WO1992012184A1

WO1992012184A1 - Syndiotactic polypropylene

Info

Publication number: WO1992012184A1
Application number: PCT/US1991/009795
Authority: WO
Inventors: Masahide Murata; Akio Yamori; Ferdinand Christian Stehling; Howard William Turner; Terry John Burkhardt
Original assignee: Exxon Chemical Patents Inc.
Priority date: 1990-12-28
Filing date: 1991-12-30
Publication date: 1992-07-23
Also published as: AU9159191A

Abstract

Syndiotactic polypropylene is formed by polymerizing propylene in the presence of a catalyst system at a temperature of no greater than about 0 °C. The catalyst system includes a structurally rigid metallocene catalyst and a cocatalyst. The syndiotactic polypropylene so produced has a polymer chain having at least 85 % racemic dyads. In the polymer chain, the racemic dyads are connected predominantly by meso dyad units.

Description

SYNDIOTACTIC POLYPROPYLENE

BACKGROUND OF THE INVENTION

The present invention relates to a process for producing syndiotactic polypropylene. As is 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 (SPP) was first disclosed in US-A-3,258,455 (Natta) as being obtained using a catalyst prepared from titanium trichloride and diethyl aluminum monochloride. US-A-3,305,538 (Natta), discloses the use of vanadium triacetylacetonate or halogenated vanadium compounds in combination with organic aluminum compounds for producing syndiotactic polypropylene. US-A-3,364,190 (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.

As is 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, e.g., the methyl groups are all above or all below the plane. Using the Fischer projection formula the stereochemical sequence of isotactic polypropylene is described as follows:

Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is ...mmmm... with each "m" representing a "meso" dyad of successive methyl groups on the same side of the plane. As known in the art, any deviation or inversion ("defect") in the structure of the chain lowers the degree of isotacticity and ciystallinity of the polymer.

In 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:

In NMR nomenclature, this pentad is described as ...rπr... in which each V represents a "racemic" dyad, i.e. successive methyl groups on alternate sides of the plane. The percentage of V dyads in the chain determines the degree of svndiotacticitv of the polymer and therefore the ciystallinity of the polymer. This ciystallinity distinguishes both syndiotactic and isotactic polymers from atactic polymers. Atactic polymers exhibit no regular order of repeating unit configurations in the polymer chain and form essentially a waxy or non-crystalline product.

Syndiotactic polypropylene produced according to US-A-

3,305,538 and 3,258,455 (Natta), wherein a vanadium based catalyst system is employed, has the following structure:

or in NMR nomenclature ...rrrrrmrrrrr.

Laboratory experiments have found that the syndiotacticity of this type of polymer is in the order of 70% racemic pentad or less. The use of traditional vanadium based catalysts as described by Natta et al., in J. Am. Chem. Soc.__84, 1488 (1962) or Doi et al. in Macromolecules, J2, 814 (1979), produces gummy, sticky product. The table below indicates the tacticity of polypropylene produced employing a traditional vanadium based catalyst system.

* Tris(2-methyl-l,3-butanedionato) vanadium catalyst with diethyl aluminum chloride cocatalyst was employed maintaining polymerization temperature at -60° C. The racemic pentad distribution of the polymer was found to be in the order or 36% with racemic dyad sequences at about 78% of the total polymer.

The problem with the above described prior art processes is that they either produce syndiotactic polypropylene in low yield or with a significant number of defects that prevent formation of a crystalline polypropylene. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process which produces a more crystalline syndiotactic polypropylene having improved properties.

The present invention provides a process for forming syndiotactic polypropylene that comprises polymerizing propylene in the presence of a catalyst system at a temperature of no greater than about 0^β C. The catalyst system comprises a structurally rigid metallocene catalyst and a cocatalyst . The crystalline syndiotactic polypropylene so produced has a polymer chain having greater than 70% crystalline racemic pentads and at least 85% racemic dyads. In the polymer chain, the racemic dyads are connected predominantly by meso dyad units.

