Article pubs.acs.org/Macromolecules
Reversible-Deactivation Radical Polymerization in the Presence of Metallic Copper. Kinetic Simulation Mingjiang Zhong, Yu Wang, Pawel Krys, Dominik Konkolewicz, and Krzysztof Matyjaszewski* Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ABSTRACT: Reversible-deactivation radical polymerization (RDRP) of methyl acrylate in DMSO in the presence of Cu0 was studied by kinetic simulations. Kinetic simulations give access to the rates and contributions of all reactions, including those of activation of alkyl halides by CuI and Cu0 species, disproportionation of CuI species, and comproportionation between CuII and Cu0. Every relevant reaction was quantified by experimentally measured rate coefficients. The rates and contributions allow the exact roles of Cu0 and CuI species to be evaluated. These simulations show that the control over the polymerization is due to the atom transfer radical polymerization (ATRP) dynamic equilibrium with CuI as the major activator and CuII as the major deactivator. The ATRP equilibrium is maintained throughout the entire process. The simulations confirmed earlier experimental findings that in dimethyl sulfoxide (DMSO) with tris[2-(dimethylamino)ethyl]amine (Me6TREN) ligand comproportionation between Cu0 and CuII species dominates disproportionation of CuI species, with both reactions being relatively slow. The contribution of Cu0 activation of alkyl halides to the overall reaction is very small, and plays only a supplemental role, since alkyl halides are predominantly activated by CuI species. The effect of Cu0 activity on polymerization rate and livingness were also studied by a series of simulations. In all cases, the rate of supplemental activation by Cu0 was similar to the rate of radical termination, with both being relatively low in order to preserve the livingness of the chains. Cu0 not only acts as a supplemental activator (SA), but also as a reducing agent (RA) and it is able to regenerate CuI from CuII, through comproportionation. Simulations based on experimentally measured rate coefficients showed that Cu0 acts as a supplemental activator and reducing agent (SARA) and the results of an RDRP in the presence of Cu0 are consistent with the SARA ATRP mechanism, and in direct conflict with the single electron transfer-living radical polymerization (SET-LRP) mechanism. The kinetic analysis also revealed that the contribution of disproportionation of CuI to the polymerization kinetics is negligible, and that the CuI species are predominantly involved in activation reactions. The effect of the surface area of Cu0, the effect of initially added CuII species, and other reaction parameters are discussed in light of SARA ATRP.
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photochemically mediated ATRP,7 have been developed to diminish the amount of catalyst used in ATRP. All of these systems regenerate the CuI activator through the reduction of CuII species by certain reducing sources such as radicals generated by conventional initiators in ICAR ATRP, reducing agents in ARGET ATRP, electrical current in eATRP, and photoinduced reduction in photochemically mediated ATRP. By contrast, the role of zerovalent metals, e.g., Cu0, is not exclusively limited to a reducing agent since it can also directly activate alkyl halides.5b−d Thus, RDRP in the presence of Cu0 has the zerovalent copper acting both as a supplemental activator and a reducing agent (SARA). However, it was recently proposed that RDRP of methyl acrylate (MA) in the presence of Cu0/Me6TREN in DMSO followed an entirely different mechanism, namely single electron transfer-living
INTRODUCTION The development of reversible-deactivation radical polymerization (RDRP) techniques has advanced the design of polymers with well-defined composition, site specific functionalities, and architecture.1 Atom transfer radical polymerization (ATRP) is one of the most robust RDRP methods, and uses transition metal catalysts to mediate the polymerization.2 In an ATRP, the metal complex, most commonly a Cu catalyst, in the lower oxidation state, e.g., CuIX/L (X and L stand for halogen and ligand, respectively) activates an alkyl halide (Pn−X) to generate a propagating radical (Pn•) and a complex with the metal in the higher oxidation state, (CuIIX2/L), which can deactivate the propagating radical Pn•. The original ATRP initiation system required over 1000 ppm of copper catalysts to achieve high conversion. Various new ATRP initiation systems, including activators regenerated by electron transfer (ARGET) ATRP,3 initiators for continuous activator regeneration (ICAR) ATRP,4 supplemental activator and reducing agent (SARA) ATRP,5 electrochemically mediated ATRP (eATRP),6 and © 2013 American Chemical Society
Received: January 22, 2013 Revised: March 31, 2013 Published: May 6, 2013 3816
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radical polymerization (SET-LRP),8 where the alkyl halide is exclusively activated by Cu0 via an outer sphere electron transfer (OSET) process, not by CuI, because in the proposed SET-LRP mechanism, the CuI species instantaneously disproportionated to CuII and Cu0. Scheme 1 presents all possible reactions involved in a RDRP conducted in the presence of Cu0. A more detailed scheme
reaction with Cu0 (via radical anion intermediates) followed by instantaneous and complete disproportionation of CuI species to reform the Cu0 activator and a Cu II deactivator. 8 Comproportionation and deactivation by CuI were not considered in SET-LRP. Thus, both mechanisms rely on CuII as the major deactivator but differ dramatically on the mode of activation, with CuI as a major activator in SARA ATRP and Cu0 as the exclusive activator in SET-LRP. The mechanisms also dramatically differ in terms of the relative contributions of disproportionation and comproportionation. SARA ATRP relies on slow comproportionation, with negligible disproportionation, whereas SET-LRP relies on instantaneous and complete disproportionation. The bottom part of Scheme 1 shows the reactions involved in polymerization, propagation (thick magenta arrow) and termination (orange arrows). Termination happens in all radical polymerizations but it was recently postulated that it does not occur in SET-LRP with claims of 100% preserved chain end functionality at 100% monomer conversion.9 This claim will be discussed in detail in the following paper of this series, however, it is in conflict with the accumulation of ∼2% of CuIIBr2/L reported in the same paper, with respect to the alkyl halide concentration.9 Because of the principle of halogen conservation, any CuIIBr2/L formed must be due to a commensurate decrease in the concentration of Br-capped chains.10 In fact, highly reactive CuI