Determination of the Radical Reactivity Ratios of 2-(N

Oct 2, 2018 - 2,2′-Azobis(isobutyronitrile) (AIBN, Aldrich, 98%) was recrystallized from methanol below 40 °C. 2-(N-Ethylperfluorooctanesulfonamido...
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Determination of the Radical Reactivity Ratios of 2‑(N‑Ethylperfluorooctanesulfonamido)ethyl Acrylate and Methacrylate in Copolymerizations with N,N-Dimethylacrylamide by in Situ 1H NMR Analysis As Established for Styrene−Methyl Methacrylate Copolymerizations Dibyendu Debnath,† Jessi A. Baughman,‡ Sujay Datta,§ R. A. Weiss,∥ and Coleen Pugh*,† Departments of Polymer Science, ‡Chemistry, §Statistics, and ∥Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States

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†

S Supporting Information *

ABSTRACT: A model system of styrene (St) and methyl methacrylate (MMA) was copolymerized in an NMR tube at 60 °C using 2,2′-azobis(isobutyronitrile) as the initiator and pyridazine as an internal standard to optimize an in situ 1H NMR spectroscopic method for determining reactivity ratios by generating data at hundreds of instantaneous comonomer compositions (244 data points from 8 to 91 mol % St) starting with only nine initial comonomer compositions. The radical reactivity ratios of styrene (rSt = 0.697 ± 0.010) and methyl methacrylate (rMMA = 0.491 ± 0.007) were determined by nonlinear least-squares fitting of a Mayo−Lewis plot of the instantaneous copolymer composition as a function of the comonomer feed composition using the terminal model and MINITAB statistical software, in which the copolymer composition was calculated by assuming that all comonomer consumed was converted to copolymer without side reactions; the results were similar to accepted literature values for the terminal and implicit penultimate models. After correcting for changes in the “lock” value at the initial stages of the copolymerization (because of solids formed in the sealed NMR tube), the same technique was used to determine the reactivity ratios of 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (FOSA; rFOSA = 1.624 ± 0.048) and 2-(N-ethylperfluorooctanesulfonamido)ethyl methacrylate (FOSM; rFOSM = 2.876 ± 0.083) in their radical copolymerizations with N,Ndimethylacrylamide (DMA; rDMA = 1.126 ± 0.031 with FOSA; rDMA = 0.859 ± 0.026 with FOSM).

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INTRODUCTION The physical properties and performance of a copolymer are determined by its chemical structure, microstructure, and chain morphology.1−5 Its microstructure, or sequence distribution, is dictated by the comonomer reactivity ratios. Therefore, many models have been developed to describe different types of copolymerizations.6 As discussed by Madruga,7 the copolymer compositions (as a function of comonomer feed) of more than 90% of all statistical radical copolymerizations are adequately described by the terminal model, in which only the identity of the terminal unit (M1* or M2* in a binary copolymerization) determines the rate of addition of comonomers to the growing chain; the copolymerization follows first-order Markov statistics, and only two comonomer reactivity ratios (r1 = k11/k12 and r2 = k22/k21)8 are required to describe a copolymerization that operates by a terminal model. Nevertheless, higher-order copolymerization models, such as the explicit9 and implicit10 penultimate, complex participation,11−13 and complex dissociation14−16 models, may be required to adequately describe their kinetics (kp vs comonomer feed).7 Moreover, Lynd et al.17 recently demonstrated that zero-order Markov18 (Bernoullian) statistics, which was unfortunately labeled as the “nonterminal” © XXXX American Chemical Society

model, is sufficient to describe copolymerizations that exhibit ideal behavior, such as ionic copolymerizations of both cyclic and vinyl monomers; in this case, the identity of the propagating chain end has no effect on the rate of (co)monomer addition, which is therefore determined only by the comonomer reactivities. For copolymerizations following the terminal model, several methods have been established to calculate the comonomer reactivity ratios by determining the copolymer composition at various comonomer feed ratios and at low comonomer conversions, while applying the instantaneous terminal copolymerization equation. The Mayo−Lewis,19 Fineman−Ross,20 and Kelen−Tüdos21 methods are linearization methods that either rearrange or refine the terminal copolymerization equation. For example, the Mayo−Lewis method rearranges the instantaneous terminal copolymerization equation into equations of r2 as a function of r1 and comonomer and copolymer compositions and then estimates the values of r1 and Received: July 17, 2018 Revised: August 29, 2018

