AU5450401A - Syndiotactic/atactic block polyolefins, catalysts and processes for producing the same - Google Patents

Description

P/00/011 Regulation 3.2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT o e oooo e ee a a. ee..

a e Invention Title: Syndiotactic/atactic block polyolefins, catalysts and Processes for producing the same The following statement is a full description of this invention, including the best method of performing it known to us: Freehills Carter Smith Beadle Melbourne\003858353 Printed 19 July 2001 (11:34) page 2 Freehills Carter Smith Beadle Melbourne\003858353 Printed 19 July 2001 (11:34) page 2 1 SYNDIOTACTIC/ATACTIC BLOCK POLYOLEFINS, CATALYSTS AND PROCESSES

FOR

PRODUCING THE SAME FIELD OF THE INVENTION: The invention relates to a metallocene catalyst useful in preparing syndiotactic/atactic block polyolefins and to the polymers so obtained..

The catalyst comprises a bridged monocyclopentadienyl metallocene in which the cyclopentadienyl ring is substituted symmetrically with respect to the active coordination positions. The invention further includes a process of preparing syndiotactic/atactic block polyolefins that comprises the use of the disclosed catalyst.

The present invention also provides for a process for polymerizing olefins having three or more carbon atoms to produce polymers with syndiotactic/atactic stereochemical configurations. The catalyst and process of the invention are particularly useful in polymerizing propylene to form a syndiotactic/atactic block polypropylene.

9**o DESCRIPTION OF THE PRIOR ART: As known in the art, syndiotactic polymers have a unique stereochemical structure in which monomeric units having enantiomorphic configuration of the asymmetrical carbon atoms follow each other alternately and regularly in the macromolecular main chain. Syndiotactic polypropylene was first disclosed by Natta et al. in U.S. Patent Number 3,258,455. The Natta group obtained syndiotactic polypropylene by using a catalyst prepared from titanium trichloride and diethyl aluminum monochloride. A later patent to Natta et al., U.S. Patent Number 3,305,538, discloses the use of vanadium triacetylacetonate or halogenated vanadiumcompounds in combination with organic aluminum compounds for producing syndiotactic polypropylene.

U.S. Patent Number 3,364,190 to Emrick discloses a catalyst system composed of finely divided titanium or vanadium trichloride, aluminum chloride, a trialkyl aluminum and a Phosphorus-containing Lewis base as producing syndiotactic polypropylene. U.S. Patent No. 4,892,851 disclosed a metallocene catalyst for producing highly crystalline syndiotactic polyolefins.

As disclosed in these patent references and as known in the art, the structure and properties of syndiotactic polypropylene differ significantly from those of isotactic polypropylene. The isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, the methyl groups are all above or below the plane. using the Fischer ~projection formula, the stereochemical sequence of isotactic polypropylene is described as follows: I I SAnother way of describing the structure is through the use of NMR.

Bovey's NMR nomenclature for an isotactic pentad is mmmm... with each representing a "meso" dyad or successive methyl groups on the same S9...side in the plane. As known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer.

SIn contrast to the isotactic structure, syndiotactic polymers are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer. Using the Fischer projection formula, the structure of a syndiotactic polymer is designated as: I1 I I l In NMR nomenclature, this pentad is described as rrrr... in which each represents a "racemic" dyad, successive methyl groups on alternate sides of the plane.

1-r- The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene.

This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer that is soluble in xylene. Atactic polymer exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. While it is possible for a catalyst to produce all three types of polymer, it is desirable for a catalyst to produce predominantly syndiotactic or isotactic polymer with some atactic block fractions.

SUMMARY OF THE INVENTION The present invention provides a catalyst and process for preparing syndiotactic/atactic block polyolefins, and more particularly syndiotactic/atactic block polypropylene. The catalyst and process can each be adapted to produce a ""polymer with differing syndiotactic-/atactic block ratios.

In one form the invention provides a catalyst for the polymerization of olefins of general formula

R"(C

4 R' mC5C4R'n) XmeQ Wherein X is an hetero-atom ligand with one or two lone pair electrons selected from the elements of Group VA or VIA which can be substituted or nonsubstituted; (C 4 R'mC 5

C

4 R'n) is a fluorenyl or a cyclopentadienyl ring which is symmetrically substituted so that the ligand exhibits bilateral symmetry, R' is hydrogen or hydrocarbyl radical having from 1- 20 carbon atoms, a halogen, an alkoxy, and alkoxy alkyl or an alkylamino or alkylsilyl radical, each R' may be the same or different and m and n independently are 0, 1, 2, 3 or 4, with the proviso that the bilateral symmetry is maintained; R" is a structural bridge between X and the (C 4 R'mC 5

C

4 R' n) ring to impart stereorigidity; Q is a hydrocarbyl radical having 1- 20 carbon atoms or is a halogen; Me is titanium and Me can be in any of its theoretically possible oxidation states.

