Collaborative Research: Stereospecific and Force-responsive Architectural Editing of Polymer Backbones via Catalyzed 3,3-sigmatropic Oxo-Rearrangements

With the support of the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Dr. Aleksandr Zhukhovitskiy of the University of North Carolina at Chapel Hill and Dr. Ian Tonks of the University of Minnesota-Twin Cities will develop catalytic methods to edit the molecular architectures of various plastics such as polyesters and polyurethanes. Architecture—e.g., the extent and type of branching—of a polymer underpins its thermomechanical properties and, consequently, applications. For instance, linear architecture of high-density polyethylene (HDPE) leads to stiff materials that could be used as milk jugs; meanwhile, highly branched linear low-density polyethylene (LLDPE) is more flexible and extensible, which supports applications like plastic bags. Accessing a spectrum of architectures for a given polymer remains a challenge. The proposed research will address this challenge by developing catalysts and new mechanisms that can rearrange the bond between atoms in the polymer skeleton, thereby turning branched chains into linear ones, and vice versa. This chemistry will allow scientists and engineers to design new types of plastics with variable and changeable properties, such as force-responsive materials that change properties upon stretching or compressing, or materials with improved degradation/recyclability properties. This project will provide interdisciplinary research training to students and help to prepare a skilled workforce for academia and industry. As a part of this work, polymer-focused educational programs will be developed that integrate concepts of sustainability and circularity. This proposal will develop branched-to-linear transformations of polymer backbones via catalyzed sigmatropic rearrangements. Transition metal- and organo-catalyzed [3,3]-sigmatropic rearrangements will be developed to isomerize a broad range of vinyl sidechain-containing polymer classes between branched and linear architectures. The specific ratio of the branched-to-linear conversion will be dictated by the percent conversion and the thermodynamics of a given system. These rearrangements will result in transformations of the thermal properties of polymers, namely lowering their glass transition temperatures and increasing their crystallinity. The stereospecific nature of concerted [3,3]-rearrangements will be utilized to enable tacticity transfer from starting polymers to rearranged polymers. Additionally, mechanical force will be utilized to alter the thermodynamic landscape of the rearrangement reaction coordinates, creating a thermodynamic bias toward linear isomers. Ultimately, this work will leverage a detailed understanding of catalyzed [3,3]-rearrangements of polymer backbones to enable broad architectural and property editing of soft materials.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.