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Single Component Iron Catalysts for Atom Transfer and Organometallic Mediated Radical Polymerizations: Mechanistic Studies and Reaction Scope Laura E. N. Allan, Jarret P. MacDonald, Gary S. Nichol, and Michael P. Shaver* School of Chemistry, University of Edinburgh, Joseph Black Building, West Mains Road, Edinburgh EH9 3JJ, United Kingdom S Supporting Information *

ABSTRACT: Tetradentate amine−bis(phenolate)iron(III) halide complexes containing chloro substituents on the aromatic ring are extremely efficient catalysts for controlled radical polymerization. Molecular weights are in good agreement with theoretical values, and dispersities are as low as 1.07 for substituted styrenes and methyl methacrylate polymerizations. Kinetic data reveal activity for styrene polymerization among the fastest reported to date, with the excellent control shown to be electronic rather than steric in origin. Mechanistic studies implicate a multimechanism system with cooperation between atom transfer radical polymerization (ATRP) and organometallic mediated radical polymerization (OMRP). The in situ reduction of the Fe(III) complex with ascorbic acid or tin octanoate allows polymerizations to be initiated by both 1-phenylethyl chloride (1-PECl, ATRP regime) and azobis(isobutyronitrile) (AIBN, OMRP regime) to isolate the mechanism of control and offer unique initiation pathways.

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INTRODUCTION Controlled radical polymerization (CRP) offers polymer chemists and engineers the ability to modify polymer macrostructure and create a unique array of materials with high functional group tolerance and defined molecular weights. In less than 20 years, CRP has developed into a powerful workhorse for scientists to produce specialized polymers with unprecedented properties and applications. Metal-mediated methods,1,2 such as (reverse) atom transfer radical polymerization ((R)ATRP)3,4 and organometallic mediated radical polymerization (OMRP),5 are especially important as altering the supporting ligand framework in a metal complex can tune the control, reactivity, monomer scope, and applications (Scheme 1).6 While ATRP has been extensively studied, OMRP remains an underdeveloped field with a paucity of active, versatile catalysts.1,5 Particularly intriguing is the interface between these controlled radical polymerization reactions, where a metal complex may

participate in more than one mechanism. Half-sandwich molybdenum complexes, 1, were the first reported examples of catalysts active in both ATRP and OMRP equilibria, with the ancillary ligand determining which mechanism dominated.7−9 Mo(III) reversibly trapped the propagating radical under OMRP conditions, and since the concentration of Mo(III) remained high under ATRP conditions, both trapping mechanisms were proposed to occur simultaneously.

Scheme 1. Exchange Equilibria Used in ATRP, RATRP, and OMRP

Figure 1. Complexes with mechanistic interplay in CRP. Received: November 19, 2013 Revised: January 29, 2014 Published: February 4, 2014 © 2014 American Chemical Society

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Similar mechanistic interplay was reported for a series of αdiimine iron complexes, R1,R2[NN]FeCl2, 2.10−15 The nature of the ligand determined the prevalent polymerization mechanism, with electron-donating groups favoring ATRP and yielding halogen-terminated polymers with well-controlled molecular weights, while electron-withdrawing groups favored an organometallic regime. The instability of the Fe(III)R species formed by the trapping of the propagating radical by the Fe(II) complex resulted in β-hydrogen elimination to give low molecular weight, olefin-terminated polymers via catalytic chain transfer. Os(PPh3)3Cl2, 3, was also shown to be active for styrene polymerization under both ATRP and OMRP conditions, although control was much better in ATRP, where dispersities were as low as 1.11. Utilizing an OMRP regime resulted in molecular weights which did increase linearly with conversion, but the presence of termination reactions resulted in broad, bimodal dispersities, Đ, of 2.8−3.2.16 Examples where control is predominantly derived from the OMRP regime have also been reported. For both the halfsandwich chromium, 4,17−19 and bis(imino)pyridine vanadium, 5,20−22 systems, control over the polymerization was imparted through the OMRP regime, despite the use of alkyl halide initiators. In the chromium system, DFT calculations showed that the ATRP dormant state was less favorable than the OMRP dormant state, as the energy barrier on going from the active radical to the ATRP dormant state was much greater than that leading to the OMRP dormant state. For the vanadium system, experimental and computational studies supported a two-step process with irreversible halogen transfer from the parent LVCl3 complex generating the active LVCl2 species in situ. This complex did not participate in ATRP, instead acting as a persistent radical and trapping the propagating radical chains in a reversible termination OMRP process. The role of iron complexes in controlled radical polymerization is of growing importance. Iron is an attractive metal to target for use in CRP as it is both highly abundant and of low toxicity, with great potential for use in low-cost, biocompatible catalysts, in parallel to the recent development in iron-catalyzed organic reactions.23,24 Several different types of ligand have been investigated for (R)ATRP with Fe(II) or Fe(III) halides, including multidentate amines,25−34 imines,10−15,35−40 phosphines,31,41−50 thiocarbamates,51−53 carbenes,54 and organic acids.55−63 Ammonium, phosphonium, imidazolium, and phosphazenium salts64−72 of iron halides have also been studied, while simple donor solvents also offer control.73−78 Many of these seemingly simple systems are complicated by multiple speciation due to the lability of the ligand which results in the formation of several potentially catalytically active species within the reaction media and the multiple role that some additives can play. Well-characterized, single site, iron-based CRP mediators with irreversibly bound ligands are less common but have offered improved control and rapid polymerization rates.37,79−81 Iron is also mechanistically interesting as it has proved capable of interacting with radicals in both ATRP and OMRP equilibria. Our preliminary report79 described the efficacy of iron(III) amine−bis(phenolate) complexes with electron-withdrawing chlorine substituents on the aromatic ring in the controlled radical polymerization of styrene (Figure S1 and Table S1). Reactions were rapid, but well controlled, with molecular weights which were in good agreement with theoretical values and Đ as low as 1.11. In this paper we describe detailed kinetic studies, reaction scope, and important mechanistic implications of CRP

