Article pubs.acs.org/IECR
ICAR ATRP for Estimation of Intrinsic Macro-Activation/Deactivation Arrhenius Parameters under Polymerization Conditions Carolina Toloza Porras, Dagmar R. D’hooge, Paul H. M. Van Steenberge, Marie-Françoise Reyniers,* and Guy B. Marin Laboratory for Chemical Technology (LCT), Ghent University, Technologiepark 914 B-9052 Zwijnaarde, Gent, East Flanders, Belgium S Supporting Information *
ABSTRACT: The potential of Initiators for Continuous Activator Regeneration Atom Transfer Radical Polymerization (ICAR ATRP) to determine reliable Arrhenius parameters for ATRP activation/deactivation of macrospecies is illustrated using styrene as monomer and CuBr2/TPMA (TPMA: tris(2-pyridylmethyl)amine) as deactivator. Regression is based on an extensive set of experimental data limited to conversions below 0.50 to avoid the interference of diffusional limitations on the activation/ deactivation process and recorded at temperatures below 90 °C to avoid the influence of thermal self-initiation. Diffusional limitations on termination are accounted for based on literature data. The activation energy for the activation and deactivation reaction involving macrospecies are respectively 29 and 1.7 kJ mol−1. The corresponding pre-exponentional factors are 6.9 × 105 and 1.8 × 107 L mol−1 s−1. At 70 °C, the corresponding rate coefficients amount to 2.2 × 10 and 9.9 × 106 L mol−1 s−1, confirming the relatively high activity of CuBr/TPMA as ATRP catalyst.
1. INTRODUCTION Atom transfer radical polymerization (ATRP) has become one of the most important controlled radical polymerization (CRP) techniques,1−8 which allow the synthesis of advanced polymer products with predetermined number-average chain length (xn), narrow chain length distribution, and high livingness.9−11 ATRP is applicable to a wide range of monomers and has been extensively used to obtain complex well-defined macromolecular architectures (e.g., block, gradient, and star copolymers) inaccessible via conventional free radical polymerization (FRP).12,13 As shown in Figure 1a, in ATRP, a transition metal complex (MtnX/L), which is typically formed in situ from a Cu salt (e.g., CuBr) and a ligand (L), is employed as a mediating agent to obtain polymer molecules with end-group functionality (X) via a catalytic cycle starting from the ATRP initiator (R0X). The presence of the ATRP catalyst allows for establishing and maintaining a pseudoequilibrium between dormant species (ΣRiX; i: chain length) and radical species (ΣRi) favoring the dormant state and thus minimizing the occurrence of inevitable termination reactions, which lead to the formation of unwanted dead polymer molecules (P). For fast ATRP initiation, a concurrent take-up of monomer molecules becomes possible so that at complete monomer consumption each dormant polymer chain has approximately a chain length equal to the targeted chain length (TCL = [M]0/[R0X]0). In order to achieve industrially realistic polymerization times, a sufficiently high Cu(I) amount (∼5000−10000 ppm with respect to monomer (molar)) is unfortunately necessary15,16 and, hence, the traditional “normal” ATRP process requires expensive postpolymerization treatments. Purification processes are indispensable, as the copper species in the polymer mixture have to be eliminated to ensure a high product quality and to fulfill environmental regulations. Additionally, the initial use of © 2014 American Chemical Society
