Detailed Kinetic and Mechanistic Insight into Radical Polymerization

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Detailed Kinetic and Mechanistic Insight into Radical Polymerization by Spectroscopic Techniques Michael Buback,* Hendrik Schroeder, and Hendrik Kattner Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstraße 6, 37077 Göttingen, Germany ABSTRACT: The in-depth understanding of the kinetics and mechanism of conventional and reversible deactivation radical polymerization (RDRP) strategies requires efficient techniques to attain accurate rate coefficients of the individual reaction steps. Within the recently developed SP-PLP-EPR technique, pulsed-laser polymerization (PLP) induced by a laser single pulse (SP) is carried out in conjunction with highly timeresolved EPR detection of the type and the concentration of radicals. The method provides an unrivaled direct access to the measurement of chain-length-dependent termination and also of propagation rate. Even for systems where as a consequence of transfer reactions different types of radical species with distinctly different properties occur, a full kinetic analysis is within reach. The SP-PLP-EPR technique is also applicable toward the measurement of RDRP-related rate coefficients for reversible addition−fragmentation transfer (RAFT) polymerization, atom transfer radical polymerization (ATRP), and organometallic-mediated radical polymerization (OMRP). Investigations into the latter two polymerization strategies benefit from additionally applying UV/vis/NIR in conjunction with Mössbauer spectroscopy to monitor reactions of the metal catalyst. of the decay of radical concentration to values as low as 10−8 mol L−1.6−8 Figure 1 illustrates SP-PLP-EPR data (red curve)

1. INTRODUCTION Despite the eminent importance of radical polymerization processes, carried out on an annual scale of multimillion tons worldwide, the kinetic knowledge about these reactions is relatively poor. The in-depth understanding of the kinetics and mechanism of conventional radical polymerization requires accurate rate coefficients for propagation, termination, and chain transfer. These coefficients also apply to reversible deactivation radical polymerization (RDRP) for which a few more RDRP-specific rate coefficients need to be known in order to estimate reaction conditions for well-controlled RDRP. So far, kinetic evidence has mostly been deduced from overall quantities such as monomer conversion vs time profiles and from polymer properties. The situation has enormously improved with the advent of pulsed-laser polymerization (PLP) techniques.1−4 Propagation rate coefficients, kp, are accurately determined by PLP in conjunction with analysis of the polymeric product by size-exclusion chromatography (SEC).4 Via SP-PLP-NIR, polymerization induced by a laser single pulse (SP) is monitored on a microsecond to millisecond scale by near-infrared (NIR) spectroscopy. This technique yields kp/⟨kt⟩ values, with ⟨kt⟩ referring to a chain-length-averaged termination rate coefficient.3 Both techniques take advantage of the instantaneous production of an intense burst of primary radicals by photodissociation of a suitable initiator. The most recent and certainly most powerful PLP method is SP-PLP-EPR.5 The beauty of the method consists of the combination of instantaneous production of an intense primary radical concentration, typically up to 10−5 mol L−1, by a laser single pulse with subsequent time-resolved sensitive detection © XXXX American Chemical Society

Figure 1. EPR spectrum of styryl radicals at 85 °C (blue) measured under continuous irradiation with a mercury-arc lamp. The red curve illustrates the decay of EPR intensity after single-pulse initiation measured in an SP-PLP-EPR experiment at constant magnetic field.

measured at constant magnetic field on styrene at 85 °C after applying a laser pulse at t = 0. The styryl radical spectrum measured prior to the SP-PLP-EPR trace under continuous irradiation is given in blue. EPR spectroscopy is unrivaled for investigations into the kinetics of radical polymerization, as the relevant radical species may be quantitatively monitored online, in both organic and aqueous solution.9 In the absence of Received: December 8, 2015 Revised: March 9, 2016

A

DOI: 10.1021/acs.macromol.5b02660 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

1). The EPR spectrometer and the laser are synchronized by a pulse generator, and the signal intensity is calibrated for absolute radical concentration by using dilute solutions of stable radicals.

transfer reactions, single-pulsed laser techniques have in common that the chain length, i, of radicals increases linearly with the time t after pulsing. As a consequence, SP-PLP-EPR is perfectly suited for investigations into the chain-length dependence of termination. The SP-PLP-EPR method also allows for the quantitative investigation of systems with more than one type of propagating radicals, as is the case, e.g., with acrylates10,11 and acrylamide.12 The secondary propagating radicals (SPRs) and the tertiary midchain radicals (MCRs), which are produced from SPRs by intramolecular transfer, exhibit clearly different EPR spectra.10,11 For these systems, SP-PLP-EPR provides access to the rate coefficients of backbiting, kbb, by which SPRs are transformed into MCRs, and of propagation from midchain radicals, kpt, which yields SPRs from MCRs. Such accurate rate coefficients are of enormous relevance, as SPRs and MCRs react very differently. The SP-PLP-EPR technique is also perfectly suited toward elucidating the detailed kinetics of RDRPs, such as reversible addition−fragmentation transfer (RAFT) polymerization,13−18 atom transfer radical polymerization (ATRP),19−21 and organometallic-mediated radical polymerization (OMRP).20,22 The metal catalyst used for ATRP23−25 and OMRP26 may additionally be monitored via UV, vis, and NIR spectroscopy.27,28 For the mechanistic studies into Fe-catalyzed polymerizations, especially for analyzing a potential interplay of ATRP and OMRP equilibria,29−36 57Fe Mössbauer spectroscopy turned out to be of great additional value.28 The Mössbauer parameters are indicative of the oxidation state and of the spin state of the iron complexes and may provide information about the ligand sphere.37−39 Moreover, the peak area of the Mössbauer doublets is expected to be proportional to the relative concentrations of the associated species. After briefly presenting a few experimental aspects of the SPPLP-EPR technique in section 2, recent results obtained by this method for conventional polymerizations, including systems which undergo backbiting, are reported in section 3, followed by section 4 on ATRP and section 5 on the interplay of ATRP and OMRP. The kinetics and mechanism of RAFT polymerization are addressed in section 6. In each section, the potential of highly time-resolved SP-PLP-EPR spectroscopy is illustrated.

3. CONVENTIONAL RADICAL POLYMERIZATION 3.1. Chain-Length Dependent Termination. Radical termination is a diffusion-controlled process. The relationship between the self-diffusion coefficient, D, and the hydrodynamic radius, r, which increases with chain length, may be quantified by the Stokes−Einstein equation: D=

kBT 6πrη

(1)

where kB is the Boltzmann constant and T the absolute temperature. For experiments at low degrees of monomer conversion, the viscosity, η, may be identified with the initial viscosity of the system, which may be measured prior to polymerization.42 The rate of radical termination decreases with increasing chain length, i, due to diffusion control.40,41 In the region of short-chain radicals where center-of-mass diffusion dominates, the chain-length dependence may be identified with the one of diffusion given by the power-law expression D ∼ i−α.40 The exponent α is above zero. Thus, D and kt are reduced toward larger chain lengths. The situation becomes more complex once long-chain radicals are involved. From a crossover chain length, ic, on, two macroradicals get entangled, and diffusion of chain segments has to take place prior to the chemical termination reaction.43,44 The transition from the center-of-mass region (at short chain lengths) to the region of segmental diffusion (at long chain lengths) is adequately taken into account by the socalled composite model introduced by Smith and Russell (eq 2).45 This model assumes a decay of kt for two radicals of identical size, kti,i, with the exponents αs and αl for the regions of short and long radicals, respectively, which are separated by the crossover chain length ic. k t i , i = k t1,1i−αs i,i

kt =

2. EXPERIMENTAL ASPECTS The essential technical aspects of the SP-PLP-EPR method are illustrated in Figure 2 (for further details see refs 6 and 7). The EPR sample is placed into the cavity and irradiated by UV laser pulses through a grid. The EPR signal intensity is measured as a function of time t after applying the laser single pulse (cf. Figure

k t1,1ic−αs + α1i−α1

i ≤ ic =

k t0i−α1

i > ic

(2)

The short-chain exponents are typically between 0.5 and 0.7, in agreement with both theory and measurements of diffusion coefficients for small molecules.46,47 The chain-length dependence is less pronounced for i > ic with αl being around 0.16− 0.20 for termination of two long chains with the radical functionality at the chain end.48−52 The understanding of ic is not yet fully clear, which poses problems toward the prediction of ic. Thus, experimental ic values are required for the kinetic modeling. The existing data suggest that higher chain flexibility reduces the size of ic.53 The crossover chain length for styrene, ic(Sty) = 30 ± 10,54 is significantly below the one of methyl methacrylate, ic(MMA) = 100,55 and of vinyl pivalate, ic(VPi) = 110 ± 30,56 but is close to the one for vinyl acetate, ic(VAc) = 20 ± 10,56 and methyl acrylate, ic(MA) = 35 ± 10. The value for butyl acrylate is ic(BA) = 65 ± 20.53 Recent experiments into CLD termination of dodecyl methacrylate and ethylhexyl methacrylate bulk homopolymerizations demonstrate that ic decreases toward higher polymerization temperature and, at temperatures around 100 °C, approaches values as with MA.57 The SP-PLP-EPR method is perfectly suited for studying CLD termination, as i increases with time t after applying the

Figure 2. SP-PLP-EPR setup entirely consisting of commercially available instrumentation; for more details see refs 6 and 7. B


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Macromolecules

The determination of the composite-model parameters αs, αl, ic, and kt1,1 proceeds in two steps as illustrated in Figure 5.7 The left-hand-side part of Figure 5 shows a double-log plot of the SP-PLP-EPR data according to eq 3, which results from integration of the termination rate law adopting the expression kti,i = kt1,1i−α with i = kpcMt.56 The plotted normalized radical concentrations require no calibration. Two straight lines may be fitted to the data with the slopes for the ranges at short and long times providing the power-law exponents for short-chain and long-chain radicals, αs and αl, respectively. The intersection of the two lines yields the crossover chain length, ic. Calibration for absolute radical concentration is needed for evaluation of kt1,1 from the SP-PLP-EPR data given on the right-hand side of Figure 5. This data is fitted to the expression based on the correlation i = kpcMt + 1 with αs and kt1,1c0R being the fitted parameters, as detailed in elsewhere.58 The term (+1) matters at small chain lengths. With the initial radical concentration, c0R, being known from calibration within ±10%, the fitted parameter kt1,1c0R,7 yields kt1,1, the rate coefficient for reaction of two radicals of chain length unity with an uncertainty of ±20%. Under control by center-of-mass diffusion, this rate coefficient may be correlated with the Smoluchowski equation

laser pulse: i = kpcMt(+1), where kp is the propagation rate coefficient and cM the monomer concentration. The term (+1) refers to the initial monomer-specific radical species produced by addition of the first monomer molecule to the photoinitiator-derived primary radical species.58 The entire set of composite-model parameters may be deduced from a single experiment. The signal-to-noise quality may be significantly enhanced by investigation of fully deuterated monomers.54,59 The coupling of the radical functionality with protons, which is complex in the case of delocalized systems like styrene radicals, results in a multiline spectrum for Sty-H8, whereas the EPR intensity is condensed essentially into one singlet in the case of Sty-d8 (Figure 3). On the other hand, replacing hydrogen by deuterium does not significantly affect the termination kinetics.54,59

k t1,1 = 2πPspinNA(DA1 + DB1)R c

(4)

in which kt1,1 is given as a function of the self-diffusion coefficient of the two small radicals, D, the spin factor Pspin = 0.25, the Avogadro constant NA, and the capture radius Rc. Combination of eq 4 with eq 1 results in eq 5, which says that kt1,1 scales with the fluidity, η−1, and with the ratio of Rc to r1. k t1,1(T ) ∝

Figure 3. EPR spectra of styrene-H8 and styrene-d8 radicals. Adapted with permission from ref 54.

1 Rc η(T ) r1

(5)

kt1,1η

Table 1 provides data for several monomer−solvent and bulk monomer systems. Section A comprises small monomers including the smallest members of the (meth)acrylate families in bulk as well as acrylamide (AAm) and nonionized acrylic acid (AA) homopolymerizing in aqueous solution at different weight percentages (wt %). The remarkable agreement of kt1,1η for different monomers and solvent environments is clearly indicative of (i) diffusioncontrolled termination operating already in the initial polymerization period, where the SP-PLP-EPR data have been measured, and of (ii) very similar ratios of Rc to r1 for the radicals associated with these monomers. Section B refers to the situation of enhanced bulkiness of the side group, which reduces kt1,1 due to enhanced shielding of the radical functionality and to a higher hydrodynamic ratio that may be expressed by a lower Rc/r1.60 In the case of the heavily shielded n-dibutyl itaconate (DBI) (section C), the bulky side groups result in a kt1,1 value which is by approximately 1 order of magnitude below the number expected from viscosity via eq 5.61 With all systems listed in Table 1, the temperature dependencies of kt1,1 and of η−1 are almost identical, which leaves the product term kt1,1η more or less insensitive toward temperature. Viscosity measurements on these systems prior to polymerization should help in estimating the composite-model parameters kt1,1 for the initial polymerization period. So far, composite-model parameters in aqueous solution have comprehensively been determined only for AAm.9 This data indicates that the termination rate coefficients for noncharged radicals in aqueous and in organic solution behave similarly, in

Deuteration clearly improves the quality of the normalized radical concentration vs t traces for styrene, as is illustrated in Figure 4 by SP-PLP-EPR data measured for 100 °C on both Sty-H8 and Sty-d8 radicals.54 2k t1,1c R0 t pα 1 − α c R0 −1= t c R (t ) 1−α

(3)

Figure 4. Normalized radical concentration vs time traces for styreneH8 and styrene-d8 at 100 °C with dicumyl peroxide (9.0 × 10−2 mol L−1) acting as the photoinitiator. The laser pulse is applied at t = 0. Signal-to-noise quality is significantly improved by using Sty-d8 rather than Sty-H8. Both traces were deduced from coadding data from 19 single pulse experiments. Reproduced with permission from ref 54. C


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Figure 5. Determination of the composite-model parameters by analysis of the normalized concentration vs time traces. Left: fitting of relative radical concentration, cR/c0R − 1, for determination of ic and αl. The slope is given by (1 − α). The relatively high temperature of 120 °C was chosen to reach chain lengths above ic. Right: determination of αs and kt1,1c0R for chain lengths below ic. The initial radical concentration, c0R, is known from the calibration procedure. The solid line represents the best fit, with αs = 0.47 and kt1,1 = 9.0 × 108 L mol−1 s−1. The dashed lines, which refer to identical kt1,1 but to different αs, indicate lower and upper bounds of αs. Reproduced with permission from ref 54.

