ARTICLES PUBLISHED ONLINE: 28 APRIL 2014 | DOI: 10.1038/NCHEM.1938 Mechanically triggered heterolytic unzipping of a low-ceiling-temperature polymer Charles E. Diesendruck1,2,GregoryI.Peterson4, Heather J. Kulik5†,JoshuaA.Kaitz1,BrendanD.Mar5, Preston A. May1,2, Scott R. White2,3,ToddJ.Martı´nez5,6, Andrew J. Boydston4 and Jeffrey S. Moore1,2* Biological systems rely on recyclable materials resources such as amino acids, carbohydrates and nucleic acids. When biomaterials are damaged as a result of aging or stress, tissues undergo repair by a depolymerization–repolymerization sequence of remodelling. Integration of this concept into synthetic materials systems may lead to devices with extended lifetimes. Here, we show that a metastable polymer, end-capped poly(o-phthalaldehyde), undergoes mechanically initiated depolymerization to revert the material to monomers. Trapping experiments and steered molecular dynamics simulations are consistent with a heterolytic scission mechanism. The obtained monomer was repolymerized by a chemical initiator, effectively completing a depolymerization–repolymerization cycle. By emulating remodelling of biomaterials, this model system suggests the possibility of smart materials where aging or mechanical damage triggers depolymerization, and orthogonal conditions regenerate the polymer when and where necessary. aterials systems with reusable building blocks are attractive results demonstrate the essential steps for the realization of for autonomous adaptive structures that have the ability to mechanically induced autonomic remodelling for adaptive and sus- Mremodel themselves in response to environmental con- tainable materials systems. ditions, including damage or degradation. A familiar example of reusable resources found in nature is the recycling of monomeric Results and discussion building blocks for polymers composed of amino acids1,2, carbo- Preparation and testing of polymers. The molecular weight hydrates1,3 and nucleic acids1,4. For synthetic materials, remodelling dependence of the chain scission of polymers in elongational flow has the potential to extend device lifetimes through the removal and fields has been well studied6. For polystyrene subjected to replacement of defective regions or by restructuring parts to meet ultrasonication, the molecular weight threshold for chain scission their changing mechanical environment. One approach to such is typically around 30 kDa (ref. 16). For our initial studies, we adaptive polymeric materials involves mechanically triggered depo- therefore used cationic polymerization to produce cyclic PPA with lymerization followed by mass transfer of the resulting monomer to molecular weights well above the limiting molecular weight its newly needed location and subsequent repolymerization. (Table 1)19. As a reference polymer, we synthesized linear However, mechanochemical bond scission has an inherent limiting poly(methyl acrylate) (PMA) of comparable molecular weight molecular weight, below which mechanochemical energy is not using controlled radical polymerization20. accumulated to a degree sufficient to break a covalent bond5,6.To Sonication was used to study the mechanochemical scission5,6 of overcome this limitation there are two possible solutions: a switch- the polymers described in Table 1. The polymers were dissolved in able monomer–polymer equilibrium (which produces large THF to a concentration where no chain entanglement is expected amounts of oligomers) or metastable polymers in which a mechan- (1 mg ml21), and pulsed ultrasound (0.5 s on, 1.0 s off, ical event activates complete depolymerization7–9. Interestingly, 8.7 W cm22) was applied under argon at 215 8C. Figure 1a–c pre- bone tissue, one of the strongest materials in living organisms, is a sents gel-permeation chromatograms of aliquots removed from the metastable composite under biological conditions10. Both the Suslick cell during the sonication experiment. morphology and properties of bone are dynamic and driven The expected decrease in molecular weight was observed for both C L L by their mechanical environment, as a consequence of PPA90 and PMA103. In great contrast to PMA103, the integrated 11–13 C continuous remodelling . PPA90 peak area decreases during the experiment, indicating a Above their ceiling temperature Tc, end-capped polymers are decline in polymer concentration with sonication time (Fig. 1a). potential candidates for mimicking this material-recycling behav- Such a decrease is consistent with mechanochemical scission fol- iour, with mechanical damage causing reversion to monomers. lowed by rapid unzipping to monomer. Moreover, a gradual Below Tc, a chemical initiator drives chain growth. Here, we show increase of a new peak close to the column dead-volume is also that polymer-to-monomer unzipping of a low-Tc poly(o-phthalal- observed in this case, presumably resulting from an increase in dehyde) (PPA)14,15 can be triggered by force-induced