Understanding the Controlled Polymerization of Methyl Methacrylate

Aix-Marseille Univ, CNRS, Institut de Chimie Radicalaire, UMR 7273, Faculté de Saint-Jerome, avenue Escadrille ... Author Present Address. Department...
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Understanding the Controlled Polymerization of Methyl Methacrylate with Low Concentrations of 9‑(4Vinylbenzyl)‑9H‑carbazole Comonomer by Nitroxide-Mediated Polymerization: The Pivotal Role of Reactivity Ratios Benoît H. Lessard,*,∥ Yohann Guillaneuf,‡ Manoj Mathew,§,⊥ Kun Liang,§ Jean-Louis Clement,‡ Didier Gigmes,‡ Robin A. Hutchinson,§ and Milan Marić*,† †

Department of Chemical Engineering, McGill Institute of Adv. Mater. (MIAM), Centre for Self-Assembled Chemical Structures (CSACS), McGill University, 3610 University Street, Montréal, Québec, Canada H3A 2B2 ‡ Aix-Marseille Univ, CNRS, Institut de Chimie Radicalaire, UMR 7273, Faculté de Saint-Jerome, avenue Escadrille Normandie-Niemen, service 542, 13397 Marseille cedex 20, France § Department of Chemical Engineering, Dupuis Hall, Queen’s University, Kingston, Ontario K7L 3N6, Canada S Supporting Information *

ABSTRACT: Previously, nitroxide-mediated controlled copolymerization (NMP) of methyl methacrylate (MMA) using BlocBuilder unimolecular initiator was found to require much less controlling comonomer when using 9-(4-vinylbenzyl)-9Hcarbazole (VBK) compared to styrene (S) as a comonomer (minimum ∼1 mol % VBK versus 4.4 mol % S). Here, we explored why this was the case. Initially, the use of dimethylformamide (DMF) solvent in the copolymerization of MMA/S was studied as the MMA/VBK copolymerizations were done in DMF. The results confirmed that the increased effectiveness of VBK as a controlling comonomer was not due to the solvent or other experimental conditions. Second, the propagation rate constant kP,VBK and various ⟨kP⟩MMA/VBK for MMA/ VBK copolymerizations were determined using pulsed laser polymerization−size exclusion chromatography (PLP-SEC), and the dissociation rate constants kd,VBK for the VBK-BlocBuilder adduct and PVBK chains were determined using electron paramagnetic resonance (EPR) spectroscopy, showing that kd for VBK was very similar to S and ⟨kP⟩MMA/VBK was very similar to kP,MMA. Finally, modeling of the system using PREDICI was done and illustrated that the difference in reactivity ratios between MMA/S and MMA/VBK was ultimately one of the major reasons for the increased control of VBK versus S. This study showed that propagation rate of the copolymerization and equilibrium constant for the dormant/active species are not the only parameters that govern controlled NMP, and the effect of reactivity ratios between the methacrylate and the controlling comonomer must also be considered for the controlled nitroxide-mediated copolymerization of methacrylate-rich mixtures.

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INTRODUCTION Nitroxide-mediated controlled radical polymerization (NMP) has experienced a rejuvenation in the past decade, with the advent of new nitroxides that has enabled a path to a wider palette of macromolecular architectures in bulk, organic solvent, or aqueous media.1−4 For example, the development of secondgeneration initiators based on stable free nitroxides such as Ntert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1, Scheme 1)5 and 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO)6 now permits the homopolymerization of previously unpolymerizable monomers by NMP such as acrylates6−10 and acrylamides.11,12 Despite these advances in new NMP initiators, the homopolymerization of methacrylates is still problematic. The complications associated with the homopolymerization of methacrylates are related to their elevated equilibrium constant (K)13,14 and their propensity for β© 2013 American Chemical Society

hydrogen transfer from poly(methyl methacrylate) propagating radicals to the SG1 nitroxide.15,16 Charleux and co-workers established that by using a commercially available SG1-based alkoxyamine, (2-methyl-2-[N-tert-butyl-N-(1-diethoxyphosphoryl-2,2-dimethylpropyl)aminoxy]propionic acid, BlocBuilder-MA9,17−19 (Scheme 1), and a small amount of styrene (S, 4.4− 8.8 mol %), as a controlling comonomer, the synthesis of welldefined poly(methyl methacrylate)-rich (PMMA-rich) copolymers was possible.20 Since then, Guillaneuf and co-workers were able to homopolymerize methyl methacrylate (MMA) using a 2,2-diphenyl-3-phenylimino-2,3-dihydroindol-1-yloxyl nitroxide (DPAIO)-based alkoxyamines to conversion approaching X = Received: November 15, 2012 Revised: December 23, 2012 Published: January 14, 2013 805

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late,27 tert-butyl methacrylate,28 methacrylic acid,29 poly(ethylene glycol)ethyl methacrylate,30 and glycidyl methacrylate.31 Since the introduction of S as a controlling comonomer, several new controlling comonomers have emerged such as acrylonitrile (AN),32 pentafluorostyrene,33 4-styrenesulfonate,34 and dimethyl acrylamide, to a lesser extent.35 Recently, 9-(4vinylbenzyl)-9H-carbazole (VBK, Scheme 1) was used as a controlling comonomer36 for the controlled synthesis of various methacrylates, while imparting hole-transport properties and fluorescence37−40 into the final methacrylate-rich copolymer. When using VBK to control the copolymerization of MMA, it was noticed that very little VBK was required to control the copolymerization; indeed as little as 1 mol % of VBK relative to MMA was required to give a linear increase in M̅ n versus X, a narrow molecular weight distribution (PDI < 1.30), and a final copolymer that had sufficient nitroxide functionality at the chain end to reinitiate a fresh batch of S.36 At the time, the observations were ascribed to the slower kinetics (i.e., the combination of K and propagation rate constant kP) associated with VBK relative to S at 80 °C obtained experimentally by the authors.36 However, these results still do not adequately answer the question: why is VBK apparently a more effective controller than S? We do not know if it is due to either the K or kP, and thus the following study is aimed at separately determining K and kP of VBK and the associated MMA/VBK copolymerization. The results are then compared to the copolymerization of MMA with S by NMP.

