Atom Transfer Radical Polymerization of Methyl Methacrylate Mediated by Grubbs 1^st and 2^nd Generation Catalysts: Insight into the Active Species

Abstract

Ruthenium benzylidene complexes were evaluated as catalysts in atom-transfer radical polymerization (ATRP) of methyl methacrylate (MMA) under different reaction conditions. The mechanism by which Grubbs 1^st and 2^nd generation catalysts mediate olefin metathesis has been studied, little is known regarding the mechanism of ATRP reaction promoted by these complexes. Conversion and semilogarithmic kinetic plots as a function of time were correlated to the different catalysts and reaction conditions; especially in the presence of Al(OiPr)₃ as additive. Molecular weight (M_n) and polydispersity index (PDI) values changed with different catalysts in the presence or absence of Al(OiPr)₃. Kinetic studies by ¹H NMR revealed that two complexes in the presence of Al(OiPr)₃ are converted into ATRP-active with the dissociation of PCy₃, but with the benzylidene group preserved. More controlled polymerizations were achieved using Grubbs 1^st and, the presence of Al(OiPr)₃ improved the control levels for both catalysts.

Keywords:
methyl methacrylate; ruthenium; ATRP; Grubbs catalysts

Introduction

Atom-transfer radical polymerization (ATRP) is a catalyst-based process which the growing radicals can be reversibly activated or deactivated via a dynamic equilibrium with a transition metal complex with an exchange of halogen atom between the chain and metal complex.^1-13 A variety of metal complexes used as catalysts in the reaction provides the control of the molecular weight distribution, which in turn can enable the facile synthesis of well-defined polymers.^14-17

Ruthenium-based metathesis catalysts have shown an excellent application profile in obtaining such polymers using the ATRP protocol.^18-21 This area of research has attracted widespread interest, since the first report of ruthenium-based catalysts, although this system itself was not effective in methyl methacrylate (MMA) polymerization, but the addition of an aluminum alkoxides such as Al(OiPr)₃ accelerated the reaction and produced polymers with narrow with narrow molecular weight distributions (PDIs).² The efficiency and versatility of Grubbs 1^st (1) and 2^nd (2) generation metathesis catalysts for promoting ATRP of vinyl monomers was already reported.^22,23 The ability of these catalysts to promote two reactions with such markedly different mechanisms has been utilized in various tandem reactions in which olefin metathesis and atom transfer radical reactions take place in one pot.²⁴ As complexes 1 and 2 were known to be highly active for ring-opening metathesis polymerization (ROMP) and ATRP reactions, however, while the mechanism of olefin metathesis with these catalysts has been well-studied, the mechanism by which ruthenium benzylidenes promote ATRP reactions remains unknown. Thus, their dual behavior prompted us to explore the yet unknown mechanism for ATRP mediated by these catalysts (Figure 1).

Figure 1
Structures of Grubbs 1^st (1) and 2^nd (2) generation catalysts.

The present study is aimed to optimize the reaction conditions for the controlled polymerization of MMA by ATRP using 1 or 2. Thus, the homopolymerization of MMA via ATRP using 1 or 2 in different reaction conditions were investigated. The complexes 1 and 2 were able to mediate these polymerizations with acceptable rate and level of control.

Experimental

General details

All reagents were purchased from Aldrich Chemical Co. All reactions and manipulations were conducted under a nitrogen atmosphere using standard Schlenk techniques. Toluene was dried overnight over calcium chloride, filtered and distilled from sodium benzophenone ketyl and degassed by three vacuum-nitrogen cycles under nitrogen before use. The monomers methyl methacrylate (MMA) and ethyl methacrylate (EMA) were washed with 5% NaOH solution, dried over anhydrous MgSO₄, vacuum distilled from CaH₂ and stored at -18 °C before use. Grubbs 1^st and 2^nd generation catalysts, anisole, 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), tetrabutylammonium hexafluorophosphate (n-Bu₄NPF₆), aluminium isopropoxide (Al(OiPr)₃) and ethyl 2-bromoisobutyrate (EBiB) were used as acquired.

