WO2024133045A1

WO2024133045A1 - Process for producing a high-flow polypropylene homopolymer

Info

Publication number: WO2024133045A1
Application number: PCT/EP2023/086301
Authority: WO
Inventors: Jingbo Wang; Markus Gahleitner; Klaus Bernreitner; Pauli Leskinen
Original assignee: Borealis Ag
Priority date: 2022-12-23
Filing date: 2023-12-18
Publication date: 2024-06-27

Abstract

The present invention relates to a process for the production of a polypropylene homopolymer, the process comprising the steps of a) polymerizing in a first reactor propylene in the presence of a metallocene catalyst yielding a first polypropylene homopolymer fraction having a MFR2 of 1 to 50 g/10min measured according to ISO 1133, b) transferring the first polypropylene homopolymer fraction to a second reactor, c) polymerizing in the second reactor propylene in the presence of the first polypropylene homopolymer fraction yielding a second polypropylene homopolymer fraction, d) withdrawing the polypropylene homopolymer comprising the first polypropylene homopolymer fraction and the second propylene homopolymer fraction from the second reactor, wherein the polypropylene homopolymer has a MFR2 of 20 to 200 g/10min measured according to ISO 1133, wherein the metallocene catalyst comprises a metallocene complex and a support, wherein the support comprises silica, and wherein a split ratio between the first polypropylene homopolymer and the second polypropylene homopolymer is 20:80 to 80:20.

Description

Process for producing a high-flow polypropylene homopolymer

The invention relates to a process for the production of a polypropylene homopolymer, in particular to a metallocene-catalyzed high-flow polypropylene homopolymer having improved stiffness.

The need for polypropylene homopolymers with excellent stiffness at high flowability is constantly increasing as down-gauging and light-weighing become more important with the need for saving energy resources. High flowability polypropylenes are typically used in molding and particularly the automotive business where injection molding is the preferred conversion-process. Especially for glass fiber reinforced applications, high flow homopolymer with good mechanical properties and high thermal stability is required. Additionally, the addition of high flow homopolymers with excellent impact stiffness balance may allow the manufacturing of automotive compounds with an increased melt flow rate (MFR), without losing the needed mechanical performance.

From a multistage propylene polymerization process perspective, a challenge may be the balance of production rate in different reactors.

For example, in multistage polymerization processes using for example a two- reactor system, two important parameters to control are a reactor balance and a reactor split between the reactors. This is important from a plant economy point of view, but also from the product properties point of view.

Typically, the reactor split (also called “the production split”) in the two-reactor system is between 40/60 and 60/40%, and if a third reactor is used to produce an elastomer for a heterophasic copolymer, the reactor split in the last reactor is typically between 5 and 30%. In a four-reactor mode, two rubber gas phase reactors (GPR) are typically used, and the total split for manufacturing a heterophasic product can be between 10 and 40% in the last reactor, with the split between rubber GPRs being between 50/50 and 90/10%. In case of tri- modal process, typically, the first three reactors are producing homo PP or random PP and the split can be for example 45/35/20%.

However, in each reactor system described above one challenge remains the same, namely to get production split controlled according to the plant design. Otherwise, the plant is running on low speed, low production and the plant economy is thus worse.

An object of the present invention may therefore be, to provide a process for the production of a polypropylene homopolymer which overcomes the above mentioned challenges.

A further object of the invention may be to provide a process for the production of a high-flow polypropylene homopolymer, in particular a metallocene-catalyzed high-flow polypropylene homopolymer, having improved stiffness.

A further object of the invention may be to provide such a process, which is beneficial from the process point of view, e.g. enables an improved production and plant speed and thus improved reactor balance and plant economy.

The expression “homopolymer” as used herein relates to a polypropylene that consists substantially, i.e. of at least 99.5 wt.%, more preferably of at least 99.8 wt.%, of propylene units. In a preferred embodiment, only propylene units in the propylene homopolymer are detectable.

The “modality” of a polymer refers to the shape of its molecular weight (Mw) distribution curve, i.e. the appearance of the graph of the polymer weight fraction as function of its molecular weight. For example, when the polymer is produced in a sequential step process, utilizing reactors coupled in series and using different conditions in each reactor, the different fractions produced in the different reactors will each have their own molecular weight distribution. When the molecular weight distribution curves of these fractions are superimposed onto the molecular weight distribution curve of the total resulting polymer product, the later will show two or more maxima or at least be distinctly broadened in comparison with the curves for the individual fractions. Such a polymer product, produced in two or more serial steps, is called bimodal or multimodal, respectively, depending on the number of steps. In the following, all polymers produced in two or more sequential steps are called for simplicity “multimodal”. It is noted that also the chemical compositions of the different fractions may be different. It now has been surprisingly found that the above-mentioned objects can be achieved by a process for the production of a polypropylene homopolymer, the process comprising the steps of a) polymerizing in a first reactor propylene in the presence of a metallocene catalyst yielding a first polypropylene homopolymer fraction having an MFR2 of from 1 to 50 g/10min measured according to ISO 1133, b) transferring the first polypropylene homopolymer fraction to a second reactor, c) polymerizing in the second reactor propylene in the presence of the first polypropylene homopolymer fraction yielding a second polypropylene homopolymer fraction, d) withdrawing the polypropylene homopolymer comprising the first polypropylene homopolymer fraction and the second propylene homopolymer fraction from the second reactor, wherein the polypropylene homopolymer has an MFR2 of from 20 to 200 g/10min measured according to ISO 1133, wherein the metallocene catalyst comprises, or consists of, a metallocene complex and a support, wherein the support comprises, or consists of, silica, and wherein a split ratio between the first polypropylene homopolymer fraction and the second polypropylene homopolymer fraction is from 20 wt.%:80 wt.% to 80 wt.%:20 wt.%, wherein the metallocene complex is an organometallic compound (C), the organometallic compound (C) has the following formula (la):

(L)₂RnMX₂ (la) wherein

“M” is Zr or Hf; each “X” is a o-ligand; each “L” is an optionally substituted cyclopentadienyl, indenyl or tetrahydroindenyl;

“R” is SiMe2 bridging group linking said organic ligands (L);

“n” is 0 or 1 , preferably 1 .

In a preferred embodiment a process for the production of an isotactic polypropylene homopolymer is provided, the process comprising the steps of a) polymerizing in a first reactor propylene in the presence of a metallocene catalyst yielding an isotactic first polypropylene homopolymer fraction, having an MFR2 of from 1 to 50 g/1 Omin measured according to ISO 1133, b) transferring the first polypropylene homopolymer fraction to a second reactor, c) polymerizing in the second reactor propylene in the presence of the first polypropylene homopolymer fraction yielding an isotactic second polypropylene homopolymer fraction, d) withdrawing the polypropylene homopolymer comprising the first polypropylene homopolymer fraction and the second propylene homopolymer fraction from the second reactor, wherein the polypropylene homopolymer has an MFR2 of from 20 to 200 g/1 Omin measured according to ISO 1133, wherein the metallocene catalyst comprises a metallocene complex and a support, wherein the support comprises silica, and wherein a split ratio between the first polypropylene homopolymer fraction and the second polypropylene homopolymer fraction is from 20 wt.%:80 wt.% to 80 wt.%:20 wt.%, wherein the metallocene complex is an organometallic compound (C), the organometallic compound (C) has the following formula (la):

(L)₂RnMX₂ (la) wherein

“R” is SiMe2 bridging group linking said organic ligands (L);

“n” is 0 or 1 , preferably 1 .

