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Is Molecular Weight or Degree of Polymerization a Better Descriptor of Ultrasound-Induced Mechanochemical Transduction? Preston A. May, Nicholas F. Munaretto, Michael B. Hamoy, Maxwell J. Robb, and Jeffrey S. Moore* Beckman Institute for Advanced Science and Technology and Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States S Supporting Information *
ABSTRACT: A detailed understanding of the fundamental processes that govern mechanical transduction in covalent polymer mechanochemistry is essential to advance innovation in this field. In contrast to progress in the development of new mechanophores, the influence of polymer structure and composition on mechanochemical activity has received relatively little attention. In order to address this gap in knowledge, a continuous flow system with synchronous UV−vis absorption capabilities was designed to quantify the ultrasound-induced mechanical activation of a spiropyran mechanophore in real-time. Measurements of reaction kinetics with polymer tethers of varying repeating unit structure demonstrate that degree of polymerization is the key descriptor of mechanochemical activity, independent of molecular weight and pendant group constitution. These results have important implications for the rationalization of mechanochemical properties and the design of new mechanochemically active polymer systems.
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of polymerization (i.e., polymer chain length) is a subtle, but important consideration for the design and analysis of mechanochemical systems. Prior to the development of the mechanophore concept, early work on the ultrasonic degradation of polymers implicated the role of chain length as a critical parameter that governs the rate of polymer chain scission.16 However, the literature reveals considerable disagreement about this issue and, to the best of our knowledge, the debate has not been satisfactorily resolved.17 Given the significant advances in polymer synthesis and analysis, as well as the availability of mechanophores to serve as standard probes of mechanochemical activation, we sought to rigorously address this elementary question. To this end, we have carried out experiments that decouple the effects of molecular weight and degree of polymerization to determine which characteristic is the better descriptor of mechanochemical transduction and ultimately responsible for governing mechanophore reactivity. We chose to investigate the mechanochemical ring-opening reaction of spiropyran as it is a model mechanophore for studying mechanochemical reactivity in diverse systems.8,18−25 The distinct changes in visible light absorption that accompany the mechanical transformation of spiropyran into its merocyanine form provide a robust handle for measuring rates of mechanochemical activation. In order to acquire kinetic information directly from a single spiropyran mechanophore
olymer mechanochemistry is a burgeoning area of scientific research.1 In this context, the fundamental mechanism of interest is the transduction of macroscopic forces into specific chemical bonds positioned within a macromolecule. The chemical transformations that result from mechanically facilitated reactions present unique opportunities to program intrinsic physical responses into materials such as self-sensing of mechanical integrity and autonomous restoration of structural damage in self-healing materials. For example, a number of mechanically induced chemical transformations have been demonstrated including activation of latent catalysts,2,3 generation of reactive groups,4−7 and the widely studied ringopening reaction of spiropyran which is accompanied by a dramatic change in color.8,9 The term “mechanophore” has been adopted to describe a molecular unit that responds in a chemoselective fashion to mechanical perturbation. Among the many methods of supplying the forces necessary to elicit mechanophore activation, ultrasound irradiation of polymer solutions is a popular technique due to the small sample quantities required, good reproducibility, achievement of high strain rates, and direct inference of mechanochemical changes using conventional analytical methods.10,11 Attachment of polymer chains is essential for transduction of mechanical energy to the mechanophore, yet few reports12−15 have investigated the effects of the physical properties of the polymers on mechanophore activation with most regarding the chains as generic handles. Despite progress in understanding mechanically coupled reactivity, fundamental questions about the transduction of forces to a mechanophore still remain. For example, the distinction between molecular weight and degree © XXXX American Chemical Society
Received: November 25, 2015 Accepted: January 7, 2016
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DOI: 10.1021/acsmacrolett.5b00855 ACS Macro Lett. 2016, 5, 177−180
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ACS Macro Letters
polymerization and vice versa due to variation in the molecular weight of the repeating units. Additionally, the poly(butyl acrylate) series are constitutional isomers, allowing investigation of the effects of side-chain branching on mechanochemical behavior. The mechanochemical reactivity of these series of polymers was characterized using the custom flow setup described above with continuous ultrasound irradiation (10.7 W cm−2) at 3−5 °C. Methyl ethyl ketone (MEK) was chosen as the solvent for these experiments due to the good solubility of all of the polymers. A constant flow rate of 4.5 mL min−1 was used, although it is important to note that variation in the flow rate was observed to have no effect on mechanochemical activity. Upon subjecting the polymers to ultrasound irradiation, an increase in the UV−vis absorption of the solution was observed with a λmax of 570 nm corresponding to the formation of the ring-opened merocyanine (Figure 3a). The position of this
embedded in a polymer chain, we developed a high-throughput flow system that couples ultrasound irradiation of a polymer solution with synchronous optical spectroscopy measurements to quantify changes in UV−vis absorption in real-time (Figure 1). The experimental setup consists of a typical ultrasonication
Figure 1. Schematic representation of the flow-cell design employed for measuring in situ ultrasonic activation rates of the spiropyran mechanophore.
