4 PropeneÀNorbornene Copolymers
PropeneÀnorbornene (PÀN) copolymers were expected to feature higher T g values
than EÀN copolymers with the same norbornene content and molar mass since
polypropene has a higher T g value than polyethene [66–68]. Moreover, differences
in stereo- and regioregularity of propene units as well as in the comonomer
distribution and the stereoregularity of the bicyclic units were expected to allow
fine tuning of copolymer microstructure and properties. However, compared to
EÀN copolymers, reports regarding PÀN copolymers are very limited [94–100].
The first report on synthesis of amorphous PÀN copolymers by Arnold with
I-2/MAO was shown to yield low polymerization activity but very high norbornene
content (up to 98 mol%) [94]. Then, Tritto and colleagues [95, 96] tackled
the synthesis and microstructural studies of PÀN copolymers with C 2 - and
C s -symmetric metallocenes and MAO as cocatalyst. Two ansa-metallocenes of
C 2 -symmetry, rac-[Et(Ind) 2 ]ZrCl 2 (I-1) and rac-[Me 2 Si(Ind) 2 ]ZrCl 2 (I-2), effective
for the synthesis of prevailingly isotactic and regioregular polypropene as well as
EÀN copolymers with a tendency to alternate, were selected [67]. II-1, which
yields prevailingly syndiotactic polypropene and is very active in EÀN copolymerization, was selected as a metallocene of C s symmetry. The polymerization activities
of I-1 and I-2 were found to be low compared to those obtained for EÀN copolymerization. Under similar polymerization conditions, II-1 allows for a lower
norbornene incorporation than catalysts I-1 and I-2. The M w values as well as T g
values of PÀN copolymers are lower than those of EÀN copolymers.
The low activity was demonstrated to result from the difficulty of inserting a
propene into the MtÀtertiary carbon bond formed after the norbornene insertion
(Mt-N), which is even more sterically crowded than the sites formed after a propene
(2,1) regioirregular insertion, less reactive than sites with a primary growing
polypropene chain. However, at low norbornene/olefin ratio it is possible to obtain
PÀN copolymers that are relatively richer in norbornene than the EÀN copolymers
prepared in similar conditions. At higher norbornene/olefin feed ratios, the great
amount of 1,3 propene misinsertions clearly revealed that the steric hindrance of the
MtÀtertiary carbon bond, when norbornene is the last inserted unit, makes the next
propene insertion difficult, causing low polymerization activities, molecular
masses, and T g .
Kaminsky and colleagues studied PÀN copolymerization with the C 2 -symmetric
I-2, two C s -symmetric [Me 2 C(Cp)(Flu)]ZrCl 2 /MAO (II-5) and [Ph 2 C(Cp)
(2,7-di
t
BuFlu)]ZrCl 2 (II-7) systems and with the constrained-geometry catalyst
IV-1. Copolymers and oligomers with a wide range of T g were produced with
satisfying activities, whereas the activities for IV-1 were much lower. These studies
confirmed that the high reactivity of the cyclic monomer makes accessible PÀN
copolymers with higher norbornene incorporation, but with lower molar masses
than EÀN copolymers [98, 101–103].
Shiono and colleagues succeeded in the synthesis of P–N copolymers with high
norbornene content up to 71 mol% with catalyst (
t
BuNSiMe 2 Flu)TiMe 2 activated by
Me 3 Al-free methylaluminoxane (dried MAO) [100]. They had previously reported
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L. Boggioni and I. Tritto
PropeneÀnorbornene (PÀN) copolymers were expected to feature higher T g values
than EÀN copolymers with the same norbornene content and molar mass since
polypropene has a higher T g value than polyethene [66–68]. Moreover, differences
in stereo- and regioregularity of propene units as well as in the comonomer
distribution and the stereoregularity of the bicyclic units were expected to allow
fine tuning of copolymer microstructure and properties. However, compared to
EÀN copolymers, reports regarding PÀN copolymers are very limited [94–100].
The first report on synthesis of amorphous PÀN copolymers by Arnold with
I-2/MAO was shown to yield low polymerization activity but very high norbornene
content (up to 98 mol%) [94]. Then, Tritto and colleagues [95, 96] tackled
the synthesis and microstructural studies of PÀN copolymers with C 2 - and
C s -symmetric metallocenes and MAO as cocatalyst. Two ansa-metallocenes of
C 2 -symmetry, rac-[Et(Ind) 2 ]ZrCl 2 (I-1) and rac-[Me 2 Si(Ind) 2 ]ZrCl 2 (I-2), effective
for the synthesis of prevailingly isotactic and regioregular polypropene as well as
EÀN copolymers with a tendency to alternate, were selected [67]. II-1, which
yields prevailingly syndiotactic polypropene and is very active in EÀN copolymerization, was selected as a metallocene of C s symmetry. The polymerization activities
of I-1 and I-2 were found to be low compared to those obtained for EÀN copolymerization. Under similar polymerization conditions, II-1 allows for a lower
norbornene incorporation than catalysts I-1 and I-2. The M w values as well as T g
values of PÀN copolymers are lower than those of EÀN copolymers.
The low activity was demonstrated to result from the difficulty of inserting a
propene into the MtÀtertiary carbon bond formed after the norbornene insertion
(Mt-N), which is even more sterically crowded than the sites formed after a propene
(2,1) regioirregular insertion, less reactive than sites with a primary growing
polypropene chain. However, at low norbornene/olefin ratio it is possible to obtain
PÀN copolymers that are relatively richer in norbornene than the EÀN copolymers
prepared in similar conditions. At higher norbornene/olefin feed ratios, the great
amount of 1,3 propene misinsertions clearly revealed that the steric hindrance of the
MtÀtertiary carbon bond, when norbornene is the last inserted unit, makes the next
propene insertion difficult, causing low polymerization activities, molecular
masses, and T g .
Kaminsky and colleagues studied PÀN copolymerization with the C 2 -symmetric
I-2, two C s -symmetric [Me 2 C(Cp)(Flu)]ZrCl 2 /MAO (II-5) and [Ph 2 C(Cp)
(2,7-di
t
BuFlu)]ZrCl 2 (II-7) systems and with the constrained-geometry catalyst
IV-1. Copolymers and oligomers with a wide range of T g were produced with
satisfying activities, whereas the activities for IV-1 were much lower. These studies
confirmed that the high reactivity of the cyclic monomer makes accessible PÀN
copolymers with higher norbornene incorporation, but with lower molar masses
than EÀN copolymers [98, 101–103].
Shiono and colleagues succeeded in the synthesis of P–N copolymers with high
norbornene content up to 71 mol% with catalyst (
t
BuNSiMe 2 Flu)TiMe 2 activated by
Me 3 Al-free methylaluminoxane (dried MAO) [100]. They had previously reported
128
L. Boggioni and I. Tritto