Hard versus Soft Materials as Supports for Metallocene and Post-Metallocene Catalysts

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456

Hard versus Soft Materials as Supports forMetallocene and Post-Metallocene Catalysts

Corinna Naundorf, Daniela Ferrari, Giovanni Rojas, Gerhard Fink,*Markus Klapper,* Klaus Mullen

The influence of organic supports on the polymerization behavior of post-metallocenecatalysts is studied and compared with similarly supported titanium and zirconium metallo-cenes. The effects of the immobilization, activation, and polymerization process were studiedby videomicroscopy, laser confocal fluorescencemicroscopy,SEM, and TEM. A model for the polymerization process for acatalyst supported on latex particleswas developed from theresults obtained. Organic supports based on latex particlesare easily adjustable for different catalysts due to the ver-satile functionalization of the surfaces and can be applied todifferent types of olefin polymerization catalysts. They canbe considered as an alternative to SiO2 or MgCl2 supports.

Introduction

Awide variety of single-site catalysts for the polymerization

of ethyleneareknowntoproducepolyethylenewithdefined

molecular weight.[1]In addition to traditional metallocenes,

the so-called post-metallocenes have proven to be attractive

catalysts due to useful properties, such as production of

branched polyethylene or copolymerization of polar mono-

mers.[2] Among post-metallocenes, zirconium, and titanium

complexesbearingphenoxy-imine ligandsstandoutforhigh

activity toward ethylene polymerization and also for

imparting control over molecular weight.[3,4]

Both metallocenes and post-metallocenes have to be

supported on suitablematerials as they produce a dust-like

C. Naundorf, G. Rojas, M. Klapper, K. MullenMax-Planck-Institute for Polymer Research, Ackermannweg 10,55128 Mainz, GermanyE-mail: [email protected]. Ferrari, G. FinkMax-Planck-Institute for Coal Research, Kaiser-Wilhelm-Platz 1,45470 Mulheim, GermanyE-mail: [email protected]

Macromol. React. Eng. 2009, 3, 456–466

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

material, which means that reactor fouling could occur in

homogeneous polymerization.[5] In addition to silica,[6]

organic materials have shown the ability to support

metallocenes for further polymerization.[7] We introduced

latex particles produced by the miniemulsion polymeriza-

tion of styrene and divinylbenzene, with a size of 50–

200nm, as organic supports. During immobilization of the

catalysts, the latex particles agglomerate reversibly to

larger secondarycatalystparticlesdueto thepresenceof the

cocatalystmethylaluminoxane (MAO).[8] Fragmentation of

the secondary particles is guaranteed by reversible cross-

linking between MAO and functional groups, such as

polyethyleneoxide chainsorpyridinegroupson the surface

of the latex particles.[9–12] Surface functionalization of the

latex particles with nucleophilic groups allows immobili-

zation of the active catalyst. The fragmentation of the

catalyst particles is one important aspect during the

polymerization of olefins via a supported catalyst. Various

methods of studying the polymerization behavior of

supported catalysts are available. By measuring the

monomer consumption, e.g., amodel for the fragmentation

process in slurry polymerizations can be developed. Such

models are complemented with video microscopy studies

DOI: 10.1002/mren.200900026

Hard versus Soft Materials as Supports for Metallocene . . .

performed in gas phase, where the growing catalyst

particles reveal additional information about the polymer-

ization and fragmentation of single particles.[13–16] A fast

method toelucidate the fragmentationand thedistribution

of the support in a product particle is laser scanning

confocal fluorescence microscopy (LSCFM).[17] As latex

particles are a rather new class of supporting material,

the polymerization in the presence of the resulting

supported catalysts is not completely understood. This

was our major driving force for intensive studies on

exemplarily selected supported metallocenes and post-

metallocenes. Thekineticsof thepolymerizationusing such

catalyst systemswas studied in a batch reactor and also by

video microscopy on the single particle level. Detailed SEM

and LSCFM studies clarified the fragmentation process of

the supports. Comparison of the results describedherewith

previously studied silica supported catalysts led to a

completely new model, which follows a different poly-

merizationcoursewhile inthepresenceoforganic supports.

Experimental Part

General Procedure and Materials

All air and water sensitive reactions were performed under dry

argon atmosphere using standard Schlenk techniques or a dry

nitrogenglovebox system.Dried solvents for supporting procedure

and polymerizations were purchased from Acros (toluene) or

Aldrich (heptane) and used without further purification. Styrene

(Aldrich), divinylbenzene (Fluka), and 4-vinyl pyridine (Aldrich)

weredriedoverCaH2anddistilledunder reducedpressure. Lutensol

AT50 (C16C18 fatty alcohol ethoxylates-EO50) was donated from

BASF AG and used without further purification. Hexadecane

(Merck-Schuchardt), a,a0-azodiiso-butyramidinedihydrochloride

(Fluka), cetyltrimethylammoniumbromide (Sigma) were used

without further purification. Deionized water was used for all

miniemulsion polymerizations. The cocatalyst,MAOwas obtained

from Albemarle as 1.2M toluene solution. DriedMAO (DMAO) was

obtained by the evaporation of the solvent and the resulting solids

were used without further purification. Ethylene (BASF AG,

Ludwigshafen) was purified by passing through columns of BASF

R3-15 deoxygenation catalyst and 4 A molecular sieves. For video

microscopy and kinetic polymerizations, ethylene was purified

additionally over a NaAlEt4 column followed by a 4 A molecular

sievescolumn.Purificationof isobutanewasachievedwithOxisorb

andHydrosorb gas purification systems. Triisobutylaluminum (1M

in hexane) and Tri-n-octylaluminum (25wt.-% in hexane) were

obtained from Aldrich and used without further purification.