air-sensitive Cu(I) activator species requires stringent experimental procedures. Therefore, in the last decades, ATRP research has been mainly focused on the development of alternative initiation procedures in which an important decrease of the amount of Cu catalyst (ideally below 50 ppm) can be achieved while preserving the ATRP activation/deactivation principle and avoiding the difficult handling of Cu(I) species, i.e. only Cu(II) species are initially present.15,17−22 For completeness it is mentioned here that for particular solvent/ ligand combinations, also Cu(0) species can be formed/added complicating the kinetic understanding of transition metal based CRP systems.21,23−26 In initiators for continuous activators regeneration (ICAR) ATRP (Figure 1b), which is the modified ATRP technique studied in this work, the reduction of the catalyst concentration is possible thanks to the presence of conventional radical initiator (I2). Upon I2 dissociation, radicals containing an I endgroup are formed which can (re)generate activator species (MtnX/L) by participation in a redox reaction with deactivator species (Mtn+1X2/L). Recent research14 has shown that typically at least one monomer unit is incorporated before this (re)generation takes place, implying a limited effect of the I related (de)activation rate coefficients, i.e. kaIX and kdaIX (Figure 1b bottom), on the overall ICAR ATRP kinetics. In contrast, the absolute values for the activation/ deactivation rate coefficients involving ATRP initiator (ka0/ kda0; Figure 1b top) and macrospecies (ka/kda; Figure 1b middle) have a significant impact on the success of (ICAR) ATRP processes. It has been especially indicated that the Received: Revised: Accepted: Published: 9674
February 24, 2014 May 14, 2014 May 16, 2014 May 16, 2014 dx.doi.org/10.1021/ie5007596 | Ind. Eng. Chem. Res. 2014, 53, 9674−9685
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Figure 1. Principle of a) “normal” or traditional ATRP and b) ICAR ATRP; ka, kda, kp, and kt are rate coefficients for activation, deactivation, propagation, and termination with subscript 0 corresponding to ATRP initiator and i to macrospecies; ka,IX, kda,I, and kp,I are rate coefficients for activation, deactivation, and propagation related to the conventional initiator I2; f, and kdis correspond to the conventional radical initiator efficiency and the dissociation rate coefficient; n(+1) is the oxidation number of the transition metal complex; X: typically halogen atom; L: ligand; the starting compounds of the polymerizations are highlighted in blue; only one termination product P is shown; dashed lines: reactions with low importance for the overall ICAR ATRP kinetics.
monomer units. Only a limited set of ATRP activation/ deactivation Arrhenius parameters for macrospecies has, however, been determined under polymerization conditions. Most kinetic modeling studies on ATRP processes are limited to a single polymerization temperature, and/or the activation/ deactivation kinetic and thermodynamic parameters are not determined based on statistical regression analysis but only assessed based on visual comparison. Only recently Wang et al.38 determined the ATRP equilibrium coefficient under ATRP conditions considering methyl methacrylate as monomer at 25 °C. However, using the traditional “normal” ATRP technique,32 it is not straightforward to obtain a wide range of response values for conversion and the control over chain length (e.g., dispersity values) necessary for a reliable estimate of the individual activation/deactivation parameters as a function of temperature, taking into account that the activator and deactivator can be characterized by relatively low solubility limits, which are difficult to measure.32 In this work, Arrhenius parameters for ATRP activation/ deactivation involving macrospecies are reported based on regression to an extensive set of experimental data obtained via the ICAR ATRP technique, i.e. in the presence of monomer and under polymerization conditions. Only monomer conversions below 0.50 are considered to avoid the influence of diffusional limitations on the CRP activation/deactivation process33,34 and to minimize the possible effect of the reaction medium on the intrinsic reactivities.31 Styrene is selected as monomer with CuBr2/TPMA (TPMA: tris(2-pyridylmethyl)amine) as deactivator, ethyl 2-bromoisobutyrate (EtBriB) as ATRP initiator, and 2,2′-azobis(2-methyl propionitrile) (AIBN) as conventional radical initiator. It is shown that the ICAR ATRP process is ideally suited for estimation of Arrhenius parameters for activation/deactivation of macrospecies in ATRP, as it allows for obtaining reliable experimental data covering a wide range of response values and without the interference of additional kinetic parameters that have to be determined. In contrast to other modified ATRP