The above objects may thus be solved by a moderate broadening of the by weight average molecular weight (Mw) of the polypropylene homopolymer product, i.e. a bimodal production. The moderate broadening and bimodal production of the polypropylene homopolymers means for example producing a higher Mw (i.e. lower MFR) polypropylene homopolymer in the first reactor and a lower Mw (i.e. higher MFR) polypropylene homopolymer in the second reactor.

The present invention may offer a number of advantages. The significantly better flowability may enable the use of the polypropylene homopolymers in molding applications, particularly injection molding applications. The excellent flowability may further be accompanied by high stiffness. High stiffness may be important for numerous polypropylene uses.

At the same time, the process according to the invention is beneficial in terms of economy of the plant and reactor balance. The fine tuning of a desired split between the reactors can be achieved by weight average molecular weight (Mw) or melt flow rate control, which in turn can be controlled by the hydrogen feed.

To illustrate the above, when for example, a high Mw polypropylene homopolymer is produced in the first reactor, a low amount of hydrogen is typically used to control the Mw and thus a lower reactivity is obtained. As consequence, a larger amount of catalyst is needed. In the second polymerization reactor a lower Mw polypropylene homopolymer is produced, thus a larger amount of hydrogen is needed which in turn accelerates the production and helping in split control. This is important since as a function of time a decay of the polymerization reaction is seen due to the catalyst living time.

In step a) propylene is polymerized in a first reactor in the presence of a metallocene catalyst yielding a first polypropylene homopolymer fraction, preferably a first isotactic polypropylene homopolymer, having an MFR2 of from 1 to 50 g/10min as measured according to ISO 1 133.

Preferably, step a) is carried out at a reactor temperature from 60 to 100 °C, more preferably from 65 to 90 °C, and most preferably from 70 to 80 °C.

Preferably, step a) is carried out at a reactor pressure of from 1 to 150 bar, more preferably from 35 to 60 bar, even more preferably from 40 to 55 bar, and most preferably from 43 to 52 bar.

The first polypropylene homopolymer fraction produced in step a) has preferably an MFR2 of from 5 to 40 g/10min, more preferably from 10 to 30 g/10min as measured according to ISO 1133.

Preferably, the first polypropylene homopolymer fraction has a xylene cold soluble content (XCS) determined according to ISO 16152 of from 0.1 to 5.0 wt.%, more preferably of from 0.5 to 4.0 wt.%, more preferably of from 1.0 to 3.0 wt.%, and most preferably of from 2.0 to 2.8 wt.%.

Step a) is preferably a slurry polymerisation step, i.e. the polymerization of the polypropylene monomer is carried out in a slurry.

The slurry polymerization in step a) is preferably a bulk polymerization process. By “bulk polymerization” is herein meant a process wherein the polymerization is conducted in a liquid monomer essentially in the absence of an inert diluent. However, as it is known to a person skilled in the art the monomers used in commercial production are never pure but always contain aliphatic hydrocarbons as impurities. For instance, the propylene monomer may contain up to 5 % of propane as an impurity. As propylene is consumed in the reaction and also recycled from the reaction effluent back to the polymerization, the inert components tend to accumulate, and thus the reaction medium may comprise up to 40 wt.% of other compounds than monomer. It is to be understood, however, that such a polymerization process is still within the meaning of “bulk polymerization”, as defined above.

Step a) of the process may be conducted in any known reactors such as loop reactors, stirred reactors or gas phase reactors. In case that step a) uses the slurry polymerization, and more preferably the bulk polymerization, then, step a) is preferably conducted in a continuous stirred tank reactor and more preferably in a loop reactor.

Preferably, the first reactor is a loop reactor. In such reactors, the slurry is circulated with a high velocity along a closed pipe by using a circulation pump. Loop reactors are generally known in the art and examples are given, for instance, in US-A-4582816, US-A-3405109, US-A-3324093, EP-A-479186 and US-A-5391654. It is thus preferred to conduct the first polymerization stage as a slurry polymerizations in a loop reactor.

Hydrogen is preferably used in the first polymerization step a) for controlling the MFR2 of the first polypropylene homopolymer fraction. The amount of hydrogen needed to reach the desired MFR2 depends on the catalyst used and the polymerization conditions, as will be appreciated by the skilled worker.

Preferably, in step a) a ratio of the feed of hydrogen to the feed of propylene is from 0.10 to 0.50 mol/kmol, preferably from 0.15 to 0.40 mol/kmol, and most preferably from 0.20 to 0.30 mol/kmol.

Preferably, step a) uses an average residence time of from 15 to 120 min, preferably from 20 to 80 min. As it is well known in the art the average residence time T can be calculated from equation (1 ) below:

T = — equation (1 ) Qo wherein

VR is the volume of the reaction space (in case of a loop reactor, the volume of the reactor, in case of the fluidized bed reactor, the volume of the fluidized bed, in gas of a gas phase reactor the volume of the gas phase reactor)

Qo is the volumetric flow rate of the product stream (including the polymer product and the fluid reaction mixture).

The production rate is suitably controlled with the catalyst feed rate. It is also possible to influence the production rate by suitable selection of the monomer concentration. The desired monomer concentration can then be achieved by suitably adjusting the propylene feed rate. In step b) the first polypropylene homopolymer fraction obtained in step a) is transferred to a second reactor, preferably directly transferred to a second reactor. Preferably, the first polypropylene homopolymer fraction is transferred to the second reactor in the form of a slurry. The slurry preferably comprises the first polypropylene homopolymer fraction, unreacted monomer and the metallocene catalyst.

The slurry may be withdrawn from the first reactor either continuously or intermittently. A preferred way of intermittent withdrawal is the use of settling legs where slurry is allowed to concentrate before withdrawing a batch of the concentrated slurry from the reactor. The use of settling legs is disclosed, among others, in US-A-337421 1 , US-A-3242150 and EP-A-1310295. Continuous withdrawal is disclosed, among others, in EP-A-891990, EP-A-1415999, EP-A- 1591460 and WO-A-2007/025640. The continuous withdrawal is advantageously combined with a suitable concentration method, as disclosed in EP-A-1310295 and EP-A-1591460. It is preferred to withdraw the slurry from the first reactor continuously.

Preferably, the slurry withdrawn from the first reactor is directly transferred into the second reactor to produce the second polypropylene homopolymer fraction. By “directly” it is meant that the slurry is introduced from the first reactor into the second reactor without any separation step in between, e.g. a flash separation step.

In step c) propylene is polymerized in the second reactor in the presence of the first polypropylene homopolymer fraction to obtain a second polypropylene homopolymer fraction, preferably an isotactic second polypropylene homopolymer fraction.

Step c) is preferably a gas phase polymerization step, i.e. step c) is carried out in a gas-phase reactor. Any suitable gas phase reactors known in the art may be used, such as preferably a fluidized bed gas phase reactor.

Preferably, step c) uses an average residence time of from 0.5 to 8 hours, more preferably 1 to 5 hours. Reference is made to equation (1 ) above.

The gas used will commonly be a non-reactive gas such as nitrogen or low boiling point hydrocarbons such as propane together with monomer, i.e. propylene. Preferably, the reactor temperature in step c) is in the range of from 60 to 90 °C, more preferably from 75 to 85 °C.

Furthermore, step c) is preferably carried out at a reactor pressure in the range of from 15 to 35 bar, more preferably from 20 to 30 bar.

Preferably, the reactor temperature in step c) is in the range of from 75 to 85 °C and step c) is carried out at a reactor pressure in the range of from 20 to 30 bar.

