Enhanced polymer mechanical degradation through mechanochemically unveiled lactonization

The mechanical degradation of polymers is typically limited to a single chain scission per triggering chain stretching event, and the loss of stress transfer that results from the scission limits the extent of degradation that can be achieved. Here, we report that the mechanically triggered ring-opening of a [4.2.0]bicyclooctene (BCOE) mechanophore sets up a delayed, force-free cascade lactonization that results in chain scission. Delayed chain scission allows many eventual scission events to be initiated within a single polymer chain. Ultrasonication of a 120 kDa BCOE copolymer mechanically remodels the polymer backbone, and subsequent lactonization slowly (~days) degrades the molecular weight to 4.4 kDa, > 10× smaller than control polymers in which lactonization is blocked. The force-coupled kinetics of ring-opening are probed by single molecule force spectroscopy, and mechanical degradation in the bulk is demonstrated. Delayed scission offers a strategy to enhanced mechanical degradation and programmed obsolescence in structural polymeric materials.

Characterization 1 H NMR spectra were collected on a Bruker Advance Neo-500 MHz multinuclear NMR spectrometer. Chemical shifts are provided in ppm (δ) and referenced to the residual 1 H peak at 7.26 ppm in CDCl3. 1 H shifts are reported as chemical shift, multiplicity, coupling constant if applicable, and relative integral. Multiplicities are reported as: singlet (s), doublet (d), doublet of doublets (dd), doublet of triplets (dt), doublet of doublet of doublets (ddd), doublet of doublet of triplets (ddt), triplet (t), triplet of doublets (td), quartet (q), pentet (p), multiplet (m), or broad (br). Coupling constants (J) are reported in Hz. High-resolution mass spectra were collected on an Agilent LCMS-TOF-DART at Duke's Mass Spectrometry Facility.
Photoreaction was conducted using a photochemical reactor from The Southern New England Ultraviolet Company (Model #RPR-100, RPR2537A/254 nm bulb). Ultrasonication was performed with a Sonics VCX 750 generator using a 13 mm tip. Pulsed ultrasound (1s on, 1s off) was applied under N2 atmosphere while cooled with an ice bath. Aliquot was taken from the solution and subjected to GPC and 1 H NMR analysis. Extrusion study was performed on a HAAKE TM MiniCTW Micro-Conical Twin Screw Compounder from Thermo Scientific TM .
Gel permeation chromatography (GPC) was performed on two Agilent PLgel mixed-C columns (10 5 Å, 7.5x300 mm, 5 μm, part number PL1110-6500) using THF (stabilized with 100 ppm BHT) as the eluent. Molecular weights were calculated using a Wyatt Dawn EOS multi-angle light scattering (MALS) detector and Wyatt Optilab DSP Interferometric Refractometer (RI). The refractive index increment (dn/dc) values were determined by online calculation based on injections of known concentration and mass.
Sharp Microlever silicon probes (MSNL) and Silicon Nitride AFM Probes (PNP-DB) were correspondingly purchased from Bruker (Camarillo, CA) and NanoAndMore(Watsonville, CA). All of the SMFS studies were conducted at ambient temperature (~23 °C) using a homemade AFM, which was constructed using a Digital Instruments scanning head mounted on top of a piezoelectric positioner, similar to the one described in detail previously. [1][2] The AFM pulling experiments were conducted in a solution of toluene. The spring constant of each cantilever was calibrated in air, using the thermal noise method, based on the energy equipartition theorem as described previously. 1-2 Measurements were carried out in a closed fluid cell with a scanning set for a series of approaching/retracting cycles. Probes were prepared by immersing in piranha solution (H2SO4: H2O2 = 3:1) for 15 minutes at room temperature and then immersing in deionized water and dried by touching them against a borohydride. Silicon substrates were prepared by first allowing each to soak in hot piranha solution for 30 minutes and then washed with deionized water and dried under a stream of nitrogen. Caution should be used when handling piranha solution: it has been reported to detonate unexpectedly. The substrate and the cantilever were then placed in a UVO cleaner (ozone produced through UV light) for 15 minutes. After ozonolysis, the cantilever was mounted in the fluid cell. 20 μL of a 0.05-0.1 mg/mL polymer solution was added to the silicon substrate surface and allowed to dry. The silicon substrate was then placed on the piezoelectric stage of the AFM. Force curves were collected in dSPACE (dSPACE Inc. Wixom, MI) and analyzed using Matlab (The MathWorks, Inc., Natick, MA). All data were filtered during acquisition at 500 Hz. After acquisition, the data were calibrated and plotted by using homemade software written in Matlab language.

