The Borderline between Simultaneous Reverse and Normal Initiation

Macromolecules , 2016, 49 (20), pp 7793–7803. DOI: 10.1021/acs.macromol.6b01765. Publication Date (Web): October 12, 2016. Copyright © 2016 America...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Macromolecules

The Borderline between Simultaneous Reverse and Normal Initiation and Initiators for Continuous Activator Regeneration ATRP Pawel Krys,† Hendrik Schroeder,‡ Johannes Buback,† Michael Buback,‡ and Krzysztof Matyjaszewski*,† †

Center for Macromolecular Engineering, Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ‡ Institut für Physikalische Chemie, Georg-August-Universität Göttingen, Tammannstraße 6, D-37077 Göttingen, Germany S Supporting Information *

ABSTRACT: Multiple methods for initiation and selecting catalyst concentration exist in atom transfer radical polymerization (ATRP). Among them, simultaneous reverse and normal initiation (SR&NI) ATRP and initiators for continuous activator regeneration (ICAR) ATRP are phenomenologically very similar. In both methods, thermal radical initiators are employed to reduce the catalyst in the higher oxidation state and generate CuI activator in situ. SR&NI and ICAR ATRP generally differ in the amount of catalyst used and in the rate of catalyst reduction. Commonly, SR&NI ATRP requires high catalyst loadings and a quick initial reduction of CuII, while ICAR ATRP relies on slow and continuous reduction of smaller catalyst loadings. However, these criteria might not be sufficient to universally distinguish among both techniques. This article investigates both methods through kinetic simulations and establishes a borderline kinetic criterion. If the polymerization rate depends on the rate of decomposition of the radical initiator, the system follows ICAR ATRP kinetics, and if it depends on the ATRP equilibrium constant, it follows SR&NI ATRP. The transition from one to the other mechanism occurred continuously with an inflection point at a ratio of rate coefficients of radical initiator decomposition to propagation of about kdc/kp ≈ 10−7 M under typical conditions. For faster initiator decomposition and slower propagation ATRP follows SR&NI ATRP, and for slower decomposition and faster propagation it obeys ICAR ATRP kinetics. The analysis to verify which mechanism is in operation is helpful for designing reaction conditions in order to obtain well-defined products.

■

Scheme 1. Mechanism of ATRPa

INTRODUCTION Over the past 20 years, the field of radical polymerization has been revolutionized by the introduction of several reversibledeactivation radical polymerization (RDRP) methods. These techniques allow polymers to be synthesized with narrow molecular weight distributions (MWDs), excellent control over molecular weight, and formation of copolymers with complex architectures. Thus, they provide similar advantages to the ones known from living ionic polymerizations but also offer tolerance to functional groups and impurities as in conventional radical polymerizations. In all RDRP methods active radicals are reversibly trapped forming dormant species, thereby extending the overall lifetime of chains and enabling the control of the macromolecular structure. Among the RDRPs, the three most commonly used are nitroxide-mediated polymerization (NMP),1,2 reversible addition−fragmentation chain transfer polymerization (RAFT),3−5 and atom transfer radical polymerization (ATRP).6−9 In ATRP, control over polymer structure is achieved by a fast activation−deactivation equilibrium in which a transition metal species in the lower oxidation state, usually CuI/L, reversibly activates alkyl halides to generate radicals and the X−CuII/L halide complex (Scheme 1). Normal ATRP reactions are conducted with a relatively large amount of the catalyst in the lower oxidation state, i.e., about 10−100 mM of CuI/L.8 However, due to the low oxidation © XXXX American Chemical Society

a Pn−X is a dormant (poly)alkyl halide, Pn• is a (macro)radical, CuI/L and X−CuII/L are the transition metal catalysts in the lower and higher oxidation states, respectively, M is the monomer, and Pn= + PnH and Pn−Pm are products of termination, by disproportionation and combination, respectively.

stability and relatively high cost of active CuI-based catalyst complexes, other advanced initiation techniques are desirable. ATRP can also be started by introducing the catalyst in the higher oxidation state, i.e., with a CuII complex. Initially, the CuI activator was formed in situ by reaction of CuII with primary radicals generated from decomposition of a thermal initiator (Scheme 2). This procedure, named reverse ATRP,10−12 has a Received: August 12, 2016 Revised: September 21, 2016

