Reevaluation of the Formation and Reactivity of Midchain Radicals in

Apr 24, 2015 - D. Sokolovski , S. Rusconi , E. Akhmatskaya , J. M. Asua. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sc...
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Reevaluation of the Formation and Reactivity of Midchain Radicals in Nitroxide-Mediated Polymerization of Acrylic Monomers Nicholas Ballard,† José Ignacio Santos,‡ and José M. Asua*,† †

POLYMAT and Grupo de Ingenierı ́a Química, Dpto. de Química Aplicada, University of the Basque Country UPV/EHU, Joxe Mari Korta Zentroa, Tolosa Etorbidea 72, 20018, Donostia/San Sebastián, Spain ‡ NMR Facility, SGIKER, University of the Basque Country UPV/EHU, Joxe Mari Korta Zentroa, Tolosa Hiribidea 72, 20018 Donostia-San Sebastián, Spain S Supporting Information *

ABSTRACT: In radical polymerization of acrylic monomers, intramolecular transfer to polymer, and the reactions of the resulting midchain radical, can have a strong impact on both the kinetics and microstructure of the resulting polymer. It has previously been stated that, in nitroxide-mediated polymerization, a substantial proportion of midchain radicals reacts with free nitroxide to form a capped species and that this can lead to a reduction in branch points, in the resulting polymer. In this article, we show that, contrary to previous evidence, the nitroxide capped midchain species cannot be observed in nitroxide-mediated polymerization of butyl acrylate. In addition, we show that, in nitroxide-mediated polymerization, lower than expected concentrations of products arising from transfer to polymer are observed.

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polymerization, is observed and a variety of causes for this have been speculated.29 Although initially considered to be of vital importance, the chain length dependence of backbiting and propagation has been shown to have a negligible effect30,31 except for reactions performed under conditions leading to extremely short chain lengths.32 The current theories for the explanation of the reduction in branching in reversible deactivation polymerizations are centered around whether (1) under conditions of reversible deactivation there is a reduction in transfer to polymer31,33 or (2) under conditions of reversible deactivation there is a reduction in propagation from the formed midchain radical.30 One theory is that midchain radicals are trapped by the control agent employed and this leads to an additional equilibrium process that can prevent propagation from the midchain radical and, in certain cases, can strongly reduce the fraction of branching. We recently showed that for ATRP of butyl acrylate, the corresponding bromide capped midchain radicals are not detectable under standard reaction conditions34 and on this basis presented a theory based on differences in the radical lifetime leading to reduced backbiting as the cause for the reduction in branching.33 However, in contradiction to what has been shown for ATRP, it had previously been reported that in nitroxidemediated polymerizations the nitroxide capped midchain species could be observed in substantial quantities.35 Nitro-

ntramolecular transfer to polymer in radical polymerization of acrylic monomers is known to have a significant effect on the polymerization kinetics and microstructure of the resulting products.1−9 Intramolecular transfer, or backbiting, is thought to occur through abstraction of a hydrogen atom via a 6membered ring transition state that results in a midchain radical. 10,11 This radical can subsequently undergo a fragmentation process via β-scission or alternatively can propagate to yield a branched structure.12,13 It has also been shown experimentally that the midchain radical can undergo a series of subsequent transfer events resulting in migration of the midchain radical along the polymer backbone.14,15 Propagation from the midchain radical proceeds slowly, due to the stability of the tertiary radical, and results in a decrease in the overall polymerization rate.1,16,17 In addition to the backbiting process, intermolecular transfer to polymer can occur, but numerous studies have highlighted that intramolecular transfer dominates over intermolecular chain transfer to polymer and is the main pathway for branch formation.18−21 The degree of branching, which is detectable by 13C NMR,12,22,23 has significant effects on the mechanical properties of the resulting polymer and is therefore of interest for targeting polymers tailored to a specific purpose.24,25 In reversible deactivation polymerizations, such as atom transfer radical polymerization (ATRP),26 reversible addition− fragmentation chain transfer polymerization (RAFT),27 and nitroxide-mediated polymerization (NMP),28 an unexpected decrease in the degree of branching of the resulting polymers, when compared to those synthesized by uncontrolled radical © XXXX American Chemical Society

Received: February 17, 2015 Revised: April 7, 2015

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DOI: 10.1021/acs.macromol.5b00347 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. NMR spectrum of poly(butyl acrylate) synthesized by SG1 mediated polymerization synthesized at 110 °C using Blocbuilder MA as initiator. [BA]/[Blocbuilder MA] = 15.6. Top left shows the 1H NMR spectrum, and the others are 13C spectra zoomed to varying areas of interest.