mediated by representative iron(III) amine−bis(phenolate)s 6a−c (Figure 2).

Figure 2. Iron(III) amine−bis(phenolate) complexes.

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RESULTS AND DISCUSSION Kinetic Studies and Monomer Scope. Initial kinetic studies with Cl,Cl,NMe2[O2NN′]FeCl, 6a (Figure 2), showed it to be the fastest iron-based catalyst for the CRP of styrene reported, reaching ca. 70% conversion in 1 h, with a kobs of 1.02 h−1 (cf. salicylaldiminato iron complexes,37 with kobs = 0.39−0.49 h−1, and α-diimine iron complexes,13,14 with kobs = 0.01−0.72 h−1). The linear semilogarithmic plot of ln[M]0/[M]t versus time and the linear increase of molecular weight with conversion, in conjunction with the narrow Đs, illustrates the excellent control imparted by this complex (Figure 3). However, in repeated kinetic experiments molecular weights were observed to be somewhat higher than the theoretical values. This can be attributed to the number of growing radical chains being lower than expected, resulting in an effective increase in the monomer concentration (f = 0.71). Polymerization is very rapid initially (reaching 32% conversion in 10 min) before a constant radical concentration is established and linear reaction kinetics are observed. End-group analysis of low molecular weight crude polystyrene samples by 1H NMR spectroscopy suggests that the polymerization mechanism of reactions mediated by Cl,Cl,NMe2 [O2NN′]FeCl, 6a, is not simply RATRP, as only 30− 35% of the chains are chlorine-terminated. No evidence of olefin end-groups is observed, and the success of start−stop reactions implies that the other polymerization pathway also operates though a controlled radical mechanism, potentially OMRP. This is supported by MALDI-MS experiments which show two families of peaks in both the crude and worked-up poly(styrene) samples, each separated by the 104 amu of styrene (Figures S2 and S3). The iron(III) amine−bis(phenolate) complexes were also screened for activity toward MMA, with the chloro-substituted complexes again proving to be the most efficient. We were initially surprised to find that the MMA polymerizations (bulk or monomer 1:1 w/w with toluene, Table S2) were slower than the corresponding styrene reactions, despite the greater inherent reactivity of MMA-derived radicals. Bulk reactions resulted in broader Đs and slightly decreased conversions when compared with the solution polymerizations (Table S2), but in all cases molecular weights were higher than the theoretical values calculated from the initial concentration of radical initiator. Kinetic studies using Cl,Cl,NMe2[O2NN′]FeCl, 6a, were conducted, although the lower boiling point of MMA meant that multiple sealed ampules were used, limiting the number of data points collected. Nonetheless, the linear semilogarithmic plot of ln[M]0/[M]t versus time, with kobs = 0.50 h−1, and the linear increase of molecular weight with conversion (Figure 4), coupled with Đs of ca. 1.3, illustrates the good control that 1250

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Figure 3. Plots of (a) ln([M]0/[M]t) vs time and (b) molecular weight and Đ vs conversion for bulk styrene polymerization at 120 °C, using Cl,Cl,NMe2 [O2NN′]FeCl, 6a, and AIBN. Initial monomer:catalyst:initiator ratio of 100:1:0.6. Solid line in (a) is least-squares fit to data. Solid line in (b) represents Mn,th, calculated by [M]0/2[I]0 × MW(monomer) × conversion, with dashed line showing linear increase of Mn with conversion.