A

DOI: 10.1021/acs.macromol.6b01765 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

species. This leads to a decrease in the termination rate but also to an unfavorable decrease in the polymerization rate. Therefore, a straightforward decrease in catalyst concentration is not a viable option because, as a consequence of termination reactions, all catalysts can be irreversibly converted to the deactivator state, preventing ATRP from reaching high monomer conversions. In normal ATRP, where the catalyst concentration is high, the PRE is not very evident because only a small fraction of the added CuIX/L is irreversibly converted to CuIIX2/L via termination reactions. With the development of more active catalyst complexes various strategies have been developed for effectively decreasing the catalyst concentration, all based on the regeneration of the CuII deactivator complex back to CuI activator, as shown in Scheme 4.

Scheme 2. Mechanism of Reverse ATRP

number of advantages, including the ease in handling the oxidatively stable catalysts and thus lower sensitivity to air. However, reverse ATRP has also its limitations, including the topologies of polymers that can be prepared and the limited functionality that can be introduced into the ω-polymer chain end by the initiator, as typically only monofunctional primary radicals are introduced as chain initiators from decomposition of standard radical initiators. In order to achieve more complex topologies such as star polymers, multifunctional chain initiators have to be used. The problem was initially resolved by the development of simultaneous reverse and normal initiation (SR&NI) ATRP.13 The precursor of an active catalyst complex was introduced in the higher oxidation state, and the activator was generated in situ as in reverse ATRP. However, the majority of the chains grow from the added alkyl halide, analogous to normal ATRP, as shown in Scheme 3. This procedure also allows for a more

Scheme 4. Mechanism of ATRP at Low Catalyst Concentrationsa

a

The CuI/L activator complex is continuously regenerated by reduction of X−CuII/L which is formed by termination reactions.

Scheme 3. Mechanism of SR&NI ATRPa

a

The low-catalyst-concentration techniques include activators regenerated by electron transfer (ARGET) ATRP,19,20 electrochemically mediated ATRP (eATRP),21−23 supplemental activator and reducing agent (SARA) ATRP,24−26 photoATRP,27−30 and initiators for continuous activator regeneration (ICAR) ATRP.31,32 In ARGET ATRP chemical reducing agents are used, such as SnII species, glucose,19,20 ascorbic acid,33 hydrazine,31 sulfites,34 or even ligands35,36 or functional monomers.37,38 In eATRP a reducing potential is applied in an electrochemically controlled reaction medium to adjust the ratio between CuI and CuII. In SARA ATRP, zerovalent metals,39 such as Cu0 or, more recently Ag0,40 are used as mild reducing agents. PhotoATRP utilizes light to photochemically (re)generate the activator complex.41 In ICAR ATRP thermal initiators, such as azobis(isobutyronitrile) (AIBN), are used to slowly and continuously regenerate CuI back from CuII. Taking into consideration all methods for initiation and selecting the catalyst concentration, some similarities can be found among the ATRP techniques. SR&NI and ICAR ATRP are phenomenologically very similar. In both techniques, thermal radical initiators are used to generate the CuI activator in situ via the reduction of the catalyst in the higher oxidation state. In general, SR&NI ATRP requires rather large amounts of the CuII catalyst, which is quickly reduced at the beginning of the reaction. The radical initiator is consumed at the onset of the reaction. In contrast, ICAR ATRP uses a low amount of CuII, targeting slow and continuous regeneration of the CuI/L complex, with many reduction cycles and with some radical initiator remaining until the end of the reaction. A similar relationship exists between AGET and ARGET ATRP.

Marked in red are reagents supplied at the beginning of the reaction.

precise control over molecular weight. The maximum amount of terminated chains is defined by the amount of CuI in the system with a direct relation to the amount of thermal radical initiator which usually represents a small fraction of alkyl halide initiator. SR&NI ATRP has successfully been carried out in miniemulsion14 and applied toward the synthesis of homopolymers,13 linear and star-shaped block copolymers,15 and gradient copolymers.16 SR&NI ATRP is very similar to the widely used activators generated by electron transfer (AGET) ATRP,17 since they both employ alkyl halides as initiators and transition metal catalysts in their higher oxidation state. In AGET ATRP, reducing agents are used instead of thermal initiators. Many efforts were then devoted to reduce the catalyst concentration in ATRP. However, ATRP similarly to all RDRP methods follows the persistent radical effect (PRE)18 in which a low fraction of unavoidable radical termination reactions cause accumulation of the CuII/L deactivator, i.e., the persistent B

DOI: 10.1021/acs.macromol.6b01765 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

selection of advanced ATRP techniques with improved control over MWD and chain-end functionality (CEF).

