recent advances in metallocene catalyzed polymerization...
TRANSCRIPT
1
RECENT ADVANCES IN METALLOCENE
CATALYZED POLYMERIZATION OF
OLEFINS AND OTHER MONOMERS
By
Albert J van Reenen
LECTURE PREPARED FOR THE 2nd
ANNUAL UNESCO
TRAINING SCHOOL, MARCH 29 – 31, 1999
2
This lecture is subdivided into 8 sections:
1. Introduction and Historical overview.
2. The role of the cocatalyst
3. Ethylene Polymerization
4. Propylene Polymerization
5. Cyclic olefin Polymerization
6. Other Monomers.
7. Catalysts based on metals other than those in Group 4.
8. New Developments
1. Introduction.
Several excellent reviews on this subject has been published in the past few years (1-4)
,
and the for the purpose of this lecture, a short summary of the salient facts covered in
these reviews and other papers is given here.
Metallocene-based catalysts, including the so-called “single-site” catalysts has
become an important technology for the global polymer industry. Although it is true
that free-radical initiated high pressure polyethylene polymerization was the
foundation for the polyolefins industry, advances in coordination Ziegler-Natta
catalysis during the past 40 or so years have been responsible for most of the growth
in production volume in polyolefin plastics. It is very likely that with the emergence
of the metallocene-type catalysts, coordination catalysts will become of even greater
importance to the polyolefin industry. The projected demand for metallocene
catalyzed polyolefins are given below (5).
3
Polymer Demand by Year: (in tons x 10-3
)
2000 2005 2101
PE 10 000 20 000 40 000
iPP and sPP 1 500 7 000 20 000
EPDM 150 200 400
SPS 80 150 300
Cyclic olefins 30 60 100
To really understand the importance of the so-called “single-site” catalysts, it
necessary to briefly look at the difference between these catalysts and the “multi-
sited” Ziegler-Natta type catalysts. In the Ziegler-Natta-type catalysts, which are
heterogeneous, the active metal centre occupies a position on the surface of the
crystal. Polymerization at the active site is influenced by the electronic and steric
environment of the crystal lattice. Because the active centers can occupy a wide
variety of lattice sites, they tend to give products with a broad molecular weight
distributions (MWD) and also, for example, non-homogeneous comonomer
distribution in olefin copolymers. Nominally metallocenes are bicomponents
consisting of group 4 transition metal compounds and cocatalysts. The bis-
cyclopentadienyl metallocene catalyst illustrated below 1 has an active centre that is
shielded to a large extent from the influence of its immediate surroundings. This kind
of catalyst yields a sharply defined product with narrow MWD and other molecular
characteristics, as well as a minimum of undesirable byproducts (eg low molecular
weight PE in LLDPE and atactic polypropylene (aPP) in isotactic PP). Even though
the narrow MWD might not be desirable for all applications, the right choice of
catalyst can lead to materials with the desired properties.
4
C Zr
Cl
Cl
.
The evolution of the metallocene catalyst structures for olefin polymerization is
shown in Table 1 (1)
.
Table 1. Timetable and historical dvelopment of metallocene research
1952 Development of the structure of metallocenes (ferrocene) by Fischer and
Wilkinson
1955 Metallocene as component of Ziegler-Natta catalysts, low activity woth
common aluminium alkyls.
1973 Addition of small amount of water to increase the activity (Al:H2O = 1:0.05
up to 1:0.3) (Reichert, Meyer and Breslow)
1975 Unusual increase in activity by adding water at the ratio Al:H2O = 1:2
(Kaminsky, Sinn and Motweiler)
1977 Using separately prepared methylaluminoxane (MAO) as cocatalyst for olefin
polymerization. (Kaminsky and Sinn)
1982 Synthesis of ansa metallocenes with C2 symmetry (Brintzinger)
1984 Polymerization of propylene using a rac/meso mixture of ansa titanocenes
lead to partially isotactic polypropylene. (Ewen)
1984 Chiral ansa zirconocenes produce highly isotactic polypropylene (Kaminsky
and Brintzinger)
Initially, it was found that using simple group 4 metallocenes like
bis(cyclopentadienyl)titanium dichloride together with a cocatalyst like
diethylaluminium chloride for the polymerization of ethylene lead to a catalyst system
that showed initial fair activity which then rapidly decreased, due to factors like alkyl
5
exchange reactions, hydrogen transfer and reduction of the transition metal species.
Reichart and Meyer (6)
found a remarkable increase in activity (20-100 times better)
by adding small amounts of water to the system Cp2TiCl2/C2H5AlCl2. An enormous
increase in activity was found in 1975 (up to 1 million times better) when water was
added in a far greater amount, and, in 1977, when MAO was used with titanocenes
and zirconocenes (7, 8)
. Thereafter the next important step was using ansa
metallocenes synthesized by Brintzinger et al in 1982 (9)
. This allowed stereospecific
polymerization of propylene. Ewen synthesized a Cs symmetric zirconocene
([Me2C(Flu)(Cp)]ZrCl2) in 1988 which allowed for the production of syndiotactic
polypropylene in high quantities. (10)
. Since 1985, a rapid world-wide industrial and
academic development began in the field of metallocene catalysts which continues
today.
Before we continue with a more detailed discussion of the cocatalyst choice and
function, it is necessary to briefly just look at the transition metal complexes that are
capable of olefin polymerization.
In general the organo-early transition metal complexes have partially ionic metal-
carbon bonds and show -agostic hydrogen interaction that somewhat stabilizes the
catalytically active species by providing electrons at a vacant site on the metal. The
organo-late transition metal complexes generally show -agostic hydrogen
interaction, and this causes easy hydrogen transfer through - hydrogen elimination
and reductive elimination, which leads to oligomerisation rather than polymerisation
of the olefins. It is therefore not surprising that a large number of the metals capable
of polymerising olefins are in fact early transition metals, particularly those in Group
4 of the period table.
2. The role of the cocatalyst
The cocatalysts are the key to the activity of the metallocenes. Methylaluminoxane
(MAO) is mostly used and is synthesised by the controlled hydrolysis of trimethyl
aluminium (TMA). Other bulky anionic complexes which show weak co-ordination,
such as borates, play an increasing role too.
6
The first function of the MAO is the alkylation of the halogenated metallocene
complex. Monomethylation takes place within seconds, and an excess of MAO leads
to dialkylated species:
Cp2MCl
ClCp2M
Cl
MeCp2M
Me
Me
MAO MAO
While the strucure of MAO is complex, it is generally accepted that it is a oligomeric
compound with a molecular weight between 1 000 and 1 500 g/mol. It would appear
as if the MAO complex can seize a methyl anion, a Cl- anion or an OR- anion from
the metallocene, forming an AlL4- anion which can distribute the electron over the
whole cage, thus stabilizing the charged system:
Cp2MMe
MeAlMAOCp2M
Me
Me
Cp2MMe
Me
AlMAO
The formed cationic L2M(CH3)+ is generally regarded as the active center in olefin
polymerization. This is evidenced by the formation of highly active metallocene
catalysts when using anionic counterions such as tetraphenyl borate (C6H5)4B-,
carborane or fluorinated borate. Typically cationic metallocene complexes can be
formed by reactions of perfluorinated triphenylborane or trityltetrakis
(pentafluorophenyl)borate:
Cp2ZrMe2
B(C6F5)3
[Ph3C][B(C6F5)4]
[Cp2ZrMe]+
[MeB(C6F5)3]-
[Cp2ZrMe]+
[B(C6F5)4]- Ph3CMe
Whereas the ratio of MAO to metallocene needs to be around 5 000:1 for active
catalyst systems, the ratio of borate to metallocene is 1:1. On the other hand, the
borate system is very sensitive to poisons and decomposition and must be stabilized
by small amounts of aluminium alkyls.
