Letter pubs.acs.org/macroletters
Following Homolytic Mechanochemical Kinetics with a Pyrenyl Nitrone Spin Trap Feng Wang, Michal Burck, and Charles E. Diesendruck* Schulich Faculty of Chemistry, Technion − Israel Institute of Technology, Haifa, 32000, Israel S Supporting Information *
ABSTRACT: The mechanochemical stability of a polymer is a fundamental parameter when choosing the ideal material for many different uses where mechanical loading may induce molecular weight reduction. The use of mechanophores has significantly improved the detection of mechanochemical reaction, but their incorporation to different polymers can be synthetically challenging. Alternatively, we return to the old strategy of using spin traps to quantify the radicals produced as a consequence of mechanochemical homolytic bond scission events. Several new spin traps have been developed in recent decades, and pyrenyl nitrones have been shown to effectively bind radicals, providing a spectroscopic methodology to follow radical concentration. Here we demonstrate the use of these probes as excellent tools to follow mechanochemical chain scission.
A
s a consequence of their high molecular weight, polymers present unique properties that make them the materials of choice for numerous day-to-day uses, both as bulk materials and in solutions. In many of these uses, polymers experience mechanical loadings at different levels during their lifetime. Already in the 1930s, Staudinger demonstrated that mechanical stress is capable of inducing covalent bond scission leading to a reduction in molecular weight.1 Given that many of the desired properties in polymers and their solutions are a consequence of their high molecular weight, mechanochemistry is a key factor behind the decay in performance over time.2 The mechanochemical stability of different polymers depends on many different parameters such as main chain bond lengths, polarity and electron density. For example, in the last 20 years, by introducing longer bonds at the center of linear chains, selective covalent scission has been developed,3 providing new opportunities in the development of mechanically responsive materials, such as damage detection4 and selfhealing,5 by triggering specifically designed molecules called mechanophores.2 Spectroscopic tools, both as external probes or mechanophores, have been developed in order to study how different parameters affect the mechanochemical kinetics. With a few exceptions,6 most polymers undergo homolytic bond scission, and therefore, spin traps such as 2,2-diphenyl-1-picrylhydrazyl (DPPH,7 1) and pentamethyl nitrosobenzene (PMNB,8 2) have been used in combination with UV−vis or ESR spectrometry to measure the degradation kinetics. With the emergence of mechanophores such as spiropyran (SP, 4),4a dioxetanes9 (5), and diarylbibenzofuranone10 (6), among others,11 important information on how stress propagates in a material also became accessible (Figure 1).4 Importantly, mechanophores have to be incorporated into specific positions © XXXX American Chemical Society
Figure 1. Spectroscopic tools used to study mechanochemical kinetics.
in the polymer chains, as they are directly triggered by the mechanical stress. Yet, as polymer chemists develop new polymers and polymer architectures, an easy and fast approach to test the mechanochemical kinetics and stability is required. Recently, Moore et al. demonstrated a simple methodology to test the mechanochemical kinetics of polymers by connecting the ultrasonication Suslick cell to a spectrophotometer, and following the change in spectrum by the conversion of SP into its merocyanine form.12 This method provides hundreds of online data measurements instead of a few which are obtained by offline measurements. A limitation of this methodology is the fact that SP still needs to be incorporated in an equimolar amount to the center of the polymer chains being compared. Typically, this functionalizaReceived: November 16, 2016 Accepted: December 21, 2016
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DOI: 10.1021/acsmacrolett.6b00874 ACS Macro Lett. 2017, 6, 42−45
Letter
ACS Macro Letters
Figure S2), due to the radicals produced by sonochemistry,14 as previously observed by De Vries.6a Nitrones are spin traps that have been successfully used to regulate radical propagation in radical polymerization.15 Recently, Studer et al. demonstrated that they can also be used to follow radical polymerization kinetics.16 Nitrones are less sensitive compared to DPPH, which may provide an advantage given that sonoradicals are very short-living. Therefore, we decided to test pyrenyl nitrones spin-traps as spectroscopic tools to measure the homolytic mechanochemical chain scission kinetics. Since short-living sonochemical radicals are always present, we started by testing the nitrones in the absence of polymers. After careful washing of the system with the nitrone solution and N2 bubbling to reduce the effect of O2, 1-pyrenecarboxaldehyde-derived nitrones bearing methyl (3a), benzyl (3b), or t-butyl (3c) substituents were sonicated in acetonitrile (ACN), a solvent commonly used in mechanochemical experiments. The slowest degradation under sonication was observed with the t-butyl substituent 3c (see SI, Figure S5), and therefore, we tested 3c in additional polar aprotic organic solvents, including DMF, 1,2-dimethoxyethane (DME), ethyl acetate (EtAc), methyl−ethyl ketone (MEK), and THF (BHT-free). In most of these solvents, a similar disappearance rate for the 391 nm peak absorbance was observed (see SI, Figure S4), with THF being the only exception: it did not provide repeatable results and is therefore unsuitable for these studies. Importantly, the
Figure 2. Picture of the flow system used for this study. Liquid flow path is indicated by yellow arrow.
