Molecular motions in metal-containing polymers: Solid-state deuterium NMR studies of polyferrocenylsilanes near their glass transition temperature

The molecular motions in polyferrocenylsilanes, a novel class of metal-containing polymers with a main chain of alternating ferrocene and organosilane units, have been probed using variable temperature solid-state H-2 NMR. Polyferrocenylsilanes with selective deuteration of either the cyclopentadienyl ligands of the ferrocene units or the side groups attached to silicon were prepared by ringopening polymerization methodologies. The polymer backbone motions in the ferrocene-deuterated materials, poly(ferrocenyl-d(8)-dimethylsilane) [Fe(eta-C5D4)(2)Si(CH3)2](n) (2a, T-g = 33 degreesC), poly(ferrocenyl-d(8)-dimethoxysilane) [Fe(eta-C5D4)(2)Si(OCH3)(2)], (2b, T-g = 19 degreesC), and poly(ferrocenyl-d(8)-dihexyloxysilane) [Fe(eta-C5D4)(2)Si(OC6H13)(2)](n) (2c, T-g = -51 degreesC) were examined over a temperature range of -50 to +125 degreesC. The 2H NMR spectra of all three polymers indicated that the cyclopentadienyl groups were static at temperatures below the particular mechanical Tg. At temperatures just above T, the cyclopentadienyl rings began to undergo slow small-angle oscillations. These motions increased in both rate and angle of excursion until, at a temperature approximately 50-70 degreesC above the particular mechanical T, the polymers underwent a transition to a regime characterized by rapid isotropic motional averaging. The silicon side group motions were also examined over the same temperature range using the corresponding poly(ferrocenyldimethyl-d(3)-silane) [Fe(eta-C5H4)(2)Si(CH3)(CD3)(n). (2d, T-g = 33 degreesC) and poly(ferrocenyldimethoxyd-6-silane) [Fe(eta-C5H4)(2)Si(OCD3)(2)], (2e, T-g = 19 degreesC). For polymer 2d the H-2 NMR spectra showed the presence of fast methyl rotation about the Si-CD3 bond even at temperatures below the mechanical Tg. For polymer 2e, the H-2 NMR spectra showed the presence of fast methyl rotations about the O-CD3 bond combined with a discrete jumping motion about the Si-O bond. Increasing temperature brought about spectral changes reflecting a superposition of backbone and side-chain motions. At temperatures 50-70 degreesC above the respective mechanical Tg the 2H NMR spectra of both 2d and 2e were isotropically narrowed, reflecting the onset of isotropic motions of the polymer backbone.