Explaining unexpected data via competitive equilibria and processes in radical reactions with reversible deactivation

Area of Science:

• Chemistry
• Polymer Science
• Chemical Kinetics

Background:

• Radical reactions with reversible deactivation exhibit unexpected and counterintuitive experimental outcomes.
• Discrepancies in product formation and reaction rates are observed when comparing reversible-deactivation radical polymerization (RDRP) with conventional radical polymerization (RP).
• Puzzling phenomena include altered copolymer composition, reduced branching in poly(acrylates), and unusual reaction rates in atom transfer radical polymerization (ATRP).

Purpose of the Study:

• To investigate the underlying mechanisms behind counterintuitive observations in radical reactions with reversible deactivation.
• To explain discrepancies in experimental results that cannot be accounted for by conventional radical polymerization theories.
• To develop a conceptual framework based on competing equilibria and processes to elucidate these unusual phenomena.

Main Methods:

• Analysis of experimental data from various radical transformation and polymerization systems, including methylcobaltamine reactions, nitroxide-mediated polymerization, and ATRP.
• Comparison of reaction outcomes between RDRP and conventional RP under similar conditions.
• Theoretical development of the concept of competing equilibria and processes to explain observed discrepancies.

Main Results:

• The presence of competing equilibria and processes in radical reactions dictates system behavior, often leading to the dominance of one pathway over others.
• This dominance can diminish the rates of competing pathways and lead to imbalanced reversible or pseudoreversible reactions.
• The concept successfully explains the near absence of bimolecular radical coupling products, differences in copolymer composition, reduced branching, and anomalous reaction rates observed in RDRP and ATRP.

Conclusions:

• Counterintuitive results in radical reactions with reversible deactivation are not anomalies but consequences of competing equilibria and processes.
• Understanding these competing pathways is crucial for accurately predicting and controlling reaction outcomes in complex chemical systems.
• The developed concept provides a unified explanation for a range of previously puzzling experimental observations in radical chemistry and polymerization.