DETAILED DESCRIPTION OF THE INVENTION

It has been determined that two types of polypropylene microstructure are typically formed during the polymerization of propylene when employing a structurally rigid metallocene catalyst and a cocatalyst, preferrably an aluminum cocatalyst; that is (1) polypropylene with a microstructure of predominantly meso triad connecting units ("mistakes" or "defects"), and (2) polypropylene with a microstructure of predominantly meso dyad connecting units.

Trace amounts of other units are also formed during the polymerization reaction. When employing the catalyst described herein, at polymerization temperatures of no greater than about 0* C, polymer (2) is formed in predominate amounts with the balance being of microstructure belonging to polymer (1).

The present invention provides a process to produce crystalline syndiotactic polypropylene consisting of blocks of racemic dyads connected predominantly by units of meso dyads. The meso dyads comprise greater than 50% of the total "defects" or mistakes produced in the PP chain, and more preferably, comprise greater than 90% of the total mistakes formed during polymerization. As described in NMR nomenclature, the structure of the polymer chain is ...rrrrmrirr.... When made by the inventive process, the polymer chain consists of greater than 70% racemic pentads and at least 85% racemic dyads and is highly ciystalline. The syndiotacticity and melting points of the polymer product increases with decreasing polymerization temperature and the tacticity of the polymer formed contains greater than 70% ciystalline racemic pentad backbone. The polymer can be produced to varying specifications for melting point, molecular weight and molecular weight distributions, depending on the precise nature of the catalyst and polymerization conditions. For example, polymer can be produced which melts in the range of 140' C - 155" C, preferrably 142* - 152' C.

When propylene or other alpha-olefins are polymerized using a catalyst consisting of a transition metal compound, the polymer product typically comprises a mixture of amorphous atactic and ciystalline xylene insoluble fractions. The ciystalline fraction may contain either isotactic or syndiotactic polymer, or a mixture of both. In contrast, the catalyst useful in producing the polymers of the present invention are syndio-specific and produce a polymer with a high syndiotactic index.

The metallocene catalysts of the present invention may be described by the formula R"(CpR_n) (CpR'_m)MXq, where each Cp is a cyclopentadienyl or substituted cyclopentadienyl ring; R is a structural bridge between the two Cp rings imparting stereorigidity to the Cp rings, and R is preferably selected from the group consisting of an alkyl radical having 1-4 carbon atoms or a hydrocarbyl radical containing silicon, germanium, phosphorus, nitrogen, boron, or aluminum; M is a group IVb, Vb, or Vlb metal from the Periodic Table of Elements; each X is a hydrocarbyl radical having 1-20 atoms or is a halogen; and 0_< q_< 3, 0_< n_< 4, 1 _< m _< 4, and each R_n and R _m having 1-20 carbon atoms; with the proviso in order to be syndio-specific, the Cp rings in the metallocene catalysts must be substituted in a substantially different manner so that there exists a steric difference between the two Cp rings. Therefore, R^» _m is selected such that (CpR' m) is a substantially different substituted ring than (CpR_n). In order to produce a syndiotactic polymer, the characteristics of the groups substituted directly on the cyclopentadienyl rings seem to be important Thus, by "steric difference" or "sterically different" as used herein, it is intended to imply a difference between the steric characteristics of the Cp rings that controls the approach of each successive monomer unit that is added to the polymer chain to produce the syndiotactic configuration.

In a preferred catalyst useful in producing polymers of the present invention, M is titanium, zirconium, or hafnium, most preferably zirconium; X is preferably a halogen, and it is most preferably chlorine; and q is preferably 2, but it may vary with the valence of the metal atoms. Exemplary hydrocarbyl radicals include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, amyl, isoamyl, hexyl, heptyl, octyl, nonyl, decyl, cetyl, phenyl, and the like. Other hydrocarbyl radicals useful in the metallocene catalysts include other alkyl, aryl, alkenyl, alkylaryl or arylalkyl radicals. Further, R_n and R' may comprise hydrocarbyl radicals attached to a single carbon atom in the Cp ring as well as radicals that are bonded to more than one carbon atom in the ring.