The term "symmetrically" shall mean that the local bilateral symmetry of the active polymerization sites is essentially maintained.

The present invention further provides a process for polymerizing an olefin monomer to form a syndiotactic/atactic block polyolefin comprising: a) selecting a catalyst according to any one of claims 1- 7 as described above; b) introducing the catalyst into a polymerization reaction zone containing an olefin monomer and maintaining the reaction zone under polymerization reaction 15 conditions; and c) extracting a syndiotactic/atactic block polymer.

In addition, a cocatalyst such as alumoxane may be introduced into the reaction zone. Further, the catalyst may also be pre-polymerized prior to introducing it into 20 the reaction zone and/or prior to the stabilization of reaction conditions in the reactor.

Metallocene catalysts are single site catalysts which, generally produce polymer having narrow molecular weight distribution.

While the present invention is mainly directed to c-olefin homopolymerization it is obvious that copolymerization with other olefins can be obtained.

The present invention also provides for a new polymer comprising alternating blocks of syndiotactic and atactic sequences, preferably long syndiotactic and short atactic sequences, most preferably comprising a fraction of syndiotactic triads (rr) of at least BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the invention and many of the attendant advantages thereof will be readily understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: Figure 1 shows the structure of a catalyst precursor, 2,7-bis-tert-butyl-fluorenyl-9-dimethylsylyl-tert-butyl-amido titanium dichloride.

Figure 2 shows the 1 H NMR spectrum of the S2,7-bis-tert-butyl-fluorenyl-9-dimethylsilyl-tert-butyl-aido titaniumdichloride DETAILED DESCRIPTION OF THE INVENTION The present invention provides for a new polymer comprising alternating blocks of syndiotactic and atactic sequences. The polymer of the invention possesses particularly long sequences of syndiotactic species and short sequences of atactic species. The syndio-/atactic ratio can easily be modified by changing the polymerization conditions and/or the catalyst structure within the invention. For example, as shown below, increasing the polymerization temperature for a given catalyst decreases the syndio-/atactic ratio. As also evidenced below, replacing a dimethylsilyl bridge by a diphenylsilyl bridge also decreases the syndio-/atactic ratio for identical polymerization conditions.

In the specific case of polypropylene, it has been noted that the new polymer has elastomeric properties. Without being bound by the theory, it may be considered that these properties are deriving from the combination of the crystalline syndiotactic part and the amorphous atactic part.

The polymer obtained has a molecular weight ranging from 100,000 to 1,000,000 depending on the polymerization conditions and on the catalyst used in the process. For example, as shown below, replacing a dimethylsilyl bridge by a diphenylsilyl bridge results in increased molecular weight under identical polymerization conditions.

The present invention provides a catalyst and process for the production Sof syndiotactic/atactic block polyolefins, particularly polypropylene.

The catalysts of the present invention produce a polymer with a syndictactic/atactic block microstructure.

The Applicants have now unexpectedly found that the catalyst of the invention can be used for the production of high molecular weight polyalphaolefin with stereoregular stereoirregular block microstructure.

When propylene or other alpha-olefins are polymerized using a catalyst consisting of a transition metal compound, the polymer product typically comprises a random mixture (reactor blend) of amorphous atactic and crystalline xylene insoluble fractions. The crystalline fraction may contain either isotactic or syndiotactic polymer or a mixture of both.

The catalysts of the present invention have syndio-/atactic specificity and produce polymeric chains with differing syndio-/atactic block ratios.

The metallocene catalysts of the present invention may be described by the formula

R"(C

4 R'mC 5

C

4 R'n) MeXQ wherein X is an hetero atom ligand with one or two lone pair electrons and selected from the elements of Group VA or VIA and is preferably nitrogen, phosphorus, oxygen or sulfur which can be substituted or not.

