the role of the support in the performance of grafted metallocene catalysts

Full Paper

The Role of the Support in the Performance ofGrafted Metallocene Catalysts

Fernando Silveira, Maria do Carmo Martins Alves, Fernanda C. Stedile,Sibele B. C. Pergher, Joao Henrique Zimnoch dos Santos*

A series of supports – differing in their textural properties, the nature of their surface sites, andin their crystallinities – were investigated in the sequential grafting reaction of Cp2ZrCl2 and(nBuCp)2ZrCl2 mixtures. The catalyst systems were analyzed by Rutherford backscatteringspectrometry, AFM, EXAFS, and nitrogen adsorp-tion. All systems were shown to be active inethylene polymerization reactions when methyl-aluminoxane was used as the cocatalyst. Experi-mental results are discussed in terms of therelationships between the grafted metal content,the catalyst activity, the surface roughness andpolarity, the interatomic Zr–C and Zr–O distancesof the grafted species and the molecular weightsof the resulting polyethylenes.

Introduction

Plastics obtained by olefin polymerization represent the

largest volume of polymer produced and commercialized

by the world’s petrochemical industry. Among the

available catalyst systems, metallocenes have generated

increasing interest in both industrial and academic

research due to the new properties that such catalyst

systems can engender in polymers and, especially,

F. SilveiraBraskem S.A., III Polo Petroquımico, Via Oeste, Lote 05, Triunfo,95853-000, BrazilM. C. M. Alves, F. C. Stedile, J. H. Z. dos SantosInstituto de Quımica, UFRGS, Av. Bento Goncalves 9500, PortoAlegre 91509-900, BrazilFax: þ55 51 3308 7304; E-mail: jhzds@iq.ufrgs.brS. B. C. PergherDepartamento de Quımica, Universidade Regional Integrada doAlto Uruguai e das Missoes (URI) – Campus Erechim, CP 743,Erechim, 99700-000, Brazil

Macromol. React. Eng. 2009, 3, 139–147

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

copolymers. It has been estimated that the polyolefin

market will grow by 64% over the next decade, that is, 3–

5% per year. It is believed that the amount of polyethylene

produced by metallocene catalysts will increase by 470%.

At present, the market for polyethylenes produced by

metallocenes represents ca. 5–6% of the polyolefin market,

which corresponds to 3–4 million tons of polymer per

year.[1]

Supported metallocenes have been widely investigated

in both the open and the patent literature.[2] Silica has been

themost widely studied support. Nevertheless, other types

of support have also been evaluated for use in the

immobilization of metallocenes with the aim of increasing

catalyst activity or generating different polymer proper-

ties. Some examples of the different supports are alumino-

silicates,[3] organotin-modified silica,[4] organosilicon-

modified magnesium-chloride-based supports, especially

in combination with Ziegler-Natta catalysts,[5] and, more

recently, nucleation agents, such as sorbitol[6] and carbon

nanotubes.[7] Among the organic supports, polystyrene

has been the most widely employed support. Examples of

DOI: 10.1002/mren.200900004 139

F. Silveira, M. C. M. Alves, F. C. Stedile, S. B. C. Pergher, J. H. Z. dos Santos

non-interacting polystyrene,[8] or spacer-modified[9] or

poly(ethylene oxide) monomethyl ether functionalized

polystyrene[10] can be found in the literature. The use of

nano-sized latex has also been reported as an alternative

organic support.[11]

As shown in Scheme 1, the supports investigated differ

in terms of their morphology, texture and surface

chemistry. In terms of morphology, such materials can

adopt granular, spherical or lamellar forms, which, in turn,

may affect the polymer morphology by means of

replication phenomena or extrusion polymerization.

Texture may influence the nature and distribution of

potential catalyst sites, as well as the accessibility of the

cocatalyst and monomer to the growing polymer chain.

Finally, supports differ with respect to the presence of

Lewis or Bronsted acidic and basic sites, which may affect

the amount of grafted material, the metallocene distribu-

tion and the reactivity of such surface species, as well the

characteristics of the resulting polymer in terms of

molecular weight, crystallinity and polydispersity.

The support plays an important role in the development

of a supported metallocene catalyst. It may affect catalyst

activity by generating bidentate inactive grafted species or

by hindering a cocatalyst’s access to the potential catalyst

species, thereby hindering the catalyst’s activation.

