Mechanically facilitated retro [4 + 2] cycloadditions

Poly(methyl acrylate)s (PMAs) of varying molecular weights were grown from a [4 + 2] cycloaddition adduct of maleimide with furan containing two polymerization initiators. Subjecting the corresponding PMA (>30 kDa) chains to ultrasound at 0 °C resulted in a retro [4 + 2] cycloaddition reaction, as observed by gel permeation chromatography (GPC) and UV-vis spectroscopy, as well as labeling of the liberated maleimide and furan moieties with appropriate chromophores featuring complementary functional groups. Similar results were obtained by sonicating analogous polymers that were grown from a thermally robust [4 + 2] cycloaddition adduct of maleimide with anthracene. The generation of anthracenyl species from these latter adducts allowed for the rate of the corresponding mechanically activated retro [4 + 2] cycloaddition reaction to be measured. No reduction in the number average molecular weight (M_n) or liberation of the maleimide, furan, or anthracene moieties was observed (1) (i) for polymers containing the cycloaddition adducts with Mn < 20 kDa, (ii) for high molecular weight PMAs (M_n > 60 kDa) featuring terminal cycloaddition adducts, or (iii) when the cycloaddition adducts were not covalently linked to a high molecular weight PMA. Collectively, these results support the notion that the aforementioned retro [4+ 2] cycloaddition processes were derived from a vectorially opposed mechanical force applied to adducts embedded within the polymer chains.