activation rate coefficients (ka(0),chem) and ATRP equilibrium coefficients (Keq(0) = ka(0),chem/kda(0),chem) are very sensitive to a temperature variation.27,28 Despite this strong influence of the polymerization temperature on the ATRP catalyst reactivity, most literature reports on activation/deactivation kinetic parameters only relate to activation of the ATRP initiator (ka0) in a monomer-free environment, i.e. in the presence of solvent only. Typically the polar acetonitrile has been used as solvent to study the ATRP initiator activation kinetics. For example, the temperature influence on the intrinsic ATRP initiator activation rate coefficient (ka0,chem) in acetonitrile has been measured by Pintauer et al.29 using the stopped-flow UV− vis technique for various initiator/Cu catalyst combinations. Additionally, Seeliger and Matyjaszewski27 reported Arrhenius parameters for a variety of ATRP initiators in the same solvent for Cu-mediated ATRP involving the commercially available ligand N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA). Furthermore, Tang et al.30 reported activation/deactivation equilibrium coefficients for these ATRP initiators in acetonitrile at 22 and/or 35 °C. More recently, using CuBr/ 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) as ATRP catalyst, Horn and Matyjaszewski31 studied the effect of the solvent on the activation rate coefficient of tertiary alkyl halide ATRP initiators at 25 °C, both in the absence and presence of monomer. For the latter case, a sufficiently low temperature was applied to exclude polymer formation. These authors indicated that it can be expected that the solvent polarity has the highest impact on the activation reactivity; for a higher solvent polarity, higher activation and ATRP equilibrium coefficients typically result. Furthermore, for the relatively nonpolar methyl methacrylate (MMA) monomer, a more or less constant activation reactivity is reported for high amounts of MMA (>50 vol %). It should however be stressed that the most important ATRP activation/deactivation parameters are those for the macrospecies (ka/kda; Figure 1b middle), as most of the activationgrowth-deactivation cycles occur with species containing several 9675
dx.doi.org/10.1021/ie5007596 | Ind. Eng. Chem. Res. 2014, 53, 9674−9685
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Table 1. Overview of Reactions Involved in ICAR ATRP of Styrene with CuBr2/TPMA as Deactivator, EtBriB as ATRP Initiator, and AIBN as Conventional Radical Initiatorf Arrhenius parametersa elementary reaction extra for ICAR ATRP
Ea
klchem (70 °C)b
ref
,k
1.3 × 10
128.6
4.7 × 10−5
41
chem k pI
4.3 × 107
32.5
4.8 × 102
42
N/A
N/A
N/A
c
N/A
N/A
N/A
c
k pchem 0
4.3 × 107
32.5
4.8 × 102
42
k pchem
4.3 × 107
32.5
4.8 × 102
42
kachem 0
2.1 × 106
29.0
6.7 × 10
d
kachem
6.9 × 105
29.0
2.2 × 10
e
chem kda 0
1.8 × 107
1.7
9.9 × 106
d
chem kda
1.8 × 107
1.7
9.9 × 106
e
A
equation dissociation propagation
f
15
chem dis
I2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2I I + M ⎯⎯⎯⎯→ R1
activation deactivation “normal” ATRP
propagation
kachem , IX
MtnX /L + IX ⎯⎯⎯⎯→ Mtn + 1X 2 /L + I chem kda ,I
Mtn + 1X 2 /L + I ⎯⎯⎯⎯→ MtnX /L + IX
R 0 + M ⎯⎯⎯⎯→ R1 R i + M ⎯⎯⎯⎯→ R i + 1 activation
MtnX /L + R 0X ⎯⎯⎯⎯→ Mtn + 1X 2 /L + R 0
MtnX /L + R iX ⎯⎯⎯⎯→ Mtn + 1X 2 /L + R i deactivation
Mtn + 1X 2 /L + R 0 ⎯⎯⎯⎯→ MtnX /L + R 0X
Mtn + 1X 2 /L + R i ⎯⎯⎯⎯→ MtnX /L + R iX termination by recomb.