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DOI: 10.1021/acs.macromol.8b01526 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules r2 from the general22 intersection of lines obtained by plotting r2 as a function of assumed values of r1 at several different feed compositions; the Mayo−Lewis method is the simplest and least reliable linearization method and is generally used only to provide rough estimates of r1 and r2. The Kelen−Tüdos method is considered the most reliable of the linearization methods. Alternatively, the reactivity ratios can be determined by nonlinear least-squares (NLLS) analysis of the experimentally determined copolymer composition as a function of comonomer feed.23 This curve-fitting method finds the values of r1 and r2 that best fit a “Mayo−Lewis plot” by minimizing the sum of the squares of the differences in the experimental and calculated copolymer compositions and is considered the most statistically correct method for determining reactivity ratios at low conversion.24 All of these methods for determining the reactivity ratios rely on accurate analysis of the copolymer sequence distributions25,26 or composition,27−31 such as by NMR spectroscopy, which generally requires careful isolation of the copolymer from a large quantity of unreacted monomer at low monomer conversions. We32−42 have developed several polymer systems that are based on the microphase separation of aliphatic fluorocarbon units from the remaining hydrocarbon structure, including physically and covalently cross-linked hydrogels that are created by copolymerization of fluorinated monomers with conventional comonomers,34−42 such as hydrophilic N,N-dimethylacrylamide (DMA). Accurate values of the comonomer reactivity ratios are required to fully understand the properties of these gels and to design new systems with advanced properties. However, the limited solubility of the fluorinated copolymers and the repulsion of the fluorinated and fluorophobic monomers make it difficult to isolate and purify the copolymer from unreacted monomer, which can result in large errors in the copolymer compositions. In this case, it would be preferable to analyze the copolymer composition by in situ NMR spectroscopy, rather than by NMR analysis of isolated copolymer. In situ NMR analyses have been used to determine the copolymer compositions at relatively low monomer conversions (4−12%) to determine the reactivity ratios of copolymerizations following the terminal model using NLLS analysis.31,43,44 Because in situ NMR analysis is ideal for following the relative rates of comonomer conversions, which correspond to the instantaneous copolymer composition,45 the Mayo−Lewis46 and Fineman−Ross47 linearization methods are also commonly used for analyses. This paper reports the reactivity ratios of 2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (FOSA) and 2-(Nethylperfluorooctanesulfonamido)ethyl methacrylate (FOSM) in their radical copolymerizations with N,N-dimethylacrylamide (DMA), after establishing an in situ NMR spectroscopy technique for the copolymerization of styrene (St) and methyl methacrylate (MMA). Our approach differs from previous in situ NMR approaches in two significant ways. First, because (co)polymer resonances are broad and barely detectable at very low monomer conversion, and require long NMR times to be quantitative, we did not determine the copolymer compositions directly as in two43,44 of the studies; instead, we assumed that all comonomer consumed at small intervals was converted to copolymer without side reactions and used that change in monomer composition at very small conversions to calculate the resulting instantaneous copolymer compositions. Second, this means that we did not use either the overall or initial relative rates of comonomer conversions to determine the

instantaneous copolymer composition,31,46,47 which provides only a single data point for each NMR tube experiment of one initial comonomer composition. Instead, we took advantage of the compositional drift throughout the copolymerizations to analyze progressively changing comonomer feed compositions with each new NMR pulse and therefore harnessed the wealth of data available at incrementally small monomer conversions throughout each copolymerization up to 75−95% monomer conversion; i.e., each pulse of the NMR experiment corresponds to a new instantaneous copolymerization experiment. Like in situ NMR studies that used an integrated form of the terminal copolymerization equation,48 this approach covers a broad comonomer composition range from a single NMR experiment but does not require further assumptions for integration or analysis of the data.