7
A further function of MAO is the reactivation of inactive complexes formed by
hydrogen transfer reactions. In solution, the combination of MAO and metallocene
leads to fast complexation and methylation, followed by the evolution of methane and
a catalytically inactive M-CH2-Al complex. This complex reacts with MAO to form
Zr-CH3+ and Al-CH2-Al structures, which explains why a large excess of MAO is
required.
3. Ethylene Polymerization
3.1 HOMOPOLYMERS
Zirconocene/MAO catalysts are about 10 to 100 times more active for ethylene
polymerisation than conventional Ziegler catalyst systems. For example,
Cp2ZrCl2/MAO, polymerising ethylene at a pressure of 8 bar and a temperature of
95°C yields 40 000 kg of PE/g Zr.h (11)
. As mentioned earlier, every Zr atom forms an
active complex (12, 13)
and produces about 46 000 polymer chains per hour.
Calculations show that one ethylene unit is inserted every 3 x 10-5
seconds.
The metallocenes generally used for ethylenes are bridged, unbridged, substituted and
half-sandwich complexes. Examples of unbridged and bridged catalysts are shown
below:
R4 R3
R5
R2R1
R2
R5
R1
R4
R3
MXX
8
No M X R1 R2 R3 R4 R5
1 Zr Cl H H H H H
2 Ti Cl H H H H H
3 Hf Cl H H H H H
4 Zr Me H H H H H
5 Ti Me H H H H H
6 Hf Me H H H H H
7 Zr Cl Me Me Me Me Me
8 Zr Cl Neomenthyl H H H H
9 Zr Cl Me Me Me Me Et
X MClCl
M = Zr, Hf X = C2H4, Me2Si
X ZrClCl
X = C2H4, Me2Si
10, 11 12, 13
9
X
R1
R2
R1
R2 ZrClCl
MClCl
X
R
No X R1 R2 No M X R
14 C2H4 Me Me 18 Zr Me2C H
15 Me2Si Me Me 19 Hf Me2C H
16 Me2Si Ph H 20 Zr Ph2C H
17 Me2Si Naph H 21 Zr Me2C Me
Zr Me2C T-Bu
X ZrClCl
X = Me2Si, C2H4
22, 23
10
In Table 2, a list of catalysts for ethylene polymerisation is given (12-15)
Table 2. Examples of catalysts for ethylene polymerisation
No Catalyst Activity (kg PE/(mol Zr.h.[Et])) Mol Mass (g/mol)
1 Cp2ZrCl2 60 900 620 000
2 Cp2TiCl2 34 200 400 000
3 Cp2HfCl2 4 200 700 000
Cp2TiMeCl 27 000 440 000
4 Cp2ZrMe2 14 000 730 000
5 Cp2TiMe2 1 200 500 000
6 Cp2HfMe2 1 600 550 000
7 (C5Me5)ZrCl2 1 300 1 500 000
Ind2ZrCl2 45 000 600 000
(neomenthylCp)2ZrCl2 12 200 1 000 000
9 (C5Me4Et)2ZrCl2 18 800 800 000
[O(SiMe2Cp)2]ZrCl2 57 800 930 000
[O(SiMe2 t-BuCp)2]ZrCl2 11 700 70 000
12 [En(Ind)2ZrCl2] 41 100 140 000
12’ [En(Ind)2HfCl2] 2 900 480 000
22 [En(Flu)2ZrCl2] 40 000
14 [En(2,4,7 Me3Ind)2ZrCl2 78 000 190 000
10 [En(IndH4)2ZrCl2] 22 000 1 000 000
11 [Me2Si(Ind)2]ZrCl2 36 900 260 000
[Ph2Si(Ind)2]ZrCl2 20 200 320 000
11
[Bz2Si(Ind)2]ZrCl2 12 200 350 000
15 [Me2Si(2,4,7 Me3Ind)2ZrCl2 111 900 250 000
13 [Me2Si(IndH4)2]ZrCl2 30 200 900 000
[Me2Si(2Me4,6iPrInd)2]ZrCl2 18 600 730 000
16 [Me2Si(2Me 4PhInd)2]ZrCl2 16 600 730 000
[Me2Si(2Me4,4BenzoInd)2]ZrCl2 7 600 450 000
[Ph2C(Ind)(Cp)]ZrCl2 3 330 18 000
[Me2C(Ind)(Cp)]ZrCl2 1 550 25 000
[Me2C(Ind)(3MeCp)]ZrCl2 2 700 30 000
[Ph2C(Flu)(Cp)]ZrCl2 2 890 630 000
18 [Me2C(Flu)(Cp)]ZrCl2 2 000 500 000
19 [Me2C(Flu)(Cp)]HfCl2 890 560 000
Important here is that partially substituted bis-indenyl systems show very high
activities. The ligands with bulky substituents like neomenthyl afford significantly
lower productivity. From the table it can be seen that electron-donating groups
enhance productivity, while steric crowding lowers it.
3.2 POLYMER PROPERTIES
Typically for PE produced by metallocene catalysts we find a Mw/Mn of around 2,
and 0.9 to 1.2 methyl groups per 1 000 carbon atoms. Depending on the catalyst used,
the molecular weight can differ by more than a factor of 50. Typically substituents in
the 2-position on the Cp ring leads to higher molecular weight; the pentamethyl-
substituted cyclopentadiene (Cp*), which has a structural similarity with a 2-
substituted indene, gives a molecular weight of 1.5 million when used in the catalyst
complex Cp*2ZrCl2. Bimodal molecular weight distributions can be achieved by
mixing different catalysts. Molecular weight can readily be decreased by increasing
12
the temperature of polymerization, raising then metallocene/ethylene ratio and by
adding small amounts of hydrogen (16)
. Melting points of these polymers are around
139 – 140.5°C, and the density decreases after initial melt pressing to 0.947- 0.953
g/cm3.
3.3 COPOLYMERS
Metallocenes are useful for copolymerizing ethylene with propylene, 1-butene, 1-
pentene, 1-hexene and 1-octene (to form LLDPE). As mentioned before, these
catalysts synthesize polymers with a more uniform comonomer distribution and fewer
extractables than in the case of Ziegler catalysts. The product of the reactivity ratios
r1.r2 is close to unity when C2 symmetric metallocenes are used, indicating a random
structure, while it is somewhat less than unity when a Cs-symmetric catalyst is used (16
- 19).
