Is Molecular Weight or Degree of Polymerization a Better Descriptor of Ultrasound-Induced Mechanochemical Transduction?

Abstract

A detailed understanding of the fundamental processes that govern mechanical transduction in covalent polymer mechanochemistry is essential to advance innovation in this field. In contrast to progress in the development of new mechanophores, the influence of polymer structure and composition on mechanochemical activity has received relatively little attention. In order to address this gap in knowledge, a continuous flow system with synchronous UV−vis absorption capabilities was designed to quantify the ultrasound-induced mechanical activation of a spiropyran mechanophore in real-time. Measurements of reaction kinetics with polymer tethers of varying repeating unit structure demonstrate that degree of polymerization is the key descriptor of mechanochemical activity, independent of molecular weight and pendant group constitution. These results have important implications for the rationalization of mechanochemical properties and the design of new mechanochemically active polymer systems.

Intramolecular Cross‐Linking: Addressing Mechanochemistry with a Bioinspired Approach

“…As previously demonstrated in Titin, [12,13] the scission of sacrificial bonds absorb applied mechanical stress without polymer fragmentation. Assuming that the stress transduced to the polymer chain is proportional to the DP, [27,28] less energy is consumed in unfolding the polymer before as acrificial bond is stretched, explaining the higher rate of mechanochemical scission events.Furthermore, intramolecular collapse inhibits the unfolding of the polymer under solvodynamic shear, reducing the maximum stress in polymer chains,b ased on the models proposed by Odell and Keller. [3,29,35] This reduction in peak stress explains the reduction in mechanochemical rates at higher cross-link densities,inw hich as teeper reduction of Rh is observed.…”

“…All SCPNs,a sw ell as the linear precursor,p resent the same DP with the only distinction being the amount of crosslinker.Importantly,ithasbeen recently shown that DP is the key parameter that defines the rate of mechanochemical reactions,a nd therefore all samples should present the same degradation rate, [27,28] unless,a sw ep ropose,t here is as tructural effect in the way the stress is transmitted to the polymer or dissipated as aconsequence of the intramolecular collapse.…”

Stress-induced colouration and crosslinking of polymeric materials by mechanochemical formation of triphenylimidazolyl radicals

“…The high molecular weight peak situated at around 200 kDa decreases rapidly and adds to the original low molecular weight peak of PMA-2 at 100 kDa. 34,35 Indeed, from the 1 H-NMR spectra it becomes clear that neither thermal dissociation nor irradiation with ultrasound of 1 lead to the formation of TPI or other considerable degradation of the HABI-moiety during the monitored time span. 1 H-NMR measurements clearly confirm that scission occurs mostly at the HABI-moiety (Fig.…”

The Intrinsic Mechanochemical Reactivity of Vinyl‐Addition Polynorbornene

“…[24] TheN MR and GPC data suggest that ring-opening olefination by sonication was concurrent with chain scission, as expected from the sonication of macromolecules. As expected, sonication of the low-molecularweight control VA -TES polymers did not result in any observable olefination or chain scission, since they are below the required degree of polymerization for sonicationinduced cleavage.…”

“…As expected, sonication of the low-molecularweight control VA -TES polymers did not result in any observable olefination or chain scission, since they are below the required degree of polymerization for sonicationinduced cleavage. [24] TheN MR and GPC data suggest that ring-opening olefination by sonication was concurrent with chain scission, as expected from the sonication of macromolecules. [25] A representative series of GPC traces taken during sonication of VA -TES are shown in Figure 3(top).…”