Table 1. Comparison of ktss(1,1)η Values for Several Monomers at 80 °Ca section

monomer

kt1,1(80 °C)η(80 °C) (L mPa mol−1 × 108)

A

AAm (10 wt % in H2O) AAm (20 wt % in H2O) Sty VAc MMA MA VPi tert-BMA DBI

3.3 3.2 3.2 3.6 3.7 3.7 1.4 1.3 ≤0.12

B C

conversion may exceed the one of the initial monomer system by several orders of magnitude.42 The qualitative change of ⟨kt⟩ as a function of monomer conversion, X, has been presented in ref 73 for MMA bulk polymerization. The initial range of segmental diffusion control is followed by a region where translational diffusion controls ⟨kt⟩. Toward even higher conversion, reaction diffusion may operate. Depending on the monomer under investigation, the individual modes of control may extend over quite different ranges of monomer conversion, thus giving rise to a wide pattern of ⟨kt⟩ vs X correlations, which additionally depend on the history of a particular polymerization. SP-PLP-EPR experiments at high monomer conversions may be carried out using two strategies: (1) A thermally initiated polymerization which does not affect the photoinitiator may go first, followed by the SP-PLP-EPR experiment at high polymer content. (2) A significant amount of polymer is added to a mixture of monomer and photoinitiator. The advantage of this second procedure relates to the fact that detailed termination kinetics may be measured for different types of polymeric background material being added. However, sample preparation for method 2 is difficult, as introducing significant amounts of polymer and preparing homogeneous polymer−monomer mixtures inside the flat EPR cells pose problems. It should be noted that the SP-PLP-EPR method also allows for studies into termination of radicals in surface polymerization, e.g., from silica nanoparticles.74 A recent such investigation into butyl methacrylate polymerization indicates that termination of surface-attached macroradicals occurs under reaction-diffusion control. The SP-PLP-EPR experiments illustrated so far deliver rate coefficients for termination of two radicals of more or less identical size. These rate coefficients are directly applicable toward reversible deactivation polymerizations, which are characterized by a very narrow distribution of radicals. In conventional and thus in most technical polymerizations termination however occurs between radicals of different size. Thus, rate coefficients, kti,j, for termination of radicals with chain lengths i and j need to be known. Three models have been used to estimate kti,j from the experimentally accessible rate coefficients for termination of two radicals of identical size, kti,i and ktj,j: the geometric mean model (gm), the diffusion

ref 9 9 54 54, 54, 53, 54, 54, 54,

56 59 54 56 60 61

a

Sty = styrene, MMA = methyl methacrylate, VAc = vinyl acetate, MA = methyl acrylate, VPi = vinyl pivalate, BMA = butyl methacrylate, and DBI = di-n-butyl itaconate. The values refer to either bulk polymerization or to polymerization in aqueous solution. For further details, see ref 9. The termination rate coefficients refer to the reaction of radicals of chain length unity with the radical functionality at the chain end.

contrast to propagation,4,62−68 where kp may differ by 1 order of magnitude for polymerization in bulk and in dilute aqueous solution. As investigations into CLD kt depend on the knowledge of kp, the recent progress in PLP-SEC work, including aqueous-solution polymerizations, will largely assist further SP-PLP-EPR studies.64,69,70 Future interest will focus on the rate coefficients for radical polymerization of fully ionized monomers. First such experiments have been carried out on sodium acrylate (NaAA)66 and trimethylaminoethyl methacrylate chloride (TMAEMA).71 These monomers are not easily subjected to PLP-SEC experiments, as the electrostatic repulsion of the equally charged radical species lowers kt by orders of magnitude.71 So far, the SP-PLP-EPR method has only been used for studies into the initial period of radical polymerizations, i.e., at low and moderate degrees of monomer conversion. As technical processes are carried out up to much higher conversion, it appears to be a matter of priority to extend the method into the region of medium to high degrees of monomer conversion. The viscosity of bulk polymerizations at high D


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Macromolecules mean model (dm), and the harmonic mean model (hm).75 To check for the applicability of such models, double-pulse experiments should be carried out, in which a second laser pulse is applied at a preselected delay time. The radical concentration vs time profiles estimated on the basis of the different models for kti,j result in different radical concentration vs time profiles. With the fully ionized monomer TMAEMA taken as an example, a DP-PLP-EPR measurement has been simulated with the program package Predici on the basis of the reported homopolymerization rate coefficients and compositemodel parameters listed in the legend to Figure 6. The simulated results in Figure 6 await experimental testing and analysis to improve the understanding of cross-termination.

given by the area under the radical concentration vs time trace as illustrated in Figure 7. Assuming kp to be independent of

Figure 7. Time-resolved radical concentration of TMAEMA radicals (20 wt % in D2O) at 60 °C. The hatched area below the curve corresponds to the integral in eq 6. Reproduced with permission from ref 71. Copyright 2015 John Wiley & Sons.

chain length, this area yields kp with the monomer conversion, X, induced by a laser pulse, being the only additional piece of required information which may be separately measured, e.g., by NIR spectroscopy. The so-obtained propagation rate coefficient for TMAEMA (20 wt % in D2O) at 60 °C is kp = (3.5 ± 0.3) × 103 L mol−1 s−1. This number from the SP-PLP-EPR measurement may be compared to the one calculated from kp/⟨kt⟩ and kp/⟨kt⟩1/2 data deduced for the same system by SP-PLP-NIR and chemically initiated experiments, respectively. Upon accounting for the difference in ⟨kt⟩ associated with these two experiments,77 the kp value from the combination of kp/⟨kt⟩ and kp/⟨kt⟩1/2 is obtained to be (3.5 ± 0.4) × 103 L mol−1 s−1 for 20 wt % TMAEMA in solution of D2O, in perfect agreement with the number from the novel SP-PLP-EPR strategy. The SP-PLPEPR method should be examined for kp determination on other difficult systems, e.g., slowly terminating ones. The SP-PLPEPR technique is however not meant to replace the PLP-SEC method but will extend the range of systems for which kp may be accurately determined. Thus, the analysis of very detailed effects, e.g., of the dependence of kp on initial monomer concentration in ionic systems may become within reach. An important point to be noted from Figure 7 is that kp and chainlength-dependent kt may be obtained from a single SP-PLPEPR experiment, which thus constitutes an extremely powerful method for the kinetic analysis of radical polymerizations. 3.3. Kinetic Analysis of Systems with the Radicals Undergoing Backbiting. Transfer reactions, e.g., transfer to solvent, to monomer, and to polymer, affect polymer properties and polymerization kinetics.78−82 Of particular relevance is intramolecular chain transfer, i.e., the so-called backbiting step, which occurs, e.g., in the radical polymerization of acrylates and acrylic acid.10,62,83−89 Secondary propagating radicals (SPRs) may undergo this backbiting step, by which a tertiary midchain radical (MCR) is produced through a concerted [1,5]-H-shift reaction via a six-membered transition state (Scheme 1). Addition of a monomer molecule to an MCR results in the formation of an SPR (not shown in the scheme). The impact of backbiting on the polymerization kinetics is enormous.90 As is illustrated in Table 2, the propagation rate coefficients from SPRs, kps, are above the ones from MCRs, kpt, by more than 3 orders of magnitude. Throughout the

Figure 6. Radical concentration vs time traces for a double-pulse experiment (DP-PLP-EPR) simulated on the basis of the known rate parameters kp = 1400 L mol−1 s−1, kt1,1 = 1 × 107 L mol−1 s−1, αs = 0.6, αl = 0.16, and ic = 30 adopting the harmonic mean (hm) and the geometric mean (gm) models for cross-termination of radicals (see text). The two models differ to a larger extent at higher chain length j and lower kt1,1.

3.2. Access to Propagation Rate Coefficients by SPPLP-EPR. The PLP-SEC method, which is perfectly suited for kp analysis of a wide range of monomers in a variety of solvent environments,4 runs into problems whenever termination becomes very slow, as is the case with fully ionized or very bulky monomers. Under such conditions, the decay in radical concentration between two subsequent pulses is too small as to induce a pronounced PLP-induced structure on the molar-mass distribution from which kp is to be accessed. These difficulties, e.g., with fully ionized monomers, may be overcome by a novel SP-PLP-EPR strategy.71 In the case of only one type of growing radicals being present, eq 6, which results from integration of the general propagation rate expression, provides the basis for this kp measurement. The method is similar to earlier attempts of kp determination by EPR detection of radical concentration under stationary polymerization conditions, where radical concentration is however mostly very low and thus difficult to be accurately detected. Moreover, this earlier EPR experiment yields kp as a mean value over an extended range of monomer-to-polymer conversion.76 Laser-pulse-induced radical concentrations are intense and thus far easier to be quantitatively detected. Moreover, the measurements are performed within a narrow monomer conversion range. ⎛ 1 ⎞ ⎟ = k ln⎜ p ⎝1 − X ⎠

∫0

∞

c R (t ) d t

(6)

The procedure is exemplified by an SP-PLP-EPR experiment on TMAEMA (20 wt % in D2O). The integral ∫ ∞ 0 cR(t) dt is E


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Macromolecules Scheme 1. Backbiting Process for the SPR-to-MCR Reaction via a [1,5]-H-Shifta

a The six-membered transition state structure is tagged by ‡. The substituent R denotes a side group, e.g., a COOR, COOH, or CONH2 moiety with acrylates, acrylic acid, and acrylamide, respectively.

Table 2. Rate Coefficients of Propagation from SPRs, kps, and from MCRs, kpt, for Acrylic Monomers at 50 °C monomer BA (1.5 M in toluene) AAm (10 wt % in H2O)

kps (L mol−1 s−1)

kpt (L mol−1 s−1)

kpt/kps

ref

28200

24.5

8.6 × 10−4

11, 91

90000

22.0

2.5 × 10−4

9

subsequent text, the superscript “s” refers to SPRs and “t” to MCRs. It is known since long that backbiting occurs in ethylene high-pressure polymerization,93,94 but it took quite a while, before the all-invasive relevance of this reaction step upon radical polymerization of acrylics was rationalized.85,88−90,95,96 EPR is perfectly suited for identifying backbiting. Shown in Figure 8 is an EPR spectrum recorded within a few seconds

Figure 9. Individual EPR spectra for SPRs and MCRs simulated on the basis of the experimentally determined hyperfine coupling constants for a BA polymerization (1.5 M in toluene).10 The simulation serves the purpose of illustrating the ability to separately monitor both types of radicals by SP-PLP-EPR experiments at the field positions indicated by the arrows. Adapted with permission from ref 11.

SPRs is transferred to typical polymerization temperatures, where SP-PLP-EPR experiments are carried out at the characteristic SPR and MCR magnetic field positions. With acrylic acid in aqueous solution, the method may be applied only in a restricted fashion, as water solidifies at temperatures where only SPRs occur. With acrylamide (AAm) polymerization in aqueous solution, the SPR-only situation can be studied at −5 °C, as backbiting occurs to a far weaker extent than with the acrylates and acrylic acid.12 The recent results for AAm will be briefly presented in what follows. The experimental overall spectra in AAm polymerization including the fitted individual spectra for SPRs and MCRs are shown in Figure 10. The EPR spectrum at −5 °C is almost entirely due to SPRs, whereas SPRs and MCRs clearly contribute to the EPR spectrum at 75 °C. With the EPR spectra of the individual types of radicals being known, the overall EPR contour may be deconvoluted into the fractions of SPRs and MCRs via the associated double integrals.12 Plotted in Figure 11 are the MCR mole fractions, xMCR, as a function of polymerization temperature for AAm (10 wt % in H2O) and BA (1.5 M, i.e., 22 wt % in toluene) as obtained from stationary photopolymerizations.10,12 At identical temperature, xMCR of BA is well above the value for AAm. However, MCRs become the dominant radical species at typical AAm polymerization temperatures above 60 °C. Other than with BA and as seen on the right-hand side of Figure 10, there is no magnetic field position from which SPRs may be separately monitored by SP-PLP-EPR in the presence of significant amounts of MCRs. There is however such a characteristic band for MCRs, indicated by the arrow. Timeresolved analysis of this MCR component in conjunction with the information from the SP-PLP-EPR experiment at −5 °C into the SPR kinetics provides the kinetic information as in the favorable situation seen with BA. Illustrated in Figure 12 are MCR traces from SP-PLP-EPR measurements at 75 and 95 °C, where sufficient amounts of MCRs and thus reasonable signal-

