Abstract
The reaction of Hf(CH
2
t
Bu)
4
with a silica surface treated at 800 °C affords the unique single site (≡SiO)Hf(CH
2
t
Bu)
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(CH
2
t
Bu)(H)
2
],
2
. For 100 ≤ θ ≤ 200 °C,
2
affords, [(≡SiO)
2
Hf(H)
2
],
3
, and [(≡SiO)
3
SiH]. For 150 ≤ θ ≤ 250 °C,
3
, is totally converted into [(≡SiO)
3
Hf(H)],
4
, and [(≡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]
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 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.