Preferably, the metallocene catalyst used in step a) is present in the second reactor during the polymerisation in step c). This is accomplished by transferring, preferably via slurry, the metallocene catalyst used in step a) into the second reactor. If needed, fresh metallocene catalyst may be added into the second reactor in step c).

A chain transfer agent, preferably hydrogen, is preferably added to step c). Preferably, in step c), a ratio of the feed of hydrogen to the feed of propylene is used of from 5.0 to 40.0 mol/kmol, preferably from 15.0 to 35.0 mol/kmol, and most preferably from 20.0 to 30.0 mol/kmol.

In the second reactor the second polypropylene homopolymer fraction is obtained. Preferably, the combined first polypropylene homopolymer fraction and the second polypropylene homopolymer fraction have a MFR2 of from 20 to 200 g/10 min, more preferably of from 30 to 100 g/10min, and more preferably of from 40 to 80 g/10 min as measured according to ISO 1133.

Preferably, the second polypropylene homopolymer fraction has a higher MFR2 than the first polypropylene homopolymer fraction.

In step d) the polypropylene homopolymer comprising the first polypropylene homopolymer fraction and the second propylene homopolymer fraction is withdrawn from the second reactor. The withdrawn polypropylene homopolymer comprising the first polypropylene homopolymer fraction and the second propylene homopolymer fraction can then be further processed, such as blending with additives.

Preferably, the process further comprises step e) of transferring the polypropylene homopolymer obtained in step d) into a third reactor downstream of the second reactor. In the third reactor, propylene and optionally one or more comonomer(s) selected from alpha-olefins having from 2 to 10 carbon atoms, more preferably from 4 to 10 carbon atoms can be polymerized. Preferably, the third reactor is a gas phase reactor.

Preferably, the propylene homopolymer obtained by the process according to the invention is an isotactic propylene homopolymer.

The polypropylene homopolymer has a MFR2 of from 30 to 100 g/10min, and more preferably of from 40 to 80 g/10 min as measured according to ISO 1 133.

Preferably, the propylene homopolymer has a xylene cold soluble content (XCS) determined according to ISO 16152 of from 0.1 to 4.0 wt.%, preferably of from 0.5 to 3.0 wt.%, more preferably of from 1 .0 to 2.5 wt.%, and most preferably of from 1.5 to 2.1 wt.%.

Preferably, the propylene homopolymer is a bimodal propylene homopolymer.

Preferably, the polypropylene homopolymer has a flexural modulus as determined according to ISO 178 of from 1500 to 1650 MPa, preferably of from 1550 to 1630 MPa.

Preferably, the polypropylene homopolymer has a melting temperature Tm determined by DSC according to ISO 11357 of 145.0 to 165.0 °C, preferably of 150.0 to 160.0 °C.

Preferably, the propylene homopolymer has a crystallization temperature T_c determined by DSC according to ISO 11357 of 100 to 140 °C.

Preferably, the split ratio between the first polypropylene homopolymer fraction and the second polypropylene homopolymer fraction is of from 30:70 to 70:30, more preferably of from 35:65 to 65:35. Split ratio is given in wt.%/wt.%.

Preferably, the first reactor is a loop reactor and the second reactor is a gas phase reactor.

A suitable process is the above-identified slurry-gas phase process, such as developed by Borealis and known as the Borstar® technology. In this respect, reference is made to the EP applications EP 0 887 379 A1 and EP 0 517 868 A1 .

Process steps a) to d) according to the invention discussed above may be preceded by a prepolymerisation step a’). Hence, the process according to the present invention preferably further comprises a prepolymerisation step a’) preceding step a), namely step a’) prepolymerizing propylene in the presence of the metallocene catalyst.

The purpose of the prepolymerisation is to polymerize a small amount of polymer onto the catalyst at a low temperature and/or a low monomer concentration, thus, controlling an initial catalyst fragmentation as well as a polymer particle growth rate. As a consequence, a smooth particle morphology may be assured, and local particle overheating, which eventually could result in particle agglomeration phenomena in the reactor, may be avoided. By controlling the above mentioned parameters, severe reactor operability issues and poor process performance could be avoided.

A further advantage of using a prepolymerisation step is that it may be possible to improve the performance of the catalyst, e.g. in particular in the slurry polymerization, and/or modify the properties of the final polymer.

The prepolymerisation step is preferably conducted as a slurry polymerization step. Preferably, the slurry polymerization is a bulk polymerization. The bulk polymerization is described above.

The reactor temperature in the prepolymerisation step is typically from 0 to 90 °C, preferably from 5 to 70°C, more preferably from 10 to 50 °C and most preferably from 15 to 30 °C.

The reactor pressure is not critical and is typically from 1 to 150 bar, preferably from 40 to 80 bar.

The average residence time in step a’) is preferably from 15 to 45 min. Reference is made to equation (1 ) above.

The amount of monomer is preferably such that from 0.1 to 1000 g of monomer per one gram of solid metallocene catalyst is polymerized in the prepolymerisation step.

As the person skilled in the art knows, the catalyst particles recovered from a prepolymerisation reactor do not all contain the same amount of prepolymer. Instead, each particle contains its own characteristic amount, which depends on the residence time of that particle in the prepolymerisation reactor, more specifically, on a residence time distribution effects (e.g., particle size, molecular properties, etc.) that are well manifested in polymerization reactors and in particular in continuous polymerization reactors. As some catalyst particles remain in the reactor for a relatively longer time and some for a relatively shorter time, then also the amount of prepolymer on different particles is different and some individual particles may contain an amount of prepolymer which is outside the above limits. However, the average amount of prepolymer on the catalyst typically is within the limits specified above.

Prepolymerizing propylene in the presence of the metallocene catalyst in step a’) yields a prepolymer. A prepolymer is a small amount of polymer produced at a low temperature and/or a low monomer concentration in the prepolymerisation reactor. The molecular weight of the prepolymer may be controlled by hydrogen as it is known in the art. Further, antistatic additives may be used to prevent the prepolymer particles from adhering to each other or the walls of the reactor, as disclosed in WO-A-96/19503 and WO-A-96/32420.

Preferably, the amount of prepolymer produced in the prepolymerisation step is from 1.0 to 5.0 wt.% in respect to the polypropylene homopolymer.

The polypropylene homopolymer is prepared in the presence of a metallocene catalyst, more preferably in the presence of at least one metallocene catalyst. A metallocene catalyst typically comprises a metallocene/activator reaction product impregnated in a porous support at maximum internal pore volume. The metallocene complex comprises a ligand which is typically bridged, and a transition metal of group IVa to Via, and an organoaluminium compound. The catalytic metal compound is typically a metal halide.

The metallocene catalyst according to the present invention may be any supported metallocene catalyst suitable for the production of isotactic polypropylene homopolymer.

Preferably, the metallocene catalyst consists of a metallocene complex and a support, wherein the support consists of silica. It is preferred that the metallocene catalyst comprises a metallocene complex, a co-catalyst system comprising a boron-containing co-catalyst and/or aluminoxane co-catalyst, and a support, preferably the support comprising or consisting of silica.

Examples of suitable metallocene compounds are given, among others, in EP 629631 , EP 629632, WO 00/26266, WO 02/002576, WO 02/002575, WO 99/12943, WO 98/40331 , EP 776913, EP 1074557 and WO 99/42497, EP2402353, EP2729479 and EP2746289.

