Hyperbranched organometallic polymers: Synthesis and properties of poly(ferrocenylenesilyne)s

A series of hyperbranched poly(ferrocenylenesilyne)s, [(η⁵-CC₅H₄)₂FeSi(R)]_n {R = CH ₃ [1(1)], CH=CH₂ [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 which 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 readility film forming. The polymers exhibited diagnostic solution properties of hyperbranched macromolecules; for example, 1(18) had a high absolute molecular weight (M_w = 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.