Thermal Ring‐Opening Polymerization of Hydrocarbon‐Bridged [2]Ferrocenophanes: Synthesis and Properties of Poly(ferrocenylethylene)s and Their Charge‐Transfer Polymer Salts with Tetracyanoethylene

Abstract The poly(ferrocenylethylene)s [Fe(η‐C 5 H 3 RCH 2 ) 2 ] n 5a and 5b ( a : R = H, b : R = Me) have been prepared by thermal ring‐opening polymerization of the corresponding strained hydrocarbon‐bridged [2]ferrocenophanes [Fe(η‐C 5 ‐H 3 RCH 2 ) 2 ] ( 4a and 4b ). An X‐ray diffraction study of 4a indicated significant strain. Polymer 5a was crystalline and insoluble in common organic solvents and was characterized by solid‐state 13 C NMR. Polymer 5b , which was soluble in organic solvents, was characterized by 1 H and 13 C NMR, UV/visible spectroscopy and elemental analysis. Its molecular weight distribution was bimodal (gel permeation chromatography: M w = 9.6 × 10 4 , M n = 8.6 × 10 4 for the high molecular weight fraction, M w = 4.8 × 10 3 , M n = 3.5 × 10 3 for the oligomeric fraction), suggesting two polymerization mechanisms. The UV/visible spectrum implied a localized structure for the polymer backbone. Cyclic voltammetry revealed that 5b undergoes two reversible oxidations in CH 2 Cl 2 solution at–0.25 and–0.16V. The redox coupling is indicative of only a small degree of interaction between the iron centres. Thermogravimetric analysis indicated that 5a and 5b are thermally stable to ca. 300–350°C under N 2 . At higher temperatures they yield ferromagnetic iron carbide ceramics 6a and 6b (ca. 50% and 32%, respectively, at 600°C) together with molecular depolymerization products. The reaction of 5b with tetracyanoethylene (TCNE) yielded insoluble and soluble oxidized products 11 and 12 , which differed in the degree of oligomerization of the TCNE y‐ x counterions. These products were characterized by IR, elemental analysis, ESR spectroscopy, and magnetic susceptibility measurements. The last revealed the presence of significant antiferromagnetic interactions in 12 . Thermal ring‐opening polymerization was used to prepare the first examples of poly‐(ferrocenylethy1ene)s from hydrocarbon‐bridged [2]ferrocenophanes (below). The polymers yield ferromagnetic ceramics on pyrolysis and, although the iron centres are only weakly interacting, TCNE‐oxidized materials display significant antiferromagnetic coupling at low temperature. magnified image