The metallocene complex is ideally an organometallic compound (C) which comprises a transition metal (M) of Group 3 to 10 of the Periodic Table (IIIPAC 2007) or of an actinide or lanthanide. The term "an organometallic compound (C)" in accordance with the present invention includes any metallocene compound of a transition metal which bears at least one organic (coordination) ligand and exhibits the catalytic activity alone or together with a cocatalyst. The transition metal compounds are well known in the art and the present invention covers compounds of metals from Group 3 to 10, e.g. Group 3 to 7, or 3 to 6, such as Group 4 to 6 of the Periodic Table, (IIIPAC 2007), as well lanthanides or actinides.

In an embodiment the organometallic compound (C) has the following formula (I):

(L)mRnMXq (I) wherein

“M” is a transition metal (M) transition metal (M) of Group 3 to 10 of the Periodic Table (IUPAC 2007); each “X” is independently a monoanionic ligand, such as a o-ligand; each “L” is independently an organic ligand which coordinates to the transition metal “M”;

“R” is a bridging group linking said organic ligands (L);

“m” is 1 , 2 or 3, preferably 2;

“n” is 0, 1 or 2, preferably 1 ; “q” is 1 , 2 or 3, preferably 2; and m+q is equal to the valency of the transition metal (M).

“M” is preferably selected from the group consisting of zirconium (Zr), hafnium (Hf), or titanium (Ti), more preferably selected from the group consisting of zirconium (Zr) and hafnium (Hf).

In a more preferred definition, each organic ligand (L) is independently

(a) a substituted or unsubstituted cyclopentadienyl or a bi- or multicyclic derivative of a cyclopentadienyl which optionally bear further substituents and/or one or more hetero ring atoms from a Group 13 to 16 of the Periodic Table (IUPAC); or

(b) an acyclic q¹- to q ⁴- or q ⁶-ligand composed of atoms from Groups 13 to 16 of the Periodic Table, and in which the open chain ligand may be fused with one or two, preferably two, aromatic or non-aromatic rings and/or bear further substituents; or

(c) a cyclic q ¹- to q ⁴- or q ⁶-, mono-, bi- or multidentate ligand composed of unsubstituted or substituted mono-, bi- or multicyclic ring systems selected from aromatic or non-aromatic or partially saturated ring systems, such ring systems containing optionally one or more heteroatoms selected from Groups 15 and 16 of the Periodic Table.

Organometallic compounds (C), preferably used in the present invention, have at least one organic ligand (L) belonging to the group (a) above. Such organometallic compounds are called metallocenes.

More preferably at least one of the organic ligands (L), preferably both organic ligands (L), is (are) selected from the group consisting of cyclopentadienyl, indenyl, tetrahydroindenyl, fluorenyl, which can be independently substituted or unsubstituted.

Further, in case the organic ligands (L) are substituted it is preferred that at least one organic ligand (L), preferably both organic ligands (L), comprise one or more substituents independently selected from Ci to C20 hydrocarbyl or silyl groups, which optionally contain one or more heteroatoms selected from groups 14 to 16 and/or are optionally substituted by halogen atom(s), The term Ci to C20 hydrocarbyl group, whenever used in the present application, includes Ci to C20 alkyl, C2 to C20 alkenyl, C2 to C20 alkynyl, C3 to C20 cycloalkyl, C3 to C20 cycloalkenyl, Ce to C20 aryl, C7 to C20 alkylaryl or C7 to C20 arylalkyl groups or mixtures of these groups such as cycloalkyl substituted by alkyl.

Further, two substituents, which can be same or different, attached to adjacent C-atoms of a ring of the ligands (L) can also taken together form a further mono or multicyclic ring fused to the ring.

Preferred hydrocarbyl groups are independently selected from linear or branched Ci to C10 alkyl groups, optionally interrupted by one or more heteroatoms of groups 14 to 16, like 0, N or S, and substituted or unsubstituted Ce to C20 aryl groups.

Linear or branched Ci to C10 alkyl groups, optionally interrupted by one or more heteroatoms of groups 14 to 16, are more preferably selected from methyl, ethyl, propyl, isopropyl, tertbutyl, isobutyl, C5-6 cycloalkyl, OR, SR, where R is Ci to C10 alkyl group,

Ce to C20 aryl groups are more preferably phenyl groups, optionally substituted with 1 or 2 Ci to C10 alkyl groups as defined above.

By "o-ligand" is meant throughout the invention a group bonded to the transition metal (M) via a sigma bond.

Further, the ligands “X” are preferably independently selected from the group consisting of hydrogen, halogen, Ci to C20 alkyl, Ci to C20 alkoxy, C2 to C20 alkenyl, C2 to C20 alkynyl, C3 to C12 cycloalkyl, Ce to C20 aryl, Ce to C20 aryloxy, C7 to C20 arylalkyl, C7 to C20 arylalkenyl, -SR", -Pr"3, -SiR"3, -0SiR"3 and -NR"2, wherein each R" is independently hydrogen, Ci to C20 alkyl, C2 to C20 alkenyl, C2 to C20 alkynyl, C3 to C12 cycloalkyl or Ce to C20 aryl.

More preferably “X” ligands are selected from halogen, Ci to Ce alkyl, Cs to Ce cycloalkyl, Ci to Ce alkoxy, phenyl and benzyl groups.

The bridging group “R” may be a divalent bridge, preferably selected from -R’2C-, -R’₂C-CR’2-, — R’2Si-, -R’2Si-Si R’2-, -R’2Ge-, wherein each R’ is independently a hydrogen atom, Ci to C20 alkyl, C2 to C10 cycloalkyl, tri(Ci-C2o-alkyl)silyl, Ce- C2o-aryl, C7- C20 arylalkyl and C7- C2o-alkylaryl . More preferably the bridging group “R” is a divalent bridge selected from -R’2C-, - R’2Si-, wherein each R’ is independently a hydrogen atom, Ci to C20 alkyl, C2 to C10 cycloalkyl, Ce- C2o-aryl, C7- C20 arylalkyl and C7- C2o-alkylaryl .

Another subgroup of the organometallic compounds (C) of formula (I) is known as non-metallocenes wherein the transition metal (M), preferably a Group 4 to 6 transition metal, suitably Ti, Zr or Hf, has a coordination ligand other than a cyclopentadienyl ligand.

The term "non-metallocene" used herein means compounds, which bear no cyclopentadienyl ligands or fused derivatives thereof, but one or more non- cyclopentadienyl q-, or o-, mono-, bi- or multidentate ligand. Such ligands can be chosen e.g. from the groups (b) and (c) as defined above and described e.g. in WO 01/70395, WO 97/10248, WO 99/41290, and WO 99/10353), and further in V. C. Gibson et al., in Angew. Chem. Int. Ed., engl., vol 38, 1999, pp 428 447, the disclosures of which are incorporated herein by reference.

However, the organometallic compound (C) of the present invention is preferably a metallocene as defined above.

Metallocenes are described in numerous patents. In the following just a few examples are listed; EP 260 130, WO 97/28170, WO 98/46616, WO 98/49208, WO 98/040331 , WO 99/12981 , WO 99/19335, WO 98/56831 , WO 00/34341 , WO00/1 48034, EP 423 101 , EP 537 130, W02002/02576, W02005/105863, WO 2006097497, W02007/116034, W02007/107448, W02009/027075, W02009/054832, WO 2012/001052, and EP 2532687, the disclosures of which are incorporated herein by reference. Further, metallocenes are described widely in academic and scientific articles.

In a preferred embodiment the organometallic compound (C) has the following formula (la):

(L)₂RnMX₂ (la) wherein

“R” is SiMe2 bridging group linking said organic ligands (L);

“n” is 0 or 1 , preferably 1.