During copolymerization with propylene, the activity is higher than that shown by the
ethylene homopolymerization. (20)
This increase in the insertion rate is probably due
to the electronic influence of the comonomer. Copolymers of industrial interest are
the E/P polymers with molar ratios of 1: 0.5 up to 1: 2. These polymers show elastic
properties and together with 2-5% of dienes are used in elastomers (21 - 23)
. Block
copolymers can be formed at low temperatures, particularly with the hafnocenes.
Long-chain branching (LCB) can be achieved by using the Dow catalyst (24 - 26)
:
Si
N
TiCl
Cl
These catalysts, in combination with MAO or borates, incorporate oligomers with
vinyl endgroups which are formed during polymerization by -hydrogen transfer.
Copolymers with 1-octene have also been made, (27)
as well as with styrene (28 – 30)
.
13
4. Propylene Polymerisation
The microstructure of this polymer in terms of the enchainment of the monomer units
is determined by the regio- and stereospecificity of the insertion of the monomer.
Both primary 1,2 insertions and secondary 2,1 insertions are possible. 1,2 insertions
(head-to tail) leads to 1,3 branching, while 2,1 insertions can lead either to 1,2
branching (head-to head) or 1,4 branching (tail-to tail). Metallocenes favour
consecutive primary insertions due to their bent sandwich structure. Secondary
insertions are based on the structure of the metallocene used and the experimental set-
up (particularly temperature and monomer concentration). Secondary insertions cause
increased steric hindrance to the next primary insertion, blocking the active center and
can be regarded as the resting state. This often leads to chain termination and
isomerization processes (31)
. Isomerization processes during the polymerization of
propylene leads to the formation of 1,3 inserted units:
MPolymer
CH2H
PolymerM
H
PolymerM
H
M Polymer
Propylene is prochiral, and polymers of this monomer have pseudochiral centers at
every tertiary carbon atom. The regularity of the configuration at these successive
chiral centers is described by the tacticity of the polymer. If two adjacent chiral
centers have the same configuration this “diad” of chiral centers is said to be meso in
arrangement. If they are dissimilar, they are said to be racemic. A polymer
comprising only meso diads is called isotactic, while a polymer comprising only
racemic diads is said to be syndiotactic. A mixture of racemic and meso diads gives
an atactic polymer.
Stereochemical control of the polymer is only possible if the active species during
polymerization is chiral. Chirality might be located at the transition metal itself, or at
the ligand attached to the transition metal, or on the growing polymer chain. Two
14
basic mechanisms of steroechemical control is possible (32, 33)
: Control can be at the
catalytic site itself, or at the chain end. Catalytic site control (also known as
enantiomorphic site control) leads to a Bernoullian distribution of stereoerrors in the
polymer, while chain end control (chirality of the last inserted monomer unit) leads to
a Markovian distribution of stereoerrors.
4.1 SYMMETRY AND STEREOSPECIFICITY
Ewen was the first to find that Cp2TiPh2/MAO produces iPP at low temperatures
(chain end control). It was then found that a mixture of rac and meso
En(Ind)2TiCl2/MAO gave a mixture of iPP and aPP. The pure rac-isomer was then
shown to produce iPP (34, 35)
. Generally 5 different catalyst symmetries may be
distinguished.
a) C2v symmetric metallocenes
These are achiral, like Cp2MCl2 or Me2Si(Flu)2ZrCl2.
b) C2 symmetric metallocenes.
These are bridged metallocenes like rac-ethylenebis(indenyl)ZrCl2, or the
tetra-H analogue thereof. Based on these catalysts, others were designed for
higher activities, higher molecular weights, tacticities and melting points (36-38)
.
Table 3: Comparison of productivity, molecular weight, melting point and isotacticity
during the polymerization of propylene at 70°C, Al/Zr ratio 15 000/1 (96)
Productivity (kg
PP/mmol Zr.h)
Mw (g/mol) Tm
(°C)
Isotacticity
(%mmmm)
Et(Ind)2ZrCl2 188 24 000 132 79
Me2Si(Ind)2ZrCl2 190 36 000 137 82
Me2Si(IndH4)2ZrCl2 48 24 000 141 84.5
Me2Si(2-MeInd)2ZrCl2 99 195 000 145 88.5
Me2Si(2-Me-4-iPrInd)2ZrCl2 245 213 000 150 88.6
15
Me2Si (2,4-Me2 Cp) 2ZrCl2 97 31 000 149 89.2
Me2Si(2-Me-4-tBuCp)2ZrCl2 10 19 000 155 94.3
Me2Si(2-Me-4,5 BenzInd)2ZrCl2 403 330 000 146 88.7
Me2Si(2-Me-4-PhInd)2ZrCl2 755 729 000 157 95.2
Me2Ge(2-Me-4-PhInd)2ZrCl2 750 1 135 000 158
Me2Si(2-Me-4-naphthInd)2ZrCl2 875 920 000 161 99.1
Main chain termination is normally achieved through -hydrogen transfer to
monomer (39, 40)
. This is effectively suppressed by substituents in position 2 on the
indenyl ring (41,42)
. Substituents in the 4-position also enhance molecular weight
because this reduces the number of 2,1 misinsertions. Bridged Cp2 zirconium and
hafnium will give high isotactic contents when there are substituents in the 2, 4, 3’
and 5’ positions, as this generates a structure similar to the bis indenyl catalysts. In
general these catalysts have much lower actvities
Table 4: Bridged bis-cyclopentadienyl zirconium dichloride catalysts and their
polymerization behaviour for propylene (43)
.
Metallocene Productivity [kg
PP/mmol Zr.h)
Mw x 10-3
(g/mol)
Tm (°C) Isotacticity
(%mmmm)
[Me2Si(2,3,5Me3Cp)2]ZrCl2 1.6 134 162 97.7
[Me2Si(2,4Me2Cp)2]ZrCl2 11.1 87 160 97.1
[Me2Si(3tBuCp)2]ZrCl2 0.3 10 149 93.4
[Me2Si(3MeCp)2]ZrCl2 16.3 14 148 92.5
[Me2Si(2,3,5Me3Cp)2]HfCl2 0.30 256 163 98.7
[Me2Si(2,4Me2Cp)2]HfCl2 0.10 139 162 98.5
[Me2Si(3tBuCp)2]HfCl2 0.63 17 157
[Me2Si(3MeCp)2]HfCl2 1.61 67 148
16
c) Cs symmetric bridged metallocenes
These generally give syndiotactic polypropylenes. In these catalysts the
chirality is centred at the transition metal itself. Due to the flipping of the
chain the metallocene alternates between the two enantiomeric configurations
and produces a syndiotactic polymer chain (44-46)
.
d) C1 symmetric bridged metallocenes.
