ziegler-natta/metallocene hybrid catalyst for ethylene polymerization

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604

Ziegler-Natta/Metallocene Hybrid Catalystfor Ethylene Polymerization

Mostafa Ahmadi,* Roghieh Jamjah, Mehdi Nekoomanesh, GholamHossein Zohuri, Hassan Arabi

A Ziegler-Natta/metallocene hybrid catalyst was produced and utilized in the polymerizationof ethylene with the aim of producing bimodal polyethylene. The MgCl2 adduct was preparedby a melt quenching method and Cp2ZrCl2 and TiCl4 catalysts were loaded, respectively, aftertreating the surface with TiBAl. The polymerization kinetics involved an induction period,followed by fragmentation and expansion ofparticles. SEM micrographs revealed that thespherical morphology was retained throughthe initial mild reaction conditions of induc-tion period. The polymers produced showedbimodal molecular weight distribution pat-terns, suggesting that both components ofthe hybrid catalyst were active over thesupport.

Introduction

Chain microstructure is an important characteristic in

determining the mechanical and rheological properties of

polymers. Virtually every aspect of chain microstructure,

such as molecular weight (MW) and its distribution

(MWD), comonomer content and its distribution (CCD),

short and long chain branching (LCB) and their distribu-

tion, can be controlled through a variety of tailoring

methods. MW largely relates to the mechanical properties

while MWD is responsible for rheological properties. MW

and MWD can be controlled in three ways: by tandem

reactor technology where two or more reactors work

in series at different polymerization conditions;[1–2] by

M. Ahmadi, R. Jamjah, M. Nekoomanesh, G. H. Zohuri, H. ArabiDepartment of Catalyst, Faculty of Polymerization Engineering,Iran Polymer and Petrochemical Institute, P. O. Box: 14965/115,Tehran, IranFax: þ98 2 144 580 0213; E-mail: [email protected]

Macromol. React. Eng. 2007, 1, 604–610

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

variation of the operating conditions during polymeriza-

tion, for example, raising temperature[3–4] or adding a

chain transfer agent;[5] by designing the catalytic system.

A common catalytic system will include three parts:

support, catalyst and cocatalyst. The desired microstruc-

ture can be achieved by manipulating each of these

parts.[6–7] For example, CCD can be controlled by changing

the cocatalyst or mixing different cocatalysts.[8] The

flexible structure of metallocene catalysts can also be

varied to produce polyolefins with a controlled micro-

structure. Favorite microstructures can also be produced

by the careful combination of different catalysts. Several

reviews and patents have been published about recent

advances in olefin polymerization using binary catalyst

systems.[9–15] Catalysts can be combined in three ways: (1)

in the catalyst transition, the second catalyst is introduced

to the reactor in the middle of polymerization and can

continue polymerization with or without (by deactivating)

the first catalyst;[16–19] (2) in the catalyst mixture, two

independent catalysts are introduced to the reactor and

DOI: 10.1002/mren.200700027

Ziegler-Natta/Metallocene Hybrid Catalyst for Ethylene Polymerization

start the polymerization together;[16–27] (3) two catalysts

can also be loaded on a support to form a new catalyst,

named a hybrid catalyst.[28–44]

Beigzadeh et al. have used a mixture of two metallocene

catalysts to manipulate LCB and its distribution in polymer

chains.[25–27] However, most of the work in the field of

catalyst combination is related to the control of MWD and

the production of a bimodally distributed MW. The MWD

of a polymer made by the combination of two catalysts

can be expressed as a weighted summation of the MWD

produced by each catalyst.[30] Based on this assumption,

Soares et al. have developed a bimodality criterion for the

combination of two single site metallocene catalysts.[30–32]

This criterion has been extended by Pinto et al. to include

the combination of multi-site catalysts.[45] The mass

fraction of the polymer produced by each catalyst is

related to its activity and quantity.[23] Agnillo et al. have

reported how a high variance in catalyst activity translates

into a difficulty in predicting the resulting global MWD

when mixing two metallocene catalysts.[23,24] On the other

hand, the stability of catalyst components on the support

in a hybrid catalyst results in a reproducible MWD.[30–42]

However, polyolefins are mostly obtained using Ziegler-

Natta catalysts, and less work has been done on the

combination of different catalysts with these traditional

catalysts.[16–21,36–44] There is great potential in combining

different metallocene catalysts with Ziegler-Natta catalysts,

not only in order to introduce a metallocene catalyst into

traditional processes, but also to combine the advantages

of both catalytic systems. In this study, a new supported

Ziegler-Natta/metallocene hybrid catalyst was prepared

and used for ethylene polymerization with the aim of

controlling the MWD. The magnesium dichloride support

was prepared by the melt quenching method, to produce

spherical polymer particles.

