Mechanistically Guided Predictive Models for Ligand and Initiator Effects in Copper-Catalyzed Atom Transfer Radical Polymerization (Cu-ATRP)

Abstract

Copper-catalyzed atom transfer radical polymerization (Cu-ATRP) is one of the most widely used controlled radical polymerization techniques. Notwithstanding the extensive mechanistic studies in the literature, the transition states of the activation/deactivation of the growing polymer chain, a key equilibrium in Cu-ATRP, have not been investigated computationally. Therefore, the understanding of the origin of ligand and initiator effects on the rates of activation/deactivation is still limited. Here, we present the first computational analysis of Cu-ATRP activation transition states to reveal factors that affect the rates of activation and deactivation. The Br atom transfer between the polymer chain and the Cu catalyst occurs through an unusual bent geometry that involves pronounced interactions between the polymer chain end and the ancillary ligand on the Cu catalyst. Therefore, the rates of activation/deactivation are determined by both the electronic properties of the Cu catalyst and the ligand-initiator steric repulsions. In addition, our calculations revealed the important role of ligand backbone flexibility on the activation. These theoretical analyses led to the identification of three chemically meaningful descriptors, namely HOMO energy of the catalyst (E HOMO), percent buried volume (V bur %), and distortion energy of the catalyst (ΔE dist), to describe the electronic, steric, and flexibility effects on reactivity, respectively.

Why Do We Need More Active ATRP Catalysts?

“…The activation of alkyl halide initiators or dormant species in ATRP by Cu I /L activators was proved to follow a concerted atom transfer inner‐sphere electron transfer pathway (ISET) . Other possible mechanisms, such as stepwise outer‐sphere electron transfer, dissociative electron transfer and oxidative addition of RX to Cu I /L via a Cu III intermediate, were found to require high energy barriers, as indicated by DFT and Marcus theory (Figure ) . Moreover, Cu I /L complexes were observed to activate RX initiators 7–10 times faster than outer‐sphere electron donors (e. g. aromatic radical anions electrochemically generated from their neutral precursors), confirming the ISET mechanism for ATRP catalysts.…”

“…The ISET process involves the formation of a late transition state (TS) that is structurally similar to the X−Cu II /L product . The optimized geometry of the TS for the reaction between several ATRP catalysts and ethyl α‐bromoisobutyrate (EBiB) as initiator (Figure ), revealed a bent Cu‐Br−C bond angle (140–155°).…”

Molecular dynamics based descriptors for predicting supramolecular gelation

“…Although the size of this dataset is modest, previous studies in different elds have shown to deliver a promising predictive model with similar dataset sizes, by using models with a low complexity or by means of chemically relevant descriptors. 14,23,24…”

Developments in the Components of Metal‐Free Photoinitiated Organocatalyzed‐Atom Transfer Radical Polymerization (O‐ATRP)

“…An extensive study on the effect of the initiator in the metal‐catalyzed ATRP can be found in the research published by Tang et al [79] . Computational analysis on the role of initiator in ATRP, including the study of electronic transitions involved and an overall mechanistic understanding, can be grasped by studying different models developed by Fang et al [83] . For the photoinduced metal‐free O‐ATRP, there is a vast scope for research considering the limited number of initiators prevalent in the current literature.…”