Oxidative regulation of the mechanical strength of a C–S bond†

The mechanical strength of individual polymer chains is believed to underlie a number of performance metrics in bulk materials, including adhesion and fracture toughness. Methods by which the intrinsic molecular strength of the constituents of a given polymeric material might be switched are therefore potentially useful both for applications in which triggered property changes are desirable, and as tests of molecular theories for bulk behaviors. Here we report that the sequential oxidation of sulfide containing polyesters (PE-S) to the corresponding sulfoxide (PE-SO) and then sulfone (PE-SO2) first weakens (sulfoxide), and then enhances (sulfone), the effective mechanical integrity of the polymer backbone; PE-S ∼ PE-SO2 > PE-SO. The relative mechanical strength as a function of oxidation state is revealed through the use of gem-dichlorocyclopropane nonscissile mechanophores as an internal standard, and the observed order agrees well with the reported bond dissociation energies of C–S bonds in each species and with the results of CoGEF modeling.


Synthesis of polymer PE-SO
The oxidation process was adapted from previous literature. 3 Polymer PE-S (78 mg, Mn = 71.7 kDa, 0.05 mmol C-S bond) was weighted in a 10 ml scintillation vial. 1 mL DCM was added to completely dissolve the polymer. Acetic anhydride (5.2 μL, 0.055 mmol, 1.1 eq) and silica gel (50 mg) was added to the solution. Then, 30% hydrogen peroxide (57 μL, 0.5 mmol, 10 eq) was added to the mixture. The reaction was further stirred at room temperature for 6 h. The resulting mixture was then diluted with 2 mL DCM and silica gel was filtered using a syringe filter. Obtained DCM solution was condensed and precipitated from methanol three time to give polymer PE-SO (74 mg, 93.9%). GPC:

Synthesis of polymer PE-SO2
To a solution of PE-SO (156 mg, Mn = 71.7 kDa, 0.1 mmol C-S bond) in 3 mL THF, mCPBA (74 mg, 70%~75%, 0.3 mmol) was added in portions. The reaction was allowed to stir at room temperature overnight. After the reaction completed, the solution was condensed and precipitated from methanol three times to give a white polymer PE-SO2 (158 mg, 99.9%). GPC

III. Sonication experiment
1. General sonication procedures A solution of 36 mg polymer (PE-S, PE-SO or PE-SO2) in 18 mL dry THF (c = 2 mg/mL) was transferred into a Suslick cell. The solution was purged with N2 for 10 min while cooled with ice bath. Pulsed ultrasound was applied (1s on, 1s off) at 30% amplitude. An aliquot of 0.8 mL sample was taken out for GPC analysis at each sonication time points: 0, 3, 6, 10, 15, 20, 30, 45 min. Each of these samples were further condensed in a 10 ml scintillation vial to give a thin layer of polymer at the vial bottom. The polymer was washed with methanol and further dried under high vacuum. Obtained dry polymers were further subjected to 1 H NMR analysis.   3) Evaluation of Φ value Φ value, the slope of ring opening percentage of gDCC versus scission cycle (SC) plot, is used to evaluate the relative strength of C-S weak bonds.

Sonication of PE-SO polymer
The analysis of PE-SO polymer is the same as PE-S polymer.

IV. CoGEF modeling
CoGEF modeling was conducted using Spartan '18 V1.4.1 software. The methyl ester form of each sulfide, sulfoxide or sulfone containing repeating unit was subjected to CoGEF modeling using DFT method on the theory level of B3YLP/61G*. The ground state geometry was first optimized, and then its end-to-end distance was constrained with step increasement of 0.1 Å. The optimized energy was plotted as a function of end-to-end distance.

V. Calculation of bond dissociation energy
The methyl ester form of each sulfide, sulfoxide or sulfone containing repeating unit was applied to calculate the C-S bond dissociation energy (BDE). The BDE was calculated using equation: BDE = E(Frag1) + E(Frag2) -E(SM).
The calculation was performed in Spartan '18 V1.4.1 software. The energy of each starting molecule was optimized using DFT method on theory level of B3LYP/6-311+G** (in nonpolar solvent, ε = 7.43), and the two radical fragments were optimized using the same method but with one unpaired electron. The obtained optimized energy for each molecule/fragment was provided in the following table.