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EXPERIMENTAL SECTION

Materials. Benzene-d6 (Cambridge Isotope Laboratories, 99.5% D), 1,4-dioxane-d8 (Cambridge Isotope Laboratories, 99.0% D), 2-(Nethylperfluorooctanesulfonamido)ethyl acrylate (SynQuest Laboratories, 99.5%), and pyridazine (Sigma-Aldrich, 98%) were used as received. 2,2′-Azobis(isobutyronitrile) (AIBN, Aldrich, 98%) was recrystallized from methanol below 40 °C. 2-(N-Ethylperfluorooctanesulfonamido)ethyl methacrylate (BOC Sciences, 97%) was passed through a plug of basic activated alumina using methanol as the eluant and then dried in a vacuum oven at 40 °C after removing solvent on a rotary evaporator. Methyl methacrylate (Acros, 99%) was passed through a plug of basic activated alumina and then stored over 4 Å molecular sieves. Styrene (Sigma-Aldrich, >99%) was distilled (21 °C/12 mmHg), and the distillate was passed through a plug of basic activated alumina and then stored over 4 Å molecular sieves. All solvents were commercially available and were used as received. St−MMA Copolymerization Preparation. The reactivity ratios of styrene and MMA copolymerizations were determined using nine NMR-tube experiments, with initial comonomer feed compositions of approximately 1/9, 2/8, 3/7, 4/6, 5/5, 6/4, 7/3, 8/2, and 9/1 [St]0/ [MMA]0. A single set of stock solutions of all components of the copolymerization were prepared in C6D6 in vials. The reported concentrations are only approximations; the exact relative concentrations were determined by 1H NMR spectroscopy at time zero of each experiment. In a typical procedure, solutions of 5.1 M St (280 μL, 1.4 mmol), 5.9 M MMA (103 μL, 0.61 mmol), 0.33 M AIBN (242 μL, 80 μmol), and 0.48 M pyridazine (90 μL, 43 μmol) were added sequentially to a 5 mm NMR tube, which resulted in a sample height of 5.5 cm in the 9 in. NMR tube. The NMR tube was vortexed for 1 min and connected to a Schlenk line, and the contents of the tube were degassed by six freeze−pump−thaw (5−10−10 min) cycles. The NMR tube was then flame-sealed under vacuum (1 mmHg). DMA−FOSA Copolymerization Preparation. The reactivity ratios of DMA and FOSA copolymerizations were determined using ten NMR-tube experiments, with initial feed compositions of approximately 0.5/9.5, 1/9, 3/7, 4.5/5.5, 5/5, 5.6/4.4, 6.2/3.8, 8/2, 8.8/1.2 and 9.2/0.8 [DMA]0/[FOSA]0. A single set of stock solutions of all components of the copolymerization were prepared in 1,4-dioxane-d8 in vials. The reported concentrations are only approximations; the exact relative concentrations were determined by 1H NMR spectroscopy at time zero of each experiment. In a typical procedure, solutions of 1.9 M DMA (31 μL, 0.058 mmol), 1.2 M FOSA (563 μL, 0.68 mmol), 0.56 M AIBN (21 μL, 12 μmol), and 0.76 M pyridazine (35 μL, 27 μmol) were added sequentially to a 5 mm NMR tube, which resulted in a sample height of 5 cm in the 9 in. tube. The NMR tube was vortexed for 1 min and connected to a Schlenk line, and the contents of the tube were degassed by six freeze−pump−thaw (5−10−10 min) cycles. The NMR tube was then flame-sealed under vacuum (1 mmHg). DMA−FOSM Copolymerization Preparation. The reactivity ratios of DMA and FOSM copolymerizations were determined using nine NMR-tube experiments, with initial feed compositions of approximately 2/8, 3/7, 4/6, 5/5, 6/4, 6.5/3.5, 7/3, 8/2, and 9/1 B

