An Investigation of the Selective Chain Scission at Centered Diels–Alder Mechanophore under Ultrasonication

It is a challenging topic to disconnect a linear polymer selectively at the mechanophore site by an external force in a “cold” fashion. In this work, the effect of the power output of ultrasonication on the selective cleavage at the centered urfuryl-maleimide Diels–Alder (DA) mechanophore of poly(methyl acrylate)s (DA-PMA -a and DA-PMA -b ) were quantitatively investigated by comparative study on experimental and simulated chain scission kinetics as well as high-resolution 1 H NMR spectroscopy (600 MHz). At low power output of the ultrasonication (2.10 W), DA-PMA- a with M n of ca. 2 M lim ( M lim, below which no further chain scission was observed) presented a DI (degradation index) −t (sonication time) plot with a turnover point at ca. 1.0 and no clear variation of the molecular weight after the turnover, which met well with the calculated center cleavage mode. At 5.52 W, DA-PMA- a and a poly(methyl acrylate) that contained two centered ester bonds (ester-PMA) presented similar DI– t plots with turnover points less than 1.0 within same sonication times, while poly(methyl acrylate) with fully carbon–carbon chain (PMA) had a turnover at DI value of ca. 0.5. By way of contrast, high power output of the ultrasonication (5.52 W) caused a possible cleavage of ester bonds of DA-PMA -a, which would mask the selective cleavage at the DA site. High-resolution 1 H NMR result of DA-PMA -b (115.8 kDa, M n was slightly higher than 2 M lim ) showed that DA conversions were up to 55% under 2.10 W and 38% under 5.52 W. The kinetics from GPC traces and 1 H NMR results of DA-PMA -b as well as 1 H NMR results of DA-PMA -c (68.4 kDa, M n was slightly higher than M lim ) under sonication confirmed the observation that low power output favored selective chain scission at DA site. The turnover point in the DI– t plot might be used as characteristic parameter to gauge the selective chain scission at mechanophore site for single mechanophore-centered polymers.