Ring-Opening Copolymerization of Cyclotetrasilanes and Silicon-Bridged [1]Ferrocenophanes: Synthesis and Properties of Polysilane−Poly(ferrocenylsilane) Random Copolymers

Abstract

Novel poly(methylphenylsilane)−poly(ferrocenyldimethylsilane) copolymers 5a − d of varying monomer composition were prepared via the thermal ring-opening polymerization of a mixture of the strained cyclic tetrasilane [MePhSi]₄ (1) and the silicon-bridged [1]ferrocenophane Fe(η-C₅H₄)₂SiMe₂ (3). The resulting materials were structurally characterized by ¹H and ²⁹Si NMR and also by gel permeation chromatography (GPC), pyrolysis mass spectrometry (MS), and cyclic voltammetry. GPC in THF indicated that the molecular weights of the polymers 5a−d were in the range M_n = (2.0 × 10⁴)−(8.9 × 10⁴) with PDI values of 2.4−3.5. Polymers 5a − c were photosensitive and GPC traces of 5a − c were studied both before and after irradiation of the sample with UV light (λ = 340 nm). The formation of short chain oligo(ferrocenylsilanes) after irradiation was consistent with exclusive photodegradation of the polysilane segments and indicated that the copolymers were random in nature. Pyrolysis MS of 5a − c also supported a random structure. Cyclic voltammetric studies of 5a − c in CH₂Cl₂ showed the presence of the characteristic two reversible oxidation waves arising from oligo(ferrocenylsilane) segments with interacting iron atoms at E_1/2 = 0.00 and 0.23 V (relative to ferrocene/ferrocenium) and an irreversible oxidation with E_p(ox) = 0.39 V arising from the polysilane segments. UV/vis spectroscopy showed that the σ-electrons in the oligosilane segments are delocalized and that λ_max increased from 325 to 333 nm as the proportion of the cyclotetrasilane 1 in the initial monomer mixture increased. These values suggest that the oligosilane segments are relatively short and do not approach the limit corresponding to ca. 30 silicon atoms (M_n = ca. 3 000), which levels off at 338 nm. Attempted anionic initiation of the copolymerization of 1 and 3 was unsuccessful and led exclusively to homopolymerization. Transition metal catalyzed copolymerization of 1 and 3 using PtCl₂ resulted in a copolymer which was derived almost exclusively from 3. The charge transport properties of the representative copolymer 5d were also investigated. Films of pristine 5d were insulating (conductivity -14 S cm^-1) but after exposure to iodine the conductivity increased by a factor of 10⁸ to (6.5−8.2) × 10^-6 S cm^-1. In comparison, the conductivities of iodine-exposed films of poly(ferrocenyldimethylsilane) (4) and poly(ferrocenyldi-n-butylsilane) (7) were ca. 2 × 10^-4 S cm^-1. The hole mobility of 5d was studied by standard time of flight techniques and was found to be appreciable with a value of 4.0 × 10^-6 cm²/V s.