Atom Transfer Radical Polymerization: From Mechanistic Investigations to Materials and Applications
Creators: name: Krys, Pawel
Publisher: name: Carnegie Mellon University
Year: 2019
Subjects: subject: 39999 Chemical Sciences not elsewhere classified
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences

description: Reversible-deactivation radical polymerization (RDRP) methods have emerged as powerful tools for synthesis of advanced functional polymers with predefined molecular weights, narrow molecular weight distributions, and highly retained chain-end functionality. RDRP methods employ fast initiation, efficient intermittent deactivation of propagating radicals, and limited chain breaking reactions. In consequence, they provide control over polymerization<br>comparable to that known from living ionic polymerizations, while maintaining advantages of the radical process, e.g. tolerance to variety of protic solvents, functional groups, and impurities. Among RDRP methods, atom transfer radical polymerization (ATRP) has gained considerable attention due to the facile experimental setup, commercial availability of all reaction components, allowing synthesis of polymers with complex architectures, including hybrid materials. My<br>contribution to this field was primarily focused on: (1) in-depth investigation of the polymerization mechanisms, exploiting kinetic simulations, (2) detailed examination of the mechanism of RDRP in the presence of Cu0, and (3) utilizing the gained understanding of the ATRP mechanisms for synthesis of various functional materials. Chapter 1 overviews the fundamentals of kinetics of ATRP. Since its discovery in 1995, ATRP was subject of detailed mechanistic studies which allowed for identification of structure property relationships of all involved components, and thus for rational design of the reaction conditions. In this chapter influence of the initiator, catalyst (transition metal, ligand, counterion), solvent, additives, type of monomer, and effect of temperature and pressure are discussed. An<br>overview of the experimental methods to determine kinetic parameters is presented, along with guidelines for efficient polymer synthesis. ATRP initiation modes are reviewed in detail, followed by an overview of recently developed methods allowing for a significant decrease of the catalyst<br>concentration. One of the methods developed with the aim to reduce the catalyst concentration employs zerovalent metals, such as Cu<sup>0</sup>. The process is straightforward and versatile, and has been broadly applied across literature for synthesis of well-defined (co)polymers with various architectures. Nonetheless, there has been a vigorous debate in the literature, concerning the mechanism of the<br>process. One mechanism, termed supplemental activator and reducing agent (SARA ATRP) has CuI/L as main activator of alkyl halides (RX), while Cu<sup>0</sup> acts as a supplemental activator. The reduction of Cu<sup>II</sup>/L by Cu<sup>0</sup> (i.e. comproportionation) is relatively slow, and disproportionation is typically kinetically insignificant. An alternate mechanism, termed single electron transfer living radical polymerization (SET-LRP) has Cu<sup>0</sup> as the exclusive activator of RX. Cu<sup>I</sup>/L species do not activate RX, but instead undergo instantaneous disproportionation, which yields Cu<sup>0</sup> and Cu<sup>II</sup><br>species. It is worthwhile mentioning that both mechanisms are based on the same reactions, although their kinetic contributions to the system is vastly different. The work presented in chapter 2 aimed to critically assess the available experimental data in both organic solvents and aqueous media, to distinguish between the two contradicting mechanisms. It was found that the RDRP in the presence of Cu0 follows SARA ATRP mechanism and none of the postulated features of SETLRP could be verified experimentally. Further studies evaluated the effect of alkyl halide, solvent, ligand, and chain length on reactions crucial to SARA ATRP. Chapter 3 describes utilization of the kinetic simulations to gain an in-depth understanding of the polymerization mechanisms. Such knowledge is crucial for optimization of the reaction conditions, and identification and diminishing of the undesired side reactions. Various areas of<br>ATRP are discussed, such as recognizing the lack of direct relationship between overall rate of ATRP and rates of activation of dormant species, or identifying the borderline between two similar ATRP mechanisms. The work presented in chapter 4 shows utilization of the knowledge of RDRP mechanisms (primarily ATRP but also reversible addition-fragmentation chain transfer (RAFT)<br>polymerization) attained in the studies described in chapters 1-3 in the synthesis of a variety of advanced functional materials. This includes rapid and efficient preparation of (co)polymers utilizing low catalyst concentration, synthesis of (co)polymer brushes from silica wafers, utilization of high pressure to facilitate ATRP of macromonomers, and aqueous RAFT synthesis of acrylonitrile-based (co)polymers for use for nanostructured carbon materials. Chapter 5 summarizes ongoing projects, for which manuscripts are currently being prepared for submission. Furthermore, future directions for several completed projects are discussed. Additionally, guidelines for PREDICI simulations, PREDICI batch program, and high pressure system are presented.<br>Finally, a summary is provided in chapter 6, with outlook on future directions. The fundamental studies presented in this thesis constitute a basis for rational design of ATRP reaction<br>conditions, aimed at efficient synthesis of advanced polymeric materials.<br>