Scheme 1. Molecular Structure Representation of BlocBuilder and 9-(4-Vinylbenzyl)-9H-carbazole (VBK) as Well as Illustration of the Necessary Experimental Conditions Used To Obtain a Controlled Copolymerization of Methyl Methacrylate (MMA) and Styrene (top) and/or of MMA and VBK (bottom)

0.60 while maintaining a linear increase of number-average molecular weight (M̅ n) vs X.21 The final PMMA homopolymers were characterized by a relatively narrow molecular weight distribution (polydispersity index, PDI = 1.3−1.4) and were able to reinitiate a fresh batch of n-butyl acrylate, even though the resulting chain extension was not controlled.21 Recently, Grubbs and co-workers were able to control the homopolymerization of MMA up to 50% conversion while maintaining narrow molecular weight distributions (PDI = 1.12−1.30) when using novel Nphenylalkoxyamines.22 However, above 40−50% conversion, the resulting homopolymers were terminated by a double bond.22 These methods involve the synthesis of new initiating systems, while the use of a small amount of S utilizes commercially available initiators and therefore has still remained of great interest. Initially, a full theoretical derivation of the copolymerization of MMA/S by NMP established that the use of a controlling comonomer can effectively control the copolymerization of methacrylate-rich feeds.22 Then, Nicolas et al. performed a comprehensive modeling study of the copolymerization of MMA and S in bulk using PREDICI kinetic modeling to get a better insight toward the mechanisms which resulted in the controlled MMA/S copolymerization by NMP.23 The addition of S decreased the overall concentration of propagating radicals20,23 resulting in a drop in the average K, ⟨K⟩. In addition, a preferential addition of S as the terminal unit has a significant effect on the dissociation temperature and contributes to the controlled copolymerization of the methacrylate-rich mixture.24,25 Later, S as a controlling comonomer was effectively used to control the copolymerization of numerous other methacrylates such as ethyl methacrylate,26 benzyl methacry-

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RESULTS AND DISCUSSION As mentioned, it was determined that a minimum of 4.4−8.8 mol % of S relative to MMA was necessary to obtain a controlled, pseudoliving copolymerization, as characterized by a linear increase in number-average molecular weight (M̅ n) versus conversion (X) and the ability to cleanly reinitiate a fresh batch of monomer, when performed in bulk at 90 °C. However, similar controlled, pseudoliving copolymerization of MMA/VBK was demonstrated experimentally when using as little as 1 mol % of VBK in a dilute N,N-dimethylformamide (DMF) solution at 80 °C.36 Effect of DMF on MMA/S Copolymerizations. The previously reported MMA/VBK copolymerizations were performed in a DMF 20 wt % solution at 80 °C;36 however, the reported MMA/S copolymerizations were synthesized in bulk at 90 °C.20 It has been reported that a significant improvement of the control of the copolymerization of methacrylate-rich/AN copolymers can result from the change in copolymerization solvent.41 Therefore, the effect of the

Table 1. Experimental Formulation for Methyl Methacrylate/9-(4-Vinylbenzyl)-9H-Carbazole (MMA/VBK) and MMA/Styrene (MMA/S) Random Copolymerizations Performed in DMF Solution exp IDa

[BlocBuilder]0 (mmol L−1)

[SG1]0 (mmol L−1)

[VBK]0 (mol L−1)

[MMA]0 (mol L−1)

[DMF]0 (mol L−1)

temp (°C)

f MMA,0b

MMA/VBK-90-5 MMA/VBK-90-2 MMA/VBK-90-1 MMA/S-80-5 MMA/S-80-3 MMA/S-80-1

9.8 9.6 9.6 8.7 8.7 8.7

1.0 1.0 1.0 0.8 0.8 0.8

0.11 0.04 0.02 0.09 0.06 0.02

2.01 2.13 2.18 1.90 1.93 1.97

10.21 10.05 9.98 10.24 10.24 10.24

90 90 90 80 80 80

0.95 0.98 0.99 0.95 0.97 0.99

a Experimental identification (exp ID) for MMA/VBK and MMA/S copolymerizations are given by MMA/X-Y-Z, with MMA representing methyl methacrylate (where X = VBK or S, representing 9-(4-vinylbenzyl)-9H-carbazole or styrene, respectively), Y representing the polymerization temperature, and Z representing the initial molar fraction of VBK in the feed. All copolymerizations were done in 20 wt % DMF solution with target average molecular weight of 25 kg mol −1 and an initial molar ratio of SG1 relative to BlocBuidler, where r = [SG1]0/[BlocBuilder]0 = 0.10. bf MMA,0 is the initial molar fraction of MMA in the feed.

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Figure 1. (a) Semilogarithmic plot of scaled conversion (ln((1 − X)−1)) (X = conversion) versus time and (b) number-average molecular weight (M̅ n) versus X for methyl methacrylate/styrene (MMA/S) copolymerizations done in a 20 wt % DMF solution using 10 mol % SG1 relative to BlocBuilder at 80 °C for various initial molar feeds: fs,0 = 0.05 (▲), fs,0 = 0.02 (●), and fs,0 = 0.01 (■) are represented and (b) the respective polydispersity index (PDI) for the corresponding MMA/S copolymerizations: fs,0 = 0.05 (△), fs,0 = 0.02 (○), and fs,0 = 0.01 (□).