Experimental Part

Materials

The metallocene catalyst (Cp2ZrCl2) and methylaluminoxane

(10 wt.-% in toluene) were purchased from Sigma-Aldrich Chemie

GmbH, (Steinheim, Germany). MgCl2 with a purity of 98%, TiCl4

with a purity of 99.9%, triethylaluminum (TEA) with a purity

of 93%, triisobutylaluminum (TiBAl) with a purity of 93% and

ethanol with a purity of 99.5% were purchased from Merck

Schuchardt OHG (Hohenburn, Germany). Silicon oil with a purity

of 99% and a viscosity of 500 cp was purchased from Dow Corning

Chemical Co. (USA). Polymerization grade ethylene with a purity

of 99.99% was supplied by Iran Petrochemical Co. and was further

purified by passing it through columns of activated 13X and 4 A

molecular sieves. Nitrogen gas with a purity of 99.99% was

purchased from Roham Co. (Tehran, Iran) and was purified by

passing it through columns of P2O5, KOH, activated silica gel and a

4 A molecular sieve. Industrial toluene, supplied by Iran Petro-



chemical Co., was further purified by repeat distillations over

sodium wire and benzophenone.

Characterization

Thermogravimetric analysis (TGA) was used to determine the

alcohol content of the support (Perkin-Elmer Pyris 1). Scanning

electron microscopy (SEM) was used to observe the morphology of

the support, catalyst and polymer particles (Cambridge instru-

ment S 360). The titanium (Ti) and zirconium (Zr) contents of

the catalyst were determined using inductively coupled plasma

(ICP, Swiss 3410 ARL). Gel permeation chromatography (GPC) with

1,2,4-trichlorobenzene as the solvent at 140 8C was used to

determine the MWD (PL-210).

Preparation of the Hybrid Catalyst

Details of the experimental procedure for producing MgCl2.nEtOH

(adduct) can be found elsewhere.[46] 5 g of adduct were introduced

into a glass reactor equipped with a mechanical glass mixer.

Temperature was controlled by the circulation of oil around the

reactor jacket using a Lauda circulator, model RP 845. 90 mL of

toluene were added as the diluent and TiBAl was added to the

reactor at �5 8C using a dropping funnel, according to the alcohol

content of the adduct. The mixture was stirred at 0, 20, 40 and

60 8C for 30 min and at 80 8C for 2 h. The mixture was then washed

five times with toluene to remove unreacted TiBAl and was

dried under a nitrogen atmosphere at 60 8C to yield the support.

2 g of the obtained support were introduced into the same

glass reactor and 100 mL of toluene were added as the diluent.

0.3 g of the metallocene catalyst of Cp2ZrCl2 were added and the

temperature was raised to 50 8C. The mixture was stirred at

180 rpm for 2 h and then washed five times with toluene. 100 mL

of toluene were added again and 20 mL of TiCl4 were introduced at

80 8C. The mixture was stirred for 2 h and then washed in the

same manner to give the final Ziegler-Natta/metallocene hybrid

catalyst. The resulting catalyst was dried under a nitrogen

atmosphere at 60 8C and suspended in 100 mL of toluene to be

used for polymerization.

