Molecular Reactivity in Responsive Polymers

Abstract

Responsive polymers can be activated by a wide range of stimuli. The underlying general principle of responsive behavior is, in many respects, similar across systems: a stimulus induces a structural change at the molecular or supramolecular level, and those structural changes result in a change in bulk material property or behavior. Within this general scheme lies a rich opportunity for diverse approaches to molecular control and optimization, wherein molecular-level responses are translated across length scales, from microscopic to macroscopic. The efficiency and magnitude of the response are determined both by intrinsic molecular reactivity and by the interplay between the responsive units and the polymer matrix. In other words, the responsive behavior can be tuned through the chemical structure of the molecules, and the effective expression also depends on the local molecular environment and on how the applied stimulus is transmitted through the material to the molecules.In this dissertation, the interplay of molecular reactivity and responsive polymer behavior is investigated across three distinct systems. First, photoacid-triggered degradation of gels and elastomers was studied by incorporating the photoacid generator, triphenyl sulfonium triflate (TPS), into polymer networks. Upon exposure to ultraviolet light, acid generation initiated the subsequent acid-mediated degradation reactions. By comparing tethered and untethered TPS molecules, the molecule-matrix interplay in gels and elastomers was further investigated. Lower efficiency was observed in elastomers than in gels. The results show that the diffusion of TPS molecules and the filter effect from the polymer matrix are key factors contributing to the differences. Second, bicyclic cyclobutane mechanophores, which release stored length when the cyclobutane ring undergoes a force-driven [2+2] cycloreversion during the extension of polymer chains, were incorporated into polymer networks. The relationship between the magnitude of this so-called reactive strand extension (RSE) effect on the bulk level and molecular details of the cycloreversion was studied by varying the size of the fused ring in the bicylic mechanophores from 5 to 12 atoms. The microscopic structural change was found to tune the macroscopic ultimate strain of the polymer networks. Polymer strands with greater RSE lead to greater stretchability and toughness, and the trend was observed in linear polymers, single-network gels, and double-network hydrogels. Finally, we consider the kinetics and equilibrium thermodynamics of polymerization and depolymerization of a model self-immolative polymer, poly(n-butanal), using in-situ infrared spectroscopy and variable-temperature nuclear magnetic resonance spectroscopy. The polymerization exhibited unusual anti-Arrhenius behavior, and polymer tacticity was found to depend on synthesis temperature. These effects are attributed to temperature-dependent aggregation during polymerization/depolymerization, which alters both rates and stereoregularity.