5) Synthesis of P5
The procedure of preparing P5 is similar to that of P3. 1  Hence, the average chain length between two adjacent BCEO mechanophore is: 66500/15 = 4433 ~ 4.4 kDa 4. Preparation of blended polymers 80 mg P4 or P5 polymer and 3.2 g commercial PCL polymer (80 kDa) were dissolved with 20 mL DCM, the viscous solution was vortexed and precipitated from methanol, and the resulting white polymer was dried under vacuum for 24 h. The polymer chunk was then cut into pieces using a scissors and further subjected to extrusion study.

General sonication procedure
A solution of polymer P2 (1.2 mg mL -1 ) and P3 (1.0 mg/mL) in dry THF was transferred into a dry Suslick cell. The solution was sparged with N2 for 10 min while cooled with ice bath. Pulsed ultrasound was applied (1s on, 1s off) at 30% amplitude. Aliquots of 0.8 mL sample at various sonication times (0, 5, 10, 20, 30, 45, 60 min) were took from the cell and analyzed by GPC. After GPC analysis, each remained samples was transferred into a 10 mL scintillation vial and condensed. Resulting polymer was further dried under high vacuum and then subjected to 1 H NMR analysis to quantify the amount of ring opened BCOE and gDCC mechanophores.  To determine RO(BCOE)%, we used the 60 min sonication sample. We used pTSA to cleave the THP protecting groups and then subjected the condensed mixture to 1 H NMR analysis. We obtained 53% activation ( Figure 3). Plugging into equation 2, we calculate:

III. Extrusion study
Extrusion study was performed on a HAAKE TM MiniCTW Micro-Conical Twin Screw Compounder. The compounder is comprised of a clamshell barrel with two conical screws and a recirculation pathway. The barrel was preheated to 65 o C and the screw rotation was set to 70 rpm. Polymer pieces (3.1g) were then added in portions using a mechanical plunger. The extrusion was performed for various of times. For P4, samples at 0, 5, 20, and 60 min extrusion were analyzed; P5 were sampled at 0, 6, 28, and 70 min extrusion.
Supplementary Figure 14. UV 332nm signal of PCL, P4 and P5 polymers from GPC analysis (2 mg mL -1 in THF).   The percentage of ring closed from in the system can be calculated using the following equation: The equilibrium at 150 o C can be estimated from the plateau (33%) of fitting: The observed rate constants is: kobs = k1 -k-1 = 1.132×10 -2 min -1 Therefore, the ring-opening rate constant is: k1 = 2k-1 = 2kobs = 2.264×10 -2 min -1 = 3.77×10 -4 s -1 According to the transition state theory, the thermal activation energy can be estimated:

S20
Mechanically induced disrotatory ring opening of BCOE is a forbidden pathway. Hence, the activation energy of forbidden reaction would be at least 4 kcal/mol more than thermally allowed path way. 14 The activation energy of disrotatory ring opening of BCOE is: ∆G ‡ > 31.7 + 4 kcal/mol = 35.7 kcal/mol

SMFS curve analysis
Force-extension curves of polymer P1 were analyzed using method reported previously. 15 Pre-and post-transition force curves were fitted with extended freely jointed chain (FJC) model to give the contour lengths of polymers before and after transition. Further analysis with Bell-Evans (BE) or Cusp models provided ∆x ‡ information.

V. CoGEF modeling
CoGEF modeling of BCOE repeating unit and corresponding forbidden ring opening product were performed on Spartan'16 V2.0.7 version at Molecular Mechanics/MMFF theory level. The distance of chain-end carbons was constrained and relaxed with step interval of 0.1 Å. The obtained energy at each relaxed step was plotted as a function of the end-to-end distance of repeating unit, which is indicated at the distance between the carbon atoms labeled with orange dots in the chemical structures. Further quadratic fitting and subsequent analysis from the first derivative gave force vs. extension relation, from which the contour length (x0) at zero force can be extrapolated.