Iron Based ATRPIron is the most abundant transition metal element in the earth’s crust, it has been employed as a catalyst for many organic reactions and is environmentally friendly (non toxic). Iron based ATRP catalyst systems go back to the initial use of zero valent metals, in conjunction with a suitable ligand, as precursors of catalyst systems for atom transfer radical polymerization in 1997.(1-2) The initial paper described the use of zero valent metals themselves or in conjunction with other, higher, oxidation state metal salts, i.e., copper(0)/copper(I), copper(0)/copper(II), iron(0)/iron(II), and iron(0)/iron(III) as suitable catalyst precursors. It was noted that the addition of these simple metals to the soluble catalyst system (e.g. copper(I), iron(II)/Ligand) provided an increase in the rate of polymerization while still maintaining good control over the polymerization of styrenes, and (meth)acrylates.
A paper submitter earlier (3) described the use of iron halide complexes under both homogeneous and heterogeneous conditions for the ATRP of styrene and methyl methacrylate with a variety of coordinating ligands including 4,4'-bis(5-nonyl)-2,2'-bipyridine, trialkylamines, triphenylphosphine, trialkylphosphines, and trialkylphosphites. Sawamoto also examined Iron(II) bis(triphenylphosphine)dichloride [FeCl2(PPh3) 2] induced living radical polymerization of methyl methacrylate (MMA)(4) and noted that the reaction did not require an aluminum based activator unlike the earlier RuCl2(PPh3) 3-based counterpart.(5)
The polymerization rate and molecular weight distribution [Mw/Mn = 1.1-1.5] were affected by the structure of the coordinating ligands and the monomers employed.
Subsequently, a simplification of iron based catalyst systems was the direct use of compounds with halide anions as complexing ligands in iron-mediated ATRP (in both direct and reverse ATRP) was reported. (6) Iron(II) bromide complexed with ammonium and phosphonium chloride, bromide, or iodide salts were shown to catalyze the polymerization of both styrene and (meth)acrylates in a controlled manner under appropriate conditions. The experimental molecular weights increased linearly with monomer conversion and were close to the theoretical values. The polymerization rates and polydispersities (Mw/Mn = 1.1-1.4) were dependent on the monomer employed.
Reverse ATRP, initiated by AIBN/FeBr3/onium salts, led to a controlled polymerization of both methyl methacrylate and methyl acrylate, but the involvement of cationic polymerization in the styrene polymerization procedure led to uncontrolled molecular weights and high polydispersities. The most frequently used iron based catalysts were FeIIBr2/FeIIIBr3 or FeIICl2/FeIIICl3 with amine, imine, or phosphine ligands.
However the catalysts can participate in multiple polymerization processes including ATRP, organometallic radical polymerization (OMRP), catalytic chain transfer (CCT) and cationic polymerization procedures. This was discussed in detail by Rinaldo Poli (7) and the effect of metal spin state of the iron complexes by Shaver and Gibson (8-9) who noted that high-spin catalysts are halogenophilic, resulting in ATRP, while intermediate-spin complexes are carbophilic giving rise to CCT.
When the effect of the electron donating ability of substituents in α-diimine iron complexes was examined it was determined that electrondonating substituents favor ATRP while electron-withdrawing substituents favor CCT. Furthermore an analysis of the radical concentrations generated by the competing
OMRP and ATRP equilibria indicates that the halogenophilicity of the Fe(II) catalyst dominates the carbophilic alkyl radical-trapping capacity of the Fe(II) species in this R-diimine catalyst system.
