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.
Highlights
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
While mastication and other mechanical processing techniques are capable of triggering polymer degradation to an extent that changes mechanical properties, the utility of mechanical force for extensive polymer degradation is limited by the fact that the tension in a parent polymer chain relaxes after a single scission event (Fig. 1a), and the ability to recapture the chain in a high stress event drops with its molecular weight
Moore and Boydston et al.[17] reported a clever strategy to amplify a single triggering event through the scission-induced cascade depolymerization of a self-immolative cyclic poly(phthalaldehyde). This strategy, is not likely to be suitable for all degradation purposes, as the mechanically depolymerizable system relies on a metastable polymer backbone, and cascade degradation happens instantaneously upon activation, which might reduce stress transfer and limit the ability to trigger events throughout all strands in a bulk material
Summary
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. Ultrasonication of P2 leads to three mechanochemical outcomes: (1) a reduction in polymer molecular weight (MW), as indicated by the steady shift of gel permeation chromatography (GPC) traces to longer retention time (Fig. 4b); (2) the conversion of gDCC mechanophores into their respective 2,3-dichloroalkene products (as previously reported38,39); and (3) the ring opening of BCOE to the corresponding diene species.
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