Article pubs.acs.org/Macromolecules
Enhanced Activity of ATRP Fe Catalysts with Phosphines Containing Electron Donating Groups Yu Wang, Yungwan Kwak, and Krzysztof Matyjaszewski* Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ABSTRACT: Fe-based atom transfer radical polymerization (ATRP) of styrene (St) with various triarylphosphines containing electron donating methoxy groups was investigated. FeIIIBr3 in the presence of tris(2,4,6- trimethoxyphenyl)phosphine (TTMPP) provided faster ATRP of St than in the presence of tris(4-methoxyphenyl)phosphine (TMPP) and much faster than with triphenylphosphine (TPP) under identical conditions. ATRP of St was carried out with initial ratio of reagents: [St]:[EBiB]:[FeIIIBr3]:[TTMPP] = 200:1:1:2, in 50% (v/v) anisole at 100 °C (EBiB is ethyl 2bromoisobutyrate). After 21 h, conversion reached 92%, yielding polystyrene with molecular weight Mn = 24 100 and Mw/ Mn = 1.25. With TMPP and TPP under the same conditions, the conversion of monomer was only 19% and 9%, respectively. With 1 equiv of TTMPP vs FeIIIBr3, control was better, Mw/Mn ∼ 1.1, but polymerization was slower. The phosphines could directly reduce FeIII to FeII but could also act as ligands complexing transition metal and forming efficient ATRP catalysts.
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catalyst complexes, the CuI or FeII species, are continuously regenerated during the polymerization. Reducing agents used for AGET, ARGET, and SARA ATRP include such organic reducing agents as glucose, ascorbic acid, hydrazine, phenols, amines, or even excess of ligands42 and inorganic reducing agents such as tin II 2-ethylhexanoate, 30 or zerovalent copper,38,43−46 zinc, magnesium, and iron.47 The added zerovalent metals can also activate alkyl halide directly, and this is the reason they act as supplemental activators. In 2008, it was reported that FeIIIX3 in the presence of various phosphines could mediate an ATRP in the absence of an additional reducing agent.48,49 The proposed mechanism was that FeII species could be generated in situ via reaction between FeIII and monomer,48,50−52 as previously reported for the CuIIX2/L system.53,54 However, FeIIIBr3 could also be reduced in the presence of phosphine to generate a FeIIBr2 and dibromophosphorane. The substituents present on the phosphines have significant influence on the activity of the transition metal complex catalysts. Faster polymerization under identical conditions could suggest intrinsically larger KATRP values. However, since phosphines can reduce FeIII to FeII, the faster polymerization could also result in more efficient reduction. Most likely, higher catalyst activity can be due to both factors. FeX n with 2-(diphenylphosphino)pyridine (DPPP) and tributylphosphine (TBP) were more active catalysts than a FeXn/TPP (TPP: triphenylphosphine) catalyst complex.55,56 While FeIIIX3/phosphine mediated ATRP of MMA was successful and relatively fast, the ATRP of St was
INTRODUCTION Controlled radical polymerization (CRP)1 is a versatile tool for the synthesis of well-defined polymeric materials with various architectures and site specific incorporated functionalities. Atom transfer radical polymerization (ATRP)2−8 is one of the most successful CRP methods, and various transition metals, such as Cu, Ru, Fe, Ni, and Os, were successfully employed as ATRP catalysts.9,10 The first iron-catalyzed ATRP, conducted in the presence of various phosphines, amines, or pyridine containing ligands, was reported in 1997.11,12 Subsequently, iron complexes containing substituted phosphines,13 half-metallocene,14 cyclic amines,15 diimines,16 carbenes,17 carboxylic acids,18,19 or halide ligands forming anionic iron species,20,21 ionic liquids,22 or solvents as ligands23,24 were reported. Transition metal complexes in a lower oxidation state, which act as the activators in ATRP, are generally sensitive to oxygen/ air. Therefore, reverse ATRP,25 simultaneous reverse and normal initiation (SR&NI) ATRP,26 and activators generated by electron transfer (AGET) ATRP27 were developed to reduce sensitivity to air. In these methods, halogen containing transition metal complexes in the higher oxidation state were added to the reaction medium and reduced to the activator state in the presence of a reducing agent or a conventional radical initiator. These initiating systems laid the foundation for the development of the next generation of ATRP procedures which can be carried out in the presence of small amounts (ppm) of transition metal complex, namely initiators for continuous activator regeneration (ICAR) ATRP,28,29 activators regenerated by electron transfer (ARGET) ATRP,28,30−37 supplemental activator and reducing agent (SARA) ATRP,38−40 and electrochemically mediated ATRP41 in which the activator © XXXX American Chemical Society
Received: May 26, 2012 Revised: July 3, 2012
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dx.doi.org/10.1021/ma3010795 | Macromolecules XXXX, XXX, XXX−XXX
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much slower with these phosphines. 52 Therefore, we investigated phosphines with electron donating groups to determine if they could enhance polymerization rate. Tris(2,4,6- trimethoxyphenyl)phosphine (TTMPP) with FeIIIX3 was very active and could mediate the polymerization of St and reach high conversion in a relatively short time.