solvent (DMF) and the polymerization temperature on the copolymerization of MMA/S must first be determined to fairly compare it to MMA/VBK copolymerizations. Therefore, various MMA/S copolymerizations were performed in DMF solutions under similar experimental conditions to those used in the MMA/VBK copolymerizations previously reported by Lessard et al.36 Three MMA/S copolymerizations with varying S initial molar feeds were studied: fs,0 = 0.05, fs,0 = 0.02, and fs,0 = 0.01 (Table 1). Figure 1a is the resulting scaled conversion (ln(1 − X)−1) versus polymerization time, and Figure 1b is the obtained M̅ n versus X with corresponding polydispersity indices (PDI) versus X for the MMA/S copolymerizations. The plateau of M̅ n with X and the relatively broad molecular distribution (PDI > 1.5) for the MMA/S copolymerizations of fs,0 = 0.02 and fs,0 = 0.01 would indicate that fs,0 > 0.02 of S was required to control the copolymerization of MMA even with the addition of DMF and a decrease in temperature. However, with fs,0 = 0.05 it is apparent that the polymerization is slowed down significantly (Figure 1a), and the M̅ n remains near the theoretical prediction with characteristic decrease in PDI with X (Figure 1b). This would indicate that MMA/S copolymerizations using fs,0 = 0.05, with 20 wt % DMF results in a controlled copolymerization. This observation is not surprising because controlled MMA/S copolymerizations done in bulk with 5 mol % S have already been reported.20 These results indicate, therefore, that the use of a DMF solution and 80 °C has little to no effect on the controlling ability of S and that the increased control reported by Lessard et al.36 for the MMA/VBK copolymerizations is not due to the use of a DMF solution at 80 °C but rather due to the use of VBK instead of S. Determination of Propagating Rate Constant (kP) of VBK. One of the hypotheses presented by Lessard et al. was that compared to the homopolymerization of S at 80 °C, the lower product of propagation rate constant and equilibrium constant, kPK, associated with the homopolymerization of VBK could decrease the ⟨kP⟩⟨K⟩ of MMA/VBK copolymerizations, resulting in a potentially increased polymerization control.36 It is essential to first determine both kP and K for VBK independently to better understand their individual effect on ⟨kP⟩⟨K⟩ of MMA/VBK copolymerization. Pulsed laser polymerization paired with size exclusion chromatography (PLP-SEC) has been established by IUPAC as the method of choice for the determination of free-radical kPs.42,43 PLP-SEC has been used to determine kPs of a variety of monomers such as styrenics, methacrylates, and acrylates.44−46

During PLP experiments, a mixture of monomer and photoinitiator is irradiated with a series of short laser pulses on the order of 20 ns, where the time between pulses, t0, is known as “dark time”. The initiation takes place during irradiation; however, during t0, the chains are only propagating and terminating. The kP is therefore evaluated from the equation DP0 = kP[M]t0

(1)

where [M] is the monomer concentration and DP0 is the chain length of the dead chains that were produced during t0. Because of the high probability for termination at the onset of the laser pulse, PLP produces a well-structured molecular weight distribution with the peaks corresponding to DP0. Propagation chains that are not terminated will continue to propagate and form chain lengths of 2 × DP0 and 3 × DP0. SEC can, therefore, be utilized in determining the molecular weight distribution. Since the value of kP is based on the molecular weight at the inflection point, SEC calibration is important in determining accurate rate constant values. Therefore, the Mark−Houwink− Sakurada (MHS) parameters determined for PVBK in THF at 35 °C were determined using SEC equipped with light scattering (LS) in collaboration with refractive index (RI) measurements. Previously synthesized PVBK homopolymers36 were utilized to determine MHS parameters of KMHS = 3.2 × 10−5 dL g−1 and α = 0.716, which gave good agreement between the RI and LS SEC results (see the Supporting Information for procedure and discussion on the MHS parameters determination). A series of VBK homopolymerizations and MMA/VBK copolymerizations were performed in dilute DMF solutions (relatively dilute solutions of 10−20 wt % were used due to solubility issues of VBK) using PLP. Because of the dilution of the samples, when the resulting polymers were analyzed using SEC, weak signals were obtained for the VBK homopolymerizations and the MMA/VBK copolymerizations that were polymerized with elevated VBK content in the feed (f VBK,0 > 0.10). However, even under these poor PLP-SEC conditions, an estimation of kP,VBK = 1610 L mol−1 s−1 was obtained at 90 °C. This means that the value for kP,VBK is even higher than kP,S in DMF (kP,S = 850 L mol−1 s−1, see Supporting Information). MMA-rich samples ( f VBK,0 < 0.10) gave good SEC structures (the molecular weight at the second inflection point was twice that of the first inflection point) and resulted in reproducible copolymerization propagating rate constants, ⟨kP⟩. As a comparison, the ⟨kP⟩ for MMA/VBK done under dilute solutions (20 or 50 wt % solution) at 90 °C were plotted as a function of 807

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f MMA,0 (Figure 2). A terminal model47 can be used to model the ⟨kP⟩ for MMA/VBK as a function of the individual comonomer

propagation rate coefficients and four reactivity ratios.50 In the case of a copolymerization mediated by NMP, Charleux and coworkers established that in some cases a penultimate model may be more appropriate to characterize the copolymerization kinetics, while a terminal model gives only a rather good estimation.20 Unfortunately, the radical reactivity ratios (s1 and s2) are not known for the MMA/VBK system, and therefore a terminal model was employed for comparison. The dotted line (MMA/S-TM) and the dashed line (MMA/VBK-TM) in Figure 2 are the fits to eq 2. It would appear that the estimation of kP,VBK is reasonable due to the relatively good agreement between the theoretical (MMA/VBK-TM, which was calculated using kP,VBK = 1610 L mol−1 s−1) and the experimental MMA/VBK ⟨kP⟩ values (Figure 2). The resulting copolymerization ⟨kP⟩ indicate that very little change is observed between the homopolymerization kP of MMA (kP,MMA) determined under dilute DMF solutions and the MMA/VBK ⟨kP⟩, where f VBK,0 = 0.00−0.05 (Figure 2). This small change in ⟨kP⟩ may not be surprising due to the small amount of VBK present. Nevertheless, we can observe in Figure 2 that due to the reactivity ratio, ⟨kP⟩ when f MMA,0 = 0.9−1 in the case of S/MMA is higher than kp,MMA, and the opposite occurs when S is replaced by VBK. This indicates that the kP of VBK (kP,VBK) does not play a substantial role in the control of the MMA/VBK copolymerization but more likely the difference in the reactivity ratios is the reason for improved control when using VBK as the comonomer. Effect of C−O Bond Homolysis on Copolymerization. The energy required for C−O bond homolysis is an important factor associated with the control of NMP and can also have substantial effects on the polymerization kinetics.51,52 It was essential that the dissociation rate constant or cleavage rate constants (kd) associated with a reversibly terminated chain containing VBK as the terminal unit be determined and compared to those associated with S chain ends. Therefore, the kd associated with a VBK-BlocBuilder adduct and the PVBK chains (M̅ n = 5.0 kg mol−1 and PDI = 1.23) were determined using electron paramagnetic resonance (EPR) and verified using 31 P NMR spectroscopy (procedure for determining kd using 31P NMR spectroscopy can be found in the literature53). To compare to literature values of PS, the kds of PVBK were going to be attempted in tert-butylbenzene; however, PVBK is completely insoluble in tert-butylbenzene, and therefore the corresponding kds had to be determined in DMF. Figure 3A shows a series of EPR spectra taken during the decomposition of PVBK in the

