Synthesis of Thermochromic Iron(II) Tris(bipyridine)-Centered Star-Shaped Polyoxazolines and Their Bipyridine-Centered Macroligand Counterparts

Abstract

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, bpyPROX2. When combined with ferrous ammonium sulfate, these bpyPROX2 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 (∼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 Mn(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. 1 H 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.

Optically Active Supramolecular Poly(L-lactide)s End-Capped with Terpyridine

“…This phenomenon has also been observed with bipyridine-based iron(II) complexes of polylactides [20] and polyoxazolines. [5] The mechanism (ligand dissociation or spin-crossover) is not yet clear.…”

Poly(2‐oxazoline)s as Smart Bioinspired Polymers

“…[27] Initiators and quenching agents carrying functional groups were used to synthesize end-functionalized or telechelic polyoxazolines; functional groups mainly included unsaturated hydrocarbon, [28][29][30] alkyne, [31] naphthalene, [32] siloxane, [33][34][35][36] hydroxyl, [33,37,38] tosylate, [39] amine, [38,40,41] trialkyl ammonium, [36,[42][43][44] bi-and terpyridine. [45][46][47][48][49][50] Statistical copolymers have been prepared by the copolymerization of mixtures of oxazoline monomers. [15,17,[51][52][53][54][55][56][57][58][59][60][61][62] The copolymers may have random, gradient, or tapered structures, depending on the relative reactivities of monomers and accordingly nature of side chains.…”

“…Figure 10. Transmission electron and scanning force micrographs (insets bottom left), and cryogenic transmission electron micrographs (insets marked with cryo) of the micellar aggregates of poly(2-methyl-2-oxazoline) 50 -stat-poly(2-phenyl-2-oxazoline) 50 in different water-ethanol mixtures: (a) 5 wt.-% ethanol, (b) 10 wt.-% ethanol, (c) 15 wt.-% ethanol. Reprinted from ref.…”

Poly(2‐oxazoline)s based on fatty acids

“…The synthesis of a variety of welldefined star-shaped homopolymers [74] and diblock copolymers [75], including poly(UndOx) (co)polymers, were reported. Interestingly, the iron(II)-centered homopolymers revealed reversible thermochromism, i.e., the characteristic red color of the metal complex disappeared upon heating, which was ascribed to dissociation of the metal complex [74]. DSC analysis of the star-block copolymers consisting of EtOx and UndOx revealed both the T g of the poly(EtOx) block and the T m of the poly(UndOx) block indicative of bulk phase separation [75].…”

“…Schematic representation of the synthesis of tris(2,2 0 -bipyridine) iron(II) centered poly(UndOx) homo-and block copolymers as reported in refs. [74] and [75].…”