chain scis- monomer concentration. The gel-permeation chromatography sion5,16–18. The resulting monomer is subsequently repolymerized (GPC) data also show that a decrease in average molecular weight under suitable conditions to complete one cycle of use. The occurs preferentially for the larger polymers after a long time 1Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA, 2Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA, 3Department of Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA, 4Department of Chemistry, University of Washington, Seattle, Washington 98195, USA, 5Department of Chemistry and the PULSE Institute, Stanford University, Stanford, California 94305, USA, 6SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA; †Present address: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachussetts 02139, USA. *e-mail: [email protected] NATURE CHEMISTRY | VOL 6 | JULY 2014 | www.nature.com/naturechemistry 623 © 2014 Macmillan Publishers Limited. All rights reserved. ARTICLES NATURE CHEMISTRY DOI: 10.1038/NCHEM.1938 Table 1 | Polymers used for initial study. molecular weight would also be expected for this polymer. If the change in Mw results from mechanical activation, the shorter polymer will retain its original molar mass, being too small to be force-activated. After 3 h of sonication, no significant change CHO O C was observed for PPA26 solutions, providing strong evidence CHO a, b O C O O that the depolymerization observed for PPA90 is triggered by a O mechanochemical process. n O L As a reference sample, PMA103 with similar molecular weight to the PPA (Fig. 1c) was sonicated under the same conditions. As previously shown, the mean molecular weight for this polymer Polymer Mn (kDa) Mw (kDa) PDI decreases, but the integrated intensity of the Mw distribution stays C nearly constant21. Figure 1d compares the normalized change in PPA26 16.5 25.6 1.6 the refractometer area with sonication time in each of the tested PPAC 58.2 89.6 1.5 90 polymers. PPAC displays a clear decrease in area while the areas PPAC 254 458 1.8 90 458 of the two controls remain unchanged, providing additional evi- L PMA103 86.4 103 1.2 dence that depolymerization to monomer is being triggered by C . mechanical force only for PPA . Preparation of PPA: BF3 OEt2,CH2Cl2, –78 8C, 2 h (a); pyridine, –78 8C, 2 h (b). Polymer collected 90 after precipitation into methanol and washing with methanol and diethyl ether. Polydispersity L index (PDI) ¼ Mw /Mn. Polymer name notation: C ¼ cyclic, L ¼ linear, no. ¼ Mw.PMA103 was Mechanistic study. For most polymers, mechanically induced chain prepared by single-electron transfer living radical polymerization21. Molecular weights were scission results in a pair of macroradicals that terminate by atom calculated by standard calibration with monodisperse polystyrene standards. abstraction or radical recombination5. PPA polymerization and depolymerization has been demonstrated through cationic and exposure to ultrasound. The residual macromolecules are chains anionic mechanisms, but not by a radical mechanism, raising from the original distribution, whose initial molecular weight was questions about the depolymerization mechanism. Unlike the close to the mechanically activated threshold. prevalence of radical intermediates in typical mechanochemical Because PPA is acid-sensitive, special care was taken to neutralize reactions, heterolytic bond scission (which, in the case of PPA, the solvent by stirring it with NaOH pellets (see Supplementary would generate reactive hemiacetalate and oxocarbenium C Section II for experimental details). As a control, PPA26 was end groups) was shown to partially occur in solid-state C sonicated under the same conditions as PPA90. If the reaction mechanochemistry of polyolefins at 77 K in the presence of is caused by acid or heat, a time-dependent reduction in tetracyanoethylene22–24. Such reactivity was also proposed for the a 1.5 b 5 min 1.4 5 min 10 min 10 min 15 min 1.2 1.2 15 min 20 min 20 min 25 min 1.0 0.9 30 min 25 min 35 min 0.8 30 min 40 min 35 min RI signal RI signal 0.6 45 min 0.6 40 min 50 min 45 min 0.4 50 min 0.3 0.2 0.0 0.0 0 50 100 150 200 250 0 50 100 150 200 250 M M w (kDa) w (kDa) c 1.8 d 1.2 5 min 1.5 10 min 1.0 15 min 20 min 1.2 25 min 0.8 30 min 0.9 35 min 0.6 40 min RI signal 45 min PMAL 0.6 0.4 103 50 min C % RI area remaining PPA 26 0.3 0.2 PPAC 90 0.0 0.0 0 50 100 150 200 250 0 1020304050 M w (kDa) Sonication time (min) C C L C C L Figure 1 | GPC studies of sonicated polymers. a,PPA90. b,PPA26. c,PMA103. d, Change in area ratio with sonication time for PPA90,PPA26 and PMA103. RI, refractive index detector. Times indicate total ‘on time’ of pulsed ultrasound conducted at 215 8C, polymer concentration 1 mg ml21 in THF, 8.7 W cm22. Final Mw values: 48.9 kDa (a), 23.6 kDa (b)and65.0kDa(c). The decrease in RI signal can only be observed for high-Mw PPA, indicating depolymerization is triggered only by mechanical force.