The metallocene catalyst complexes of the invention are preferably asymmetrical. Asymmetrical means simply that the two ligands forming the metallocene are different, that is, each ligand bears a set of substituents that are chemically different.

The metallocene catalyst complexes of the invention are typically chiral, racemic bridged bisindenyl C-i-symmetric metallocenes in their anti-configuration. Although such complexes are formally C-i-symmetric, the complexes ideally retain a pseudo-C2-symmetry since they maintain C2-symmetry in close proximity of the metal center although not at the ligand periphery. By nature of their chemistry both anti and syn enantiomer pairs (in case of C-i-symmetric complexes) are formed during the synthesis of the complexes. For the purpose of this invention, racemic-anti means that the two indenyl ligands are oriented in opposite directions with respect to the cyclopentadienyl-metal-cyclopentadienyl plane, while racemic-syn means that the two indenyl ligands are oriented in the same direction with respect to the cyclopentadienyl-metal-cyclopentadienyl plane, as shown in the scheme below.

Racemic Anti Racemic Syn

Formula (I), and any sub formulae, are intended to cover both syn- and anticonfigurations. Preferred metallocene catalyst complexes are in the anti configuration. The metallocene catalyst complexes of the invention are generally employed as the racemic-anti isomers. Ideally, therefore at least 95%mol, such as at least 98%mol, especially at least 99%mol of the metallocene catalyst complex is in the racemic-anti isomeric form.

More preferably, the metallocene catalyst is of formula (II)

Formula (II)

Mt is Hf or Zr; each X is a sigma-ligand; each R¹ independently are the same or can be different and are a CH2-R⁷ group, with R⁷ being H or linear or branched Ci -6-alkyl group, C3-8 cycloalkyl group, Ce- 10 aryl group, each R² is independently a -CH=, -CY=, -CH2-, -CHY- or -CY2- group, wherein Y is a C1-10 hydrocarbyl group and where n is 2-6, each R³ and R⁴ are independently the same or can be different and are hydrogen, a linear or branched Ci -Ce-alkyl group, an OY group or a C7-20 arylalkyl, C7-20 alkylaryl group or C6-20 aryl group, whereby at least one R³ per phenyl group and at least one R⁴ is not hydrogen, and optionally two adjacent R³ or R⁴ groups can be part of a ring including the phenyl carbons to which they are bonded,

R⁵ is a linear or branched Ci -Ce-alkyl group, C7-20 arylalkyl, C7-20 alkylaryl group or Ce-C2o-aryl group,

R⁶ is a C(R⁸)₃ group, with R⁸ being a linear or branched Ci-Ce alkyl group, and each R is independently a Ci-C2o-hydrocarbyl. It is preferred if Mt is Zr.

Preferably, each X is independently a hydrogen atom, a halogen atom, C1-6 alkoxy group or an R' group, where R' is a C1-6 alkyl, phenyl or benzyl group. Most preferably, X is chlorine, benzyl or a methyl group. Preferably, both X groups are the same. The most preferred options are two chlorides, two methyl or two benzyl groups, especially two chlorides.

Each R is independently a Ci-C2o-hydrocarbyl, such as Ce-C2o-aryl, C7-C20- arylalkyl or C?-C2o-alkylaryl. The term C1-20 hydrocarbyl group also includes Ci- 20 alkyl, C2-20 alkenyl, C2-20 alkynyl, C3-20 cycloalkyl, C3-20 cycloalkenyl, C6-20 aryl groups, C7-20 alkylaryl groups or C7-20 arylalkyl groups or mixtures of these groups such as cycloalkyl substituted by alkyl. Unless otherwise stated, preferred C1-20 hydrocarbyl groups are C1-20 alkyl, C4-20 cycloalkyl, C5-20 cycloalkyl-alkyl groups, C7-20 alkylaryl groups, C7-20 arylalkyl groups or C6-20 aryl groups.

Preferably, both R groups are the same. It is preferred if R is a C1-C10- hydrocarbyl or Ce-C -aryl group, such as methyl, ethyl, propyl, isopropyl, tertbutyl, isobutyl, Cs-6-cycloalkyl, cyclohexylmethyl, phenyl or benzyl, more preferably both R are a Ci-Ce-alkyl, C3-8 cycloalkyl or Ce-aryl group, such as a Ci-C4-alkyl, C5-6 cycloalkyl or Ce-aryl group and most preferably both R are methyl or one is methyl and another cyclohexyl. Most preferably the bridge is - Si(CH₃)₂-.

Each R¹ independently are the same or can be different and are a CH2-R⁷ group, with R⁷ being H or linear or branched Ci -e-alky I group, like methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl and tert. -butyl or C3-8 cycloalkyl group (e.g. cyclohexyl) , Ce- aryl group (preferably phenyl).

Preferably, both R¹ groups are the same and are a CH2-R⁷ group, with R⁷ being H or linear or branched Ci-C4-alkyl group, more preferably, both R¹ groups are the same and are a CH2-R⁷ group, with R⁷ being H or linear or branched C1-C3- alkyl group. Most preferably, both R¹ are both methyl.

Each R² is independently a -CH=, -CY=, -CH2-, -CHY- or -CY2- group, wherein Y is a C1-10 hydrocarbyl group, preferably a C1-4 hydrocarbyl group and where n is 2-6, preferably 3-4. Each substituent R³ and R⁴ are independently the same or can be different and are hydrogen, a linear or branched Ci-Ce-alkyl group, an OY group or a C7-20 arylalkyl, C7-20 alkylaryl group or C6-20 aryl group, preferably hydrogen, a linear or branched Ci-Ce-alkyl group or C6-20 aryl groups, and optionally two adjacent R³ or R⁴ groups can be part of a ring including the phenyl carbons to which they are bonded. More preferably, R³ and R⁴ are hydrogen or a linear or branched Ci- 04 alkyl group or a OY-group, wherein Y is a is a C1-4 hydrocarbyl group. Even more preferably, each R³ and R⁴ are independently hydrogen, methyl, ethyl, isopropyl, tert-butyl or methoxy, especially hydrogen, methyl or tert-butyl, whereby at least one R³ per phenyl group and at least one R⁴ is not hydrogen.

Thus, preferably one or two R³ per phenyl group are not hydrogen, more preferably on both phenyl groups the R³ groups are the same, like 3',5'-di-methyl or 4'- tert-butyl for both phenyl groups.

For the indenyl moiety preferably one or two R⁴ on the phenyl group are not hydrogen, more preferably two R⁴ are not hydrogen and most preferably these two R⁴ are the same like 3',5'-di-methyl or 3 ', 5 '-di-tert-buty I.

R⁵ is a linear or branched Ci-Ce-alkyl group such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, sec-butyl and tert-butyl, C7-20 arylalkyl, C7-20 alkylaryl group or C6-C20 aryl group. R⁵ is a preferably a linear or branched C1-C6 alkyl group or C6-20 aryl group, more preferably a linear C1-C4 alkyl group, even more preferably a C1-C2 alkyl group and most preferably methyl.

R⁶ is a C(R⁸)₃ group, with R⁸ being a linear or branched Ci-Ce alkyl group.