(C

4 R'mC 5

C

4 R'n) is a fluorenyl ring, preferably substituted symmetrically; each R' is hydrogen or hydrocarbyl radical having from 1-20 carbon atoms, a halogen, an alkoxy, and alkoxy alkyl or an alkylamino or alkylsilylo radical, each R' may be the same or different, m and n independently are 0, 1, 2, 3 or 4, and with the proviso that the bilateral symmetry is maintained; R" is a structural bridge between the hetero atom X and

(C

4 R'mCsC 4 R'n) ring to impart stereorigidity, and preferably is a silyl or hydrocarbyl biradical having at least one silicon or carbon atom to form the bridge; Q is a hydrocarbyl radical, such as an alkyl, aryl, alkenyl, alkylaryl or arylalkyl radical having 1-20 carbon atoms or is a halogen; Me is a Group IIIB, IVB, VB, or VIB metal as positioned in the Periodic Table of. Elements; the transion metal can adopt any of the possible oxidation states. Preferred metals for the catalysts of the invention are Ti, Zr, Hf. A particularly suitable metal is Ti.

(C

4

R'

4

C

5

C

4

R'

4 comprises hydrocarbyl biradicals bonded to two adjacent carbon atoms in a cyclopentadienyl ring to form a fused ring.

In order to obtain syndiospecificity the cyclopentadienyl or fluorenyl rings in the metallocene catalysts must be substituted in an essentially symmetric manner with respect to the active coordination positions so that the metallocene exibits bilateral symmetry at least around the active coordination site. It has been unexpectedly noted that if in the original syndiospecific catalysts described in US patent No.4,892,851 the non-substituted cyclopentadienyl is replaced by a heteroatom ligand with one or two lone pair electrons and selected from elements of Group V A or VI A, the stereospecificty of catalyst will be periodically changed from syndiospecificity to aspecificity during the polymerization giving rise to the formation of atactic blocks within the predominantly syndiotactic chain.

Bilateral symmetry is defined as the condition in which there is no substituents or one or more substituents on one side and no substituents or.one or more substituents on the other side in the same relative position such that a mirror image is formed from one side to another.

One preferred example of such a compound is 2,7 di-tert-butyl-9-fluorenyl dimethyl silyl, tert-butyl amido titanium dichloride.

An illustration of the ligands of this compound is shown below: i **ee Bilateral symmetry is illustrated by a plane bisecting the fluorenyl and the bridge resulting the right side of each ligand being a mirror image of its left side. The numerical positions of the fluorenyl ring represent the position of possible substituents on the fluorenyl ring; 9 being the position of the bridge.

Whilst not wishing to. be bound by a theory and without intending to limit the scope of the present invention as indicated by the claims, it is believed that during the polymerization reaction the growing polymer chain migrates from one coordination position to the other after each monomer insertion as the catalyst simultaneously isomerizes and enchains sequences of polymer of the syndiotactic structure; occasionally this mechanism is disturbed. by a haptocity change of the fluorenyl-transition metal bonding or through inversion of the chiral nitrogen center, leading to the loss of steric control and formation of short sequences of atactic blocks whithin a predominantly syndiotactic polymer chain. This mechanism differs from that put forward for syndiospecific catalysts such as the active species formed from isopropylidene [cyclopentadienyl-9-fluorenyll zirconiu m dichloride as disclosed in U.S. Patent no. 4,892,851 wherein the fluorenyl ring exhibits mainly pentahapto bonding, all five carbon atoms of the fluorenyl rings are coordinated with the zirconium atom.

wi n c e i c ni m a o It is believed that the catalysts of the present invention demonstrate a reversible transformation of syndiospecific sites to atactic specific sites through a haptocity change related stereoregulation/stereoderegulation mechanism.

In theory, the steric condition for a preferred orientation of the growing chain, adoption of one of the two possible conformation; intermittently disappears and the coordination and insertion of either of the propylene prochirai faces at both lateral coordination positions becomes accidental. This mechanism enchains atactic sequences in a predominantly syndiotactic polymer chain by a spontaneous change in haptocity of the bonding between the fluorenyl and the transition metal from pentahapto to tri- and possibly mono-hapto bonding which results in the loss of stereoregulating property of the sites.

oo This electro-dynamic behavior of the active species provides in effect a temporary transformation of enantiotopic, equivalent sites to non-enantiotopic, non-equivalent environment at the lateral coordination positions, atactic sequences being formed whenever this occurs.

Theoretically, only when both events, periodic loss of stereoregularity and the stereospecific character of the catalyst occur simultaneously, is a syndiotactic/atactic blok homopolymer formed.