Figure 4. Plots of (a) ln([M]0/[M]t) vs time and (b) molecular weight and Đ vs conversion for solution (1:1 w/w monomer:toluene) MMA polymerization at 120 °C, using Cl,Cl,NMe2[O2NN′]FeCl, 6a, and AIBN. Initial monomer:catalyst:initiator ratio of 100:1:0.6. Solid line in (a) is leastsquares fit to data. Solid line in (b) represents Mn,th calculated by [M]0/[I]0 × MW(monomer) × conversion.

styrenes,82 the polymerization rate increased significantly with pCl-St and decreased significantly with p-Me-St and p-MeO-St (Table 1). Linear semilogarithmic plots of ln[M]0/[M]t versus time (Figures S4, S6, and S8) support controlled processes with

Cl,Cl,NMe2

[O2NN′]FeCl, 6a, exerts over MMA polymerizations. Notably, molecular weights correlate well with theoretical values which were calculated assuming that each AIBN molecule initiates only one growing polymer chain. This low initiator efficiency ( f = 0.50) is attributed to the reactive nature of the propagating polymer chains, which results in significant termination reactions. While 1:1 solution polymerizations in toluene offered improved control, higher monomer concentrations gave molecular weights that were still considerably higher than theoretical values and dispersities increased slightly as the monomer concentration (and thus effective radical concentration) increased (Table S3). Building from the surprisingly slow reaction kinetics for MMA, we explored the polymerizations of styrene derivatives with variable inherent reactivities to determine if slower rates and control were related to the kp of the specific monomer or the chemical functionality of the methacrylate unit. Contrary to MMA kinetics, but as expected from the kp of para-substituted

Table 1. Data for Polymerization of Para-Substituted Styrene Derivativesa para substituent

% conv

Mn,th

Mn

Đ

kobs/h−1

Cl H Me MeO

81 60 46 23

9240 5240 4530 2520

13060 7470 6090 3230

1.09 1.14 1.19 1.90

2.20 1.02 0.34 0.033

a Bulk p-substituted styrene polymerizations, using Cl,Cl,NMe2[O2NN′]FeCl, 6a, with an initial complex:AIBN:monomer ratio of 1:0.6:100, 120 °C for 1 h. Rate data from kinetic experiments run until conversion >90%. M n,th = [M]0 /2[I] 0 × MW(monomer) × conversion.

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Scheme 2. Interplay between OMRP and ATRP Equilibria with [O2NN′]FeCl

theoretical values (Mn = 3980, Mn,th = 2880), control was decreased in comparison to the other chlorine-substituted complexes, with Đs of 1.34, suggesting inefficient initiation due to the required dissociation of a strongly bound dimer. The presence of the donor arm likely also helps to stabilize the unsaturated Fe(II) intermediate and may prevent catalyst decomposition. Mechanistic Studies: Reaction Conditions. With these unusual kinetic results, coupled with our early observation that polymer chains were not universally capped with halogen endgroups, we pursued a more thorough mechanistic study of the reaction, aided by the robust single-site nature of the complex. We suggest that an interplay between ATRP and OMRP equilibria is possible for these [O2NN′]FeCl systems, as shown in Scheme 2. We first investigated the effect of changing the catalyst:initiator ratio on styrene polymerizations (Figure 5, Figure S11 and Table

constant radical concentrations throughout the polymerizations. Molecular weights increased linearly with conversion and Đs were narrow for both the Cl- and Me-substituted styrenes (Figures S5 and S7), only deviating from CRP with the extremely slow p-MeO-St polymerizations (Figure S9). Interestingly, the best control over the substituted styrenes was achieved using pCl-St, which polymerized extremely rapidly reaching high conversion in 1 h, with Đs of 1.11. Of note, more reactive acrylate monomers are poorly controlled (Table S4), while less reactive monomers such as vinyl acetate (VAc) and Nvinylpyrrolidone (NVP) irreversibly terminate, with no productive polymerization observed using these iron(III) amine−bis(phenolate) catalysts. One of the potential challenges of a catalyst system that may operate via a dual mechanism is clean reinitiation of polymer chains. Attempted block copolymerizations using methyl methacrylate were unsuccessful, suggesting sluggish chain reinitiation, potentially due to the stable organometallic intermediates. However, block copolymers are readily prepared using styrene derivatives, with good control over molecular weights and narrow Đs (Table S5). The reinitiation appears complete, with no evidence of styrene homopolymer (poly(styrene-block-p-methylstyrene), Figure S10) and kinetics consistent with the homopolymerization rates of the respective monomers. The initial, rapid polymerization is seen only during the formation of the first block, likely due to the rapid decomposition of the thermal initiator and establishment of the dynamic equilibrium. Mechanistic Studies: Sterics vs Electronics. While our tentative conclusions from the screening reactions suggested that the success of the chlorine-substituted amine-bis(phenolate) iron(III) complexes in CRP was an electronic effect, we recognized that our initial studies only investigated alkylsubstituted complexes with bulky ortho-tert-butyl groups. In order to ascertain whether the excellent control over styrene polymerization obtained using chlorine-substituted complexes was predominantly a steric or electronic effect, we synthesized the complex with ortho-methyl groups through modification of a literature procedure.83 Me,Me,NMe2[O2NN′]FeCl, 6b, proved to be an inefficient mediator of styrene and MMA polymerizations, yielding molecular weights which were much higher than the theoretical values with broad Đs of 2.17−2.21. Given the similar sizes of the methyl and chlorine substituents (2.0 Å for Me, cf. 1.8 Å for Cl),84 the loss of control in the alkyl-substituted complexes can be attributed primarily to electronic effects. Our initial study also indicated that the donor arm had little effect, except in the absence of a Lewis basic site. The only alkylsubstituted complex to yield reasonable control over styrene polymerization has a noncoordinating n-propyl group as the Nsubstituent and adopts a dimeric solid-state structure (V, Figure S1 and Table S1).85 We pushed the electron deficiency of the system further by synthesizing the chlorine-substituted analogue (6c, Figure 2 and Table S1). However, although styrene polymerization mediated by Cl,Cl,Pr[O2NN′]FeCl, 6c, reached 33% conversion in 1 h, with molecular weights close to