Each R is independently a Ci-C2o-hydrocarbyl, Ce-C2o-aryl, C7-C2o-arylalkyl or C7-C2o-alkylaryl. Preferably each R⁸ are the same or different with R⁸ being a linear or branched Ci-C4-alkyl group, more preferably with R⁸ being the same and being a Ci-C2-alkyl group. Most preferably, all R⁸ groups are methyl. In a further preferred embodiment the organometallic compound (C) has the following formula (III):

Formula (III) wherein Mt is Zr or Hf, preferably Zr; each R³ and R⁴ are independently the same or can be different and are hydrogen or a linear or branched Ci-Ce-alkyl group, whereby at least on R³ per phenyl group and at least one R⁴ is not hydrogen.

Specific metallocene catalyst complexes include: rac-anti-dimethylsilanediyl[2- methyl-4,8-bis-(4’-tert-butylphenyl)-1 ,5,6,7-tetrahydro-s-indacen-1 -yl][2-methyl- 4-(3’,5’-dimethyl-phenyl)-5-methoxy-6-tert-butylinden-1 -yl] zirconium dichloride; rac-anti-dimethylsilanediyl[2-methyl-4,8-bis-(3’,5’-dimethylphenyl)-1 ,5,6,7- tetrahydro-s-indacen-1 -yl][2-methyl-4-(3’,5’-dimethylphenyl)-5-methoxy-6-tert- butylinden-1 -yl]zirconium dichloride; rac-anti-dimethylsilanediyl[2-methyl-4,8-bis-(3’,5’-dimethylphenyl)-1 ,5,6,7- tetrahydro-s-indacen-1 -yl][2-methyl-4-(3’,5’-ditert-butyl-phenyl)-5-methoxy-6- tert-butylinden-1 -yl] zirconium dichloride or their corresponding zirconium dimethyl analogues.

The ligands required to form the metallocene catalysts of the invention can be synthesized by any process and the skilled organic chemist would be able to devise various synthetic protocols for the manufacture of the necessary ligand materials. W02007/116034, for example, discloses the necessary chemistry.

Synthetic protocols can also generally be found in W02002/02576, WO201 1/135004, WO2012/084961 , WO2012/001052, WO2011/076780 and WO201 5/158790.

To form an active catalytic species it is normally necessary to employ a cocatalyst as is well known in the art.

According to the present invention preferably a cocatalyst system comprising a boron containing cocatalyst and/or an aluminoxane cocatalyst is used in combination with the above defined metallocene catalyst.

The aluminoxane cocatalyst can be one of formula (IV):

where n is usually from 6 to 20 and R has the meaning below. Aluminoxanes are formed on partial hydrolysis of organoaluminum compounds, for example those of the formula AIR3, AIR2Y and AI2R3Y3 where R can be, for example, C1-C10 alkyl, preferably C1-C5 alkyl, or C3-C10 cycloalkyl, C7-C12 arylalkyl or alkylaryl and/or phenyl or naphthyl, and where Y can be hydrogen, halogen, preferably chlorine or bromine, or C1-C10 alkoxy, preferably methoxy or ethoxy. The resulting oxygen-containing aluminoxanes are not in general pure compounds but mixtures of oligomers of the formula (III).

The preferred aluminoxane is methylaluminoxane (MAO). Since the aluminoxanes used according to the invention as cocatalysts are not, owing to their mode of preparation, pure compounds, the molarity of aluminoxane solutions hereinafter is based on their aluminium content.

According to the present invention, also a boron containing cocatalyst can be used instead of the aluminoxane cocatalyst or the aluminoxane cocatalyst can be used in combination with a boron containing cocatalyst.

It will be appreciated by the person skilled in the art that where boron based cocatalysts are employed, it is normal to pre-alkylate the complex by reaction thereof with an aluminium alkyl compound, such as TIBA. This procedure is well known and any suitable aluminium alkyl, e.g. AI(Ci-Ce alkyl)s can be used. Preferred aluminium alkyl compounds are triethylaluminium, tri- isobutylaluminium, tri-isohexylaluminium, tri-n-octylaluminium and tri- isooctylaluminium.

Alternatively, when a borate cocatalyst is used, the metallocene complex is in its alkylated version, that is for example a dimethyl or dibenzyl metallocene complex can be used.

Boron based cocatalysts of interest include those of formula (V)

BY₃ (V) wherein Y is the same or different and is a hydrogen atom, an alkyl group of from 1 to about carbon atoms, an aryl group of from 6 to about 15 carbon atoms, alkylaryl, arylalkyl, haloalkyl or haloaryl each having from 1 to 10 carbon atoms in the alkyl radical and from 6-20 carbon atoms in the aryl radical or fluorine, chlorine, bromine or iodine. Preferred examples for Y are methyl, propyl, isopropyl, isobutyl or trifluoromethyl, unsaturated groups such as aryl or haloaryl like phenyl, tolyl, benzyl groups, p-fluorophenyl, 3,5- difluorophenyl, pentachlorophenyl, pentafluorophenyl, 3,4,5-trifluorophenyl and 3,5- di(trifluoromethyl)phenyl. Preferred options are trifluoroborane, triphenylborane, tris(4-fluorophenyl)borane, tris(3,5-difluorophenyl)borane, tris(4- fluoromethylphenyl)borane, tris(2,4,6-trifluorophenyl)borane, tris(penta- fluorophenyl)borane, tris(tolyl)borane, tris(3,5-dimethyl-phenyl)borane, tris(3,5- difluorophenyl)borane and/or tris (3,4,5-trifluorophenyl)borane.

Particular preference is given to tris(pentafluorophenyl)borane.

However it is preferred that borates are used, i.e. compounds containing a borate 3⁺ ion.

Such ionic cocatalysts preferably contain a non-coordinating anion such as tetrakis(pentafluorophenyl)borate and tetraphenylborate. Suitable counterions are protonated amine or aniline derivatives such as methylammonium, anilinium, dimethylammonium, diethylammonium, N- methylanilinium, diphenylammonium, N,N-dimethylanilinium, trimethylammonium, triethylammonium, tri-n- butylammonium, methyldiphenylammonium, pyridinium, p-bromo-N,N- dimethylanilinium or p-nitro-N,N-dimethylanilinium.

Preferred ionic compounds which can be used according to the present invention include: triethylammoniumtetra(phenyl)borate, tributylammoniumtetra(phenyl)borate, trimethylammoniumtetra(tolyl)borate, tributylammoniumtetra(tolyl)borate, tributylammoniumtetra(pentafluorophenyl)borate, tripropylammoniumtetra(dimethylphenyl)borate, tributylammoniumtetra(trifluoromethylphenyl)borate, tributylammoniumtetra(4-fluorophenyl)borate, N,N-dimethylcyclohexylammoniumtetrakis(pentafluorophenyl)borate, N,N-dimethylbenzylammoniumtetrakis(pentafluorophenyl)borate,

N,N-dimethylaniliniumtetra(phenyl)borate,

N,N-diethylaniliniumtetra(phenyl)borate,

N,N-dimethylaniliniumtetrakis(pentafluorophenyl)borate,

N,N-di(propyl)ammoniumtetrakis(pentafluorophenyl)borate, di(cyclohexyl)ammoniumtetrakist(pentafluorophenyl)borate, triphenylphosphoniumtetrakis(phenyl)borate, triethylphosphoniumtetrakis(phenyl)borate, diphenylphosphoniumtetrakis(phenyl)borate, tri(methylphenyl)phosphoniumtetrakis(phenyl)borate, tri(dimethylphenyl)phosphoniumtetrakis(phenyl)borate, triphenylcarbeniumtetrakis(pentafluorophenyl)borate, or ferroceniumtetrakis(pentafluorophenyl)borate.

