Enhanced polymer mechanical degradation through mechanochemically unveiled lactonization

Abstract

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.

Mechanochemical tools for polymer materials

“…15). For instance, gem-dihalocyclopropane mechanophores, reported by Craig's group, [123][124][125][126][127] generate a 2,3-dihaloalkene sequence after activation as shown in Fig. 15a.…”

Mechanochemical generation of acid-degradable poly(enol ether)s

“…15,16 Subsequently, the Craig group reported a clever alternative design of a tetrasubstituted [4.2.0]bicyclooctene based mechanophore that, upon mechanochemical ring-opening into 1,3-cyclooctadiene, can spontaneously undergo intramolecular lactonization and result in backbone cleavage (Scheme 1C). 17 These recent examples showcased mechanically enhanced polymer degradation as a new modality of response, but in these designs, the hydrolysable functionalities already exist in the polymers before mechanoactivation. As a result, deleterious hydrolysis prior to the desired mechanoactivation would inevitably hydrolyze these same functionalities in the precursor polymers and change their properties, leading to the generation of undesired scissile cyclobutane linkages or premature polymer cleavage that disrupts the transduction of force and thus limits the extent of mechanoactivation.…”

Triggering Forces at the Nanoscale: Technologies for Single‐Chain Mechanical Activation and Manipulation

“…c) Reproduced with permission under the terms of the Creative Commons license. [ 48 ] Copyright 2020, The Authors. Published by Springer Nature.…”

“…The authors reported the mechanical activation of a new mechanophore based on [4.2.0] bicyclooctene (BCOE) (Figure 5c). [ 48 ] Several copolymers of BCOE, g DCC‐COD, and epoxy‐COD were prepared. All the synthesized copolymers had 14 mol% of BCOE.…”