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Figure 1. (a) Stacked 1H NMR spectra at representative time intervals of the AIBN-initiated radical copolymerization of styrene (29 mol % St) and methyl methacrylate (71 mol % MMA) in benzene-d6 at 60 °C using pyridazine as an internal standard. (b) The corresponding kinetic curves of monomer conversions normalized to pyridazine concentration, with the corrected styrene and MMA curves fitted to the data for t > 16000 s, and the proton assignments for (c) pyridazine, (d) styrene, and (e) methyl methacrylate. [DMA]0/[FOSM]0. A single set of stock solutions of all components of the copolymerization were prepared in 1,4-dioxane-d6 in vials. The reported concentrations are only approximations; the exact relative concentrations were determined by 1H NMR spectroscopy at time zero of each experiment. In a typical procedure, solutions of 1.9 M DMA (250 μL, 0.47 mmol), 1.4 M FOSM (325 μL, 0.47 mmol), 0.56 M AIBN (17 μL, 9.5 μmol), and 0.76 M pyridazine (58 μL, 44 μmol) were added sequentially to a 5 mm NMR tube, which resulted in a sample height of 5 cm in the 9 in. tube. The NMR tube was vortexed for 1 min and connected to a Schlenk line, and the contents of the tube were degassed by six freeze−pump−thaw (5−10−10 min) cycles. The NMR tube was then flame-sealed under vacuum (1 mmHg). NMR Instrumentation and T1 Measurements. 1H NMR experiments were performed on a Varian INOVA 400 MHz spectrometer with a 5 mm 1H/31P−15N switchable probe for the St− MMA system. A Varian VNMRS 500 MHz spectrometer with a 5 mm broadband probe with pulse field gradients was used to collect 1H NMR data for DMA−FOSA and DMA−FOSM systems. The spin−lattice relaxation times (Tables S1−S3 in the Supporting Information) of all protons of each monomer and pyridazine were measured at 30 °C using the inversion recovery sequence (relaxation delay−180°−τ−90°− acquire) on degassed and sealed solutions before performing the copolymerizations. Relaxation experiments for the St−MMA system were performed with the following parameters: 90°/180° pulse widths of 6.75/13.50 μs, 6.39 kHz spectral width, acquisition time of 1.5 s, and a 10 s relaxation delay. The relaxation experiments for the DMA−FOSA and DMA−FOSM systems were collected with the following parameters: 90°/180° pulse widths of 14.5/29.0 μs, 8.01 kHz spectral width, acquisition time of 2 s, and a 5 s relaxation delay. Ten values of τ were arrayed exponentially from 0.0625 to 32 s using four transients per increment for all T1 experiments. Data were analyzed using the standard three-parameter exponential fitting program in VnmrJ 3.2A. In Situ 1H NMR Copolymerization Experiments. All of the NMR copolymerization experiments were conducted with a 6.4 kHz spectral window and a 2 s acquisition time. In addition, the St−MMA experiments were conducted with a 6.75 μs pulse (tip angle: 90°) and a 108 s recycle delay, which corresponds to 110 s relaxation time (recycle delay plus 2 s acquisition time). The DMA−FOSA experiments were conducted with a 14.5 μs pulse (tip angle: 90°) and a 48 s recycle delay (50 s relaxation time). The DMA−FOSM experiments were conducted with a 5.8 μs pulse (tip angle: 36°) and a 48 s recycle delay (50 s

relaxation time). The transient parameter in each experiment was arrayed to produce multiple, single-scan increments. The NMR tube was placed in the instrument at 30 °C, and a singlescan 1H spectrum was recorded to quantify the initial concentrations of the comonomers, initiator, and internal standard. The sample tube was removed, and the probe was heated to 60 °C, as calibrated using neat ethylene glycol. The probe was retuned and shimmed at the temperature as rapidly as possible to minimize the delay before the first acquisition. The sample was reintroduced after the probe temperature equilibrated, and spectra were recorded for about 12 h until the monomer peak heights had decreased 75−95% from the initial height. The monomer and internal standard resonances of the NMR spectra obtained were integrated using VnmrJ 3.2A. The peak integral of protons of interest were normalized to the internal standard with magnitude of 2. The monomer conversions and instantaneous copolymer compositions were calculated from the monomerconsumption data. The 1H resonance of pyridazine at 8.63 ppm (Ha) was used as an internal standard in the St−MMA copolymerizations in benzene-d6, and that at 9.17 ppm (Ha) was used as an internal standard in the DMA−FOSA and DMA−FOSM copolymerizations in 1,4-dioxane-d8; the upfield shift of this resonance in C6D6 may be due to π−π stacking interactions.49 For the St−MMA copolymerizations, monomer conversions were monitored using the 5.56 ppm (He) and 6.10 ppm (Hg) vinyl resonances of styrene and MMA, respectively. For the DMA−FOSA copolymerizations, monomer conversions were monitored using the 6.68 ppm (Hk) and 6.38 ppm (Hn) vinyl resonances of DMA and FOSA, respectively. For the DMA−FOSM copolymerizations, monomer conversions were monitored using the 6.69 ppm (Hz) and 6.12 ppm (Hu) vinyl resonances of DMA and FOSM, respectively. The copolymer compositions (F1 and F2) were calculated after each increment of time by the change in the comonomer feed compositions (f1 and f 2). For example, the compositions of styrene in the feed and copolymer of the styrene−MMA copolymerizations were calculated using the normalized intensities (I) of two resolved resonances (He and Hg, Figure 1) at two different acquisition times (t1 and t2) according to eqs 1 and 2, respectively. The time increments were such that the intensity changes were 1%−4%. fSt = C