The ionic nature of the iron complexes means that they could be removed easily from the reaction mixture by washing with water. This simplified purification procedure was also reported for iron mediated ATRP carried out in the presence of ionic liquids. (10) The ionic liquids containing transition metal salts could be readily separated from the polymerization media and regeneration of the catalyst in the ionic liquid was successful. A successful approach to more active iron based catalysts was reported by O’Reilly and Gibson when they discussed the preparation and use of a series of five-coordinate complexes of iron(II) containing tridentate nitrogen donor ligands for the ATRP of styrene. (11) Cyclic voltammetric studies showed that the redox potential for the FeII/FeIII couple strongly depended on the donor capacity of the complexing ligand. The polymerizations revealed that ligands derived from alkyl amine or pyridine groups were the most active although slower than their four-coordinate relatives due to unfavorable steric interactions. In general, catalyst activity decreases in the order of donor group: alkyl amine ~ pyridine > alkyl imine ≫aryl imine > aryl amine, with the aryl derivatives being almost completely inactive in ATRP.(12) The trend in activity correlated with the redox potential of the corresponding complexes. Indeed α-diimines are excellent ligands for four-coordinate iron ATRP catalysts due to their ease of preparation and their amenability to modification. (13) The activity is relative to reducing power with higher reducing power providing more active catalysts.
In summary high spin is favored to avoid OMRP and CCT; addition of a Lewis base is needed to shut down cationic polymerization for styrene. Ligands forming catalysts with higher reducing power gives higher activity (halide affinity should also be considered and larger kdeact values gives better control.
A novel ATRP procedure using activators generated by electron transfer (AGET ATRP) mediated by iron catalyst, was developed, using an oxidatively stable FeCl3/triphenylphosphine as a catalyst complex, ascorbic acid as a reducing agent, and methyl methacrylate as a monomer. The polymerization could be successfully conducted in the presence of a limited amount of air and displayed the features of "living"/controlled free-radical polymerizations such as Mn increasing linearly with monomer conversion and narrow molecular weight distributions. The “living” feature of the obtained polymer was further verified by a chain extension experiment. (14) AGET ATRP was used for MMA polymerization using a novel catalyst system based on iron FeCl3 complexes with iminodiacetic acid and using ascorbic acid as a reducing agent. The kinetics of AGET ATRPs of MMA with different amounts of reducing agent in the presence of air was investigated. (15)
ATRP of MMA has also been reported to occur in the presence of polar solvents where the reaction is catalyzed by FeBr2 without the need for additional ligand. (16-17) The amount of NMP could be reduced to as low as 10% and still dissolve most of the FeBr2 and maintained control over polymerization. The addition of a small amount of FeBr3 further improved control over molecular weight distribution.
In 2008 it was also reported that FeIIIX3 with various phosphines could mediate an ATRP in the absence of an additional reducing agent. (18-19) The proposed mechanism was that the FeII species could be generated in-situ via a reaction between FeIII and the monomer, (1) as previously reported for an CuIIX2/L system. (20) However, FeIIIBr3 could be also reduced in the presence of phosphine to generate a FeIIBr2 and dibromophosphorane. (2)
The substituents present on the phosphines have significant influence on the activity of the transition metal complex catalysts. Higher activity means a larger value for KATRP, which is correlated with faster polymerization under identical conditions. However, since phosphine can reduce FeIII to FeII, the faster polymerization could be a result of both larger value for KATRP and faster reduction of FeIII.
Within the group it was recently determined that iron based ATRP of styrene with various triarylphosphines containing electron donating methoxy groups led to different rates of polymerization. (21) Three different phosphines: tris(2,4,6- trimethoxyphenyl)phosphine (TTMPP), tris(4-methoxyphenyl)phosphine (TMPP) and triphenylphosphine (TPP), Scheme 1, were employed with FeIIIBr3 to control the polymerization of St in order to determine whether phosphines with more electron donating groups on the aromatic rings will accelerate an ATRP.
ATRP of styrene with an FeIIIBr3 complex formed with TTMPP ligand led to a faster polymerization than one formed with TMPP and much faster than with TPP under identical conditions. The polymerizations were carried out with initial ratio of reagents equal to [St]:[EBiB]:[FeIIIBr3]:[TTMPP] = 200:1:1:2, in 50% (v/v) anisole at 100 oC, (EBiB is ethyl 2-bromoisobutyrate). After 21 h, in the presence of the FeIIIBr3/TMPP catalyst complex 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% after 21 h, respectively. Control was better with one equiv TTMPP vs. FeIIIBr3, Mw/Mn ~ 1.1, but polymerization was slower. The phosphines could directly reduce FeIII to FeII, and the FeII species acted as the actual activator. The FeIIIBr3/phosphine catalyzed ATRP can be considered as an activator generated by electron transfer (AGET) ATRP process, in which the phosphine acts as the reducing agent.
