Quantifying Activation Rates of Scissile Mechanophores and the Influence of Dispersity

Abstract

The ability to accurately and quantitatively characterize structure−mechanochemical activity relationships is important for informing the fundamental understanding of mechanochemical reactivity and, in turn, the successful advancement of the rapidly growing field of polymer mechanochemistry. Ultrasound-induced mechanical activation of polymers remains one of the most general methods for studying mechanophore reactivity; however, the activation rates of scissile mechanophores are still routinely deduced from changes in polymer size using gel permeation chromatography (GPC) that indirectly report on mechanophore activation with questionable accuracy. Here, the rates of ultrasound-induced mechanochemical activation of two distinct scissile and fluorogenic mechanophores are measured using photoluminescence spectroscopy and compared directly to rates determined using various methods for analyzing chain scission kinetics from GPC measurements. This systematic study confirms that the conventional method for analyzing chain scission kinetics is inaccurate and that it provides a misleading picture of mechanophore activity. Instead, time-dependent changes in the GPC refractive index response closely reproduce the rates of mechanophore activation determined spectroscopically. These results expand on prior work by providing a systematic evaluation of the methods used to characterize mechanophore activation kinetics and emphasize the need for a unified approach to kinetic analysis in the field of polymer mechanochemistry. Moreover, analysis of mechanophore activation efficiency reveals an important insight into the consequences of molecular weight dispersity on the characterization of mechanophore reactivity.

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

“…It is reasonable to expect that each polymer has a similar degree of chain-centeredness of the mechanophore unit; however, characterizing the trend in mechanophore activation efficiency over the entire range of polymers in each series further minimizes error associated with random termination or other processes. Dilute solutions of each polymer were subjected to pulsed ultrasonication (2 mg/mL, 1 s on/2 s off, 13.6 W/cm 2 ) in an ice bath and aliquots were removed periodically for analysis by PL spectroscopy following previously reported procedures (see the Supporting Information for details) . Ultrasonication was stopped in each case once the PL signal from the mechanically generated product reached a nearly constant value, and the data were fit to an expression of first-order kinetics to determine the maximum mechanophore activation in each experiment (Figures S1 and S2).…”

“…Strikingly, a mechanophore activation efficiency of 96% is observed for the AM mechanophore while only 32% mechanophore activation is achieved for the CD mechanophore, on average, across the entire range of polymer molecular weights. We demonstrated previously that the activation efficiency for an AM mechanophore decreases to 64% for a 43 kDa PMA-AM polymer, consistent with having a significant fraction of chains in the molecular weight distribution that experiences insufficient mechanical force to achieve activation . Above this threshold molecular weight, the nearly constant activation efficiency measured is notable considering it is in opposition to the molecular weight dependent trend in mechanophore activation previously observed for multimechanophore polymers .…”

“…Polymerizations were well-controlled with polymer molecular weights ( M n ) in the range 60.2–221 kDa for the PMA-AM series and 78.7–206 kDa for the PMA-CD series with all polymers having a Đ ≤ 1.10. These molecular weights are all well above the threshold molecular weight for each mechanophore . It is reasonable to expect that each polymer has a similar degree of chain-centeredness of the mechanophore unit; however, characterizing the trend in mechanophore activation efficiency over the entire range of polymers in each series further minimizes error associated with random termination or other processes.…”

“…The rate of mechanochemical activation was determined to be significantly faster for PMA-FM compared to PMA-CD based on the trends for three different molecular weight samples (Figure S3). Similarly, we also confirmed that the mechanically induced scission of PMA-CD is faster than backbone cleavage in a PMA homopolymer that does not contain any mechanophore. Analysis by 1 H NMR spectroscopy revealed a mechanophore activation efficiency of 69–74% for PMA-FM after extended sonication, which is significantly higher than that observed for PMA-CD (Figure S4).…”

“…The anthracene−maleimide (AM) 16 and coumarin dimer (CD) 14 mechanophores were initially identified due to their scissile nature and fluorogenic properties, which enable the straightforward determination of activation efficiency as demonstrated previously. 14,17 Density functional calculations using the constrained geometries simulate external force (CoGEF) method 18 predict that the AM and CD mechanophores exhibit significantly different mechanochemical reactivity, with values of F max predicted to be 4.1 and 5.9 nN, respectively (Figure 1a). 7 Values of F max from CoGEF calculations have been demonstrated to be good indicators of the relative mechanochemical reactivity of mechanophores.…”