Synthesis of thermochromic iron(II) tris(bipyridine)-centered star-shaped polyoxazolines and their bipyridine-centered macroligand counterparts

Iron tris(bipyridine) complexes [Fe(4,4'-bis(chloromethyl)-2,2'-bipyridine}(3)], 1a, and the corresponding iodide analogue generated in situ using NaI, 1b, were explored as initiators for the polymerization of a series of 2-R-2-oxazoline monomers (R = ethyl, EtOX; methyl, MeOX; phenyl, PhOX; and undecyl, UnOX). The resulting labile core, red-violet Fe-centered star polymers fragment during molecular weight analysis by gel permeation chromatography (GPC). Thus, samples were subjected to chemical cleavage in aqueous K2CO3 to generate metal-free bipyridine-centered polyoxazolines, bpyPROX(2). When combined with ferrous ammonium sulfate, these bpyPROX(2) macroligands chelate to Fe(II), regenerating the [Fe(bipyridine)(3)](2+) chromophores. Both preparative and analytical kinetics experiments generally produce polymers with reasonably narrow molecular weight distributions (similar to 1.1-1.5). Molecular weight vs % monomer conversion plots with either the iodide or chloride initiating system were nearly linear for all monomers; however, only PEOX and PUOX products show good correlation with M-n(calcd) based on monomer/initiator loading. For most monomers, reactions with iodide initiators are faster than the chlorides, and linear first-order kinetics plots were observed. Polymerization of oxazolines with 4,4'-bis(halomethyl)-2,2'-bipyridines produced polymers with very narrow molecular weight distributions but with molecular weights higher than targeted based on monomer loading. H-1 NMR data illustrates that termination with dipropylamine is efficient for EtOX polymerizations. Thermal analysis by DSC and TGA reveal few differences between Fe-centered stars and their bpy-centered PROX macroligand counterparts. Variable-temperature UV/vis data is provided for an Fe-centered PEOX thin film sample.