Figure 8. EPR spectrum recorded during a butyl acrylate polymerization at 20 °C under laser-pulse initiation at a repetition frequency of 20 Hz.85

during a butyl acrylate radical polymerization under laser-pulseperiodic irradiation at 20 Hz.97 EPR intensities are seen which oscillate with the laser pulse, whereas part of the EPR contour is insensitive toward pulsing. The oscillating behavior is characteristic of SPRs which are instantaneously produced and may rapidly terminate, whereas MCRs, as transient species, are slowly built up from SPRs and undergo slow termination. Since SPRs and MCRs differ significantly in EPR hyperfine coupling,10,12,85 fitting of the overall EPR spectrum by coadding individual simulated SPR and MCR spectra allows for easily distinguishing these species and thus for safely establishing whether and to which extent the two types of radicals occur. A simulation of individual EPR spectra is shown for butyl acrylate polymerization in 1.5 M solution of toluene in Figure 9. With BA, the favorable situation of characteristic magnetic field positions for both SPRs or MCRs is met.85 The EPR spectra of the two radicals are simulated on the basis of the experimentally determined hyperfine coupling constants. The two characteristic EPR field positions are highlighted by the arrows in Figure 9. The measurements are first carried out at low temperature (−40 °C) where SPRs are the by far dominating species. The so-obtained kinetic information for F


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Macromolecules

Figure 10. EPR spectra recorded during radical polymerizations of AAm (10 wt %) in aqueous solution under stationary UV irradiation at −5 and 75 °C with Darocur as the photoinitiator. The magnetic field positions used for SP-PLP-EPR investigation are indicated by the arrows. The simulated EPR signal for SPRs (red) and MCRs (blue) have been estimated from the hyperfine coupling constants in ref 12 and scaled by the molar MCR fraction determined in separate experiments (see Figure 11).12 Adapted with permission from ref 9.

termination, ktst(1,1), as detailed elsewhere.9 Extensive variation of kttt(1,1), the rate coefficient of homotermination for two (hypothetical) MCRs of chain length unity, tells that this reaction may be neglected under the reaction conditions of the present study. The temperature dependence of the measured maximum in MCR concentration is essentially related to the activation energy of kbb, Ea(kbb). The right-hand side of Figure 12 demonstrates the consecutive production of MCRs from SPRs. Analysis of the MCR concentration vs time traces yields an activation energy for backbiting of Ea(kbb) = 49 kJ mol−1 for both 10 and 20 wt % AAm in aqueous solution.9 As may be seen from the entries in Table 3, the backbiting rate coefficient Table 3. Rate Parameters for Backbiting of Acrylamide and Butyl Acrylate

Figure 11. Mole fraction of midchain radicals, xMCR, deduced from fitting the experimental EPR spectra recorded during polymerizations of AAm (10 wt %) in aqueous solution (circles) between −5 and +100 °C. Experimental values of xMCR for BA polymerization (1.52 M in toluene)10 are included for comparison (triangles).12 Reproduced with permission from ref 9.

monomer AAm (10 wt %, 20 wt % in H2O) BA (1.5 M in toluene)

to-noise quality for time-resolved MCR detection are obtained.9 On the right-hand side of Figure 12, the experimental and fitted SP-PLP-EPR signals for MCRs are plotted together with the signal for SPRs which is obtained by simulation on the basis of the SP-PLP-EPR data for SPRs measured at −5 °C. The fitting procedure yields the rate coefficients of backbiting, kbb, for MCR propagation, kpt, and for cross-

Ea(kbb) (kJ mol−1) 49 34.7

A(kbb) (108 s−1) 37 0.484

kbb(50 °C) (s−1)

ref

44

9

393

11

at 50 °C is significantly higher for BA (in toluene solution) than for AAm (in aqueous solution). The smaller backbiting rate which is consistent with the observed lower fraction of MCRs in AAm polymerization (see Figure 11) is due to the larger activation energy. With vinyl acetate (VA), no backbiting has been observed at typical polymerization temperatures. The

Figure 12. Left: MCR concentration vs time traces from SP-PLP-EPR and from associated Predici fitting for AAm (10 wt % in H2O) at 75 and 95 °C. Right: experimental and simulated MCR and simulated SPR concentration vs time traces for 95 °C. The simulated SPR trace is based on the αs, αl, ic, and ktss(1,1) data for SPRs from the SP-PLP-EPR experiment at −5 °C together with kbb, kpt, and ktst(1,1) determined by fitting the MCR trace, i.e., from the blue trace on the left. Reproduced with permission from ref 9. G


Perspective

Macromolecules Scheme 2. Mechanism of Cu-Mediated ATRPa

decrease in backbiting reactivity from BA through AAm to VA is understood as being mainly due to an increase in the activation energy for MCR formation. As is illustrated by the entries in Table 3, the entropy-motivated pre-exponential factor A(kbb) also contributes to the difference in absolute kbb. The detailed understanding of backbiting as a function of the type of monomer, of the size of the acrylate ester side group, and of the solvent environment is still in its infancy. Additionally, the SP-PLP-EPR experiments provide data for the cross-termination rate coefficient of an SPR with an MCR, ktst(1,1), i.e., for the termination rate coefficient of an SPR with a (hypothetical) MCR also of chain length unity. Clearly, a minimum size of three repeat units is required to allow for backbiting of an SPR and thus producing an acrylate MCR of the smallest possible size. The so-obtained numbers for ktst(1,1) are listed in Table 4 together with the homotermination rate coefficients, ktss(1,1),

a L represents the ligand to Cu, Rn−X refers to the dormant alkyl halide species, Rn• to the propagating radical, and M to monomer.

media.111 Measurements of KATRP and of kact were carried out for monomer-free model systems by monitoring the accumulation of the CuII persistent radical species via online FT-nearIR (FT-NIR) and visible spectroscopy.98−100,112 More recently, KATRP has also been determined under polymerization conditions in a wide range of pressure, p, and temperature, T, and with various solvents.113−115 The vis/NIR spectroscopic detection runs into difficulties with very fast reactions, as for ATRP deactivation. Most of the available data for kdeact thus rest on indirect measurements116−118 or on estimates from KATRP and kact (model system) values.99,112 Highly time-resolved EPR spectroscopy in conjunction with laser-pulse-induced radical production (SPPLP-EPR) is, however, perfectly suited for direct measurements of kdeact.19 The SP-PLP-EPR measurement of kdeact and the NIR spectroscopic determination of KATRP under polymerization conditions will be illustrated for Fe-catalyzed ATRP. 4.1. Kinetics of Fe-Catalyzed ATRP. Fe-based ATRP23,24 is an attractive alternative to the extensively used Cu catalysis due to the broad availability and low toxicity of iron.23,24,119,120 Fe-mediated ATRP of styrene and of methacrylates is already feasible at ppm levels of the Fe catalyst.121−126 To foster Fe catalysis, it is particularly desirable to identify Fe complexes with enhanced KATRP to allow for the efficient ATRP of monomers such as acrylates, which produce less active alkyl halides.127 KATRP should be measured under polymerization conditions, where an FeII/L species typically operates as the ATRP activator and an X−FeIII/L complex as the ATRP deactivator.28,128 The procedure for measuring KATRP is based on the online NIR spectroscopic detection of monomer conversion (Figure 13B) in conjunction with monitoring the FeIII deactivator complex (Figure 13A) in the visible region.129 The associated concentrations of FeII and Rn−Cl required for estimating KATRP via eq 7 are obtained from the relationship [FeII] = [Rn−Cl] = [FeIII]0 − [FeIII]. The propagation rate coefficient, kp, is known from pulsed-laser polymerization experiments.130

Table 4. Rate Coefficients of Homo-Termination, ktss(1,1) = kt1,1, and Cross-Termination, ktst(1,1), and the Associated Arrhenius Parameters for the Polymerization of 10 and 20 wt % AAm in Aqueous Solution12 Ea (kJ mol−1) ktss(1,1) ktst(1,1)

17.8 18.6

ktss(1,1) ktst(1,1)

19.0 21.2

A (L mol−1 s−1 × 1011) 10 wt % 3.9 1.2 20 wt % 5.0 2.1

kt (50 °C) (L mol−1 s−1 × 108) 5.2 1.1 4.2 0.8

for two AAm concentrations in aqueous solution.9 The homotermination rate coefficient, ktss(1,1) = kt1,1 exceeds ktst(1,1) by a factor of about 5, which is understood to be a consequence of enhanced shielding of the MCR radical functionality as reflected by the smaller A(ktst(1,1)) value.9 The similarity of Ea(ktst(1,1)) and Ea(ktss(1,1)) indicates that both processes are mainly affected by diffusion control. The ratio of ktst(1,1) for the two initial monomer concentrations, 10 and 20 wt %, is in close agreement with the measured ratio of the associated fluidities. Despite the success reached so far, an extended amount of work remains to be spent, e.g., into the detailed kinetics of various types of monomers, including partially and fully ionized monomers with different counterions, as well as into systems at moderate and high degrees of monomer conversion and in different solvent environments, not to talk about copolymerizations.

Rp = −

4. ATOM TRANSFER RADICAL POLYMERIZATION The kinetics of ATRP (Scheme 2) is superimposed on a conventional radical polymerization scheme. The metalmediated formation of propagating radicals, Rn•, from the organohalide, Rn−X, is given by the activation rate coefficient, kact. The deactivation rate coefficient, kdeact, refers to the associated reverse reaction. The equilibrium constant, KATRP, is defined as the ratio kact/kdeact. The mechanism and kinetics of Cu-mediated ATRP in organic phase have been extensively studied.98−106 Cumediated ATRP is also feasible in aqueous phase107−110 with a beneficial increase in both KATRP and kact by about 3 orders of magnitude in passing from organic to predominantly aqueous

[Fe II][R n−Cl] d[M] [M] = k p[R •n][M] = k pKATRP dt [Fe III] (7)

The spectra shown in Figure 13A,B were recorded during reverse ATRP (R-ATRP) of styrene catalyzed by an amine− bis(phenolate)iron(III) chloride complex, Cl−FeIII/L (for the structure see Figure 13A). The R-ATRP was initiated at 90 °C by the rapid decomposition of the azo initiator 2,2′-azo-bis(4methoxy-2,4-dimethylvaleronitrile (V-70).129 The time-dependent concentration of Cl−FeIII/L is found by calibration against the absorbance measured at known Cl−FeIII/L content in the absence of the thermal initiator (dash-dotted line in Figure 13A). During polymerization, the Cl−FeIII/L concentration increases with time according to the persistent radical ef fect.131 H


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Macromolecules

Figure 13. (A) Online vis/NIR spectroscopic measurement of a reverse ATRP carried out at 90 °C starting with 14.5 mM of the amine− bis(phenolate)iron(III) chloride catalyst and 8.7 mM of the azo initiator V-70 in solution of styrene/anisole (1:1, v/v). The Cl−FeIII/L concentration is measured between 27 000 and 12 000 cm−1 at an optical path length of 0.5 mm. The initial spectrum (dashed-dotted line) was recorded in the absence of the thermal initiator. For better presentation, the intensity of this spectrum was reduced by a factor of 3. (B) Styrene concentration monitored via the characteristic peak absorbance at around 6137 cm−1 at an optical path length of 5 mm. The dashed line represents the reference absorbance for full conversion of styrene. Reproduced with permission from ref 129.

The associated FeII/L complex is not easily detected via vis/ NIR spectroscopy, since one part of the absorption (i.e., the d− d transition) is of very low NIR intensity and the other (i.e., the charge transfer transition) in the UV region is overlapped by the solvent absorption. 57Fe Mössbauer spectroscopy has turned out to be extremely helpful toward the characterization of the individual FeII and FeIII species and toward the provision of reliable information on the oxidation and spin states as well as on the relative amounts of these species without any need for calibration. Mössbauer spectra may be recorded on flash-frozen solutions. The FeII/L and the Cl−FeIII/L species were first monitored individually in reference solutions of each complex but without the ATRP initiator (Figures 14A and 14B,

monomeric FeII/L species, whereas the dominant component refers to the dimeric complex, which is otherwise identical.28 Based on the detailed insight emerging from speciation analysis, the online vis/NIR spectroscopic measurement was used to determine KATRP for a variety of Fe and Cu catalysts with some representative values being tabulated in Table 5. The highest KATRP in Fe catalysis, which is associated with the highest Fe catalyst activity, has so far been found for a chlorinesubstituted amine−bis(phenolate)iron complex (entry 1 in Table 5). KATRP (60 °C) = 1.6 × 10−7 is close to the value for the CuBr(PMDETA) complex (entry 5), KATRP (60 °C) = 2.1 × 10−7, with both values referring to styrene polymerization. The associated value for Fe-mediated MMA polymerization, KATRP = 2.0 × 10−5 (entry 2), results from an estimate based on the relative bond strength of the alkyl halides for MMA and styrene polymerization.132−134 The direct measurement of KATRP with this system is complicated because of the interplay of ATRP and OMRP as detailed in the next section. Fe-mediated ATRP is also feasible without external ligand systems, i.e., with the solvent acting as a ligand to Fe.121,123,128,135−137 With nonpolar solvents such as the monomer MMA itself, KATRP (60 °C) = 9.7 × 10−6 (entry 3 in Table 5) of the associated [FeBr3(MMA)]− complex is only by about a factor of 2 below the one of the amine− bis(phenolate)iron complex (cf. entry 2) but is reduced by 3 orders of magnitude when polar solvents such as N-methylpyrrolidin-2-one, NMP (entry 4), are used.27,128 The lowering of KATRP for [FeBr3(NMP)]− is accompanied by the formation of ineffective [Fe(NMP)6]2+ species. KATRP for [FeBr3(solv)]− catalyst species (entries 3 and 4) is strongly enhanced toward higher temperature due to the large reaction enthalpy, ΔrH(KATRP), of about 60 kJ mol−1 (entries 3 and 4). The positive reaction volumes, ΔrV(KATRP), indicate that catalyst activity is higher at ambient than at elevated pressure.128 Applying high pressure, however, turns out to be beneficial with the amine−bis(phenolate)iron complex (entry 1) and with Cu/ligand complexes (entry 5 and refs 112, 114, and 115 for further examples): Due to the large negative ΔrV(KATRP) and to the negative ΔV‡(kp),4 ATRP rate may be enhanced by more than 2 orders of magnitude in passing from 1 to 6000 bar.129 Monodentate ligands such as substituted phosphines or carbenes have frequently been used for the formation of Febased catalysts.23−25 The structure of the resulting Fe/ligand