tion has been achieved using SP diinitiators for radical polymerization;4a however, these initiators cannot be used in cationic or anionic polymerization. SP has also been incorporated in polycondensates,13 but controlling their location and distribution in the chain is tricky. Instead, we decided to return to the classic DPPH spin trap, which was used in the past in combination with ultrasonication to study the mechanochemistry of PMMA7 and PDMS.6a Unfortunately, DPPH is consumed quite rapidly under sonication, even in the absence of polymers (see the Supporting Information (SI),
Figure 3. Sonication of different PMAs (5 mg/mL) with 3c (30 uM) in ACN. (a) Change in UV−vis spectra as a function of sonication time with PMASS271. (b) Second order kinetic graphs for the PMASSs tested. (c) Change in UV−vis spectra as a function of sonication time with PMA360. (d) Second order kinetic graphs for the PMAs tested. 43
DOI: 10.1021/acsmacrolett.6b00874 ACS Macro Lett. 2017, 6, 42−45
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ACS Macro Letters
3c against the first order kinetic constants obtained by the change in molecular weight method. The rates show a direct correlation, indicating the probe is indeed providing correct relative information, but only when comparing polymers that form the same type of radical. PMAs and PMASSs do not fall in the same curve, indicating that the lifetime of the radical produced affects their detection by the spin-trap. To conclude, we tested a known nitrone spin trap that was recently used to measure polymerization kinetics to follow mechanochemical chain scission kinetics by ultrasonication of polymers in dilute solutions. We optimized the probe and solvents to obtain the minimum baseline signal due to sonochemistry. The method, which uses a flow cell and a spectrophotometer, allows for a fast, clean, and simple collection of hundreds of signal points for the comparison of the mechanochemical rates of different polymers, without the need of synthesis of mechanophores or polymers. The use of this simple method will allow a fast screening of new polymers for uses where mechanochemical scission is an important factor on the lifetime and performance of the material.
Figure 4. Comparison of kinetic constants measured with new trap and by change in molecular weights.
solvent parameters that affect the mechanochemical reaction are different from those that affect sonochemistry.17 Therefore, we decided to continue our studies in ACN, a solvent that combines low vapor pressure, low gas solubility, and reasonable viscosity and, therefore, presents a suitable environment for high mechanochemical rates.18 We prepared a series of poly(methyl acrylates) (see SI, Table S1) with different degrees of polymerization (PMAMn) and, therefore, different mechanochemical rates,12 as well as PMAs with a chain-centered disulfide mechanophore (PMASSMn), which not only undergo faster mechanochemical scission,19 but also produce a longer lived radical. Each polymer was dissolved in ACN (5 mg/mL) with the spin trap 3c (30 μM) and circulated using a peristaltic pump and Teflon tubing, from the Suslick cell to a flow cell in the spectrophotometer and back (Figure 2). Under flow conditions only, no change in the 3c spectrum was observed (see SI, Figure S3). Then, the solution was sonicated (1 s on, 2 s off, 2 h in total) at −2 °C, and the full spectrum was measured to follow the kinetics. As observed in Figure 3a, the peaks at 370 and 391 nm decrease with time as radicals are being produced. Assuming second order kinetics, following the bimolecular reaction mechanism described by Studer et al., radical production was followed by the decay of the 391 nm signal.16 Importantly, the spectrum change kinetics was significantly accelerated in the presence of PMAs compared to the baseline decay of the probe due to sonochemistry only. As expected, similar polymers with higher degree of polymerization (molecular weight) presented faster kinetics (see SI, Figures S9−S12). Also, the disulfide containing polymers presented significantly faster rates (see SI, Figures S13−S16), indicating that the spin trap is useful in the study of both polymers that undergo scission to carbon and sulfur radicals in their mechanochemical reaction. In order to understand if the probe is really providing meaningful relative results between the polymers, we repeated the sonication experiments and took aliquots every 15 min to be measured offline using GPC. Using the method for direct calculation of mechanochemical rate developed by Malhorta,20 we obtained the rate constants for each polymer under our sonication conditions by measuring the change in numberaverage molecular weight (see SI, Figures S17−S32). Importantly, these are real rates of mechanochemical scission, while the ones obtained using the probe are relative. Figure 4 shows a plot of the second order rate constants measured with
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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/acsmacrolett.6b00874. Polymer syntheses, GPCs, UV−vis spectra, kinetic curves (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Charles E. Diesendruck: 0000-0001-5576-1366 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based on work supported by the Israel Science Foundation (Grant No. 920/15). The authors thank Chemada Fine Chemicals for the kind donation of high-purity methyl 2bromo-2-methylpropionate. C.E.D. is grateful to the American Technion Society for a Women’s Division Career Advancement Chair. F.W. is grateful to the GTIIT for a postdoctoral fellowship.
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