Both ICAR and SR&NI ATRP are widely studied and reported in the literature and are subject of detailed kinetic

■

MODEL AND COMPUTATIONAL METHODS Simulation Parameters. Simulations were carried out with the program package PREDICI51 (v. 6.3.2). The modeled reaction steps are presented in Scheme 5. Herein, R−X and R• are used to represent small-molecule initiators and radicals introduced in the form of the alkyl halide ATRP initiator, while I−X and I• represent alkyl halide and primary radicals formed from the thermal initiator, I2. In some cases, the primary radicals I• may be identical to R• (e.g., with dimethyl 2,2′azobisisobutyrate and methyl 2-bromoisobutyrate), which reduces the number of reaction steps required for modeling. The detailed model presented herein covers both cases, I• = R• and I• ≠ R•.

Table 1. Rate Coefficients Used for Modeling ICAR ATRP and SR&NI ATRP of MMA rate coefficient

k [M−1 s−1]

ref

kdeact kt1,1 ktR ktd

1.0 × 10 1.0 × 109 1.0 × 108 DP ≤ 100: (2/3) × 109 × DP−0.63 DP > 100: (2/3) × 109 × 100−0.63+0.16 × DP−0.16 DP ≤ 100: (1/3) × 109 × DP−0.63 DP > 100: (1/3) × 109 × 100−0.63+0.16 × DP−0.16

57 58 58 59

ktc

7

59

investigations.42−45 However, there are no clear boundaries established to distinguish between them. Finding the borderline between AGET and ARGET systems is challenging due to a more complex reduction kinetics and side reactions such as the possible ligand displacement from the Cu complex by tin(II) octoate, a commonly used reducing agent.20,37 In contrast, the reduction process in SR&NI and ICAR systems is directly related to the rate of thermal initiator decomposition and can be tuned by selecting the reaction temperature or an appropriate thermal initiator. In this article, we present the different kinetic features between SR&NI and ICAR and establish criteria to distinguish between the two systems. It must be noted that both SR&NI and ICAR start with alkyl halide initiator and usually use a small amount (ca. 10 times less, similar to RAFT) of thermal radical initiator (AIBN). However, typically in SR&NI, the amount of CuII is similar to or larger than the one of AIBN and CuII is reduced nearly entirely to CuI at low monomer conversion. In contrast, in ICAR, the amount of CuII is much smaller, CuI is regenerated several times during polymerization, and AIBN remains until the end of the reaction. As in RAFT or conventional RP, once AIBN is depleted, reaction halts. Therefore, kinetics of ICAR resembles conventional RP or RAFT and depends on the rate of AIBN decomposition.46,47 Thus, there are four typical distinct features for each process: (a) the kinetics in SR&NI follows PRE and shows a dependence on the catalyst activity, but in ICAR, the kinetics obeys the conventional radical polymerization rules with low/no dependence on the nature/ amount of Cu catalyst; (b) the concentration of CuII is high in SR&NI but is low in ICAR; (c) CuI is generated once at low conversion in SR&NI but is regenerated several times during the entire polymerization in ICAR; and (d) the thermal radical initiator is consumed at low monomer conversion in SR&NI but remains until the end of polymerization in ICAR. It is interesting to determine whether all these criteria are followed in each SR&NI and ICAR process or whether some of them are not obeyed. Therefore, kinetic simulations of ATRP over a wide range of conditions were carried out to find and define the borderline between both processes. Modeling was carried out with the PREDICI package, which has a documented record of accurately simulating radical polymerization processes and was proven to be able to supplement or substitute experimental data.25,26,42,44,46−50 The simulations were evaluated for the influence of catalyst concentration, thermal initiator concentration, initiator decomposition rate, and ATRP activation rate coefficient on the transition between both systems. Such mechanistic insight is important for the