Furthermore, the support can promote the generation of

Scheme 1. The potential effects of the support on supported metallo

active sites with lower propagation rates resulting from

interactions with the support’s surface, or restrict mono-

mer access to the active sites, thereby hindering chain

propagation.[12] Generally, polymers produced with sup-

ported metallocenes have higher average molecular

weights, due to a reduction in the rates of the termination

reactions.[13] Clearly, the nature of the support plays a very

relevant role in the performance of the supported

metallocene.

We have been studying the heterogenization of

metallocenes for ca. fifteen years in terms of the influence

of experimental parameters (grafting time and tempera-

ture, contact time, solvent, etc) on the grafting reac-

tion;[14,15] the effect of alternative immobilization techni-

ques, such as the synthesis of supported metallocenes on

hybrid silicas,[16] in situ immobilization[17] or metallocene

encapsulation;[18] the nature of the surface species;[19,20]

and the effect of surface modification with compounds

such as organosilanes[21] or tin chloride.[22] More recently,

we have investigated the effect of the textural properties

of silicas produced by different techniques (aerogel,

xerogel, precipitated silicas, etc),[23,24] as well as the

effect of the support surface’s chemical composition

(alumino-silicates, silica-zirconia, magnesium silica sup-

ports) and its crystallinity (amorphous, lamellar and

crystalline materials)[25] on the catalysts’ performance.

cene catalysts.

DOI: 10.1002/mren.200900004

The Role of the Support in the Performance of Grafted Metallocene Catalysts

An investigation of the leachability in those supported

systems showed it to be negligible.[23,24]

In this paper, we systematize our previously reported

results with the aim of extracting some trends related to

the support’s texture and polarity, and to its effect on the

grafted metal content, the nature of the surface metallo-

cene species, the catalyst’s activity and the morphologies

and characteristics of the resulting polymers.

Experimental Part

Materials

All chemicals were manipulated under an inert atmosphere using

Schlenk techniques. Commercial silicas (Grace 560, Grace 550 and

Grace 480) were provided by Grace Chemical. Chrysotile was

kindly provided by SAMA (Goias, Brazil). SBA-15 was prepared at

the Instituto de Tecnologıa Quımica de Valencia (ITQ-UPV, Spain).

Methylaluminoxane (MAO)-modified silica (SMAO) with 23 wt.-%

Al/SiO2 (Witco) and alumina (INLAB, Brazil) were used without

further purification. MCM-41, MCM-22 and ITQ-2 were prepared

according to literature procedures.[26–28] Silica xerogel and silica-

zirconia were prepared, respectively, by hydrolytic and non-

hydrolytic sol-gel processes.[23] Silica aerogel was synthesized at

CENERG (France). Leached chrysotile was obtained according to a

procedure in the literature.[29] Furthermore, the catalysts

(nBuCp)2ZrCl2 (Aldrich) and Cp2ZrCl2 (Aldrich) and the cocatalyst

MAO (Witco; 10.0 wt.-% toluene solution) were used as received.

Toluene was deoxygenated and dried by standard techniques

before use. Ethylene and argon (White Martins) were passed

through molecular sieves (13 A) prior to use. Toluene was purified

by refluxing over sodium and distilled under nitrogen just before

Synthesis of the Supported Hybrid Catalysts

All of the supports were activated under vacuum (P<10�5 bar) for

16 h at 450 8C. In a typical experiment, a toluene solution

of Cp2ZrCl2, corresponding to 0.25 wt.-% Zr/SiO2, was added to ca.

1.0 g of the pre-activated support and stirred for 30 min at room

temperature. The solvent was then removed under vacuum

through a fritted disk. A toluene solution of (nBuCp)2ZrCl2,

corresponding to 0.75 wt.-% Zr/SiO2, was added, and the resulting

slurry was stirred for at least 30 min at room temperature, then

filtered through a fritted disk, and the solids were washed with

toluene (10�2.0 cm3) and dried under vacuum for 4 h.

The same procedure was carried out on all of the activated

supports, such as MCM-22, ITQ-2, SBA-15, silica-MAO, alumina,

silica-zirconia produced by the non-hydrolytic sol-gel route, as

well as on natural chrysotile. The resulting supported catalyst

systems were named M22, IT2, S15, SMAO, ALU, NHI and nCR,

respectively, after catalyst grafting. For the silicas, the label

corresponds to the commercial source: G56 for silica Grace 560,

SMAO for MAO-modified silica; or to the synthetic route: AER for

aerogel, HID for xerogel produced by the hydrolytic sol-gel route.