I(He)t1 × 100 I(He)t1 + I(Hg )t1

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Figure 2. Mayo−Lewis plots of the copolymer composition as a function of the feed composition and the 95% confidence intervals for the free-radical copolymerization of styrene and methyl methacrylate in benzene-d6 at 60 °C using AIBN as the initiator: (a) using the first data point at 1−4% conversion from each of the nine NMR-tube experiments; (b) using ∼10% conversions from each of the nine NMR-tube experiments; (c) using multiple incremental conversions of 1−4% from all nine NMR-tube experiments; (d) using multiple incremental conversions of 1−4% from four of the nine NMR-tube experiments; (e) using multiple incremental conversions of 1−4% from the remaining . FSt =

I(He)t1 − I(He)t 2 × 100 {I(He)t1 − I(He)t 2 } + {I(Hg )t1 − I(Hg)t 2 }

three copolymerization systems, five different nonlinear regressions were run, taking (a) the first data point from all of the experiments (i.e., n = 9, 10), (b) the data point at 10% comonomer conversion from all of the experiments, (c) all of the data points from all of the experiments, (d) all of the data points from four (or five) of the nine (or ten) different experiments, and (e) all of the data points from the other five of the nine or ten different experiments. To fit each nonlinear regression model, Hartley’s modified version51 of the Gauss−Newton algorithm was used, with the initial values of r1 and r2 being 1; convergence did not appear to depend on the initial value assigned. Point estimates as well as 95% confidence intervals (via Wald’s approximation) were obtained for each parameter in each of the data sets of the three systems (St−MMA, DMA−FOSA, and DMA−FOSM).

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Statistical Analysis of Mayo−Lewis Plots. The Mayo−Lewis plots of F1 vs f1 were analyzed using the MINITAB50 statistical software. In addition to being readily available, the advantage of using this program is that it provides detailed documentation of the algorithm used for the least-squares fitting and the rationale behind the confidence intervals. For each of the three cases (St−MMA, DMA−FOSA, and DMA−FOSM), the reactivity ratios, r1 and r2, were estimated via a nonlinear regression model relating a response variable (FSt in the case of St−MMA, FDMA in the cases of DMA−FOSA and DMA−FOSM systems) and an explanatory variable (f St in the case of St−MMA, f DMA in the cases of DMA−FOSA and DMA−FOSM systems). In each of the D

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Table 1. Reactivity Ratios Determined by MINITAB Statistical Analysis of the Data Sets Corresponding to Figure 2a−e for the Free-Radical Copolymerization of Styrene (St) and Methyl Methacrylate (MMA) in Benzene-d6 at 60 °C Using AIBN as the Initiator Figure 2 data

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type of data points

no. of data points (n) used in regression

2a

1−4% conversion from each of nine experiments

9

2b

10% conversion from nine experiments

9

2c

incremental conversion across all conversions from nine experiments

244

2d

incremental conversion across all conversions from four experiments (subset of 9)

107

2e

incremental conversion across all conversions from five experiments (different subset of 9)

