Polymerization of α-olefins and Hydrogenolysis of the Resulting Polyolefins with Hafnium Hydrides Supported on Silica
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The reaction of Hf(CH2 tBu)4 with a silica surface treated at 800 °C affords the unique single site (≡SiO)Hf(CH2 tBu)3, 1, ((≡SiO) = silica surface ligand). Reaction of 1, with dihydrogen at temperatures (θ) ranging from 100 °C to 250 °C leads to different surface hydrides. At θ ≤ 100 °C, there is formation of [(≡SiO)Hf(CH2 tBu)(H)2], 2. For 100 ≤ θ ≤ 200 °C, 2 affords, [(≡SiO)2Hf(H)2], 3, and [(≡SiO)3SiH]. For 150 ≤ θ ≤ 250 °C, 3, is totally converted into [(≡SiO)3Hf(H)], 4, and [(≡SiO)2Si(H)2]. These different surface hafnium hydrides 2–4, which are obtained simultaneously but in various proportions were referred to as [Hf]θ–H, (θ = 75, 100, 125, 150 and 250 °C). [Hf]150–H is the most active catalyst in α olefin (ethylene, propene, isobutylene) polymerization at 25 °C and in polyolefin depolymerization at 150 °C under H2 atmosphere. Although, there is a clear correlation between the catalytic activity both in polymerization and in depolymerization, and the amount of 3 and 4, it is not possible to establish which between the mono and the bis-hydride is the most active.
KeywordsSupported hafnium hydride Silica Polymerization Depolymerization
We are grateful to the Rhône–Alpes Region for its financial support (G. T.), to CEZUS Company for HfCl4 sample, to Anne Baudouin for the solid state NMR, and to Dr Spitz for our fruitful discussion.
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