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Table 1. Activities of FeIIIBr3 with Various Triarylphosphinesa
EXPERIMENTAL SECTION
Materials. Styrene (St, 99%, Aldrich), n-butyl acrylate (BA, 99%, Aldrich), and methyl methacrylate (MMA, 99%, Aldrich) were passed through a column filled with basic alumina prior to use. Ethyl 2bromoisobutyrate (EBiB, 98%, Aldrich), ethyl 2-bromopropionate (EBrP, 99%, Aldrich), triphenylphosphine (TPP, 99%, Aldrich), tris(4methoxyphenyl)phosphine (TMPP, 95%, Aldrich), tris(2,4,6trimethoxyphenyl)phosphine (TTMPP, Aldrich), tetrabutylammonium bromide (TBABr, >98%, Aldrich), FeIIBr2 (98%, anhydrous, Alfa), FeIIIBr3 (98%+, anhydrous, Alfa), N,N-dimethylformamide (DMF), and anisole (99% Aldrich) were used as received without further treatment. Phosphines and iron salts were stored in a glovebox to avoid oxidation. PSt-Br macroinitiator was prepared via normal ATRP with CuIBr/PMDETA as the catalyst (Mn = 12 000, Mw/Mn = 1.10). Characterization. Molecular weight and its distribution were determined by GPC, conducted with a Waters 515 pump and Waters 2414 differential refractometer using PSS columns (Styrogel 105, 103, 102 Å) in THF as an eluent at 35 °C and at a flow rate of 1 mL/min. Linear PSt and PMMA standards were used for calibration. Conversions of all monomers were determined with known concentrations of polymers in THF. Spectroscopic measurements were performed on a Varian Cary 5000 UV/vis/NIR spectrometer. General Polymerization Procedures. Iron halides were weighed in the glovebox and placed into a dried Schlenk flask equipped with a stir bar. The flask was filled with dry nitrogen and sealed. Phosphine, monomer, initiator, and anisole were added to a sealed flask and degassed by four freeze−pump−thaw cycles then the solution was transferred to the Schlenk flask via a syringe. The Schlenk flask was placed in a thermostated oil bath at the desired temperature. Samples were taken periodically under a N2 atmosphere using a N2-purged syringe, diluted with THF, passed through a column filled with neutral alumina to remove the iron complex, and analyzed by GPC.
phosphine (ratio to FeIII)
Mn,GPC
Mn,th
conv (%)
Mw/Mn
TPP (1) TMPP (1) TTMPP (1) TPP (2) TMPP (2) TTMPP (2)
900 1300 8100 1200 1900 24100
873 2454 9235 1830 3890 19094
4 12 44 9 19 92
1.05 1.10 1.11 1.09 1.15 1.25
a
[St]:[EBiB]:[FeIIIBr3]:[phosphine] = 200:1:1:(1 or 2), in 50% (v/v) anisole at 100 °C, after 21 h polymerization.
