Hyperbranched Organometallic Polymers:  Synthesis and Properties of Poly(ferrocenylenesilyne)s

A series of hyperbranched poly(ferrocenylenesilyne)s, [(η5-C5H4)2FeSi(R)]n {R = CH3 [1(1)], CHCH2 [1(V)], n-C8H17 [1(8)], n-C12H25 [1(12)], n-C16H33 [1(16)], n-C18H37 [1(18)]}, was prepared in good isolation yields (up to 77 wt %) by one-pot coupling reactions of dilithioferrocene with trichlorosilanes. While the polymers with small R groups [1(1) and 1(V)] were partially soluble, those with long alkyl chains [1(m) with m ≥ 8] were completely soluble and readily film forming. The polymers exhibited diagnostic solution properties of hyperbranched macromolecules; for example, 1(18) had a high absolute molecular weight (Mw = 5 × 105 Da) but a low intrinsic viscosity ([η] = 0.02 dL/g). Spectroscopic analyses revealed that the polymers possessed rigid skeleton structures with extended conjugations, with their absorption spectra tailed into the infrared region (>700 nm). With an increase in the alkyl chain length, the polymer changed from glassy to rubbery state. The polymers lost little of their weights when heated to ∼400 °C but ceramized when pyrolyzed at higher temperatures, with ceramization yield increasing with a decrease in the alkyl chain length. Sintering 1(1) and 1(V) in 700−1200 °C produced ceramics in ∼50% yields. Higher temperature pyrolyses favored the formation of ceramics with bigger inorganic nanoclusters and better magnetic performances. The ceramic prepared from the calcination of 1(1) at 1200 °C contained large iron silicide nanocrystals and exhibited high magnetizability (up to ∼51 emu/g) but near-zero remanence and coercivity. This ceramic is thus an outstanding soft ferromagnet with a high magnetic susceptibility and practically nil hysteresis loss.