These are variations of Cs symmetric metallocenes. If a methyl group is
introduced at position 3 of the Cp ring, stereospecificity is disturbed at one of
the reaction sites, leading to every second insertion being random, and a
hemiisotactic polymer is produced. The introduction of a larger group like t-
butyl, causes inversion of stereoselectivity and iPP is formed.
e) Oscillating Metallocenes
Waymouth et al have shown that polypropylene containing blocks of atactic
and syndiotactic material can be formed when consecutive insertions take
place as well as chain migratory insertion reactions. They have shown that
bis-2-phenylindene zirconium dichloride catalysts are particularly suited to
this purpose. This work will be discussed in more detail under the New
Developments section of this document.
4.3 POLYMER PROPERTIES
With conventional Ziegler catalysts, only low molecular weight atactic
polypropylene (aPP) waxes with broad MWD are produced. With metallocene
catalysts, aPP covering the whole spectrum molecular weights and with narrow MWD
can be produced (47)
.
The properties and melting point of isotactic polypropylene (iPP) polymers are
determined by the amount of irregularities which are distributed randomly along the
polymer chain (unlike the extractable PP with Ziegler catalysts). Metallocene iPP has
a melting point varying from 125°C to 165°C. Even with high pentad isotacticities,
low Tm can be found in metallocene iPP polymers, due to 2,1 and 1,3 misinsertions.
17
The amount of extractables in metallocene homo-and copolymers is far lower than for
conventional PP, which makes it excellent for food wrapping and applications at
cooking temperature. When highly stereoselctive metallocenes are used, highly
crystalline, stiff PP is produced, exhibiting a stiffness 25 – 33% above that of
conventional PP, in fact resembling the properties of conventional PP’s filled with talc
or other minerals. Metallocene PP’s are also easier to recycle.
Apart from stiff iPP’s, metallocenes can also produce waxy iPP for pigment
dispersions etc (48)
. These waxes have molecular weights of 10 000 to 70 000 and
melting points of 140°C to 160°C. The vinyl endgroups may be utilized for
funtionalization.
Syndiotactic polypropylene (sPP) has a higher degree of irregularities than iPP.
Generally we see a lower density, lower Tm (2)
. The smaller crystal size of sPP leads
to higher clarity than iPP, but inferior gas barrier properties makes these polymers
unsuitable for food packaging applications. However, the good resistance to radiation
of sPP makes it suitable for medical applications. These polymers have good impact
strength.
Commercially, a silica-supported metallocene in bulk suspension at 50 –70°C is used
at a pressure of 30kg/m3 (2)
.
Elastomeric polypropylenes (ePP) can be produced in two different ways. ePP can
be produced due to 1,3 insertions in the polymer backbone, or ePP can be produced by
oscillating catalysts or C1 symmetric catalysts.
18
Table 5: Properties of elastomeric polypropylenes produced by catalysts:
[MeH(Ind)(Cp*)]TiCl2 (1), [Me2C(Ind)(Cp)]HfCl2 (2), [Me2C(Ind)(Cp)]ZrCl2 (3) and
(2PhInd)2ZrCl2 (4) (49, 50)
.
Catalysts: 1 2 3 4
Mw (g/mol) x10-3
127 30 380 889
Mmmm% 40 38 52 28
Tm (°C) 47/61 53/84 125/145
Crystallinity (%) 6.7 7.2 16.7 0.2
Strain to break (%) 525 200 800 1210
4.4 HETEROGENIZATION AND POLYMERIZATION IN THE PRESENCE
OF FILLERS.
Current technology is based on gas phase and slurry processes. Thus metallocenes
have to be fixed on a carrier to be used as “drop-in” catalysts in existing plants.
Carriers may be divided into 3 groups.
Metals have been used as fillers.
Inorganics like silica, alumina and zeoliths (51 - 58)
.
Organic materials like cyclodextrins (59)
, and polymers (polyamides, polystyrenes).
The most common method of heterogenizing the catalyst is by heterogenization of the
cocatalyst, followed by mixing the cocatalyst-modified carrier with the catalyst and
subsequent activation by a trialkylaluminium. The MAO is either generated by
reacting a carrier containing hydroxyl groups with trimethylaluminium, or by treating
the carrier with MAO. (Table 6).
19
Table 6: Comparison of metallocenes in the homogeneous phase and supported on
silica-fixed MAO at 40°C. Catalysts used (Figures on pp 7 – 9)): (I) = 10, (II) = 14,
(III) = 1
Catalyst Cocatalysts Activity
(kg/mol Zr.h)
Tm (°C) Mn
(kg/mol)
Mw/Mn Mmmm %
or rrr%
(I)/homog. MAO 3mmol 2070 111 3.3 1.9 71
(I)/MAO/SiO2 TMA 1mmol 313 126 2.2 1.8
(I)/MAO/SiO2 TEA 1mmol 77 140 5.3 2.5 90
(I)/MAO/SiO2 TIBA 1mmol 556 136 14.2 2.0
(I)/MAO/SiO2 TIBA 0.5mmol > 1500 128 4.7 3.1
(I)/MAO/SiO2 TIBA 2mmol 382 105 6.6 1.8 69
(II)/Homog. MAO 13mmol 758 123 39.3 1.8 77
(II)/MAO/SiO2 TIBA 2mmol 141 133 45.2 1.9 83
(III)/Homog. MAO 10mmol 132 0.3
(III)/MAO/SiO2 TIBA 0.4mmol 99 1.8
5. Cycloolefin Polymerization
5.2 HOMOPOLYMERS
Strained cycloolefins like cyclobutene, cyclopentene and norbornene can be used as
monomers and comonomers in a wide variety of polymers. Vinyl addition
polymerization of the cyclic olefins is possible by metallocene catalysis. The
polymers are known as ditactic, as they have two chiral centers per monomer unit.
Metallocene polycycloolefins tend to be highly crystalline materials with high melting
points (sometimes even above the decomposition temperature) and good chemical
resistance.
20
Polymerization of norbornene shows that enchainment occurs through cis exo
insertion, while cyclopentene shows quite unique cis and trans 1,3 enchainment (no
1,2 enchainment):
X M
ClCl
5.2 COPOLYMERS
Homopolymers of cycloolefins like norbornene and tetracylodecene are not
processable due to high melting points and insolubility in common organic solvents.
Copolymerization with ethylene and -olefins yield cycloolefin copolymers (COC) (60
– 66). Copolymers tend to be amorphous if more than about 15 mole% of the
cycloolefin is introduced into the polymer chain. The glass transition temperatures of
the COC polymers may be varied according to the amount of cycloolefin being
produced. With ethylene/norbornene polymers it has been illustrated that very narrow
MWD polymers can be produced (MWD = 1.1 – 1.4) (67)
. These copolymers are
transparent and optically anisotropic. They are stable against hydrolysis and chemical
degradation, processable and have a high refractive index. They are interesting
materials for optical applications, like optical fibres, compact discs and lenses (68)
.