Characterization of Supported Catalysts

Rutherford Backscattering Spectrometry (RBS)

Zirconium loadings in the catalysts were determined by RBS

using Heþ beams with 2.0 MeV of energy incident on

homogeneous compressed (12 MPa) tablets of the catalyst

material. The method is based on the determination of the

number and energy of the detected particles that are elastically

scattered by the Coulombic field of the atomic nuclei in the target.

In this study, the Zr/Si or Zr/Al atomic ratios were determined by

the heights of the signals corresponding to each of the elements in

the spectra and converted to a wt.-% of Zr/SiO2 or Al2O3. For an

introduction to thismethod and applications of this technique, the

reader is referred elsewhere.[30]

Nitrogen Adsorption/Desorption Isotherms

Samples were degassed (10�2 mbar) at 120 8C (silica) or 85 8C(supported catalysts) for 8 h. Adsorption/desorption nitrogen

isotherms were measured at �196 8C in a Gemini 2375 (Micro-

meritics) instrument. Specific surface areas (SBET) were determined

by the Brunauer-Emmett-Teller equation (P/P0¼0.05–0.35). The

mesopore size and distribution were calculated by the Barrett-

Joyner-Halenda (BJH) method using the Halsey standards.

Desorption branches were used.

Atomic Force Microscopy (AFM)

Images of the supported catalyst surfaces were obtained using a

Nanoscope IIIa1 atomic force microscope, manufactured by Digital

Instruments Co., using the contact mode technique with silicon

nitride probes. The WS M 4.0 software from Nanotec Electronic

S.L.[31]wasused to process the images and to calculate the roughness

of the surface (RMS). The samples were compressed in the form of

tablets and fragments of roughly 16mm2were used for the analysis.

Extended X-Ray Absorption Fine Structure (EXAFS)

The EXAFS measurements were performed around the Zr K edge

(E¼17 998 eV) using the Si(220) channel-cut monochromator at

the XAFS 1 beamline (LNLS, Campinas, Brazil). The spectra were

collected in fluorescencemode using one ionization chamber filled

with argon and a Si(Li) detector. In order to perform the EXAFS

experiments, the supported metallocene powder was compacted

into a pellet and covered with Kapton1 tape. All of the

manipulations were performed in a dry box to avoid any

oxidation reactions. The EXAFS spectra were acquired from

17900 to 18900 eV in 3 eV intervals. Several scans were averaged

in order to improve the signal-to-noise ratio.

The IFEFFIT analysis package[32] and the Winxas program[33]

were used to analyze of the EXAFS data. The EXAFS signals

between 1.0 and 10.0 A�1 were Fourier transformed with a k1

weighting and a Bessel window. Structural parameters were

obtained from least-squares fitting in k and R space using

theoretical phase shift and amplitude functions deduced from the

FEFF7 code.[29] The input for the FEFF7 code was provided by

the ATOMS program.[34] In the fitting procedure, the amplitude

reduction factor (S20) was close to 1.0 for all samples, and the

threshold energies (E0) for the Zr–C and Zr–O pairs were �7.5 and

�3.5 eV, respectively.

www.mre-journal.de 141

Polymerization Reactions

Polymerizations were performed in toluene (0.15 L) in a 0.30 L

Pyrex glass reactor connected to a constant-temperature circulator

equipped with mechanical stirring and inlet ports for argon and

monomers. For each experiment, the catalyst system (a mass

corresponding to 10�5 mol � L�1 of Zr) was suspended in toluene

(0.01 L) and transferred into the reactor under argon. The

polymerizations were performed under an atmospheric pressure

of ethylene at 60 8C for 30 min at an Al/Zr ratio of 1 000:1, using

MAO as the cocatalyst. Acidified ethanol (HCl) was used to quench

the reactions. The reaction products were separated by filtration,

washed with distilled water and finally dried under reduced

pressure at 60 8C.

Polyethylene Characterization

The molar masses and molar mass distributions were determined

with a Waters CV plus 150C high-temperature gel permeation

chromatography (GPC) instrument, equipped with a viscosimetric

detector, and three Styragel HT-type columns (HT3, HT4 and HT6)

with an exclusion limit of 1�107 for polystyrene. 1,2,4-

trichlorobenzene was used as the solvent with a flow rate of

1 cm3 �min�1, and the analyses were performed at 140 8C. Thecolumns were calibrated with polystyrenes that had standard

narrow molar mass distributions and with linear low-density

polyethylenes and poly(propylene)s. Scanning electron micro-

scopy (SEM) experimentswere carried out on a JEOL JSM/6060. The

powders were initially fixed on a carbon tape and then coated

with gold by conventional sputtering techniques.

