Abstracts Chemistry

Chromium atom routes to 2-vinylpyridine polymers and composites

by Laura. Deakin

The reaction between atomic chromium and neat 2-vinylpyridine leads to the polymerization of the monomer, forming a polymer glass. The product of the rapid polymerization of neat monomer is atactic poly(2-vinylpyridine) in which the chromium has been incorporated as a complex. From cyclic voltammetry and electronic absorption studies, it was proposed that the chromium forms a neutral complex with a severely distorted octahedral geometry and consists of a formally Cr2+ metal center. The metal would then form an organometallic complex that is also bound via the nitrogen atom to pendant pyridine rings. The polymerization of 2-vinylpyridine in 2-methyltetrahydrofuran at -78°C after reaction with chromium atoms leads to the formation of isotactic poly(2-vinylpyridine) with 50% mmm isotactic content from the 13C NMR spectrum. A coordination polymerization mechanism at a chromium metal center is proposed to occur during this reaction. Semicrystalline poly(2-vinylpyridine) was formed by annealing the isotactic polymer. The highest melting temperature reported for this polymer by differential scanning calorimetry was that observed with our sample at 217°C. The atactic polymer from the rapid polymerization of neat monomer displayed a glass transition at 95°C at a heating rate of 20°C/minute, but did not melt. The morphology of the crystalline portions of the isotactic polymer after annealing consisted of spherulites displaying both "Maltese Cross" and banding under cross polarizers. Bis(eta6-1-methylnaphthalene)chromium was synthesized by the metal atom route. This complex was reacted with 2-vinylpyridine producing an atactic polymer glass from the resulting rapid polymerization of the monomer. This rapid polymerization was not inhibited by CO2 (g), but was quenched upon addition of hydroquinone. These results support a radical propagation mechanism that forms the polymer glass. A cryogenic vacuum station was designed and constructed for the low-temperature (15 K) electron paramagnetic resonance characterization of the reaction products from the cocondensation of atomic vanadium with 2-vinylpyridine. The cocondensation of benzene with vanadium atoms produced bis(eta6-benzene)vanadium that displayed the characteristic axial spectrum. No signal was observed for reactions between vanadium atoms and nitrogen heterocycles including 2,6-dimethylpyridine and 2-vinylpyridine. The incorporation of atomic iron and cobalt into 2-vinylpyridine following the addition of atomic chromium leads to a rapid polymerization reaction forming a polymer composite consisting of magnetic metal particles embedded in poly(2-vinylpyridine). The particle sizes were measured by transmission electron microscopy to be 1.5 – 22 nm in diameter. Magnetometry studies indicated the additional presence of smaller particles not identified by microscopy. The observation of a ferromagnetic resonance spectrum from the cobalt sample at X-band frequency is consistent with the cobalt particles having undergone an allotropic phase transition to the…