and Cu0 species react not only with alkyl halides but also with alkyl radicals, causing some additional termination reactions.11 Another chain breaking reaction in the radical polymerization of acrylates is chain transfer to polymer that predominantly occurs by backbiting.12 This reaction was neglected in the scheme due to its relatively low contribution to the overall process at ambient temperature.13 A proper understanding of the polymerization mechanism is necessary to optimize the reaction conditions for synthesis of specific targeted polymers. The two models for RDRP in the presence of Cu0, SARA ATRP and SET-LRP, differ in their assumptions of the dominant activator and the contributions of comproportionation and disproportionation. Therefore, two questions need to be answered in order to discriminate between these two models: (1) whether Cu0 or CuI is the predominant activator; and (2) whether under typical polymerization conditions, disproportionation or comproportionation is thermodynamically favored and if these processes are kinetically important. In other words, which Cu species is responsible for the activation of alkyl halides, and what is the position of the comproportionation/disproportionation equilibrium, as well as how fast are these processes? In addition, to discriminate between these two models, it is also important to precisely define and quantify the contribution of Cu0 to the RDRP process, and to determine whether CuI participates predominantly in the activation of alkyl halides or in a disproportionation reaction. It is also imperative to understand the mechanism of alkyl halide activation by both Cu0 and CuI, and determine whether it follows an inner-sphere electron transfer (ISET) or OSET mechanism. An earlier report showed that the activation of alkyl halides by CuI via ISET had an activation energy ca. 14 kcal/mol lower than one occurring via OSET. This implies that the rate of alkyl halide activation by CuI in an ISET process should be ca. 1010 times faster than that in the proposed OSET process.14 A recent study has also shown that the reduction of alkyl halides in a hypothetical OSET process must occur via concerted dissociation of the
Scheme 1. Schematic Illustration of All Possible Reactions in RDRP in the Presence of Cu0 a
a
Here bold lines represent dominant reactions, thin lines represent contributing reactions, and dashed lines represent minor or negligible reactions for the RDRP of MA in DMSO with Me6TREN ligand and Cu0.
Scheme 2. Kinetic Model for RDRP in the Presence of Cu0 Used in PREDICI Simulations
distinguishing the small molecule (R• and R−X) and polymeric chains (Pn• and Pn−X) is shown in Scheme 2. The reactions shown in Scheme 1 are involved in both SARA ATRP and SETLRP but their contributions to the overall mechanism are dramatically different. In SARA ATRP the fastest and dominating reactions are those maintaining the ATRP equilibrium, i.e., activation by CuI and deactivation by CuII (thick blue and red arrows). Activation by Cu0 and comproportionation (thin blue and green solid arrows) only play a supplementary role. Disproportionation and deactivation by CuI (purple and red dashed arrows) are very minor contributing reactions that can be neglected. In SET-LRP, the activation was postulated to occur exclusively via an OSET 3817
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respectively, based on the entire population of CuI and CuII species without taking into account association with ligand and halide. Values app for the rate coefficients, kapp comp and kdisp, are taken from the first paper of this series. The second paper of this series provided the rate coefficients for activation of methyl 2-bromopropionate (MBrP), and Br-terminated poly(methyl acrylate) by Cu0 (kapp a0 ), in the presence of Me6TREN. For convenience, an overall rate coefficient (kover) was calculated for heterogeneous reaction from kapp by taking into account the effect of Cu0 surface area and reaction volume (eq 1).
alkyl halide rather than through a radical anion intermediate, as postulated in SET-LRP.5d,15 In order to create a full picture of RDRP in the presence of Cu0, a detailed kinetic study was undertaken. The first and second paper of this series systematically studied the comproportionation and disproportionation reactions, and the activation reactions of alkyl halides by Cu 0 and CuI, respectively. These studies were performed in both pure dimethyl sulfoxide (DMSO) as well as in the presence of methyl acrylate (MA) monomer, i.e., under the conditions relevant to polymerization.16 These studies were designed to reduce the complex process of RDRP in the presence of Cu0 to a series of simpler model reactions. This article combines information gained from the model studies and focuses on a holistic and detailed understanding of the reaction mechanism, using kinetic simulations. All simulations employed experimentally determined rate coefficients, under conditions relevant for polymerization, with very good agreement between the experimental data and the simulated data. Thus, a complex system was accurately described by combining a series of simpler model reactions. The simulations and subsequent calculations quantitatively illustrate the contribution of each reaction shown in Scheme 1, and confirm the validity of the SARA ATRP mechanism.
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kover = kapp
where S and V are the surface area of Cu and total reaction volume, repectively. In all cases, the Cu0 surface area was taken to be only the area of the Cu0 wire, since there is no direct experimental evidence of “nascent” Cu0 being formed under polymerization conditions.16a This is because the extent of disproprotionation under experimental conditions is below the detection limit of UV−vis,16a and the lifting experiments of Percec et al.18 show that only a very small fraction of the total Cu0 surface area, ca. 1%, is from Cu0 particles not attached to the Cu0 wire. The rate coefficient of MBrP activation by CuI (kapp a1 ) was measured by the stopped-flow method as reported in the second paper of this series.16b The rate coefficient of deactivation by CuII (kapp d1 ) was 19 through then calculated from kapp a1 and the reported values of KATRP app kapp d1 = ka1 /KATRP. It is necessary to emphasize that all these measurements were performed in a representative polymerization medium of MA and DMSO. According to the principle of microscopic reversibility,20 five of the six rate coefficients for the processes shown in Scheme 1 (top) can be independently measured, while the sixth one, in this case the rate coefficient of deactivation by CuI (kapp d0 ) can be estimated using eq 2. By dimension analysis, kd0 has the same dimension as kapp disp.