reaction vessel (Suslick cell) equipped with an argon inlet and a peristaltic pump to transport the reaction solution through a UV−vis flow-cell and return the fluid to the reaction vessel continuously throughout the course of each sonication experiment. Using this technique, we were able to systematically investigate a series of spiropyran-linked polymers and demonstrate that degree of polymerization, not molecular weight, is the key criterion that determines the kinetics of mechanochemical activation. A series of acrylate polymers containing a spiropyran mechanophore positioned near the chain midpoint with a range of molecular weights (Mn = 49−309 kDa) were synthesized from a bis-functional spiropyran initiator by living radical polymerization8 (see the Supporting Information (SI) for details). Five different acrylate monomers with varying ester substituents (i.e., methyl, ethyl, n-butyl, iso-butyl, and tert-butyl) were polymerized to investigate the effect of polymer composition and side-chain constitution on mechanophore activation while keeping the mechanophore structure and polymer attachment sites constant (Figure 2). Critically, the differences in side chain composition/constitution allow us to compare the mechanochemical effects of molecular weight and degree of polymerization independently since polymers with similar molecular weights will have different degrees of Figure 3. (a) UV−vis absorption measurements performed in realtime during ultrasound irradiation demonstrate the mechanochemical ring-opening reaction of spiropyran to generate merocyanine with a λmax at 570 nm. (b) Absorbance of merocyanine monitored as a function of ultrasound irradiation time. Fitting the data to eq 1 provides a quantitative analysis of the kinetics of the mechanochemical reaction. Data shown is for PMA (Mn = 156 kDa).
peak was consistent for all polymers as well as the small molecule initiator (activated with UV light) under the experimental conditions employed. Importantly, ultrasonication of control polymers, which contained spiropyran only at the chain-end and therefore should not be susceptible to mechanical forces, did not show any increase in absorption
Figure 2. Structures of spiropyran-linked acrylate polymers with different repeating unit composition and side chain constitution. 178
DOI: 10.1021/acsmacrolett.5b00855 ACS Macro Lett. 2016, 5, 177−180
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ACS Macro Letters upon ultrasound irradiation (see Figures S1−S5 in the SI); however, these polymers did exhibit the characteristic absorption spectrum of the merocyanine after irradiation with UV light which confirmed the presence of the spiropyran unit. The rate of mechanochemical transformation of the spiropyran mechanophore into its merocyanine form was measured by plotting the absorbance of the merocyanine species at 570 nm as a function of sonication time (Figure 3b). Nonlinear least-squares fitting of the data with eq 1 gives the rate constant for the reaction, k: A t = B(1 − e−kt )
(1)
where t is sonication time, At is the absorbance at time t (λ = 570 nm), and B is the amplitude (maximum absorbance value).26 The absorbance of merocyanine was observed to asymptotically level off as the maximum concentration of merocyanine species was reached. It is important to note that spiropyran and merocyanine exist in equilibrium, which can be biased by a number of different stimuli including light, heat, solvent interactions, etc.27 Upon irradiation with ultrasound, the ring-opening reaction of spiropyran proceeds until the system reaches a new dynamic equilibrium, a mechanostationary state, where the merocyanine form is favored. Detailed experiments were performed to characterize the kinetics of thermal reversion of the merocyanine species to spiropyran under similar conditions employed for ultrasound-induced mechanical activation (see the SI for details). Critically, these data confirmed that the rates of thermal reversion are significantly slow (order of magnitude) compared to the forward mechanochemical reaction for all polymer compositions. Accordingly, the rate constants for thermal reversion were excluded from calculations of mechanochemical activation rates. The rate constant for mechanochemical activation of spiropyran was measured for each polymer and plotted against either molecular weight (Mn) or degree of polymerization (Figure 4). The latter values were obtained simply by dividing Mn by the molecular weight of the repeating unit. First, examination of the rate of mechanochemical activation as a function of molecular weight reveals a threshold Mn for each polymer as expected for mechanical activation.28 More importantly, differences in reactivity commensurate with changes in the composition of the polymer are observed (Figure 4a). For example, the rate of activation of spiropyran in PMA polymers was fastest for a given molecular weight, followed by the PEA series, and finally the PBA series which was least reactive. It is interesting to note that this trend agrees with prior data for the nonspecific ultrasonic degradation of a similar series of polymers.29 Each rate measurement was performed in duplicate and pairwise t tests confirmed a statistically different slope between PMA, PEA, and all other polymers. Furthermore, we found no statistical difference in slope or x-intercept between any of the butyl acrylate polymers, indicating that the connectivity of alkyl groups in the side chain does not significantly influence the mechanochemical reactivity. Alternatively, when the rate of mechanochemical activation is plotted with respect to degree of polymerization, which gives the average number of repeating units per chain, the data for all polymer series collapse onto a single linear regression (Figure 4b). Statistical analysis confirmed that there is no difference in slope or x-intercept for any of the individual regression lines. Critically, these data indicate that degree of polymerization is the fundamental criterion that determines the rate of
Figure 4. Rate of mechanochemical activation of spiropyran as a function of (a) number-average molecular weight, and (b) degree of polymerization. All rate data collapse onto a single linear regression when plotted against degree of polymerization, independent of the composition or side chain constitution of the individual polymers. Each data point is the average of two measurements with the error bars denoting the range of the two values.
