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  • 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

    95C 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.

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