ATRP procedure

A ruthenium complex (23.5 µmol) was placed in a Schlenk tube containing a magnet bar and capped with a rubber septum. Air was expelled by three vacuum-nitrogen cycles before the monomer (MMA; 4.71 mmol) and the initiator solution (EBiB; 48.2 µmol) were added. All liquids were handled with dried syringes under nitrogen. The tube was capped under N₂ atmosphere using Schlenk techniques, then the reaction mixture was magnetically stirred and heated in a thermostated oil bath at 85 °C. Aliquots (20 µL) were removed at appropriate intervals.

Analyses

Conversion was determined from the concentration of residual monomer measured by gas chromatography (GC) using a Shimadzu GC-2010 gas chromatograph equipped with flame ionization detector and a 30 m (0.53 mm i.d., 0.5 µm film thickness) SPB-1 Supelco fused silica capillary column. Anisole was added to polymerization and used as an internal standard. Analysis conditions: injector and detector temperature, 250 °C; temperature program, 40 °C (4 min), 20 °C min^-1 until 200 °C, 200 °C (2 min). The molecular weights and the molecular weight distribution of the polymers were determined by gel permeation chromatography using a Shimadzu Prominence LC system equipped with a LC-20AD pump, a DGU-20A5 degasser, a CBM-20A communication modulo, a CTO-20A oven at 40 °C, and a RID-10A detector equipped with two Shimadzu columns (GPC-805: 30 cm, Ø = 8.0 mm). The retention time was calibrated with standard monodispersed polyMMA using HPLC-grade THF as an eluent at 40 °C with a flow rate of 1.0 mL min^-1. Theoretical molecular weights (M_n,th) were calculated without considering the end groups according to the following equation: M_n,th = ([Monomer]₀/[Initiator]₀) × Conversion × Monomer molecular weight. Electrochemical measurements were performed using an Autolab PGSTAT 204 potentiostat with a stationary platinum disk and a wire as working and auxiliary electrodes, respectively. The reference electrode was Ag/AgCl. The measurements were performed at 25 ± 0.1 °C in CH₂Cl₂ with 0.1 mol L^-1 of n-Bu₄NPF₆. The E_1/2 values were the arithmetic average of the anodic and cathodic potential peaks (E_p,a + E_p,c)/2. UV-Vis measurements were performed on a Cary 400 UV-Vis spectrophotometer (Varian) using 1 cm path length quartz cells. Toluene solutions of the complexes of 0.1 mM concentrations were used for these measurements. The ¹H and ³¹P{¹H} nuclear magnetic resonance (NMR) spectra were obtained in CDCl₃ at 298 K on a Bruker DRX-400 spectrometer operating at 400.13 and 161.98 MHz, respectively. The obtained chemical shifts were reported in ppm relative to TMS or 85% H₃PO₄.

Results and Discussion

The polymerizations were performed in the presence or in the absence of Al(OiPr)₃ with an initial molar ratio of [Monomer]/[EBiB]/[Ru] = 200/2/1. With 1 as catalyst in the absence of Al(OiPr)₃ at 85 °C, the MMA polymerization took 4 h to reach 10% conversion, and considerably increased to 90% after 16 h of reaction, whereas with 2 under similar conditions, only 5% of conversion was reached after 4 h, increasing to 25% in 16 h of reaction. With Al(OiPr)₃, the conversion values for both catalysts were similar, increasing from 30 to 60% as the reaction time increased from 4 to 16 h (Figure 2). The plot of ln ([M]₀/[M]) as a function of the reaction time shows an asymptotic relationship for both catalysts, revealing that the radical concentration is not constant during the MMA polymerization (Figure 2, insert).