The catalyst may be prepared by any method known in the ar Two methods of preparing the catalyst are disclosed herein with the second method being preferred as it produces a more stable and active catalyst. It is important that the catalyst complex be "clean" as usually low molecular weight, amorphous polymer is produced by impure catalysts. Generally, the preparation of the catalyst complex consists of forming and isolating the Cp or substituted Cp ligands which are then reacted with a halogenated metal to form the complex.

Preparation of the Catalyst - Method A

The synthesis procedures for the catalyst were performed under an inert gas atmosphere using a Vacuum Atmospheres glove-box or Schlenk technique. The synthesis process generally comprises the steps of 1) preparing the halogenated or alkylated metal compound, 2) preparing the ligand, 3) synthesizing the complex, and 4) purifying the complex.

In Method A, the halogenated metal compound was prepared using tetrahydrofuran ("THF') as a solvent, resulting in THF being bound in with the final catalyst complex. Specifically, MCL, THF was prepared as described in Manzer, L.. Inorg. Synth.. 21,135-36 (1982). In the Examples below, M is zirconium and hafnium, but it may also include titanium or other transition metals. The substituted dicyclopentadienyl ligand may be prepared using various processes known in the art depending upon the selection of the specific bridge or ring substituents. In the preferred embodiments shown in the Examples below, the ligand is 2,2-isopropyl(fluorene)cyclopentadiene. To prepare this ligand. 44 gms (0.25 mol) of fluorene were dissolved in 350 ml THF in a round bottom flask equipped with a side arm and dropping funnel. Contained within the funnel were 0.25 mol of methyl lithium (CFfeLi) in ether (1.4M). The CH3U was added dropwise to the fluorene solution and the deep orange-red solution was stirred for several hours. After gas evolution had ceased, the solution was cooled to -78* C and 100 ml of THF containing 26.5 gms (0.25 mol) of 6,6-dimethylfulvene was added dropwise to the solution. The red solution was gradually warmed to room temperature and stirred overnight. The solution was treated with 200 ml of water and stirred for ten minutes. The organic fraction of the solution was extracted several times with 100 ml portions of diethylether, and the combined organic phases were dried over magnesium sulfate. Removal of the ether from the organic phases left a yellow solid which was dissolved in 500 ml of chloroform and recrystallized by addition of excess methanol at 2* C to yield a white powder.

Catalyst complexes produced in accordance with Method A are noted to be somewhat impure and extremely air and moisture sensitive. As a result, further purification is necessary.

Catalyst Synthesis Procedure - Method B

As an alternative synthesis Method B provides catalysts that are more air stable, more active, and produce a higher percentage of syndiotactic polypropylene. In this process, methylene chloride is used as a non-coordinating solvent. The process described below uses hafnium as the transition metal, but the procedure is adaptable for use with zirconium, titanium or other transition metals. The substituted dicyclopentadienyl ligand was synthesized in THF in the same manner as described in Method A above. The red dilithio salt of the ligand (0.025 mol) was isolated as disclosed in Method A by removing the solvents under vacuum and by washing with pentane. The isolated red dilithio salt was dissolved in 125 ml of cold methylene chloride and an equivalent amount (0.025 mol) of HfCl* was separately slurried in 125 ml of methylene chloride at -78° C. The HfCU slurry was rapidly cannulated into the flask containing the ligand solution. The mixture was stirred for two hours at -78* C, allowed to warm slowly to 25* C and stirred for an additional 12 hours. An insoluble white salt (LiCl) was filtered off. A moderately air sensitive, yellow powder was obtained by cooling the brown/yellow methylene chloride solution to -20* C for 12 hours and cannulating away the supernatant. The bright yellow product was washed on the sintered glass filter by repeatedly filtering off cold supernatant that had been cannulated back over it. The catalyst complex was isolated by pumping off the solvents using a vacuum, and it was stored under dry, deoxygenated argon.