tion with styrene, and (iii) a disapperance of the peak when SG1 was exchanged with TEMPO. It was highlighted that such an effect will also impact greatly on the use of equilibrium constants obtained from homopolymerization of acrylic monomers in copolymerizations where intramolecular transfer is reduced.37,38 Although these experiments would seem to confirm the interpretation put forward by the authors, there are potential alternative explanations that would subsequently alter the expected polymer microstructure and reaction kinetics. Furthermore, Guillaneuf et al. showed that no characteristic signal could be distinguished when comparing small, model alkoyamines with secondary or tertiary alkyl fragments.36 In order to shed further light on the subject, a sample of poly(butyl acrylate) synthesized by SG1 mediated polymerization has been fully characterized using a combination of NMR techniques in order to determine whether the trapped midchain radical can be observed. It is shown that under standard conditions, the midchain nitroxide species is not

xide-mediated polymerizations are especially sensitive to the presence of midchain radicals since both the formation (by backbiting) and subsequent side reactions (notably β-scission) of midchain radicals are more prevalent at the higher temperatures commonly employed in NMP and can result in disturbing effects to molecular weight distributions at high conversions.36 The observation that in ATRP the midchain species cannot be detected, while it is seemingly abundant in nitroxide polymerization, led us to revisit previous experimental work on the subject. Hlalele and Klumperman performed nitroxide-mediated polymerization of butyl acrylate using N-tert-butyl-N-[1diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1) and observed a peak around 4.4 ppm in the 1H NMR spectrum which was attributed to the α proton of SG1 attached reversibly to a midchain radical (see structure in Figure 1).35 This assignment was based on: (i) prediction of the spectrum based on a chemical shift substituent increment scheme, (ii) a reduction in the observed peak in SG1-mediated copolymerizaB

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HPLC grade THF at concentrations of about 4 mg mL−1 and then was injected into the equipment. The dn dc−1 used for the molar mass calculation was the one corresponding to pBA (dn dc−1 = 0.064 mL g−1).39 The SEC/MALS data was analyzed by using the ASTRA software version 6.0.6. (Wyatt Technology Corp., USA). The absolute molar mass was calculated from the MALS/RI data using the Debye plot (with first order Zimm formalism). Nitroxide-Mediated Polymerization. Bulk nitroxide-mediated polymerizations were carried out in a 25 mL round-bottom flask. In a typical reaction, n-butyl acrylate (10 g, 78 mmol) was added to Blocbuilder MA (1.95 g, 5 mmol) and SG1 (30 mg, 0.1 mmol) and the oxygen was removed by continuous nitrogen bubbling for 1 h. The reaction mixture was heated to 110 °C in an oil bath with continuous stirring and left for 7 h. Fractional conversion, X, was measured by NMR by comparison of the integral of the vinyl protons to the combined signal of the OCH2 signals from both monomer and polymer. The polymer solution was precipitated in an ice cold mixture of MeOH:water (50:50 wt/wt) and was centrifuged. The polymer was separated from the solvent by decantation and dissolved in dichloromethane. The precipitation−centrifugation process was repeated and the resulting polymer was dried under vacuum. Nitroxide Exchange Experiments. A sample of poly(butyl acrylate) synthesized at [BA]/[Blocbuilder MA] = 15.6 (0.5 g, Mn =1300, 4 × 10−4 mol) and TEMPO (0.13 g, 8 × 10−4 mol) were dissolved in toluene (1 g) and deoxygenated by constant nitrogen bubbling for 1 h.40 The reaction mixture was heated to 100 °C for 2 h. The polymer solution was precipitated in an ice cold mixture of MeOH:water (50:50 wt/wt) and was centrifuged. The polymer was separated from the solvent by decantation and dissolved in dichloromethane. The precipitation−centrifugation process was repeated, and the resulting polymer was dried under vacuum.

present and, furthermore, the concentrations of other species known to result from midchain radicals are much lower than may be expected. We postulate that the cause of the reduction of these products is due to a lower rate of intramolecular transfer to polymer as a result of the reversible deactivation process and highlight the positive effects this may have on the kinetics of nitroxide-mediated radical polymerizations.