6. Other Monomers
6.1 STYRENE
Since 1985, when the first pure syndiotactic polystyrene was synthesized using a
metallocene/MAO catalyst system (69)
, a great number of patents claiming the use of
sPS for certain applications have been filed (see for example 70
). This polymer melts
21
at 270°C, which is the highest melting point of all metallocene homopolymers. The
half sandwich titanium compound catalysts (with Cp ligands) have proven to be the
most active for styrene polymerization. In contrast to olefin polymerization,
titanocenes are more active than zirconocenes and fluoro ligands are better than
chloro ligands. Summary is given in Table 7.
Table 7: Synthesis of sPS using metallocene/MAO catalysts.
Temp (°C) Activity Tm (°C) Mw (Kg/mol) Mw/Mn
CpTiCl3 10 109 267 390 3.6
30 477 263 230 2.2
(C5Me5)TiCl3 30 3.5 277 186 2.3
50 15.4 275 169 3.6
(C5Me5)ZrCl3 30 0.01 249 20 2.2
(C5Me5)TiCl3 50 690 275 660 2.0
CpTiF3 30 2400 261 380 1.8
50 1700 257 100 2.0
6.2 DIENE POLYMERISATION
Metallocenes polymerize non-conjugated and conjugated dienes.
Cyclopolymerization affording ring structures separated by methylene groups was
observed for non-conjugated dienes (1,5 dienes) (71 - 73)
. 1,2 Insertion of the terminal
double bond is followed by an intramolecular cyclization forming a ring (74)
. The
polymerization of 1,5- hexadiene affords four different structures, simple bis-Cp
metallocenes giving predominantly trans-isotactic structures, while pentamethyl-
substituted zirconocenes give cis-connected polymers which are highly crystalline and
melt at 90°C. Optically active ansa-metallocenes polymerise the dienes to optically
active trans-isotactic polymers (in contrast to the olefins, which give only oligomers
that are optically active). Functionalized olefins were prepared by the polymerisation
22
of a 1,6-diene, 4-trimethylsilyloxy-1,6 heptadiene, using Cp2* zirconocenes and
borate cocatalysts. After hydrolysis with HCl of the cyclopolymer, polymethylene-3-
hydroxycyclohexane is formed. Waymouth et al (75)
using the same catalyst also
polymerized 5-N,N-diisopropyl-amino-1-pentene and 4-t-butyldimethylsiloxy-1-
pentene. Functional polyolefins were also prepared by copolymerization of olefins
with borane monomers. Chung (76, 77)
polymerized 5-hexenyl, 9
borabicyclo(3,3,1)nonane together with various -olefins. Borane groups were
subsequently converted to hydroxyl groups. The functionalized monomers are further
discussed under new developments.
Conjugated dienes are polymerized by half-sandwich titanocenes (as for styrene) (78)
.
6.3 METHYLMETHACRYLATE
If MAO is replaced by other Lewis acid cocatalysts like tetraphenylboranes, catalysts
which may tolerate functional groups are obtained. Collins (79, 80)
polymerized MMA
with a metallocene dimethyl using borate cocatalysts. In the case of a chiral
metallocene (En(IndH4)2ZrCl2) highly isotactic polymer was produced. The polymer
appeared to have living character. When a third component, an aluminium or zinc
alkyl, was added to a chiral zirconocenedimethyl/borate catalyst system, highly
isotactic PMMA was made (80)
. Of course, lanthanocenes, which are isoelectronic
with alkylzirconocenium ions are also capable of polymerizing methyl methacrylate
(81, 82).
7. Metallocene Complexes From Metals Other Than Those
in Group 4
7.1 METALLOCENE COMPLEXES FROM GROUP 3 METALS
Neutral iso-electronic Group 3 complexes such as Cp2*MR (M = Sc, Y, and
Lanthanide metals) show isolobal analogy to cationic Group 4 metallocenes.
Highly active catalysts for the polymerization of ethylene are found in this Group (83 -
89). The activity is in the order La > Nd >> Lu. These catalysts show high initial
activity, but the activity decreases rapidly with time.
23
7.2 ORGANOMETALLIC COMPLEXES OF GROUP 5 METALS
Whereas the complexes NbCl3Cp2, NbCl2Cp2 and NbCl4Cp show no polymerization
activity, the types of complex [MRCp(1,3 diene)]+
where M = Nb and Ta were found
to give “living” polymerization of ethylene (90 - 93)
. Much of this work was done in the
period 1985 – 1992. Similar substituted Cp-based catalysts (94 – 96)
polymerized
ethylene to Mn = 23 000 and MWD = 1.05 at Tp = -20°C, while the same
polymerization at Tp = 20°C gives PE with Mn = 83 000 and MWD = 1.3.
MR M
RM
R
Group 3 Group 4 Group 5
7.3 GROUP 6 ORGANOMETALLIC COMPOUNDS
Chromium-based ethylene polymerisation catalysts have of course been developed
commercially. Also soluble organochromium complexes are able to polymerise
ethylene (97 - 99)
.
The complex [Cp*CrMe(THF)2]+(PPh4)
- shows very low activity, but the
isoelectronic analogue (RN)2CrX2 with R = t-Bu or Ph, X = CH2Ph, Cl, are active for
ethylene polymerization. Tris(butadiene) complexes of Mb and Tungsten (0) also
show activity for the polymerization of ethylene (100)
.
7.4 POLYMERIZATION OF ORGANOMETALLIC COMPOUNDS OF
OTHER TRANSITION METALS
The metals of groups 8 – 10 tend to catalyze the dimerization and oligomerization of
the olefins, they prefer -hydrogen elimination followed by reductive elimination.
(101). There are however some exceptions, in particular complexes of Pd and Ni. Of
particular interest here are the Ni-diimine complexes which are reported to give the
living polymerization of ethylene and -olefins. These will be discussed in much
more detail in the New Developments section.
24
8. New Developments in Transition Metal Catalyzed
Polymerisation of Olefins and Ethylene.
This section can be further subdivided into the following sections:
1. New catalysts for olefin and ethylene polymerisation.
2. Functionalized and other “new monomers”
3. “New Polymers” from old monomers.
4. Cationic polymerisation by metallocenes.
5. Living polymerization by metallocenes.
6. Olefin elastomers by metallocene catalysis.
8.1 NEW CATALYSTS FOR OLEFIN AND ETHYLENE
POLYMERISATION.
Most metallocene catalysts are based on substituted Cp, Ind or Flu ligands. Single-
site catalysts and half-sandwich titanocenes and late T/M catalysts without Cp ligands
have been reported for ethylene and -olefin polymerisation. Ewen et al (102)
reported
the preliminary findings for catalysts for propene polymerization bearing Cp rings
with 5-membered heterocyclic compounds fused to the Cp ligand. The new ansa
metallocenes contain the ligands:
NS
SSL1
L2L3
In contrast to ferrocenes with heterocyclic ligand L1 and others which decompose in
solution, Group 4 heterocenes are stable and both highly active and sterospecific for
propylene polymerisation with MAO.