Preparation of the PEO-pyridine Functionalized Latex

Particles, Stained with Dye

Lutensol AT50 (400mg, 0.16mmol, 0.15mol-%, 3.5wt.-%) and

cetyltrimethylammoniumbromide (22mg, 0.06mmol) were dis-

solved in distilled water (100mL) at 40 8C and stirred for 30min.

Styrene (5 g, 48.0mmol, 47mol-%), 4-vinylpyridine (5 g, 47.3mmol,



46mol-%), divinylbenzene (0.91 g, 7.0mmol, 6mol-%), and hex-

adecane (0.42 g, 1.86mmol) ashydrophobweremixedandadded to

the water/emulsifier mixture at room temperature. The solution

was stirred for 30min and then sonicated with a Branson Sonifier

450W 70% power under ice cooling for 7min to form a

miniemulsion. The miniemulsion was degassed with argon for

20min and then heated to 70 8C. The initiator for radical

polymerization a,a0-azodiiso-butyramidinedihydrochloride (210mg,

0.77mmol) was dissolved in 10mL dist. water, degassed for 20min

and added to theminiemulsion. After 12h themixturewas filtered

and purified by forced dialysis with at least 300mL of water and

then frozen and lyophilized.

Supporting Procedure for Catalysts 1 and 2

(Systems A and B)

The latex-supportwasdried for12 hat 70 8C invacuo. Latex support

(300mg)wasmixedwithDMAO (120mg) and suspended in toluene

(5mL).Todispersethe latexparticlesthesuspensionwasplaced inan

ultrasonic bath for 20min and stirred for an additional 10min.

The catalyst (0.03mmol)was dissolved in toluene (5mL) and the

catalyst solution (2mL) added to the latex particle–cocatalyst

mixture at 0 8C. The mixture was stirred and allowed to warm to

room temperature. The suspensionwas filtered through a glass frit

under nitrogen to remove the solvent. The resulting cakewas dried

in vacuumand transferred into a glovebox. The cakewasmortared

and the resulting powder sieved through analytical sieves with

pore sizes of 60 and40mm.The fractionbetween40 and60mmwas

used for videomicroscopyand the fraction smaller than40mmwas

used for slurry polymerization. The calculated support loading

was estimated to be 29mmol metal g�1 het cat. The aluminum to

metal ratio was estimated to be 170:1.

Supporting Procedure for Catalyst 3 (System C)

The latex supportwasdried for 12 hat 70 8C in vacuo. Latex support

(300mg) was mixed with 3mL of MAO (1.5M, 10wt.-% in toluene)

and an additional 5mL of toluene was added. To disperse the latex

particles thesuspensionwasplaced inanultrasonicbath for20min

and stirred for an additional 10min.

The catalyst (0.032mmol) was dissolved in 3mL of MAO (1.5M,

10wt.-% in toluene) and the catalyst solution (2.1mL) added to the

latex particle–cocatalyst mixture. The mixture was stirred for 1h

and then filtered over a glass frit under nitrogen to remove the

solvent. The resulting cake was dried in vacuum und transferred

into theglovebox. The cakewasmortaredand the resultingpowder

sieved through analytical sieves with pore sizes of 60 and 40mm.

The fraction between 40 and 60mmwas used for videomicroscopy

and the fraction smaller than 40mm was used for slurry

polymerization. The calculated support loading was estimated to

be 30mmol metal g�1 het cat. The aluminum to metal ratio was

estimated to be 340:1.

Polymerization Under High Ethylene Pressure

In a typical ethylene polymerization, the stainless steel reactor

equipped with a propeller-like stirrer was charged with isobutane

www.mre-journal.de 457

C. Naundorf, D. Ferrari, G. Rojas, G. Fink, M. Klapper, K. Mullen

458

(400mL), tri-n-octylaluminum (2mL, 0.95mmol) ortriisobutylalu-

minum (5mL, 5mmol), and poly(propylene glycol)-block-poly-

(ethylene glycol)-block-poly(propylene glycol) (PPG-b-PEG-b-PPG)

(1mg) in toluene (1mL) in case of catalyst (1). The reactor was

heated to the desired polymerization temperature under stirring

(400 rpm) using a thermostat. An ethylene pressure of 40bar was

applied. The catalyst was injected into the reactor under argon

through a pressure lock. After 60min the polymerization was

stoppedby the releaseof ethyleneand isobutane. Thepolyethylene

product was stirred in acidic methanol (400mL), filtered, stirred

again in methanol (400mL), filtered again, and dried in a vacuum

oven at 75 8C under reduced pressure.