Figure 5. Molecular weight vs conversion plots for bulk styrene polymerizations using 0.8 (●), 1.0 (◆), and 2.0 equiv (■) of Cl,Cl,NMe2 [O2NN′]FeCl, 6a, and AIBN. Initial monomer:initiator ratio of 100:0.6. Solid line indicates Mn,th, calculated by [M]0/2[I]0 × MW(monomer) × conversion.

S6) mediated by Cl,Cl,NMe2[O2NN′]FeCl, 6a. Increasing the amount of trapping agent, 6a, had the expected effect of slowing the rate of polymerization, which significantly improved the dispersities. When 2 equiv of catalyst was used, nonlinear plots of ln([M]0/[M]t) were obtained (Figure S11) with the polymerization taking 8 h to reach 50% conversion, albeit with Đs which were below 1.16 throughout and 1.45, although the molecular weights continue to increase linearly with conversion. At these high temperatures, AIBN decomposes very rapidly to afford the initiating radicals, with the high radical concentrations leading to irreversible termination reactions through radical coupling. Lower temperatures may favor the formation of dormant chains, will decrease kp values and can offer improved control, but may also give inefficient radical generation depending upon the azo initiator used. Bulk MMA polymerizations using 3 or 6 equiv of AIBN as the initiator at lower temperatures (Table S8) gave low conversions at 50 °C, as expected, and broader Đs (1.4−1.5), indicating inefficient exchange and a slow generation of new chains as the polymerization proceeds (the 10 h half-life decomposition temperature of AIBN is 65 °C). These broad dispersities indicate that we are not accessing an efficient DT-OMRP under these conditions. Increasing the temperature up to 70 °C gave higher conversions but continued poor control. To complement these expected results, we explored other radical initiators with lower 10 h half-life decomposition temperatures, V-65 (51 °C) and V70 (30 °C), as reported in Table S8. Polymerizations were reasonably well-controlled (Đ = 1.24) by 0.6 equiv of V-65 at temperatures as low as 90 °C, indicating that trapping is efficient and reversible at these lower temperatures. Conversions were considerably lower, reaching only 35% conversion after 1 h at 120 °C, compared to 51% in an analogous reaction initiated by AIBN. At lower temperatures, conversion is decreased further to ca. 25%. Molecular weights remain higher than theoretical values at all temperatures ( f = 0.31−0.36). In contrast, reactions initiated by V-70 (0.6 or 6 equiv) were not well controlled over all conditions tested. Đs were broad, only decreasing to 1.45 when 6 equiv of initiator was used at 65 °C. These experiments with different initiators and temperatures explored MMA polymerizations mediated by Cl,Cl,NMe2[O2NN′]FeCl, 6a, at a range of radical concentrations to assess the effect on the position of the equilibria. Irreversible trapping is not observed, even at the lower temperatures, although DT-OMRP is inefficient. However, lower temperatures are not beneficial to the control over the polymerizations, with dispersities broadening due to inefficient initiation, even with initiators such as V-65 and V-70. The optimal conditions are 6 equiv of AIBN at 120 °C, in both bulk and solution MMA polymerizations. Mechanistic Studies: Initiation Routes. If the OMRP pathway is important to control, targeting the coordinatively unsaturated Cl,Cl,NMe2[O2NN′]Fe complex could provide an avenue to independently assess polymerizations under ATRP

decreasing from 1.27 at the beginning of the polymerization to 1.17 at the end. The initial rate over the first 20 min of polymerization is faster when 0.8 equiv of catalyst is used, again suggesting that increased amounts of catalyst reduce the uncontrolled events at the beginning of the polymerization. Changing the concentration of the initiator allowed for a more expansive view of the effect that the initial radical concentration has on the polymerization efficiency. In styrene polymerizations, use of 6 equiv of AIBN led to a loss of control, where high conversions were rapidly achieved as a result of the high radical concentrations (Table S6). The excess radicals cannot be deactivated by the catalyst fast enough, leading to classic uncontrolled radical polymerization with prevalent termination reactions. The use of 1.5 equiv of AIBN (3 radicals generated per iron) surprisingly resulted in only a slight loss of control, with Đs broadening to 1.29 and molecular weights higher than the theoretical values due to low initiation efficiency. However, the catalyst still imparts a surprising level of control over the polymerization, even at this significantly elevated radical concentration. At radical concentrations below our standard 0.6 equiv of AIBN, excellent control is observed, although the polymerizations are slower. With 0.3 equiv of AIBN, the catalyst is in excess and deactivation of the propagating radicals is favored. The polymerization is significantly slower with correspondingly excellent Đs of 1.11. The effect of initiator concentration on bulk MMA polymerizations was also investigated using Cl,Cl,NMe2[O2NN′]FeCl, 6a (Table 2), yielding significantly different results to the styrene Table 2. Effect of AIBN Concentration on MMA Polymerizationa AIBN (equiv)