MODEL AND COMPUTATIONAL METHODS
app kd0 =
ka0 kd0 kcomp kdisp ka1 kd1 kp kadd ktc ktRd kt0
1.0 × 10−4 cm s−1 1.2 × 10−1 cm s−1 3.5 × 10−3 cm s−1 3.1 × 10−6 cm s−1 2.0 × 102 M−1 s−1 2.7 × 108 M−1 s−1 15600 M−1 s−1 5.8 × 105 M−1 s−1 1.0 × 108 M−1 s−1 1.0 × 108 M−1 s−1 2.0 × 109 M−1 s−1
koverb 2.8 3.4 9.9 8.7 − − − − − − −
× × × ×
10−5 10−2 10−4 10−7
reference s−1 s−1 s−1 s−1
app app app ka0 kd1 kdisp app app ka1 kcomp
(2)
Table 1 lists all the rate coefficients used in this study, including those for propagation (kp), radical termination (kt), addition of R• to monomer (kadd), and coupling of primary radicals, R• (kt0), taken from the literature.21 It is worth noting that the solvent effect (viscosity)21c was taken into account for kt0 and the viscosity of polymerization medium, i.e. MA/DMSO = 2/1 (v/v), was estimated using the Refutas equation.21d To simplify simulations, backbiting processes and chain length dependent termination12a,22 were neglected in this work but will be considered in forthcoming publications. Backbiting should have smaller contributions at lower (ambient) temperatures. Also, due to the relatively low molecular weight range studied here and the presence of solvent, the influence of chain length dependence is expected to be relatively small. Therefore, an average value of kt = 108 M−1 s−1 was used, as in an earlier work that modeled the influence of various parameters on loss of chain end functionality.23 Since termination between two radicals may proceed through either radical disproportionation or combination processes, it is worth clarifying that the dead chain fraction (Tmol %) is defined as the fraction of lost living chains out of all the initial living/dormant chains, i.e. Tmol % = ([R−X]0 − [R−X]t − [Pn−X]t)/[R−X]0. In addition to conventional biradical termination, other reactions that lead to radical loss, such as CuI and Cu0 induced terminations (kt,Cu(I), kt,Cu(0) or an apparent rate coefficient of termination kapp t ) were introduced and will be discussed later.
Table 1. Rate Coefficients for Modeling RDRP in the Presence of Cu0 at 25 °Ca k or kapp
(1) 0
PREDICI (version 6.3.2) was used for all kinetic modeling. An adaptive Rothe method was employed in PREDICI as a numerical strategy for time discretization.17 The number average degree of polymerization (DPn) and molecular weight distribution (MWD), Mw/Mn, were calculated from all the polymer populations, including the dormant species (Pn−X), propagating species (Pn•) and dead chains (T). Scheme 2 shows the kinetic model used in the PREDICI simulations and all rate coefficients are listed in Table 1. Herein, R−X
rate coefficients
S V
ref 16b this work ref 16a ref 16a ref 16b refs 16b, 19 ref 21a ref 21b ref 24 ref 24 ref 21c, d
a
L = Me6TREN, X = Br, R−X = MBrP, reaction medium is MA/ DMSO = 2/1 (v/v). bkover = kapp × S/V, kover values in this table were calculated for the system with S = 1.27 cm2 and V = 4.5 mL. ckt = ktc + ktd, where ktc and ktd stand for the combination and disproportionation termination rate coefficients respectively, with the assumption of exclusive combination of macroradicals. dIt was also assumed that ktR ≈ k t.
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RESULTS AND DISCUSSION Kinetic Simulation of RDRP of MA in the Presence of Cu0. This paper gives detailed kinetic simulations for the RDRP in the presence of Cu0 of methyl acrylate in DMSO, using experimentally derived rate coefficients. Previously, RDRP in the presence of Cu0 has been simulated by Monteiro et al. and Haehnel et al.,25 but the rate coefficients used in the simulations were not consistent with experimental data. For example, a
and R• are used to represent the small molecule initiators and radicals, and distinguishes these small molecules from their polymeric analogues. In this model, X and L stand for Br and the tris[2(dimethylamino)ethyl]amine (Me6TREN) ligand, respectively. As discussed in the first two papers of this series, for the reactions involving CuI and CuII, the notation kapp is used to describe the app app app app apparent kinetic rate coefficients, i.e. kapp a1 , kd1 , kcomp, kdisp, and ka0 , 3818
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disproportionation rate coefficient of kdisp = 107 M−1 s−1 was used and the possibility of comproportionation was completely ignored.25 However, as demonstrated in the first paper of this series, the disproportionation of CuIBr/Me6TREN in DMSO, and especially in MA/DMSO, is slow and far from complete.16a That paper showed that comproportionation dominates over disproportionation in both DMSO and MA/DMSO, especially in the presence of a large excess of ligand. This makes the studies of Monteiro et al. and Haehnel et al.25 purely theoretical, since the experimental data show that comproportionation, not disproportionation, dominates in the polymerization of MA in DMSO. Furthermore, the rapid rate of alkyl halide activation by CuI species indicates that activation by CuI is more likely than disproportionation of CuI In all cases, experimentally derived rate coefficients were used in the simulations, and the excellent consistency of the data reported in the first two papers of this series highlights the reliability of these data. The experimentally measured apparent rate coefficients of comproportionation and disproportionation −3 −6 are kapp cm s−1 and kapp cm s−1, comp = 3.5 × 10 disp = 3.1 × 10 respectively, at 25 °C in MA/DMSO = 2/1 (v/v) (Table 1), where the dimensions of cm s−1 account for the fact that these two reactions are heterogeneous and depend on the surface area of the Cu0 and volume of the reaction mixture. In a specific system, for instance, with the Cu0 wire surface area of S = 1.27 cm2 (length (l) = 4 cm, diameter (d) = 1 mm) and reaction volume of V = 4.5 mL, the overall rate coefficients can be −4 −1 calculated using eq 1, giving values of kover s and comp = 9.9 × 10 −7 −1 over s for comproportionation and k disp = 8.7 × 10 disproportionation, respectively. In the previously published simulations, activation of alkyl halides by CuI was either completely neglected or strongly (>1000-fold) underestimated. −1 −1 s was used for In these papers, the value of kapp a1 = 0.09 M I the activation of MBrP by Cu Br/Me6TREN.25 However, the activation rate coefficients of MBrP by CuIBr/Me6TREN in 3 −1 −1 s MeCN was reported to be as fast as kapp a1 = 1.1 × 10 M 26 app from direct stopped-flow measurements and ka1 = 28 M−1 s−1 from scaling methodology.27 The rate coefficient of activation of ethyl 2-bromoisobutyrate (EBiB) in pure DMSO by a cationic complex of CuI/Me6TREN was reported to be ka1 = 8.7 × 104 M−1 s−1, as estimated by cyclic voltammetry coupled with kinetic simulations.28 The second paper of this series quantified the activation of MBrP by CuIBr/Me6TREN by stopped-flow methods, generating an activation rate 2 −1 −1 s in MA/DMSO = 2/1 coefficient of kapp a1 = 2.0 × 10 M 2 −1 −1 app (v/v) and ka1 = 3.2 × 10 M s in pure DMSO at 25 °C.16b In addition, the earlier simulations25 assumed exclusive activation by Cu0 using a Cu0 activation rate coefficient of ka0 = 4.33 M−1 s−1, with a Cu0 concentration of 3.9 mM whereas the results reported in the second paper of this series considered both contributions and showed that activation by Cu0 was much slower than activation by CuI.16b The measured value for −4 MBrP activation by Cu0, kapp cm s−1 in MA/ a0 = 1.0 × 10 DMSO, is consistent with recently published results by Nicolas −4 cm s−1 for the activation of EBiB et al,29 i.e., kapp a0 = 2.5 × 10 by Cu0 with N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) in DMSO. To clarify this difference if one uses the Cu0 activation rate coefficient of ka0 = 4.33 M−1 s−1, with a Cu0 concentration of 3.9 mM, or 2.5 mg of Cu0 in 10 mL of solvent, this would give an overall Cu0 activation rate coefficient of 1.7 × 10−2 s−1. This overall activation rate coefficient of 1.7 × 10−2 s−1 is equivalent to a ratio of the surface area of Cu0 to −4 cm s−1. volume of solvent of 170 cm2/mL, for kapp a0 = 1.0 × 10
In 10 mL of solvent, this ratio requires a surface area (S) of 1700 cm2. Achieving such a high surface area for Cu0 would need a Cu0 wire with a length (l) of 200 m, and a diameter (d) of 0.25 mm, calculated as l = S/(πd) in 10 mL of solvent. Alternatively, if the mass of Cu0 used of ∼2 mg, in 10 mL of solvent, were stretched into a cylinder with S = 1700 cm2, this cylinder would have d = 6 nm, and l = 8 × 106 m.30 Our simulations of RDRP in the presence of Cu0 were first carried out under the following conditions: 25 °C, [MA]0: [MBrP]0:[Me6TREN]0 = 200:1:0.1, MA/DMSO = 2/1 (v/v), V = 4.5 mL, with a Cu0 surface area of S = 1.27 cm2, which is equivalent to a Cu0 wire with l = 4 cm, d = 1 mm (Figure 1).
Figure 1. Simulated kinetic plots for RDRP of MA in the presence of Cu0 in DMSO. (a) Semilogarithmic kinetic plot; (b) DPn vs monomer conversion plot; (c) Mw/Mn vs monomer conversion plot; (d) Tmol % vs monomer conversion plot. Conditions: 25 °C, [MA]0:[MBrP]0: [Me6TREN]0 = 200:1:0.1, MA/DMSO = 2/1 (v/v), V = 4.5 mL, S = 1.27 cm2 (l = 4 cm, d = 1 mm).
The resulting linear semilogarithmic kinetic plot, shown in Figure 1a, indicated that the radical concentration was nearly constant for the whole polymerization. At the early stages of the polymerization (conversion < ∼20%), a large deviation of DPn from theoretical DPn and relatively broad MWD was observed. (Figure 1b,c) This is attributed to the formation of long polymer chains caused by slow generation of the CuIIX2/L deactivator species, as reported by other groups.25b Sufficient amount of CuI and CuII species were generated above 20% monomer conversion and the polymerization became well controlled with good agreement between the theoretical and measured DPn and a significant decrease of Mw/Mn. Control was attained through the ATRP equilibrium, as will be demonstrated later. Figure 1d shows the evolution of Tmol %, based on a constant value of termination rate coefficient of 108 M−1 s−1. In these and all subsequent simulations the surface area of Cu0 was taken to be the total surface area of the Cu0 wire only. In this way, the “nascent” Cu0 proposed in other work was not considered,8a since there is no experimental verification of 3819
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Figure 2. Comparison of experimental kinetic data with simulated results, using various values of kt and ka0. (a) Semilogarithmic kinetic plot; (b) DPn vs monomer conversion plot; (c) Mw/Mn vs monomer conversion plot. Conditions: 25 °C, [MA]0:[MBrP]0:[Me6TREN]0 = 200:1:0.1, MA/ DMSO = 2/1 (v/v), V = 4.5 mL, S = 1.27 cm2 (l = 4 cm, d = 1 mm).