Figure 2. Copolymerization propagating rate constants, ⟨kP⟩, with respect to the initial molar feed ratio of methyl methacrylate ( f MMA,0). The⟨kP⟩ values obtained for MMA/VBK copolymerizations done at 90 °C using either a 50 wt % DMF solution (open circle, ○) or a 20 wt % DMF solution (open triangle, △) were performed using pulsed laser polymerization paired with size exclusion chromatography (PLP-SEC). The dotted red line and the dashed black line represent the fitting of eq 2 using the reactivity ratios for MMA/VBK (rVBK = 2.7, rMMA = 0.24)36 and the homopolymerization kP done in a DMF solution for MMA/S and MMA/VBK, respectively.

kP (see the Supporting Information for determination of kP for MMA, S, and VBK in DMF): ⟨kP⟩ =

r1f12 + 2f1 f2 + r2f2 2 r1f1 kP,11

+

r2f2

(2)

kP,22

where r1, r2, f1, and f 2 are the reactivity ratios and the molar feed compositions of monomer 1 and 2, respectively. It has been shown that the terminal model mostly fails to describe copolymer composition and propagation rate simultaneously.48,49 If done separately, there was a good agreement but two sets of reactivity ratios for a single system are obtained. Fukuda et al.50 recommended the use of the penultimate model that increases the number of parameters to be determined to eight for the explicit penultimate model (EPUE), two homopropagation rate coefficients and six reactivity ratios, and six for the implicit penultimate model (IPUE), two homo-

Table 2. Molecular Characterization of the Methyl Methacrylate/9-(4-Vinylbenzyl)-9H-Carbazole (MMA/VBK) and MMA/ Styrene (MMA/S) Random Copolymerizations Performed in DMF Solution exp IDa

Xb

tpolymerization (min)

M̅ nc (kg/mol)

PDIc

FMMAd

⟨kP⟩⟨K⟩e (s−1)

MMA/VBK-90-5 MMA/VBK-90-2 MMA/VBK-90-1 MMA/S-80-5 MMA/S-80-3 MMA/S-80-1

0.32 0.36 0.42 0.18 0.95 0.59

99 91 96 195 120 190

7.8 8.5 10.1 3.1 2.1 5.2

1.36 1.44 1.46 1.32 1.52 1.50

0.914 0.987 >0.99 0.31 0.95 0.89

(4.27 ± 0.06) × 10−6 (5.15 ± 0.02) × 10−6 (7.68 ± 0.04) × 10−6 (9.0 ± 0.9) × 10−6

a

Experimental identification (exp ID) for MMA/VBK and MMA/S copolymerizations are given by MMA/X-Y-Z, with MMA representing methyl methacrylate (where X = VBK or S, representing 9-(4-vinylbenzyl)-9H-carbazole or styrene, respectively), Y representing the polymerization temperature, and Z representing the initial molar fraction of VBK in the feed. All copolymerizations were done in DMF solution with target average molecular weight of 25 kg mol −1 and an initial molar ratio of SG1 relative to BlocBuidler, where r = [SG1]0/[BlocBuilder]0 = 0.10. bMonomer conversion, as determined by 1H NMR spectroscopy. cNumber-average molecular weight (M̅ n) and polydispersity index (PDI) were determined using size exclusion chromatography (SEC) run in THF at 35 °C and calibrated against poly(methyl methacrylate) standards. dFMMA is the final molar composition of MMA in the copolymer, as determined by 1H NMR spectroscopy. eProduct of the average propagation rate constant, ⟨kP⟩, and the average equilibrium constant, ⟨K⟩. Error derived from the standard error in the slope of scaled conversion (ln(1 − X)−1) versus time plots. 808

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Figure 3. (A) A series of EPR spectra taken during the decomposition of PVBK (M̅ n = 5.0 kg mol−1 and PDI = 1.23) in the presence of O2, (B) time dependence of the SG1 concentration during thermolysis of PVBK−SG1 and PS−SG1 in the presence of O2 in excess at 70 °C, and (C) is the linearized signal using eq 3.

