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. 2024 Sep 27;146(40):27594–27599. doi: 10.1021/jacs.4c08765

Breaking Strong Alkynyl-Phenyl Bonds: Poly(para-phenylene ethynylene)s under Mechanical Stress

Maximilian Elter †, Matthias Brosz ‡, Daniel Sucerquia ‡, Andrei Kuzhelev ∥, Denis C Kiesewetter ‡, Markus Kurth ‡,§, Andreas Dreuw ⊥, Thomas F Prisner ∥, Jan Freudenberg †,*, Uwe H F Bunz †,*, Frauke Gräter ‡,§,*
PMCID: PMC11468784  PMID: 39332820

Abstract

graphic file with name ja4c08765_0008.jpg

Stronger chemical bonds withstand higher mechanical forces; thus, the rupture of single bonds is preferred over the rupture of double or triple bonds or aromatic rings. We investigated bond scission in poly(dialkyl-p-phenylene ethynylene)s (PPEs), a fully conjugated polymer. In a scale-bridging approach using electron-paramagnetic resonance spectroscopy and gel permeation chromatography of cryomilled samples, in combination with density functional theory calculations and coarse-grained simulations, we conclude that mechanical force cleaves the sp–sp2 bond of PPEs (bond dissociation energy as high as 600 kJ mol–1). Bond scission primarily occurs in shear bands with locally increased shear stresses. The scission occurs in the middle of the PPE chains. Breaking sp–sp2 bonds into free radicals thus is feasible but requires significant mechanical force and an efficient stress concentration.

Introduction

Understanding and controlling mechanochemical abrasion of polymers is important1−3—annually more than 3 million tons of microplastics are formed by weathering and/or friction and deposited in the environment.4 It is generally accepted that polymer chains break close to their center.5−10 From a chemical perspective, homolytic bond scission, resulting in two radicals, occurs preferentially at the weakest bonds of the backbone, that is, at its single sp3-sp3 bonds, with varying scission propensities (e.g., S–S > C–S > C–C).11 Incorporation of functional groups12−17 can further serve as mechanochemical breaking points by design, which are an additional point of interest.18 Scission, however, will strongly differ when moving from backbones of nonconjugated, flexible polymers, as mostly studied so far, to rigid-rod conjugated or ladder-type polymers, as those lack sp3-sp3 bonds and would typically form highly unstable radicals.

Poly(p-phenyleneethynylene)s (PPEs) are fluorescent19−22 and semiconducting23−25 rigid-rods,26−28 with backbones consisting of sp and sp2 hybridized carbons. PPEs are difficult to postfunctionalize. The drastic conditions (75 h, 520 bar of H2 and 300 °C, Wilkinson’s catalyst) to generate poly(p-phenyleneethylene) in a “simple” catalytic hydrogenation illustrates PPEs’ lack of reactivity.29

Homolytic bond scission of PPEs is dictated by the bond dissociation energies (Csp2–Csp: ∼600; double Car–Car: >800 and C≡C: ∼800 kJ/mol, Figure 1b).30 Scission must occur at the Csp2–Csp bond between a phenylene ring and a triple bond among the very stable and conjugated backbone, resulting in highly reactive radicals. In this work, we study the mechanochemical degradation of PPEs (Figure 1a): phenyl and alkynyl radicals were generated via mechanical activation of dodecyl-substituted PPE-1(25) in a cryomill operated at liquid nitrogen temperature. The primary and secondary radicals were characterized via X- and G-band EPR spectroscopy, and their generation and decay were confirmed via quantum-chemical calculations. We locate scission to the center of the PPE chains and identify a sufficiently long chain as critical for breaking such strong rigid-rod polymers.

Figure 1.

Figure 1

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(a) Illustration of the goal of this study. (b) Bond dissociation energiesa of different bonds in PPE-1. aRough values taken from computation (BMK/6-31+g) of a PPE-Trimer (Table S1).