Scheme 5. Kinetic Model for SR&NI and ICAR ATRP Used in the PREDICI Simulations

The catalytic radical termination (CRT) reaction, i.e., the CuI-catalyzed termination of radicals via an organometallic intermediate,50,52,53 was not included in the model. In order to demonstrate the borderline for a wide range of propagation rate coefficients, simulations were performed for two monomers, methyl methacrylate (MMA) and a hypothetical acrylate monomer with the kinetic properties of methyl acrylate, which however does not undergo backbiting reactions (MA*). Listed in Table 1 are those rate coefficients which were kept constant throughout the modeling procedure for MMA. The deactivation rate coefficient kdeact was set to 1.0 × 10 7 M −1 s −1. Chain-length-dependent termination rate coefficients (ktd and ktc) were used to account for the significant lowering of termination rate with increasing radical chain length and thus with the degree of monomer conversion. A constant short-long termination rate coefficient, ktR, was used. Diffusional limitations on thermal initiator decomposition54 and on ATRP activation and deactivation,55,56 which in most cases are both chemically controlled processes as well as density changes were disregarded for model simplicity. The analogous rate coefficients for the MA* simulations are presented in Table S1.49 The activation rate coefficient kact was varied, reflecting various ATRP equilibrium constants, KATRP, depending on the choice of the ligand in ATRP (Table 2 for MMA and Table S2 C

DOI: 10.1021/acs.macromol.6b01765 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Table 2. Parameter Variation Used within the Modeling of ICAR ATRP and SR&NI ATRP of MMA T [°C] 45 60 65 70 80 85 95 105 115 130 a

kdca [s−1] 1.0 1.0 2.0 3.0 1.0 3.0 1.0 3.0 1.0 1.0

× × × × × × × × × ×

−6

10 10−5 10−5 10−5 10−4 10−4 10−3 10−3 10−2 10−1

kpb [M−1 s−1]

kadd,RXc [M−1 s−1]

kadd,AIBNc [M−1 s−1]

[CuIIX2/L]0 [mM]

[I2]0 [mM]

× × × × × × × × × ×

× × × × × × × × × ×

× × × × × × × × × ×

20 10 5 2 1 0.5

10 5 2.5 1 0.5 0.25

5.69 8.33 9.38 1.05 1.31 1.46 1.79 2.17 2.61 3.38

2

10 102 102 103 103 103 103 103 103 103

6.64 9.72 1.10 1.23 1.54 1.71 2.10 2.55 3.06 3.96

3

10 103 104 104 104 104 104 104 104 104

2.14 3.29 3.77 4.30 5.53 6.24 7.87 9.80 1.21 1.62

3

10 103 103 103 103 103 103 103 104 104

kact [M−1 s−1] 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

× × × × × × × ×

10−3 10−2 10−1 100 101 102 103 104

kdc value for AIBN in benzene.60 bReference 61. cReference 62.

Figure 1. Simulation of (a) ln([M]0/[M]) vs time traces and (b) dispersity vs monomer conversion curves in SR&NI ATRP at initial molar ratios of [MMA]:[RX]:[CuIIX2/L]:[I2] = 250:1:0.5:0.25, [MMA]0 = 5 M, estimated for a thermal initiator decomposition rate coefficient of kdc = 1 × 10−2 s−1 (T = 115 °C, kp = 2.61 × 103 M−1 s−1) and various values of the ATRP equilibrium constant, KATRP. The kinetic model is presented in Scheme 5. All other simulation parameters are listed in Tables 1 and 2.

for MA*). The rate coefficient of initiator decomposition kdc was also varied, mimicking the choice of radical initiator and/or reaction temperature. The propagation rate coefficient (kp), and the coefficients of addition of radicals to monomer (kadd,RX and kadd,AIBN) were varied, depending on the temperature. All reactions were simulated with a ratio of monomer to initiator of [M]0:[R−X]0 = 250:1 and an initial monomer concentration of [M]0 = 5 M and for [R−X]0 = 0.02 M. The initial concentrations of [CuIIX2/L]0 and [I2]0 were varied according to the entries listed in Table 2 for MMA and in Table S2 for MA*. The variation of two types of monomer (MMA and MA*), temperature (45−130 °C), activation rate coefficient (1.0 × 10−3 − 1.0 × 104 M−1 s−1), and two initial concentrations ([CuIIX2/L]0 = 0.5−20 mM and [I2]0 = 0.25−10 mM) resulted in 2 × 10 × 8 × 6 × 6 = 5760 combinations. Because of the large number of simulations performed, a previously developed external software based on the Python programming language was used48,63,64 to run multiple automated PREDICI processes, which significantly reduced the computation time.