Consistent with prior disclosure of metallocene catalysts for the production of isotactic polymer, the catalysts of the present invention are particularly useful in combination with an aluminum cocatalyst, preferably an aluminoxane, an alkyl aluminum, or a mixture thereof. In addition, a complex may be isolated between a metallocene catalyst as described herein and an excess amount of aluminum cocatalyst in accordance with the teachings of EP-A- 226,463. The aluminoxanes useful in combination with the catalysts of the present invention may be represented by the general formula R-A1-0- in the cyclic form and R(R-Al-0)_n-AlR2 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, although useful, aluminoxanes are not limited to those of this formula. Most preferably R is a methyl group. The aluminoxanes 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 benzene. The water may also be contained in a carrier such as silica. Another preferred method includes the preparation of aluminoxane in the presence of a hydrated copper sulfate as described in US-A-4,404,344. 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

EP-A-351,392 (Ewen e al) discloses SPP having a polymer chain consisting of blocks of repeating racemic dyads (preferably with at least 85% racemic dyads) which are connected predominantly by meso triad units, preferably at least 40% of the total mistakes produced in the PP chain are such meso triads. Such an SPP is disclosed in EP 351.391 as being produced at 20^c C polymerization temperature in order to achieve the required meso triad defect. The present invention employs a process for forming SPP having 70% racemic pentads and/or at least 85% or more racemic dyads connected by meso dyads wherein the meso dyads comprise 50% or more of the total mistakes or defects formed during the polymerization reaction. It has been discovered that site control defect i.e. production of meso dyad versus triad defects, is directly related to polymerization temperature. In order to achieve the predominantly meso dyad defect structure disclosed herein it is important to maintain the polymerization temperature at between 0* C - ^"80* C or less, preferably at ^"30 - ^"70* C

Polymerization temperatures greater than about 10* C produce a SPP with predominantly meso triad defects as disclosed in EP-A-351,391.

The SPP produced by the inventive process differs from the prior art and especially that disclosed in EP-A-351,391 by the predominance of meso dyad defects formed when polymerized at less than or equal to O' C and by its extreme ciystallinity. SPP produced by the present inventive process contains about 70% or more racemic pentads when compared to that produced by Natta or Doi processes which employ vanadium based catalyst systems. This tacticity in turn leads to a more crystalline polymer than previously made. The following non-limiting examples illustrate the preparation of polymers of the present invention and the polymers' various advantages in more detail. The polymerization conditions expressed in the examples have been normalized or adjusted so that productivity of the catalyst system is sufficient to produce enough polymer to permit analysis. It is common, general knowledge that at increased temperatures, catalyst activity is maximized. Therefore at lower temperatures, a need exists to increase the amount of catalyst to obtain approximately the same amount of polymer product. It is believed that the difference in microstructure of SPP produced by the inventive process versus prior art procedures is due to polymerization temperature differences and not to concentration of catalyst or monomer differences. EXAMPLE 1

A catalyst system comprising 0.055 grams of isopropyl (fluorenyl)(cyclopentadienyl) zirconium dichloride, 5 cc of IM methylaluminoxane, and 2 ml of toluene were combined and placed in a 75 ml pressure vessel. This was added to a second pressure vessel (500 ml) containing 20 cc of IM methylaluminoxane and then was cooled to -60" C in a chlorofoπn/diy ice bath and stirred for 30 minutes. Propylene vapor was slowly introduced into the pressure vessel unit and a pressure of 12-14 psi was reached. The flask was then stirred for 5 hours. The polymer yield after deashing with HC1 was 0.40 grams. The polymer contained a polymer chain with greater than 70% racemic pentads, at least 85% racemic dyads, connected by blocks of meso dyads comprising greater than 78% of the total mistakes of the PP formed.