Figure 14. 57Fe Mössbauer spectra of (A) the amine−bis(phenolate)iron(II) catalyst, FeII/L, and (B) the associated Cl−FeIII/L species recorded at 80 K. (C) Spectrum recorded after initiation of an RATRP of styrene indicating the presence of FeII/L (80%) and of Cl− FeIII/L (20%). Reproduced in part with permission from ref 28.

respectively).28 Both species occur in the high-spin state as may be inferred from the associated isomer shift (see ref 39). In a spectrum recorded after initiation of an R-ATRP, i.e., after the rapid decomposition of the azo initiator, the FeII/L and the Cl− FeIII/L species were observed at a ratio of 80:20 (Figure 14C).28 The Mössbauer analysis confirms that only a single FeIII and one predominant FeII species are present. The minor subfunction for FeII shown in Figure 14A is most likely due to a I


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Macromolecules

Table 5. Equilibrium Constants, KATRP, at 1 bar and 60 °C, Reaction Enthalpies, ΔrH, and Reaction Volumes, ΔrV, for ATRPs Mediated by the Indicated Catalyst entry 1 2 3 4 5 6

FeII catalyst [O2NN′]Fe [O2NN′]Fe [FeBr3(MMA)]− [FeBr3(NMP)]− CuIBr(PMDETA)d CuIBr(PMDETA)d

monomera Sty MMA MMA MMA Sty MMA

KATRP at 60 °C/1 bar 1.6 2.0 9.7 1.1 2.1 2.6

× × × × × ×

−7

10 10−5b,c 10−6 10−8 10−7 10−5 b

ΔrH (kJ mol−1)

ΔrV (cm3 mol−1)

ref

25 ± 5

−17 ± 2

62 ± 7 58 ± 6 22

1±2 8±2 −20 −22

129 138 27 128 114 115

a

Solvents: bulk (entries 1−3), 33 vol % NMP (entry 4), 50 vol % MeCN (entries 5 and 6). bEstimates based on the reactivity ratio for MMA and Sty ATRP.114,115 cInterplay of ATRP and OMRP. dPMDETA = N,N,N′,N″,N″-pentamethyldiethylenetriamine.28

generate the deactivated alkyl halide, Rn−Cl, and the FeII/L complex. In addition, the radicals may terminate and produce dead polymer. Since only minor amounts of FeII/L are produced in the laser-pulse-controlled reaction, the reverse reaction, ATRP activation, does not occur to any significant extent. Shown in Figure 16 are two relative radical concentration vs time traces for 2-ethylhexyl methacrylate (EHMA) at 40 °C.21

complexes127,139,140 is, however, very similar to the systems with the solvent acting as the ligand (see Figure 15),27,141 which

Figure 15. Fe-based catalysts which are similar in KATRP.127

appears to be the reason why KATRP of such complexes is close to the one of [FeBr3(MMA)]−.27 Some of these ligands may be preferable in that they improve catalyst robustness142,143 at ppm concentrations and also solubility.124 Substituted triarylphosphines, in addition, serve as internal reducing agents for the iron bromide catalyst.127 To identify Fe-based systems with even higher KATRP, the measurements should be extended to further types of substituted amine−bis(phenolate)iron complexes (see also next section). Moreover, catalase-based or heme-based Fe complexes appear to be promising systems with enhanced performance already at ambient conditions.144−147 4.2. SP-PLP-EPR Measurement of ATRP Deactivation. An efficient ATRP catalyst is associated with rapid deactivation. EPR spectroscopy is perfectly suited for measuring the deactivation rate coefficient, kdeact, with both reactants, Rn• and Cl−FeIII/L, being monitored in a single experiment. Shown in Scheme 3 is the mechanistic scenario for the SP-PLP-EPR

Figure 16. Relative EHMA• radical concentration, [R•]/[R•]0, vs time recorded at 40 °C with the laser single pulse being applied at time zero. Two [R•]/[R•]0 vs time traces were recorded: one in the absence57 and one in the presence of 3.0 mM Cl−FeIII/L. The black line illustrates the simulated data for the experiment with Cl−FeIII/L. Reproduced with permission from ref 21.

In the absence of Cl−FeIII/L (gray curve), the decay in radical concentration after applying a laser single pulse at t = 0 is entirely due to radical−radical termination. In the presence of 3 mM amine−bis(phenolate)iron(III) chloride (red line), the decay is much faster and dominated by ATRP deactivation. Deactivation is pseudo-first-order, since the Cl−FeIII/L concentration greatly exceeds the one of Rn•. The ln([Rn•]0/ [Rn•]) vs time data from the SP-PLP-EPR measurement thus yields the product kdeact[Cl−FeIII/L] according to eq 8.21

Scheme 3. SP-PLP-EPR Measurement of kdeacta (Reproduced with Permission from Ref 21)

d ln([R n•]0 /[R n•]) = kdeact[Cl−Fe III /L] dt

a

The starting components, i.e., the photoinitiator MMMP, monomer M, and the Cl−FeIII/L complex, are marked in red. The primary radicals generated via laser pulsing produce propagating radicals, Rn•, of chain length n. Deactivation of Rn• yields alkyl chloride, Rn−Cl, and FeII/L. The scheme includes termination to dead polymer.

(8)

III

The signal intensities of the Cl−Fe /L species recorded before and after laser-pulse application (Figure 17) indicate that the concentration remains close to the known preselected concentration of Cl−FeIII/L, which allows for an easy estimate of kdeact from the product kdeact[Cl−FeIII/L]. Along these lines, the estimate of kdeact is based on relative concentrations of [Rn•] and thus on relative EPR intensities. The SP-PLP-EPR method thus requires no calibration for absolute radical concentration. The analysis may be accompanied and checked by simulations with the program package Predici as illustrated by the black line in Figure 16 for the experiment with Cl−FeIII/L.21

measurement of kdeact.21 As in R-ATRP, the catalyst is employed in the higher oxidation state, i.e., Cl−FeIII/L. The starting reagents are marked in red. A photoinitiator such as α-methyl4-(methylmercapto)-α-morpholinopropiophenone, MMMP (for the structure see Scheme 3), is used for producing primary radicals, which rapidly add to monomer molecules, M. The propagating radicals, Rn•, react with the Cl−FeIII/L complex to J


Perspective

Macromolecules

stationary polymerization conditions, ATRP and OMRP reactions occur simultaeously.21 4.3. SP-PLP-EPR Measurement of Catalytic Radical Termination. The ATRP scheme (cf. Scheme 2) contains the deactivation reaction of propagating radicals with the metal catalyst in the higher oxidation state, i.e., CuII or FeIII. However, an organometallic reaction of propagating radicals may also occur with the catalyst in the lower oxidation state, CuI or FeII. SP-PLP-EPR was applied to measure the termination rate in the absence and in the presence of an FeII catalyst, [FeBr3(solv)]− (Figure 18).20 As seen from Figure 18a, termination rate in DMA polymerization is not significantly enhanced in the presence of FeII. Conversely, the decay in radical concentration is enormously enhanced with increasing FeII content in the case of butyl acrylate polymerization (Figure 18B). The underlying reaction was found to be the catalytic termination of two secondary propagating radicals by FeII, according to Scheme 4. The rate-determining step of catalytic radical termination (CRT) is the formation of the R−FeIII/L intermediate, denoted with the rate coefficient kadd, which may quickly enter a followup reaction regenerating the FeII catalyst and producing dead polymer. Currently it cannot be decided whether the R−FeIII/L intermediate is Rn−FeIII/L (R = Rn) or is H−FeIII/L (R = H). The latter compound may be produced either directly or in a consecutive step from Rn−FeIII/L.31,149 Further insight into the structure, e.g., from X-ray studies or quantum-chemical calculations, and into the relative importance of Rn−FeIII formation and subsequent β-H elimination should be helpful to understand the different behavior seen for acrylates and methacrylates. Irrespective of this missing piece of information, the precise knowledge of the rate coefficients for CRT is indispensible for RDRP, since CRT may significantly reduce the degree of chain-end functionality of the polymeric material. The CRT reaction is also found in Cu-catalyzed ATRP of acrylates.57,149 The associated organometallic intermediate, [R−Cu I I (TPMA)] + , results from the reaction of [CuI(TPMA)]+ with secondary propagating radicals, by using either CuBr or CuPF6 as the copper salt. As with the abovementioned Fe system, it can currently not be decided whether the species observed via EPR is R−CuII/L with R = Rn or with R = H. The intermediate is stable at −40 °C and is formed by applying a sequence of laser pulses for radical production (Figure 19). At 0 °C or at even higher temperatures, as are relevant for ATRP processes, the organometallic intermediate becomes labile (cf. Figure 19) and reacts according to the CRT pathway.57 In Cu catalysis, the reverse reaction, i.e., the decomposition of the organometallic intermediate to CuI/L and Rn•, also takes place at these higher temperatures. This should be the reason why the apparent CRT rate, kCRT, is lower than for [FeBr3(solv)]−, even though the addition reaction, denoted with kadd, is faster (see Table 7).

Figure 17. EPR spectrum (red line) recorded on a flash-frozen solution of 3.0 mM Cl−FeIII/L and 50 mM MMMP in EHMA and anisole (3:1, v/v) at −153 °C (120 K). The second spectrum (black line) was recorded on the same solution and also at −153 °C, but after applying 40 laser pulses at −40 °C. Reproduced with permission from ref 21.

Listed in Table 6 are kdeact values reported for three Fe- and Cu-based ATRP catalysts involving polymeric methacrylatetype radicals.21,27,57 Further kdeact values for the deactivation of methacrylate- and acrylate-type radicals with some frequently used Cu catalysts are found in the thesis of N. Soerensen.57 It is apparent from both the absolute value of kdeact and from the measured activation energy, Ea(kdeact), that deactivation occurs under chemical control. At 60 °C, kdeact data reported for the amine−bis(phenolate)iron(III) chloride complex, [O2NN′]FeIIICl, is by approximately 1 order of magnitude below the value reported for [FeIIIBr4]− and by about 2 orders of magnitude below kdeact for the CuIIBr2(HMTETA) complex.19,57 This difference decreases toward higher temperature because of the high Ea(kdeact) for [O2NN′]FeIIICl of about 35 kJ mol−1. RDRPs with this complex should be carried out at elevated temperature providing an increased ratio of deactivation to propagation rate (Ea(kp,EHMA) ≈ 20 kJ mol−1),148 which is associated with better control. The deactivation rate is expected to be higher for the bromine analogue, [O2NN′]FeIIIBr,21 and thus closer to the values for [FeBr4]− and CuIIBr2(HMTETA). The difference in the size of the ester side chain, i.e., in between MMA, EHMA, and DMA, should result in only minor effects on kdeact, since deactivation occurs under chemical control and the side chain is located relatively far off the carbon-centered radical site.21 The kinetic data available so far show that the precise knowledge of kdeact in an extended temperature range is essential for identifying suitable polymerization conditions. The SP-PLP-EPR measurement of kdeact should thus be applied to further ATRP catalysts and monomer classes. The technique allows for the analysis of kdeact even for systems in which, under

Table 6. kdeact at 60 °C and Activation Energy, Ea(kdeact), for ATRP of Methacrylate-Type Radicals Mediated by Different Deactivator Species entry

deactivator

monomer

kdeact at 60 °C (L mol−1 s−1)

1 2 3

[O2NN′]FeIIICl [TBA][FeIIIBr4]a CuIIBr2(HMTETA)b

EHMA MMAc DMA

2.7 × 104 5.0 × 105 2.2 × 106

Ea (kJ mol−1) 35 ± 5 21.5 ± 5

ref 21 27 19

a TBA = tetrabutylammonium. bHMTETA = 1,1,4,7,10,10-hexamethyltriethylenetetramine. cPMMA-Br of molar mass 8000 g mol−1 dissolved in 2butanone is used as the initiator.

K


Perspective

Macromolecules

Figure 18. (A) Relative radical concentration, [R]/[R]0, vs time traces for SP-PLP-EPR measurements at 0 °C in solution of DMA:2-butanone (70:30 v/v) with 10 mM [FeIIBr3(solv)]− and without FeII. (B) SPR concentration vs time traces measured at −60 °C in a solution of BA:2butanone (85:15 v/v) at different levels of [FeIIBr3(solv)]−. Adapted with permission from ref 20.

Scheme 4. Mechanism of FeII/L-Catalyzed Radical Termination (CRT)a

amounts of the metal catalyst. ATRP procedures like ICAR or activator regenerated by electron transfer (ARGET) ATRP using ppm levels of the catalyst are thus preferable for acrylate polymerization.20,149

a

kadd refers to the rate-controlling step of formation of the organometallic intermediate.

Tmol % =

( 1 −1 X )

k tFe[Fe II] ln

k p[RX]0

(9)

It appears rewarding to check for CRT with monomers such as vinyl acetate, which are difficult to polymerize via ATRP. It should be noted that the CRT reaction presents just one type of organometallic reaction that may occur simultaneously with ATRP. As will be shown in section 5, also OMRP processes may occur.