Rp = −

d[M] [Cu I/L][RX] [M] = k p[R•][M] = k pKATRP dt [X−Cu II /L] (1)

Simulations of SR&NI ATRP were carried out with initial molar ratios of [M]:[RX]:[CuIIX2/L]:[I2] = 250:1:0.5:0.25, with kdc = 1 × 10−2 s−1 (this corresponds to ca. 1 min AIBN half-life at ≈115 °C), and with a variation of kact between 1.0 × 10−2 and 1.0 × 101 M−1 s−1 which is associated with KATRP being between 10−9 and 10−6. The increase of Rp upon increasing KATRP is illustrated by the simulated ln([M]0/[M]) vs time traces in Figure 1a. Furthermore, not only Rp but also the dispersity is strongly dependent on KATRP (Figure 1b). For KATRP = 10−7 the best compromise between high polymerization rate and narrow MWD (Mw/Mn = 1.04 at 99% monomer conversion) was obtained. Molecular weights corresponded well to the values based on the ratio of concentrations of converted monomer to the sum of the introduced initiators. Analogous results were obtained in simulations of MA* (Figure S1). Because of the much higher kp, MA* polymerizations were significantly faster. However, their rate and dispersity were still largely dependent on the value of KATRP. A disparate scenario occurs in ICAR ATRP. In contrast to normal and SR&NI ATRP, almost identical ln([M]0/[M]) vs time plots were observed for the simulations of ICAR ATRP, regardless of the value selected for KATRP (Figure 2a). The slight differences are a result of chain-length-dependent termination (see Table 1). At low KATRP, high molecular weight chains are produced at early stages of the reaction in consequence slowing down termination. Simulations carried

■

RESULTS AND DISCUSSION Kinetics of SR&NI and ICAR ATRP. SR&NI and ICAR ATRP differ by several distinct kinetic features. The kinetics of SR&NI ATRP is essentially identical to the normal ATRP process once the thermal initiator is depleted (typically at low monomer conversion). Equation 165 shows that the polymerization rate (Rp) in normal or SR&NI ATRP depends on the value of the equilibrium constant, KATRP, and on the preselected catalyst and initiator concentrations. D

DOI: 10.1021/acs.macromol.6b01765 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 2. Simulation of (a) ln([M]0/[M]) vs time traces and (b) dispersity vs monomer conversion curves in ICAR ATRP at initial molar ratios of [MMA]:[RX]:[CuIIX2/L]:[I2] = 250:1:0.025:0.125, [MMA]0 = 5 M, estimated for a thermal initiator decomposition rate coefficient of kdc = 2 × 10−5 s−1 (T = 65 °C, kp = 9.38 × 102 M−1 s−1) and various values of the ATRP equilibrium constant, KATRP. The kinetic model is presented in Scheme 5. All other simulation parameters are listed in Tables 1 and 2.

Figure 3. Plots of ln([M]0/[M]) vs time in (a) an ICAR ATRP at a high Cu concentration and initial molar ratios of [MMA]:[RX]:[CuIIX2/L]:[I2] = 250:1:0.5:0.25, [MMA]0 = 5 M, with a radical initiator decomposition rate coefficient of kdc = 2 × 10−5 s−1 (T = 65 °C, kp = 9.38 × 102 M−1 s−1) for various activation rate coefficients kact, and (b) SR&NI ATRP at low Cu concentration and initial molar ratios of [MMA]:[RX]:[CuIIX2/L]:[I2] = 250:1:0.025:0.0125, [MMA]0 = 5 M, with a radical initiator decomposition rate coefficient kdc = 1 × 10−2 s−1 (T = 115 °C, kp = 2.61 × 103 M−1 s−1) and various activation rate coefficients kact. The kinetic model is presented in Scheme 5. All other simulation parameters are listed in Tables 1 and 2.

