Characterizing the Mechanical Strengths of Chemical Bonds via Sonochemical Polymer Mechanochemistry
Mechanically induced chemical bond scission underlies the fracture and macroscopic failure of polymeric materials. Thus, the mechanical strength of scissile chemical bonds plays a role in material failure and in the mechanical initiation of cascade reactions, but quantitative measurements of mechanical strength are rare. This dissertation describes research that quantifies relative mechanical strengths of polymers that possess a variety of chemical and topological functionalities in order to assess the strength of putative "weak bonds" along their backbones.
First, relative mechanical strengths of "weak" bonds that break by homolytic scission were assessed: the carbon-nitrogen bond of an azobisdialkylnitrile (< 30 kcal mol-1), the sulfur-sulfur bond of a disulfide (54 kcal mol-1), and the carbon-oxygen bond of a benzylphenyl ether (52-54 kcal mol-1). The mechanical strengths were assessed in the context of chain scission triggered by pulsed sonication of polymer solutions, by using the competing non-scissile mechanochemical reaction of gem dichlorocyclopropane mechanophores as a gauge of the force required for chain scission. The relative mechanical strengths of the three weak bonds are found to be: azobisdialkylnitrile (weakest) < disulfide < benzylphenyl ether. The greater mechanical strength of the benzylphenyl ether relative to the disulfide is ascribed in part to poor mechanochemical coupling as a result of the rehybridization that accompanies carbon-oxygen bond scission.
Studies of bond scission were extended to include the effect of topology. The introduction of mechanical bonding in polymers can sufficiently affect physical properties, but experimental measurements of the relative strengths of topological bonding are rare. We studied the relative mechanical strengths of three bonding topologies formed from the same set of chemical functionalities: a catenane, a symmetrical macrocycle, and a linear construct. Mechanical strengths were obtained by analysis of molecular weight of polymers with embedded topological molecules after 4 h of pulsed sonication (M4h). The obtained M4h was converted to the length (L4h) using the calculated force free length of each monomer. The relative mechanical strengths of these topological molecules are nearly identical based on L4h, and we conclude that the mechanical strength of a mechanical bond (catenane) is as high as that of a linear analog.
Using these methods, the relative mechanical strengths of triazoles were also investigated. Random copolymers containing either 1,4-triazole, 1,5-triazole, or indole were synthesized via entropy driven ring opening metathesis co-polymerization. Solutions of those polymers were subjected to pulsed ultrasound for 4 hours and M4h measured was less than the M4h of poly(gDCC). Taken together, these results suggest that the introduction of these heterocycles does weaken the polymer main chain, but not through mechanically assisted cycloreversion.
As an extended study of mechanical strength of different molecular topologies, the sonochemistry of a polymeric trefoil knot was also investigated. A zinc-templated polymeric trefoil knot was subjected to pulsed ultrasound, to determine whether demetallation can be mechanically triggered by tightening the trefoil knot under high forces of tension. The products of sonication of the polymeric trefoil knot were analyzed by 1H NMR and by the color change of dithizone solution used to coordinate any released zinc. No evidence for mechanical demetallation or knot scission was obtained, suggesting that the presence of the zinc template in the trefoil knot can prevent knot tightening and subsequent weakening and scission.
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