k tcchem ,00
RAFT-CLD-T
40
k tcchem ,0i
RAFT-CLD-T
40
k tcchem , ij
RAFT-CLD-T
40
R 0 + R 0 ⎯⎯⎯⎯⎯→ R 0R 0 R 0 + R i ⎯⎯⎯⎯⎯→ Pi R i + R j ⎯⎯⎯⎯⎯→ Pi + j
Pre-exponential factor A in L mol−1 s−1 or s−1 and activation energy Ea in kJ mol−1. bIn s−1 or L mol−1 s−1. cKinetically insignificant (see ‘Results and Discussion’) and in agreement with ref 14. dImproved based on measurement of initiator conversion via GC at very low conversions after parameter estimation with ka0chem = kachem and kda0chem = kdachem. eEstimated in this work based on experimental data for conversion, xn and Đ while keeping ka0chem = kachem and kda0chem = kdachem (no impact on estimated values after refining of ka0chem); confidence intervals are given in Table 2. fAlso given corresponding Arrhenius parameters and intrinsic values at 70 °C, intrinsic initiator efficiency as the simulations are limited to conversions below 0.50; only diffusional limitations on termination have to be accounted for: For “cross-termination”, the geometric mean of the corresponding apparent “homotermination” rate coefficients is calculated; no intrinsic kinetic parameters are needed for these apparent termination rate coefficients. a
selected for parameter estimation is given in the Supporting Information (Table S1; temperature range: 60−80 °C; [M]0: [R0X]0: 50−500; initial Cu(II) levels: 10−50 ppm; [I2]0: [R0X]0: 0.2−2). As shown in the Supporting Information, the reproducibility of the experimental data is good (average error below 5%). A typical polymerization (e.g., entry 6 in Table S1 in the Supporting Information) was performed as follows: CuBr2 (4.63 mg, 0.021 mmol) and TPMA (6.02 mg, 0.021 mmol) were first dissolved in DMF (2.38 mL, 0.031 mol; catalyst solution), and a major part of the total amount of styrene (43 mL, 373 mmol) was mixed with the catalyst solution in a 100 mL three-neck Schlenk flask. Next, a coldfinger was attached to one of the necks as a part of the temperature control. A stopcock was attached to the second neck, and the last neck was capped with a rubber septum allowing sampling along the polymerization. Additionally, a thermocouple for temperature control was inserted into the reaction flask through this rubber septum. This solution was bubbled three times with argon while applying intermediate vacuum periods. After oxygen removal, the flask was backfilled with argon via the stopcock valve. The reaction flask was immersed in an oil bath thermostated at 70 °C under mild agitation. The remaining small volume of styrene (4.8 mL, 41.6 mmol) was poured into a 10 mL two-neck Schenk flask together with
techniques, which also start from the air insensitive deactivator, in ICAR ATRP, the well-known conventional initiation mechanism is applied implying no use of an external source, such as a reducing agent or an electrode, for which currently only a limited number of kinetic parameters are available.35,36 It is also shown that Cu levels above 10 ppm have to be applied to ensure a good control over chain length.
2. EXPERIMENTAL PROCEDURE 2.1. Materials. Styrene (Sty, monomer (M), ≥99%, SigmaAldrich) was passed through a column filled with basic aluminum oxide to remove stabilizer. Copper(II) bromide (CuBr2, 99.999%), tris(2-pyridylmethyl)amine (TPMA, 98%), ethyl 2-bromoisobutyrate (EtBriB, 98%), 2,2′-azobis(2-methylpropionitrile) (AIBN, 98%), N,N-dimethylformamide (DMF, 99.5%), tetrahydrofuran (THF, ≥99.9%), and dichloromethane (DCM, ≥99.5%) were purchased from Sigma-Aldrich and used without further purification. 2.2. Batch Isothermal ICAR ATRP of Styrene. The batch isothermal polymerizations of styrene under ICAR ATRP conditions were performed with 5% (v/v; with respect to monomer) of internal standard (DMF) for gas chromatography (GC) analysis. In situ temperature control was performed via a proportional−integral−derivative controller (PID) using water as cooling agent. An overview of the experimental conditions 9676