% conv

Mn,th,Fe

Mn

Đ

0.3 0.6 1.5 3 6

29 48 88 90 93

2900 4810 8810 9010 9310

9990 8120 7710 5070 3780

1.29 1.28 1.29 1.24 1.18

a Bulk MMA polymerizations, using Cl,Cl,NMe2[O2NN′]FeCl, 6a, with an initial complex:monomer ratio of 1:100, 120 °C for 1 h. Mn,th,Fe = [M]0/[cat]0 × MW(monomer) × conversion.

polymerizations. A decrease in the initial AIBN concentration to 0.3 equiv resulted in little change to the dispersity (Đ = 1.28 and 1.29 for 0.6 and 0.3 equiv of AIBN, respectively), with no significant change in resultant molecular weights other than decreased conversions as a result of reduced radical concentrations. Increasing the AIBN concentration to 1.5 equiv resulted in largely unchanged Đs, but conversion was considerably higher (88% vs 48%) after 1 h, attributed to the higher number of radicals generated initially. Further increasing the AIBN concentration to 3 equiv actually resulted in improved control, illustrated by a decrease in Đ to 1.24. Increasing the initial AIBN concentration to 6 equiv (12 radicals per metal center) improved control further (Đ = 1.18) although the molecular weights were much higher than expected if the theoretical molecular weights were calculated from [AIBN]0. Control in these systems is derived from the strongly bound dormant state, likely through the aforementioned organometallic intermediate. The metal complex could trap a single alkyl radical efficiently, with rapid coupling of the unused radical chains. Supporting this hypothesis, basing theoretical molecular weights on the monomer-to-catalyst ratio (Mn,th,Fe), as in a DT-OMRP 1253

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and OMRP conditions. While an ATRP reaction set up using an alkyl halide initiator such as 1-PECl could still involve the OMRP equilibrium, by virtue of the presence of the Fe(II) OMRP trapping agent, a halogen-free polymerization setup with Cl,Cl,NMe2 [O2NN′]Fe and AIBN will separate the OMRP contribution to the reaction. Despite extensive efforts, direct synthesis of the Fe(II) complexes proved unexpectedly difficult. Best results were achieved through reaction of either the lithium or sodium salts of the ligand with FeCl2(THF)1.5 and yielded a white solid which decomposed to purple in both solid and solution states upon exposure to trace amounts of air or moisture. Polymerizations run under both ATRP and OMRP conditions with these Fe(II) complexes were poorly controlled (Table S9) with bimodal Đs. Structural elucidation of the Fe(II) species revealed that the complex was dimeric, bridged by 2 equiv of LiCl (Figure 6). Crystals were obtained from both hot toluene and hot

Table 3. Data for Polymerization of Styrene Using Varying Equivalents of Ascorbic Acid as an in Situ Reducing Agenta equiv

time/h

% conv

Mn,th

Mn

Đ

1 1 10 0.1

1 4 1 6

13 32 30 75

1350 3330 3130 7810

1700 3770 4080 10140

1.14 1.16 1.32 1.13

Bulk styrene polymerizations at 120 °C using Cl,Cl,NMe2[O2NN′]FeCl, 6a, and 1-PECl, with an initial complex:initiator:monomer ratio of 1:1:100. Mn,th = [M]0/[I]0 × MW(monomer) × conversion. a