Table 1. Grafted metal content, textural characteristics and catalytcharacteristics for several supported metallocenes.[22,24]

Supported catalyst Grafted metal content Surface area

wt.-%a) m2 � g�1

G56 0.51 200� 3

G55 0.33 244� 2

G48 0.35 249� 3

HYD 0.50 341� 0

AER 0.15 462� 2

M41 0.84 768� 1

lCR 0.20 226� 0

NHI 0.41 18.0� 0

SMAO 1.00 99� 0

ALU 0.90 103� 1

S15 0.42 463� 0

IT2 0.61 478� 0

M22 0.51 313� 0

nCR 0.21 14� 0

a)Expressed relative to Zr/SiO2 or Al2O3.

Results and Discussion

Table 1 shows some data on the supported metallocenes,

such as their grafted metal content, their textural

characteristics, their catalytic activity in ethylene poly-

merization and the resulting polymer characteristics. More

information can be found elsewhere.[23,25] According to

Table 1, the grafted metal content varied from 0.15 to

1.00 wt.-% Zr/SiO2 or Zr/Al2O3, depending on the support,

while their textural characteristics were very distinct. As

shown in Figure 1, grafted metal content was expressed as

a Zr density by taking into account the immobilized metal

content (determined by RBS) and the specific surface area

(determined by the BET method, after nitrogen adsorp-

tion).

Figure 1 clearly shows that three catalyst categories can

be identified: i) catalysts NHI (which possesses 12.9 wt.-%

Zr/SiO2 incorporated into the silica backbone) and nCR

(which possesses 10.4 wt.-% Mg); ii) catalyst ALU, which

uses alumina as a support; and, iii) catalysts containing

silica or alumino-silicates as their support. The latter

group, independent of the specific support, has Zr densities

near 0.2 nm�2. It is likely that the steric repulsion caused

by the metallocene itself may restrict the number of metal

centers grafted to the OH-group-bearing surface. As

already shown by IR spectroscopy, the surface reaction

between metallocene centers and the silanol groups on a

silica surface is affected by steric repulsion from the

ic activity in ethylene polymerization and the resulting polymer’s

Pore diameter Catalyst activity Mw Mw=Mn

A kgPE �mol�1Zr �h�1 kg �mol�1

145 4 280 423 2.0

128 2 220 338 2.1

118 5 310 258 1.9

39 1 680 356 3.0

58 3 537 343 2.0

24 880 484 1.9

39 1 927 378 2.1

36 720 306 2.4

84 6 560 388 2.2

42 853 351 2.2

50 3 200 273 2.5

14 1 260 477 2.0

19 1 467 275 2.1

39 400 254 2.0

DOI: 10.1002/mren.200900004

Figure 1. Zr density as function of surface area for several sup-ported metallocenes.

grafted metallocene, which hinders further reaction

between the metallocenes in solution and the remaining

silanol groups on the surface.[20]

A higher concentration of Zr per square nanometer

could produce catalysts having higher activity due to

either the greater quantity of potential catalyst (metal)

centers or the higher number of catalyst sites, which make

it more robust with respect to catalyst poisoning. Figure 2

shows catalyst activities expressed as function of Zr

density.

It indicates that the supported catalysts’ activities vary

significantly, although the majority of the supports have

similar Zr densities. Catalysts NHI, nCR and ALU – despite

having a high Zr density on the support – showed the

lowest catalytic activities. It is worth noting that a higher

Zr density may result in catalytic centers that are closer

Figure 2. Catalytic activity of the supported metallocenes as afunction of Zr density.

together; this proximity may promote the formation of

bimolecular metallocene species, which are known to be

inactive in olefin polymerization reactions.[35]

A large number of ions associated with bimolecular

species were detected by matrix-assisted laser desorption/

ionization time-of-flight mass spectrometry (MALDI-TOF-

MS) analysis of catalyst supports having pore diameters

smaller than 100 A.[24] In such pores, the proximity of

intraglobular OH groups makes it easier to generate

bimolecular species on the surface, as shown in Scheme 2.

Catalyst activity can also be affected by the surface’s

roughness. As previously proposed, small diameter pores

may act as obstacles on the surface, imposing a diffusion

constraint on both the monomer and the cocatalyst.[36,27]

The higher the proportion of small diameter pores on the

surface, the greater the number of obstacles and, conse-

quently, the lower the catalyst activity. Obstacles on the

surface of the support afford it roughness. AFM allows us

to estimate this, expressed as the root-mean-square (RMS)

roughness, for the support surface on the nanometer scale.