Video Microscopy

A small 50mL volume reactor with a borsilicat window in the top

was evacuated for 4 h at 80 8C and transferred into a glovebox. A

silver plate serving as catalyst holder was prepared with catalyst

particles. After reconnection to the gas/vacuum line the reactor

was heated to 55 8C. Simultaneously with picture recording, the

polymerization was started by the addition of monomer to the

reactor. Pictures were recorded every 10 s over a period of 180min.

The polymerization was stopped by release of the monomer. The

recorded pictures were analyzed by Analysis five.

Slurry Polymerization

The slurrypolymerization at lowethylenepressurewas carried out

in heptane slurry in a 250mL glass autoclave equipped with a

variable speed stirrer (1 400 rpm) under constant pressure (1, 2, or

4 bar). 130mL of anhydrous heptane was mixed with the desired

amount of scavenger (trioctylaluminum or triisobutylaluminum)

and added to the reactor. The solution was thermostated and

saturated with monomer up to the desired ethylene pressure. The

heterogeneous catalyst system was injected to the reactor as a

suspension in heptane with argon pressure. The consumption of

monomer gas was registered by mass flow meters. The polymer-

ization was stopped by releasing the monomer and quenched by

the addition ofmethanol. The resulting polymerwaswashedwith

methanol/HCl and puremethanol, filtered and dried at 75 8C under

reduced pressure.

Scheme 1. (1) bis[N-(3-t-butylsalicylidene) cycloheptylaminato]titanium (IV) dichloride, (2) bis[N-(5-methoxy-3-t-butylsalicyli-dene)-2-methylcyclohexylaminato] zirconium (IV) dichloride, and(3) dimethylsilyl-bis-[2-methylbenzindyl] zirconium (IV) dichloride.

Characterization of Polyethylene

Polymer melting points (Tm) were determined on a differential

scanning calorimeter (DSC) using a heating rate of 10 8C �min�1 in

the temperature range of 20–200 8C. The heating cycle was

performed twice, but only the results of the second scan were

reported.

Gel permeation chromatography (GPC) of polyethylene from

catalyst 1 was performed on a Waters 150-C gel permeation

chromatograph at 145 8C using three TSKgel columns (two sets of

TSKgelGMHHR-H(S)HTandTSKgelGMH6-HTL)with refractive index

detection and calibration versus narrow polystyrene standards.

Polyethylene from catalysts 2 and 3 was performed.



Scanning electron micrographs were obtained by using an LEO

Zeiss. The polyethylene was applied on a graphite tape.

Results and Discussions

Immobilization of the Catalysts

To study the olefin polymerization in the presence of post-

metallocenes immobilized on latex particles, titanium (1)

anda zirconium (2) catalyst bearingphenoxy-imine ligands

(FI-catalysts, see Scheme 1) were chosen as examples.

MetalloceneMe2Si(2-MeBenzInd)2ZrCl2 (3) was selected for

comparisonwith the two FI-catalysts because it has shown

excellent properties in ethylene polymerization when

supported on functionalized latex particles. All catalysts

are activatedwithmethylaluminoxane (MAO),which is the

most common cocatalyst formetallocenes and FI-catalysts.

The chosen catalysts were supported on latex particles

functionalized with PEO-chains and pyridine-groups on

their surfaces. These organic supports have already shown

suitability for two selected catalysts. Metallocene 3,

supported on latex particles, produces polyethylene with

high activity, productivity, and narrow molecular weight

distributions.[10,18] The FI-titanium catalyst 1 also exhibits

excellent properties for producing ultrahigh molecular

weight polyethylene (UHMWPE) with narrow molecular

weight distribution.[19] The particles were synthesized by

the miniemulsion polymerization of styrene, 4-vinylpyr-

idine, divinylbenzene, and a styryl-functionalized perylene

monoimide derivative (see Scheme 2). PEO-ethoxy fatty

alcohol was used as an emulsifier to stabilize the

miniemulsion. Alongwith the pyridine groups, PEO-chains

also helped to anchor the activated catalyst. The latex

particle size (primary particles) was about 150nm, deter-

mined by dynamic light scattering. During the immobiliza-

tion process of the catalysts, the primary particles

agglomerate reversibly to a secondary catalyst particle.

During ethylene polymerization, these secondary catalyst

particles fragmented down to primary latex particles,

DOI: 10.1002/mren.200900026


Scheme 2. Synthesis of the organic support containing a fluorescent perylene dye.

which were then spread homogeneously within the

resulting polyethylene particle.[16,17]

For immobilization on theorganic supports, the catalysts

were dissolved in toluene and pre-activated with the

addition of MAO. The organic supports were mixed with

MAO and the pre-activated catalysts were added. The

immobilization procedure of catalysts 1 and 2was slightly

different from the immobilization of catalyst 3, as the FI-

catalysts decomposed during the pre-activationwithMAO.