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EXPERIMENTAL SECTION

Materials. N-Butyl acrylate (Quimidroga, technical grade) was purified by distillation and was kept at −20 °C until use. Blocbuilder MA and SG1 were provided by Arkema and were used as received. TEMPO (98%, Aldrich) was used as received. All other solvents were purchased from Scharlab, were of technical grade, and were used without purification. Methods. NMR spectra were recorded at 25 °C on a Bruker AVANCE 500 MHz equipped with a z-gradient double resonance probe at a concentration of approximately 300 mg mL−1 in CDCl3. The central solvent peak was used as an internal reference (δ C 77.0; δ H 7.27). 1D 1H spectra were acquired by use of 32k data points which were zero-filled to 64k data points prior to Fourier transformation. 1D 13 C spectra were recorded at a 13C Larmor frequency of 125.77 MHz. The spectra were recorded using 20000 transients. Quantitative 13C spectra were recorded using single pulse excitation, using a 5.5 μs 90° pulse, inverse gated waltz16 decoupling to avoid NOE effects and a relaxation delay of 10 s. Apodization was achieved using an exponential window function equivalent to a line width of 10 Hz. 1D 13C DEPT135 spectra (distortion-less enhancement by polarization transfer) were acquired for 16000 transients using single pulse excitation, using a 5.5 μs 90° pulse, inverse-gated waltz16 decoupling to avoid NOE effects, and a relaxation delay of 10 s. Apodization was achieved using an exponential window function equivalent to a line width of 3 Hz. 2D NMR spectra were recorded in HSQC (heteronuclear single quantum coherence) and HMBC (heteronuclear multiple bond coherence) experiments. The spectral widths for the HSQC experiment were 5000 and 25000 Hz for the 1H and the 13C dimensions, respectively. The number of collected complex points was 2048 for the 1H dimension with a recycle delay of 5 s. The number of transients was 64 and 256 time increments were recorded in the 13C dimension. The 1JCH used was 140 Hz. The J-coupling evolution delay was set to 3.2 ms. The squared cosine−bell apodization function was applied in both dimensions. Prior to Fourier transformation the data matrices were zero filled to 1024 points in the 13C dimension. For the HMBC experiment the spectral widths were 2000 and 32000 Hz for the 1H and the 13C dimensions, respectively. The 1JCH used was 145 Hz and n JCH was 10 Hz. The J-coupling evolution delay was set to 3.4 and 50 ms. The number of transients was 1024 and 16 time increments were recorded in the 13C dimension. The spectrum was processed with a cosine window function in each dimension and one level of zero filling in F1 and is presented in magnitude mode. The average molar mass was analyzed by SEC/MALS. The equipment was composed by a LC20 pump (Shimadzu) coupled to a miniDAWN Treos multiangle( three angles) light scattering laser photometer equipped with an He−Ne laser (λ = 658 nm), an Optilab Rex differential refractometer (λ = 658 nm) (all from Wyatt Technology Corp., USA). Separation was carried out using one column (Styragel HR2, with pore size of 102 Å). Filtered toluene (HPLC-grade from Sigma-Aldrich) was used for the calibration of the 90° angle scattering intensity. The detectors at angles other than 90° in the MALS instrument were normalized to the 90° detector using a standard (PS 28 770 g mol−1, Polymer Laboratories), which is small enough to produce isotropic scattering, at a flow rate of THF through the detectors of 1 mL min−1. In addition, the same standard and conditions were used to perform the alignment (interdetector delay volume) between concentration and light scattering detectors and the band broadening correction for the sample dilution between detectors. The analysis was performed at 35 °C and THF was used as mobile phase at a flow rate of 1 mL min−1. The dried polymer was diluted in

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RESULTS In order to test the original hypothesis that a significant amount of dormant midchain nitroxide species are formed, poly(butyl acrylate) was synthesized by SG1-mediated polymerization starting from a high concentration of a commercial alkoxyamine initiator, and the resulting low molecular weight polymer was analyzed in detail by a combination of NMR techniques. Figure 1 shows the 1H and 13C NMR spectra of the poly(butyl acrylate) polymer measured under quantitative conditions with the assigned structures. The assignment was based on a combination of 1D and 2D NMR experiments including DEPT-135, COSY, HMBC, and HSQC. Both the spectra and a summary of the results leading to the assignments in Figure 1 are given in the Supporting Information. Two key peaks in this work are those located in the 1H spectrum at approximately 3 ppm which relate to the proton next to the nitrogen atom from an end chain capped species (14 in Figure 1) and at approximately 4.3 ppm which has previously been assigned to the same proton of a midchain capped species35 (see highlighted structure in Figure 1). However, the assignment of this proton to the midchain nitroxide is incompatible with several features of the NMR spectra and is more in line with the present assignment of being a hydrogen next to the nitroxide unit (12 in Figure 1). This assignment was based on four key features: (1) The relative integral value of the two signals is close to 1. Such a value would be extremely unlikely (although not impossible) if it is assumed that the peak at 4.3 ppm is from the midchain species, but it would be expected if the two signals are from different protons of the same species, namely hydrogen 12 in Figure 1. (2) By comparison with the corresponding peaks in 13C NMR, as determined by the HSQC spectrum, the C