5. INTERPLAY OF ATRP AND OMRP Various studies have outlined the potential interplay of ATRP and organometallic reactions,24,27−36 specifically with catalytic chain transfer mediated by Fe−diimine species30,32,33,150,151 or with OMRP mediated by amine−bis(phenolate)iron complexes.29,36 As illustrated in the upper part of Figure 20, an FeII/ L complex may participate in both the ATRP and the OMRP equilibrium. Mössbauer and UV/vis spectroscopy are particularly useful to identify the Fe species expected for either of the two mechanisms, as shown for studies into RDRP mediated by amine−bis(phenolate)iron catalysts.28 While styrene polymerizations are ATRP-controlled (cf. Figure 14), MMA polymerizations operate via both ATRP and OMRP equilibria; i.e., both Cl−FeIII/L and Rn−FeIII/L are present during the reaction (Figure 20). The ATRP deactivator, Cl−FeIII/L, with maximum absorption at around 520 nm (Figure 20A, purple lines), may react with two propagating MMA-type radicals to the organometallic species, Rn−FeIII/L (orange lines), with maximum absorption at around 430 nm. After extended reaction times of about 20 h at 80 °C, the Rn−FeIII/L species decomposes and Cl−FeIII/L is formed back (see Figure 20A). The Rn−FeIII/L species is also produced by reaction of FeII/L

Figure 19. EPR spectra at −40 °C of the Rn−CuII/L and/or H−CuII/ L complex after 0, 700, and 1100 laser single pulses (SPs) being applied (top) and after heating to 0 °C (bottom). The experiments were performed as described in ref 149, with the data taken from ref 57.

The CRT reaction complicates the ATRP of acrylates when, as in the case of [FeIIBr3(Solv)]−, kCRT is of the order of 1 × 104 L mol−1 s−1 or even above. The percentage of dead chains, Tmol %, produced in ATRP at such fast CRT rates, where catalytic termination is faster than conventional radical−radical termination, may be estimated according to eq 9, which applies irrespective of the selected ATRP method, e.g., normal, reverse or ICAR (initiators for continuous activator regeneration) ATRP.20 Tmol % for a given degree of monomer conversion, X, is a function of the FeII (or CuI) concentration. To reach an acceptable degree of chain-end functionality and thus of low Tmol %, ATRP of acrylates should be carried out at small

Table 7. kadd(BA) at −40 °C and kCRT(BA) at 25 °C for Different Cu-Based and Fe-Based Complexesa entry 1 2 3 a

catalyst II

−

[Fe Br3(Solv)] [CuI(TPMA)]+ [CuI(PMDETA)]+

kadd at −40 °C (L mol−1 s−1)

kCRT at 25 °C (L mol−1 s−1)

ref

(1.5 ± 0.2) × 10 (3.0 ± 0.8) × 105 (9 ± 3) × 103

(7.2 ± 0.7) × 104 (7.0 ± 1.2) × 103 b

20 57, 149 57

4

Cosolvents: MeCN (with Cu) and 2-butanone (with Fe). bApparent value of kCRT measured under stationary conditions. L


Perspective

Macromolecules

Figure 20. (top) Reaction scheme for an Fe-mediated radical polymerization with ATRP and OMRP equilibria being simultaneously involved. Both types of reaction include the FeII/L activator complex and growing radicals, Rn•. Bottom: (A) In MMA polymerization, rapid transformation of Cl− FeIII/L at around 520 nm to Rn−FeIII/L at around 430 nm was observed as represented by the increasing line thickness and the color transition to orange. Cl−FeIII/L is formed back at extended reaction times. (B) Rn−FeIII/L may also be formed by the reaction of FeII/L with propagating radicals in the absence of alkyl halide. Adapted with permission from ref 28.

of styrene under OMRP conditions turned out to be uncontrolled (see Table 8) due to the weak interaction between the growing polymer chain and the Fe mediator.28 The methods and mechanistic insights presented here should be helpful for studies into further Fe-based RDRP systems, such as Fe protein or heme-based catalysts which may be used for the RDRP of water-soluble methacrylates, acrylates, and of Nisopropylacrylamide under biorelevant conditions in aqueous phase at ambient temperature.144−147 The heme-based catalysts may occur in various oxidation states,152,153 and an interplay between ATRP and OMRP should be taken into account. 5.1. Perspectives of Kinetic Studies into OMRP. A variety of transition metals may be used for OMRP catalysis.26,154−156 EPR spectroscopy has already been applied toward the characterization of organometallic species,22,157 but the SP-PLP-EPR method has eluded research efforts so far. This technique should however be well suited for studies into the kinetics of the formation of organometallic species from metal species in the lower oxidation state, including the timeresolved monitoring of propagating radicals via SP-PLP-EPR in combination with the pointwise probing of catalyst concentration via EPR or UV/vis/NIR spectroscopy. OMRP is an interesting alternative to ATRP for monomers, which are difficult to be activated via ATRP but produce organometallic intermediates of sufficient reactivity. Vinyl acetate, e.g., may be polymerized via Fe(acac)2-mediated OMRP,22 whereas ATRP procedures have not been reported for vinyl acetate so far.

with a propagating MMA-type radical (Figure 20B). The reactions in Figure 20 were initiated by V-70. Mössbauer spectroscopy is perfectly suited for confirming UV/vis spectroscopic assignments and for obtaining quantitative information on the relative amounts of individual Fe species. The Mössbauer spectra were recorded on flash-frozen solutions at 80 K during experiments as carried out with UV/vis spectroscopic detection.28 For the reaction of Cl−FeIII/L with MMA-type radicals, the spectrum recorded after 30 min (Figure 21B) indicates the presence of the two FeIII species: Rn−FeIII/L (80%) and Cl−FeIII/L (20%). After 20 h at 80 °C, Cl−FeIII/L (65%) is (partially) regenerated from Rn−FeIII/L (35%) (see Figure 21C). The formation of the organometallic Rn−FeIII/L complex (82%) by the reaction of FeII/L with Rn• may also be observed as shown in Figure 21D,E. It should be noted that the relative importance of ATRP and OMRP may be controlled by proper selection of the polymerization conditions. The formation of dormant species in OMRP requires stoichiometric amounts of the metal catalyst and of propagating radicals, whereas substoichiometric amounts of the catalyst are sufficient to activate the alkyl halide in ATRP (Figure 22). Thus, polymerizations with an excess of alkyl halide relative to Fe are predominantly ATRP-controlled, whereas polymerizations carried out in the absence of (alkyl) halide are exclusively OMRP-controlled. It was also found that higher temperature favors ATRP control due to the lability of the organometallic species. 28 Processes with just one mechanism being dominant may be preferable in that polymer with only one type of chain-end functionality is produced. Catalysts such as the amine−bis(phenolate)iron complexes may be used for different RDRP protocols: Both the ATRP of styrene and the ATRP-OMRP of MMA are well controlled yielding polymeric material with dispersities of Đ = 1.14 and 1.19 (see Table 8), respectively. Moreover, the OMRP of MMA operates in the absence of any (alkyl) halide in a controlled fashion yielding a dispersity, Đ, below 1.3. The polymerization

6. MECHANISTIC ASPECTS OF RAFT POLYMERIZATION The RAFT-specific rate coefficients of addition, kad, and of fragmentation, kfrag, are superimposed on the conventional scheme for radical polymerization. The RAFT equilibrium constant is given by Keq = kad/kfrag. M


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Table 8. Polymerizations of Styrene (Sty) and MMA under ATRP and/or OMRP Control with [O2NN′]FeII 28 entry 1 2 3 4

monomer Sty Sty MMA MMA

initiatora b

PECl V-40c EBrPAd V-40 or AIBNc

T (°C)

mechanism

Đ

100 110 120 110

ATRP uncontrolled ATRP−OMRP OMRP

1.14 >1.5 1.19 1.29

a PECl = 1-phenylethyl chloride, V-40 = 2,2′-azobis(4-methoxy-2.4dimethylvaleronitrile), EBrPA = ethyl α-bromophenyl acetate, AIBN = 2,2′-azobis(2-methylpropionitrile). bReaction conditions: molar concentrations [Sty]:[PECl]:[FeII]:[Cl−FeIII] = 100:1.00:0.17:0.04, styrene:anisole = 3:1 (v/v), reaction time 23 h. cReaction conditions: [MMA]:[FeII]:[azo initiator] = 100:1.00:1.00, MMA:anisole = 1:1 (v/ v), reaction time 1 h. dReaction conditions: [MMA]:[FeII]:[EBrPA] = 100:1.00:1.00, MMA:anisole = 1:1 (v/v), reaction time 1 h.

Scheme 5. RAFT Main Equilibrium

Figure 21. 57Fe Mö ssbauer spectrum of (A) the amine−bis(phenolate)iron(III) chloride catalyst, Cl−FeIII/L, recorded at 80 K. (B) Spectrum recorded on an MMA polymerization carried out with V-70 (200 mM) and Cl−FeIII/L (50 mM). After 30 min at 70 °C, both Rn−FeIII/L (80%) and Cl−FeIII/L (20%) were observed. (C) After 20 h at 80 °C, Cl−FeIII/L (65%) and Rn−FeIII/L (35%) were found. (D) Powder spectrum of FeII/L. (E) Rn−FeIII/L in 82% yield resulting from reaction of FeII/L (50 mM) with MMA-type radicals generated via V-70 (500 mM) at 60 °C. Reproduced in part with permission from ref 28.

Figure 23. SP-PLP-EPR traces of INT• and P• species after applying a laser single pulse, at t = 0, during a trithiocarbonate-mediated butyl acrylate polymerization at −40 °C.15 Reproduced with permission from ref 15. Copyright 2013 Wiley-VCH.

trithiocarbonate-mediated polymerization of butyl acrylate at −40 °C. The SP-PLP-EPR traces for INT• and for P• have been measured after firing a laser single pulse at t = 0 in separate experiments, each at a magnetic field position, which is characteristic of INT• and of P•, respectively.15 Kinetic information may be obtained from EPR by two methods: (1) Time-resolved traces as in Figure 23 are fitted, via Predici, to a kinetic scheme including initiation, propagation, and chain-length-dependent termination with this information being available from the literature, plus the RAFT rate coefficients, kad and kfrag, and the cross-termination rate coefficient, ktcross, for reaction between INT• and P•.15 As shown in Figure 25 below, the SP-PLP-EPR traces are almost insensitive toward the size of ktcross. Thus, kad and kfrag are accessible as the only unknowns from simultaneous fitting of the INT• and P• concentration vs time traces. The SP-PLPEPR method is, however, laborious in that calibration of both types of radicals is required under conditions as close as possible to the ones of the RAFT experiment. (2) Within the

Figure 22. Ratio of the metal catalyst, FeII, to alkyl halide may be up to 1:100 in ATRP, whereas stoichiometric amounts of FeII and propagating radicals are required for OMRP.

Only the so-called main equilibrium is illustrated in Scheme 5. For the initial pre-equilibrium period, the Pm moiety has to be replaced by the leaving group R of the RAFT agent Z−C( S)SR. The situation of having two such equilibria may be simplified by selecting R such that the leaving radical is more or less identical to the type of propagating radical of the monomer under investigation. The advantage of SP-PLP-EPR studies into the time evolution of more than one type of radicals enables the quantitative measurement of both propagating radicals, Pn•, and intermediate radicals, INT• (see Scheme 5). This favorable EPR spectroscopic situation is illustrated in Figure 23 for a N


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radicals of chain length i. Assuming kt,cross = kti,i corresponds to an upper limiting value for kt,cross. The data in Figure 25 demonstrate that cross-termination does not significantly affect the radical kinetics in BMPTmediated BA polymerization at −40 °C. The rate coefficients obtained upon selecting kt,cross ≈ 0.1kti,i read kad = (4.1 ± 0.9) × 106 L mol−1 s−1 and kfrag = 45 ± 5 s−1, which yields Keq = (9 ± 3) × 104 L mol−1. Applying the second EPR-based method to the same system, i.e., to BMPT-mediated BA polymerization (1.5 mol L−1 in toluene) at −40 °C using MMMP (1.0 × 10−2 mol L−1) as the photoinitiator, yields the data in Figure 26. The ratios of the

second EPR-based method, the ratio of the characteristic EPR intensities is measured, which is identical to the ratio of the INT• to P• concentrations, cINT•/cP•. This ratio has to be determined for several initial RAFT concentrations during quasi-stationary conditions of continuous laser pulsing. The method needs no EPR calibration for absolute concentration. Plotting the ratio of the two EPR intensities vs the initial RAFT concentration yields the ratio of the two RAFT rate coefficients, kad/kfrag = Keq, according to eq 10.158 c INT •/c P • = kad /k fragc RAFT = Keqc RAFT

(10)

158

As pointed out by Kwak et al., eq 10 is valid in the case of cross-termination and self-termination rates being negligible compared to the addition and fragmentation rates.158 These requirements are mostly met but need to be checked for each system under investigation. SP-PLP-EPR data for a butyl acrylate polymerization (1.5 mol L−1 in toluene) mediated by S-S′-bis(methyl-2-propionate) trithiocarbonate (BMPT; for the structure see Figure 24) at

Figure 26. Ratio of intermediate radical and propagating radical concentrations plotted vs the initial BMPT concentration for BA polymerizations (1.5 mol L−1 in toluene) at −40 °C using MMMP (1.0 × 10−2 mol L−1) as the photoinitiator. The slope to the straight line fit yields Keq. Reproduced with permission from ref 15. Copyright 2013 Wiley-VCH.