reagents [St]:[EBiB]:[FeIIIBr3]:[phosphine] = 200:1:1:1, in 50% (v/v) anisole. After 21 h polymerization, the conversion reached 44%, 12%, and 4% for FeIIIBr3 complexes with TTMPP, TMPP, and TPP, respectively. When 2 equiv of the phosphines vs FeIIIBr3 was used, the polymerization rate increased and conversion reached 92%, 19%, and 9% for TTMPP, TMPP, and TPP, respectively. The level of control was good for all systems, with Mw/Mn values ranging from 1.05 to 1.25. However, because of concurrent thermal self-initiation of St, some of the experimental molecular weights were lower than theoretical values. ATRP of St with FeIIIBr3/TTMPP. ATRP of St withFeIIBr2/ TTMPP was well controlled with initial ratio of reagents [St]: [EBiB]:[FeIIBr2]:[TTMPP] = 200:1:1:1, in 50% (v/v) anisole at 100 °C. The observed molecular weights of PSt were in agreement with the theoretical values. After 21 h polymerization, the conversion reached 71%, giving PSt with molecular weight Mn = 16 300 and Mw/Mn = 1.24. ATRP of St with FeIIIBr3/TTMPP was slower than that with FeIIBr2 under identical conditions, and the dispersity was lower, Mw/Mn < 1.1 below 50% conversion. The molecular weight of the obtained PSt was 9400 with Mw/Mn = 1.12 after 21 h polymerization when the conversion reached 50%. When 2 equiv of TTMPP was used in conjunction with FeIIIBr3, the polymerization was much faster; however, the molecular weight became higher than the theoretical value after 70% conversion, indicating some plausible contribution of termination by coupling. The Mw/Mn values were slightly higher than with 1 equiv of TTMPP with Mw/Mn = 1.25 at 92% conversion, Mn = 24 100, in 21 h. This suggests that excess of phosphine can reduce concentration of FeIII species to a level too low for efficient control.
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RESULTS AND DISCUSSION St Polymerization with Different Phosphines. Three phosphines (TPP, TMPP, and TTMPP, Scheme 1) were
Scheme 1. Structures of Phosphines
employed with FeIIIBr3 to control the polymerization of St in order to determine whether phosphines with more electron donating groups will accelerate ATRP. The results shown in Table 1 indicate that the activity of FeIIIBr3 complexes with TMPP was higher than that with TPP, and the activity of a FeIIIBr3/TTMPP was much higher than that with TMPP. The experiments were carried out at 100 °C, with the ratio of
Figure 1. (a) Kinetic plots of ln([M]0/[M]) vs time and (b) plot of number-average molecular weights Mn and Mw/Mn values vs conversion for ATRP of St with (FeIIIBr3 or FeIIBr2)/TTMPP. [St]: [EBiB]:([FeIIIBr3] or [FeIIBr2]):[TTMPP] = 200:1:1:(1 or 2) at 100 °C in 50% (v/v) anisole. B
dx.doi.org/10.1021/ma3010795 | Macromolecules XXXX, XXX, XXX−XXX
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the theoretical values and with dispersities Mw/Mn ≈ 1.25. Increasing the ratio of TTMPP/FeIIIBr3 to 1.5 resulted in the experimental molecular weights twice higher than the theoretical values and with dispersities Mw/Mn ≈ 1.5. When 2 equiv of TTMPP was used, the molecular weights were higher than 20 000 and did not increase with conversion, with values of Mw/Mn ≈ 1.5−1.7. Excess of phosphine could reduce concentration of FeIII species to a level too low for efficient control, and/or the catalytic complexes with TTMPP were too active for MMA, leading to significant levels of termination. This indicates, as previously reported, that less active phosphines could perform better in the polymerization of MMA.48,50
The polymerization was also well controlled, albeit slower, at a lower concentration of catalyst, at the ratio [FeIIIBr3]: [TTMPP] = 1:2, and a ratio of initiator to transition metal, [EBiB]:[FeIIIBr3] = 1:0.5. Conversion reached 52% after 20 h, Mn = 13 800, Mw/Mn = 1.19. However, with only 0.1 equiv of catalyst, [EBiB]:[FeIIIBr3] = 1:0.1, very slow polymerization was observed, and conversion reached 29% after 20 h, producing a polymer with Mn = 4900 and Mw/Mn = 1.19.
