A Polymer with Mechanochemically Active Hidden Length

Incorporating hidden length into polymer chains can improve their mechanical properties, because release of the hidden length under mechanical loads enables localized strain relief without chain fracture. To date, the design of hidden length has focused primarily on the choice of the sacrificial bonds holding the hidden length together. Here we demonstrate the advantages of adding mechanochemical reactivity to hidden length itself, using a new mechanophore that integrates (Z)-2,3-diphenylcyclobutene-1,4-dicarboxylate, with hitherto unknown mechanochemistry, into macrocyclic cinnamate dimers. Stretching a polymer of this mechanophore more than doubles the chain contour length without fracture. DFT calculations indicate that the sequential dissociation of the dimer, followed by cyclobutene isomerization at higher forces yields a chain fracture energy 11 times that of a simple polyester of the same initial contour length and preserves high energy-dissipating capacity up to ∼3 nN. In sonicated solutions cyclobutene isomerizes to two distinct products by competing reaction paths, validating the computed mechanochemical mechanism and suggesting an experimental approach to quantifying the distribution of single-chain forces under diverse loading scenarios.

Gel permeation chromatography (GPC) data were calibrated on two columns in series (7.8 × 300 mm, 2 GMHHRM17932 and 1 GMHHRH17360) with tetrahydrofuran (THF, HPLC grade) as eluent at 40 o C with a LC-20AD pump. The facility was equipped with two detectors (RID-10A refractive index detector; SPD-20A UV detector) and the molecular weight was calibrated against polystyrene standards.
Silicon substrate and silicon nitride AFM tips (Veeco Instruments, now Bruker Nano, Santa Barbara, CA, MLCT) were used in the SMFS experiments. Before modification, the AFM tips and silicon slides were treated with piranha solution (H2SO4 (98%)/H2O2 (30%) = 7:3 in volume), and thoroughly rinsed with deionized water, followed by drying in an oven at 115 o C for 90 min to remove any remaining water.

(Caution: Piranha solution that may result in explosion or skin burns is a very hazardous oxidant. This solution must be handled with extreme care.)
The vapor-phase deposition method was used for the silanization of clean AFM tips and silicon slides by placing them in the atmosphere of the 3-aminopropyldimethylmethoxysilane (APDMMS) in a dry nitrogen-purged desiccator for 1.5 h at 25℃. Immediately after being taken out, the silanized tips and silicon slides were rinsed three times with methanol and then placed in a 110 o C oven for 10 min for activation. The silanized substrates and AFM tips were immersed in 1 mL DMF solution of P1 (0.5 umol/mL) and N,N'dicyclohexylcarbodiimide (DCC) (0.75 umol/mL) and kept for 2.5 h at room temperature. Then the they were rinsed with DMF thoroughly to remove any loosely adsorbed molecules. Both the amino functionalized AFM tips and polymer anchored substrates were newly prepared just before SMFS experiments.
Ultrasound experiments were performed on a Vibra Cell 505 liquid processor with a 12.8 mm (diameter) titanium solid probe (Sonics and Materials). For typical sonication experiment, polymers were dissolved in THF with a concentration of 5 mg/mL in 15 mL of solvent. The solution was then transferred to a 3-necked Suslick cell in an ice bath and sparged with nitrogen for 20 min. A pulse sequence of 1 s on/1 s off was applied to the solution at a power of 8.7 W/cm 2 . The temperature of the system was maintained at 0 -5 o C. The sonication was carried out under a nitrogen atmosphere. Aliquots (0.5 mL, 4 mg/mL) at given time were removed from the Suslick cell for GPC and UV-Vis test. To monitor the cinnamic moiety, UV absorbance at 280 nm in GPC test was recorded. After sonication, the residual solution was precipitated by methanol and redissolved in CH2Cl2 (~ 25 mg/mL) for 1 H NMR measurement.

Synthesis of compound 2'
bis(2-(2-hydroxyethoxy)ethyl) (1R,2S)-3,4-diphenylcyclobut-3-ene-1,2-dicarboxylate, 2': 7 a solution of 1,2diphenylacetylene (5.34 g, 30 mmol, 3 equiv.), maleic anhydride (0.98 g, 10 mmol, 1 equiv.), benzophenone (5.5 S4 g, 30 mmol, 3 equiv.) in acetonitrile (200 mL) placed in a cylindrical water-cooled reactor was degassed with a nitrogen stream for 15 min, then sealed. The solution was then irradiated for 24 h with a 400 W mediumpressure mercury lamp fitted with a Pyrex filter. The solution was then evaporated under reduced pressure to get a yellow oil. Then it was diluted with 30 mL dry THF in a 100 mL round bottom flask and added 8 mL diethylene glycol. Concentrated H2SO4 (1.5 mL) was carefully added, then the solution was heated at 75 o C overnight. After cooling, the reaction was quenched by pouring into 100 mL sat. NaHCO3 and extracted with CH2Cl2 (100 mL). The organics were washed with 200 mL sat. NaCl and dried over Na2SO4, then evaporated under reduced pressure to give a crude product which was purified with column chromatography (ethyl acetate) to give compound 2' (1.5 g) as a yellow oil in 30% yield. 1

Calculations of the fractions of conversion
The fraction of dissociated cinnamate dimers was calculated by the ratio of the integrals of peaks h' and h'' at various times (see Figure 4A-B) to half that of peak l (the proton of methylene group, δ=2.70 ppm, see Figure S7). The fraction of isomerized cyclobutene was calculated by the ratio of the integrals of peaks (d'+d''+d*) at various times (see Figure 4A,C) to half that of peak l (the proton of methylene group, δ=2.70 ppm, see Figure S7).
The ratios of the two products of cyclobutene isomerization were calculated by the ratios of peak d* of the E,E-diene to the sum of peaks d' and d" of the E,Z -diene, Fig. S13. To exclude the thermal effect during ultrasonication, we sonicated a shorter counterpart P4, which should experience much less stretching force but the same thermal effect. No change of 1 H NMR was seen therefore the two dienes solely come from the mechanochemical ring-opening of Z-2.
S22 GPC at various sonication times The various sonication times are indicated in the legend. The concentration of P1 is 5 mg/mL in THF.