Mechanophore-linked addition polymers.

There is growing interest in the use of mechanical energy to alter the molecular and supramolecular structure of polymers to create stress-responsive materials.1a-l Chemical reactions that are accelerated by force remain poorly understood, and there is a need for the rapid discovery of new mechanophores (i.e., stress-sensitive units). Screening putative mechanophores, however, is a slow process that requires a high molecular weight polymer having a single testable unit positioned near the midpoint of the chain, the location where stress under elongation is greatest. Here we show that the required mechanophore-linked addition polymers are easily prepared using bifunctional initiators and a living polymerization method. The approach is demonstrated with benzocyclobutene1k and spiropyran mechanophores that undergo stress-induced 4π and 6π electrocyclic ring opening, respectively. Mechanophore-linked addition polymers thus show considerable promise for rapidly identifying new mechanophores and will lead to a greater, molecular-level understanding of mechanochemical transduction in polymeric materials. Single electron transfer living radical polymerization (SET-LRP)2 was employed for the synthesis of mechanophore-linked polymers, as this method has been shown to generate high molecular weight macromolecules with narrow polydispersity indices (PDIs). cis1,2-Bis(R-bromopropionyloxy)-1,2-dihydrobenzocyclobutene (1), capable of initiating bidirectional SET-LRP, was synthesized and used to produce a series of benzocyclobutene (BCB)-linked PMAs (Scheme 1). Polymerizations were performed at room temperature in DMSO with Cu(0) catalyst and a hexamethylated tris(2aminoethyl)amine (Me6TREN) ligand. Low (18 kDa), medium (91 kDa), and high (287 kDa) molecular weight BCB-linked PMAs (PMA-BCB-PMA) with PDIs around 1.3 were synthesized and used to investigate the ultrasound-induced electrocyclic ring opening reaction. Mechanochemical activation was analyzed by trapping the intermediate ortho-quinodimethide with UV-active N-(1-pyrene)maleimide via cycloaddition (Scheme 1).1k PMA end-functionalized with a BCB unit (PMA-BCB) was prepared as a mechanochemical control polymer since ultrasound-generated forces at the chain ends are minimal. Specifically, the monofunctional initiator cis-1acetoxy-2-(R-bromopropionyloxy) 1,2-dihydrobenzocyclobutene was used to produce a PMA-BCB with a PDI of 1.3 and molecular weight of 190 kDa. This control polymer dispels the notion that the chemical changes are thermally induced, rather than the result of mechanical force. The BCB-containing polymers and PMA homopolymer were subjected to an acoustic field to probe for mechanical activity. Each polymer was dissolved in CH3CN with a large excess of N-(1pyrene)maleimide and radical trap 2,6-di-tert-butyl-4-methylphenol (BHT) and exposed to pulsed sonication3 for 45 min under Ar at 6-9 °C. Aliquots were withdrawn at the beginning and end of each experiment and analyzed by analytical gel permeation chromatography (GPC) using a refractive index (RI) detector. The remainder of the sonicated solution was analyzed by preparatory GPC having both UV (set to 345 nm) and RI detectors.