Impact of Catalyzed Radical Termination (CRT) and Reductive Radical Termination (RRT) in Metal‐Mediated Radical Polymerization Processes

Abstract

Atom Transfer Radical Polymerization (ATRP), a metal‐catalyzed process, is a most powerful method for macromolecular engineering, producing polymers with targeted and low‐dispersity molar masses and with high chain‐end fidelity. This is due to the persistent radical effect, which dramatically reduces the spontaneous radical terminations, prolonging the lifetime of radical chains and the concurrent growth of all polymer chains. Two additional reaction modes that involve metals and organic radicals, however, may negatively affect the controlled polymerization. These are the Catalyzed Radical Termination (CRT) and the Reductive Radical Termination (RRT) and were shown to be particularly important for the polymerization of acrylate monomers. The scope and the mechanistic investigations carried out to elucidate how these processes interplay with ATRP and with each other are outlined in this Minireview.

A journey into metal–carbon bond homolysis

“…We do not even know, as yet, what fractions of coupling and disproportionation products are produced by the CRT process, since the product distribution is skewed by other competing phenomena (e.g. conventional radical termination, reductive radical terminations) [153][154][155]. Further investigations aimed at determining this product distribution are ongoing.…”

p‐Substituted Tris(2‐pyridylmethyl)amines as Ligands for Highly Active ATRP Catalysts: Facile Synthesis and Characterization

“…The [R−Cu II (L)] + intermediate can 1) dissociate back to R . and [Cu I (L)] + (OMRP activation, k a,OMRP ); 2) react with a propagating radical to form dead chains and regenerate [Cu I (L)] + (CRT, k CRT ); or 3) react with proton donors in solution (such as impurities, excess ligand) forming a saturated chain and a Cu II species that is inactive in ATRP (RRT, k RRT ) …”

Photo‐mediated metal‐free atom transfer radical polymerization: recent advances in organocatalysts and perfection towards polymer synthesis

“…However, most active Cu(I) species may react with propagating radicals to form organometallic intermediates via organometallic‐mediated radical polymerization, leading to copper catalysed termination. When more active catalysts are used in ATRP, the concentration of Cu(I) activators falls drastically, thereby preventing the generation of organometallic intermediates and metal catalysed termination 57,58 . ATRP provides flexibility in many domains, such as the ability to carry out reactions in a variety of conditions and solvents, tolerance to most functional groups, and the ability to grow polymers from organic materials, proteins, surfaces and inorganic materials.…”