Carbodiimide Ring-opening Metathesis Polymerization: Precision Synthesis of Nitrogen-Rich Polymer Backbones

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

With the support of the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry, Aleksandr Zhukhovitskiy of the University of North Carolina at Chapel Hill is developing ring-opening polymerization reactions for the synthesis of nitrogen-rich polymer backbones. Polymers—macromolecular constituents of plastics and rubber materials— are essential and ubiquitous in our daily lives: from food packaging to clothing to vehicle parts. The vast majority of plastics that are currently being produced contain predominantly carbon atoms along the main polymer backbone. Meanwhile, further developing the chemistry associated with systems containing nitrogen-rich polymer backbones is critically needed. This improved control over their assembly, folding, thermodynamic and dielectric properties, as well as catalytic activity. These concepts are illustrated well in proteins, which are naturally occurring polymers. However, incorporation of nitrogen into the polymer backbone is difficult to achieve synthetically, and the majority of current methodologies are non-selective and generally lack structural precision. In this research, highly active catalysts based on iridium metal will be used in ring-opening polymerization of a variety of carbodiimides to prepare polymers with abundant and precisely positioned nitrogen atoms along the polymer backbone. Strong emphasis will be placed on detailed kinetic and mechanistic understanding of the catalytic cycle, monomer design, and control over the length of polymer chains. Complementary computational methods will also be utilized to guide experimental design and understand how these reactions work and how to improve them. This research provides unique opportunities to educate students in mechanistic analysis, physical organic chemistry, organometallic chemistry and polymer chemistry. The research will also transform the design and synthesis of academically and industrially relevant nitrogen-rich polymer backbones, opening the door to a deeper understanding of the structure-property relationships in these polymers. In addition, the team will continue to expand its educational polymer-chemistry focused program that targets K-12 students and science teachers with a focus on talent pools that remain under-resourced and underrepresented in the STEM fields. This program includes virtual and in-person science demonstrations at K-12 schools in Chapel Hill and will expand to include Durham, NC. These efforts will be instrumental to nurturing within future generations of students an appreciation for and identification with polymer science research. This research will focus on carbodiimide ring-opening metathesis polymerization (CDI ROMP) catalyzed by iridium guanidinate complexes for the precise synthesis of nitrogen-rich polymer backbones. The first objective will focus on the investigation of the reaction mechanism through a combination of experimental and theoretical tools. The following aspects will be explored: verification of chain growth, the type of metathesis process, energetics of the carbodiimide-iridium coordination, the nature of energy barrier of turnover limiting step, and the geometry of the corresponding transition state. Controlled and living CDI ROMP will be developed in the second objective and utilized to prepare block copolymers. Computational and experimental studies will shed light on the effect of ancillary cyclopentadienyl ligand structure in the iridium catalyst on the backbiting/chain transfer process. The last objective will focus on the derivatization of precise main-chain polycarbodiimides to form libraries of other classes of nitrogen-rich polymer backbones, including polyureas, polythioureas, polyguanidines, polyamidines and polyamidinates. The mechanism of insertion/elimination in this research is fundamentally different from traditional metathesis of C-C bonds that proceeds via cyclo-addition/elimination. As such, the gained mechanistic insights will be instructive in the future development of other types of metathesis transformations, such as diazene metathesis and ring-closing carbodiimide metathesis. 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.