25
Catalyst Activitya Mn (x10
3)
b Tm (°C) Pentad %
Me2C(3-tBu1-1Cp)(7-L3)ZrCl2 13 91 130 84
Me2C(3-tBu-1-Cp)(9Flu)ZrCl2 13 91 125 80
Me2C(1-Cp)(7-L3)ZrCl2 14 98 110 74
Me2C(1-Cp)(9-Flu)ZrCl2 89 133 140 82
Me2Si(4-L2)(2-Me-4-Ph-1-Ind)ZrCl2 550 997 161 97
Polymerization at 70°C:
Me2Si(4,6 L1)2ZrCl2 97 113 125 80
Me2Si(2-Me-1-Ind)2ZrCl2 99 195 145 88
Me2Si(4-L2)2ZrCl2 560 198 155 96
Me2Si(4-L2)(2-Me-4-Ph-1-Ind)ZrCl2 865 709 156
Me2Si(2-Me-4-Ph-1-Ind)2ZrCl2 955 1 287 156 95
a = activity in kg Polymer/mol Zr.h, b = molecular weight in g/mol
Another exciting development in new catalyst systems are the work of Brookhart et
al, some examples of which are included in other sections. Based on their earlier
work with Pd(II) and Ni(II) diimine catalyst systems, they recently reported (103)
on
new catalysts they had prepared. These iron(II) and cobalt(II) catalysts are based on
tridentate pyridine bisimine ligands, in which the imine moieties are bulky ortho-
substituted aryl rings.
The key to the polymerization activity of these late transition metal catalysts are the
bulky ortho substituents on the aryl group in the catalyst systems. The catalyst
preparation and structures are presented below:
26
R'
R
NH2N
O O
N
N NR
R' R'
R
1: R = R' = iPr
2: R = R' = Me
3: R = tBu, R' = H
N
N N ArAr
MX2(H2O)
N
N N
M
XX
a: M = Fe
b: M = Co
Additionally, Brookhart et al (104)
have reported on the use of Ni(II) and Pd (II)
caalysts for the polymerization of cyclopentene. Unlike the case when cyclopentene
is polymerized with C2-symmetric metallocenes and highly crystalline materials with
Tm = 395°C is obtained, the Ni and Pd catalysts below give lower melting
temperatures, probably due to lower tacticities of the polymers.
N
N
Pd
OOMe
N
N
NiBr2
Cl
Cl
N
N
NiBr2
N
N
NiBr2
OMe
Cl Cl
OMe
Cl Cl
Tm = 245°C Tm = 265°C Tm = 285°C Tm = 320°C
A 2-methylbenz(e)indenyl-based ansa-monocyclopentadienylamido complex catalyst
was reported by Xu and Ruckenstein (105)
. This catalyst was used copolymerize
ethylene and 1-octene and gave high activities and good 1-octene inclusion, high
molecular weight (Mw = 327 000 and MWD = 1.8):
27
SiMe2
N
TiCl
Cl
Schaffer et al (106)
reported the copolymerization of ethylene and isobutylene with
metallocene catalysts. Previously En(Ind)2ZrCl2/MAO had been used to
copolymerise ethylene and IB, but the IB incorporation was very low. Schaffer et al
claimed incorporation of IB up to levels of 45%. The catalyst used was
Me2Si(Cp*)cyclodecylamidodimethyl titanium, activated by either dimethylanilinium
tetrakis(pentafluorophenyl)borate, triphenylmethyltetrakis(pentafluorophenyl)borate
or MAO. The cocatalysts were chosen to prevent any possible carbocationic
polymerization of IB. Activities of 100 to 400 kg polymer/mol Ti.h were reported
(very similar to those for ethylene/styrene polymerisation) and molecular weights
varied between 9 000 and 50 000 g/mol. The copolymerisation of IB with propylene
using the same catalyst was also attempted, but failed. Small amounts of ethylene
present lead to the IB/E/P terpolymer.
Sita and Babcock (133)
described the synthesis of a wide range of derivatives of
(C5R5)TiMe2-[NR1-C(Me)NR
3] in high yield and that some of these compounds are
catalyst precursors for the polymerization of ethylene upon activation by MAO:
TiMe
MeMe
R1N=C=NR
3
TiMe
Me
Me
N
N
R1
R3
Doubly bridged metallocenes (134)
were reported. Previously these catalysts (135)
were
reported to polymerize propylene with high activities. These (Me2Si)2{5-C5H-3-
(CHMe2)-5-Me}2-MCl2 (M=Zr, Ti) are interesting in that the zirconocenes
28
polymerize propylene to sPP, while the titanocene produces only aPP. The last paper
(134) presented evidence that the titanocene catalyst undergoes rac-meso
interconversion.
Si SiMCl Cl
Amor et al (136)
report the synthesis of alkyl complexes of Group 4 metals containing
tridentate-linked amide Cp ligands. These catalysts are between the metallocenes and
the half-sandwich metallocenes:
Si
N
X
Ti Cl
Cl
Si
N
Ti R
RX
MgR2
X = OMe, NMe2
These catalysts were found to polymerize ethylene with low activities.
Another exciting new catalyst system was reported where a combination of the Dow
catalyst and boratobenzene metallocene mimics was used (137)
. Then boratobenzene
type zirconocenes had previously been shown to give, with ethylene, an mixture of 2-
alkyl-1-alkenes (138)
or 1-alkenes (139)
.
29
B O
B O
ZrCl2 ZrCl2
B
B
ETHYLENE
nR
n
Combining the boratobenzene on the left (see above) with the DOW catalyst they
obtained a branched polyethylene. Activities were in the region of 1 200 kg PE/mol
Zr.h.
Kim et al (140)
also reported on Ni(II) and Pd(II) catalysts for the production of
hyperbranched polymers from ethylene, but they claim, that unlike the claim by
Brookhart, bulky substituents were not necessary to produce branched PE.They used
simple, unencumbered metal catalysts like [Ni( -methallyl)Br]2 or Pd(1,5
cyclooctadiene)(Me)(Cl) with an excess of an aluminium compound (AlCl3 or AlEt3).
The products they obtained very of very low molecular weight (400 – 1000 g/mol).
Self-activating catalysts were developed by Ducahteau et al (141)
.
B(C6F5)2
MMe
MeL
AlR3
MR
RL
B(C6F5)2
-
M
MeL
B(C6F5)2
R
Brintzinger’s group also reported on a whole series of modified
Me2Si(benz(e)Ind)2ZrCl2 catalysts (142)
. These catalysts were evaluated for propylene
polymerization:
30
ZrCl2 (Me2)Me2SiMe2Si ZrCl2 (Me2)Me2Si
R
R
ZrCl2 (Me2)ZrCl2 (Me2)Me2Si
3 4a (R = H)
4b (R = Me)
5
Catalyst 3 gave activity (a) of 5 800 kg polymer/molZr.h, 82% mmmm, Tm = 126°C,
Mw = 28 100 g/mol, MWD = 1.62 and 2,1 insertions were 1.2%. Similarly (same
units) for 4a: a = 24 100, 80% mmmm, Tm = 126, Mw = 49 400, MWD = 1.59, 2,1%
= 0.5; for 4b, a = 55 600, 97% mmmm, Tm = 152, Mw = 132 000, MWD = 1.86,
2,1% = 0.4; for 5 a = 28 000, 92% mmmm, Tm = 150, Mw = 72 000, MWD = 1.57.