This may be attributed to the decomposition of the FI-

catalysts by the presence of free trimethylaluminum (TMA)

in MAO.[20] Therefore, the content of free TMA in the

cocatalyst was reduced by co-evaporation of TMA and

toluene. DMAO with a lower content of free TMA

redissolved in toluene was then used as an activator for

the FI-catalysts 1 and 2. For immobilization of the FI-

catalysts, a mixture of dried PEO-pyridine-functionalized

latex particles and DMAO was redispersed in toluene,

followedby the addition of catalysts 1 or 2 to themixture of

the carrier and activator. The obtained catalyst systems A

andBwerefiltrated throughaglass frit.A clearandcolorless

filtrate was obtained for both catalysts, indicating that

most of the catalyst, which had originally shownan orange

or yellow color in solution, was immobilized on the carrier.

Table 1. Elemental analysis for the catalyst systems A–C.

Catalyst System Calculated Calculated Observeda) result Observeda)

wt.-% of

met

wt.-% of

Al

wt.-% of

met

wt.-%

Al

A [Ti] 0.14 15.1 0.12 (86%) 14.9 (98

B [Zr] 0.26 13.3 0.23 (88) 12.8 (96

C [Zr] 0.27 27.5 0.16 (60) 24.0 (87

a)Elemental analysis was performed by ICP-OES.



The remaining filter cake—the supported

catalyst—was ready to use after drying

under vacuum and controlled crushing.

Immobilization of metallocene 3 (cat-

alyst system C) was performed by pre-

activation of 3with MAO and addition to

a mixture of carrier and MAO. Upon

filtration, the yellow filtrate indicated

that not all the catalyst was immobilized.

Since leaching of the catalyst results in

reactor fouling and dust formation, all

three catalyst systemsA–Cwereanalyzed

for their composition to determine

whether complete immobilization had

been achieved. Elemental analysis of

aluminum, titanium, or zirconium

(Table 1) of the resulting catalyst systems

showed that in the case of FI-catalysts 1 and 2 almost all the

catalyst and cocatalyst added to the supporting material

had been immobilized. For catalystA, 98%of the aluminum

added was adsorbed on the latex particles surface, while

86% of the catalyst added to the latex particles was

immobilized. In the case of catalyst 2, FI-zirconiumcatalyst,

nearly all aluminumof theDMAOwas also absorbed on the

organic carrier, while 88% of the catalyst was immobilized.

In contrast, based on the elemental analysis from catalyst 3,

87% of aluminum and only 60% of the added zirconiumwas

immobilized to the carrier.

An explanation for the lower degree of immobilization

for metallocene 3might be an incomplete formation of the

activated cationic catalyst complex. Only the positively

charged species can be immobilized by electrostatic

interaction with the nucleophilic surface of an inorganic

or organic support. Catalyst complexes that exist as neutral

species are not able to be adsorbed on the carrier as they

cannot interact directly with the supports or with the

cocatalyst MAO.[19] For FI-titanium catalysts, it has been

proved by NMR studies that at very low aluminum to

titanium ratios of 50:1, the methylated cationic complex,

which is the active species in ethylene polymerization,was

formed.[21] Thisverifies thatall catalystmoleculesof1and2

result Catalyst loading Al:met-ratio

of mmol metal g�1

het cat

%) 25 220

%) 25 190

%) 17 500



460

were transformed to cationic complexes at the chosen

aluminum to metal ratio of around 200:1. For catalyst 3,

however, the formation of cationic active species was only

observed at aluminum to zirconium ratios of more than

300:1[22] Therefore, it can be assumed that themetallocene

3 was not completely converted into the cationic species

andnon-activatedmetallocenecomplexeswerefiltratedoff

along with the solvent.

Slurry Polymerization of the Catalysts A, B, and C atHigh Ethylene Pressure (40 bar)

The three different catalyst systemsA, B, and Cwere tested

toward ethylene polymerization at a pressure of 40 bar.

Polymerizations were performed in liquid isobutane in the

presence of a scavenger suitable for the catalysts (Table 2).

For systems A and B trioctylaluminum was chosen as a

scavenger, since previous studies have shown that narrow

molecular weight distributions are obtained using this

scavenger and FI-titanium catalyst.[19] For catalyst C,

triisobutylaluminum was used as scavenger. In polymer-

izations performed using the catalyst system A, a hydro-

philic block copolymer [5mg PPG-b-PEG-b-PPG (Mw ¼ ca.

2 500 gmol�1)] had to be added to the polymerization

medium to avoid static agglomeration of the produced

polyethylene, which would result in reactor fouling.[23]

System C showed an activity of 8 150 kg PE mol�1

Zr h�1 � bar�1 at 75 8C, which was slightly higher than the

activity of the FI-zirconium catalyst, 6 100 kg PE mol�1

Zr h�1 � bar�1 at the same polymerization temperature. At

70 8C the FI-titanium catalyst (1) showed an activity of

1 300 kg PE mol�1 Ti h�1 � bar�1, which was significantly

lower than that of the zirconium catalysts. This is in good

agreement with the results obtained for the correspond-

ing homogeneous polymerizations, which indicates that

both catalysts are influenced in a similar way by the

supportingmaterial.[24] CatalystCproducedpolyethylene

with a weight average molecular weight of

1 250 kg �mol�1 and a molecular weight distribution of

3.2. Catalyst A produced UHMWPEwith a weight average

molecular weight of 4 710 kg �mol�1 and a molecular

Table 2. Polymerization results of the three catalysts.