Figure 3 demonstrates the correlation between catalyst

activity in ethylene polymerization and the surface

roughness of the different supported catalyst systems

investigated.

As shown, greater surface roughness correlates with

lower catalyst activity. It is worth noting that SMAO

exhibited the smoothest surface and afforded the highest

catalyst activity. This type of support is known to produce

very active catalysts. Its high activity has been attributed

to the nature of the surface species generated. It is believed

that the MAO creates a liquid layer that allows the

metallocene species to disperse over the surface and

renders the distribution of the surface species much more

Scheme 2. Proposed nature of grafted species in small diameterpores.

Figure 3. Surface roughness (RMS) of the supported metallocenecatalysts and its effect on catalyst activity in ethylene polymeri-zation.

homogeneous.[38] This work contributes to the field by

highlighting another reason why metallocenes supported

on MAO-modified silicas always exhibit a high activity. It

seems that coating a silica surface, for example with MAO,

promotes the generation of a smoother support surface,

which, in turn, aids the diffusion of both themonomer and

the cocatalyst.

However, the textural properties of the support do not

only affect catalyst activity by means of steric

effects. Cp2ZrCl2 and (nBuCp)2ZrCl2 possess Zr–C inter-

atomic distances of 2.34 and 2.35 A, respectively.[39] In the

corresponding supported systems, this distance contracts

to between 1.5 and 2.2 A, which suggests that the pore

diameter of the support may influence the structure of

Figure 4. Influence of pore diameter on the mean Zr–C inter-atomic distance in the grafted metallocene species.

metallocene catalyst within the pores. Figure 4 shows the

correlation between the Zr–C interatomic distance and the

pore diameter.

Iler[40] proposed that, in silicas with pore diameter

smaller than 100 A, the negative curvature keeps silanol

groups closer, favoring the formation of hydrogen bonds

and, therefore, increasing the stability of silanol groups

against dehydroxylation. The presence of a larger number

of silanol groups can favor their interaction with the

grafted zirconocene species. IR bands assigned to the

interaction between anisole and surface silanol groups

have already been reported in the literature[41] and

attributed to the interaction between hydroxyl groups

and the p-electron system of the aromatic ring. The

interaction of intraglobular silanol groups within small

diameter pores could cause grafted species to becomemore

closely packed and, thus, increase the Zr–C distance of the

cyclopentadienyl rings in the supported metallocenes due

to the kind of interaction mentioned above. Such an

increase would influence the olefin coordination and chain

propagation steps, which in turn would affect the overall

catalyst activity. This proposition is depicted in Scheme 3.

Figure 5 shows the relationship between Zr–C intera-

tomic distance and catalyst activity; increases in the Zr–C

distance are correlated with a reduction in catalyst

activity.

The polarity of the support can also influence the

performance of the immobilizedmetallocenes. Polarity can

be estimated by nitrogen adsorption by determining the

constant C in the BET method.[42] Figure 6 shows the

relationship between the support’s polarity (determined

by BET) and the Zr–O distance (determined by EXAFS).

Two opposing trends are observed: for silica-based

catalysts, an increase in surface polarity engenders an

Scheme 3. Proposed interaction between grafted metallocenespecies within small diameter pores.

DOI: 10.1002/mren.200900004

Figure 6. Influence of surface polarity on the mean Zr–O inter-atomic distance in the grafted metallocene species.

Figure 5. Correlation between Zr–C interatomic distance andcatalyst activity.

increase in the Zr–O interatomic distance; however,

supports made from alumino-silicates, alumina, silica-

zirconia and natural chrysotile show the opposite trend –

with these support materials increases in surface polarity

result in decreased catalyst activity. It is noteworthy that

the second group of supports has significantly higher C

values (greater than 100), while the silica-based materials

have lower values (70–100). The increase in polarity in the

second group of supports is probably caused by the

presence of Lewis acid sites (e.g., Zrþ and Alþ), and may

affect the oxygen bonding in the surface–O–Zr (from the

zirconocene) triad. In the case of silica-based supports, the

Zr–O interatomic distances were between 1.98 and 2.10 A.

It is worth noting that the constant C in the BET equation is

related to the adsorption heat. A large value of C may be

observed when the pore size is small. Nevertheless, taking

into account only pore size, one cannot account for the

trends observed in Figure 6 (see Table 1). The textural

properties of the support could also cause the differences

observed in the mean Zr–O interatomic distances in the

surface-supported metallocene species.