Preference is given to triphenylcarbeniumtetrakis(pentafluorophenyl) borate,

N,N- dimethylcyclohexylammoniumtetrakis(pentafluorophenyl)borate or

N,N- dimethylbenzylammoniumtetrakis(pentafluorophenyl)borate.

It has been surprisingly found that certain boron cocatalysts are especially preferred.

Preferred borates of use in the invention therefore comprise the trityl ion. Thus the use of N,N-dimethylammonium-tetrakispentafluorophenylborate and Ph3CB(PhFs)4 and analogues therefore are especially favoured.

According to the present invention, the preferred cocatalysts are aluminoxanes, more preferably methylaluminoxanes, combinations of aluminoxanes with Al- alkyls, boron or borate cocatalysts, and combination of aluminoxanes with boron- based cocatalysts.

Suitable amounts of cocatalyst will be well known to the person skilled in the art. The molar ratio of boron to the metal ion of the metallocene may be in the range 0.5: 1 to 10:1 mol/mol, preferably 1 : 1 to 10:1 mol/mol, especially 1 : 1 to 5:1 mol/mol.

The molar ratio of Al in the aluminoxane to the metal ion of the metallocene may be in the range 1 : 1 to 2000:1 mol/mol, preferably 10: 1 to 1000: 1 mol/mol, and more preferably 50:1 to 900:1 mol/mol, and most preferably 600: 1 to 800:1 mol/mol.

The metallocene catalyst used in the polymerization process of the present invention is used in supported form. The support used comprises, preferably consists of, silica. In other words, the support is preferably a silica support. The person skilled in the art is aware of the procedures required to support a metallocene catalyst.

Especially preferably, the support is a porous material so that the metallocene complex may be loaded into the pores of the support, e.g. using a process analogous to those described in WO 94/14856 (Mobil), WO 95/12622 (Borealis) and WO 2006/097497.

The average particle size of the support can be typically from 10 to 100 pm. However, it has turned out that special advantages can be obtained if the support has an average particle size from 15 to 80 pm, preferably from 18 to 50 pm.

The particle size distribution of the support is described in the following. The silica support preferably has a D50 of between 10 and 80 pm, preferably 18 and 50 pm. Furthermore, the silica support preferably has a D10 of between 5 and 30 pm and a D90 of between 30 and 90 pm. Preferably, the support has a SPAN value of 0.1 to 1.1 , preferably 0.3 to 1 .0.

The average particle size of the metallocene catalyst is preferably of from 20 to 50 pm, more preferably of from 25 to 45 pm, and most preferably of from 30 to 40 pm.

The particle size distribution of the metallocene catalyst is described in the following. The metallocene catalyst preferably has a D50 of from 30 to 80 pm, preferably of from 32 to 50 pm and most preferably of from 34 to 40 pm. Furthermore, the metallocene catalyst preferably has a D10 of at most 29 pm, more preferably of from 15 to 29 pm, more preferably of from 20 to 28 pm, and most preferably of from 25 to 27 pm. The metallocene catalyst preferably has a D90 of at least 45 pm, more preferably of from 45 to 70 pm and most preferably of from 40 to 60 pm.

The average pore size of the support can be in the range 10 to 100 nm, preferably 20 to 50 nm and the pore volume from 1 to 3 ml/g, preferably 1.5 to 2.5 ml/g. BET surface area of silica support materials are determined according to ASTM D3663 and porosity parameters based on BJH according to ASTM D4641. Examples of suitable support materials are, for instance, ES757 produced and marketed by PQ Corporation, Sylopol 948 produced and marketed by Grace or SUNSPERA DM-L-303 silica produced by AGC Si-Tech Co. Supports can be optionally calcined prior to the use in catalyst preparation in order to reach optimal silanol group content.

It will be appreciated, that the propylene homopolymers may contain standard polymer additives. These typically form less than 5.0 wt.%, such as less than 2.0 wt.% of the polymer material. Additives, such as antioxidants, phosphites, cling additives, pigments, colorants, fillers, anti-static agents, processing aids, nucleating agents, clarifiers and the like may thus be added after the polymerisation process in the pelletization step. These additives are well known in the industry and their use will be familiar to the artisan. Any additives which are present may be added as an isolated raw material or in a mixture with a carrier polymer, i.e. in so called master batch.

Experimental Part

Measurement methods

Any parameter mentioned above in the detailed description of the invention is measured according to the tests given below. a) Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and is indicated in g/10 min. The MFR is an indication of the melt viscosity of the polymer. The MFR is determined at 190 °C for PE and 230 °C for PP. The load under which the melt flow rate is determined is usually indicated as a subscript, for instance MFR2 is measured under 2.16 kg load (condition D).

The MFR2 of the second polypropylene homopolymer fraction (PPH2), produced in the second reactor is determined according to equation (2): log equation ₍₂₎

wherein

MFR(PPH) is the MFR2 of the polypropylene homopolymer (PPH), w(PPH1) and w(PPH2) are the weight fractions of the first polypropylene homopolymer fraction (PPH1 ) and the second polypropylene homopolymer fraction (PPH2) in the polypropylene homopolymer (PPH), and

MFR(PPH1) is the MFR2 of the first polypropylene homopolymer fraction (PPH1 ) produced in the first reactor. b) Particle size and particle size distribution

The particle size distribution was determined using laser diffraction measurements by Coulter LS 200. The particle size and particle size distribution is a measure for the size of the particles. The D-values (D10 (or d10), D50 (or d50) and D90 (or d90)) represent the intercepts for 10%, 50% and 90% of the cumulative mass of sample. The D-values can be thought of as the diameter of the sphere which divides the sample’s mass into a specified percentage when the particles are arranged on an ascending mass basis. For example the D10 is the diameter at which 10% of the sample's mass is comprised of particles with a diameter less than this value. The D50 is the diameter of the particle where 50% of a sample's mass is smaller than and 50% of a sample's mass is larger than this value. The D90 is the diameter at which 90% of the sample's mass is comprised of particles with a diameter less than this value. The D50 value is also called median particle size. From laser diffraction measurements according to ISO 13320 the volumetric D-values are obtained, based on the volume distribution. The distribution width or span of the particle size distribution is calculated from the D-values D10, D50 and D90 according to equation (3):

Span = (D90-D10)/D50 equation (3) c) Density

Density of the polymer was measured according to ISO 1183 / 1872-2B. For the purpose of this invention the density of the blend can be calculated from the densities of the components according to:

where

Pb is the density of the blend,

Wi is the weight fraction of component '/’ in the blend and

Pi is the density of the component

d) Bulk density

The bulk density is measured according to ASTM D1895. e) Differential scanning calorimetry (DSC)

Differential scanning calorimetry (DSC) analysis, melting temperature (Tm) and melt enthalpy (Hm), crystallization temperature (T_c), and heat of crystallization (He, Her) are measured with a TA Instrument Q200 differential scanning calorimetry (DSC) on 5 to 7 mg samples. DSC is run according to ISO 11357 I part 3 /method C2 in a heat I cool I heat cycle with a scan rate of 10 °C/min in the temperature range of -30 to +225 °C.

Crystallization temperature (T_c) and heat of crystallization (H_c) are determined from the cooling step, while melting temperature (Tm) and melt enthalpy (Hm) are determined from the second heating step.

Throughout the patent the term Tc or (Ter) is understood as Peak temperature of crystallization as determined by DSC at a cooling rate of 10 K/min (i.e. 0.16 K/sec). f) Xylene cold soluble fraction The xylene cold soluble fraction (XCS) is determined according to ISO 16152 at 25 °C.