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Figure 2. 13C NMR spectrum of poly(butyl acrylate) synthesized by SG1 mediated polymerization synthesized at 110 °C using Blocbuilder MA as initiator ([BA]/[Blocbuilder MA] = 15.6) before (top) and after nitroxide exchange with TEMPO at 100 °C for 2 h in toluene.

the idea that this peak arose from the SG1 moiety.35 However, knowing the related signals in the 13C NMR this experiment was repeated showing a shift of signal assigned to the terminal unit upon nitroxide exchange, but no decrease in intensity like the other signals related to SG1 (see Figure 2). In the 1H NMR a similar shift upon nitroxide exchange will result in the signal being obscured by the presence of the much larger OCH2 peak from the main butyl acrylate structure. It is for this reason that the intensity of the peak is decreased in the 1H NMR spectrum. This experiment further supports the idea that the proton signal in the NMR spectrum is not related to the midchain nitroxide but rather to the terminal butyl acrylate unit and the combination of experiments performed herein show that no midchain nitroxide exists under standard reaction conditions at high concentrations of initiator. In order to confirm that this was not a chain length dependent effect, a further experiment was conducted at longer target chain length ([BA]/ [Blocbuilder MA] = 236). The 13C NMR spectra of the two polymers are shown in Figure 3. It can be observed that while there is a decrease in intensity of all peaks related to the

chemical shift of the peak is in excellent agreement with the chemical shift predicted by chemical shift substituent increment scheme. (3) The number-average degree of polymerization from NMR analysis assuming the given structure (DPn = 7, Mn = 1300 Da) is in good agreement with data from SEC/ MALLS analysis of the polymer (Mn = 1300 Da, PDI = 1.104). If, as originally postulated, both the hydrogens at 4.3 ppm (from midchain) and 3 ppm (from endchain) were originated from the SG1 molecule, then the number-average degree of polymerization from NMR would be around DPn = 3.5 which is unrealistically small, given the results from SEC and based on the theoretical degree of polymerization for the given conversion value (X = 0.52, Mn,theo = 1420, DPn = 8). (4) From the number-average molecular weight above, assuming that the peak at 4.3 ppm is due to midchain radicals, the ratio of backbiting to propagation events, can be approximated from the ratio of the assumed midchain nitroxide to the number of propagation events giving a value of 9.95%. This value is unrealistically high given that even at extremely high temperatures (>140 °C) and high conversions (>99%) where backbiting is favored a value of 10% is rarely exceeded.9 The assignment shown in Figure 1 is in contradiction to previous work35 on the key peaks in the 1H NMR spectrum previously assigned to the midchain nitroxide structure. The previous assignment was based not only on the chemical shift of the signal but also on experiments designed to minimize the formation of the midchain species and thus observe a reduction in the peak at 4.4 pm which was assigned to the nitroxide capped midchain species. It was shown that copolymerization of styrene results in a decrease of the peak in the proton located at 4.3 ppm and this was assumed to be caused by the known reduction in chain transfer to polymer in the presence of comonomers.4,41 Given the current assignment, a similar effect would be observed since the majority of terminal groups in BA/ styrene copolymerization are styrene and thus the chemical shift of the terminal proton would be changed. In addition, it has been previously shown that nitroxide exchange of the SG1 moiety for TEMPO results in a loss of signal at 4.3 ppm in the 1H NMR, which was used to support

Figure 3. 13C NMR spectrum of poly(butyl acrylate) synthesized by SG1 mediated polymerization synthesized at 110 °C using Blocbuilder MA as initiator (top, [BA]/[Blocbuilder MA] = 15.6, Mn = 1300, PDI = 1.1) and (bottom, [BA]/[Blocbuilder MA] = 236, Mn = 20700, PDI = 1.5). D

