Mechanophore-Linked Addition Polymers

Abstract

The use of mechanical energy to increase reaction rates and alter the distribution of products has gained considerable interest of late. However, the discovery of new mechanophores (i.e., mechanochemically reactive units) is currently time-consuming, expensive, low-yielding, and a process that is not easily scaled to large quantities. Here we show that mechanophore-linked addition polymers are easily prepared using bifunctional initiators with a living radical polymerization method. The mechanophore is positioned close to the center of the polymer, where ultrasound-generated forces are the largest. Since these forces are strongly dependent on molecular weight, the use of controlled polymerization enables fine-tuning of the mechanical activity so that mechanophore reactions are initiated while minimizing chain scission. The approach is illustrated first with the synthesis and investigation of a 1,2-disubstituted benzocyclobutene mechanophore that is incorporated into the center of a polymethacrylate (PMA) chain. Selected molecular weights are probed using ultrasound to illustrate that the 4π electrocyclic ring opening of the benzocyclobutene link is stress-induced. To demonstrate the broad applicability of this method for mechanophore screening, we also report a new spiropyran-linked PMA that undergoes an ultrasound-induced 6π-electron electrocyclic ring opening. The straightforward synthesis of mechanophore-linked addition polymers presented here shows considerable promise for the investigation of new mechanophores and will lead to a greater understanding of mechanochemical reactivity within polymer systems.