increase in polymerization rate was unexpected for the lower loading (0.1 equiv) of ascorbic acid but is likely due to the formation of low concentrations of Fe(II) species facilitating controlled initiation while also making the contribution of trapping via the OMRP mechanism negligible and allowing ATRP to dominate. Polymerizations using 1 equiv of tin octanoate as the reducing agent were faster than those using ascorbic acid, reaching 70% in 1 h (cf. 13% with ascorbic acid) but were less well controlled. We attribute this to the increased solubility of tin octanoate in the polymerization medium, resulting in more rapid formation of the Fe(II) species and faster initiation. Inefficient trapping leads to molecular weights which are higher than the theoretical values and broad dispersities. Two control experiments were also performed: (i) polymerization of styrene using Cl,Cl,NMe2[O2NN′]FeCl, 6a, and 1-PECl is unsuccessful in the absence of a reducing agent, indicating that the Fe(III)/Fe(IV) redox couple is, as expected, not accessible, and (ii) reaction of Cl,Cl,NMe2[O2NN′]FeCl, 6a, and ascorbic acid in the presence of styrene produces no polymer. Styrene polymerizations attempted under ARGET (activator regenerated by electron transfer)87 conditions using both ascorbic acid and tin octanoate were unsuccessful. Although polymerizations were productive, lowering the concentration of catalyst to 0.1 and 0.01 equiv resulted in extremely broad Đs and molecular weights which were much higher than the theoretical values (Table S11). Decoloration of the reaction suggests that formation of the Fe(II) complex predominates, with the excess reducing agent favoring rapid reduction of the Fe(III) species and leading to uncontrolled propagation, high molecular weights, and broad dispersities. To decouple the OMRP equilibria, we targeted styrene polymerization from the Fe(II) species prepared in situ with ascorbic acid, initiated by AIBN. Rapid polymerization was observed, reaching 81% conversion in 1 h at 120 °C. Molecular weights were higher than the theoretical values (Mn,th = 7080, Mn = 20 760), but dispersities showed good control (Đ = 1.32). This low initiation efficiency ( f = 0.34) is most likely to be due to early termination reactions, with radical coupling reducing the effective initiator concentration and thus increasing the effective monomer concentration. Decreasing the temperature to 100 °C slowed the polymerization, favoring the formation of the dormant Fe(III)−R complex and reducing the termination reactions. After 1 h at 100 °C, 52% conversion was achieved with molecular weights which were in better agreement with the theoretical values than at the higher polymerization temperature (Mn,th = 4550, Mn = 9370; f = 0.49), although the dispersities were essentially unchanged (Đ = 1.31). Further investigation and optimization of the conditions is ongoing, but these results represent the best OMRP control over styrene reported for an iron complex, presuming full reduction of Fe(III) and

Figure 6. Molecular structure of Fe(II) species crystallized from THF.

THF, showing isostructural Fe2Li2 cores and ligands (Figure 6 and Figure S14). The structure obtained from THF was of low quality but sufficient for connectivity. The crystals were observed to desolvate when removed from the mother liquor (resulting in poorly refined partial THF molecule occupancies). The structure obtained from toluene was of higher quality and was solved easily by direct methods; refinement was less straightforward due to the behavior of Fe1 which lies in a hole −2.17 e Å−3 deep (Figure S15). Allowing the iron occupancy to refine freely gave a value which was very close to that of the occupancy ratio of lithium and chloride, both of which refined independently to around 75% (for further details see Supporting Information). The bimetallic dimer is surprisingly stable in this crystalline form, and this stability is the likely cause of sluggish initiation to give poor CRP behavior. Rather than targeting an isolable Fe(II) species, we decided to investigate in situ formation of the Fe(II) complex from the reaction of the Fe(III) complex with a reducing agent, an AGETtype ATRP (activator generated by electron transfer).86 Polymerization of styrene initiated by 1-PECl using Cl,Cl,NMe2 [O2NN′]FeCl, 6a, in the presence of ascorbic acid or tin octanoate was productive (Table 3 and Table S10). Despite its poor solubility in the bulk monomer, best results were obtained with 1 equiv of ascorbic acid as the additive, giving slow but well-controlled styrene polymerization, with Đ as low as 1.14. Both increasing and decreasing the concentration of ascorbic acid resulted in faster polymerization, but broadened Đs of 1.32 were obtained with the increased additive loading. The apparent 1254

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recognising that ATRP equilibria will still play a key role. Indeed, there is a paucity of literature on Fe complexes in OMRP, with a recent report by Poli on the RT and DT OMRP of vinyl acetate using Fe(acac)2 illustrating the potential of iron in this regime.80 OMRP may follow two different mechanisms: reversible termination OMRP where the metal complex reversibly caps the propagating chain (as shown in Scheme 1), or degenerative transfer OMRP, a thermodynamically neutral bimolecular exchange between a low concentration of growing radical chains and a dormant species. DT-OMRP requires a constant influx of radicals throughout the polymerization, whereas RT-OMRP typically requires an external initiator to start the polymerization, but the homolytic cleavage of the M−R dormant species then provides the only source of radicals. Our system was ineffective for the RATRP of vinyl acetate (Table S4), but we wondered whether the OMRP contribution of the amine−bis(phenolate) iron(III) complexes could be accessed using DT-OMRP conditions. VAc was successfully polymerized under DTOMRP conditions (Table S12), reaching 44% conversion after 6.5 h at 60 °C. However, molecular weights were much higher than the theoretical values, and the broad Đ of 1.58 indicated a lack of control. Theorizing that this system appears to benefit from rapid initiation, we combined an initiator that would decompose quickly at the polymerization temperature (V-70) with an initiator operating at its 10 h half-life temperature (AIBN). The results are shown in Table S12, for both styrene and MMA, and the data from these experiments suggest that our system is not operating via DT-OMRP. Although molecular weights were in fairly good agreement with those calculated using the monomer-to-catalyst ratio, dispersities were broad (Đ = 1.50−2.61) in all cases, indicating a lack of control. Complementing this work, we used DT-OMRP conditions in conjunction with ascorbic acid, to reduce the Fe(III) complex in situ, allowing the DT-OMRP scope of the Fe(II) species to be assessed individually. Poor results were obtained for both styrene and vinyl acetate, with molecular weights that were much higher than the theoretical values and broad dispersities (Đ = 1.72− 1.76), again indicating that these conditions do not suit the amine−bis(phenolate) iron(III) catalysts. It is likely that our iron catalysts can only function under RT-OMRP conditions. Our in situ Fe(II) generation greatly expands the potential applications of our iron system, allowing us to access OMRPtype controlled radical polymerizations and the traditional initiation pathways used in Cu-mediated ATRP. These AGET initiation reactions, as well as expanding the application scope of our sustainable Fe catalysts, are a focus of our current efforts.

iron-mediated OMRP and the fastest iron-based ATRP catalysts reported to date.