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azobis(isobutyronitrile) (AIBN) as conventional radical initiator. To avoid the interference of styrene thermal selfinitiation and the related chain transfer reactions, the maximal polymerization temperature is taken equal to 80 °C (Table 1). As indicated in previous kinetic studies3,37,38 such side reactions are only relevant for temperatures higher than 100 °C. The corresponding continuity equations are integrated similarly as described by Toloza Porras et al.14 in their theoretical study of ICAR ATRP of n-butyl acrylate (nBuA). The intrinsic Arrhenius parameters, except for termination, are also listed in Table 2 together with the values of the
the conventional radical initiator, AIBN (136 mg, 0.829 mmol), and the ATRP initiator, EBriB (0.62 mL, 4.146 mmol). This solution was deaerated by three vacuum-argon cycles and stirred at room temperature. When the temperature in the reaction flask was stable (ca. 40 min later), the polymerization was initiated by injecting the content of the 10 mL flask solution into the reaction flask. At distinct polymerization times, 1.4 mL samples were withdrawn from the flask with an stainless-steel needle, poured into a 2 mL vial, and immediately quenched in liquid nitrogen for a few seconds to prevent further polymerization. At the final polymerization time considered, the reaction flask was opened, and chilled THF was added to stop the reaction. 2.3. Analysis. Gas chromatography (GC) and gravimetric analysis were used for the experimental determination of monomer and ATRP initiator conversion, whereas size exclusion chromatography (SEC) analysis was used to measure xn and Đ as a function of conversion. A trace-GC ultra-Gas Chromatograph equipped with an AS3000 autosampler, flame ionization detector (FID) detector, and a CP Wax 52 CB 30m capillary column was employed for GC analysis. The injector and detector temperature were 275 °C. Helium (flow rate: 30 mL min−1) was used as carrier gas, and a stepwise temperature program was set as follows: 50 °C during 3 min, followed by a heating ramp of 10 °C min−1 until a temperature of 110 °C was reached. DMF was used as internal standard and DCM as solvent to prepare the samples. A representative GC curve is shown in the Supporting Information. To determine the number-average chain lengths and dispersities SEC analysis was performed with a PL-GPC50 plus instrument equipped with a PL-AS RT autosampler, refractive index (RI) detector, and the following columns connected in series: Resipore 50 × 7.5 mm guard column and two Resipore 300 × 7.5 mm columns. Calibration was performed with narrow polystyrene standard samples (Agilent Technologies) ranging from 162.0 to 3.7 × 105 g mol−1 and THF was used as eluent (flow rate: 1 mL min−1). For the experimental points, error bars (calculated as explained in the Supporting Information) are indicated. Note that for the number-average chain length and dispersity values two error bars are shown, since these data are presented as a function of conversion. A representative SEC curve is shown in the Supporting Information. For completeness it is mentioned here that reliable experimental data on end-group functionality X could not be obtained. For the studied ICAR ATRPs, the interpretation of the nuclear magnetic resonance (NMR) spectra is not straightforward due to overlap of signals, taking into account the high viscosity of the samples related to the high TCLs studied and the fact that polymer chains containing both I and R0 groups are formed during ICAR ATRP processes (Figure 1b). It should however be emphasized that from a parameter estimation point of view enough variation in the multiresponse range can be obtained by selecting the conversion, numberaverage chain length, and dispersity as responses and by considering a wide range of ICAR ATRP conditions (see Results and Discussion).
Table 2. Estimated Parameters for (De)activation Involving Macrospecies (Middle of Figure 1b; Also Included in Table 1) and the Corresponding 95% Confidence Intervalsa parameter ‑1
estimated value ‑1
pre-exponential factor (L mol s ) activation energy (kJ mol‑1)
activation deactivation activation deactivation
(6.9 ± 1.5) × 105 (1.8 ± 0.1) × 107 29 ± 1 1.7 ± 0.1
a
Estimates are obtained under the assumption that kda0chem = kdachem and ka0chem = kachem (which is justified based on Figure 11; the right part in Figure 2 (regression approach)), the F-value for the global significance of 8.1 × 105 (tabulated value: 3.36).