Figure 24. Structure of S,S′-bis(methyl-2-propionate) trithiocarbonate (BMPT).

two radical EPR intensities, i.e., of cINT·/cP· plotted against initial BMPT concentration, closely fit to a straight line. The slope to this line yields Keq = (7.0 ± 0.2) × 104 L mol−1, which is in very satisfactory agreement with Keq = (9 ± 3) × 104 L mol−1 obtained from the individual traces in Figure 25. It should further be noted that the two experimental Keq values are fairly close to the ab initio Keq value estimated for BMPT-mediated methyl acrylate polymerization at −30 °C.159 This agreement demonstrates that quantum chemistry allows for adequate predictions in the case of trithiocarbonatemediated RAFT polymerization. Other than with trithiocarbonates, the mechanism of RAFT polymerizations mediated by dithiobenzoates (DTBs, C6H5C(S)−SR) has been very controversially discussed (for the associated RAFT equilibrium see Scheme 6). The Figure 25. Comparison of simulated and experimental concentration vs time profiles of propagating and intermediate radicals for a BA polymerization (1.5 mol L−1 in toluene) at −40 °C with BMPT being the RAFT agent (3.5 × 10−5 mol L−1) and MMMP being the photoinitiator (1.0 × 10−2 mol L−1). The simulations were carried out for the full range of reasonable cross-termination rate coefficients, ktcross, i.e., between 0 and kti,i. Reproduced with permission from ref 15. Copyright 2013 Wiley-VCH.

Scheme 6. RAFT Equilibrium for Dithiobenzoates

status of this dilemma has been summarized by Moad in a recent trend report.160 The observed lowering of DBTmediated polymerization rate has been assigned to the limiting situations of either slow fragmentation (SF) of the INT• species161 or cross-termination (CT) of INT• with propagating radicals, i.e., to intermediate radical termination,158,162 thereby reducing overall radical concentration and thus polymerization rate.

−40 °C are shown in Figure 25.15 The RAFT rate coefficients, kad and kfrag, were deduced from kinetic modeling, e.g., with the program package Predici. As the rate coefficient for crosstermination of INT• and P• is not available from the literature, the impact of kt,cross on the kinetics has been elucidated by simulation adopting the limiting cases of kt,cross = 0 and kt,cross = kti,i, with kti,i referring to the termination rate coefficient of two O


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ETTP-mediated acrylate polymerization. RAFT polymerizations with ETTP are capable of providing a general picture of DTB-mediated acrylate polymerizations, as the INT• species from ETTP is identical to the one occurring during cumyl DB (CDB)- and cyanoisopropyl DB (CPDB)-mediated butyl acrylate polymerizations. This common resonance-stabilized INT• species is depicted on the right-hand side of Scheme 7.18 Shown in Figure 27 are the concentrations of propagating and, on the right-hand side, of intermediate radicals measured via the SP-PLP-EPR technique during an ETTP-mediated BA polymerization (1.5 mol L−1 in toluene) at −40 °C.18 As detailed in ref 18, modeling of the experimental data in Figure 27 yields the following RAFT rate coefficients for −40 °C: kad = (1.4 ± 0.4) × 106 L mol−1 s−1 and kfrag = 4.7 ± 1.5 s−1 with the cross-termination rate coefficient being ktcross = 0.25kti,i. From kad and kfrag, the equilibrium constant at −40 °C is obtained to be Keq = (3.4 ± 0.6) × 105 L mol−1. The SF model is obviously in conflict with the measured INT• concentration vs time trace, as is indicated by the dashed line in the righthand-side plot of Figure 27, which refers to an upper limiting value for kfrag of 0.01 s−1 according to the slow-fragmentation model. Typical SF-model values for kfrag would result in an even weaker decay of the INT• concentration with time after pulsing. The second method for Keq determination, via cINT·/cP·, yields the straight-line plot shown in Figure 28 with the slope corresponding to Keq = (2.3 ± 0.6) × 105 L mol−1.

The discrepancy between the SF and CT models, which both account for the observed rate reduction, is enormous in that the associated kfrag values differ by several orders of magnitude,17,18 which is a nightmare for each kineticist. The SP-PLP-EPR method should be perfectly suited toward solving this issue, as will be demonstrated by experiments into butyl acrylate polymerized with ethyl S-thiobenzoyl-2-thiopropionate (ETTP) as the RAFT agent. The structure of ETTP (see Scheme 7) is advantageous in that the leaving group is Scheme 7. Structure of Several Dithiobenzoate (DTB) RAFT Agents Together with the Common Resonance-Stabilized INT• Radical Occurring during Butyl Acrylate RAFT Polymerizations Mediated by These DTBs (Reproduced with Permission from Ref 18)

Figure 28. Ratio of intermediate radical and propagating radical concentrations, cINT·/cP·, plotted vs ETTP concentration for BA polymerizations (1.5 mol L−1 in toluene) at −40 °C. Reproduced with permission from ref 18.

chemically equivalent to the one occurring during acrylate polymerizations mediated by ETTP. Thus, the difference between the pre-equilibrium and main equilibrium situations becomes minor. Associated to this is the advantage of only two types of radical species being present during the course of an

Figure 27. Experimental and Predici-simulated concentration vs time traces for propagating (left) and intermediate (right) radicals during an ETTPmediated BA polymerization (1.5 mol L−1 in toluene) at −40 °C. The initial ETTP and MMMP concentrations were 2.0 × 10−5 and 1.0 × 10−2 mol L−1, respectively. The kinetic parameters introduced into the Predici simulation were kp = 2.27 × 103 L mol−1 s−1, ki = 10 × kp, c0R = 1.30 × 10−5 mol L−1, kt(i,i) = 1.65 × 108 L mol−1 s−1, αs = 0.85, αl = 0.22, ic = 30, and ktcross(i) = 0.25 × kt(i,i).18 The dashed red line in the right figure refers to kfrag of 0.01 s−1, which would be a typical value according to the slow-fragmentation model. Adapted with permission from ref 18. P


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products of acrylate polymerizations170 but may occur in the case of DTB-mediated styrene polymerizations.158 The validity of the CT model, despite the missing significant amount of three-arm star products in DTB-mediated acrylate polymerizations, may be understood by adding a new reaction step to the CT model, for which we coined the term “missing step”.171 This reaction, which is illustrated in Scheme 8, occurs

The Keq values from the two independent EPR-based methods again agree within the limits of experimental accuracy. Recent ab initio calculations163 in which local coupled cluster calculations have been applied in a tailored conjugate method, including basis set extrapolation and solvation effects, yield an equilibrium value for ETTP-mediated BA polymerization in a toluene environment at −40 °C of 1.5 × 105 L mol−1, which is close to the experimental Keq values and meets the expectations from the intermediate radical termination model but is in conflict with the SF model.163 For a further comparison of experimental data with quantumchemical estimates, the RAFT equilibrium constant, Keq, has been determined for the model system cyanoisopropyl DTB− cyanoisopropyl radical at temperatures as used in RAFT polymerizations.17 The experimental value for 70 °C, Keq = 9 ± 1 L mol−1, is remarkably close to the one from recent ab initio calculations also referring to a toluene environment, Keq = 5.5 L mol−1.163 This ab initio value should be correct within 1 order of magnitude. Both values are by orders of magnitude below the numbers from earlier ab initio calculations for the same temperature: Keq = 2.35 × 105 L mol−1 (in the gas phase)164 and 2.31 × 104 L mol−1 (after correction to the experimental solvent environment).165 Such high Keq values, which are not reproduced by the recent quantum-chemical estimates for DTBs, had been considered as support of the slowfragmentation model. The experimental and the recent ab initio value for Keq demonstrate that intermediate radical termination rather than slow fragmentation is the correct explanation for the observed reduction of polymerization rate with DTB-mediated acrylate (and styrene) polymerizations.158 That intermediate radical termination is the major reason for rate retardation has also been shown by a comparison of experimental data for polystyryl-mediated styrene RAFT polymerizations in miniemulsion and in bulk.166 The recent trend article by Moad also concludes that there appears to be overwhelming evidence for intermediate radical termination being the primary cause for retardation in DTB-mediated RAFT polymerization.160 The failure of the slow-fragmentation model to represent DTB-mediated acrylate and styrene polymerizations does not exclude that slow fragmentation may occur with some dithiobenzoate INT• species. Such a situation is expected in the case of poorly stabilized leaving moieties, e.g., of methyl or tert-butyl radicals. As has been reported by Chernikova et al.167 and by Ranieri et al.168 on the basis of EPR trapping experiments, tert-butyl DTB is slowly fragmenting. This finding should however not be transferred to acrylate and styrene polymerizations mediated with the DTBs listed in Scheme 7, which have been the systems under debate in the discussion about slow-fragmentation vs cross-termination models. The tert-butyl radical is far less stabilized than styryl and (meth)acrylate radicals and does not resemble the radicals encountered in DTB-mediated RAFT polymerizations of typical monomers, such as styrene and (meth)acrylates.169 The controversy on DTB-mediated RAFT polymerization between supporters of the slow-fragmentation model on the one side and supporters of the cross-termination or intermediate radical termination model on the other side was about the fact that the SF hypothesis was in conflict with the measured low INT• concentrations whereas the CT model appeared unsatisfactory, as no significant amount of three-arm star material from cross-termination was found among the

Scheme 8. Illustration of One out of a Plethora of Missing Step Reactions between a Growing Radical and a CrossTermination Product, Pk-Int, in Dithiobenzoate-Mediated Radical Polymerization of Acrylates171

between a highly reactive acrylate radical and a labile crosstermination product to form a resonance-stabilized INT• species and a stable product from reaction of two radicals. There is a strong driving force behind this “missing step”. Scheme 9 demonstrates the catalytic cycle with the net reaction Scheme 9. Catalytic Radical Termination in Acrylate Polymerizations Mediated by Dithiobenzoate-Type RAFT Agents (Reproduced with Permission from Ref 171. Copyright 2006 Wiley-VCH)

being the termination of two radicals. The reaction proceeds via (several types of) Pk-Int species and may occur by combination, as shown in Scheme 8, but also by disproportionation. The catalytic reaction accounts for the concentration of INT• staying more or less constant despite the occurrence of crosstermination and explains the low concentration of the product from CT, Pk-Int, in the final reaction mixture of RAFT polymerizations. The reduction of RAFT polymerization rate is understood as a consequence of the loss of propagating radicals due to catalyzed termination. Products of such “missing step” reactions have recently been detected with several DTBmediated RAFT polymerizations and with model reactions.13−18 The two EPR methods, in particular, the SP-PLP-EPR method which provides direct access to the individual RAFT rate coefficients of addition and of fragmentation, are available for general and extensive use in solving kinetic and mechanistic questions of RAFT polymerization. Q


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7. SUMMARY AND CLOSING REMARKS The potential of the SP-PLP-EPR technique for kinetic and mechanistic studies into the entire field of radical polymerization is enormous. The time evolution of the reacting species, i.e., of the radicals, is directly and quantitatively monitored after instantaneous production of an intense burst of primary radicals. SP-PLP-EPR provides unrivaled access to the measurement of chain-length-dependent termination in organic and aqueous solution. Even for systems with two types of radical species, e.g., with SPRs and MCRs occurring as a consequence of backbiting, SP-PLP-EPR enables the comprehensive analysis of the rate coefficients of termination, transfer, and propagation of SPRs and MCRs. In case that the conventional PLP-SEC method runs into difficulties, i.e., with very slowly terminating monomers, SP-PLP-EPR may allow for the determination of propagation rate coefficients. The SP-PLP-EPR technique has also been applied toward measuring the rate coefficients of addition and fragmentation in RAFT polymerization, thus providing evidence in favor of the intermediate radical termination mechanism with dithiobenzoate-mediated RAFT polymerizations. SP-PLP-EPR furthermore allows for the direct measurement of ATRP deactivation rate. A recently discovered reaction relevant to ATRP is the catalytic termination of propagating radicals by FeII or CuI species, which was quantified via SP-PLP-EPR. The technique is also applicable toward studies into other types of organometallic reactions, e.g., the ones associated with OMRP. Investigations into ATRP and OMRP benefit from additionally applying UV/vis/NIR in conjunction with Mössbauer spectroscopy, particularly in situations where both mechanisms occur simultaneously. Recent reports have demonstrated applications of UV/vis spectroscopy toward ATRP in aqueous phase172−179 with emphasis on comproportionation and disproportionation issues of Cu species and on Cu-mediated activation of alkyl halides.179,180 The detailed information provided by SP-PLPEPR should also be valuable for checking novel ideas about radical polymerization kinetics.181−186

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awarded a Heisenberg Fellowship by the German Science Foundation (DFG). He became Professor for Applied Physical Chemistry at the University of Göttingen in 1981, Full Professor for Technical and Macromolecular Chemistry in 1995, and Member of the Göttingen Academy of Sciences in 2000. Michael Buback received the Dechema Award, the Bunsen-Denkmünze, and the Herman F. Mark Medal. He has published over 300 peer-reviewed papers. His research interests cover the entire field of radical polymerization with a focus on detailed kinetic studies via pulsed laser initiation carried out in conjunction with highly time-resolved IR, near-IR, and EPR spectroscopy. Further activities address the kinetics and the phase behaviour of chemical processes in extended ranges of pressure and temperature. Special expertise centers around the quantitative monitoring, via online vibrational spectroscopy, of species occurring during chemical transformations at pressures up to 7000 bar.

Hendrik Schröder earned his doctorate degree at the University of Göttingen, Germany, in 2015 under the supervision of Michael Buback. He was a graduate student in the Catalysis for Sustainable Synthesis (CaSuS) International Ph.D. program and a recipient of the Fonds der Chemischen Industrie scholarship. During this time, he authored and coauthored 13 publications in international peerreviewed journals. His research interest revolves around the mechanism and kinetics of metal-mediated radical polymerizations and employs various state-of-the-art spectroscopic techniques, such as highly time-resolved EPR spectroscopy in conjunction with laser pulse initiation, 57Fe Mössbauer spectroscopy, and online NIR spectroscopy at pressures up to 7000 bar.

