Structure Activity Relationships in Reactive Strand Extension Mechanophores

Abstract

Historically, polymer network fracture was studied primarily or solely through the lens of an engineering perspective. Recent investigation has broadened this perspective into one that considers the chemical reality of such materials, as covalent scission is necessary for polymer network failure to occur. For over half a century, Lake-Thomas theory has remained the dominant theory connecting molecular details to the critical failure of a network. Prior advances from our group include adjustments of the molecular energy parameter and modifications of the theory to account for the complex, molecular details of a polymer network. Chemical reactivity which can be mapped onto macromolecular properties is therefore desirable.Reactive Strand Extension (RSE) has been established as a stress-relieving, mechanochemical response wherein polymer strands elongate under force rather than undergoing scission. The response has been documented on the single polymer strand level by Atomic Force Microscopy (AFM) and by pulsed ultrasonication, and incorporated into complex multinetwork architectures. Since the energy dissipation per mechanophore unit can be estimated, RSE-reinforced polymers hold the potential to assist in bridging the quantitative gap between molecular and mechanical frameworks. Chapter 2 discusses the synthesis of end-linked networks containing RSE units and the consequence on their resultant toughness. We perform ROMP of cyclooctene and a cyclooctene-derivative containing RSE to construct two highly similar polyolefins, P1 and P2, differing only in the content of RSE loading, with P1 at 0% RSE and P2 at 20% RSE. We then exploit activated alkyne-hydroxyl “click chemistry” to crosslink them into networks, and prepare them as organogels N1 and N2. Network characterizations via oscillatory rheology and comparison of swelling ratios and sol fractions suggest the networks differ only in the latent mechanochemical ability of the networks. N2 exhibits a tearing energy of 9.6 ± 0.7 J·m-2, and N1 a tearing energy of 6.9 ± 1.1 J·m-2, (p = 0.01, t-test). We attribute the difference of 30% tearing to the installation of RSE. Chapter 3 describes a workflow leveraging advances in machine learning to identify useful scissile mechanophores that can be adapted into tougher polymer networks. We report that the installation of trimethysilyl groups in the meta position to the polymerization/crosslinking handles (NUSZEG) decreases the mechanochemical stability of the resultant metallocene. We synthesize copolymers of similar size containing ferrocene + gDCC and NUSZEG + gDCC and sonicate them, observing ring opening values of ~20% and ~5% respectively. The lowered ring opening is consistent with competition from a weaker bond, one of the lowest phi values we have reported for this sort of competition experiment. We also report that installation of NUSZEG as a crosslinker into acrylate-based networks enhances the tearing energy by to four times. Chapter 4 reviews the mechanochemistry of the four-membered, nitrogen containing heterocycle azetidine. Azetidine undergoes a force-mediated [2+2] cycloelimination to reveal a linear oxime and alkene, and in the presence of water, creates complex hydrolysis products. Its potential in dynamic networks and future RSE-related systems is also discussed.