Figure 2
Dependence of conversion on the reaction time for ATRP of MMA in the presence and absence of Al(OiPr)₃ using 1 or 2 in toluene at 85 °C. (●) [MMA]/[EBiB]/[1]/[Al] = 200/2/1/0; (■) [MMA]/[EBiB]/[2]/[Al] = 200/2/1/0; (○) [MMA]/[EBiB]/[1]/[Al] = 200/2/1/4; (□) [MMA]/[EBiB]/[2]/[Al] = 200/2/1/4. Insert: semilogarithmic plots on the reaction time for ATRP of MMA in the presence or absence of Al(OiPr)₃ using 1 or 2.

Similar result was observed in ATRP mediated by a Ru compound and CCl₄ as initiator; this fact was interpreted as an indicative that the propagating species were consumed only in the termination reactions. In addition, the literature highlights that, in similar cases, no M_n dependence on conversion could be observed.²⁵ However, the M_n values obtained in the reaction grew with conversion accompanied by decreasing PDI values when using both catalysts (Figure 3). When analyzing the control on the polymerization of MMA when using 1 or 2, the ratio between the experimental and theoretical molecular weight values shows that the molecular weights of polyMMA were far below or above than the predicted one. The narrowest molecular weight distribution was obtained with 1 for [MMA]/[Ru] = 800, although the M_n was higher than the calculated value (initiation efficiency factor (f) = 0.6).²² With [MMA]/[Ru] = 200, the initiation efficiency factor was then higher than 1 (f = 1.5), indicating the generation of additional polymer chains through transfer reactions (Table 1). Broader PDI values resulted from substitution of one PCy₃ in 1 by a N-heterocyclic carbene in 2.

Table 1
ATRP^a of MMA in the presence or absence of Al(OiPr)₃ using 1 or 2 in toluene

	Condition	Conversionb / %	Mnc × 103	PDI	f d
1	without Al	95f	66.0	1.28	0.6
		90	6.0	1.32	1.5
	with Ale	63	9.1	1.32	0.7
2	without Al	25	3.5	1.92	0.6
	with Ale	59	13.8	1.86	0.4

• ^a
  [monomer]/[initiator]/[Ru] = 200/2/1 molar ratio, T = 85 ºC, reaction time = 16 h;
• ^b
  determined from the concentration of residual monomer measured by gas chromatography (GC);
• ^c
  determined with size exclusion chromatography (SEC) with polystyrene calibration;
• ^d
  initiation efficiency f = M_n,theor/M_n,exp with M_n,theor = ([monomer]/[initiator])× M_w,(monomer)× conversion:
• ^e
  [Al(OiPr)₃]/[Ru] = 4 molar ratio;
• ^f
  [monomer]/[initiator]/[Ru] = 800/2/1 molar ratio, T = 85 ºC, reaction time = 16 h.²²

Polymerizations were also carried out with 1 and 2 under similar conditions in the presence of Al(OiPr)₃. For both catalysts, the linear dependence of ln ([M]₀/[M]) on time, with k_obs = 1.25 × 10^-(5 s^-1 for 1 and k_obs = 1.24 × 10^-(5 s^-1 for 2, and the linear increase of molecular weight with conversion coupled with lower PDIs (Figures 2 and 3), illustrate an improvement of the control that 1 or 2 exert over MMA polymerization. However, the improvement was more pronounced for 1, which M_n values are in good agreement with those theoretically calculated, with initiation efficiency factor nearer of 1 (f = 0.7) when compared that performed in the absence of Al(OiPr)₃ (Table 1).

Figure 3
Dependence of M_n (solid) and PDI (hollow) values on conversion for ATRP of MMA in the presence or absence of Al(OiPr)₃ using 1 or 2 in toluene at 85 °C. [MMA]/[EBiB]/[Ru] = 200/2/1; (●,○) [1]/[Al] = 1/0; (■,□) [2]/[Al] = 1/0; (■,□) [1]/[Al] = 1/4; (■,□) [2]/[Al] = 1/4.

