Highly alternating stereoregular (isotactic) EÀN copolymers were synthesized
with the C 1 -symmetric, bridged metallocenes II 2-4, II-6 [61, 71] in the presence of
an excess of norbornene. The copolymers obtained are crystalline if the norbornene
content is higher than 37 mol%, and have melting points of 270–320
C.
Metallocene I-5 gave a mainly alternating isotactic copolymer with trace
amounts of ENNE sequences and a surprising significant amount of norbornene
(up to ~10%) belonging to NNN triads [74]. Isotactic alternating copolymers with
norbornene content above 37 mol% are semicrystalline. They feature T g values of
100–130
C and T m values of 270–320
C. They are still transparent owing to the
small size of their crystalline regions (5 nm). Atactic alternating copolymers are
amorphous with T g values up to about 130
C.
Constrained geometry catalysts also copolymerize ethene and norbornene
with lower activity than ansa-zirconocenes, but can give perfectly alternating
copolymers [45]. C 1 -symmetric, bridged monocyclopentadienyl titanium amido
complexes Me 2 Si(Cp
0 )(N
t
Bu)TiCl 2 (where Cp
0
¼ 2,4-Me 2 Cp, 3t
BuCp, indenyl)
(IV 2-4) have also been shown to yield mainly alternating EÀN copolymers by
Waymouth and coworkers [46].
Random E–N copolymers with high molar masses and high norbornene content
were produced by Shiono [53] by using the half-sandwich titanocene precatalyst
Me 2 Si(Flu)(N
t
Bu)TiMe 2 (IV-5) activated with MAO free from Me 3 Al. The complex
produces E–N copolymers with high molecular weights (up to 63,000 g/mol), narrow
molecular weight distributions (M w /M n ¼ 1.32) and an incorporation of norbornene
up to 58 mol%.
Nomura [56] showed that non-bridged half-titanocenes containing anionic donor
ligand of the type, Cp
0 TiX 2 (Y) (where Cp
0
¼ cyclopentadienyl group; X ¼ halogen,
alkyl; Y ¼ anionic ancillary donor ligands such as aryloxo, ketimide etc.) displayed
unique characteristics such as efficient incorporation of bulky olefins. Random
EÀN copolymers with high norbornene contents could be obtained with (Ind)
TiCl 2 (O-2,6-iPr 2 C 6 H 3 ) (V-1) that exhibited higher norbornene incorporation than
the Cp*-aryloxo analogue (V-3), the 1,2,4-Me 3 C 5 H 2 -aryloxo analogue (V-4), and
the constrained geometry catalyst (CGC, IV-1).
Although living α-olefin polymerizations with transition metal catalysts are still
quite rare despite the modern efficient examples recently developed (for a recent
review, see [69]), some EÀN copolymerizations have been shown to be quasi-living
or living. The molecular mass of E–N copolymers by catalyst I-1, at temperatures
between 30 and 50
C and high norbornene feed fractions, increases with time for up
to 1 h. The polydispersity can be as narrow as 1.1 at [N]/[E] ¼ 28 [49], indicating
that EÀN copolymerizations are quasi-living under these conditions.
Fujita [55, 57, 76–78] and coworkers developed bis(pyrrolide-imine)Ti
complexes (named PI catalysts), which give highly alternating E–N copolymers.
These group 4 transition metal [57, 76–79] catalysts with electronically flexible
nonsymmetric [N^N] chelate ligands displayed a marked tendency to produce E–N
copolymers with a stereoirregular structure despite the C 2 -symmetric nature of the
catalysts. The E–N copolymerization with PI catalysts is living [55].
126
L. Boggioni and I. Tritto
with the C 1 -symmetric, bridged metallocenes II 2-4, II-6 [61, 71] in the presence of
an excess of norbornene. The copolymers obtained are crystalline if the norbornene
content is higher than 37 mol%, and have melting points of 270–320
C.
Metallocene I-5 gave a mainly alternating isotactic copolymer with trace
amounts of ENNE sequences and a surprising significant amount of norbornene
(up to ~10%) belonging to NNN triads [74]. Isotactic alternating copolymers with
norbornene content above 37 mol% are semicrystalline. They feature T g values of
100–130
C and T m values of 270–320
C. They are still transparent owing to the
small size of their crystalline regions (5 nm). Atactic alternating copolymers are
amorphous with T g values up to about 130
C.
Constrained geometry catalysts also copolymerize ethene and norbornene
with lower activity than ansa-zirconocenes, but can give perfectly alternating
copolymers [45]. C 1 -symmetric, bridged monocyclopentadienyl titanium amido
complexes Me 2 Si(Cp
0 )(N
t
Bu)TiCl 2 (where Cp
0
¼ 2,4-Me 2 Cp, 3t
BuCp, indenyl)
(IV 2-4) have also been shown to yield mainly alternating EÀN copolymers by
Waymouth and coworkers [46].
Random E–N copolymers with high molar masses and high norbornene content
were produced by Shiono [53] by using the half-sandwich titanocene precatalyst
Me 2 Si(Flu)(N
t
Bu)TiMe 2 (IV-5) activated with MAO free from Me 3 Al. The complex
produces E–N copolymers with high molecular weights (up to 63,000 g/mol), narrow
molecular weight distributions (M w /M n ¼ 1.32) and an incorporation of norbornene
up to 58 mol%.
Nomura [56] showed that non-bridged half-titanocenes containing anionic donor
ligand of the type, Cp
0 TiX 2 (Y) (where Cp
0
¼ cyclopentadienyl group; X ¼ halogen,
alkyl; Y ¼ anionic ancillary donor ligands such as aryloxo, ketimide etc.) displayed
unique characteristics such as efficient incorporation of bulky olefins. Random
EÀN copolymers with high norbornene contents could be obtained with (Ind)
TiCl 2 (O-2,6-iPr 2 C 6 H 3 ) (V-1) that exhibited higher norbornene incorporation than
the Cp*-aryloxo analogue (V-3), the 1,2,4-Me 3 C 5 H 2 -aryloxo analogue (V-4), and
the constrained geometry catalyst (CGC, IV-1).
Although living α-olefin polymerizations with transition metal catalysts are still
quite rare despite the modern efficient examples recently developed (for a recent
review, see [69]), some EÀN copolymerizations have been shown to be quasi-living
or living. The molecular mass of E–N copolymers by catalyst I-1, at temperatures
between 30 and 50
C and high norbornene feed fractions, increases with time for up
to 1 h. The polydispersity can be as narrow as 1.1 at [N]/[E] ¼ 28 [49], indicating
that EÀN copolymerizations are quasi-living under these conditions.
Fujita [55, 57, 76–78] and coworkers developed bis(pyrrolide-imine)Ti
complexes (named PI catalysts), which give highly alternating E–N copolymers.
These group 4 transition metal [57, 76–79] catalysts with electronically flexible
nonsymmetric [N^N] chelate ligands displayed a marked tendency to produce E–N
copolymers with a stereoirregular structure despite the C 2 -symmetric nature of the
catalysts. The E–N copolymerization with PI catalysts is living [55].
126
L. Boggioni and I. Tritto