intrinsic rate coefficients at 70 °C. To stress their intrinsic nature, the superscript “chem” is used throughout the remainder of the text. For the reactions in common with free radical polymerization (FRP), i.e. conventional radical initiator dissociation and propagation, these parameters are taken from the literature,39,41 whereas the activation/deactivation Arrhenius parameters are determined in this work. For simplicity, the propagation reactivity of ATRP initiator radicals is taken equal as those for the macroradicals. It can be expected that in reality these values are different, but simulations revealed that this assumption has no impact on the obtained results. Similarly a single value for the initiator efficiency is selected ( fchem = 0.75). Based on literature reports, for termination, the intrinsic rate coefficients are replaced by conversion and chain length dependent apparent rate coefficients to account for diffusional limitations.33,43,44 Apparent “homotermination” rate coefficients (equal chain lengths) as determined with the so-called composite kt model45,46 are used. The parameters of the corresponding correlations are taken from reversible addition− fragmentation chain transfer chain length dependent termination (RAFT-CLD-T) measurements at 90 °C.40 To a first approximation it can be assumed that these parameters also hold at the lower temperatures studied in this work (60−80 °C; Table 1), as termination of polystyryl radicals is characterized by a low activation energy.47 For “cross-termination”, the geometric mean of the corresponding apparent “homotermination” rate coefficients is considered. The equations are provided in the Supporting Information. Figure 2, presents the regression approach followed in this work to estimate ATRP activation/deactivation Arrhenius parameters involving macrospecies. As will be demonstrated in the section ‘Results and Discussion’, a reliable estimation of the Arrhenius parameters for the calculation of intrinsic (de)activation rate coefficients involving macrospecies (k(d)achem; middle Figure 1b) is possible by assuming in a first instance that ka0chem = kachem and kda0chem = kdachem (Figure 2 left). In a subsequent step, k(d)a0chem can be improved based on ATRP
3. KINETIC MODEL AND REGRESSION APPROACH Table 1 summarizes the considered reactions for ICAR ATRP of styrene using CuBr2/TPMA as deactivator, ethyl 2bromoisobutyrate (EtBriB) as ATRP initiator, and 2,2′9677
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Figure 2. Regression approach to estimate intrinsic (de)activation Arrhenius parameters involving macrospecies (calculation of kachem and kdachem; middle Figure 1b); kaIXchem and kdaIchem (bottom Figure 1b) are kinetically insignificant for the studied ICAR ATRP system.
Figure 3. Kinetic insignificance of (de)activation intrinsic parameters involving I species (k(d)aIchem; bottom Figure 1b) as evidenced by the almost coinciding lines for the a) the conversion profile and for b) the change of the dispersity with conversion; simulation conditions: entry 6 (Table S1 in the Supporting Information); (green): kaIchem = 0 and kdaIchem = 0; (black): kaIchem = 0 and kdaIchem equal to 100 times kdachem from Table 1; (red): kaIchem = kachem and kdaIchem = kdachem with kachem and kdachem from Table 2; c) corresponding rate of propagation with I species ( (black), (red), and (green)) and the corresponding rate of deactivation with I species (−·−·− (black), −·−·− (red), and −·−·− (green)) as a function of conversion.
mainly takes place at higher conversions.53,54 The study is not restricted to only very low conversions to have also available reliable dispersity data for the multiresponse regression analysis. For completeness it is mentioned here that small deviations in the efficiency factor have a limited impact on the parameter estimates.
initiator GC measurements at low polymerization times/ monomer conversions (Figure 2 middle) without any influence on the previous estimates for the macrospecies (Figure 2 right). During the regression procedure, as will be also illustrated further, no attention has to be paid to the activation/ deactivation Arrhenius parameters involving I species, i.e. the Arrhenius parameters to calculate kaIXchem and kdaIchem (bottom Figure 1b), as they are kinetically insignificant, in contrast to ka(0)chem and kda(0)chem and in agreement with earlier work.14 To ensure a reliable estimation of the intrinsic (de)activation Arrhenius parameters involving macrospecies, the comparison between simulation and experimental results is limited to conversions below 0.50, and thus the kinetic study focuses on relatively low average chain lengths. As indicated in previous kinetic modeling studies, at higher conversions, the activation/ deactivation process and the conventional radical initiation can become influenced by diffusional limitations due to a viscosity increase, and thus the regular activation-growth-deactivation process can become less efficient.48−51 A similar approach has been successfully followed by D’hooge et al.52 for the determination of activation/deactivation parameters for secondary macrospecies in the ATRP of isobornyl acrylate (iBoA) with CuBr/PMDETA as catalyst in order to avoid the formation of tertiary radical species via backbiting, which
4. RESULTS AND DISCUSSION In this section, a detailed kinetic study of ICAR ATRP of styrene with CuBr2/TPMA as deactivator using EtBriB as ATRP initiator and AIBN as conventional radical initiator is presented up to intermediate conversions (