Recently, the importance of the characteristics of complexes in relation to the nature of ligands in the coordination sphere has been highlighted in the radical polymerization of methacrylates when proceeding in the presence of Al(OiPr)₃.^26-28 As previously seen, the presence of Al(OiPr)₃ plays an important role in the control of polymerizations; therefore, we proceed the UV-Vis spectroscopy study to observe what occurs in the reaction mixture (Figure 4). UV-Vis spectra were registered in the interval of 20 min for 3 h with 1 or 2 in presence or in the absence of Al(OiPr)₃. No variation was observed when comparing the spectra in presence or in the absence of Al(OiPr)₃ for up to 3 h. It was interpreted that there was no coordination of the additive to the complex, because other bands were not observed, at least in the metal with oxidation state +2. The stability of 1 and 2 in solution was also evaluated as a function of time at 50 °C by NMR (Figures S1 and S2). In both complexes, the appearance of the sign at 48.9 ppm shows that a PCy₃ leaves the ruthenium center, which the PCy₃ dissociation from 1 is faster than 2. Thus, the decrease in the absorption observed in the electronic spectra of the catalysts in the presence or absence of Al additive can be related to the release of PCy₃ from the coordination sphere of the benzylidene complexes.

Figure 4
Absorption spectra of 1 (above) and 2 (below) in the (a) absence and (b) presence of Al(OiPr)₃ in toluene; [Ru] = 0.1 mmol L^-1; [Al]/[1] = 4.

Since ATRP depends on a single electron redox couple, electrochemical techniques provide useful insight into which complexes should be suitable catalysts for ATRP.^2-12 The cyclic voltammogram of 1 and 2 (Figure 5) presented a redox process in the potential range from 0.2 to 0.9 V, corresponding to the Ru^II/Ru^III conversion with E_1/2 = 0.66 V for 1 and E_1/2 = 0.55 V for 2 vs. Ag/AgCl. The cyclic voltammetry of 1 showed the anodic process corresponding to the Ru^II/Ru^III conversion at E_pa = 0.74 V, and a small cathodic process was detected at E_pc = 0.59 V. Although the Ru^II/Ru^III redox cycle of 1 seems chemically irreversible during the electrochemical redox process, the absence of a complete reduction indicates that the Ru^III species is not stable, and immediately decomposes into other species that cannot regenerate the Ru^II species. In contrast, the cyclic voltammogram of 2 shows a quasi-reversible redox couple where the anodic and cathodic process were clearly observed at E_pa = 0.69 V and E_pc = 0.40 V, respectively.

Figure 5
Cyclic voltammogram of 1 (left) and 2 (right) in the absence (solid line) and presence (dash line) of Al(OiPr)₃ in CH₂Cl₂ at 25 °C. [Ru] = 10 mM; [n-Bu₄NPF₆] = 100 mM (supporting electrolyte). Scan rate = 100 mV s^-1.

The cyclic voltammogram data confirm the greater reducing power of 2 containing a more electron-donating ligand. However, the correlation does not directly extend to the quality of an ATRP catalyst. The reversibility of the redox couple is also important. For example in the case of 2 the redox potential is lower than that for 1, but the peak-to-peak separation is almost twice larger (∆E_p = 150 mV for 1 and ∆E_p = 290 mV for 2) and, as consequence, 2 undergoes a more sluggish electron transfer, possibly as a result of a more substantial reorganization of the Ru center during the electron-transfer process.