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EXPERIMENTAL SECTION

Materials. The amine−bis(phenolate) iron(III) complexes were prepared through modified literature procedures.83,85,88,89 Toluene, hexane, dichloromethane, diethyl ether, and THF were obtained from either an MBraun or Innovative Technologies solvent purification system, consisting of columns of alumina and copper catalyst. The solvents were degassed by three freeze−pump−thaw cycles prior to use. Triethylamine was obtained from Sigma-Aldrich, dried over calcium hydride, distilled under inert atmosphere, and degassed before use. nBuLi was purchased from Acros Organics (2.0 M in hexanes) and used as received. Monomers styrene, p-chlorostyrene, p-methylstyrene, pmethoxystyrene, methyl methacrylate, methyl acrylate, tert-butyl acrylate, N-vinylpyrrolidone, and vinyl acetate were purchased from Sigma-Aldrich, Fisher, or VWR and dried by stirring over calcium hydride for 24 h, before being vacuum transferred or distilled, degassed, and stored at −35 °C under inert atmosphere. Radical initiators were purchased from Aldrich or Wako: azobis(isobutyronitrile), AIBN, and azobis(2,4-dimethylvaleronitrile), V-65, were recrystallized from methanol prior to use and then stored at −35 °C under inert atmosphere. Azobis(4-methoxy-2,4-dimethyl valeronitrile), V-70, was stored at −35 °C under inert atmosphere and used as received. Ascorbic acid was purchased from VWR, recrystallized from methanol, and dried under vacuum prior to use. Tin octanoate, purchased from SigmaAldrich, was distilled under vacuum prior to use and then stored under inert atmosphere. General Considerations. All experiments involving moisture- and air-sensitive compounds were performed under a nitrogen atmosphere using an MBraun LABmaster sp glovebox system equipped with a −35 °C freezer and [H2O] and [O2] analyzers or using standard Schlenk techniques. Gel permeation chromatography (GPC) was carried out in THF at a flow rate of 1 mL min−1 either at 50 °C with a Polymer Laboratories PL-GPC 50 Plus integrated GPC system using two 300 × 7.8 mm Jordi gel DVB mixed bed columns and a refractive index detector coupled to a Wyatt Technology mini-DAWN TREOS multiple angle light scattering (MALS) detector or at 35 °C on a Malvern Instruments Viscotek 270 GPC Max triple detection system with 2× mixed bed styrene/DVB columns (300 × 7.5 mm). Absolute molecular weights were obtained using dn/dc values of 0.185 for poly(styrene),90 p oly (c hl oro styrene) , po ly( m ethy lsty rene), 9 1 a n d p oly (methoxystyrene),91 0.088 for poly(methyl methacrylate),92 0.063 for poly(methyl acrylate),93 0.049 for poly(tert-butyl acrylate),94 and 0.052 for poly(vinyl acetate).90 1H and 2D NMR spectra were recorded at 298 K with Bruker Avance spectrometers (300 or 400 MHz) in CDCl3. MALDI-TOF mass spectra were obtained on a Bruker Daltonics UltrafleXtreme MALDI-TOF-TOF instrument. Elemental analyses were performed by the Science Centre at London Metropolitan University. Crystal data were collected on an Agilent Technologies SuperNova diffractometer using molybdenum/copper radiation and a crystal temperature of 120 K. The structures were solved by Patterson/ direct/charge flipping methods and refined using full-matrix leastsquares on F-squared using SHELXL.95 General Polymerization Procedure for Screening Reactions. Monomer, catalyst, and initiator in the ratio 100:1:0.6 were placed in an ampule under inert atmosphere. The ampule was stirred in a preheated oil bath at 120 °C for the required length of time, then removed from the heat, and cooled quickly under running water. Workup procedures were dependent on the monomer: poly(styrene)s, poly(methyl methacrylate), poly(methyl acrylate), and poly(tert-butyl acrylate) samples were dissolved in 5 mL of THF and precipitated into 150 mL of acidified methanol (1% HCl). Monomer conversion for these reactions was determined by 1H NMR spectroscopic analysis of crude samples, by comparing the integration of the polymer versus monomer resonances. For poly(vinyl acetate), excess monomer was removed under reduced pressure, and the samples were dried to constant mass and then weighed to determine monomer conversion gravimetrically.