137

RESULTS AND DISCUSSION The monomer concentrations of each copolymerization were followed by real-time 1H NMR spectroscopy at 60 °C using AIBN as the initiator and pyridazine as an internal standard. Pyridazine can be used as an internal standard in all three copolymerizations because at least one of its aromatic resonances is resolved in the spectrum of each comonomer pair. Based on its similar chemical structure to that of pyridine, which has a negligible chain transfer constant (Cs ∼ 10−5) in radical polymerizations of styrene52 and MMA,53 pyradizine should also cause negligible chain transfer, as is typical of aromatic structures like benzene. However, in contrast to pyridine (bp 115 °C), whose aromatic 1H NMR resonances overlap those of styrene, it is nonvolatile (bp 208 °C), and its concentration should therefore not vary with time at 60 °C. The chain transfer constants to 1,4-dioxane, which was used in deuterated form as the solvent in the copolymerizations of FOSA and FOSM with DMA, are low in radical polymerizations of acrylates (Cs ∼ 10−4 for acrylic acid54,55) and methacrylates (Cs ∼ 10−5 for MMA56). The corresponding instantaneous copolymer compositions (F1, F2) for small increments of time, corresponding to different instantaneous comonomer feed compositions (f1, f 2), were calculated by assuming that all comonomer consumed was converted to copolymer without side reactions. In order for each pulse of the NMR experiment to correspond to a separate instantaneous copolymerization experiment with quantitative data, the delay time between each NMR pulse must be sufficiently long. The relaxation time (Tr) of each experiment is the sum of the acquisition time and the recycle delay. Traditionally, a relaxation time of 5 times T1 is used to attain 99.5% accuracy.57 As summarized in Tables S1−S3, the longest spin−lattice relaxation times of the protons monitored were 18.8 ± 0.2, 10.4 ± 1.2, and 14.4 ± 0.3 s at 30 °C for the St−MMA, DMA−FOSA, and DMA−FOSM copolymerizations, respectively. Based on the room temperature T1 measurements, delay times of 108 s (Tr = 110 s; quality of integration57 > 99.7%), 48 s (Tr = 50 s; quality of integration57 > 98.7%), and 48 s (Tr = 50 s; tip angle = 36°; quality of integration57 > 99.3%) were used for the St−MMA, DMA−FOSA, and DMA−FOSM experiments, respectively. St−MMA Model Copolymerization. Figure 1a presents representative stacked 1H NMR spectra of the copolymerization of 29 mol % styrene with 71 mol % MMA in benzene-d6 at 60 °C. The resonance of pyridazine at 8.63 ppm (Ha) was used as the internal standard, and the vinyl resonances at 5.56 ppm (He) and

reactivity ratio(s)

standard deviation (±)

95% confidence interval

rSt = 0.686 rMMA = 0.505 rSt = 0.596 rMMA = 0.503 rSt = 0.697 rMMA = 0.491 rSt = 0.698 rMMA = 0.515 rSt = 0.683 rMMA = 0.453

0.042 0.034 0.029 0.026 0.010 0.007 0.022 0.011 0.009 0.008

0.593−0.793 0.432−0.589 0.531−0.669 0.446−0.568 0.679−0.716 0.478−0.504 0.655−0.742 0.494−0.536 0.665−0.701 0.438−0.469

6.10 ppm (Hg) were monitored to determine the concentrations of styrene and MMA, respectively, throughout the copolymerization. A delay time of 108 s was used between pulses, which more than meets the requirements for quantitative NMR analysis with >99.7% accuracy.57 Figure 1b presents the corresponding monomer conversions. Although the peak intensity of the internal standard (Ha) should be constant throughout the copolymerization, Figure 1b shows that it fluctuated at times 16000 s to first-order exponential decay equations and then extrapolated those consumption curves back to time zero; this restored the starting ratio of He and Hg at time zero at 30 °C. We therefore used values of [St]t and [MMA]t from these extrapolation curves at times 99.3% at room temperature. As the reaction progressed, the Hu and Hs resonances started to overlap, and eq 3 was used to find the intensity of Hu.