Studies show that the polymerization of vinyl monomer via ATRP mediated by Ru catalysts in the presence of metal alkoxides, for instance Al(OiPr)₃, in some cases, increases the polymerization rate and affords polymers of controlled molecular weights by interaction with the ruthenium complex and thereby stabilizes the higher oxidation state Ru^III species to facilitate radical generation from a dormant species.²⁹ The electrochemical properties of 1 and 2 in the presence of Al(OiPr)₃ were also investigated by cyclic voltammetry to investigate possible interactions between complex (1 or 2) and the Al additive (Figure 5). However, changes in redox processes and the appearance of new processes were not observed in both cases, corroborating the UV-Vis studies. Moreover, the presence of Al(OiPr)₃ did not provide an increase in the electrochemical reversibility of 1 and 2. The ³¹P{¹H} NMR spectrum of 1 obtained from the experiment in the presence of EBiB, Al(OiPr)₃ and MMA at 50 °C showed the decrease of the initial peak at 35.3 ppm while a new peak arose at 35.8 ppm assigned to bromide complex [RuCl₂Br(=CHPh)(PCy₃)], with the appearance of the signal at 48.9 ppm attributed to the free PCy₃ produced in the solution after discoordination from 1 (Figure 6, right). This is a clear indication that the PCy₃ leaves the complex to produce a five-coordinated bromide-Ru species via a dissociative type mechanism. Similar results were observed with 2 (Figure 6, left).

Figure 6
³¹P{¹H}-NMR spectra of 1 (left) and 2 (right) in the presence of EBiB, Al(OiPr)₃ and MMA as a function of time in CDCl₃ at 50 °C; [MMA]/[EBiB]/[Al] = 200/2/4/1.

In order to investigate whether ATRP is mediated by ruthenium species without decomposition of the benzylidene moiety, the signal of carbene metal was monitored during polymerization by ¹H NMR (Figure S3). In fact, there is a consumption signal assigned to the benzylidene group from the initial species, but the appearance of a new signal was also observed, confirming that the metal carbene is preserved in the active species during ATRP of MMA under these conditions. Thus, the ATRP of MMA mediated by 1 or 2 in the presence of Al(OiPr)₃ will occur when the PCy₃ molecule undergoes discoordination from the metal center (Scheme 1). We believe that the Al additive plays an important factor in ATRP mechanism mediated by 1 or 2, preserving the benzylidene group in the active species, different from the polymerization performed in the absence of additive, wherein the benzylidene group is decomposed.^22.23 Although a detailed study has been conducted to try to elucidate the action of Al additive in the ATRP reaction, the origins of these effects are still under investigation.

Scheme 1
Possible reaction routes for ATRP of MMA mediated by 1 or 2 in the presence of Al(OiPr)₃.

The cyclic voltammetry, UV-Vis and NMR studies help us build an understanding about the differences in the reactivity of the catalysts 1 and 2 in the polymerization of MMA. Initially, it should be emphasized that the redox properties of 1 are 2 for ATRP are important, but not decisive for their efficiency, as proof, electrochemical profiles confirm a greater reversibility to the precursor 2, however, the best results came from 1. On the other hand, kinetic studies show the output of PCy₃ from 1 is faster than 2, accessing the active species efficiently.

Conclusion

The catalysts 1 or 2 were successfully applied for ATRP of MMA in the presence or in the absence of Al(OiPr)₃. Both catalysts showed reasonable control of MMA polymerization in the absence of Al(OiPr)₃ but the control in ATRP is improved in the presence of Al additive. The ¹H and ³¹P NMR studies of 1 or 2 obtained from the experiment in the presence of EBiB, Al(OiPr)₃ and MMA showed that the PCy₃ leaves the initial complex to produce a five-coordinated bromide-Ru species without decomposition of the benzylidene moiety via a dissociative type mechanism. Kinetic studies show the output of PCy₃ from 1 is faster than 2, accessing the active species efficiently and that Al additive plays as an important tool during the catalysis.

Acknowledgments

The authors are indebted to the financial support from FAPESP (Proc. 2013/10002-0) and Prope (Proc. 2180/002/14-PROPe/CDC). The 400 MHz NMR analyses were performed at the Instituto de Química de São Carlos, IQSC/USP, São Carlos, SP, Brazil.

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br as PDF file.

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Publication Dates

• Publication in this collection
  Aug 2017

History

• Received
  05 Aug 2016
• Accepted
  06 Dec 2016