Catalyst Catalyst

systems

Scavenger TP

mmol -C

1 A [Ti] 1-TOA 70

2 B [Zr] 2-TOA 75

3 C [Zr] 5-TIBA 75

Polymerization conditions: 1 L stainless steel reactor, 400mL isobutan



weight distribution of 2.3, which is rather narrow for

supported catalysts. The FI-zirconium catalyst system B

producedUHMWPEwith ahighermolecularweightMw of

5 450 kg �mol�1 and a molecular weight distribution of

more than 30, unusually broad for single-site catalysts.

This phenomenon might be caused by the formation of

different conformations of the FI-zirconium catalyst.[25]

For most catalysts only the lowest level of energy, an

octahedral conformation, is obtained by calculations,

which indicates a single site-catalyst resulting in a

polyethylene with a narrow molecular weight distribu-

tion. However, for a few FI-zirconium catalysts a multi-

modal distribution of the polyethylene is obtained during

homogeneous polymerization.[25] It is assumed that in

these casesmore than one conformationwith aminimum

of energy exists, which results in a multi-site catalyst

system.

Closer insight into the influence of the support and the

typeof catalystonthepolymerizationprocesswasobtained

by studying the overall kinetics[26] in a batch reactor (slurry

polymerization). Single grain kinetics[14,27] were obtained

from gas phase polymerizationsmonitored in situ by video

microscopy. Additional informationwasmade available by

visualizing the morphology of the obtained particles by

SEM.

Kinetic Investigations in Slurry Polymerization

For kinetic investigation, evaluating the activity and

productivity of the supported catalysts, slurry polymeriza-

tions were performed in heptane using catalyst systems A,

B, and C. Ethylene consumption was controlled by a mass

flow-meter. The experiments were run at 50 and 75 8C. Apressureof4 bar (Table3)wasapplied inorder to slowdown

the polymerization process and to obtain accurate data at

the beginning of the polymerization. B showed the highest

polymerization activity [13 000 kg PE mol�1 Zr h�1 �bar�1]

in heptane. Interestingly, the activity was significantly

higher than that at high ethylene pressure in isobutene

(Table26 100 kgPEmol�1Zrh�1 �bar�1. In contrast, catalyst

C showed a similar activity under high (40bar) and low

Activity Mwa) Mn=Mw

a)

kg PE mol�1

met h�1 � bar�1T 103kg �mol�1

1.30 4 710 2.3

6.10 5 450 37

8.15 1 250 3.2

e, 40 bar ethylene pressure.a)Analyzed by GPC (PS-standard).

DOI: 10.1002/mren.200900026


Figure 1. Slurry polymerization of ethylene by three differentcatalyst systems (4 bar ethylene pressure, A: 50 8C, 2 mmolTOA; B: 75 8C, 2 mmol TIBA; C: 75 8C, 1 mmol TOA).

(4 bar) monomer pressure, 8 150 and 6 900 kg PE mol�1

Zr h�1 �bar�1, respectively.

Although the three catalyst systems yielded different

overall activities toward ethylene polymerization (Table 3),

the course of the polymerizationwas very similar. After the

addition of catalysts to the reaction system, the polymer-

ization started immediately, with the highest rate of

polymerization occurring a few seconds later (see Figure 1).

After reaching its maximum, the rate of polymerization

slowly decreased. Catalyst systems B and C showed a

significantly higher polymerization rate than system A,

whichexhibiteda constant lossof activityduring thewhole

polymerization. In contrast,A lost only a small portion of its

activity and then remained constant after reaching the

maximum rate. This behavior has been observed for

ethylene polymerizations at 40 bars, where a catalyst

system similar to A showed constant activities over

polymerization times of up to 6h.[19]

Kinetic Investigation by Video Microscopy

The polymerization behavior of the catalyst systems was

studied via video microscopy. This technique is of increas-

ing importance for investigations dealing with the kinetic

behavior of growing catalyst or polymer particles. It was

first applied for the study of polymerization of butadiene in

the gas phase.[28] Later on, video microscopy was used to

study single catalyst particles during ethylene polymeriza-

tion or copolymerizations by evaluating the product

particle sizes.[27] In the present work, we used video

microscopy to investigate the polymerization behavior of

catalyst systems A, B, and C in the gas phase (see Figure 2

and 3). Ethylene polymerizations were performed at 55 8Cand 4bars. A snapshot of the catalyst particles was taken

every 10 s over a period of 3 h. The growing of seven

particles were investigated for every catalyst systems, as

shown in Figure 2. The projection area of every single

catalyst particle, therefore, was measured and converted

into the equivalent circle diameter (ECD). As the observed

Table 3. Polymerization conditions for kinetic investigations. (Polyme

Cat-no. Loading of

catalyst system

Amount of

catalyst

mmol metal g�1

het cat

mg mm

A [Ti] 25 50 1.2

B [Zr] 25 25 0.6

C [Zr] 17 25 0.4

a)Trioctylaluminum; b)Triisobutylaluminum.



particles had adifferent size, the EDCwasnormalized to the

initial ECD (ECD0) to achieve comparable results.

