Iron(III) complexes of bidentate nitrogen ligands as catalysts in reverse atom transfer radical polymerization of styrene

Three iron(III) complexes of bidentate nitrogen ligands, namely 3,5-dimethyl-bispyrazolylmethane (bpzm), 2,2'-dipyridyl (bipy) and 1,1-bis(4,4-dimethyl-1,3-oxazolyn-2-yl)-ethane(box), have been tested in atom transfer radical polymerization (ATRP) of styrene at 120 degrees C using the 1,1,2,2-tetraphenyl-1,2-ethanediol (TPED) initiator under reverse protocol. Good control of the polymerization reaction and narrow average molecular weight distribution (PDI) have been found for the TPED/FeCl3/box catalyst whereas TPED/FeCl3/bpzm and TPED/FeCl3/bipy produce linear dependence of M. vs polymerization time but PDI values broader than that expected for a controlled mechanism. Attempts to clarify the performances of these catalysts were carried out, evaluating the formation constants beta(III) and the electrochemical parameters E-1/2 of the FeCl3/L adducts (L=bpzm; bipy; box) by means of UV-vis spectroscopy and cyclic voltammetry, respectively. A regular increase of both beta(III) and E-1/2 values was observed as the Lewis basic properties of the ligands increase in the order bpzm < bipy < box. The best performances of the FeCl3/box catalyst have been thus attributed to the highest formation constant that produce easy and reversible reduction of the iron(III) metal centre to the corresponding iron(II). Lower reducibility and reversibility were found for the iron(III)/iron(II) redox couple in FeCl3/bipy and FeCl3/bpzm in agreement with the modest performances in ATRP of styrene of these complexes. (C) 2009 Elsevier B.V. All rights reserved.