such a case, the deactivation process was not sufficiently fast, and many monomers could be added during a single ATRP activation−deactivation cycle, resulting in a broad MWD. In contrast, with KATRP = 10−7 the majority of the Cu/L complex remained as CuII, providing sufficiently fast deactivation. Analogous results were observed for simulations of ICAR ATRP of MA* (Figure S3b). However, even higher KATRP values, of 10−6 or 10−5, were required to obtain narrow MWDs due to the higher propagation rate coefficient of MA*.

out with constant termination rate coefficients show almost perfectly overlapping monomer conversion vs time curves (Figure S2). In the case of MA* (Figure S3a), induction periods were observed at low KATRP, as some time is required to reduce CuII/L. Nevertheless, after these initial periods, the slopes of ln([M] 0/[M]) curves were almost identical, irrespective of KATRP. In ICAR ATRP, the slow decomposition of the thermal initiator becomes the rate-limiting step for radical generation. Consequently, radical concentration, [R•], and thus polymerization rates primarily depend on the rate of initiator decomposition (eq 2). Note that eq 2 is not a function of KATRP. The kinetics of ICAR ATRP is comparable to RAFT, in that the polymerization rate also depends on the rate of thermal initiator decomposition.

[X−Cu II /L] [RX] = KATRP • [R ] [Cu I/L]

Active catalysts with a high activation rate coefficient, and thus with high KATRP, are desirable in low ppm ATRP processes to provide a sufficiently fast reactivation of dormant chains to yield polymers with narrow MWD. This results from the relationship shown in eq 4, which correlates dispersity with conversion (x), rate coefficients, and initiator and deactivator concentrations.66

2kdc[I 2] = 2k t[R•]2 [R•] =

kdc[I 2] kt

(3)

(2)

⎛ k p[RX]0 ⎞⎛ 2 Mw ⎞ 1 ⎟⎜ − 1⎟ =1+ +⎜ II ⎝ ⎠ Mn DPn ⎝ kd[Cu /L] ⎠ x

Interestingly, eq 1 is also obeyed, and the concentrations of CuI and CuII species adjust such as to follow the ATRP equilibrium. The [CuI]/[CuII] ratio is defined by KATRP, [RX], and [R•], the latter quantity defined by eq 2. Control over MWD was however largely affected by the choice of the ligand, as seen in Figure 2b. With KATRP = 10−9, the Cu catalyst was predominantly present as the CuI activator, with very low amounts of the CuII deactivator species (eq 3). In

⎛ k k [Cu I/L] ⎞ 0 ⎟ + ⎜⎜ t a ⎟x II ⎝ 4k pkd[Cu /L]0 ⎠

(4)

SR&NI and ICAR ATRP may be distinguished by analyzing the ln([M]0/[M]) vs time correlation for different catalyst− E

DOI: 10.1021/acs.macromol.6b01765 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. Simulated CuII, CuI, and I2 concentration vs time curves for (a) ICAR ATRP at initial molar ratios of [MMA]:[RX]:[CuIIX2/L]:[I2] = 250:1:0.025:0.125, [MMA]0 = 5 M, for a thermal initiator decomposition rate coefficient of kdc = 2 × 10−5 s−1 (T = 65 °C, kp = 9.38 × 102 M−1 s−1) and an activation rate coefficient of kact = 1 × 102 M−1 s−1, (b) SR&NI ATRP at initial molar ratios of [MMA]:[RX]:[CuIIX2/L]:[I2] = 250:1:0.5:0.25, [MMA]0 = 5 M, with a thermal initiator decomposition rate coefficient of kdc = 1 × 10−2 s−1 (T = 115 °C, kp = 2.61 × 103 M−1 s−1) and an activation rate coefficient of kact = 1 M−1 s−1. Shown in (c) is the calculated number of CuII reductions for both reactions. The kinetic model is presented in Scheme 5. All other simulation parameters are listed in Tables 1 and 2.