Polymerization

All polymerizations were performed in a 1 L stainless steel Buchi

reactor, model bmd 300. The reaction medium was mixed with a

paddle mixer rotating at 800 rpm. The temperature was controlled

by the circulation of water using a Huber circulator, model Polysat

CC3. The reactor was purged with nitrogen gas at 90 8C for about

30 min to ensure the absence of moisture and oxygen. The reactor

was then charged with 500 mL of toluene and was subsequently

evacuated five times. Nitrogen was used to refill it, and this was

followed by purging with ethylene at least five more times. The

temperature was raised to the desired value and the reactor

was filled with ethylene to saturate the toluene. The cocatalyst

was added to the reactor by means of a syringe and, after a few

minutes agitation, the catalyst was introduced in the same

manner. The reaction was started by filling the reactor with

www.mre-journal.de 605

M. Ahmadi, R. Jamjah, M. Nekoomanesh, G. H. Zohuri, H. Arabi

606

ethylene to the desired pressure and mixing. The monomer

consumption was monitored using a Pressflow gas controller.

After the specified polymerization time, the reaction was

terminated by degassing the reactor. The polymer produced

was washed with ethanol and then dried.

Figure 1. TGA thermograms of: (a) MgCl2; (b) adduct; (c) support.

Results and Discussion

Characterization of Catalyst

Ethanol was used in the process of adduct preparation as a

solvent. Because the alcoholic groups in the support serve

as deactivating materials for the metallocene catalyst,

they should be eliminated. On the other hand, in a similar

situation, dos Santos et al. found that the maximum

catalyst loading on silica decreased when decreasing

the alcohol content of the support.[47] The metallocene

compound is believed to react with the hydroxyl groups on

silica and attach to the surface through M–O–Si bonds.[48]

Therefore the highest catalyst activity is achieved at an

optimum alcohol content. Two methods are generally

used to reduce the alcohol content of the support to the

Figure 2. SEM micrographs of: (a) adduct 50�; (b) support 100�; (c)



optimum level. The first involves physically heating under

a nitrogen atmosphere[46] and the second involves a

chemical reaction between alcoholic groups and alkylalu-

minum compounds.[36–42] In our work, TiBAl was used

in stoichiometric amounts according to the alcohol

content of the adduct. TiBAl has been found to be the

most effective material for impregnation of the Cp2ZrCl2

support 2 500�; (d) catalyst 3 000�.

DOI: 10.1002/mren.200700027


Table 1. Elemental analysis of the hybrid catalyst.

Element wt.-%

Ti 1.620

Zr 0.023

Al 0.888

catalyst, among other alkylaluminum compounds like TEA

and TMA.[40] In addition, because of milder reaction

characteristics, TiBAl is preferred to yield catalyst particles

with a higher mechanical strength.

TGA was used to calculate the alcohol content of the

adduct. Figure 1 shows that the alcohol content of MgCl2

was increased to 59% during preparation of the adduct and

was then reduced to 24% by treatment with TiBAl. Figure 2

shows SEM photographs of the adduct, the support after

treatment and the hybrid catalyst. The MgCl2 particles

were spherical in shape as they were converted to adduct.

It is obvious that, after treatment with TiBAl, the spherical

morphology was saved, but the support became more

porous, which is necessary to achieve higher activity. The

spherical shape is also saved after the loading of

metallocene and Ziegler- Natta catalysts. Table 1 shows

the elemental analysis of the hybrid catalyst according to

ICP results. The Ti content in the catalyst is about 70 times

higher than the Zr content. Considering the higher activity

of the Cp2ZrCl2 catalyst, which was observed in our

Table 2. Polymerization conditions and results of catalyst activity. Ptoluene at 800 rpm for 1 h.

Considered parameter Run T P C

-C bar

1 60 3

2a) 60 2

Temperature 3 40 2

4 50 2

5 60 2

6 70 2

Pressure 7 60 1

5 60 2

8 60 3

9 60 4

[Al]/[TiRZr] 10 60 2

5 60 2

11 60 2

12 60 2

a)2 h.



laboratory to be about a hundred times higher than

Ziegler-Natta catalyst, it seems that this ratio is suitable for

the production of bimodal polyethylene. There is also a

considerable amount of aluminum in the catalyst which

suggests that TiBAl has entered into the structure of the

support. An exact interpretation of how the aluminum

compounds are bonded to the structure needs further

investigation.