EXAMPLE 2

The procedure described in Example 1 was carried out using only 0.020 grams of iso-propyl (fluorenyl) (cyclopentadienyl) zirconium dichloride. The yield was 0.1 g of product. The polymer produced had the same characteristics as that of Example

1. The reduced catalyst amount did not affect the microstructure.

EXAMPLE 3

A catalyst system comprising 10 mg of isopropyl-

(fluorenyl)(cyclopentadienyl)zirconium dichloride and 20 cc. of methylaluminoxane in toluene were combined. This mixture was added to a pressure vessel (1.5 1 sas auto clave) containing 400 cc of liquid propylene monomer at 0" C. The polymer was purged of unreacted monomer and vacuum dried. The polymer produced had a polymer chain containing greater than 70% racemic pentads, at least 85% racemic dyads connected by blocks of meso dyad units comprising greater than 60% of the total defects formed in the polymer. MP: TMi = 151" C, TM2 = 144" C. Comparative Examples Example A

The procedure as described in Example 3 was carried out utilizing 5 mg of catalyst, 10 cc of methylaluminoxane and a polymerization temperature of 10" C. The polymer produced had a polymer chain as per Example 3 however the meso dyad units comprised approximately 52% of the total defects formed in the polymer. MP: TMi = 149" C, TM2 = 140" C.

Example B The procedure as described in Example 3 was carried out utilizing 2 mg of catalyst, 5 cc of methylaluminoxane, 500 cc of liquid propylene and a polymerization temperature of 25" C. The polymer produced had a polymer chain as per Example 3 however the meso dyad units comprised approximately 56% of the total defects formed in the polymer. MP: TMi = 149" C. TM2 = 140" C.

Example C The procedure as described in Example 3 was carried out utilizing 3.5 mg of catalyst, 5 cc of methylaluminoxane, 500 cc of liquid propylene and a polymerization temperature of 50" C. The polymer produced had a polymer chain as per Example 3 however the meso dyad units comprised approximately 36% of the total defects formed in the polymer. MP: TMi = 134" C, TM2 = 124" C

Claims

1. A process for producing syndiotactic polypropylene by polymerizing propylene in the presence of (1) a structurally rigid metallocene catalyst, said catalyst having one substituted cyclopentadienyl group and one substituted or unsubstituted cyclopentadienyl group, the two cyclopentadienyl groups being sterically different and (2) a cocatalyst, characterized in that polymerization is carried out at a temperature of not greater than 0" C and in that the polypropylene produced is highly crystalline and syndiotactic comprising a polymer chain at least 85% of which is repeating blocks of racemic dyads and/or at least 70% of which is repeating blocks of racemic pentads, the blocks being connected by defect structural units of predominantly meso dyads.

2. The process of claim 1 wherein the metallocene catalyst is represented by the following formula:

R' ^• (CpRn)(CpR' m) Xq

Wherein Cp is a cyclopentadienyl or substituted cyclopentadienyl ring; R¹ ' is a structural bridge between the two Cp rings imparting stereorigidity thereto;

M is a group IVb, Vb, VIb metal; X is a hydrocarbyl radical having 1-20 carbon atoms or is a halogen; 0_< q_< 3, 0.< n_< 4, l < m_< 4; and R and R' are independently radicals having 1-20 carbon atoms; provided that (CpR_n) and (CpR _m) are selected to be sterically different

3. The process of any of the preceding claims wherein an aluminum containing cocatalyst is used.

4. The process of claim 3 wherein the cocatalyst is an aluminoxane, an alkyl aluminum or a mixture thereof.

5. The process of claims 2, 3 or 4 wherein R¹ * is an alkyl radical having 1-4 carbon atoms or a hydrocarbyl radical containing silica, germanium, phosphorus, nitrogen, boron or aluminum.

6. The process of any preceding claim wherein the metallocene is a zirconocene or a hafhocene.

7. The process of any of claims 2- 6 wherein MXq is ZrC .

8. The process of any of the preceding claims wherein the catalyst system comprises iso-propyl (fluorenyl)(cyclopentadienyl)zircomum dichloride and methylalumoxane.

9. The process of any preceding claim wherein polymerization is carried out at a temperature of -60" C or less.

10. The process according to any of the preceding claims wherein in the syndiotactic polypropylene produced, the meso dyads connecting the blocks comprise greater than 50% of the total defects produced in the chain.

11. The process according to claim 10 wherein the meso dyads connecting the blocks comprise greater than 60%, preferably greater than 78% of the total defects produced in the chain.

12. The process according to any preceding claims wherein the syndiotactic polypropylene produced melts in the temperature range of from 140" C - 155" C, preferably 142 - 152" C.