A weighed amount of a sample is dissolved in hot xylene under reflux conditions at 135°C. The solution is then cooled down under controlled conditions and maintained at 25°C for 30 minutes to ensure controlled crystallization of the insoluble fraction. This insoluble fraction is then separated by filtration. Xylene is evaporated from the filtrate leaving the soluble fraction as a residue. The percentage of this fraction is determined gravimetrically.

XCS = (100 x ml x V0) / (m0 x V1 ), wherein mO designates the initial polymer amount (g), ml defines the weight of residue (g), VO defines the initial volume (ml) and V1 defines the volume of the analyzed sample (ml). g) Flexural Modulus

Flexural modulus was determined in a 3-point-bending according to ISO 178 on injection molded specimens of 80 x 10 x 4 mm, prepared at 200 °C or 230°C in accordance with EN ISO 19069-2.

Materials a) Catalyst

The following catalyst was used in the processes according to the comparative and inventive examples as described in Table 1 . A metallocene complex has been used as described in WO 2019/179959 A1 :

A steel reactor equipped with a mechanical stirrer and a filter net was flushed with nitrogen and the reactor temperature was set to 20 °C. Next silica grade DM-L-303 from AGC Si-Tech Co, pre-calcined at 600 °C (10 kg) was added from a feeding drum followed by careful pressuring and depressurising with nitrogen using manual valves. Then toluene (43.5 kg) was added. The mixture was stirred for 30 min. Next 30 wt.% solution of MAO in toluene (17.5 kg) from Lanxess was added via feed line on the top of the reactor within 140 min. The reaction mixture was then heated up to 90 °C and stirred at 90 °C for additional two hours. The slurry was allowed to settle, and the mother liquor was filtered off. The catalyst was washed twice with toluene (43.5 kg) at 90 °C, following by settling and filtration.

Finally, MAO treated SiO2 was dried at 60 °C under nitrogen flow for 2 hours and then for 14 hours under vacuum (~0.5 barg) with stirring. MAO treated support was collected as a free-flowing white powder found to contain 15.0% Al by weight. 30 wt.% MAO in toluene (2 kg) was added into a steel nitrogen blanked reactor via a burette at 20 °C. Toluene (12.8 kg) was then added under stirring. 129 g of the metallocene was added from a metal cylinder followed by flushing with 1 kg toluene. The mixture was stirred for 60 minutes at 20 °C. Trityl tetrakis(pentafluorophenyl) borate (127.2 g) was then added from a metal cylinder followed by a flush with 1 kg of toluene. The mixture was stirred for 1 h at room temperature. The resulting solution was added to a stirred cake of MAO-silica support prepared as described above over 4-2 hour. The cake was stirred for 30 minutes and then allowed to stay without stirring for 30 minutes, followed by drying under N2 flow at 60 °C for 2 h and additionally for 15 h under vacuum (~0.5 barg) under stirring. b) Additives

Additive 1 : Irganox 1010(FF) (CAS-No. 6683-19-8) commercially available from BASF,

Additive 2: Irgafos 168(FF) (CAS-No. 31570-04-4) commercially available from BASF, Additive 3: Irganox 1098 (CAS-No. 23128-74-7) commercially available from BASF.

Examples

The following three examples (Comparative Example CE01 , and Inventive Examples IE01 and IE02) were carried out in a Borstar pilot plant, comprising a reactor sequence consisting of a prepolymerisation reactor, a loop reactor and a gas phase reactor. CE01 was a high-flow unimodal polypropylene homopolymer, whereas both IE01 and IE02 were bimodal high-flow polypropylene homopolymers. Process conditions and properties are given in Table 1 below.

The pelletization of the powder of the base polymers is done in a twin screw extruder with a screw diameter of 18 mm at a melt temperature of 240 °C and a throughput of 7 kg/h. Additives are added as indicated in Table 1.

Table 1

*APS (average particle size) is a D50 value, determined by camera eye by camsizer device.

As can be seen from Table 1 above, the moderate broadening of the molecular weight distribution, expressed as polydispersity index, and bimodal production lead to an increase of the stiffness, expressed as flexural modulus, by about 100 to 120 MPa. This is a significant improvement in particular in terms from a process point of view.

Claims

1. Process for the production of a polypropylene homopolymer, the process comprising the steps of a) polymerizing in a first reactor propylene in the presence of a metallocene catalyst yielding a first polypropylene homopolymer fraction having a MFR2 of 1 to 50 g/10min measured according to ISO 1133, b) transferring the first polypropylene homopolymer fraction to a second reactor, c) polymerizing in the second reactor propylene in the presence of the first polypropylene homopolymer fraction yielding a second polypropylene homopolymer fraction, d) withdrawing the polypropylene homopolymer comprising the first polypropylene homopolymer fraction and the second propylene homopolymer fraction from the second reactor, wherein the polypropylene homopolymer has a MFR2 of 20 to 200 g/10min measured according to ISO 1133, wherein the metallocene catalyst comprises a metallocene complex and a support, wherein the support comprises silica, and wherein a split ratio between the first polypropylene homopolymer and the second polypropylene homopolymer is 20:80 to 80:20, wherein the metallocene complex is an organometallic compound (C), the organometallic compound (C) has the following formula (la):

(L)₂RnMX₂ (la) wherein

“M” is Zr or Hf; each “X” is a o-ligand; each “L” is an optionally substituted cyclopentadienyl, indenyl or tetrahydroindenyl; “R” is SiMe2 bridging group linking said organic ligands (L);

“n” is 0 or 1 , preferably 1 .

2. The process according to any of the preceding claims, wherein the support has an average particle size of 10 to 100 pm.

3. The process according to any of the preceding claims, wherein the first polypropylene homopolymer fraction has a MFR2 of 5 to 40 g/10minmeasured according to ISO 1133.

4. The process according to any of the preceding claims, wherein the polypropylene homopolymer has a MFR2 of 30 to 100 g/10min measured according to ISO 1 133.

5. The process according to any of the preceding claims, wherein the split ratio between the first polypropylene homopolymer fraction and the second polypropylene homopolymer fraction is 30:70 to 70:30.

6. The process according to any of the preceding claims, wherein the first reactor is a loop reactor and/or wherein the second reactor is a gas phase reactor.

7. The process according to any of the preceding claims, wherein step a) is carried out at a temperature from 60 to 100 °C and/or at a pressure of 1 to 150 bar.

8. The process according to any of the preceding claims, wherein step c) is carried out at a temperature from 60 to 90 °C and/or at a pressure of 10 to 140 bar.

9. The process according to any of the preceding claims, wherein the first polypropylene homopolymer fraction has a xylene cold soluble content (XCS) determined according to ISO 16152 of 0.1 to 5.0 wt.%.

10. The process according to any of the preceding claims, wherein the propylene homopolymer has a xylene cold soluble content (XCS) determined according to ISO 16152 of 0.1 to 4.0 wt.%.

11. The process according to any of the preceding claims, wherein the propylene homopolymer is a bimodal propylene homopolymer.

12. The process according to any of the preceding claims, wherein the polypropylene homopolymer has a flexural modulus of from 1500 to 1650 MPa determined according to ISO 178.

13. The process according to any of the preceding claims, wherein the polypropylene homopolymer has a melting temperature Tm determined by

DSC according to ISO 11357 of 145.0 to 165.0 °C.

14. The process according to any of the preceding claims wherein the propylene homopolymer has a crystallization temperature T_c determined by DSC according to ISO 1 1357 of 100.0 to 140.0 °C.