“nascent” Cu0 being formed under polymerization conditions. In particular, the first paper of this series showed that in the reaction medium of MA/DMSO = 2/1 (v/v), the extent of disproportionation was less than 3%,16a with the excess of ligand to soluble Cu typically used in experiments such as those under the conditions in Figure 1.16b Furthermore, the experiments of Percec et al.18 showed that when lifting the Cu0 wire out of the reaction solution, the polymerization rate dropped to 10% of the original rate. Since the polymerization rate depends on the square root of the Cu0 surface area,31 this implies the amount of Cu0 particles not attached to the Cu0 wire, or “nascent” Cu0, corresponds to 1% of the original wire area. Thus, under typical polymerization conditions, 99% of the Cu0 surface area is represented by the Cu0 wire, or other added Cu0 source, and that “nascent” Cu0 may be safely neglected. Other Factors Affecting Kinetics. Although the simulations in Figure 1 provided overall results similar to those observed experimentally, simulated polymerization rates, shown in Figure 2a (dashed blue line), were larger than experimental values (black squares). The higher simulated rate of polymerization is due to higher radical concentration, [Pn•]. This could be caused by either an overestimation of the rate coefficient of activation of Pn−X by Cu0 or an underestimated rate coefficient of termination.16b The model studies of activation of R−X by Cu0 evaluated in the second paper of this series did not take into account conversion and chain length dependent effects. For instance, the concentration of CuII measured in the model study of Cu0 activation increased linearly with time.16b This differs from the progressively reduced rate of evolution of CuII reported in Figure 7 in ref 32. The differences between the model study and polymerization results can be due to chain length or viscosity dependent rate coefficients. The green dotdashed line in Figure 2 represents kinetic plot simulated using a value of ka0 for Pn−X 10 times smaller than for MBrP. This highlights the effect of values of ka0 on kinetics. The polymerization slowed down and fitted the experimental results relatively well. Another reason for differences between simulated and experimental results could be an underestimation of the rate coefficient of termination. We have previously reported CuI/L induced catalytic radical termination.11a A similar reaction could occur in the presence of Cu0/L. In RDRP in the presence of Cu0, Tmol % can be experimentally quantified by measuring the increase of [CuII], according to the principle of halogen
conservation.10,11 This value can be compared with values predicted for conventional radical termination based on polymerization rate (i.e., [Pn•] and known values of kp and kt). Thus, in a polymerization of MA with the ratio of reagents [MA]0:[MBrP]0:[Me6TREN]0 = 200:1:0.1, in MA/DMSO = 1/1 (v/v), with Cu0 wire (S = 7.95 × 10−2 cm2, l = 1 cm, d = 0.25 mm) at 25 °C, after 240 min the conversion of monomer was 55% and [CuIIBr2/Me6TREN] = 0.26 mM.16a This corresponds to a value for Tmol % = 2%. This value exceeds by a factor of 10 the value Tmol % = 0.2% predicted for termination between two radicals with kapp = 108 M−1 s−1.23 t This suggests accelerated termination in the presence of Cu0/L. Interestingly, previous electrospray ionization mass spectrometry analysis of the end groups of a well-defined PMA prepared in the presence of Cu0 and Me6TREN in DMSO showed 94% of Br-capped chains, 4% of H-terminated chains and 2% of unsaturated chains and essentially no chains formed by coupling between two radicals.33 This is quite an unexpected result, since in a standard radical polymerization of acrylates termination predominately occurs by coupling not by disproportionation. Moreover, at [MA]0:[EBiB]0 = 26:1, 25 °C at 99% conversion in 2 h, the predicted value for conventional termination should be only Tmol % = 1.1%, much smaller than the measured value of Tmol % = 6%.33 This suggests some other processes are responsible for accelerated termination that generate saturated and unsaturated end groups, resembling termination by disproportionation, although at a 2:1 ratio rather than the 1:1 ratio expected for pure termination by disproportionation. The accelerated loss of end group functionality could be due to a reduction of radicals to anions by strongly reducing Cu0/L species, followed by quenching with protic impurities5d or due to a process, resembling catalytic radical termination, previously reported for some reactive ATRP CuI catalysts In order to explore and quantify the termination process, a conventional radical polymerization of MA initiated by AIBN was studied without any additive or with Cu0 alone, with Me6TREN alone, or with Cu0/Me6TREN. As expected, in all cases the Mn decreased with conversion, which is typical for uncontrolled free radical polymerizations,34 due to monomer depletion occurring faster than initiator consumption.35 When the Cu0 or Me6TREN was used alone, there was minimal impact on the polymerization kinetics and molecular weights. However, in the presence of both Cu0 and ligand, the rate of 3820
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Figure 3. Influence of Cu0/Me6TREN on conventional radical polymerization. (a) Plot of ln([M]0/[M]) vs time. (b) Plot of Mn (filled symbols) and Mw/Mn (open symbols) vs conversion. Conditions: [MA]0:[AIBN]0 = 800:1 in MA/DMSO = 1/9 (v/v) at 60 °C, without any additive or in the presence of Cu0 wire (S = 1.27 cm2), or with 3.3 mM Me6TREN, or with 4 cm Cu0 wire (S = 1.27 cm2) and 3.3 mM Me6TREN.