mechanochemical reactivity. That is, in order to increase the rate of mechanophore activation, a proportional increase in the number of repeating units per chain is needed, independent of the molecular weight of the individual repeating units. This relationship can be rationalized in terms of the bead−rod model30 in which the force experienced by a (fully extended) polymer chain is dependent upon the number of beads along the contour length. This model also predicts a dependence on the diameter of the bead; however, the effect of significantly longer alkyl substituents on the activation rate remains a question for further investigation. Polymer chains are paramount for mechanical transduction of forces to a covalently linked mechanophore. Nevertheless, the influence of compositional and structural properties of the polymers on the efficiency of mechanochemical activation has received relatively little attention. Here, the use of a custom designed flow setup to measure the rate of mechanochemical activation of a spiropyran mechanophore in real-time during ultrasonic irradiation has enabled the investigation of a series of acrylate polymers with different repeating unit compositions and side-chain branching. These studies demonstrate that degree of polymerization is the fundamental property underlying the kinetics of mechanical transduction for this series of polymers. These results are in contrast to the molecular weight dependence of mechanochemical activation rates typically 179
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(18) Lee, C. K.; Davis, D. A.; White, S. R.; Moore, J. S.; Sottos, N. R.; Braun, P. V. J. Am. Chem. Soc. 2010, 132, 16107−16111. (19) O’Bryan, G.; Wong, B. M.; McElhanon, J. R. ACS Appl. Mater. Interfaces 2010, 2, 1594−1600. (20) Beiermann, B. A.; Davis, D. A.; Kramer, S. L. B.; Moore, J. S.; Sottos, N. R.; White, S. R. J. Mater. Chem. 2011, 21, 8443−8447. (21) Kingsbury, C. M.; May, P. A.; Davis, D. A.; White, S. R.; Moore, J. S.; Sottos, N. R. J. Mater. Chem. 2011, 21, 8381−8388. (22) Beiermann, B. A.; Kramer, S. L. B.; Moore, J. S.; White, S. R.; Sottos, N. R. ACS Macro Lett. 2012, 1, 163−166. (23) Lee, C. K.; Diesendruck, C. E.; Lu, E.; Pickett, A. N.; May, P. A.; Moore, J. S.; Braun, P. V. Macromolecules 2014, 47, 2690−2694. (24) Gossweiler, G. R.; Hewage, G. B.; Soriano, G.; Wang, Q.; Welshofer, G. W.; Zhao, X.; Craig, S. L. ACS Macro Lett. 2014, 3, 216−219. (25) Gossweiler, G. R.; Kouznetsova, T. B.; Craig, S. L. J. Am. Chem. Soc. 2015, 137, 6148−6151. (26) Wohl, C. J.; Kuciauskas, D. J. Phys. Chem. B 2005, 109, 21893− 21899. (27) Minkin, V. I. Chem. Rev. 2004, 104, 2751−2776. (28) Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Chem. Rev. 2009, 109, 5755−5798. (29) Mahalik, J. P.; Madras, G. Ind. Eng. Chem. Res. 2005, 44, 6572− 6577. (30) Odell, J. A.; Keller, A. J. Polym. Sci., Part B: Polym. Phys. 1986, 24, 1889−1916.
reported in the literature and serve as a guide for understanding, and disseminating, mechanochemical phenomena, particularly when comparing the mechanochemical properties of disparate polymeric materials.
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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.5b00855. Experimental details, GPC data, UV−vis characterization, control experiments, and kinetic analyses (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by the National Science Foundation (DMR 1307354) and a ARO MURI (W911NF-071-0409). The authors thank Dr. Charles Diesendruck, Dr. Matt Kryger, and Dr. Koushik Ghosh for helpful discussions and Dorothy Loudermilk for assistance with graphics. M.J.R. gratefully acknowledges the Arnold and Mabel Beckman Foundation for a Beckman Institute Postdoctoral Fellowship.
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