Polymerization of α-olefins and Hydrogenolysis of the Resulting Polyolefins with Hafnium Hydrides Supported on Silica

The reaction of Hf(CH2 tBu)(4) with a silica surface treated at 800 A degrees C affords the unique single site (a parts per thousand SiO)Hf(CH2 tBu)(3,) 1, ((a parts per thousand SiO) = silica surface ligand). Reaction of 1, with dihydrogen at temperatures (theta) ranging from 100 A degrees C to 250 A degrees C leads to different surface hydrides. At theta A a parts per thousand currency signA 100 A degrees C, there is formation of [(a parts per thousand SiO)Hf(CH2 tBu)(H)(2)], 2. For 100 a parts per thousand currency sign theta a parts per thousand currency sign 200 A degrees C, 2 affords, [(a parts per thousand SiO)(2)Hf(H)(2)], 3, and [(a parts per thousand SiO)(3)SiH]. For 150 a parts per thousand currency sign theta a parts per thousand currency sign 250 A degrees C, 3, is totally converted into [(a parts per thousand SiO)(3)Hf(H)], 4, and [(a parts per thousand SiO)(2)Si(H)(2)]. These different surface hafnium hydrides 2-4, which are obtained simultaneously but in various proportions were referred to as [Hf](theta)-H, (theta A = 75, 100, 125, 150 and 250 A degrees C). [Hf](150)-H is the most active catalyst in alpha olefin (ethylene, propene, isobutylene) polymerization at 25 A degrees C and in polyolefin depolymerization at 150 A degrees C under H-2 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.