polymerization and Mn were significantly decreased (Figure 3). This suggests that Cu0/L facilitates radical loss, and under these conditions, Cu0, and CuI induced radical loss dominates over conventional biradical termination. It is important to describe these data in light of the recent experiments of Percec et al.9 which showed that the chain end functionality, and consequently the rate of termination, is lower for a polymerization in MA/DMSO = 2/1 (v/v), than in MA/ MeCN = 2/1 (v/v), which in turn is lower than in MA/toluene = 2/1 (v/v). As highlighted in the first paper of this series, even in MA/DMSO = 2/1 (v/v) the extent of disproportionation is minimal, and in MeCN and toluene the disproportionation is expected to be even lower. Therefore, the relative fractions of retained chain end functionality can be explained by the higher KATRP in DMSO, compared to MeCN and toluene,19,21c rather than disproportionation. The higher the KATRP, the lower the ratio of CuI to CuII needed for a given rate of polymerization. The highest values of KATRP are in DMSO followed by MeCN, and by toluene.19,21c Therefore, the lowest ratio of CuI to CuII is in DMSO, followed by MeCN, and by toluene. Therefore, the rate of CuI induced radical loss,11 which decreases chain end functionality, should be highest in toluene, followed by MeCN followed by DMSO. Similar results for Cu0 induced radical loss are expected. The detailed investigation of additional termination in RDRP in the presence of Cu0 will be reported separately. In this paper app only the apparent rate coefficient of termination (kapp = t : Rt app • 2 2kt [R ] ) was used. This apparent rate coefficient includes contributions of all termination processes (conventional biradical, CuI, and Cu0 induced), and it may also account for some effects due to backbiting. In the second paper of this series the experimental procedures and calculations were provided to quantify the value of kapp t , giving an average apparent rate coefficient of kapp = 1.4 × 109 M−1 s−1 in MA/ t DMSO = 1/1 (v/v) with Me6TREN as the ligand. The into the simulation model provided good incorporation of kapp t agreement between the simulated kinetics and experimental −4 results (Figure 2, solid red line), when using kapp a0 = 1.0 × 10 −5 cm s−1. It is important to note that using kapp = 1.0 × 10 cm a0 s−1, with kapp = 1.4 × 109 M−1 s−1 would result in a simulated t polymerization rate far below the experimental one. In the case −4 −1 −1 where kapp cm s−1 and kapp s were a0 = 1 × 10 t = 1.4 × 109 M
used, there was good agreement between the kinetics of polymerization, evolution of Mn and Mw/Mn. Nevertheless, reaction conditions (conversion, viscosity and chain length) can affect the rate coefficients of both termination and activation with Cu0, and this could explain the slight downward curvature of the experimental ln([M]0/[M]) plot with time. The subsequent sections will present the effect of initial addition of CuII species and surface area (activity) of Cu0, followed by the analysis of contributions of rates of relevant reactions and concentration of various species and discussion of the role of major reagents. This discussion will be based on simulation results using values of rate coefficients from Table 1. Effect of Initially Added CuII Deactivator to the Reaction. In some polymerizations in the presence of Cu0, the CuII deactivator is initially added to improve the level of control over the reaction. Simulations with different initial concentrations of CuIIX2/L were conducted to understand the influence of [CuII X 2 /L] 0 on polymerization. In these simulations, other conditions and rate coefficients were the same as those presented previously. Figure 4 summarizes the results of simulations with 0, 5, and 50 ppm of CuIIX2/L, as a molar ratio to monomer. The rate of polymerization was not affected by the initial concentration of the deactivator and was almost the same under these three conditions, (Figure 4a) although a few minutes of induction period was observed with 50 ppm of initially added CuIIX2/L. This corresponds to the time needed to reduce excess CuII to CuI and establish a steadystate radical concentration. The addition of the deactivator at the start of the reaction significantly improved the agreement between measured and theoretical DPn and reduced the Mw/ Mn (Figure 4b,c), especially at low monomer conversion. In particular, with an initial CuIIX2/L concentration of 50 ppm, the DPn fitted very well with the theoretical DPn for essentially the whole polymerization. Furthermore, the Mw/Mn value at 20% conversion changed from 3.3 for 0 ppm, to 1.6 for 5 ppm, and 1.1 for 50 ppm of initially added CuIIX2/L. Since all systems have a similar rate of polymerization, the rate of termination, and loss of chain end functionality should be also similar. Figure 4d showed that the Tmol % was not affected by [CuIIX2/L]0. The similar rate of polymerization, with higher initial concentrations of CuII added at the start of the reaction is consistent with the experiments in ref.36 3821
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Figure 5. Simulated kinetic plots for RDRP in the presence of Cu0 of MA in DMSO with 0.127, 1.27 amd 12.7 cm2 surface areas of Cu0 wire. (a) Semilogarithmic kinetic plots; (b) DPn vs monomer conversion plot; (c) Mw/Mn vs monomer conversion plot; (d) Tmol % vs monomer conversion plot. Conditions: 25 °C, [MA]0: [MBrP]0:[Me6TREN]0 = 200:1:0.1, MA/DMSO = 2/1 (v/v), V = 4.5 mL.
Figure 4. Simulated kinetic plots for RDRP of MA in the presence of Cu0 in DMSO with 0, 5, and 50 ppm [CuIIX2/L]0. (a) Semilogarithmic kinetic plot; (b) DPn vs monomer conversion plot; (c) Mw/Mn vs monomer conversion plot; (d) Tmol % vs monomer conversion. Conditions: 25 °C, [MA]0:[MBrP]0:[Me6TREN]0 = 200:1:0.1, MA/ DMSO = 2/1 (v/v), V = 4.5 mL.
