Mechanism of Halogen Exchange in ATRP

Abstract

Detailed mechanistic studies reveal that halogen exchange (HE) in ATRP can occur not only by a radical pathway (atom transfer) but also by an ionic pathway (SN2 reaction) because Cu(I)(L)X and Cu(II)(L)X2 complexes contain weakly associated halide anion that can participate in the SN2 reaction with alkyl halide (ATRP initiator). Both pathways were kinetically studied, and their contributions to the HE process were quantitatively evaluated for seven alkyl halides and three Cu(I)(L)Cl complexes. Radical pathway dominates the HE process for 3° and 2° alkyl bromides with more active complexes such as Cu(I)(TPMA)Cl. Interestingly, ionic pathway dominates for 1° alkyl bromides and less active ATRP catalysts. These studies also revealed that degree of association of alkyl halide anion depends on the structure of copper complexes. In addition, radical pathway is accompanied by the reverse reactions such as deactivation of radicals to alkyl bromides and also activation of alkyl chlorides, reducing the efficiency of halogen exchange.

Synthesis of cationic star polymers by simplified electrochemically mediated ATRP

“…The rate of activation of a bromo-terminated PDMAEMA chain end is faster than the activation of a 2-bromoisobutyrate β-CD chain ends (ω-functional arms) [55,56]. Typically, halogen exchange procedures [55][56][57][58] are employed to solve this problem, but it can't be used for systems with low concentrations of catalyst, because of poor initiation efficiency.…”

MALDI‐TOF Analysis of Halogen Telechelic Poly(methyl methacrylate)s and Poly(methyl acrylate)s Prepared by Atom Transfer Radical Polymerization (ATRP) or Single Electron Transfer‐Living Radical Polymerization (SET‐LRP)

“…Despite the dispersity of 1.5 (Table – entry 3) determined by GPC the polymer chains are well‐defined. The major species is the expected product—a bis ‐chloro‐telechelic PMA—and the second product is a bromo‐/chloro‐telechelic PMA . The chloro‐telechelic PMA was estimated at 81 mol%.…”

Methacrylic Zwitterionic, Thermoresponsive, and Hydrophilic (Co)Polymers via Cu(0)-Polymerization: The Importance of Halide Salt Additives

“…Moreover, tris(2‐pyridylmethyl)amine (TPMA) and bipyridine (Bpy) were utilized as the ligands in previous reports (as opposed to PMDETA herein), hence different behavior can be expected. It is important to stress that a chloride salt was preferred to a bromide salt to avoid unsatisfactory halogen‐exchange throughout polymerization . Although the quantity of salt present in the reaction medium is greater than in previous reports, the speed of the polymerization was not affected (as opposed to previous investigations), as full monomer conversions could still be attained within 30 min.…”