Chien et al (143)
reported the catalyst [2-(dimethylamino)ethyl]CpTiCl3, which was
reported to polymerise ethylene, ethylidine norbornene, vinylcyclohexane and 1,4
hexadiene.
8.2 FUNCTIONALIZED POLYMERS.
The limited tolerance of metallocene catalysts to functional olefins limits the end uses
of polyolefins. Compatability between polymers can be obtained when block or graft
polymers are used as compatibilizing agents. Stehling et al (107)
reports the
polymerization of sterically hindered alkoxyamines. These functionalized
alkoxyamines were then used as unimolecular initiators in nitroxide-mediated “living”
free radical polymerizations. They synthesized following alkene-substituted
aloxyamine:
31
OO N
A B
O
O
N
X
OR2
O
R1
R1 = R2=Me (a)
R1 = H, R2 = nBu (b)
X = Cl, Me
"Living" Free radical
Polymers were made with Mn = 210 000, and MWD = 2.0.
Polyolefins are stable to solvolysis, photo-degradation and microbial growth, mostly
due to their lack of functional groups. In certain applications, like where adhesion is
important, the lack of functional groups become a liability. Functionality can be
introduced by direct copolymerization of functional olefins. Unfortunately, free-
radical polymerizations involving the -olefins are limited to those where the
monomers do not have -hydrogens. This drawback can be overcome with
coordination-type catalysts. Brookhart et al (108)
reported the copolymerization of
ethylene and propylene with functionalized vinyl monomers using Pd(II) catalysts of
the type:
32
N
R R
NAr Ar
Pd
CH3OEt2
+
N NAr Ar
Pd
CH3OEt2
+B(Ar')4
-
B(Ar')4-
Copolymerization of ethylene with acrylates lead to high-molecular weight polar
polymers. These polymers were amorphous and highly branched materials, with
about 100 branches per 1 000 carbon atoms. The ester groups occurred typically at
the end of the branches. This could only occur through 2,1 insertion of the acrylate
into the Pd-C bond. Other polymers synthesised were ethylene/methyl vinyl ketone
and propylene/acrylate polymers. In general productivities were low and Mn values
varied between 10 000 and 140 000 g/mol.
Conventional Ziegler-Natta catalysts are intolerant to most functional groups.
Zirconocene/MAO catalyst systems have been shown to be moderately successful in
the copolymerization of ethylene and propylene with 1-hydroxy-10-undecene (109)
, 1-
chloro-10-undecene (110)
, N,N-bis(silyltrimethyl)-1-amino-10-undecene (111)
,
silsesquioxane-functionalized decene (112)
, an o-heptenyl phenol derivative (113)
, and
borane-functinalized -olefins (114, 115)
.
Kesti et al (75)
earlier carried out the homopolymerization of silyl-protected alcohols
and different tertiary amines. Recently, in 1998, Stehling et al (116)
reported the
polymerization of 5-amino1-pentenes and one 4-amino-1-butene using
metallocene/borate catalyst systems:
NR
R
m
R = Me, Et, iPr, Ph
m = 1,2
In general, only low molecular weight (oligomeric) products were obtained (Mn = 900
– 3 000). Polymerization with a Cp2*ZrMe2/borate catalyst system indicated that 5-
(N,N-diisopropylamino)-1-pentene gave the highest molecular weight, and this
monomer was then polymerized with bridged metallocenes (iPr(t-BuCp)(Flu)ZrMe2
33
(1), rac-En(Ind)2ZrMe2 (2), iPr(Cp)(Flu)ZrMe2 (3)), and this yielded tactic polymers
with up to 99 mmm% (1) and 86 rrr% (3). Melting points were between 110 and
115°C, and the molecular weights were reported to be above 14 000. Activities of the
catalysts were around 50 kg Polymer/mol Zr.h. Interestingly enough, while the
activity of the catalyst systems to the functionalized monomers were understandably
low, the tacticity was unaffected by the presence of functional groups.
In 1999, Stehling et al (117)
reported on the copolymerization of 5-(N,N-
diisopropylamino)-1-pentene. These monomers were polymerized with 1-hexene and
4-methyl-1-pentene using rac-En(IndH4)ZrMe2/Borate catalyst system, as well as
Cp2*ZrMe2/Borate.
8.3 NEW POLYMERS
Of these systems, many groups have concentrated on the copolymers of ethylene with
styrene.
Sernetz et al (118)
reported on the use of a number of different half-sandwich catalysts
to produce ethylene/styrene (E/S) copolymers:
Si
NTi Cl
ClSi
NTi Cl
ClSi
NTi Cl
Cl
R
Si
NTi Cl
Cl
R= H, SiMe3
Typically molecular weights (Mn, g/mol) of 51 000 to 121 000 were obtained, MWD
= 2.2 – 3.5 and catalyst activities varied from 35 to 3 000 kg polymer/mol Ti.h.
Subsequently, Sernetz et al (119)
used half-sandwich catalysts (DOW) and synthesised
ethylene/styrene (E/S) copolymers and terpolymers of ethylene/styrene and other
olefins, like 1-octene (O), propene (P), norbornene (N) and 1,5 hexadiene (HD). E/S
copolymers of Mn = 77 000 with MWD = 2.3 and activity of 10 500 kg Polymer/mol
Ti.h were reported. Other examples were E/S/N terpolymers, where catalyst activity
34
of 4 500 kg Polymer/mol Ti.h were found, Mn = 41 000, MWD = 2.2, E/S/O
terpolymers (Activity 170, Mn = 90 000, MWD =2.4), E/S/HD (a = 6 800, Mn = 30
000, MWD = 14.5). It was also demonstrated that Tg could be tailored by varying the
styrene content in the co- and terpolymers.
In 1998, Xu (120)
reported on the copolymerization of ethylene with styrene using a
titanocene catalyst based on an amide-fluoroenyl ligand bridged by a dimethylsilene
group:
Si
NTi
CH3
CH3
Hou et al (121)
reported the one-step block copolymerization of ethylene and styrene,
using a C5Me5/ER ligated Sm(II) complex. They prepared copolymers with Mn = 130
000 – 160 000 g/mol and claim that NMR data reveals that the copolymers are block
copolymers.
Venditto et al (144)
also reported the preparation of stereoregular E/S copolymers using
rac-En(Ind)2ZrCl2, while atactic copolymers were produced using CpTiCl3.
Ethylene/norbornene copolymers were reported polymerising with homogeneous
catalysts, iPr[(Ind)(Cp)]ZrCl2 (145)
.
Henschke et al (146)
used Me2Si(2-Me-BenzInd)2ZrCl2/MAO to polymerize
polystyrene macromers with polypropylene. The PS macromers were prepared by
anionic polymerization.