Table 3. Activation Energy and Dissociation Rate Constant Determined by EPR at 70 °C in Various Solutions for 9-(4Vinylbenzyl)-9H-Carbazole (VBK) and Styrene (S) SG1 Adducts and Homopolymers exp IDa

solvent

S-SG1 S-SG1 VBK-SG1 VBK-SG1 PVBK-SG1 PVBK-SG1 PS-SG1 PS-SG1

t-Bu-Benz DMF t-Bu-Benz DMF t-Bu-Benz DMF t-Bu-Benz DMF

kd,70b (10−4 s−1) 7 43 37 52 0.52 0.22 0.51

Eac (kJ mol−1)

kd,120c (10−3 s−1)

ref

115.5 116.5 116.9 116.0

106.7 78 68 92

122.5 125 122.6

12 5.6 12

14 this work this work this work insoluble this work 24 this work

a

Experimental identification (exp ID) for SG1 containing homopolymers and adducts where VBK and S represent 9-(4-vinylbenzyl)-9H-carbazole and styrene, respectively. bkd,70 was determined by EPR, using oxygen as a scavenger at 70 ± 0.1 °C (unless the value was taken from literature as indicated by a reference). ckd,120 = A exp(−Ea/393R), using R = 8.314 J mol−1 K−1 and A = 2.4 × 1014 s−1.

s−1.36 Dividing this value by kP,VBK = 1610 L mol−1 s−1 which was obtained using PLP-SEC in the previous section, we find KVBK,90 °C = 2.5 × 10−10 mol L−1. This value for KVBK,90 °C is rather similar to that of S at the same temperature (K90 °C = 5.56 × 10−10 mol L−1),23,24,58 which is not surprising due to the similar kds obtained for both PVBK and PS and both the BlocBuilder-VBK and S adducts (discussed in previous section). The small difference is thus related to a small increase of the recombination rate constant (factor of 2 approximately). Therefore, we now have all the required kinetic constants to model the ⟨kP⟩⟨K⟩ for MMA/VBK and the ⟨kP⟩⟨K⟩ for MMA/S as a comparison. Equation 4, which was derived from a terminal model, can be used to plot the ⟨kP⟩⟨K⟩ and ⟨K⟩ with respect to the molar feed composition.20

presence of O2. Previous studies have shown that a sufficient concentration of dissolved O2 is a reliable scavenger in alkoxyamine homolysis experiments, and therefore no additional radical scavenger was required.54−56 The increase in the concentration of SG1 nitroxide radicals could be monitored until all the SG1 was dissociated from the chains (Figure 3B). This increase in concentration of SG1 followed the expected first order kinetics and the associated kds could easily be extrapolated using eq 3 (Table 3). ⎛ [SG1]∞ − [SG1]t ⎞ ln⎜ ⎟ = kdt [SG1]∞ ⎠ ⎝

(3)

The concentration of SG1 for both PVBK and PS were linearized using eq 3 and can be found in Figure 3C. As a comparison, the kds associated with the poly(styrene) (PS, M̅ n = 5.2 kg mol−1 and PDI = 1.20) and styrene−BlocBuilder adduct were both determined in DMF as well (Table 3). From the determined kds, the associated activation energies could be calculated using a simple Arrhenius relationship and by assuming the preexponential factor, A = 2.4 × 10 14 s −1, as previously discussed.54,57 When comparing the kd values obtained by EPR for PVBK and PS in DMF, it becomes apparent that no significant difference was observed (Table 3 and Figure 3b). These results indicate that the energy required for C−O bond homolysis plays no substantial role in VBK’s ability to control the copolymerization of MMA more effectively than styrene’s ability to do so. Modeling the MMA/VBK Copolymerization. At 90 °C, the experimentally obtained kPK for VBK is (4.0 ± 0.1) × 10−7

⟨kP⟩⟨K ⟩ =

r1f12 + 2f1 f2 + r2f2 2 r1f1 kP,11K11 r1f1

⟨K ⟩ =

kP,11 r1f1 kP,11K11

+ +

+

r2f2 kP,22K 22

r2f2 kP,22 r2f2 kP,22K 22

(4)

Here, r1, r2, f1, f 2, K11, K22, kP,11, and kP,22 are the reactivity ratios, the molar feed compositions, the equilibrium constants, and the propagating rate constants of monomer 1 and 2, respectively. As mentioned, in some cases it may be more accurate to apply a penultimate unit effect model such as eq 5.20 809

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Figure 4. Predicted values of the (A) product of average activation−deactivation equilibrium constant ⟨K⟩ and the average propagating rate constant, ⟨kP⟩, ⟨kP⟩⟨K⟩ and of the (B) ⟨K⟩, as a function of the as a function of the initial molar feed fraction of methyl methacrylate ( f MMA,0), for the copolymerization of MMA/styrene (MMA/S) and MMA/9-(4-vinylbenzyl)-9H-carbazole (MMA/VBK) at 90 °C, using either the terminal model (TM) or the implicit penultimate unit effect model (IPUE). The filled circles are experimentally obtained ⟨kP⟩⟨K⟩ values for MMA/VBK copolymerizations done at 90 °C in a 50 wt % DMF solution. The error bars were determined from the standard deviation of the slope from the semilogarithmic kinetic plots.

Figure 5. Evolution of the (A) polydispersity index, PDI, and (B) M̅ n versus conversion for the copolymerization of MMA with 9 mol % of a controlling comonomer (either VBK or S, i.e., f MMA,0 = 0.91) using BlocBuilder at 90 °C under different scenarios. The black line represents MMA/S copolymerization taken from Nicolas et al.,23 while the blue line values corresponds to elevated kc values (scenario i: kc = 2 × kc,S); the solid red line values corresponding to a change in reactivity ratios (scenario ii: rVBK = 2.7, rMMA = 0.24). The solid purple line represents both the elevated kc and modified reactivity ratios (scenario iii). Part C represents the evolution of PDI versus conversion for the copolymerization of MMA/VBK using scenario iii for different initial molar feed ratio indicated in the figure.