Results and Discussion

Dodecyl-substituted PPE-1 was synthesized via Sonogashira-Hagihara coupling (Mn = 47 kDa, Mp = 106 kDa, PDI = 3.46, Pn = 107) on a multigram scale25 and used for the ball-milling (250 mg PPE, milling conditions: multiples of 6 min up to a total of 72 min at 30 Hz) in a 25 mL screw-top jar with a zirconium oxide inlet and a single 10 mm diameter zirconium oxide ball under liquid nitrogen (LN2) cooling. At −196 °C, the normally rubbery and fibrous poly(p-phenylene ethynylene) is highly brittle, forms powder, and slows decay reactions after homolysis. Extraneous radical species were avoided by sample preparation in a nitrogen-filled glovebox with zirconium oxide tools to avoid steel abrasion. After ball-milling, the powders were analyzed via EPR and SEC/GPC.

GPC analyses (Figure 2a; see SI, Figure S7, for chromatograms) depict a trend of mass loss proportional to the milling time; after 36 min, the peak molecular weight (Mp) went down from 106 to 49 kDa, which at longer milling times reached a limit of 44 kDa (Pn decreasing from 107 to 44). The peak retention time in GPC is inverse to the milling-time-dependent increase of the EPR signal intensity (Figure 2b, after 72 min), which is increased by a factor of ∼19 compared to the weak EPR signal of the nonmilled reference).

Figure 2.

Figure 2

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(a) Milling-time-dependent evaluation of molecular weight. Inset: fit parameters and the structure of PPE-1. (b) EPR spectra of samples after varying ball-mill durations.

The observed narrow EPR line for the powder system (X-Band EPR, see SI, Figure S13 and Table S8) indicates formation of carbon-based radicals.31,32 GPC and EPR analyses confirm backbone scission of the PPE. The pronounced mass losses cannot be explained by side chain scission. We hypothesized that triple or aromatic bond scission is not observed due to the much higher bond dissociation energies, so we anticipated sp–sp2 bond scission to occur, resulting in phenyl and alkynyl radicals33−35 as primary scission fragments.

To test this scenario, we performed additional G-band EPR measurements. The high frequency (180 GHz in the G-band compared to ∼9.5 GHz in the X-band) results in higher sensitivity, which allows one to shed light on the anisotropic nature of the formed radical species. Due to the correlation of frequency and energy difference, the Zeeman splitting between energy levels widens.36 The spectra exhibit low gradients; a variation of the time delay in the Hahn echo experiment (see SI, Figure S15a) or temperature (Figure 3a) decreases the signal width. This behavior supports the presence of several radical species with unique longitudinal electron spin relaxation times (T1) and magnetic susceptibilities (see SI, Figure S16).37 Simulation of the G-band spectrum with EasySpin38 (Figure 3b) indeed reveals two dominant radical species to be present in the ball-milled samples, with unique g-tensors. For the prefitted simulation, see Figure S15d.

Figure 3.

Figure 3

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(a) Temperature-dependent G-Band EPR spectra from 50 to 230 K; (b) deconvolution of the low-temperature radical signal with the g-tensor for the alkynyl radical A: [2.0057 2.00344 2.0022] and the benzylic radical E: [2.00289 2.00264 2.00227].

We identified five possible radical structures A–E (Scheme 1, A and B: primary phenyl and alkynyl radicals, C–E: benzylic secondary radicals after radical migration). The alkynyl radical A and the benzylic radical E (similarly C or D) have calculated g-factor tensors (DFT, basis sets: cc-pVDZ or EPRII; see SI for Calculation Methods) that match the G-band spectrum best, regardless of the calculation method (Figure 4). The ratio of the radicals A:E here is around 33:5, however, varies among experiments, which we attribute to the sample preparation for G-band EPR samples under air before cooling and different ambient temperatures for sample transportation upon repetition of the experiment (A:E = 8:5 in a second measurement, see SI, Figure S15b).