Ethylene Polymerization

The operational conditions of all runs and the results

of catalyst activity in the polymerization of ethylene

are depicted in Table 2. Temperature, monomer partial

pressure, cocatalyst type and its concentration were

varied, as the parameters with the most effect on activity

and MWD. It has been reported in some references that

metallocene catalysts are inactive in the presence of

traditional cocatalysts like TEA.[17,36] Therefore it was

expected that the contribution of the metallocene

component of the hybrid catalyst in the polymerization

of ethylene would become negligible when TEA was used

as the cocatalyst. Thus MAO was also used as a cocatalyst

to activate the metallocene component and to produce

bimodal polyethylene.

Monomer partial pressure was increased in run 3, 7, 8

and 9. Table 2 shows that the activity increases with

olymerization conditions: [TiþZr]¼ 5.75� 10�6 mol � L�1 in 500 mL

ocatalyst [Al]/[TiRZr] Activity

kg � (mol TiRZr)S1 �hS1

TEA 1 740 21 867.07

TEA 1 740 8 350.19

MAO 1 260 1 663.08

MAO 1 260 10 300.31

MAO 1 260 8 839.03

MAO 1 260 5 801.65

MAO 1 260 7 770.90

MAO 1 260 8 839.03

MAO 1 260 11 618.95

MAO 1 260 23 568.42

MAO 945 6 883.70

MAO 1 260 8 839.03

MAO 1 575 16 642.98

MAO 1 890 23 577.12



Figure 3. Effect of temperature on polymerization kinetics: (a) run3: 40 8C; (b) run 4: 50 8C; (c) run 5: 60 8C; (d) run 6: 70 8C. For detailsof polymerization conditions, see Table 2.

608

monomer pressure, as would be expected. This indicates

that the cooling system used was competent for removing

the heat which builds up from the polymerization reaction.

Table 2 shows that the activity also increases when

Figure 4. SEM micrographs of polymer particles produced in run12: (a) 100�; (b) 3 000�.



increasing the [Al]/[M] molar ratio, when MAO was used

as the cocatalyst. This suggests that there are still inactive

centers which become active by complexation with

further cocatalyst molecules. It cannot yet be determined

whether these sites are Ti or Zr, because both the

Ziegler-Natta and metallocene component of the hybrid

catalyst are active in the presence of MAO.[36–42]

The polymerization temperature was increased from

40 to 70 8C, through run 3 to 6. Table 2 shows that the

highest activity was obtained at about 53 8C. The solubility

of ethylene in toluene decreases with temperature,

while rate constants increase, according to the Arrhenius

equation. Therefore the catalyst activity increased with

temperature, because of the growth in the propagation

rate constant, and decreased above the optimum tem-

perature as a result of the reduction in monomer solubility

and growth of the deactivation rate constant, which leads

to a decrease in the concentration of active centers. The

effects of temperature on the polymerization kinetics are

more obvious in Figure 3 which shows the polymerization

yield versus time. The rise in the deactivation rate constant

with increasing temperature led to a reduction in the

polymerization rate during the last stages of polymeriza-

tion. Diffusion limitations are more dominant in the initial

stages of polymerization. The coating of the surface of

catalyst particles with the polymer produced results in a

high initial polymerization rate, followed by an induction

period as the coating limits the diffusion of the mono-

mer.[49] Under these mild reaction conditions, catalyst

particles undergo conditions similar to a prepolymeriza-

tion process. It was therefore expected that a long

induction period helps the particles to retain their

spherical shape during fragmentation and expansion

when enough monomer molecules diffuse to the inner

active centers. This complex behavior cannot be inter-

preted by a simple kinetic model and a diffusion model

should be combined with a kinetic model to interpret the

whole polymerization process.

Figure 5. Effect of cocatalyst type on polymerization kinetics: (a)run 2; (b) run 12. For details of polymerization conditions, seeTable 2.

DOI: 10.1002/mren.200700027


Figure 6. MWDs of polyethylene produced by the hybrid catalystunder different polymerization conditions: (a) run 2; (b) run 12; (c)run 5; (d) run 9; (e) run 3. For details of polymerization conditions,see Table 2.

Figure 4 shows SEM micrographs of the polymer

particles produced in run 12, which indicate that the

spherical shape of the catalyst particles has replicated

through uniform fragmentation. Figure 5 shows the effect

of cocatalyst type at approximately similar concentrations

on polymerization kinetics. Catalyst activity was persist-

ent even after 2 h when TEA was used as a cocatalyst.