The Effect of Cu0 Activity or Surface Area. The central question is whether Cu0 or CuI is predominantly responsible for the activation of Pn−X. Earlier work demonstrated that the contribution of Cu0 to activation was much smaller than that of CuI under typical conditions of RDRP in the presence of Cu0.16b,29 However, the rate of Cu0 activation can be enhanced by increasing the surface area (S) of Cu0. Polymerizations with different S were simulated to study the effect of Cu0 activity on RDRP in the presence of Cu0. Table 2 lists both the apparent and overall rate coefficients used for specific S/V values for heterogeneous reactions that involve Cu0 as either a reactant or a product. Figure 5 presents the results of the simulation with different surface areas of Cu0. Polymerizations using higher Cu0 surface areas resulted in faster polymerization (Figure 5a). The polymerization rate expressed by the apparent values kpapp increased from 6.0 × 10−4 s−1, to 1.8 × 10−3 s−1, and to 5.2 × 10−3 s−1, with surface area, 0.127 cm2, 1.27 cm2, and 12.7 cm2 respectively. There was an improvement in the initial agreement between the measured and theoretical DPn, and the initial Mw/Mn values for the higher surface areas of Cu0 (Figure 5b,c). However, Figure 5d shows that larger S resulted in a faster polymerization and, therefore, was accompanied by a
larger loss of end-group functionality.23 At 80% conversion, the Tmol % increased from 1% for the Cu0 surface of S = 0.127 cm2 to 10% for the 100 times larger Cu0 surface area. Therefore, the claim that 100% livingness can be achieved in a system with superfast Cu0 activators is incorrect. The slope of the semilogarithmic plot in Figure 5a increased by a factor of ∼3 between a surface area of 0.127 cm2 and 1.27 cm2 and by another factor of 3 between a surface area of 1.27 cm2 and 12.7 cm2. These results are consistent with the square root dependence of the polymerization rate with the surface area of Cu0 reported by Percec et al.,31 our group,5b and predicted by Nicolas et al.29 The kapp p values calculated from simplified kinetic model of Nicolas et al.29 with surface area, 0.127 cm2, 1.27 cm2, and 12.7 cm2 are 5.0 × 10−4 s−1, 1.6 × 10−3 s−1, and 5.0 × 10−3 s−1, respectively, which agree well with the simulated values of 6.0 × 10−4 s−1, 1.8 × 10−3 s−1, and 5.2 × 10−3 s−1. Since the simplified model neglected the contribution of comproportionation, the calculated values were always smaller than simulated values and the discrepancy decreased with the larger surface area, and lower contribution of comproportionation.
Table 2. Apparent and overall rate coefficients of heterogeneous reactions with various surface area of Cu0 a kover b k
rate coefficient ka0 kd0 kcomp kdisp a
1.0 1.2 3.5 3.1
× × × ×
app
10−4 10−1 10−3 10−6
S = 0.127 cm cm cm cm cm
s−1 s−1 s−1 s−1
2.8 3.4 9.9 8.7
× × × ×
10−6 10−3 10−5 10−8
2
s−1 s−1 s−1 s−1
S = 1.27 cm2 2.8 3.4 9.9 8.7
× × × ×
10−5 10−2 10−4 10−7
s−1 s−1 s−1 s−1
S = 12.7 cm2 2.8 3.4 9.9 8.7
× × × ×
10−4 10−1 10−3 10−6
s−1 s−1 s−1 s−1
25 °C in MA/DMSO = 2/1 (v/v). bkover = kapp × S/V, V = 4.5 mL. 3822
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Figure 6. (a) Plot of concentrations of all species vs monomer conversion; (b) plot of reaction rates vs monomer conversion. Conditions: 25 °C, [MA]0:[MBrP]0:[Me6TREN]0 = 200:1:0.1, MA/DMSO = 2/1 (v/v), V = 4.5 mL, S = 1.27 cm2 (l = 4 cm, d = 1 mm).
disproportionation of CuI (Rdisp), comproportionation between Cu0 and CuII (Rcomp), and deactivation of Pn• by CuII (Rd1), respectively.
This square-root dependence may be explained by the fact that Cu0 acts as both a supplemental activator of alkyl halides and reducing agent of CuII species, with both components giving a square root dependence of the polymerization rate with the surface area of Cu0.5b As highlighted in ref 37, the supplemental activator role of Cu0 is kinetically similar to the role of the conventional initiator in ICAR ATRP. Since ICAR ATRP gives a square-root dependence of the polymerization rate with the concentration of conventional initiator,4,38 the supplemental activator component of SARA ATRP should have a square-root dependence with the surface area of Cu0. Similarly, since Cu0 can also act as a reducing agent for CuII, the kinetics should also follow a square-root dependence with the surface area of the Cu0, as observed in ARGET ATRP processes.39 Concentrations of Soluble Species and Calculation of Reaction Rates. The PREDICI simulations were used to determine time-dependent concentrations of all species under the typical polymerization conditions: 25 °C, [MA]0:[MBrP]0: [Me6TREN]0 = 200:1:0.1, MA/DMSO = 2/1 (v/v), V = 4.5 mL, S = 1.27 cm2 (l = 4 cm, d = 1 mm). The results obtained using rate coefficients from Table 1 are shown in Figure 6a. Typical polymerizations are carried out under conditions different from those used in the model studies discussed in paper 2 of this series.16b Polymerization obviously includes the monomer MA, and a lower concentration of ligand to slow down the activation of alkyl halides by Cu0 and thereby decrease the concentration of radicals. The reactions were also run for a shorter time to reduce fraction of terminated chains. During the reaction, the concentration of halide end groups was nearly constant, indicating that a small amount of termination. [Pn•] quickly reached a steady-state and stayed at that constant value, consistent with the first order polymerization kinetics shown in Figure 1(a). Throughout the entire reaction, the [CuI] and [CuII] increased with conversion. The concentration of terminated chains, [T] followed the relation [T] = [CuI] + 2[CuII], since halogens in CuIX and CuIIX2 species (XCu) must originate from the alkyl halides lost to termination reactions, since the alkyl halide is the only original source of halogen atoms in the system.10 The reactions rates were calculated based on rate coefficients from Table 1. The rates of all reactions are plotted in Figure 6b. Equations 3 to 7 were used to calculate the rates of Pn−X activation by Cu0 (Ract0), Pn−X activation by CuI (Ract1),