Scheme 1. Proposed Homolysis Pathway of the sp–sp2 Single Bond in PPE, Initially Forming Alkynyl Radical A and Phenyl Radical B with Plausible Follow-Up Reactions.

Scheme 1

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Figure 4.

Figure 4

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Initially formed radicals (A and B) and benzylic radicals (C–E) with calculated g-factor tensors gz, gy, and gx compared with the experimental g-factor tensors (dashed lines). For the DFT calculations, the dodecyl chains in the PPE-1 were substituted by butyl groups.

Is the formation of a benzylic and an alkynyl radical plausible? We assume homolysis of the (weakest) single bond (vide supra) initially gives an alkynyl (A) and a phenyl (B) radical (Scheme 1). The phenyl radical B is not detected via G-band EPR–fast intra- or intermolecular radical migration must occur. As both radicals are approximately equally reactive, we were surprised that A was still observed. From that observation, we rule out intermolecular hydrogen radical abstraction (from a neighboring polymer chain) as that decomposition pathway would also deplete the reservoir of A to the same extent. Instead, we assume an intramolecular radical 1,3 hydrogen migration (see SI, Figure S4), transforming B into the stable benzylic radical E. Experimentally, we can rule out a 1,3-alkyl migration to radical D through mismatch with calculated g-factor tensors. The same 1,3 hydrogen shift is not available for alkynyl radical A, which, despite its high reactivity, does not (easily) find a reaction partner. The alkynyl group acts as a spacer unit (see SI, Figure S5), and the fact that barriers for hydrogen atom migration increase strongly with the distance of migration.39 The remaining polymer strands act as a matrix for the radical centers, shielding them from decomposition in the absence of oxygen. Our experiment cannot rule out radical hopping in the dodecyl side chain ending in E. Additionally, a hydrogen shift converting alkynyl radical A to C seems unlikely but cannot be experimentally excluded. Neither NMR characterization after ball-milling (see SI, Figure S11) nor radical trapping with 4-hydroxy-TEMPO-benzoate provided any evidence for the benzylic species (see SI, Figure S12). This is not unexpected, as quenching with the solvent occurs (NMR), and radical concentrations are rather low.

Is there a preferred position for chain scission in the PPE chain, and does the strand fission depend on the length and structural assembly of the PPE chains? To this end, we resorted to coarse-grained molecular dynamics simulations, using our previously developed MARTINI-based model for PPEs.40 In this model, ∼ 4 heavy atoms are described as one coarse-grained bead so that key chemical properties are properly captured. We constructed bulk systems composed of 1500, 11,000, or 25,000 chains, with 20, 60, and 120 monomer units, respectively. After equilibration, each system was sheared at rates of 0.01 ns–1; 0.1 ns–1; and 1 ns–1. Shearing served as a simplified mimic of the mechanical loading in the ball mill. As expected, we observed a pronounced alignment of the polymer chains along the shear flow, which was stronger for longer chains (Figure 5, left). Remarkably, shear bands formed, reflected by alternating regions of aligned (ordered) and entangled (disordered) chain configurations. We quantified shear banding by calculating velocity profiles and nematic order along the shearing axis (Figure 5, right; see SI, Figure S2). This analysis shows that shear banding, as evident from highly nonlinear profiles of these observables, requires sufficient shear rates and long polymer chains (N = 60 and 120). While it was not observed for N = 20 chains, independent of the shear rate. we find nearly perfect interchain parallel alignment in regions with high changes in velocity, i.e., extreme local shearing (see SI, Figure S2).

Figure 5.

Figure 5

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Left: Bulk cells containing 1500 PPEs with n = 20 (top), 11,000 with n = 60 (center), and 25,000 with n = 120 (bottom) while under a shear rate of 1 ns–1. Right: Velocity profiles of the bulk cells under varying shear rates.