the reactivity ratios obtained with all ten NMR-tube experiments (data in Figure 4c). The variability, mean, and median of both f DMA and FDMA are significantly different based on the T-, variance, and Mann−Whitney tests, which also demonstrates that neither of the produce the same equivalent points (rDMA and rFOSA) and interval estimates (CI) as the ten NMR-tube experiments. This means that we did not successfully choose a priori which subset of five starting comonomer compositions would provide data that are as statistically correct as the full set of ten NMR-tube experiments. In addition, the reactivity ratios obtained using the full set of data from all ten NMR-tube experiments in Figure 4c, with 114 data points, are almost the mean values of the two subsets of data from Figures 4d and 4e. The most accurate reactivity ratios are therefore those based on the full set of data from all ten NMR-tube experiments: rDMA = 1.126 ± 0.031 and rFOSA = 1.624 ± 0.048. These reactivity ratios demonstrate that the propagating radicals of both DMA and FOSA prefer to homopropagate, rather than add the other monomer unit to the growing chain. Nevertheless, the preference is not great, and the blockiness from either monomer unit within the polymer chains will not be large, especially for the sequences of DMA homodyads.

I(H u) = I

I(H t + H w ) + I(Hs + H u) − I(H z) 2

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Figure 6. Mayo−Lewis plots of the copolymer composition as a function of the feed composition and the 95% confidence intervals for the free-radical copolymerization of N,N-dimethylacrylamide (DMA) and 2-(N-ethylperfluorooctanesulfonamido)ethyl methacrylate (FOSM) in 1,4-dioxane-d8 at 60 °C using AIBN as the initiator: (a) using the first data point at 1−4% conversion from each of the nine NMR-tube experiments; (b) using ∼10% conversions from each of the nine NMR-tube experiments; (c) using multiple incremental conversions of 1−4% from all nine NMR-tube experiments; (d) using multiple incremental conversions of 1−4% from four of the nine NMR-tube experiments; and (e) using multiple incremental conversions of 1−4% from the remaining five NMR-tube experiments.

comonomer feed compositions. The points in Figure 6a correspond to the first data point measured in each of the nine NMR-tube experiments, following the first NMR pulse, and correspond to conversions of 1−4%. Figure 6b corresponds to the data points generated at ∼10% conversion from each of the nine NMR-tube experiments. Figure 6c involves 167 data points at incremental conversions of 1−4% over the feed composition range of 17−96 mol % DMA, which were possible due to compositional drift with increasing conversion from the nineNMR-experiments; the 167 data points cover essentially the entire line, except for f DMA < 0.17. Figures 6d and 6e correspond to the data points from incremental conversions using only four

The peak intensity of the internal standard (Ha in Figure 5b) fluctuated and was not constant at the beginning of the experiment (times 1, rFOSA > 1) produce copolymers with relatively short homodyad sequences of both comonomers, whereas free radical copolymerizations of DMA and 2-(NK

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Macromolecules

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ethylperfluorooctanesulfonamido)ethyl methacrylate (rFOSM > 1 > rDMA) will produce gradient copolymers if the growing chains are living throughout the copolymerization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01526. Room temperature 1H NMR relaxation times of the relevant protons of pyridazine, styrene, MMA, DMA, FOSA, and FOSM; first-order exponential decay equations that were used to fit and correct the intensities of the relevant protons from the St−MMA, DMA−FOSA, and DMA−FOSM copolymerizations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (330) 972-6614 (C.P.). ORCID

R. A. Weiss: 0000-0002-5700-6871 Coleen Pugh: 0000-0002-1476-0890 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Acknowledgment is made to the National Science Foundation for support of this research through CHE-1112326 and The University of Akron Magnetic Resonance Center for the donation of NMR time. We also appreciate the Polymer Chemistry Division of the American Chemical Society and their many cosponsors, and Wiley (Journal of Polymer Science), for recognizing D.D. with the best poster presentation award and a book award, respectively, for this work at the 13th Excellence in Graduate Polymer Research Symposium at the 2017 National ACS meeting in San Francisco. D.D. thanks Dr. Alberto Gallardo (CSIC, Spain) and Dr. Nathaniel A. Lynd (University of Texas - Austin) for helpful discussions at the start of this project.

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REFERENCES

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DOI: 10.1021/acs.macromol.8b01526 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01526 Macromolecules XXXX, XXX, XXX−XXX