that this metallocene when activated with dried MAO yields living propene and
norbornene homopolymerization [99]. PÀN copolymerizations were carried at 20
C
under atmospheric pressure of propene. Dimethylmetallocene IV-2 was activated by
dried MAO, modified methylaluminoxane (MMAO), and Ph 3 CB(C 6 F 5 ) 4 /Oct 3 Al. The
system activated by Ph 3 CB(C 6 F 5 ) 4 /Oct 3 Al was the most active and yielded the
copolymer with the lowest molecular weight and broadest molecular weight distribution. Dried MAO yielded the copolymer with the narrowest polydispersity. The
norbornene content in the copolymer was almost proportional to the [N]/[P] feed
ratio and the T g of the P–N copolymers increased linearly against the norbornene
content in the copolymers, from 53
C to 249
C.
The influence of propene pressure and temperature on activity, norbornene
content, M w , and T g of P–N copolymers by I-1 was assessed [101]. A decrease in
norbornene content, M w , and T g was observed at high temperatures and pressures.
The great number of 1,3 propene insertions found, especially at high temperature and
pressure, occurring after an inserted norbornene unit confirmed that the difficulty in
inserting a propene after norbornene is the limiting step in P–N copolymerization and
that chain transfer reactions are likely to occur more often at a propene-last insertedMt bond. This originates the lower M w values of PÀN copolymers with respect to
those of EÀN copolymers. The highest molar masses obtained at room temperature
were in the range of 40,000 g/mol.
With the aim of obtaining PÀN copolymers with high norbornene content and
high molar masses, Tritto and colleagues investigated the synthesis of PÀN
copolymers using rac-Me 2 Si(2-Me-Ind) 2 ZrCl 2 (I-4) [102]. Indeed the 2-alkyl
indenyl substitutions of C 2 -symmetric zirconocenes are key in considerably
increasing polypropene molar masses of the produced polymers. Methyl
2-substitution on the indenyl ligand in I-4 was found to cause an unexpected and
strong decrease in catalytic activity, molar fractions f N , T g , and M n values. P–N
copolymers with a maximum of 16 mol% of norbornene were obtained by I-4 in
contrast to those highly alternating copolymers obtained by I-1. Chain-end group
analysis revealed a greater amount of 2-butenyl end groups, arising from termination at a Mt-P 21 , than of vinylidene groups arising from termination at a Mt-P 12 . The
greater amount of 2-butenyl end groups in samples with lower molar fractions of
triads containing the P 21 unit and in samples obtained with I-1 gives an evidence
that the limiting step in P–N copolymerization is the difficulty in inserting a
propene after norbornene, which causes 2,1 insertions with subsequent isomerization to 1,3 propene insertions as well as chain epimerization in starved propene
conditions with I-4. The great decrease in the tacticity of the PP blocks in the
copolymers prepared with I-4 with increasing the norbornene content in the feed
revealed that the difficulty of this catalyst in accommodating a norbornene into
Mt-P 12 N makes unimolecular epimerization events probable.
13 C NMR experiments and ab initio theoretical chemical shift calculations,
combined with rotational isomeric state (RIS) statistics of the P–N chain, gave
the first assignment of the
13 C NMR spectra of P–N copolymers [95–97]. Cis-2,3exo norbornene insertion is considered to occur into the metal–carbon bond as in
Polyolefins with Cyclic Comonomers
129
norbornene homopolymerization [99]. PÀN copolymerizations were carried at 20
C
under atmospheric pressure of propene. Dimethylmetallocene IV-2 was activated by
dried MAO, modified methylaluminoxane (MMAO), and Ph 3 CB(C 6 F 5 ) 4 /Oct 3 Al. The
system activated by Ph 3 CB(C 6 F 5 ) 4 /Oct 3 Al was the most active and yielded the
copolymer with the lowest molecular weight and broadest molecular weight distribution. Dried MAO yielded the copolymer with the narrowest polydispersity. The
norbornene content in the copolymer was almost proportional to the [N]/[P] feed
ratio and the T g of the P–N copolymers increased linearly against the norbornene
content in the copolymers, from 53
C to 249
C.
The influence of propene pressure and temperature on activity, norbornene
content, M w , and T g of P–N copolymers by I-1 was assessed [101]. A decrease in
norbornene content, M w , and T g was observed at high temperatures and pressures.
The great number of 1,3 propene insertions found, especially at high temperature and
pressure, occurring after an inserted norbornene unit confirmed that the difficulty in
inserting a propene after norbornene is the limiting step in P–N copolymerization and
that chain transfer reactions are likely to occur more often at a propene-last insertedMt bond. This originates the lower M w values of PÀN copolymers with respect to
those of EÀN copolymers. The highest molar masses obtained at room temperature
were in the range of 40,000 g/mol.
With the aim of obtaining PÀN copolymers with high norbornene content and
high molar masses, Tritto and colleagues investigated the synthesis of PÀN
copolymers using rac-Me 2 Si(2-Me-Ind) 2 ZrCl 2 (I-4) [102]. Indeed the 2-alkyl
indenyl substitutions of C 2 -symmetric zirconocenes are key in considerably
increasing polypropene molar masses of the produced polymers. Methyl
2-substitution on the indenyl ligand in I-4 was found to cause an unexpected and
strong decrease in catalytic activity, molar fractions f N , T g , and M n values. P–N
copolymers with a maximum of 16 mol% of norbornene were obtained by I-4 in
contrast to those highly alternating copolymers obtained by I-1. Chain-end group
analysis revealed a greater amount of 2-butenyl end groups, arising from termination at a Mt-P 21 , than of vinylidene groups arising from termination at a Mt-P 12 . The
greater amount of 2-butenyl end groups in samples with lower molar fractions of
triads containing the P 21 unit and in samples obtained with I-1 gives an evidence
that the limiting step in P–N copolymerization is the difficulty in inserting a
propene after norbornene, which causes 2,1 insertions with subsequent isomerization to 1,3 propene insertions as well as chain epimerization in starved propene
conditions with I-4. The great decrease in the tacticity of the PP blocks in the
copolymers prepared with I-4 with increasing the norbornene content in the feed
revealed that the difficulty of this catalyst in accommodating a norbornene into
Mt-P 12 N makes unimolecular epimerization events probable.
13 C NMR experiments and ab initio theoretical chemical shift calculations,
combined with rotational isomeric state (RIS) statistics of the P–N chain, gave
the first assignment of the
13 C NMR spectra of P–N copolymers [95–97]. Cis-2,3exo norbornene insertion is considered to occur into the metal–carbon bond as in
Polyolefins with Cyclic Comonomers
129