Shear banding implies particularly high shearing stresses in specific regions of the sample, and we hypothesized bond scission to dominate there. We indeed observed strongly stretched molecular bonds in the backbone of PPEs within shear bands (Figure 6, left) but only for longer chains and high shear stresses. Instead, in absence of shear banding, i.e., at low shear rate or for the shorter chains, the backbone forces are constant along the sheared coordinate. The same analysis also allows to track the forces along the PPE chain, averaged over time and all chains. The forces are on average highest in the center of a given polymer chain (Figure 6, right) and higher for longer chains. Bond rupture thus is favored in the center of the chains, as also observed for other polymers.5−10

Figure 6.

Figure 6

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Left: Rupture force profiles of each bulk system under shear stress. Right: Rupture forces along the PPE backbone are the same for each bulk system.

Our results from coarse-grained simulations are perfectly in line with the observed halving of the average molecular weight (Figure 2a). For further validation, we investigated the mechanochemistry of PPE-2 (Mp ∼ 51,000; Pn = 67 and 107,000; Pn = 158) with shortened side chains (R = EtHex).41 We observe a loss of molecular weight only for the polymer of higher molecular weight (see the SI, Figure S8). Similarly, a shorter chain length of PPE-1 (Pn = 35) shows no fragmentation when ball-milled (see SI, Figure S9). These results support our simulations and highlight the drastic stress concentration required to break conjugated PPEs.

Conclusions

Mechanical forces homolytically cleave the strong single bond in fully conjugated poly(dialkyl-p-phenylene ethynylene)s (PPEs), resulting in scission of a strong sp–sp2 bond, with a bond dissociation energy as high as ∼140 kcal mol–1 (∼590 kJ mol–1). The cleavage products, i.e., radicals, were characterized via EPR spectroscopy and identified using DFT calculations. To our surprise, alkynyl radicals, despite their instability, could be unambiguously identified—the surrounding PPE chains, and the lack of nearby hydrogens for intermolecular or intramolecular migration, must act as a matrix stabilizing these otherwise fleetingly and hardly characterized species. Phenyl radicals were not observed probably due to an intramolecular H-Shift, which is not possible for the alkynyl radicals. Coarse-grained simulations underlined the likelihood of the main chain scission in the middle of the polymer chain, while a threshold mass is needed to induce enough strain in the chain’s central region for scission of the strong sp–sp2 bond. Our data suggest that shear banding is an important mechanism for further stress concentration, which is likely at play also in other rigid-rod polymers. In the future, we will introduce photochemically cleavable propiolic peroxyanhydride moieties to oligo-para-phenylene ethynylenes as model compounds to further shed light on the nature and follow-up reaction of alkynyl radicals in these systems.

Acknowledgments

F.G. and U.H.F.B. acknowledge key funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy for the Excellence Cluster “3D Matter Made to Order” (EXC-2082/1-390761711). F.G. thanks the Klaus Tschira Foundation for generous funding through SIMPLAIX. We thank Peter Comba for insightful discussions.

Data Availability Statement

Research data are available from heiData, an institutional repository for open research data, under https://doi.org/10.11588/data/YPHTIY. Additional DFT data is available at https://github.com/graeter-group/g-tensors-PPEpaper.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c08765.

  • Experimental details, CG-simulations, computation, UV/vis and Fluorescence, g-factor calculations, further G-Band and X-Band EPR spectra, NMR and IR spectra, GPC analyses, and photographs of the samples (PDF)

Author Contributions

The manuscript was written through contributions of all authors./All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ja4c08765_si_001.pdf (1.3MB, pdf)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ja4c08765_si_001.pdf (1.3MB, pdf)

Data Availability Statement

Research data are available from heiData, an institutional repository for open research data, under https://doi.org/10.11588/data/YPHTIY. Additional DFT data is available at https://github.com/graeter-group/g-tensors-PPEpaper.

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