reduction of CuII was not the rate-controlling step and the polymerization rate was dependent on KATRP, as is typical for SR&NI ATRP. Analogous plots for both cases were obtained for simulations using MA* (Figure S4). The setup for SR&NI described above is not unusual. SR&NI ATRP may be carried out at even lower catalyst concentrations when the catalyst−ligand system operates with a high KATRP value, e.g., of 10−6. A high degree of monomer conversion, e.g., about 70% in 24 h as in Figure 3b, narrow MWD (Đ = 1.05) and a relatively high CEF (96%) may be reached due to the excess of alkyl halide vs Cu. If the thermal initiator is used in excess, i.e., [I2]0 = 1 mM, the analogous reaction yields 80% monomer conversion in 24 h and polymer with Đ = 1.13 and CEF = 84%. Examination of these reaction conditions clearly shows that catalyst concentration cannot be used to universally distinguish between ICAR and SR&NI ATRP. Regeneration Criterion. Since the total amount of copper catalyst is kept at low levels in an ICAR ATRP, maintaining the ATRP equilibrium usually requires that each copper atom is reduced more than once during the reaction; i.e., CuI transformed into CuII as a consequence of radical termination is regenerated. The number of reduction cycles of CuII within a given reaction time can be defined by eq 5.

ligand combinations and KATRP values. In the case that the rate of polymerization is independent of KATRP, the system should follow ICAR ATRP kinetics. Conversely, if the rate of polymerization is proportional to KATRP, SR&NI ATRP is in operation. Catalyst Concentration Criterion. The primary difference between SR&NI and ICAR ATRP is commonly seen in the concentration of copper catalyst. SR&NI ATRP is considered as a high-catalyst-concentration system, whereas catalyst concentration in ICAR ATRP is generally much lower. We assessed the validity of this assumption. Simulations were carried out to verify whether high-Cu-concentration systems may also follow ICAR ATRP behavior and whether low-Cuconcentration systems may operate in a SR&NI ATRP manner. Figure 3a shows simulated ln([M]0/[M]) vs time data for an ATRP with a large initial amount of [CuII]0 = 10 mM (2000 ppm) and 5 mM of a slowly decomposing thermal initiator with kdc = 2 × 10−5 s−1 (ca. 10 h AIBN half-life time at ≈65 °C). In the case presented in Figure 3a, the Cu concentration was relatively high, which is usually indicative of an SR&NI experimental setup. Additionally, there was not sufficient radical initiator present to reduce the CuII complex more than once. However, the polymerization rate was dependent on the slow decomposition rate of the thermal initiator resulting in slow Cu II reduction and in the polymerization rate being independent of KATRP. Therefore, the reaction followed ICAR ATRP kinetics. The opposite situation is illustrated in Figure 3b by a low concentration of [CuII]0 = 0.5 mM (100 ppm). At such low copper concentration one would expect ICAR ATRP behavior. However, due to the large decomposition rate coefficient of kdc = 1 × 10−2 s−1 (ca. 1 min AIBN half lifetime at ≈115 °C),

red Cu(t ) = 2 ×

[I 2]0 − [I 2](t ) [Cu I]0 + [Cu II]0

(5)

where [I2](t) is the radical initiator concentration at time t, and [I2]0, [CuI]0, and [CuII]0 are the initial concentrations of the radical initiator, copper(I), and copper(II), respectively. The concentration of radicals [I•] terminating or initiating new F

DOI: 10.1021/acs.macromol.6b01765 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

were carried out with only 0.5 mM CuII and slow initiator decomposition, kdc = 2 × 10−5 s−1 (ca. 10 h AIBN half-life at ≈65 °C). Because of the slow and continuous reduction of the deactivator complex under ICAR ATRP conditions, the concentration of CuII quickly equilibrated and remained nearly constant as shown in Figure 4a and Figure S5a. In contrast, under SR&NI ATRP conditions a rapid reduction of CuII at the beginning of the reaction was observed, along with fast depletion of the thermal initiator as seen in Figure 4b and Figure S5b. Only after the majority of I2 was depleted, the concentrations of CuI and CuII began to follow the persistent radical effect (t > 0.1 h in Figure 4b). The calculated number of reductions in Figure 4c and Figure S5c confirms the immediate reduction of CuII in the case of SR&NI ATRP as redCu quickly reached unity and remained constant throughout the reaction. CuII was never regenerated due to the stoichiometric limitation between CuII and thermal initiator supplied to the system. For ICAR ATRP, several reduction processes occurred, and the calculated number of reductions progressively increased to values above unity. Such regeneration compensated for the transformation of CuI to CuII due to radical termination. The number of reductions calculated at 90% monomer conversion and a wider range of conditions is presented in Figure 5 for MMA polymerizations. When the [CuII]:[I2] ratio was