app S R a0 = ka0 [Pn−X] V
(3)
app R a1 = ka1 [Cu IX/L][Pn−X]
(4)
app R disp = kdisp
S [Cu IX/L]2 V [L]
(5)
app S R comp = kcomp [Cu IIX 2/L] V
(6)
app R d1 = kd1 [Cu IIX 2/L][Pn•]
(7)
Values of Ra1 and Rd1 are essentially equal during the entire polymerization, meaning that the ATRP equilibrium is established very early in the reaction and maintained during the whole process. Interestingly, initially the rate of termination (Rt) was essentially equal to the rate of activation by Cu0 (Ra0). This suggests that hypothetically fast activation by Cu0 should result in strongly enhanced termination and loss of livingness. This is a common feature of all RDRP processes, including RAFT, ARGET and ICAR ATRP and also SARA ATRP. Radicals introduced to the system must terminate and faster generation of radicals must be compensated by faster termination. Thus, faster activation by Cu0 (higher intrinsic reactivity or larger surface area) must be accompanied by concurrently faster termination. At much longer reaction times,16b due to progressive termination, the rate of supplemental activation decreases (lower [Pn−X]) and the rate of comproportionation increases (higher [CuIIX2/L]). Under these conditions, Rt roughly equals to the sum of Ra0 and Rcomp.29,40 Major Activating Species of Alkyl Halides. The first question posed to discriminate between SARA ATRP and SETLRP was whether Cu0 or CuI species were predominantly responsible for the activation of alkyl halides. The reaction contributions can be defined in two ways, by their instantaneous fractions (IF) or by their cumulative fractions (CF). The instantaneous fraction refers to the contribution of a given reaction in a narrow time window, whereas the cumulative fraction refers to the total contribution of a given 3823
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factor of 104 to 108. The time needed to reach equilibrium is much longer than the polymerization time. As reported in the first paper of this series, disproportionation is facilitated by low ligand concentrations since Rdisp is inversely proportional to [L], indicating that the disproportionation process is suppressed at high ligand concentrations. The in situ formed CuI species participate predominantly in the activation of alkyl halides, rather than in disproportionation, although a small extent of disproportionation is possible. In the next section, the role of CuI will be quantified by calculating the contributions of each reaction. Role of CuI. The CuI species could be involved in four reactions: activation of Pn−X, disproportionation, deactivation of propagating radical Pn•, and CuIX/L induced termination. The last two reactions will be neglected in the following discussion since their contributions are very small, and not important for understanding SARA ATRP. The contributions of disproportionation (IF1_disp and CF1_disp) and activation (IF1_act and CF1_act) by CuI were calculated using eqs 12 to 15 and plotted in Figure 8. The contribution of disproportionation
process from the start of the reaction until that point in time. These contributions are given in eqs 8 to 11 where the subscripts “act_0” and “act_1” stand for the activation of Pn−X by Cu0 and CuI, respectively. R a0 IFact_0 = R a0 + R a1 (8)
IFact_1 =
R a1 R a0 + R a1
(9)
t
CFact_0 =
∫0 R a0dt t
t
∫0 R a0dt + ∫0 R a1 dt
(10)
t
CFact_1 =
∫0 R a1dt t
t
∫0 R a0dt + ∫0 R a1dt
(11) I
As shown in Figure 7, despite the initial absence of Cu , the in situ generated CuI is active enough to dominate the
Figure 7. Plot of relative contributions of CuI and Cu0 to activation vs monomer conversion; Inset: Zoom-in of the plot at conversion range of 0 to 1%. Conditions: 25 °C, [MA]0:[MBrP]0:[Me6TREN]0 = 200:1:0.1, MA/DMSO = 2/1 (v/v), V = 4.5 mL, S = 1.27 cm2 (l = 4 cm, d = 1 mm).
Figure 8. Role of CuI: relative contributions of activation and disproportionation of CuI vs monomer conversion. Conditions: 25 °C, [MA]0:[MBrP]0:[Me6TREN]0 = 200:1:0.1, MA/DMSO = 2/1 (v/v), V = 4.5 mL, S = 1.27 cm2 (l = 4 cm, d = 1 mm).
activation throughout the whole polymerization. The inset in Figure 7 is a zoom-in at low monomer conversion that reveals that the switch of major activator from Cu0 to CuI occurs below 0.1% monomer conversion, and therefore the RDRP is governed by ATRP, since the major activator is CuI. However, since the system is under ATRP equilibrium, as seen in Figure 6a, the rate of activation by CuI is balanced by the rate of deactivation by CuII. This implies that the high rate of activation by CuI allows the radical centers to be efficiently exchanged between the different polymeric alkyl halides, through the ATRP activation−deactivation reactions. In contrast, Cu0 activation compensates for radicals lost to termination. Position of Disproportionation/Comproportionation Equilibrium. Instantaneous and complete disproportionation of CuI species is the major postulate in the SET-LRP mechanism.8b,25b The first paper of this series proved that the comproportionation dominates over disproportionation, and both are slow.16a This was already clearly shown in Figure 6b. In fact, during polymerization, Rdisp is slower than Rcomp, by a
is very small and is observed only at full conversion. This indicates that the instantaneous disproportionation does not occur in RDRP in the presence of Cu0/Me6TREN in DMSO under conditions typical for polymerization and the CuI species predominantly act as activators of alkyl halides. The contribution of disproportionation is below