The particles from catalyst system A showed a relative

uniform growth and increased their ECD during 3h of

polymerization toabout280–320%of thestartingECD0. The

size of particles formed by catalyst system B increased to

about 480–500% of the initial ECD0. Similar to system B,

which showed a very uniform growth of its particles,

catalyst system C increased about 400% in size. Although

the catalyst particles of the three systems present different

growing rates, the normalized EDC is very similar. All

observed particles started growing directly after exposure

to themonomergasandshowedaveryrapid increase insize

during the first 20min, e.g., all catalyst particles doubled

their diameter within the first 5min. The growth rate

sloweddownafter 20min, but particle growth could still be

observed even at 3 h of polymerization.

Study of the Particle Fragmentation by LaserScanning Confocal Fluorescence Microscopy (LSCFM)

For the evaluation of the kinetic results, it is essential that

all catalyst particles arehomogeneouslyandequally loaded

rization conditions: 140 mL heptane, 4 bar ethylene pressure).

Scavenger TP Activity

ol mmol -C kg PE mol�1

met h�1 � bar�1

5 1.0a) 50 950

3 2.0a) 75 13 000

3 2.0b) 75 6 900



Figure 2. Initial catalyst particles system (A) [Ti] and after gasphase polymerization (B) [4 bar ethylene, 55 8C, 30mmol Ti g�1 CatAl:Ti 170:1].

Figure 3. ECD/ECD0 versus time obtained from gas phasepolymerization of ethylene with (a) catalyst A, (b) catalyst B,and (c) catalyst C at 55 8C and 4 bar.

462Macromol. React. Eng. 2009, 3, 456–466


with thepolymerization catalysts. It has tobeexcluded that

the macroscopically observed activity does not result from

only a part of the catalyst. Additionally, it has to be

confirmed that the expected fragmentation process of the

secondary particles is taking place. Both can be easily

visualized by LSCFM.[17] The organic supports, therefore,

were tagged directly during the particle formation via

emulsion polymerization with a fluorescent styryl functio-

nalized perylene monoimide (see Scheme 2).[17] The

particles of all catalyst systems yielded a homogeneous

intense fluorescence before being applied in the polymer-

ization process. After polymerization olefin beads were

obtained showing a very homogeneous distribution of

fluorescent spots (Figure4). Depending on the reaction time

these spots increase in quantity but decrease in size, which

proves that fragmentation of the supports occurs in the

required manner.

The activities obtained for the supported catalyst

systems and the initial studies by LSCM indicate that the

supported systems here studied are suitable for olefin

polymerization. All catalysts show excellent activities

yielding high-molecular polyethylenes.

Comparison of Soft and Hard Materials forSupporting Catalyst

Apart from the different activity and the different relative

growth of the catalyst particles, no significant difference in

the course of the polymerization of the three catalyst

systems could be observed. However, in comparison to

silica-supported catalysts,[6,29] a dramatic difference in the

polymerization process was found. In the slurry and gas

phases, the olefin polymerizations with a silica-supported

catalyst canbedivided into fourdifferentphases. In thepre-

polymerization phase, a first slight polymerization activity

of the supported catalyst can be observed.[14] During this

Figure 4. LSCM saturated images of the middle slice of a productparticle obtained from catalyst A after (a) 5 min and (b) 15 min ofethylene polymerization [magnification of the middle layer(objective, plan Neofluar 10; wavelength, 488 nm; 70% trans-mission; scale bar, 20mm)].

DOI: 10.1002/mren.200900026


phase, a thin crystalline layer of polyethylene is formed

round the catalyst particle. In the second phase, there is a

dramatic reduction inactivity. This effect is causedbya lack

of monomer inside the particles because the monomer has

to diffuse through the crystalline layer of polyethylene

encapsulating the catalyst particle to reach the inner active

centers (diffusion phase). The monomer reaches the active

centers in the core of the particles slowly to form polymer

inside the bead. The generated polymer imparts hydraulic

forces to the support, fragmenting the catalyst particle

(secondary particle) from outside to inside (fragmentation

phase). More active sites become accessible to the mono-

mer, yielding more centers for ethylene polymerization. A

steady increase in activity is observed until the catalyst

particle is completely fragmented and the catalyst system

reaches a maximum rate of polymerization. In the

following particle expansion phase, the polymer particle

expands, andaslowdecrease in thepolymerization rate can

be observed. This effect originates from the increasing

diffusion ways due to the expanding particle of the

monomers to the active centers. This model of describing

the polymerization in the presence of silica-supported

catalysts is called ‘‘polymer-growth and particle-expan-

sion-model’’.[29]