⟨kP⟩⟨K ⟩ =

r1f12 + 2f1 f2 + r2f2 2 r1f1

+

kP,11K11 r1f1 kP,11

⟨K ⟩ =

r1f1 kP,11K11

+

MMA/VBK exhibit an exponential increase at much higher f MMA,0 content (Figure 4), for example: the feed composition at which ⟨kP⟩⟨K⟩MMA/S = 5.0 × 10−6 s−1 is f MMA,0 ≈ 0.975, while the composition at ⟨kP⟩⟨K⟩MMA/VBK = 5.0 × 10−6 s−1 is f MMA,0 ≈ 0.993. Figure 4B indicates that the reason for the decrease in ⟨kP⟩⟨K⟩ is a result of both ⟨K⟩ and ⟨kP⟩, even if the main parameter seems to be ⟨K⟩. This observation is not surprising because as mentioned in the previous section, kP,VBK is almost 2 times larger than that of kP,S, but due to the reactivity ratios, the ⟨kP⟩ is lower at high MMA content and therefore contributes to a decrease the ⟨kP⟩⟨K⟩MMA/VBK, relative to ⟨kP⟩⟨K⟩MMA/S. Second, in the previous sections we also proved that kd, and ultimately KVBK, is rather similar to that of KS (an increase of kc by a factor of 2 was noticed) and could not significantly decrease the ⟨kP⟩⟨K⟩MMA/VBK, which was observed (Figure 4). Therefore, when studying eq 4, the only logical conclusion is that the reactivity ratios are playing a major role, and in fact the reactivity ratios for MMA/VBK (rVBK = 2.7, rMMA = 0.24)36 are very different from those of MMA/S (rS = 0.489, rMMA = 0.492).59 The favorable reactivity ratios result in an increase in addition of VBK monomers as the terminal units of propagating chains resulting in a decreased equilibrium with SG1 (⟨K⟩MMA/VBK < ⟨K⟩MMA/S). These findings are consistent with the hypothesis previously proposed by Lessard et al.36 This decrease in ⟨K⟩ at higher f MMA,0

r2f2 kP,22K 22

r2f2 kP,22 r2f2

+

kP,22K 22

(5)

where kP,11 = kP,11

r1f1 + f2 r1f1 +

f2 s1

and kP,22 = kP,22

r2f2 + f1 r2f2 +

f1 s2

The radical reactivity ratios, s1 and s2, have been reported for the MMA/S system and were therefore used to calculate the ⟨kP⟩⟨K⟩ and ⟨K⟩ for MMA/S but are not known for the MMA/VBK system. Figure 4 illustrates the ⟨kP⟩⟨K⟩ and ⟨K⟩ with respect to molar feed composition, plotted for MMA/VBK and MMA/S at 90 °C using eqs 4 and 5 with newly obtained data as well as literature values. The experimentally obtained ⟨kP⟩⟨K⟩MMA/VBK,90 °C values are also plotted (filled circles) as comparison and resemble closely the theoretical model. As can be seen from Figure 4, the theoretical prediction for ⟨kP⟩⟨K⟩MMA/VBK,90 °C and the experimental ⟨kP⟩⟨K⟩MMA/VBK,90 °C are both lower than those of MMA/S. Second, the curves for 810

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dimethylpropyl]amino]oxidanyl (SG1, >85%) were obtained from Arkema and used without further purification. Synthesis of 9-(4-Vinylbenzyl)-9H-Carbazole BlocBuilder (VBK−SG1) Adduct. The synthesis of the styrene−SG1 adduct (S− SG1) and the VBK−SG1 adducts were adapted from the literature.61 As an example the synthesis of VBK−SG1 is reported. Both the VBK monomer (2.1 g, 7.8 mmol) and the BlocBuilder (3 g, 7.8 mmol) unimolecular initiator were added to a 50 mL Schlenk glass reactor along with 10 mL of a 1:1 mixture of THF and anisole. The mixture was bubbled with argon for 30 min and then carefully sealed without allowing any air into the vessel. The reactor was then heated to 100 °C using an oil bath for 1.2 h. The mixture was cooled to room temperature before prior to reducing its volume by half. Cold pentane was added to the resulting oil, and the mixture was allowed to precipitate in the freezer overnight. The precipitant was collected and dried under vacuum overnight. VBK-SG1: Yield = 1.5 g (30%). 1H NMR spectroscopy (ppm, CDCl3): major diastereomer: 8.18−8.04 (m, 2H), 7.47−7.00 (m, 6H), 5.54−5.33 (m, 2H), 4.82 (d, 1H), 4.36−3.86 (m, 4H), 3.30 (d, 1H), 3.13−2.73 (m, 1H), 2.16 (dd, 1H), 1.40−0.85 (m, 16H), 0.75 (s, 9H); minor diastereomer: 8.18−8.04 (m, 2H), 7.47−7.00 (m, 6H), 5.54− 5.33 (m, 2H), 5.13 (d, 1H), 4.36−3.86 (m, 4H), 3.33 (d, 1H), 3.13−2.73 (m, 1H), 2.09 (dd, 1H), 1.40−0.85 (m, 21H), 0.75 (s, 9H). 31P NMR (ppm, CDCl3): major diastereomer: 23.62 (57%); minor diastereomer: 24.81 (43%). Copolymerization of 9-(4-Vinylbenzyl)-9H-Carbazole (VBK) with Methyl Methacrylate (MMA). The MMA/VBK and MMA/S copolymerizations were performed in a similar setup to that previously reported by Lessard et al.36 The copolymerizations were performed in a 50 mL three-neck round-bottom glass reactor equipped with a condenser, thermocouple in temperature well, and heating mantel connected to a temperature controller. All copolymerizations were done in a DMF solution with target average molecular weight at complete conversion of roughly 25 kg mol−1 and with varying molar feed compositions. As an example, the synthesis of MMA/VBK-90-2 was performed by first adding VBK (0.16 g, 0.57 mmol), MMA (2.90 g, 29.0 mmol), BlocBuilder (0.05 g, 0.13 mmol), SG1 (0.004 g, 0.015 mmol), and DMF (10.0 g) to the reactor. All experimental formulations can be found in Table 1. The reaction mixture was bubbled for 20 min using a nitrogen purge and stirred using a magnetic stir bar and stir plate. The reactor was heated to 90 °C and t = 0 min was arbitrarily assigned as the time when the reactor temperature reached 90 °C. Samples were drawn throughout the polymerization and characterized by SEC and 1H NMR spectroscopy directly (no precipitation was performed). The crude polymer was later recovered by precipitation in methanol and dried in the vacuum oven at 60 °C to remove the residual solvent. For the specific example cited, the final copolymer was characterized by a yield of 0.62 g (X = 0.36) with number-average molecular weight, M̅ n = 8.6 kg·mol−1, and a polydispersity index, PDI = 1.44, determined relative to PMMA standards in THF at 35 °C. Characterization. The molecular weight distribution of the MMArich copolymers was determined using size exclusion chromatography (SEC, Waters Breeze) equipped with both ultraviolet (UV 2487) and differential refractive index (RI 2410) detectors. The SEC used THF as the mobile phase heated to 40 °C and a flow rate of 0.3 mL min−1. Calibration of the detectors against narrow molecular weight distribution poly(methyl methacrylate) standards was performed using a guard column and with three Waters Styragel HR columns in series (HR1 with molecular weight measurement range of 0.1−5 kg mol−1, HR2 with molecular weight measurement range of 0.5−20 kg mol−1, and HR4 with molecular weight measurement range (5−6) × 102 kg mol−1). Final copolymer composition was estimated by 1H NMR spectroscopy, which was obtained using a 400 MHz Varian Mercury spectrometer using CDCl3 solvent in 5 mm Up NMR tubes and by performing a minimum of 32 scans per sample. The copolymer composition was determined by the ratio of the area associated with the resonance of the proton adjacent to the ester on the methyl groups of MMA (δ = 4.0−4.1 ppm, O−CH2−CH2) and the resonances corresponding methylene protons corresponding to VBK units (δ = 5.2−5.4 ppm, C−CH2−C). The individual monomer conversion was determined by comparing the vinyl peaks (δ = 6.6, 5.6, and 5.1 ppm for

