Hyperbranched poly(ferrocenylene)s containing groups 14 and 15 elements: Syntheses, optical and thermal properties, and pyrolytic transformations into nanostructured magnetoceramics

A series of hyperbranched poly(ferrocenylene)s containing elements (E) of groups 14 [E = Si (hb-1), Ge (hb-2)] and 15 [E = P (hb-3), Sb (hb-4)] are prepared in good isolation yields (up to 82 wt%) by the salt-eliminative polycoupling of dilithioferrocene with tri-(RECl ₃) or tetrachlorides of the elements (ECl ₄). While the polymers with no or small R groups are insoluble or partially soluble, those with long alkyl chains (R = C _nH _2n+1 with n ≥ 8) are completely soluble and film forming. The polymers exhibit solution properties characteristic of hyperbranched macromolecules: e.g. hb-1(18) shows a low intrinsic viscosity ([η] = 0.02 dL/g) despite its high absolute molecular weight (M _w = 5 × 10 ⁵). Spectroscopic analyses reveal that the polymers possess rigid skeleton structures with extended conjugations, with their absorption spectra tailing into the infrared region (> 700 nm). The polymers show good thermal stability with T _d up to ∼400°C and can be graphitized into iron-containing ceramics when pyrolyzed at high temperatures, with char yields up to ∼60 wt%. While calcinations of the Si-containing polymers (hb-1) at 1000°C under nitrogen give ceramics containing mostly α-Fe nanoparticles, those of Ge-(hb-2) and Sb-containing polymers (hb-4) are completely transformed into their iron-alloys. The ceramics from the P-containing polymers (hb-3) show diffraction patterns of iron phosphides. Iron silicide nanocrystals of "large" sizes are obtained when the pyrolysis of hb-1 is conducted at a high temperature of 1200°C under argon. This ceramic is highly magnetizable (M _s up to ∼51 emu/g) and shows near-zero remanence and coercivity; in other words, it is an outstanding soft ferromagnet with a high magnetic susceptibility and practically nil hysteresis loss.