the comonomer response with increasing polymerization temperature. Furthermore,
the substituent-free ligands do not promote any stereocontrol for the 1-olefin
coordination, and the approaching 1-olefin can coordinate in regio- or stereoirregular fashion. The resulting misinsertions inhibit chain growth and decrease
the molecular weight drastically. Polypropylene produced with this catalyst is
atactic and has a low molecular weight [30]. Pentamethyl substitution (2) of
cyclopentadienyl significantly reduces comonomer response whereas an alkyl
[e.g., methyl, ethyl, propyl or n-butyl (3)] substitution of the Cp ring reduces
comonomer response only slightly compared to the unsubstituted 1. These changes
can be attributed to the increase in steric bulk around the active center [31].
The introduction of an interannular bridge between the ligands and the substitution of the cyclopentadienyl ligand with a C 2 -symmetric ligand eliminates the free
rotation and promotes stereocontrol for 1-olefin insertion. The very first synthesized
metallocenes belonging to this category were rac-Et[Ind] 2 ZrCl 2 (4) and its mesoisomer 5. Hydrogenation of indenyl ligand yields rac-Et[H 4 Ind] 2 ZrCl 2 (6), which
was the first metallocene catalyst shown to produce isotactic polypropylene
[39, 40]. The comonomer response of this catalyst is also clearly higher than the
response of 1 [33, 41].
Table 1 Ethylene and 1-olefin reactivity ratios (r E ) for selected metallocenes in ethylene/1-olefin
copolymerization
Metallocene
r E (80
C)
r E (40
C)
C 2 /C 6
C 2 /C 6
C 2 /C 16
C 2 /C 6
C 2 /C 8
1
114
112 Æ 9
–
–
32.8 (0.05)
2
1,012
–
–
–
–
3
–
–
160 Æ 13
–
–
4
–
4 8Æ 4 (<0.02)
51 Æ 7
–
–
5
–
–
–
–
–
6
–
7 1Æ 4 (<0.03)
–
–
–
7
–
–
–
–
–
8
–
6Æ 1
–
–
–
9
–
2 6Æ 4
–
–
18.9 (0.014)
10
–
–
–
–
–
11
–
–
–
–
4.1 (0.29)
12
–
–
–
–
19.5 (0.013)
13
–
–
–
–
10.1 (0.118)
14
–
–
–
–
–
15
–
5 5Æ 3 (0.005)
63 Æ 2 (0.002)
36 Æ 3 (0.003)
–
16
–
3 0Æ 5 (0.004)
–
14 Æ 4 (0.004)
–
17
–
1 9Æ 4 (0.006)
25 Æ 1
1 6Æ 1 (0.005)
–
18
–
–
–
1 1Æ 1 (0.005)
–
19
–
–
–
1 0Æ 1 (0.001)
–
References
[31]
[51, 66, 88]
[ 66, 74]
[ 66, 74]
[ 36]
Values in parentheses (if shown) represent comonomer reactivity (r C )
186
J. Seppa ¨la ¨ et al.
the substituent-free ligands do not promote any stereocontrol for the 1-olefin
coordination, and the approaching 1-olefin can coordinate in regio- or stereoirregular fashion. The resulting misinsertions inhibit chain growth and decrease
the molecular weight drastically. Polypropylene produced with this catalyst is
atactic and has a low molecular weight [30]. Pentamethyl substitution (2) of
cyclopentadienyl significantly reduces comonomer response whereas an alkyl
[e.g., methyl, ethyl, propyl or n-butyl (3)] substitution of the Cp ring reduces
comonomer response only slightly compared to the unsubstituted 1. These changes
can be attributed to the increase in steric bulk around the active center [31].
The introduction of an interannular bridge between the ligands and the substitution of the cyclopentadienyl ligand with a C 2 -symmetric ligand eliminates the free
rotation and promotes stereocontrol for 1-olefin insertion. The very first synthesized
metallocenes belonging to this category were rac-Et[Ind] 2 ZrCl 2 (4) and its mesoisomer 5. Hydrogenation of indenyl ligand yields rac-Et[H 4 Ind] 2 ZrCl 2 (6), which
was the first metallocene catalyst shown to produce isotactic polypropylene
[39, 40]. The comonomer response of this catalyst is also clearly higher than the
response of 1 [33, 41].
Table 1 Ethylene and 1-olefin reactivity ratios (r E ) for selected metallocenes in ethylene/1-olefin
copolymerization
Metallocene
r E (80
C)
r E (40
C)
C 2 /C 6
C 2 /C 6
C 2 /C 16
C 2 /C 6
C 2 /C 8
1
114
112 Æ 9
–
–
32.8 (0.05)
2
1,012
–
–
–
–
3
–
–
160 Æ 13
–
–
4
–
4 8Æ 4 (<0.02)
51 Æ 7
–
–
5
–
–
–
–
–
6
–
7 1Æ 4 (<0.03)
–
–
–
7
–
–
–
–
–
8
–
6Æ 1
–
–
–
9
–
2 6Æ 4
–
–
18.9 (0.014)
10
–
–
–
–
–
11
–
–
–
–
4.1 (0.29)
12
–
–
–
–
19.5 (0.013)
13
–
–
–
–
10.1 (0.118)
14
–
–
–
–
–
15
–
5 5Æ 3 (0.005)
63 Æ 2 (0.002)
36 Æ 3 (0.003)
–
16
–
3 0Æ 5 (0.004)
–
14 Æ 4 (0.004)
–
17
–
1 9Æ 4 (0.006)
25 Æ 1
1 6Æ 1 (0.005)
–
18
–
–
–
1 1Æ 1 (0.005)
–
19
–
–
–
1 0Æ 1 (0.001)
–
References
[31]
[51, 66, 88]
[ 66, 74]
[ 66, 74]
[ 36]
Values in parentheses (if shown) represent comonomer reactivity (r C )
186
J. Seppa ¨la ¨ et al.