due to the favorable reactivity ratios means less VBK compared to S is required to control the copolymerization of MMA. Further investigation into the copolymerization of MMA/ VBK and MMA/S was performed using the PREDICI modeling software to simulate the kinetics and evolution of the molecular weight distribution (see Supporting Information for all parameters used in the modeling). It has to be noted that it is not possible to compare rigorously the modeling with the experimental data since most of the kinetic rate constants used are not determined in DMF and such solvent effects could induce non-negligible changes that are difficult to take into account. The modeling was rather used to investigate qualitatively the impact on the control of the polymerization of various parameters such as the increase of the rate of reversible combination between propagating chain and SG1, kc, the variation of the reactivity ratio, and both parameters simultaneously. The reference modeling was performed using the PREDICI scheme and the set of parameters that has been established by Nicolas et al. for the bulk polymerization of MMA/S.23 However, in the study the set of parameters was chosen to test three different scenarios: (i) an increase of the kc value by a factor of 2 for the second monomer, (ii) a change of the reactivity ratio and the kp value to mimic the value determined for the VBK, and (iii) a combination of cases i and ii since both phenomena are occurring in the case of VBK/MMA copolymerization. Figures 5A and 5B represents the evolution of PDI versus X and M̅ n versus X ( f MMA,0 = 0.91), respectively, under the condition of scenarios i, ii, and iii. Figure 5C represents the evolution of PDI versus X for different f MMA,0 (1.00, 0.99, 0.98, 0.97, 0.95, and 0.91) using the scenario iii. As can be seen in Figure 5, the influence of both the increase of kc value and the change in reactivity ratio improved similarly the control of the polymerization. For example, the PDI value decreased from 1.6 at 20% conversion to 1.5 using either scenario i or ii. The combination of the two parameters led to a drastic decrease of the PDI that is in good agreement with the experimental result obtained by replacing S with VBK. The influence of the amount of comonomer was studied in Figure 5C and the modeling shows that a threshold value (i.e., 3 mol % for the amount of comonomer) is necessary to have a clear decrease of PDI with increasing conversion. The model did not exactly fit the experimental data; however, a better fit would be expected if more parameters were known, for example, the radical reactivity ratios (sMMA and sVBK) or the effect of DMF on the rate of dissociation and combination of BlocBuilder. Regardless, the results of the modeling are in good agreement with the experimental observations, indicating that the favorable reactivity ratios of MMA/VBK compared to MMA/S and the increase of the kc due to VBK compared to S both have a significant increase on the control of the polymerization of MMA.

■

EXPERIMENTAL SECTION

Materials. Methyl methacrylate (MMA, 99%) and styrene (>99%) were obtained from Sigma-Aldrich and were purified by passage through a column containing a 1:40 mixture of basic alumina to calcium hydride. 9-(4-Vinylbenzyl)-9H-carbazole (VBK, >95%) was synthesized according to the literature.60 N,N-Dimethylformamide (DMF, >95%) and tetrahydrofuran (THF, >99.5%, HPLC grade) were obtained from Fisher Scientific while the deuterated chloroform (CDCl3, >99%) was obtained from Cambridge Isotopes Laboratory and were used as received. All other chemicals were used as received unless otherwise mentioned. Finally, 2-([tert-butyl[1-(diethoxyphosphoryl)-2,2dimethylpropyl]amino]oxy)-2-methylpropanoic acid (BlocBuilderMA, 99%) and [tert-butyl[1-(d ietho xyphosphoryl)-2,2811