When catalysts of the present invention are used in polymerization of olefins, the polymer which results is of a syndio-/atactic microstructure as illustrated below: In a preferred embodiment of the catalyst of the present invention

(C

4 R'mC 5

C

4

R'

n is preferably a substituted cyclopentadienyl ring with bulky substituents in 2 and 7 or positions combined 1,8 or 3,6 or positions are also envisageble) such as tert-butyl and trimethylsilyl;

(C

4 R'mCsC 4 R'n) is more preferably a substituted fluorenyl radical; Me is preferably titanium, zirconium or hafnium, more preferably zirconium or titanium and most preferably titanium; Q is preferably a halogen and is most preferably chlorine; and R" is preferably a silyl or hydrocarbyl biradical of at least one silicon or carbon atom which is coordinated with the heteroatom ligand and with (C4R'mCsC 4 most preferably a silyl or hydrocarbyl biradical of at least one silicon or carbon atom being connected with heteroatom ligand and (C4R'mCSC 4 R'Rn) moiety.

Exemplary hydrocarbyl biradicals for the structural bridge include ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, and the like.

Other hydrocarbyl radicals useful as the structural bridge in the present catalysts include linear alkyl radicals of 1-10 carbon atoms or branched alkyl radicals of 1-20 carbon atoms, preferably one carbon atom, which may be substituted or unsubstituted, preferably substituted.

Exemplary silyl biradicals for structural bridge include dimethylsilyl (which is preferred), diphenylsilyl of the general formula

R

2 Si=, with each R independently being any CI-C 20 hydrocarbyl or C 1

-C

20 mono-, di- or trialkyl silyl radicals.

The catalyst may be prepared by any method known in the art. Generally, the preparation of the catalyst complex consists of forming and isolating the substituted cyclopentadienyl or fluorenyl ligands which are then reacted with a halogenated metal to form the complex. The preferred method is that disclosed in U.S. Patent nos. 4,892,851. The catalyst may further be supported on an organic or inorganic carrier. The synthesis process generally comprises the steps of preparing the halogenated or alkylated metal compound, preparing the ligand, (3) synthesizing the complex, and purifying the complex.

The metallocene catalysts of the present invention are useful in many of the polymerization processes (solution, slurry or gas phase) known in the art including many of those disclosed for the preparation of crystalline or amorphous polypropylene. When the catalysts of the present invention are used in these types of processes, the processes produce syndiotactic/atactic block polymers.

11 Further examples of polymerization processes useful in the practice of the present invention include those disclosed in U.S. Patent No.

4,767,735 and European Patent Application Publication No. 310,734, the disclosures of which are hereby incorporated herein by reference. These preferred polymerization procedures include the step of prepolymerizing the catalyst and/or precontacting the catalyst with a cocatalyst and an olefin monomer prior to introducing the catalyst into a reaction zone.

The ionizing agent is an alumoxane, an aluminum alkyl, other Lewis acid or a combination thereof which will ionize a neutral metallocene compound to form a cationic metallocene catalyst. Examples of such ionizing agents are methyl alumoxane (MAO), triethyl aluminum (TEA1) and .o tris(pentafluorophenyl)boron. Other ionizing agents are disclosed in oEuropean Patent Publication Nos. 277003 and 277004 which are hereby incorporated by reference.

The syndio-/atactic-specific catalysts of the present invention are particularly useful in combination with an aluminum alkyl cocatalyst or with an alumoxane, preferably with the latter.

In addition, a complex may be isolated between a metallocene catalyst as described herein and an aluminum cocatalyst in accordance with the teachings of European Patent Publication Number 226,463. As disclosed therein, a metallocene is reacted with an excess of alumoxane in the presence of a suitable solvent. A complex of the metallocene and alumoxane may be isolated and used as a catalyst in the present invention.

The alumoxanes useful in combination with the catalysts of the present invention, either in the polymerization reaction or in forming the complex disclosed above, may be represented by the general formula in the cyclic form and R(R-Al-0-)nALR 2 in the linear form wherein R is an alkyl group with one to five carbon atoms and n is an integer from 1 to about 20. Most preferably, R is a methyl group and the preferred alumoxane is methylalumoxane (MAO). The alumoxanes can be represented structurally as follows: 12

H

3

CH

3 Al 0-Al-0 Al

CH

3 CH 3 n

CH

3 n=15-20 The alumoxanes can be prepared by various methods known in the art.