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specific reaction on the midchain radical that results in deactivation/decomposition of the nitroxide, on branching level is limited by the quantity of nitroxide used. For example, the presence of midchain nitroxide capped species can only affect the total branching density when its concentration is on the same order of magnitude as the branching density itself. For the highest concentration used here, this may be envisaged since the amount of nitroxide used can account for around 6 mol % of the total concentration. However, in the reaction conducted at low initiator concentration, the total amount of nitroxide in the system was less than 0.5 mol %; thus, it can only reduce the branching by a maximum of this amount, assuming that all the nitroxide molecules are trapping the midchain radicals. Thus, even if patching of the midchain radical by the nitroxide did occur, which the evidence presented above shows is not the case, then it would be insufficient to explain the reduction in branching density when compared to a standard free radical polymerization. One additional effect to consider is that of molecular weight, which varies substantially between samples and may partially explain differences between branching fractions. Given that backbiting tends to occur via a 6-membered ring transition state that requires a minimum of 3 monomeric units, at low chain lengths this requirement can severely impact on the degree of branching.32 A growing oligomeric chain of 7 monomer units, for example, will only have had competition between propagation and backbiting after the chain length exceeds 3. Thus, there is an expected reduction in the branching fraction of around 40% since a number of steps have proceeded in the complete absence of backbiting. However, as chain length increases and (DPn − 3)/ DPn ≈ 1, then this effect becomes negligible and is insufficient to explain the differences in branching that are observed experimentally. Thus, the polymerization at lower intitator concentration with DPn = 162, has an estimated reduction of branching of 2%, compared to the 85% reduction observed experimentally (see Table 1). That the reduction in branching is not linked directly to molecular weight can be further observed by comparison with experiments using high concentrations of chain transfer agents at similar temperatures in which the molecular weight was significantly smaller than that of the lower initiator concentration polymerization conducted here but branching was significantly higher (Mn = 2600 Da, Cq = 1.9%).43 In addition, experiments conducted at higher free nitroxide concentration but at constant initiator concentration have similar average molecular weights but lower branching fractions.28 Thus, the present results suggest that under conditions of reversible deactivation, imposed by the presence of the nitroxide species, the rate of backbiting is reduced compared to that of propagation. Although with classical reaction kinetics this is not physically possible, it is a natural result of a recently proposed model that uses a hypoexponential function for the probability density function of each individual reaction.33 It should, however, be noted that this model has yet to be conclusively proven and thus must be treated with caution. Under the assumption of this theory, changing the transient lifetime, as occurs when an additional kinetic event such as radical deactivation in controlled radical polymerization is possible, can effect the ratio of backbiting to propagation. This is particularly important in nitroxide polymerizations where the high temperatures typically employed favor formation of midchain radicals. It may therefore be expected that the decrease in midchain radical fraction will have positive

polymer end groups, as expected due to the increased target chain length, there are no additional unaccounted peaks in the spectrum. One aspect to consider is that if the midchain nitroxide cannot be observed as the experimental evidence herein suggests, then the equilibrium constant between active and dormant midchain radicals must be relatively high with respect to the equivalent secondary radical equilibrium, since in a typical radical polymerization of butyl acrylate a large fraction (>70%) of the radical species exist as tertiary radicals yet the corresponding capped radical is not observed. That the equilibrium constant is higher is perhaps unsurprising given the observed effect in SG1 mediated polymerization of methyl methacrylate (MMA), where the tertiary MMA radicals have equilibrium constant of K = 10−7 M compared to SG1 mediated polymerization of butyl acrylate where the secondary radicals have equilibrium constant of K = 10−11 M.42 Furthermore, given that the nitroxide in the midchain could not be observed, it is interesting to consider the consequential effects on both kinetics and microstructure of acrylic (co)polymers produced by NMP. Under the assumptions of classical reaction kinetics, the rate of intramolecular transfer to polymer is identical in both controlled and free radical polymerization. Thus, the sum of the products of the reactions of resulting midchain radicals (branch points formed by propagation, macromonomers formed through β scission and midchain nitroxide formed by deactivation of a midchain radical by combination with nitroxide) should be identical in both cases. In a typical bulk free radical polymerization of butyl acrylate conducted at 100 °C propagation is dominant pathway of the midchain radical and a typical branching fraction is 4%.43 In the experiments conducted herein, no measurable products of beta scission or midchain capped nitroxide were observed while the branching levels were 0.6% for the higher molecular weight butyl acrylate and