Polymerization of α-olefins and hydrogenolysis of the resulting polyolefins with hafnium hydrides supported on silica

The reaction of Hf(CH₂tBu)₄ with a silica surface treated at 800 °C affords the unique single site (≡SiO) Hf(CH ₂tBu)₃,1, ((≡SiO) = silica surface ligand). Reaction of 1,with dihydrogen at temperatures (h) ranging from100 °C to 250 °C leads to different surface hydrides. At h B 100 °C, there is formation of [(≡SiO)Hf(CH₂tBu)(H)₂], 2. For 100 B h B 200 °C, 2 affords, [(≡SiO)₂ Hf(H)₂], 3, and [(≡SiO)₃SiH]. For 150 ≤ h ≤ 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, (h = 75, 100, 125, 150 and 250 °C). [Hf]₁₅₀-His 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.