The support may also affect the morphology of the

polymer produced. In the case of commercial supported

Figure 7. Micrographs of polymers produced by the M22 and G48 catalyst systems.

Ziegler-Natta catalysts, it is well known

that the spherical polymer formed is a

morphological result of the spherical

magnesium chloride adduct morphol-

ogy. The metallocenes grafted onto

alumino-silicates catalyze a process of

‘‘polymerization through extrusion,’’ in

which the polymer chain grows out of

the mesopores up to the particle’s sur-

face. This type of polymerization often

produces polymers in fiber form.[37,43,44]

In our studies, M22 catalysts did indeed

produce nanofibers, as shown in Figure 7. Similar

morphologies were observed for NHI, M41 and lCR

catalysts. The width of the nanofibers varied between

12.5 and 39.9 nm.

The replication of a spherical morphology has been

reported previously.[16] In contrast, supports with unde-

fined morphology or porosity produce polymers with

irregular morphology, as shown in the case of the G48

catalyst.

The Zr–O interatomic distance of the grafted metallo-

cene seems to correlate with the properties of the polymer

it produces. Scheme 4 shows the correlation between

mean Zr–O interatomic distance and themolecular weight

(Mw) of the resulting polyethylene.

It has been suggested in the literature that blocking one

of the catalyst’s sides by grafting the metallocene to a

support disfavors the termination step of the polymeriza-

tion, which causes an increase in the final molecular

weight.[12] As a result, polymers produced by supported

metallocenes usually have a much higher molecular

weight than polymers produced by homogeneous analogs.

As shown in Scheme 4, an increase in the Zr–O interatomic

distance results in polyethylene with a lower molecular

weight. In other words, when themetallocene species feels

Scheme 4. Correlation between the mean Zr–O interatomic distance in the grafted species and the molecular weight of the resultingpolyethylene.

less of the surface’s steric effect (i.e., increasing Zr–O

distance), the chain termination step is likely to be favored,

thereby causing a decrease in the resulting polymer’s

molecular weight. Clearly, when designing a supported

metallocene catalyst, the choice of support may tune the

molecular weight or morphology of the resulting poly-

ethylene.

Despite the potential for heterogeneity intrinsic to a

surface, which might contain several distinct species that,

in turn, might be capable of producing polymers of

different molecular weights, we observed relative uniform

values of polydispersity. As shown in Table 1, the

polydispersity of the polymers varied from 1.9 to 3.0

and no clear indications of bimodality were observed in

the chromatograms.

Conclusion

The internal environment of the support’s pores affects the

molecular structure of the metallocenes grafted into the

pores. This can be seen in the reduction of the interatomic

distances between the coordination sphere and the Zr

center observed in supported catalysts, as compared to the

distances determined for the analogous homogeneous

catalysts. This change seems to be caused by the polarity of

the support’s surface. Intraglobular silanol groups occur-

ring at a relatively high density within small diameter

pores may interact with the grafted zirconocene centers,

increasing the distance between the carbon atoms of the

aromatic rings and the metal center. This increase in the

Zr–C interatomic distance is accompanied by a loss in

catalyst activity. In addition to facilitating the generation

of inactive bimolecular species and increasing the rough-

ness of the surface, which retards the diffusion of both

monomer and cocatalyst, small pores alter the nature of

the metallocene species grafted to the surface, further

reducing catalyst activity.

In summary, the textural properties of a support may

influence catalyst activity through the roughness exhib-

ited by the surface or by altering the structure of the

grafted metallocenes, which, in turn, may affect the

molecular weight of the resulting polyethylenes. Further-

more, the support can enforce a morphology on the

DOI: 10.1002/mren.200900004

polymer produced, as is the case with the nanofibers

produced by alumino-silicate crystalline supports. Ulti-

mately, the surface area of the support, which is an

integral textural property in the development of supported

catalysts, does not affect the amount of metallocene

grafted to the support, but alters the nature of the catalytic

species.

Acknowledgements: Thisworkwas partially funded by CNPq. Theauthors are thankful to LNLS for measurements performed in theEXAFS beamline (Project D04B XAFS1#5839). William BretasLinares from SAMA is especially thanked for providing thechrysotile samples.

Received: January 26, 2009; Revised: February 28, 2009; Accepted:March 3, 2009; DOI: 10.1002/mren.200900004

Keywords: AFM; EXAFS; mesoporous materials; silica; supportedmetallocene

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