Unlike silica-supported catalysts, the latex-supported

catalyst reaches a maximum in the polymerization rate

immediately after polymerization begins. No significant

drop in the polymerization activity was found after the

maximum rate was reached, which indicates that no

crystalline layerofpolymerwas formedaround thecatalyst

particle. As shown in Figure 1 for latex-supported catalysts,

the maximum rate of polymerization was observed a few

seconds after polymerization began. This can be attributed

to the fact that there was no diffusion barrier. After the

maximum polymerization rate was reached, the activity

decreased steadily into the followingphase. Thedecrease in

activity couldbeobservedduring thewholepolymerization

time for catalyst systems B and C. For catalyst system A,

which showed the lowest activity, the polymerization rate

remained constant for the rest of the polymerization after a

slight drop in the first minutes. This is in agreement with

the results obtained from polymerizations at high pressure

where a constant activity for catalyst system A was

observed over 6 h. There are two explanations for the

decrease in polymerization activity. One possible explana-

tion is that all active metal centers in the catalyst particle

take part in the polymerization right from the beginning.

The continuous drop in polymerization can then be

explained by the diffusion way of the monomer into the

catalyst particle,whichbecomes longerwith the expansion

of the resulting polymer.

It is obvious that the polymerization behavior of latex-

supported catalysts in slurry polymerization in a batch

reactor, video microscopy, and LSCFM leads to a different



mechanism. After the addition of monomer to the reactor,

no initiation phase was found but catalyst particles began

to grow immediately. During polymerization, the growing

rateof theparticles slowlydecreasesbutevenafter60mina

significant polymerization, indicated by the expansion of

the catalyst/polymer particles, was observed.

The different activity of latex- and silica-supported

catalysts at the beginning of the polymerization can be

explained by the different consistence of the carrier

material. Silica is a hard material, which can adsorb the

monomeronly in thepores. Theorganic latexparticles swell

in the polymerization solvent andmonomer is thereby also

absorbed in the carrier material itself. In the case of silica-

supported catalysts, after the small amount of monomer

stored in the pores is consumed, new monomer has to

diffuse into the particle through the crystalline polyethy-

lene layer. The active centers supported on the latex

particles can be fedwithmonomer directly from the carrier

material until new monomer diffuses into the particle.

Another important factor is the composition of the

catalyst particles. While the silica primary particles

agglomerate to a very hard and strong particle, the primary

latex particles build a secondary particle by agglomeration

only through nucleophilic interaction with the cocata-

lyst.[18] Therefore, the catalyst particle based on organic

material consists only of a loose network, which can be

fragmentedmuch easier through the hydraulic force of the

polymer formed inside the catalyst particles. The ability of

the latex particles to swell with monomer and the loose

network of latex particles are the two factors that result in

the very high activity for latex-supported catalysts

immediately after initiation of the polymerization. Since

the organic support is able to swell, presence of monomer

from the beginning of the polymerization is guaranteed

along the whole particle. Polymerization of the polyolefin,

therefore, starts not only on the surface of the secondary

particle but also at the same time in the interior. In contrast

to a porous silica support, the physical cross-linking of the

organic secondary particles is much weaker and the

polyethylene formed during polymerization can break up

the supporting material much more easily. As a result, the

fragmentation can take place throughout the organic

support without an induction period.

Polymer Morphology of the ObtainedPolyolefin Particles

Study of the morphology of the product particles (e.g.,

particle size and surface morphology) gives detailed

information about the polymerization process. Therefore,

polymerizations performed in thevideomicroscopy reactor

in gas phase at different times were developed. The

polymerizations were stopped by releasing the monomer

after the desired polymerization time, and the polymer



Figure 6. Polyethylene of (a) catalyst A and (b) catalyst C after 15 min of polymerization time.

Figure 5. Polyethylene obtained from (a) catalyst A and (b) catalyst C after 30 s of polymerization time.

464

particles were exposed to air

to deactivate the catalyst.

The surfaces of the obtained

polymer particles were inves-

tigated by SEM.

Figure 5(a) shows an SEM-

imageofapolymer sampleof

catalyst systemAafter30 sof

polymerization. On the left,

nano-sized spherical parti-

cles can be seen. These par-

ticles can be identified as the

primary latex particles that

serve as supporting material

for the catalyst. Due to the

defined spherical shape of

the latex particles, the pro-

duced polyethylene can be

distinguished from the origi-

nal supporting material.

Polyethylene seems to grow

likebubblesoutof thecarrier.

Polyethyleneparticles from

catalyst system C, obtained

after 30 s of polymerization,

differed significantly from

PE-particles of catalyst sys-

tem A. On the surface of the

polymer sample from cata-

lyst C, primary latex particles could also be identified, but

the polyethylene produced was not cauliflower-like

[Figure 5(b)]. Some of the latex particles do not seem to

participate in the polymerization as they are still clearly

visible on the surface of theproduct particle in their original

size and not surrounded by polyethylene. On the PE

particles of both catalyst systems, areas could be found

where no polymer was formed during the first seconds,

while inotherareas thepolymerizationhadalreadystarted.

For both types of supported catalysts, itmight be suggested

that every single latex particle in the secondary particle

can be seen as a mini reactor where initiation of the

polymerizationneededmoreor less time.One reasonmight

bea localdifference in theconcentrationofmetalloceneand

activator or in a different accessibility to the monomer.

