Selective scission of pyridine-boronium complexes: mechanical generation of Bronsted bases and polymerization catalysts

Coupling two pyridine-capped poly(methyl acrylate) (PMA) chains of varying molecular weights to bis(pentafluorophenyl) boron chloride afforded the first examples of boronium-based polymers. Ultrasonication of CH3CN solutions of these polymers with number average molecular weights (M-n) > 40 kDa induced selective scission of a boron-pyridine bond, affording a two-fold reduction in M-n. The liberated pyridine was used to effect a colorimetric change, via a stoichiometric Bronsted acid-base reaction with an indicator, and to catalyze the polymerization of alpha-trifluoromethyl-2,2,2-trifluoroethyl acrylate. No reduction in M-n, colorimetric response, or polymerization activity were observed (i) in the absence of sonication, (ii) for polymers with M-n < 40 kDa, (iii) for a high molecular weight PMA (Mn 110 kDa) containing a terminal boronium species, or (iv) when the boron-pyridine adduct was not covalently linked to a PMA chain. Collectively, these results support the notion that the aforementioned scission processes were induced by an applied mechanical force.