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 2 H NMR. Polyferrocenylsilanes with selective deuteration of either the cyclopentadienyl ligands of the ferrocene units or the side groups attached to silicon were prepared by ring-opening polymerization methodologies. The polymer backbone motions in the ferrocene-deuterated materials, poly(ferrocenyl- d 8 -dimethylsilane) [Fe(η-C 5 D 4 ) 2 Si(CH 3 ) 2 ] n ( 2a, T g = 33 °C), poly(ferrocenyl- d 8 -dimethoxysilane) [Fe(η-C 5 D 4 ) 2 Si(OCH 3 ) 2 ] n ( 2b, T g = 19 °C), and poly(ferrocenyl- d 8 -dihexyloxysilane) [Fe(η-C 5 D 4 ) 2 Si(OC 6 H 13 ) 2 ] n ( 2c, T g = −51 °C) were examined over a temperature range of −50 to +125 °C. The 2 H NMR spectra of all three polymers indicated that the cyclopentadienyl groups were static at temperatures below the particular mechanical T g . At temperatures just above T g 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 °C above the particular mechanical T g, 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(η-C 5 H 4 ) 2 Si(CH 3 )(CD 3 )] n ( 2d, T g = 33 °C) and poly(ferrocenyldimethoxy- d 6 -silane) [Fe(η-C 5 H 4 ) 2 Si(OCD 3 ) 2 ] n ( 2e, T g = 19 °C). For polymer 2d the 2 H NMR spectra showed the presence of fast methyl rotation about the Si−CD 3 bond even at temperatures below the mechanical T g . For polymer 2e, the 2 H NMR spectra showed the presence of fast methyl rotations about the O−CD 3 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 °C above the respective mechanical T g the 2 H NMR spectra of both 2d and 2e were isotropically narrowed, reflecting the onset of isotropic motions of the polymer backbone.

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