Electrochemical Investigation of Iron-Catalyzed Atom Transfer Radical Polymerization

Abstract

Use of iron-based catalysts in atom transfer radical polymerization (ATRP) is very interesting because of the abundance of the metal and its biocompatibility. Although the mechanism of action is not well understood yet, iron halide salts are usually used as catalysts, often in the presence of nitrogen or phosphorous ligands (L). In this study, electrochemically mediated ATRP (eATRP) of methyl methacrylate (MMA) catalyzed by FeCl3, both in the absence and presence of additional ligands, was investigated in dimethylformamide. The electrochemical behavior of FeCl3 and FeCl3/L was deeply investigated showing the speciation of Fe(III) and Fe(II) and the role played by added ligands. It is shown that amine ligands form stable iron complexes, whereas phosphines act as reducing agents. eATRP of MMA catalyzed by FeCl3 was investigated in different conditions. In particular, the effects of temperature, catalyst concentration, catalyst-to-initiator ratio, halide ion excess and added ligands were investigated. In general, polymerization was moderately fast but difficult to control. Surprisingly, the best results were obtained with FeCl3 without any other ligand. Electrogenerated Fe(II) effectively activates the dormant chains but deactivation of the propagating radicals by Fe(III) species is less efficient, resulting in dispersity > 1.5, unless a high concentration of FeCl3 is used.

Effects of Solvent and Monomer on the Kinetics of Radical Generation in Atom Transfer Radical Polymerization

“…[11] Recently, iron-based catalysts, albeit being much less active than copper-amine complexes, have attracted considerable attention because Fe is a cheap and abundant metal that can be used even without any added organic ligand. [12][13][14][15][16][17] Photo-induced ATRP using copper catalysts [18][19][20] or without any metal catalyst is also widely used. [21][22][23] The activation step, in which the propagating radical is generated, plays a crucial role in the rate of polymerization.…”

“…However, the most widely used catalysts are copper complexes with polydentate nitrogen‐based ligands because of ease of preparation (often by in situ mixing of a Cu(II) salt with a ligand) and high activity, which is tunable through modification of ligand structure [11] . Recently, iron‐based catalysts, albeit being much less active than copper‐amine complexes, have attracted considerable attention because Fe is a cheap and abundant metal that can be used even without any added organic ligand [12–17] . Photo‐induced ATRP using copper catalysts [18–20] or without any metal catalyst is also widely used [21–23] …”

Copper(II)bromide/2,4,6‐tri‐(2‐pyridyl)‐1,3,5‐triazine Complex for the Photo Atom Transfer Radical Polymerization of Acrylate and Methacrylate Monomers

“…[4] Therefore, researchers mostly gravitate towards the techniques that will circumvent these problems either by decreasing the transition metal loadings to parts per million (ppm) level or in situ generation of active catalyst species. Some polymerization techniques incorporating catalysts at ppm level include Activator Generated by Electron Transfer (AGET) ATRP, [5] electrochemical ATRP, [6] me-chanical ATRP, [7] photoATRP, [3d,8] etc. The advantageous attributes of these processes are the fast polymerization rate, in situ generation of active catalyst, and low concentration of catalyst.…”