN was used as the Cu binding ligand (see Fig. S3 in the ESI†); the asymmetry of the voltammograms as well as the amplification of the cathodic current with increasing amounts of EBiB are apparent. The full mechanistic details of the Cu/Me₆TREN system are not the focus of this paper and they will be published separately. It can be seen for the Cu/AMME-N₃S₃sar system that regardless of the concentration of EBiB (Fig. 2 and Fig. S2†) there is little change in the voltammograms, suggesting that activation by the CuBr/AMME-N₃S₃sar complex was much slower than for the CuBr/Me₆TREN complex. This was in agreement with kinetic data found for the polymerization of styrene in toluene at 100 °C,⁵ and will be discussed below for the kinetic polymerizations carried out in DMSO at 25 °C

Kinetics of MA activated with CuBr/AMME-N₃S₃sar in DMSO at 25 °C

Polymerizations of MA were carried out varying the CuBr concentration (Fig. 3). The conversion-time data shows that the maximum conversion was 50% for the lowest CuBr concentration (curve a in Fig. 3A) and 75% conversion for the higher concentrations (curve c). The rate increased with the concentration of CuBr and was obviously non-first order (Fig. 3B). This suggests that the equilibrium between activating and deactivating species was not established in this system, leading to non-living radical behavior. The molecular weight distribution (MWD) data (Fig. 3C and D) confirms the non-living behavior with high number-average molecular weight (M_n) and polydispersity index (PDI) values. These polymerizations are more indicative of conventional free-radical polymerizations.

Fig. 3 Kinetic data for the polymerization of methyl acrylate (3.67 M) in DMSO (9.36 M) at 25 °C with EBiB (1.83 × 10⁻² M), CuBr and AMME-N₃S₃sar. (A) conversion versus time, (B) ln[M]₀/[M] versus time (C) number-average molecular weight (M_n,SEC) versus conversion, and (D) polydispersity index versus conversion. The concentration ratios of reactants [MA] : [EBiB] : [CuBr] : [AMME-N₃S₃sar] are (a) 200 : 1 : 0.1 : 0.1, (b) 200 : 1 : 0.5 : 0.5, and (c) 200 : 1 : 1 : 1.

The external orders of the polymerization in [CuBr], [CuBr₂] and [AMME-N₃S₃sar] for MA in DMSO were given in Fig. 4, and provides information on the effect of AMME-N₃S₃sar on deactivation and whether AMME-N₃S₃sar has an effect on the kinetics. The external order of the reaction in [CuBr] is 0.42, which was close to 0.5 for activation only by CuBr. However, the order for [AMME-N₃S₃sar] was −2.08 suggesting that the ligand retards the rate contrary to what is found for the styrene system (order = 0.47).⁵ This was seen more clearly in the rate data given in the ESI (Fig. S4).† For CuBr₂, the order was very low and close to zero, supporting the postulate that deactivation using the cage ligand was negligible and in accord with what was found⁵ from the styrene data.

Fig. 4 Determination of the external order of reaction in [Cu(I)]₀, [Cu(II)]₀ and [AMME-N₃S₃sar]₀ for the CuBr/AMME-N₃S₃sar -catalyzed polymerization of methyl acrylate (MA) in DMSO at 25 °C initiated with ethyl 2-bromoisobutyrate (EBiB). [MA] : [EBiB] = 200 : 1; (A) ln k_p^appversus ln [CuBr]₀, (B) ln k_p^appversus ln [CuBr₂]₀, (C) ln k_p^appversus ln [AMME-N₃S₃sar]₀. The concentration of ligand was equal to the total concentration of copper (i.e. CuBr and CuBr₂) used in each experiment except for ligand order reactions.

RAFT-mediated polymerization of MA initiated with EBiB and AMME-N₃S₃sar/CuBr in DMSO at 25 °C

The kinetic and uncontrolled MWD data of MA polymerizations above shows that the EBiB in the presence of CuBr/AMME-N₃S₃sar complex can be used as a unique redox couple to initiate polymerizations at low temperatures. The advantage of this redox couple is the little or no deactivation of the polymeric chain end with Cu(II), and can be used to initiate RAFT-mediated polymerizations at low temperatures with high chain-end fidelity with RAFT moieties. Fig. 5 shows the RAFT-mediated polymerization of MA with varying concentrations of a highly reactive CTA (MCEBTTC, a trithioester RAFT agent). The monomer concentration in all cases was kept constant, and therefore the polymerizations become slower with increased [CTA]_o as a result of the lower [CuBr]_o. The M_n for all polymerizations increased linearly with conversion and was close to theory for an ideal ‘living’ polymerization. The polydispersity index decreases with conversion down to below 1.1, showing that well-defined polymers were synthesized. Carrying out polymerizations at this low temperature significantly reduces side reactions that would otherwise produce short and long chain branches or crosslinks at temperatures normally used to polymerize MA.

Fig. 5 Kinetic data for the polymerisation of methyl acrylate (3.67 M) in DMSO (9.36 M) at 25 °C with EBiB, MCEBTTC, CuBr and AMME-N₃S₃sar at various degrees of polymerization. (A) Conversion vs. time, (B) number-average molecular weight (M_n,SEC) vs. conversion, and (C) polydispersity index vs. conversion. The concentration ratios of reactants [MA] : [EBiB] : [CuBr] : [AMME-N₃S₃sar] : [MCEBTTC] are (a) 200 : 0.1 : 0.1 : 0.1 : 1, (b) 400 : 0.1 : 0.1 : 0.1 : 1, and (c) 800 : 0.1 : 0.1 : 0.1 : 1. (--- represents M_n,theory).

Conclusion

The redox potential of the copper/N₃S₃ cage ligand complex was found to be similar to many of the nitrogen based ligands (e.g. Me₆TREN in acteonitrile). Even though it has been found that the greater the redox potential the greater the activation capacity of the complex for nitrogen-based ligands, the Cu/AMME-N₃S₃sar system showed little or no activation of the alkyl halide. This low activation capability of the Cu/AMME-N₃S₃sar was confirmed from the very small change in the voltammograms when an alkyl halide was introduced into the electrochemical cell. The polymerization kinetic data also supported this conclusion, and further showed that deactivation was negligible. This led us to use this Cu/AMME-N₃S₃sar complex to initiate MA polymerizations at room temperature in the presence of a RAFT agent. The results showed that the molecular weight distribution was controlled and followed ideal ‘living’ radical behavior.

Acknowledgements

M.J.M acknowledges financial support from the ARC Discovery grant (DP0987315).

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Footnote

† Electronic supplementary information (ESI) available: Mechanisms for AMME-N₃S₃sar mediated initiation and RAFT mediated polymerizations, redox potentials of CuBr₂/AMME-N₃S₃sar, CuBr₂/Me₆TREN and ferrocene, cyclic voltammograms of [Cu(AMME-N₃S₃sar)]^2+/+, cyclic voltammograms of [Cu(Me₆TREN)]^2+/+, kinetic data for the polymerization of methyl acrylate, and size exclusion chromatograms of MCEBTTC mediated polymerization of methyl acrylate. See DOI: 10.1039/b9py00315k

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