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42987-AC7
Studies on Mechanochemical Activation in Polymer Molecules
During the course of chemical reactions, reactant molecules need to surmount an energy barrier to allow their transformation into products. The energy is usually provided by heat, light, pressure, or electrical potential. A different way of initiating or accelerating reactions is to use force to deform molecules along a specific direction of the reaction coordinate. Recently, we prepared a mechanophore (i.e. mechanochemically reactive unit) linked polymer, and showed that it is possible to use mechanical forces generated by ultrasound to affect a polymer solution. We were able to accelerate rearrangement reactions and bias reaction pathways to yield products not obtainable from thermal or light-induced reactions.
The trans and cis isomers of a 1,2-disubstituted benzocyclobutene (BCB) mechanophore were incorporated into well-defined poly(ethylene glycol) (PEG) polymers, which were exposed to ultrasound in the presence of dienophile N-(1-pyrene)maleimide. This dienophile trap allowed the reaction to be monitored by gel permeation chromatography (GPC) using ultraviolet (UV) and refractive index (RI) detectors. GPC characterization of the product showed a strong UV signal coincident with the RI signal from the polymer, indicating incorporation of the pyrene trap via an ultrasound-induced electrocyclic ring opening of the BCB and subsequent Diels-Alder reaction. A polymer below the molecular weight threshold for mechanical activation and a PEG homopolymer were subjected to the same conditions. The absence of a UV signal in these polymers indicated that the reaction was indeed due to mechanical activation.
To determine the specific chemical structures of the pyrene-modified LFPs and investigate the stereochemical consequences of mechanical activation, we synthesized pyrene-labeled maleimide enriched with 13C isotope in the carbonyl positions. The 40 kDa trans and cis BCB-linked polymers were then subjected to ultrasound in the presence of 13C labeled pyrene maleimide. After sonication, each polymer was analyzed by 13C nuclear magnetic resonance (NMR). Both the trans and cis BCB-linked polymers yielded a single resonance at δ 174.2 ppm, which matched the signal of a small molecule model compound. It was found that the observed NMR signal was due to the Diels-Alder adduct that results from endo addition of the maleimide to the E,E-oQDM intermediate. The trans and cis BCB-linked PEGs were found to give identical products, which contrasts with reaction initiation by light or heat, in which the isomers follow mutually exclusive pathways to different products.
These initial studies demonstrated that ultrasound can be applied to polymer solutions to investigate mechanochemical reactions and can be used to accelerate and alter the course of chemical reactions. To expand the scope of this research, we sought to discover new mechanophores. However, this process is time-consuming, expensive, low-yielding, and not easily scaled to large quantities. In our recent studies, we showed 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. The approach was illustrated first by investigation of a BCB mechanophore that was incorporated into the center of a polymethacrylate (PMA) chain.
Single electron transfer living radical polymerization (SET-LRP), which has been shown to generate high molecular weight macromolecules with narrow polydispersity indices (PDIs), was employed for the synthesis of mechanophore-linked PMA. Cis-1,2-bis(α-bromopropionyloxy)-1,2-dihydrobenzocyclobutene capable of initiating bidirectional SET-LRP, was synthesized and used to produce low, medium, and high molecular weight BCB-linked PMAs. After sonication with N-(1-pyrene)maleimide, no UV signal was apparent with the polymer below the threshold molecular weight for mechanical activation. The medium and high molecular weight polymers showed UV signals, corresponding to incorporation of the pyrene via the mechanochemical 4π electrocyclic ring opening. PMA having an end-functionalized BCB unit was prepared as a mechanochemical control polymer, since ultrasound-generated forces at the chain ends are minimal. No incorporation of the pyrene was observed, consistent with the absence of mechanical activity.
To demonstrate the broad applicability of this method for mechanophore screening, we investigated a spiropyran-linked PMA that was shown to undergo an ultrasound-induced 6π-electron electrocyclic ring opening. A spiropyran (SP) bis-functionalized with α-bromo-α-methyl-propionyloxy groups was used as an SET-LRP initiator to produce spiropyran linked PMA (PMA-SP-PMA). When subjected to pulsed ultrasound, the originally colorless PMA-SP-PMA solution changed to a visible pink hue. Examination of the sonicated solution with a UV spectrophotometer showed a new band at 550 nm, corresponding to the open form of spiropyran. Exposure to ambient light for 40 minutes at room temperature caused the color to disappear, consistent with the known photolytic reversion to the closed form. Both end-functionalized and low molecular weight control polymers were sonicated and showed no color change. In the future, we plan to further investigate the mechanochromic properties of PMA-SP-PMA by studying the SP-containing polymer in the solid state and also incorporating the SP into other polymers.