Preferably, they are prepared by contacting water with a solution of trialkyl aluminum, such as trimethyl aluminum, in a suitable solvent *such as a benzene. Another preferred method includes the preparation of alumoxane in the presence of a hydrated copper sulfate as described in the U.S. Patent Number 4,404,344 the disclosure of which is hereby incorporated by reference. This method comprises treating a dilute solution of trimethyl aluminum in toluene with copper sulfate. The preparation of other aluminum cocatalysts useful in the present invention may be prepared by methods known to those skilled in the art.

13 The Examples given below illustrate the present invention and its various advantages and benefits in more detail.

Example 1 The synthesis procedure was performed under an inert gas atmosphere using a Vacuum Atmospheres glovebox or Schlenk techniques.

Step 1 Preparation of the 2,7-di-tert-butylfluorenyl-dimethylsilyl-chloride (1) a) Aromatization of the fluorene: In a one liter flask equipped with magnetic stirring bar, nitrogen inlet and reflux condenser, there was added a solution of 0.1 mol 2 7 -di-tert-butylfluorene in 300 cc of diethyl ether. To this solution was added at room temperature dropwise 0.1 equimol of methyllithium in ether (1.6 molar). The reaction was completed after gas evolution stops. The orange solution was used in the next step.

b) Reaction with dimethyldichlorosilane The orange solution prepared in step la was added dropwise to a solution of 0.1 mol of :'ooo dimethyldichlorosilane in 200 cc of ether. The mixture was stirred for several hours at room temperature until the reaction was completed.

Step 2 Preparation of tert-butyllithiumamide In a one liter flask equipped with magnetic stirring bar, nitrogen inlet and reflux condenser 0.1 mol of tert-butylamine was dissolved in 200 cc of diethyl ether. The solution was cooled to -78"C. One equimol of methyllithium in ether was added dropwise to the solution. The temperature was increased to room temperature slowly. The reaction mixture was stirred for several hours at room temperature until the formation of the title compound was complete.

Step 3. preparation of 2, 7 -di-tert-butylfluorenyl-dimethylsilyl-tert-butylamine 14 The reaction products prepared in step 2 and step 3 were added together and stirred for several hours at room temperature. The LiCl formed as byproduct was filtered off and the ether filterate was subjected to evaporation. The title compound was obtained as a yellow oil.

Step 4. Preparation of 2,7-di-tert-butyl-dimethyl-tert-butylamido titanium dichloride a) Formation of dianion 0.25 mol of 3 was dissolved in 200 cc of diethylether. To this solution was added dropwise 0.5 mol of methyllithium in ether. The solution became red. The reaction was stopped after gas evolution had stopped oo..

b) Reaction of the dianion with TiC14 0 The red powder obtained after evaporation of the ether from the solution .04. resulting from step 4b was washed several times with pentane and then reacted with 0.25 mol of TiC14 in 200 cc pentane. The mixture was stirred for several hours at room temperature and filtered to separate a the LiC1. Evaporation of the solvent led to the isolation of a brown powder. Recrystalization of the product from dichloromethane provided spectroscopically pure product (Cf Figure 2 HNMR) 2,7-di-er-butyl-fluorenyl-dimethysilyl-ert-butylamido-zirconium dichloride and the corresponding hafnium derivative were formed by repeating example .1 while in step 4b respectively ZrC14 or HfC14 were used instead of TiC1 4 Examples 4 to 6 Examples 1 to 3 were repeated while using diphenyldichlorosilane instead of dimethyldichlorosilane in step lb. The following metallocenes were formed 2, 7 -dicert-butylfluorenyldiphenylsilyltertbutyiamido titaniumdichloride 2, 7 -ditert-butylfluorenyldiphenylsilyltertbutylamido zirconiumdichloride 2, 7 -ditert-butylfluorenyldiphenylsilyltertbutylamido hafniumdichloride Examples 7-14 Propylene was polymerized using the metallocenes produced by examples 1 to Two liters of liquid propylene were introduced into a 4- liter bench reactor at room temperature. The metallocene was placed in three milliliters of a 11% solution of MAO in toluene to form a solution which *o* was added to the reactor and the temperature increased to 40'C. The polymerization reaction was allowed to run for 60 minutes during which time the reactor was maintained at the temperature indicated in Tab. 1 for each individual run. The reaction was terminated by venting the reactor of monomer. The catalyst activity in grams of polypropylene per grams of catalyst per hour was calculated. The molecular weight, molecular weight distrubution and 1 3 C NMR analysis of the polymer were determined. The results are shown in Table 1.

Example 7 polymerization of propylene with at 60 Figure 3 shows the C NMR spectrum of the syndiotactic/atactic block polypropylene; signals at about 15.3, 17.5, 31.2 and 35.7 may be due to long-chain branching.

