ratio for the transformation of model 3f was much higher than that of model 9f,
which can be rationalized by assuming that reduction of the hexavalent chromate
model 3f into a lower valence state should consume more MAO. It was noteworthy
that only a small amount of PE but no liquid oligomer was produced in the case of
TiBA as cocatalyst for both model catalysts. Larger amounts of TiBA only
completely deactivated the two model catalysts, probably due to its much stronger
reducing power than MAO.
The transformation from ethylene polymerization to ethylene nonselective oligomerization over the two model catalysts (3f and 9f) in the presence of Al-alkyl
cocatalyst MAO with the increase in Al/Cr molar ratio is shown in Scheme 14. Such
interesting transformation phenomenon could not be found using the same catalysts
combined with TiBA. Similar polymerization/oligomerization transformation
behavior has also been reported recently on Cr-based ethylene trimerization
Table 3 Distributions of ethylene oligomerization products over BC (3f) and [(Ph 3 SiO)Cr ·
(THF)] 2 (μ-OSiPh 3 ) 2 (9f) with Al-alkyl cocatalysts
a
Entry
b Cocatalyst Al/Cr Oligomer
c (g)
Oligomer distribution
d (%)
Vinyl
e (mol%)
C 6
C 8
C 10 C 12 C 14 C 16
3
f
MAO
500
2.4
12.5 25.7 26.8 18.3 9.5 4.3 90.3
4
f
MAO
1,000 5.0
9.5 19.2 18.4 25.3 14.6 7.3 81.6
5
f
MAO
1,500 2.7
10.5 23.5 11.4 26.2 15.1 8.0 84.6
11
MAO
200
4.8
13.5 16.1 21.8 21.5 14.9 7.2 92.6
12
MAO
500
5.2
10.1 35.6 11.5 18.9 11.1 6.2 84.2
13
MAO
1,000 5.8
12.3 31.6 16.1 11.0 9.1 9.0 89.1
a
Standard conditions: T ¼ 22
C, V ¼ 10 mL, P ¼ 20 atm., catalyst ¼ 10 mg, time ¼ 30 min
b
Entries 1–8 for BC catalyst, entries 9–16 for [(Ph 3 SiO)Cr · (THF)] 2 (μ-OSiPh 3 ) 2 catalyst
c
By integration of the NMR olefinic resonances with respect to the Me of the toluene solvent
d
By GC-MS, values of C 4 are not given due to volatility, remainder is C 4 and C 18+
e
By integration of the NMR olefinic resonances
f
Bimodal distribution from GPC analyses
Scheme 14 Transformation of ethylene polymerization to ethylene nonselective oligomerization
over BC (3f, upper) and [(Ph 3 SiO)Cr · (THF)] 2 (μ-OSiPh 3 ) 2 (9f, lower) complexes
174
R. Cheng et al.
which can be rationalized by assuming that reduction of the hexavalent chromate
model 3f into a lower valence state should consume more MAO. It was noteworthy
that only a small amount of PE but no liquid oligomer was produced in the case of
TiBA as cocatalyst for both model catalysts. Larger amounts of TiBA only
completely deactivated the two model catalysts, probably due to its much stronger
reducing power than MAO.
The transformation from ethylene polymerization to ethylene nonselective oligomerization over the two model catalysts (3f and 9f) in the presence of Al-alkyl
cocatalyst MAO with the increase in Al/Cr molar ratio is shown in Scheme 14. Such
interesting transformation phenomenon could not be found using the same catalysts
combined with TiBA. Similar polymerization/oligomerization transformation
behavior has also been reported recently on Cr-based ethylene trimerization
Table 3 Distributions of ethylene oligomerization products over BC (3f) and [(Ph 3 SiO)Cr ·
(THF)] 2 (μ-OSiPh 3 ) 2 (9f) with Al-alkyl cocatalysts
a
Entry
b Cocatalyst Al/Cr Oligomer
c (g)
Oligomer distribution
d (%)
Vinyl
e (mol%)
C 6
C 8
C 10 C 12 C 14 C 16
3
f
MAO
500
2.4
12.5 25.7 26.8 18.3 9.5 4.3 90.3
4
f
MAO
1,000 5.0
9.5 19.2 18.4 25.3 14.6 7.3 81.6
5
f
MAO
1,500 2.7
10.5 23.5 11.4 26.2 15.1 8.0 84.6
11
MAO
200
4.8
13.5 16.1 21.8 21.5 14.9 7.2 92.6
12
MAO
500
5.2
10.1 35.6 11.5 18.9 11.1 6.2 84.2
13
MAO
1,000 5.8
12.3 31.6 16.1 11.0 9.1 9.0 89.1
a
Standard conditions: T ¼ 22
C, V ¼ 10 mL, P ¼ 20 atm., catalyst ¼ 10 mg, time ¼ 30 min
b
Entries 1–8 for BC catalyst, entries 9–16 for [(Ph 3 SiO)Cr · (THF)] 2 (μ-OSiPh 3 ) 2 catalyst
c
By integration of the NMR olefinic resonances with respect to the Me of the toluene solvent
d
By GC-MS, values of C 4 are not given due to volatility, remainder is C 4 and C 18+
e
By integration of the NMR olefinic resonances
f
Bimodal distribution from GPC analyses
Scheme 14 Transformation of ethylene polymerization to ethylene nonselective oligomerization
over BC (3f, upper) and [(Ph 3 SiO)Cr · (THF)] 2 (μ-OSiPh 3 ) 2 (9f, lower) complexes
174
R. Cheng et al.