However, the activity was higher when MAO was used. It

can be concluded that both components of the hybrid

catalyst are more active in the presence of MAO.

Figure 6 shows the changes in MWD when the

polymerization conditions were varied. The MWDs of

polyethylene produced using the hybrid catalyst showed

a bimodal pattern. Curve a belongs to run 2, when TEA

was used as a cocatalyst. It is obvious that the broad

asymmetric MWD is yielded from two sources. The lower

MW region is the result of the metallocene component and

the higher MW region is from the Ziegler-Natta compo-

nent. It can be concluded that the metallocene compound

was active, even in presence of TEA. There are many

reports on the deactivation of metallocene catalysts in the

presence of traditional cocatalysts,[17,36] but some reasons

can be suggested for this unprecedented behavior. It has

been reported that during prealumination of the support,

alkylaluminum compounds can attach to the surface

through a support–O–Al bond. The loaded alkylaluminum

can methylate metallocene molecules during impregna-

tion of the metallocene catalyst. Thus metallocene

molecules are immobilized on the surface by an ionic

interaction with the bonded alkylaluminum, and become

active species at the same time.[48] Another reason is the

reaction of TiBAl with the alcoholic groups of the adduct in

the treatment stage and the production of supported

alkylaluminoxane species.

Curve b belongs to run 12 when MAO was used as

cocatalyst, at an approximately similar concentration to



run 2 under the same operational conditions. It can be

concluded that the MWs of the polymer produced by both

components of the hybrid catalyst increased when using

MAO. This difference arises from the bulkier structure

of MAO relative to TEA, which impedes chain transfer

reactions. The reduction of the MAO concentration in run

5 leads to the broader MWD depicted in curve c. The MWs

of polymer produced by both components of the hybrid

catalyst have increased because chain transfer to the

cocatalyst has decreased. It is also obvious that the amount

of polymer produced by the metallocene component of

the catalyst has decreased. Therefore, it can be suggested

that the inactive centers that become active at higher

cocatalyst concentrations were metallocene species.

Curve d was obtained by the enhancement of monomer

pressure in run 9. There are no significant changes in the

shape of the MWD, which suggests that the importance

of chain transfer to monomer is the same for both

components of the hybrid catalyst. Curve e shows the

effect of reducing the temperature in run 3. The amount of

polymer produced in the lower MW region decreased, thus

the overall MW has increased and the MWD has become

narrower. The higher activation energies of chain transfer

and deactivation reactions relative to the propagation

reaction cause an increase in MW at lower tempera-

tures.[50]

Conclusion

The new Ziegler-Natta/metallocene hybrid catalyst pro-

duced appeared to be compatible on the MgCl2.nEtOH

support produced by a melt quenching method. The specific

polymerization kinetics, which involved an induction

period followed by fragmentation and expansion of

particles, were believed to be responsible for the uniform

fragmentation of spherical particles. Replication phenom-

ena were indicated by SEM micrographs of the adduct,

support, catalyst and polymer particles. Ethylene poly-

merization was carried out at different operational

conditions and the effects of polymerization conditions

on MW and MWD were investigated to find alternative

ways to controlling MW and its distribution. The resulting

polyethylenes showed bimodal MWD patterns, suggesting

that the components of the hybrid catalyst acted as

individual active species over the support and polymers

were mixed at molecular level. It was found that the

metallocene component was active in the presence of

TEA, and this will be further investigated in future work. A

bimodal MWD was also obtained when MAO was used as a

cocatalyst. The negligible variation in MWD after raising

the monomer pressure suggests that the importance of

chain transfer to monomer is similar in both components

of the hybrid catalyst. The MW increased as chain transfer



610

and deactivation reactions decreased on reduction of

temperature, and MWD converted to unimodal as a result

of reduction of the polymer produced in the lower MW

region.

Received: June 13, 2007; Revised: August 7, 2007; Accepted:August 22, 2007; DOI: 10.1002/mren.200700027

Keywords: ethylene polymerization; hybrid catalysts; metallo-cene catalysts; polymerization kinetics; Ziegler-Natta catalysts

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DOI: 10.1002/mren.200700027

ziegler-natta/metallocene hybrid catalyst for ethylene polymerization

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