8.4 CATIONIC POLYMERISATION BY METALLOCENES
The highly electrophilic character of cationic metallocenes (Cp2MR)+ and their
behaviour as strong Lewis acids suggests that they may be able to mediate
polymerisation reactions by acting as carbcationic initiators. Commercially
isobutylene and isoprene uses an AlCl3/H2O initiator at –100°C. Carr et al (122)
35
reported the use of “halide free” Cp2*ZrMe2/B(C6F5)3 catalyst system which rapidly
polymerises IB and IP to high molecular weight at temperatures as high as –30°.
Barsan et al (123)
also reported the polymerization of isobutylene and isoprene using a
Cp*TiMe2( -Me)B(C6F5)3 catalyst system:
Ti
MeMe
C B
C6F5
C6F5
C6F5
H
H
H
They polymerised isobutylene in methylene chloride and toluene at temperatures
varying between –40 and –75°C, while IB/IP copolymerisations were carried out in
toluene.
8.5 LIVING POLYMERISATION OF ETHYLENE AND OLEFINS
Hagihara et al (124)
(OMNP4) describe the living polymerization of propene and 1-
hexene with Me2Si(t-BuN)(Flu)TiMe2/B(C6F5)3 as catalyst. Earlier work on living
systems were reported by Brookhart (125)
who reported the living polymerization of
propene, 1-hexene and 1-octadecene using a Ni(II)- -diimine catalyst system.
Scollard and McConville (126)
reported the living polymerization of 1-hexene, 1-octene
and 1-decene by chelating diamido complexes of Ti. Schrock, (127)
reported the living
polymerization of 1-hexene with a zirconium complex that contains a tridentate
diamido ligand.
Details on the Brookhart paper (125)
:
The authors used a Ni(II)- -diimine catalyst to prepare diblock and triblock -
olefins:
36
B(Ar')4-
+
N NAr Ar
Pd
CH3OEt2
R'
MAO
-10°C
Poly(propylene) Mn = 161 000, MWD = 1.13
Poly(1-hexene) Mn = 44 000, MWD = 1.09
During polymerization, a significant fraction of the -olefin insertions occur in a 2,1
fashion. Metal migration ensures a 1, enchainment. They also synthesized
propylene (P) with 1-hexene (H) and 1-octadiene (O) to form P-b-H and O-b-PrO-b-
O, the latter being an elastomer with Mn = 253 000.
The same catalyst type as the type used by Hagira et al also gives ethylene/styrene
copolymers (117)
. The same authors as above (128)
initially studied the syndiospecific
polymerization of propylene using the same catalyst, when they realised that chain
transfer to MAO was predominant. They then switched to a borate cocatalyst and
lowered the reaction temperature to –50°C to find the living system. PP with Mn = 20
000 g/mol, MWD = 1.15 and catalyst activity of 590 kg PP/mol Ti.h were reported.
For poly(1-hexene), low activity (0.152kg polymer/mol Ti.h), Mn = 26 000 g/mol and
MWD = 1.10 were found.
In 1996, Scollard and McConville (126)
reported the living polymerisation of -olefins
by chelating diamide complexes of Titanium. The catalysts of the type:
N
N
Ti
R
R
CH3
CH3B(C6F5)3
R'
23°C
R
x
1 a, b a R = 2,6-iPr2C6H3
b R = 2,6-Me2C6H3
R' = n-Bu, n-Pr, n-Hex
were investigated. Previously the same catalyst, when activated by MAO was shown
by these authors to be highly active in the polymerization of 1-hexene. They then
replaced the MAO as cocatalyst with a borate anion, which resulted in the living
37
polymerization of -olefins at room temperature. Fairly high activities (around 750kg
polymer/mol catalyst.h), Mn values of up to 160 000 and consistent MWD values of
1,05 to 1.09 were achieved.
In 1997, Baumann et al (127)
reported on the Ti and Zr complexes that contain the
tridentate ligand:
O
NH NH C CH3
CD3
CD3
CH3C
CD3
CD3 O
N N
TiN N
tBu d6 tBu d6
O
N N
TiCl Cl
tBu d6 tBu d6
O
N N
TiMe Me
tBu d6 tBu d6
SiMe3Cl
MeMgCl
([NON]2-
) and the living polymerization of 1-hexene by activated [NON]ZrMe2.
These catalyst complexes can be activated by B(C6F5)3. These catalysts were shown
to be active for ethylene polymerization (activity 800 kg polymer/mol catalyst.h) and
1-hexene (a = 200 kg polymer/mol catalyst.h). Molecular weight for the poly(1-
hexene) is reported to be around 45 000 g/mol and the MWD = 1.2.
Nomura et al (129)
used analogous catalysts for the homopolymerization of ethylene
and the copolymerization of ethylene with 1-butene, as well as the copolymerization
of ethylene with 1-hexene (130)
:
38
N
N
TiCl2
R
R
S
O O
Ti
Cl Cl
R = Si iPr3 (a)
R = Si tBu3 (b)
R = SiMe3 (c)
These catalysts were used with MAO and borates as cocatalysts.
8.6 OLEFIN ELASTOMERS OF BY METALLOCENE CATALYSIS
8.6.1 “EPDM” ELASTOMERS
EPDM is the most important commercial polyolefin elastomer. Normally it is a
copolymer comprising a ethylene/propylene ratio of 55:45, and a third component,
either 1,5 hexadiene or 5-ethylidine norbornene. Polymers normally have a Tg of less
than –45°C. A new generation of butyl rubber was recently commercialized by
Exxon, comprising isobutylene copolymerized with several % para-methyl styrene.
This yields a completely saturated elastomer, with the “active” methyl group allowing
crosslinking reactions through the benzylic hydrogens. Chung et al (131)
have made
copolymers of ethylene and pMS. In 1998, they reported (132)
the synthesis of new
polyolefin elastomers. These were ethylene/propylene/para-methyl styrene and
ethylene/1-octene/para-methyl styrene terpolymers, using a constrained geometry
catalyst/MAO cocatalyst. The octene containing terpolymers show promise as the
presence of the octene allows the incorporation of more pMS while still maintaining
a Tg of below –45°C.
8.6.2 ELASTOMERIC POLYPROPYLENE
The first elastomeric polypropylene (ePP) was generated by a C1-symmetric bridged
titanocene and was described by Chien et al. (catalyst 1) Gauthier et al (146)
prepared
catalysts 2 – 6, and showed that only the hafnocenes 4 and 6 produced ePP.
39
In 1995 Waymouth and Coates (147)
reported on the production of stereoblock PP with
a non-bridged metallocene, bis (2-phenylindenyl)ZrCl2:
ZrCl
ClZr Cl
Cl
Isotactic blocks Atactic blocks
The catalyst is able to switch its coordination geometry from aspecific to isospecific
during the cause of polymerisation in order to generate isotactic and atactic blocks. A
series of papers by the same authors followed (148-150)
. Amongst others some of the
catalysts they evaluated for the production of ePP were the following:
40
ZrCl2 ZrCl2ZrCl2
ZrCl2 ZrCl2 ZrCl2
ZrCl2 ZrCl2
All of these catalysts produced essentially amorphous PP.
41
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