3 Ethene–Norbornene Copolymers
Norbornene can be copolymerized with olefins such as ethene and propene. Among
these new cyclic olefin copolymers, made accessible from metallocenes [22, 28,
38–93], the ethene–norbornene (E–N) copolymers are the most versatile and
interesting ones.
E–N copolymers are usually amorphous and display a wide range of T g , from room
temperature to about 220
C. They are characterized by high chemical resistance as
well as good processability. They show excellent transparency and high refractive
index, owing to their high carbon/hydrogen ratio, e.g., the refractive index is 1.53 for a
50:50 E–N copolymer. These properties make them suitable for optical applications
such as coatings for high-capacity CDs and DVDs, for lenses, medical equipment,
blisters, toner binder, and packaging. The industrially produced copolymers have
norbornene contents between 30 and 60 mol% and T g values of 120–180
C. After
their first synthesis by Kaminsky [22, 28], E–N copolymers have been developed to
commercial products such as TOPAS (see http//www.topas.com: [40]) from Ticona,
while Mitsui produces APEL [41] by using vanadium-based catalysts.
The group 4 metallocene catalysts [22, 28] show much higher activity than
traditional heterogeneous TiCl 4 /AlEt 2 Cl or vanadium catalysts, and the fine-tuning
of ligand substituents allows the control of copolymer properties and their
structures, from random to alternating [23–25].
A variety of metallocene catalysts having C 1 , C 2 , C 2v , and C S symmetry were
studied for E–N copolymerization. Subsequently, homogeneous organometallic
catalysts, including half-sandwich and cyclopentadienyl (Cp)-free group 4 metal
catalysts, late transition metal catalysts, and more recently cationic rare earth metal
half-sandwich alkyls have been reported to catalyze E–N copolymerization (Fig. 3)
[24, 25, 42–57].
The resultant copolymer properties depend on different parameters, such as
comonomer content and distribution throughout the polymer chain, as well as the
configuration of the asymmetric carbons of the comonomer units. The microstructure
of the copolymer can be controlled by the appropriate choice of reaction conditions
and catalyst structure.
A description of the microstructure by
13
C NMR spectroscopy of these
copolymers, as well as a detailed understanding of the processes and mechanisms
involved in these copolymerizations, proved difficult to achieve. A number of groups
took on this challenge using various methodologies, which included synthesis of
model compounds, NMR pulse sequences, synthesis of series of copolymers with
different norbornene content and using catalysts of different symmetries, synthesis
of copolymers selectively
13
C-enriched, chemical shift prediction, and ab initio
chemical shift computations. Such assignments enabled detailed information to be
obtained on copolymerization mechanisms by Tritto et al. [24]. They employed a
computer optimization routine, which allows a best fit to be obtained for the
microstructural analysis by
13
C NMR spectra in order to derive the reactivity ratios
for both first- and second-order Markov models (M1 and M2, respectively).
Polyolefins with Cyclic Comonomers
123
Norbornene can be copolymerized with olefins such as ethene and propene. Among
these new cyclic olefin copolymers, made accessible from metallocenes [22, 28,
38–93], the ethene–norbornene (E–N) copolymers are the most versatile and
interesting ones.
E–N copolymers are usually amorphous and display a wide range of T g , from room
temperature to about 220
C. They are characterized by high chemical resistance as
well as good processability. They show excellent transparency and high refractive
index, owing to their high carbon/hydrogen ratio, e.g., the refractive index is 1.53 for a
50:50 E–N copolymer. These properties make them suitable for optical applications
such as coatings for high-capacity CDs and DVDs, for lenses, medical equipment,
blisters, toner binder, and packaging. The industrially produced copolymers have
norbornene contents between 30 and 60 mol% and T g values of 120–180
C. After
their first synthesis by Kaminsky [22, 28], E–N copolymers have been developed to
commercial products such as TOPAS (see http//www.topas.com: [40]) from Ticona,
while Mitsui produces APEL [41] by using vanadium-based catalysts.
The group 4 metallocene catalysts [22, 28] show much higher activity than
traditional heterogeneous TiCl 4 /AlEt 2 Cl or vanadium catalysts, and the fine-tuning
of ligand substituents allows the control of copolymer properties and their
structures, from random to alternating [23–25].
A variety of metallocene catalysts having C 1 , C 2 , C 2v , and C S symmetry were
studied for E–N copolymerization. Subsequently, homogeneous organometallic
catalysts, including half-sandwich and cyclopentadienyl (Cp)-free group 4 metal
catalysts, late transition metal catalysts, and more recently cationic rare earth metal
half-sandwich alkyls have been reported to catalyze E–N copolymerization (Fig. 3)
[24, 25, 42–57].
The resultant copolymer properties depend on different parameters, such as
comonomer content and distribution throughout the polymer chain, as well as the
configuration of the asymmetric carbons of the comonomer units. The microstructure
of the copolymer can be controlled by the appropriate choice of reaction conditions
and catalyst structure.
A description of the microstructure by
13
C NMR spectroscopy of these
copolymers, as well as a detailed understanding of the processes and mechanisms
involved in these copolymerizations, proved difficult to achieve. A number of groups
took on this challenge using various methodologies, which included synthesis of
model compounds, NMR pulse sequences, synthesis of series of copolymers with
different norbornene content and using catalysts of different symmetries, synthesis
of copolymers selectively
13
C-enriched, chemical shift prediction, and ab initio
chemical shift computations. Such assignments enabled detailed information to be
obtained on copolymerization mechanisms by Tritto et al. [24]. They employed a
computer optimization routine, which allows a best fit to be obtained for the
microstructural analysis by
13
C NMR spectra in order to derive the reactivity ratios
for both first- and second-order Markov models (M1 and M2, respectively).
Polyolefins with Cyclic Comonomers
123