Example 8 polymerization of propylene with at 16 Example 9 :Polymerization of Propylene with "4"1 at Example 10 polymerization of propylene with at 60 *C in 1 liter of cyclohexane Example 12. copolymerization of propylene, and ethylene with "14" at

.C

Example 12 polymerization of propylene with at Example 13 polymerization of propylene with at Example 14 polymerization of propylene with at The palymerization conditions and results are summarized in Table 1.

CATA Polym. Temp .~ye Acivt n fluf Mw flf .z flf *WD r* Table i 1:6 2-4 mr~ato 124 tn 5256 114 lt .2t7 631 a2.d 3.1 746 216 4 60 Bulk2 1.1 180 160000 98 317 831 3.2 235 79.86 16.3 60 Bulk 3.8 250 66000 1.1 19 -3.8 85.54 10.67 6 60 Bulk 4.2 2 952 7. 60 Bulk 2.4 45 18750 190 687. 1M 3.6 6.03 72.08 21.891 cC6: Cyohexane not datermined

Claims (12)

1. Catalyst for the polymerization ofolefins of general formula R"(C 4 R'mC 5 C 4 R'n) XmeQ Wherein X is an hetero-atom ligand with one or two lone pair electrons selected from the elements of Group VA or VIA which can be substituted or non- substituted; (C 4 Rm'C 5 C 4 Rn' is a fluorenyl or a cyclopentadienyl ring which is symmetrically substituted so that the ligand exhibits bilateral symmetry, R' is hydrogen or hydrocarbyl radical having from 1-20 carbon atoms, a halogen, an alkoxy, and alkoxy alkyl or an alkylamino or alkylsilylo radical, each R' may be the same or different and m and n independently are 0, 1, 2, 3 or 4, with the proviso that the bilateral symmetry is maintained; R" is a structural bridge between X and the (C 4 R'mC 5 C 4 R'n) ring to impart stereorigidity; Q is a hydrocarbyl radical having 1 20 carbon atoms or is a halogen; Me is a Group IIIB, IVB, VB, or VIB metal as positioned in the Periodic Table of Elements; 15 and Me can be in any of its theoretically possible oxidation states.

2. Catalyst according to claim 1 wherein (C 4 R'mCsC 4 R'n) is a substituted fluorenyl radical, preferably symmetrically substituted.

3. Catalyst according to claim 2 wherein the substituent of the fluorenyl radical is 2,7 di-tert-butyl-9-fluorenyl.

4. Catalyst according to claims 1 to 3 wherein the heteroatom ligand is selected from the group consisting of N, P, O and S. Catalyst according to claims 1 to 4 wherein Me is selected from the group consisting of Ti, Zr and Hf.

6. Catalyst according to claims 1 to 5 wherein R" is preferably a silyl or hydrocarbyl biradical having at least one silicon or carbon atom to form the bridge. 19

7. A catalyst according to any of claims 1- 6, wherein Q is selected from alkyl, aryl, alkenyl, alkylaryl and arylalkyl radicals.

8. A process for polymerizing an olefin monomer to form a syndiotactic/atactic block polyolefin comprising: a) selecting a catalyst according to any one of claims 1- 7 as described above; b) introducing the catalyst into a polymerization reaction zone containing an olefin monomer and maintaining the reaction zone under polymerization reaction conditions; and c) extracting a syndiotactic/atactic block polymer. o

9. A process according to claim 8, wherein the monomer is propylene. 15 10. Syndiotactic/atactic block polyolefin prepared according to the process of claim 8.

11. A syndiotactic/atactic block polyolefin of claim 10 being a homopolymer of an a-olefin having 3 or more carbon atoms wherein the fraction of the syndiotactic 20 triads is at least

12. A syndiotactic/atactic block homopolymer according to claim 10, wherein the a(-olefin is polypropylene.

13. A syndiotactic/atactic block copolymer of two a-olefins, comprising one or more blocks as defined in claim 11 or claim 12.

14. A catalyst as defined in claim 1 and substantially as hereinbefore described with reference to any one of the non-comparative examples. A process as defined in claim 8 for polymerizing an olefin monomer to form a syndiotactic/atactic block polyolefin substantially as hereinbefore described with reference to any one of the non-comparative examples. DATED: 11 July 2001 FREEHILLS CARTER SMITH BEADLE Patent Attorneys for the Applicant: FINA RESEARCH S.A. *ooo