Samples after 15min of polymerization also showed

differences in polymer morphology. While the polymer

particles fromcatalyst C [see Figure 6(b)]were replica of the

original secondary catalyst particles, the polymer particles

from catalyst A [see Figure 6(a)] seemed to be an

agglomeration of small single polymer spheres replicating

the shape of the primary particles.

At longer polymerization times, characteristic morphol-

ogy structures were found on the polymer particles of all

catalyst systems. On the surface of polymer samples from



catalystBaswell as fromcatalystC, filamentsorfibrilswere

observed. The fibrils on the surface of PE particles were

formedduetoexpansionof thePEparticles. Polymer formed

in the inside led to expansion of the particle. The surface of

the polymer particle could not expand due to minor

polymerproduction, and thepolymer surfacewasstretched

to fibrils (Figure 7).

The formation of polyethylene filaments led to the

suggestion that a high concentration of inter-crystalline

links was formed during polymerization. Another indica-

tion of high concentrations of inter-crystalline links was

found by analyzing the melting temperature of the

produced polyethylene by DSC (Table 4). A marked

difference in the melting temperature of the polyethylene

samples between the first and second heating cycles of the

measurement was observed. For example, in the first

heating cycle of the DSC analysis, PE of catalyst B showed a

melting temperature of 147 8C, while after cooling in the

second heating cycle a Tm of 140 8C was observed. This is a

strong indication for the high concentration of inter-

crystalline links in the nascent PE. After melting of the

polymer sample, cooling, and recrystallization, a normal

concentration of inter-crystalline links was formed and

therefore Tm was significantly lower. The inter-crystalline

linkswereprobably causedbyacombinationof two factors.

DOI: 10.1002/mren.200900026


Table 4. Melting temperatures of PE-samples from catalyst sys-tems 1–3.

PE from

cat-no

Tm first

heating cycle

Tm second

heating cycle

DTm

-C -C -C

A [Ti] 146 140 6

B [Zr] 147 140 7

C [Zr] 139 135 5

Figure 7. Polyethylene obtained from (a) catalyst A and (b) catalyst C after 60 min of polymerizationtime at high ethylene pressure.

One factorwas ahigh concentration of active centers on the

surface of the supportingmaterial. Thesemight have led to

crystallization of polymer chains surrounded by polyethy-

lene spherolithes. Another factor might have been that the

polymerization rate of ethylene by the catalysts was faster

than the crystallization rate of the produced polyethylene

chains. Therefore, the polymer chains are entangled and

crystallized indifferent spherolites to form inter-crystalline

links.

Conclusion

Only hard inorganic supports such as MgCl2 or SiO2 are

typically applied for supporting metallocene and post-

metallocene catalysts in industrial applications. Although

they are well understood due to their technical relevance,

they still have disadvantages, e.g., pre-polymerization is

sometimes required, incomplete fragmentation occurs

during the polymerization, and an inhomogeneous course

of thepolymerization (e.g., an inductionperiod) is observed.

In this paper, we have reported on the systematic

investigation of latex particles as a rather new class of

soft organic support. Immobilization of different types of

catalysts on the particles was elaborated and their

polymerization behavior compared to silica supported



catalysts. It becomes obvious

that the immobilization pro-

cedure and the activation

process have a dramatic

influence on the polymeriza-

tion process. While tradi-

tional metallocenes had to

be pre-activated to result in

good polymerization per-

formance, the FI-catalysts

reacted sensitive to MAO

and decomposed. A detailed

understanding of the poly-

merization process by a sup-

ported catalyst is essential

for improving and enhancing a catalyst system.However, a

comprehensive description of the polymerization process

requires many different methods. While polymerization

experiments in larger reactor systems provide the overall

activity and productivity values, a more detailed look was

necessary to elucidate the processes inside and on the

surface of a single catalyst particle. Video microscopy and

LSCFM were particularly suitable methods to study the

homogeneity of the loading of the supports and the

fragmentation. Opticalmethods such as SEM or TEM reveal

additional information about particle growth, surface

changes, and also inner morphology of the polyolefin

particles obtained. Additionally, organic supports offer the

possibility to optimize each catalyst system simply by

varying the nature of the surface of the organic nanopar-

ticles. In contrast to silica orMgCl2-based supports, organic-

based supports can be synthesized in amuchwider variety

just by altering the type of surfactants, monomers and also

by varying the reaction conditions in the emulsion and

miniemulsion process. The incorporation of pyridine into

the particles drastically improves the activity of the Zr-FI

catalyst.

Acknowledgements: The financial support of Mitsui Chemicals isgratefully acknowledged.

Received: April 24, 2009; Revised: July 10, 2009; DOI: 10.1002/mren.200900026

Keywords: catalysis; catalysts; metallocene; polyolefine; reactionkinetics

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DOI: 10.1002/mren.200900026

Hard versus Soft Materials as Supports for Metallocene and Post-Metallocene Catalysts

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Transcript of Hard versus Soft Materials as Supports for Metallocene and Post-Metallocene Catalysts