Examining the Impact of Relative Mechanophore Activity on the Selectivity of Ultrasound-Induced Mechanochemical Chain Scission

Abstract

Despite recent advances in polymer mechanochemistry, a more complete understanding of the factors that dictate the ultrasound-induced mechanochemical activation efficiency of mechanophores is necessary. Here, we examine how the identity of a mechanophore, and hence its unique force-coupled reactivity, affects the competition between mechanophore activation and nonspecific polymer backbone scission. Polymers incorporating distinct mechanophores but with putatively similar “chain-centeredness” exhibit widely different mechanochemical activation efficiencies. Furthermore, we employ mechanophores that can be orthogonally cleaved following ultrasonication using heat or light to report on the degree of nonspecific backbone scission that occurs for different mechanophore-containing polymers subjected to ultrasound-induced mechanical force. Our results illustrate that the identity of the mechanophore as well as its position in the polymer chain are inextricably important parameters that together control the selectivity of mechanophore activation during ultrasonication.

Regiochemical effects for the mechanochemical activation of 9‐π‐extended anthracene‐maleimide Diels–Alder adducts

“…Such dispersity-related phenomena were recently found to profoundly hinder kinetic analysis by Robb and coworkers. 45,46 While possibly minor mechanochemical reactivity differences might be inferred from CoGEF simulations, our results highlight the capabilities and limitations of the employed experimental techniques for such delicate variances and raise the question whether these would be relevant for mechanochemical applications in a material context.…”

Competitive Activation Experiments Reveal Significantly Different Mechanochemical Reactivity of Furan–Maleimide and Anthracene–Maleimide Mechanophores

“…Importantly, the forces calculated using CoGEF provide a framework for comparing the relative reactivity of different mechanophores that agrees well with experimental trends . For example, a recent study investigating the selectivity of mechanophore activation compared to nonspecific polymer chain scission confirmed that a coumarin dimer mechanophore is much less reactive than an anthracene–maleimide (AM) mechanophore, ,− consistent with the significantly different predicted F max values of 5.9 and 4.1 nN, respectively . It should be noted that relevant F max values from CoGEF are in a narrow range < 6.0 nN, which represents an apparent upper limit for selective mechanophore reactivity .…”

“…42 Formation of the anthracene product was monitored by photoluminescence (PL) spectroscopy and quantified using a calibration curve (Figure 2b). 42 A separate set of sonication experiments was performed under nearly identical conditions to measure the mechanochemical activation of the FM subunit in each bis-adduct mechanophore using 1 H NMR spectroscopy, 25,40 but in the absence of BHT to prevent spectral contamination (Figure 2c). Ultrasonication experiments performed on a low molecular weight control polymer (M n = 18 kDa; Đ = 1.10) containing a chain-centered AM−FM(endo) bis-adduct mechanophore demonstrate negligible reactivity, consistent with a mechanochemical transformation (Figure S6).…”

“…The selectivity of the mechanically induced retro-Diels–Alder reactions was determined by evaluating the percent activation of the FM and AM subunits in each bis-adduct mechanophore upon ultrasonication. , The plateau value obtained from fitting the time-dependent activation data for each adduct to an expression of first order kinetics provides an estimate of the total mechanophore activation, which is illustrated for the AM and FM adducts in Figure a and b, respectively. Both bis-adducts exhibit a much higher selectivity for activation of the FM subunit than for the AM subunit.…”

“…35 For example, a recent study investigating the selectivity of mechanophore activation compared to nonspecific polymer chain scission confirmed that a coumarin dimer mechanophore 36 is much less reactive than an anthracene−maleimide (AM) mechanophore, 17,37−39 consistent with the significantly different predicted F max values of 5.9 and 4.1 nN, respectively. 40 It should be noted that relevant F max values from CoGEF are in a narrow range < 6.0 nN, which represents an apparent upper limit for selective mechanophore reactivity. 35 On the other hand, it is much more challenging to make reliable predictions about differences in mechanochemical reactivity for mechanophores with similar predicted values of F max .…”