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CONCLUSION In summary, chloro-substituted amine−bis(phenolate) iron(III) complexes are highly efficient mediators for the controlled radical polymerization of styrenes and methyl methacrylate, affording excellent control over molecular weights and dispersities of both homo- and copolymerizations. Kinetic studies illustrate the controlled nature of these polymerizations and polymer endgroup analysis supports a dual-control mechanism, with cooperation between ATRP and OMRP. Further mechanistic studies reveal that the excellent control is primarily an electronic phenomenon, and manipulation of the radical concentrations allows control over the polymerizations to be further improved. Investigation of alternative initiation routes shows that formation of the Fe(II) species in situ allows efficient ATRP and OMRP initiation to occur. These complexes offer both the best control in 1255

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Macromolecules Representative Polymerization Procedure. Cl,Cl,NMe2[O2NN′]FeCl, 6a (0.025 g, 0.05 mmol), AIBN (0.005 g, 0.03 mmol), and styrene (0.500 g, 5 mmol) were added to an ampule containing a microstirrer bar under inert atmosphere, which was then sealed and heated at 120 °C with stirring for 1 h. 1H NMR spectroscopic analysis of the crude residue indicated 51% monomer conversion, with GPC analysis of the crude material giving an Mn of 5570 and a Đ of 1.15. Precipitation into acidified methanol gave white poly(styrene), with Mn = 6070 and Đ = 1.14. General Polymerization Procedure for Styrene Kinetics. Monomer, catalyst, and initiator in the ratio 100:1:0.6 were placed in a Schlenk flask under inert atmosphere and sealed with a rubber septum (Suba-Seal). The Schlenk flask was placed in an oil bath preheated to 120 °C, at which point timing commenced. Samples were removed from the Schlenk via a degassed syringe at designated intervals and quenched with CDCl3. Analysis of the crude samples by 1H NMR spectroscopy gave the monomer conversion, while GPC analysis gave the molecular weights and Đs of the samples. General Procedure for Styrene Copolymerization. Monomer, catalyst, and initiator in the ratio 100:1:0.6 were placed in an ampule under inert atmosphere. The ampule was stirred in a preheated oil bath at 120 °C for the required length of time, then removed from the heat, and cooled quickly under running water. A sample was removed via a degassed syringe, and any residual monomer was removed in vacuo. The second monomer was added via cannula, and then the ampule was returned to the oil bath at 120 °C and stirred for the required length of time before cooling and removing a second sample. Analysis of these crude samples by 1H NMR spectroscopy gave the monomer conversion, while GPC analysis gave the molecular weights and Đs of the samples. Synthesis of Cl,Cl,Pr[O2NN′]FeCl, 6c. FeCl3 (0.12 g, 0.7 mmol) was dissolved in 10 mL of THF and stirred at −78 °C as a solution of Cl,Cl,Pr [O2NN′]Li2 (0.30 g, 0.7 mmol) in 15 mL of THF was added dropwise. After addition was complete, the red-brown solution was allowed to warm to ambient temperature and stirred for 2 h. The solvent was removed under reduced pressure, and the complex extracted into 30 mL of CH2Cl2, filtered, and concentrated to ca. 10 mL. The complex was precipitated through the addition of hexane (ca. 50 mL), collected by filtration, and dried under vacuum, 0.11 g, 31%. No assignable resonances were observed in the 1H NMR spectrum. Anal. Calcd for C17H15Cl5FeNO2: C, 40.97; H, 3.03; N, 2.81. Found: C, 41.15; H, 3.16; N, 2.92. μeff = 6.0 μB. MS (FAB+): m/z 518.9 [M + Na]+, 461.9 [M − Cl]+. Synthesis of (Cl,Cl,NMe2[O2NN′]FeLiCl)2. FeCl2(THF)1.5 (0.16 g, 0.7 mmol) was suspended in 10 mL of toluene and stirred at −78 °C as a solution of Cl,Cl,NMe2[O2NN′]Li2 (0.30 g, 0.7 mmol) in 40 mL of toluene was added dropwise. After addition was complete, the solution was allowed to warm to ambient temperature and stirred overnight, during which time a white precipitate formed. The solution was filtered, and the precipitate was washed with toluene (2 × 15 mL) and then extracted into ether (ca. 50 mL) before being filtered through Celite. The solvent was removed under reduced pressure to yield a white solid, 0.22 g, 62%. Crystals suitable for X-ray analysis were obtained through recrystallization from hot toluene and from hot THF. Anal. Calcd for C36H36Cl10Fe2Li2N4O4: C, 40.46; H, 3.40; N, 5.24. Found: C, 40.56; H, 3.52; N, 5.16. MS (ESI+): m/z 1068.9 [M]+.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by an NSERC Discovery Grant and the Universities of Prince Edward Island and Edinburgh through a Research Fellowship and Chancellor’s Fellowship, respectively. We thank the University of Edinburgh for funding the diffractometer purchase.

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ASSOCIATED CONTENT

S Supporting Information *

Additional kinetic plots, polymerization data, experimental data, and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +44 (0) 131 650 4726 (M.P.S.). Notes

The authors declare no competing financial interest. 1256

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