AUTHOR INFORMATION

Corresponding Author

*Fax +49 551 3912709; e-mail [email protected] (M.B.). Notes

The authors declare no competing financial interest. Biographies

Hendrik Kattner is a PhD Student at the University of Göttingen, Germany, working under the supervision of Michael Buback since 2012. He received the Gustav-Tammann award of the Göttingen Faculty of Chemistry and is the author and coauthor of eight papers published in international peer-reviewed journals so far. His primary research interests focus on the application and development of highly

Michael Buback studied chemistry at the University of Karlsruhe, where he received his PhD in 1972. After Habilitation in 1978 he was R


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(28) Schroeder, H.; Lake, B. R. M.; Demeshko, S.; Shaver, M. P.; Buback, M. Macromolecules 2015, 48, 4329−4338. (29) Allan, L. E. N.; MacDonald, J. P.; Nichol, G. S.; Shaver, M. P. Macromolecules 2014, 47, 1249−1257. (30) Shaver, M. P.; Allan, L. E. N.; Gibson, V. C. Organometallics 2007, 26, 4725−4730. (31) Poli, R. Chem. - Eur. J. 2015, 21, 6988−7001. (32) Shaver, M. P.; Allan, L. E. N.; Rzepa, H. S.; Gibson, V. C. Angew. Chem., Int. Ed. 2006, 45, 1241−1244. (33) Gibson, V. C.; O’Reilly, R. K.; Wass, D. F.; White, A. J. P.; Williams, D. J. Macromolecules 2003, 36, 2591−2593. (34) Poli, R. Eur. J. Inorg. Chem. 2011, 2011, 1513−1530. (35) Poli, R. Angew. Chem., Int. Ed. 2006, 45, 5058−5070. (36) Allan, L. E. N.; MacDonald, J. P.; Reckling, A. M.; Kozak, C. M.; Shaver, M. P. Macromol. Rapid Commun. 2012, 33, 414−418. (37) Mössbauer, R. L. Eur. Phys. J. A 1958, 151, 124. (38) Mössbauer, R. L. Naturwissenschaften 1958, 45, 538. (39) Gütlich, P. Z. Anorg. Allg. Chem. 2012, 638, 15−43. (40) Barner-Kowollik, C.; Russell, G. T. Prog. Polym. Sci. 2009, 34, 1211−1259. (41) Achilias, D. S. Macromol. Theory Simul. 2007, 16, 319−347. (42) Barth, J.; Buback, M. Macromolecules 2011, 44, 1292−1297. (43) Benson, S. W.; North, A. M. J. Am. Chem. Soc. 1959, 81, 1339− 1345. (44) Benson, S. W.; North, A. M. J. Am. Chem. Soc. 1962, 84, 935− 940. (45) Smith, G. B.; Russell, G. T.; Heuts, J. P. A. Macromol. Theory Simul. 2003, 12, 299−314. (46) Mahababi, H. K.; O’Driscoll, K. F. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 283−300. (47) Piton, M. C.; Gilbert, B. C.; Kuchel, P. W. Macromolecules 1993, 26, 4472−4477. (48) Johnston-Hall, G.; Monteiro, M. J. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 3155−3173. (49) Friedman, B.; O’Shaughnessy, B. Macromolecules 1993, 26, 5726−5739. (50) Karatekin, E.; O’Shaughnessy, B.; Turro, N. J. Macromol. Symp. 2002, 182, 81−101. (51) Khokhlov, A. Makromol. Chem., Rapid Commun. 1981, 2, 633− 636. (52) Olaj, O. F.; Zifferer, G. Macromolecules 1987, 20, 850−861. (53) Barth, J.; Buback, M.; Russell, G. T.; Smolne, S. Macromol. Chem. Phys. 2011, 212, 1366−1378. (54) Kattner, H.; Buback, M. Macromolecules 2015, 48, 309−315. (55) Johnston-Hall, G.; Theis, A.; Monteiro, M. J.; Davis, T. P.; Stenzel, M. H.; Barner-Kowollik, C. Macromol. Chem. Phys. 2005, 206, 2047−2053. (56) Kattner, H.; Buback, M. Macromol. Chem. Phys. 2014, 215, 1180−1191. (57) Soerensen, N. Kinetics and Mechanism of Cu-Catalyzed Atom Transfer Radical Polymerization. Dissertation, Georg-August-University, Goettingen, 2015. (58) Smith, G. B.; Russell, G. T. Z. Phys. Chem. 2005, 219, 295−323. (59) Barth, J.; Buback, M. Macromol. Rapid Commun. 2009, 30, 1805−1811. (60) Barth, J.; Buback, M.; Hesse, P.; Sergeeva, T. Macromolecules 2009, 42, 481−488. (61) Buback, M.; Egorov, M.; Junkers, T.; Panchenko, E. Macromol. Chem. Phys. 2005, 206, 333−341. (62) Wittenberg, N. F. G.; Preusser, C.; Kattner, H.; Stach, M.; Lacík, I.; Hutchinson, R. A.; Buback, M. Macromol. React. Eng. 2015, DOI: 10.1002/mren.201500017. (63) Buback, M. Macromol. Symp. 2009, 275−276, 90−101. (64) Lacík, I.; Ucnová, L.; Kukucková, S.; Buback, M.; Hesse, P.; Beuermann, S. Macromolecules 2009, 42, 7753−7761. (65) Schrooten, J.; Buback, M.; Hesse, P.; Hutchinson, R. A.; Lacik, I. Macromol. Chem. Phys. 2011, 212, 1400−1409. (66) Lacík, I.; Beuermann, S.; Buback, M. Macromol. Chem. Phys. 2004, 205, 1080−1087.

time-resolved single-pulse EPR-spectroscopic studies into radical polymerization. He is a task group member of the IUPAC project Critically evaluated EPR spectra of important polymerization-related radicals.

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ACKNOWLEDGMENTS The authors are grateful to German Science Foundation (DFG) for generous funding of the original research summarized in this article and for support and stimulation by BASF SE over many years. H.S. gratefully acknowledges a Ph.D. fellowship granted by the Fonds der Chemischen Industrie. We are very grateful for stimulating discussions and common research activities with many excellent colleagues resulting in a multitude of joint papers as listed in the references section.

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REFERENCES

(1) Olaj, O. F.; Bitai, I.; Hinkelmann, F. Makromol. Chem. 1987, 188, 1689−1702. (2) Buback, M.; Gilbert, R. G.; Russell, G. T.; Hill, D. J. T.; Moad, G.; O’Driscoll, K. F.; Shen, J.; Winnik, M. A. J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 851−863. (3) Buback, M.; Hippler, H.; Schweer, J.; Vögele, H.-P. Makromol. Chem., Rapid Commun. 1986, 7, 261−265. (4) Beuermann, S.; Buback, M. Prog. Polym. Sci. 2002, 27, 191−254. (5) Buback, M.; Egorov, M.; Junkers, T.; Panchenko, E. Macromol. Rapid Commun. 2004, 25, 1004−1009. (6) Barth, J.; Buback, M. Macromol. React. Eng. 2010, 4, 288−301. (7) Kattner, H.; Buback, M. Macromol. Symp. 2013, 333, 11−23. (8) Yamada, B.; Westmoreland, D. G.; Kobatake, S.; Konosu, O. Prog. Polym. Sci. 1999, 24, 565−630. (9) Kattner, H.; Buback, M. Macromolecules 2015, 48, 7410−7419. (10) Barth, J.; Buback, M.; Hesse, P.; Sergeeva, T. Macromol. Rapid Commun. 2009, 30, 1969−1974. (11) Barth, J.; Buback, M.; Hesse, P.; Sergeeva, T. Macromolecules 2010, 43, 4023−4031. (12) Kattner, H.; Buback, M. Macromol. Rapid Commun. 2015, 36, 2186−2191. (13) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2005, 58, 379−410. (14) Sidoruk, A.; Buback, M.; Meiser, W. Macromol. Chem. Phys. 2013, 214, 1738−1748. (15) Meiser, W.; Buback, M.; Sidoruk, A. Macromol. Chem. Phys. 2013, 214, 2108−2114. (16) Meiser, W.; Buback, M.; Ries, O.; Ducho, C.; Sidoruk, A. Macromol. Chem. Phys. 2013, 214, 924−933. (17) Meiser, W.; Buback, M. Macromol. Rapid Commun. 2011, 32, 1490−1494. (18) Meiser, W.; Barth, J.; Buback, M.; Kattner, H.; Vana, P. Macromolecules 2011, 44, 2474−2480. (19) Soerensen, N.; Barth, J.; Buback, M.; Morick, J.; Schroeder, H.; Matyjaszewski, K. Macromolecules 2012, 45, 3797−3801. (20) Schroeder, H.; Buback, M. Macromolecules 2014, 47, 6645− 6651. (21) Schroeder, H.; Buback, M. Macromolecules 2015, 48, 6108− 6113. (22) Xue, Z. G.; Poli, R. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3494−3504. (23) Xue, Z.; He, D.; Xie, X. Polym. Chem. 2015, 6, 1660−1687. (24) Poli, R.; Allan, L. E. N.; Shaver, M. P. Prog. Polym. Sci. 2014, 39, 1827−1845. (25) di Lena, F.; Matyjaszewski, K. Prog. Polym. Sci. 2010, 35, 959− 1021. (26) Allan, L. E. N.; Perry, M. R.; Shaver, M. P. Prog. Polym. Sci. 2012, 37, 127−156. (27) Schroeder, H.; Buback, J.; Demeshko, S.; Matyjaszewski, K.; Meyer, F.; Buback, M. Macromolecules 2015, 48, 1981−1990. S


Perspective

Macromolecules

(100) Horn, M.; Matyjaszewski, K. Macromolecules 2013, 46, 3350− 3357. (101) Pintauer, T.; Braunecker, W.; Collange, E.; Poli, R.; Matyjaszewski, K. Macromolecules 2004, 37, 2679−2682. (102) Seeliger, F.; Matyjaszewski, K. Macromolecules 2009, 42, 6050− 6055. (103) Braunecker, W. A.; Tsarevsky, N. V.; Gennaro, A.; Matyjaszewski, K. Macromolecules 2009, 42, 6348−6360. (104) Bortolamei, N.; Isse, A. A.; Di Marco, V. B.; Gennaro, A.; Matyjaszewski, K. Macromolecules 2010, 43, 9257−9267. (105) Fischer, H. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 1885− 1901. (106) Goto, A.; Fukuda, T. Prog. Polym. Sci. 2004, 29, 329−385. (107) Wang, X. S.; Armes, S. P. Abstr. Pap. Am. Chem. Soc. 2000, 219, U395−U395. (108) Fantin, M.; Isse, A. A.; Gennaro, A.; Matyjaszewski, K. Macromolecules 2015, 48, 6862−6875. (109) Konkolewicz, D.; Magenau, A. J. D.; Averick, S. E.; Simakova, A.; He, H. K.; Matyjaszewski, K. Macromolecules 2012, 45, 4461−4468. (110) Averick, S.; Simakova, A.; Park, S.; Konkolewicz, D.; Magenau, A. J. D.; Mehl, R. A.; Matyjaszewski, K. ACS Macro Lett. 2012, 1, 6− 10. (111) Smolne, S.; Buback, M. Macromol. Chem. Phys. 2015, 216, 894−902. (112) Buback, M.; Morick, J. Macromol. Chem. Phys. 2010, 211, 2154−2161. (113) Wang, Y.; Kwak, Y.; Buback, J.; Buback, M.; Matyjaszewski, K. ACS Macro Lett. 2012, 1, 1367−1370. (114) Morick, J.; Buback, M.; Matyjaszewski, K. Macromol. Chem. Phys. 2011, 212, 2423−2428. (115) Morick, J.; Buback, M.; Matyjaszewski, K. Macromol. Chem. Phys. 2012, 213, 2287−2292. (116) Porras, C. T.; D’Hooge, D. R.; Van Steenberge, P. H. M.; Reyniers, M. F.; Marin, G. B. Ind. Eng. Chem. Res. 2014, 53, 9674− 9685. (117) Zerk, T. J.; Bernhardt, P. V. Inorg. Chem. 2014, 53, 11351− 11353. (118) Matyjaszewski, K.; Paik, H. J.; Zhou, P.; Diamanti, S. J. Macromolecules 2001, 34, 5125−5131. (119) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104, 6217−6254. (120) Gopalaiah, K. Chem. Rev. 2013, 113, 3248−3296. (121) Wang, Y.; Zhang, Y. Z.; Parker, B.; Matyjaszewski, K. Macromolecules 2011, 44, 4022−4025. (122) Chen, X. X.; Khan, M. Y.; Noh, S. K. Polym. Chem. 2012, 3, 1971−1974. (123) Mukumoto, K.; Wang, Y.; Matyjaszewski, K. ACS Macro Lett. 2012, 1, 599−602. (124) Okada, S.; Park, S.; Matyjaszewski, K. ACS Macro Lett. 2014, 3, 944−947. (125) Zhu, G. H.; Zhang, L. F.; Zhang, Z. B.; Zhu, J.; Tu, Y. F.; Cheng, Z. P.; Zhu, X. L. Macromolecules 2011, 44, 3233−3239. (126) Zhang, L. F.; Miao, J.; Cheng, Z. P.; Zhu, X. L. Macromol. Rapid Commun. 2010, 31, 275−280. (127) Schroeder, H.; Matyjaszewski, K.; Buback, M. Macromolecules 2015, 48, 4431−4437. (128) Schroeder, H.; Buback, M.; Matyjaszewski, K. Macromol. Chem. Phys. 2014, 215, 44−53. (129) Schroeder, H.; Buback, M.; Shaver, M. P. Macromolecules 2015, 48, 6114−6120. (130) Buback, M.; Kuchta, F. D. Macromol. Chem. Phys. 1995, 196, 1887−1898. (131) Fischer, H. Chem. Rev. 2001, 101, 3581−3610. (132) Tang, W.; Matyjaszewski, K. Macromolecules 2007, 40, 1858− 1863. (133) Gillies, M. B.; Matyjaszewski, K.; Norrby, P. O.; Pintauer, T.; Poli, R.; Richard, P. Macromolecules 2003, 36, 8551−8559. (134) Nanda, A. K.; Matyjaszewski, K. Macromolecules 2003, 36, 8222−8224.