[St]:[EBiB]:[FeBr3]:[Ligand] = 200:1:1: (1 or 2), in 50%(v/v) anisole at 100 oC, after 21 h
The proof that phosphine can reduce FeIII to FeII was provided by the following set of experiments. A PSt-Br macroinitiator with Mn = 12,000 and Mw/Mn = 1.10 was prepared via normal ATRP. A reaction mixture with 0.1 g PSt-Br macroinitiator in 2 mL anisole was heated at 110 oC. The molecular weight and Mw/Mn value remained the same after 18 h. However, when 0.1 g PSt-Br was mixed with a catalyst complex with the ratio of reagents [PSt-Br]:[FeIIIBr3]:[TTMPP] = 1:1:2 in 2 mL anisole and heated at 110 oC for 18 h, a bimodal GPC curve was observed. The peak molecular weight increased from 14,000 to 26,800, central GPC trace in the following figure.
This observation suggested that PSt-Br could be activated by FeII species forming a radical and the resulted macroradicals terminated via coupling, forming PSt with double the molecular weight. Since there was no additional reducing agent or monomer in this reaction mixture, it is clear that TTMPP reduced FeIII to FeII which activated the dormant species. For comparison, the same experiment was performed with tetrabutylammonium bromide, TBABr, which was previously used as a successful co-catalyst for Fe based ATRP of St, instead of TTMPP. In this case, the macroinitiator could not be activated and the molecular weight did not change, even after 18 h, right hand GPC trace. These experiments suggested that in Fe based ATRP processes phosphines can act as reducing agents.
The reduction process was studied via UV/VIS/NIR spectroscopy of 30 mM FeIIIBr3 in DMF in the presence of 1 equiv TTMPP at room temperature (23 oC) and the change of absorbance corresponding to FeIII and FeII was followed over time. The absorption bands of FeIII with halide ligands between 680 and 735 nm decreased strongly after addition of 1 equiv of TTMPP. The absorbance continued to decrease in the following 2 h, indicating a continuous gradual reduction process. The broad absorbance of FeII, which could be observed at 2000 nm, appeared quickly after addition of TTMPP and continued to increase during 2 h. The reduction of FeIII by phosphine should result in the corresponding dibromophosphorane.
A successful ICAR ATRP of styrene was conducted with iron(III) bromide and 1,1'-azobis(cyclohexanecarbonitrile) (ACHN) as the thermal initiator. The polymerization was started with 50 ppm of FeBr3 and 50 mol equiv. of ACHN in 33% (vol./vol.) anisole at 90 °C, reached 70% conversion in 24 h and was well controlled, giving a polymer with a narrow MWD, Mw/Mn = 1.15. which corresponded well to theoretical values, as conversion increased.(22)
The rate of polymerization was dependent on the amount of ACHN initially added to the reaction. A polymer with a relatively narrow molecular weight distribution, Mw/Mn = 1.29 at 65% of conversion, was obtained with 5 ppm of FeBr3 and the appropriate amount of ACHN. This procedure therefore provides an efficient controlled polymerization in addition to creating a robust, cheap, and environmentally friendly catalytic system. The level of control over the polymerization with ACHN was better than with tert-Butyl peracetate as a thermal initiator or tin(II) 2-ethylhexanoate, Fe0, or Zn0 wire as reducing agents. (23)
An FeBr2,/FeBr3, tetrabutylammonium bromide catalyst was employed for an ATRP grafting-from reaction resulting in formation of brush macromolecules with a narrow molecular weight distribution (Mw/Mn = 1.18-1.28).(24) Initiation efficiencies were calculated by cleaving the side chains by alcoholysis and then injecting to product into gel permeation chromatography. The initiation efficiencies were ca. 80-95 %, showing relatively higher values for the “grafting from” polymerization with an iron catalyst. These results indicate that iron catalyzed ATRP can provide well controlled polymerizations even when targeting dense grafting from procedures.
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