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VBK and δ = 5.45 and 6.05 ppm for MMA) of the respective monomers, to the methoxy group (δ = 4.0 and 4.1 ppm, for MMA) and the methylene group (δ = 5.2−5.4 ppm, corresponding to the VBK) corresponding to the respective monomers and copolymers. The overall monomer conversion (X) was then determined by X = f VBK,0XVBK + XMMA f MMA,0, where XVBK, XMMA, f VBK,0, and f MMA,0 are the individual monomer conversion and initial molar percent in the feed for VBK and MMA, respectively. Pulsed Laser Polymerization (PLP) Measurements. Propagation rate constants (kP) for VBK homopolymerization and MMA/VBK copolymerizations were performed by pulsed-laser polymerization paired with size exclusion chromatography (PLP-SEC) using a similar setup as previously reported.62,63 A Spectra-Physics Quanta-Ray 100 Hz Nd:YAG laser that is capable of producing a 355 nm laser pulse of duration 7−10 ns and energy of 1−50 mJ per pulse was used as the laser source, which is reflected twice (180°) to shine into a 10 mm square quartz cuvette. A digital delay generator (DDG, Stanford Instruments) is connected to the laser to regulate the pulse output repetition rate to a tunable value between 10 and 100 Hz, and the temperature is controlled by a circulating oil bath. 3−5 mmol L−1 of the photoinitiator, 2,2dimethoxy-2-phenylacetophenone (DMPA), was added to a solution of VBK or MMA/VBK monomers in DMF prior to being injected into a quartz cell, which was then heated to 90 °C and exposed to the laser at 20 Hz. For the MMA/VBK copolymerizations, MMA mole fraction in the monomer mixture was varied between 1.0 ≥ f MMA,0 > 0.6. Temperature was monitored throughout the laser pulsing and never increased more than 0.5 °C during polymerization. Once the polymers were synthesized by PLP, their molecular weight distribution was analyzed by SEC. A Waters 2960 separation module instrument equipped with four Styragel HR columns (HR 0.5 with molecular weight measurement range 0.1−10 kg·mol−1, HR 1, HR 3 with molecular weight measurement range 0.5− 30 kg·mol−1, and HR 4), a Waters 410 differential refractometer (RI), and a Wyatt Instrument Dawn EOF light scattering detector (LS) were used to analyze the polymers. The RI detector was calibrated using PS standards while a flow rate of 1.0 mL min−1 of THF was maintained at 35 °C. Copolymer calibrations were performed using a compositionweighted average of homopolymer calibrations (RI data) or homopolymer refractive indices (LS data).62,63 Electron Paramagnetic Resonance (EPR) Measurements. Similar to previous reports,57,64 the kinetic parameters were determined by following the evolution of the double integrals of the signal resulting from the captured SG1 nitroxide during time-resolved EPR experiments. The measurements were run on a CW-ESR spectrometer (Bruker EMX). Prior to heating, pure oxygen was bubbled in the mixtures for 10 min, to increase the dissolved oxygen content, and the vial was sealed using Parafilm. Kinetic Modeling. The modeling of the MMA/VBK was performed using PREDICI (CIT, version 6.34) software,65 and the model for this study was identical to the one used by Nicolas et al.23 The PREDICI model was performed on a 2.53 GHz Intel Core i3Macintosh computer using the CrossOver software. For all simulations, a PREDICI model comprising all the reaction steps and respective rate constants has been used. The kinetic parameters related to VBK, which were used as a comparison, can be found in the Supporting Information.

■

DMF. The results showed that not only was kP,VBK > kP, but the resulting ⟨kP⟩MMA/VBK were found to change very little with a change in f MMA,0; however, the change was not significant enough to explain the decrease in ⟨kP⟩⟨K⟩MMA/VBK at f MMA,0 ≈ 0.98−0.99. Third, the kd,VBK values for the VBK−BlocBuilder adduct and PVBK chains was determined using EPR experiments and were found, again, to be very similar to those of S. As KVBK was already determined, this implies that the kc value, for the PVBK chains reversibly terminated with SG1, is slightly higher than the kc value of PS chains (also reversibly terminated with SG1), by a factor 2. In summary, VBK and S homopolymerize in a very similar fashion, and the only major difference is their behavior when copolymerized with methacrylates at f MMA,0 ≈ 0.96−0.99. Next, modeling of the system using PREDICI illustrated that the different reactivity ratio between MMA/S and MMA/VBK was one of the significant differences between the two systems and the reason for the increased control. The influence of a kc value higher by a factor 2 was also highlighted. The modeling not only illustrated the decrease in ⟨kP⟩⟨K⟩ values for MMA/VBK copolymerizations at (f MMA,0 ≈ 0.99) but also the favorable M̅ n versus X profile at similar feed compositions. This study reinforces the importance of having favorable homopolymerization kinetics of the controlling comonomer in NMP of methacrylates (relatively low kP, low K) as expected, but interestingly, the reactivity ratios between the methacrylates and the controlling comonomers can play an important role in NMP of methacrylates. We studied a specific controlling comonomer, VBK, in this study that will provide future guidance to the evaluation of other controlling comonomers for NMP of methacrylates.

■

ASSOCIATED CONTENT

S Supporting Information *

Procedure and discussion on the MHS parameters determination, determination of kP,S done in DMF, and all parameters used in the PREDICI modeling. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.H.L.); milan.maric@ mcgill.ca (M.M.). Present Addresses

∥ Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada. ⊥ Department of Chemical Engineering, University of Waterloo, Kitchener Waterloo, Canada.

CONCLUSION

Notes

The authors declare no competing financial interest.

Recently, it was shown experimentally that richer MMA compositions (f MMA,0 ≈ 0.99) with VBK as the controlling comonomer result in more controlled BlocBuilder-mediated NMP than when using S as a controlling comonomer ( f MMA,0 ≈ 0.96).36 In this study, the reason for this observation was explored. First of all, we showed that the copolymerization of MMA/S in a dilute DMF solution at 80 °C did not result in a controlled copolymerization when f S,0 < 0.05. The enhanced control of MMA/VBK copolymerizations is therefore, a result of the VBK substitution and not the experimental conditions. Second, the kP,VBK as well as various ⟨kP⟩MMA/VBK for the copolymerizations were determined using PLP-SEC at 90 °C in

■

ACKNOWLEDGMENTS We thank the Canadian Foundation for Innovation (CFI) New Opportunities Fund, NSERC Discovery Grant, AIX-Marseille Université, and CNRS for financial support. We also thank the Graduate Travel Funding Program from the faculty of engineering at McGill University for financial support towards B.L.’s travel. We also thank Scott Schmidt and Noah Macy of Arkema, Inc., for their aid in obtaining the BlocBuilder initiator and SG1 nitroxide. 812

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