(67) Degirmenci, I.; Ozaltin, T. F.; Karahan, O.; Van Speybroeck, V.; Waroquier, M.; Aviyente, V. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2024−2034. (68) Beuermann, S. Macromol. Rapid Commun. 2009, 30, 1066− 1088. (69) Stach, M.; Lacik, I.; Kasák, P.; Chorvát, D.; Saunders, A. J.; Santanakrishnan, S.; Hutchinson, R. A. Macromol. Chem. Phys. 2010, 211, 580−593. (70) Santanakrishnan, S.; Hutchinson, R. A.; Ucnová, L.; Stach, M.; Lacik, I.; Buback, M. Macromol. Symp. 2011, 302, 216−223. (71) Kattner, H.; Drawe, P.; Buback, M. Macromol. Chem. Phys. 2015, 216, 1737−1745. (72) Drawe, P.; Buback, M. Macromol. Theory Simul. 2016, 25, 74− 84. (73) Buback, M. Makromol. Chem. 1990, 191, 1575−1587. (74) Moehrke, J.; Vana, P. Macromolecules 2015, 48, 3190−3196. (75) Olaj, O. F.; Kornherr, A.; Zifferer, G. Macromol. Theory Simul. 1998, 7, 501−508. (76) Kubota, N.; Kajiwara, A.; Zetterlund, P. B.; Kamachi, M.; Treurnicht, J.; Tonge, M. P.; Gilbert, B. C.; Yamada, B. Macromol. Chem. Phys. 2007, 208, 2403−2411. (77) Drawe, P.; Kattner, H.; Buback, M. 2016, to be published. (78) Lonsdale, D. E.; Johnston-Hall, G.; Fawcett, A.; Bell, C. A.; Urbani, C. N.; Whittaker, M. R.; Monteiro, M. J. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3620−3625. (79) Britton, D.; Heatley, F.; Lovell, P. A. Macromolecules 1998, 31, 2828−2837. (80) Coote, M. L.; Davis, T. P.; Klumperman, B.; Monteiro, M. J. J. Macromol. Sci., Polym. Rev. 1998, 38, 567−593. (81) Potnis, B. S. P.; Deshpande, A. M. Makromol. Chem. 1969, 125, 48−58. (82) McCord, E. F.; Shaw, W. H.; Hutchinson, R. A. Macromolecules 1997, 30, 246−256. (83) Dossi, M.; Storti, G.; Moscatelli, D. Macromol. Symp. 2010, 289, 119−123. (84) Liu, S.; Srinivasan, S.; Grady, M. C.; Soroush, M.; Rappe, A. M. Int. J. Quantum Chem. 2014, 114, 345−360. (85) Willemse, R. X. E.; van Herk, A. M.; Panchenko, E.; Junkers, T.; Buback, M. Macromolecules 2005, 38, 5098−5103. (86) Barth, J.; Meiser, W.; Buback, M. Macromolecules 2012, 45, 1339−1345. (87) Scott, G. E.; Senogles, E. J. Macromol. Sci., Chem. 1970, 4, 1105−1117. (88) Scott, G. E.; Senogles, E. J. Macromol. Sci., Chem. 1974, 8, 753− 773. (89) Scott, G. E.; Senogles, E. J. Macromol. Sci., Polym. Rev. 1973, 9, 49−69. (90) Junkers, T.; Barner-Kowollik, C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 7585−7605. (91) Asua, J. M.; Beuermann, S.; Buback, M.; Castignolles, P.; Charleux, B.; Gilbert, R. G.; Hutchinson, R. A.; Leiza, J. R.; Nikitin, A. N.; Vairon, J. P.; van Herk, A. M. Macromol. Chem. Phys. 2004, 205, 2151−2160. (92) Lacík, I.; Beuermann, S.; Buback, M. Macromolecules 2003, 36, 9355−9363. (93) Roedel, M. J. J. Am. Chem. Soc. 1953, 75, 6110−6112. (94) Woodbrey, J. C.; Ehrlich, P. J. Am. Chem. Soc. 1963, 85, 1580− 1584. (95) Toh, J.; Huang, D.; Lovell, P. A.; Gilbert, B. C. Polymer 2001, 42, 1915−1920. (96) Ahmad, N. M.; Heatley, F.; Lovell, P. A. Macromolecules 1998, 31, 2822−2827. (97) Buback, M.; Hesse, P.; Junkers, T.; Sergeeva, T.; Theis, T. Macromolecules 2008, 41, 288−291. (98) Tang, W.; Tsarevsky, N. V.; Matyjaszewski, K. J. Am. Chem. Soc. 2006, 128, 1598−1604. (99) Tang, W.; Kwak, Y.; Braunecker, W.; Tsarevsky, N. V.; Coote, M. L.; Matyjaszewski, K. J. Am. Chem. Soc. 2008, 130, 10702−10713. T


Perspective

Macromolecules (135) Wang, Y.; Matyjaszewski, K. Macromolecules 2011, 44, 1226− 1228. (136) Wang, Y.; Matyjaszewski, K. Macromolecules 2010, 43, 4003− 4005. (137) Teodorescu, M.; Gaynor, S. G.; Matyjaszewski, K. Macromolecules 2000, 33, 2335−2339. (138) Schroeder, H. Metal-Catalyzed Radical Polymerization up to High Pressure. Ph.D. Dissertation, Georg-August-University, Goettingen, 2015. (139) Przyojski, J. A.; Arman, H. D.; Tonzetich, Z. J. Organometallics 2012, 31, 3264−3271. (140) Dunbar, K. R.; Quillévéré, A. Polyhedron 1993, 12, 807−819. (141) Gill, N. S. J. Chem. Soc. 1961, 3512−3515. (142) Xue, Z. G.; He, D.; Noh, S. K.; Lyoo, W. S. Macromolecules 2009, 42, 2949−2957. (143) Nishizawa, K.; Ouchi, M.; Sawamoto, M. Macromolecules 2013, 46, 3342−3349. (144) Simakova, A.; Mackenzie, M.; Averick, S. E.; Park, S.; Matyjaszewski, K. Angew. Chem., Int. Ed. 2013, 52, 12148−12151. (145) Ng, Y. H.; di Lena, F.; Chai, C. L. L. Chem. Commun. 2011, 47, 6464−6466. (146) Ng, Y. H.; di Lena, F.; Chai, C. L. L. Polym. Chem. 2011, 2, 589−594. (147) Sigg, S. J.; Seidi, F.; Renggli, K.; Silva, T. B.; Kali, G.; Bruns, N. Macromol. Rapid Commun. 2011, 32, 1710−1715. (148) Hutchinson, R. A.; Beuermann, S.; Paquet, D. A.; McMinn, J. H. Macromolecules 1997, 30, 3490−3493. (149) Wang, Y.; Soerensen, N.; Zhong, M. J.; Schroeder, H.; Buback, M.; Matyjaszewski, K. Macromolecules 2013, 46, 683−691. (150) O’Reilly, R. K.; Shaver, M. P.; Gibson, V. C.; White, A. J. P. Macromolecules 2007, 40, 7441−7452. (151) Gibson, V. C.; O’Reilly, R. K.; Reed, W.; Wass, D. F.; White, A. J. P.; Williams, D. J. Chem. Commun. 2002, 1850−1851. (152) Tabard, A.; Cocolios, P.; Lagrange, G.; Gerardin, R.; Hubsch, J.; Lecomte, C.; Zarembowitch, J.; Guilard, R. Inorg. Chem. 1988, 27, 110−117. (153) Guilard, R.; Boisselier-Cocolios, B.; Tabard, A.; Cocolios, P.; Simonet, B.; Kadish, K. M. Inorg. Chem. 1985, 24, 2509−2520. (154) Shipp, D. A. Polym. Rev. 2011, 51, 99−103. (155) Hurtgen, M.; Detrembleur, C.; Jerome, C.; Debuigne, A. Polym. Rev. 2011, 51, 188−213. (156) Peng, C. H.; Yang, T. Y.; Zhao, Y. G.; Fu, X. F. Org. Biomol. Chem. 2014, 12, 8580−8587. (157) Chiang, L.; Allan, L. E. N.; Alcantara, J.; Wang, M. C. P.; Storr, T.; Shaver, M. P. Dalton Trans. 2014, 43, 4295−4304. (158) Kwak, Y.; Goto, A.; Tsujii, Y.; Murata, Y.; Komatsu, K.; Fukuda, T. Macromolecules 2002, 35, 3026−3029. (159) Lin, C. Y.; Coote, M. L. Aust. J. Chem. 2009, 62, 1479−1483. (160) Moad, G. Macromol. Chem. Phys. 2014, 215, 9−26. (161) Feldermann, A.; Coote, M. L.; Stenzel, M. H.; Davis, T. P.; Barner-Kowollik, C. J. Am. Chem. Soc. 2004, 126, 15915−15923. (162) Monteiro, M. J.; de Brouwer, H. Macromolecules 2001, 34, 349−352. (163) Oliveira, J. C. A.; Meiser, W.; Buback, M.; Mata, R. A., to be published. (164) Coote, M. L.; Izgorodina, E. I.; Krenske, E. H.; Busch, M.; Barner-Kowollik, C. Macromol. Rapid Commun. 2006, 27, 1015−1022. (165) Junkers, T.; Barner-Kowollik, C.; Coote, M. L. Macromol. Rapid Commun. 2011, 32, 1891−1898. (166) Suzuki, K.; Kanematsu, Y.; Miura, T.; Minami, M.; Satoh, S.; Tobita, H. Macromol. Theory Simul. 2014, 23, 136−146. (167) Chernikova, E.; Golubev, V.; Filippov, A.; Lin, C. Y.; Coote, M. L. Polym. Chem. 2010, 1, 1437−1440. (168) Ranieri, K.; Delaittre, G.; Barner-Kowollik, C.; Junkers, T. Macromol. Rapid Commun. 2014, 35, 2023−2028. (169) Meiser, W. Investigation of the Kinetics and Mechanism of RAFT Polymerization via EPR Spectroscopy. Ph.D. Dissertation, Georg-August-Universität, Göttingen, 2012.

(170) Feldermann, A.; Ah Toy, A.; Davis, T. P.; Stenzel, M. H.; Barner-Kowollik, C. Polymer 2005, 46, 8448−8457. (171) Buback, M.; Vana, P. Macromol. Rapid Commun. 2006, 27, 1299−1305. (172) Levere, M. E.; Nguyen, N. H.; Percec, V. Macromolecules 2012, 45, 8267−8274. (173) Alsubaie, F.; Anastasaki, A.; Nikolaou, V.; Simula, A.; Nurumbetov, G.; Wilson, P.; Kempe, K.; Haddleton, D. M. Macromolecules 2015, 48, 6421−6432. (174) Alsubaie, F.; Anastaski, A.; Nikolaou, V.; Simula, A.; Nurumbetov, G.; Wilson, P.; Kempe, K.; Haddleton, D. M. Macromolecules 2015, 48, 5517−5525. (175) Harrisson, S.; Nicolas, J. ACS Macro Lett. 2014, 3, 643−647. (176) Konkolewicz, D.; Wang, Y.; Zhong, M. J.; Krys, P.; Isse, A. A.; Gennaro, A.; Matyjaszewski, K. Macromolecules 2013, 46, 8749−8772. (177) Zhong, M. J.; Wang, Y.; Krys, P.; Konkolewicz, D.; Matyjaszewski, K. Macromolecules 2013, 46, 3816−3827. (178) Wang, Y.; Zhong, M. J.; Zhu, W. P.; Peng, C. H.; Zhang, Y. Z.; Konkolewicz, D.; Bortolamei, N.; Isse, A. A.; Gennaro, A.; Matyjaszewski, K. Macromolecules 2013, 46, 3793−3802. (179) Peng, C. H.; Zhong, M. J.; Wang, Y.; Kwak, Y.; Zhang, Y. Z.; Zhu, W. P.; Tonge, M.; Buback, J.; Park, S.; Krys, P.; Konkolewicz, D.; Gennaro, A.; Matyjaszewski, K. Macromolecules 2013, 46, 3803−3815. (180) Ribelli, T. G.; Krys, P.; Cong, Y.; Matyjaszewski, K. Macromolecules 2015, 48, 8428−8436. (181) Konkolewicz, D.; Sosnowski, S.; D’Hooge, D. R.; Szymanski, R.; Reyniers, M. F.; Marin, G. B.; Matyjaszewski, K. Macromolecules 2011, 44, 8361−8373. (182) Konkolewicz, D.; Krys, P.; Matyjaszewski, K. Acc. Chem. Res. 2014, 47, 3028−3036. (183) Ahmad, N. M.; Charleux, B.; Farcet, C.; Ferguson, C. J.; Gaynor, S. G.; Hawkett, B. S.; Heatley, F.; Klumperman, B.; Konkolewicz, D.; Lovell, P. A.; Matyjaszewski, K.; Venkatesh, R. Macromol. Rapid Commun. 2009, 30, 2002−2021. (184) Reyes, Y.; Asua, J. M. Macromol. Rapid Commun. 2011, 32, 63− 67. (185) Ballard, N.; Salsamendi, M.; Santos, J. I.; Ruiperez, F.; Leiza, J. R.; Asua, J. M. Macromolecules 2014, 47, 964−972. (186) Ballard, N.; Rusconi, S.; Akhmatskaya, E.; Sokolovski, D.; de la Cal, J. C.; Asua, J. M. Macromolecules 2014, 47, 6580−6590.

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