Figure 2. (a) Kinetic plots of ln([M]0/[M]) vs time and (b) plot of number-average molecular weights Mn and Mw/Mn values vs conversion for ATRP of St with FeIIIBr3/TTMPP. [St]:[EBiB]: [FeIIIBr3] = 200:1:(1, 0.5 or 0.1), [FeIIIBr3]:[TTMPP] = 1:2 at 100 °C in 50% (v/v) anisole. Figure 4. (a) Kinetic plots of ln([M]0/[M]) vs time and (b) plot of number-average molecular weights Mn and Mw/Mn values vs conversion for ATRP of MMA with FeIIIBr3/TTMPP. [MMA]: [EBiB]:[FeIIIBr3]:[TTMPP] = 200:1:1:(1, 1.5 or 2) at 60 °C in 50% (v/v) anisole.
ATRP of BA and MMA with FeIIIBr3/TTMPP. A wellcontrolled ATRP of BA was carried out at 100 °C in 50% (v/v) anisole when using 1 equiv of TTMPP and ratio of reagents [BA]:[EBiB]:[FeIIIBr3]:[TTMPP] = 200:1:1:1. The molecular weights of the PBA agreed very well with theoretical values, and Mw/Mn was around 1.2. The rate of polymerization decreased after 10 h, but the polymerization was accelerated at higher ratio of TTMPP to FeIIIBr3. When using 1.5 equiv of TTMPP, the polymerization was still well controlled with molecular weights slightly higher than theoretical values and the Mw/Mn values around 1.3. However, when the ratio of TTMPP was increased to 2 equiv, the polymerization lost control, resulting in production of polymers with molecular weights more than twice higher than theoretical values and the Mw/Mn values larger than 1.6. The polymerization of MMA was well controlled when 1 equiv of TTMPP was used at [MMA]:[EBiB]:[FeIIIBr3]: [TTMPP] = 200:1:1:1, in 50% (v/v) anisole, at 60 °C. The experimental molecular weights were in good agreement with
Phosphines as Reducing Agents. A PSt-Br macroinitiator with Mn = 12 000, Mw/Mn = 1.10 was prepared via normal ATRP. A reaction mixture with 0.1 g of PSt-Br macroinitiator in 2 mL of anisole was heated at 110 °C. The molecular weight and Mw/Mn value remained the same after 18 h (Figure 5b). However, when 0.1 g of PSt-Br was mixed with a catalyst complex at the ratio of reagents [PSt-Br]:[FeIIIBr3]: [TTMPP] = 1:1:2 in 2 mL of anisole and heated at 110 °C for 18 h, a bimodal GPC curve was observed. The peak molecular weight increased from 14 000 to 26 800 (Figure 5c). This observation suggested that PSt-Br could be activated by FeII species and the resulted macroradicals terminated via coupling, forming PSt with doubled molecular weight (Figure 5a). Since there was no additional reducing agent (including monomer) in this reaction mixture, TTMPP should reduce FeIII to FeII to activate dormant species. For comparison, the same experiment was performed with tetrabutylammonium bromide (TBABr), which was previously used as a successful cocatalyst for Febased ATRP of St,20,29 instead of TTMPP. In this case, the macroinitiator could not be activated, and the molecular weight did not change, even after 18 h (Figure 5d). These experiments suggested that in Fe-based ATRP processes phosphines can act as reducing agents. The polymerization of St was also performed with the ratio of reagents [St]:[EBrP]:[FeIIIBr3]:[TBABr] = 200:1:1:1, in 50% (v/v) anisole, at 110 °C. After 23 h, the conversion was very low,
