Correlation between Catalyst Surface Structure and Polypropylene

Supported Titanium-Magnesium Catalysts for Polymerization of Olefins. Tatiana B. Mikenas , Evgeny I. Koshevoy , Vladimir A. Zakharov , Marina I. N...
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J. Phys. Chem. B 2001, 105, 3922-3927

Correlation between Catalyst Surface Structure and Polypropylene Tacticity in Ziegler-Natta Polymerization System† Seong Han Kim and Gabor A. Somorjai* Department of Chemistry, UniVersity of California at Berkeley and Materials Science DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: August 22, 2000; In Final Form: NoVember 28, 2000

The correlation between the surface structure of the Ziegler-Natta catalysts and the stereospecificity in propylene polymerization was shown experimentally using model catalysts and temperature-programmed desorption (TPD) of mesitylene as a nondestructive surface probe. Two types of titanium chloride model catalysts, with MgCl2 support (TiClx/MgCl2) and without MgCl2 (TiCly), were fabricated on an Au substrate in ultrahigh vacuum. Once activated with AlEt3 vapor, both catalysts were active for propylene polymerization in the absence of excess AlEt3 during polymerization. The TiClx/MgCl2 catalyst produced both atactic and isotactic polypropylene, while the TiCly catalyst without the MgCl2 support produced exclusively isotactic polypropylene. For the same catalysts, the mesitylene TPD revealed that TiClx/MgCl2 had two adsorption site structures, a basal plane structure and a nonbasal plane structure, while TiCly had only the latter structure. To our knowledge, this was the first direct experimental evidence revealing the structural difference of the catalyst surfaces that exhibit different stereospecificity in propylene polymerization.

I. Introduction The most prominent feature of heterogeneous Ziegler-Natta polymerization catalysts is the ability to produce isotactic polyolefins exhibiting outstanding mechanical properties. If a combination of Lewis bases is used to modify the catalyst surface, the MgCl2-supported TiCl4 catalysts produce polypropylene with an isotactic index of higher than 95%.1,2 However, full knowledge of the reaction mechanism producing isotactic polypropylene has not yet been achieved. Theoretical studies have been aimed at depicting the mechanism of the stereospecific polymerization reaction and the structure of the catalytically active site.3-10 Most of these theoretical predictions remain still to be proved because direct experimental methods to discern various surface sites of the catalyst surface were not possible until now.1,2,11 Recently, we discovered that thermal desorption profile of a mesitylene molecule can be used to differentiate the structures of the adsorption sites present on model Ziegler-Natta polymerization catalysts without altering the surface structure and composition.12,13 The nondestructive nature of this technique allowed monitoring the surface changes of a single catalyst before and after the aluminum alkyl activation and carrying out propylene polymerization using the same catalyst.13 As a first step to experimentally unveil the stereospecific polymerization mechanism, we applied the mesitylene temperature-programmed desorption (TPD) technique to study the correlation between the adsorption site distribution of the model catalysts and the stereospecificity of the polypropylene produced. Two types of titanium chloride model catalysts, with and without MgCl2 support, were fabricated by codeposition of Mg and TiCl4 and electron-induced deposition of TiCl4, respectively, on an Au substrate in ultrahigh vacuum (UHV).14-17 Once activated with triethylaluminum (AlEt3) vapor, both catalysts †

Part of the special issue “John T. Yates, Jr., Festschrift”. * Corresponding author. E-mail: [email protected].

were active for propylene polymerization without excess AlEt3 or any Lewis base during the polymerization. The model catalyst supported on MgCl2 (TiClx/MgCl2) produced both atactic and isotactic polypropylene, while the other catalyst without the MgCl2 support (TiCly) produced exclusively isotactic polypropylene. The mesitylene TPD found a clear difference in the adsorption site structure of the catalyst surface that is reflected into the polypropylene structure produced with it. II. Experimental Section The UHV chamber used in this study was equipped with a sputter ion gun for surface cleaning, an X-ray source and a double-pass cylindrical mirror analyzer (CMA) with a coaxial electron gun for X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), a quadrupole mass spectrometer (QMS) for residual gas analysis and TPD. The chamber also had three leak valves for gas exposure, a Mg evaporation source for Mg dosing, an electron flood gun for electron beam irradiation onto the sample, and an internal high-pressure reaction cell (HP) for in situ polymerization. More details of the chamber have been described elsewhere.14 A polycrystalline Au foil (1.2 cm2) was used as an inert substrate. The Au foil was cleaned with Ar ion sputtering followed by annealing at 900 K. Surface cleanliness was checked with XPS and AES. TiCl4 and mesitylene were purified by several freeze-pump-thaw cycles. A triethylaluminum (AlEt3) cylinder was purified by pumping to a few millitorrs while being cooled with an ice-water bath. Polymer-grade propylene was purified by flowing through an oxygen/water trap before use. Two types of model catalyst films were prepared in UHV: one composed of titanium chloride and magnesium chloride, TiClx/MgCl2, and the other composed of only titanium chloride, TiCly. The TiClx/MgCl2 film was produced by codeposition of TiCl4 and Mg on the Au foil at 300 K.14,16 First, the substrate

10.1021/jp002997y CCC: $20.00 © 2001 American Chemical Society Published on Web 01/19/2001

Catalyst Surface Structure and Polymer Tacticity was positioned to face both the Mg source and the TiCl4 doser at 60° with respect to the surface normal. Then, Mg was evaporated from the Mg source during the TiCl4 exposure. The Mg atom flux on the surface was calculated to be about 6 × 1012 atoms/cm2 s at a source temperature of 600 K.14 The TiCly film was fabricated by electron-induced deposition of TiCl4.15,17 A 500 eV electron beam with a flux of 1.5 × 1014 electrons/ cm2 was irradiated onto the substrate at 100 K during the TiCl4 exposure. After deposition at 100 K, the TiCly film was annealed at 580 K. The deposited films were characterized with XPS and TPD. In XPS analysis, the Al KR radiation (1486.6 eV) was used and the angle between the surface normal and the CMA axis was 35°. The Au 4f7/2 peak at 84 eV was taken as a reference for the energy scale. The XPS spectra of Ti 2p reported in the following section have been background subtracted and deconvoluted into a series of synthetic peaks (67% Gaussian and 33% Lorentzian; fwhm ) 2.3 eV) that represent the photoelectron emission from different oxidation states.14 In mesitylene TPD experiments, the catalyst film at 100 K was exposed to a given exposure of mesitylene and then resistively heated at a ramp rate of 4 K/s. The QMS ionizer was covered with a shroud that had an aperture of about 5 mm, positioned about 1 cm from the surface, to discriminate background desorption. After XPS and mesitylene TPD characterization in UHV, the model catalyst was enclosed in the HP cell and exposed to ∼1 Torr of AlEt3 at 300 K. After evacuation of the HP cell to remove excess AlEt3, the HP cell was open and the activated catalyst surface was analyzed with XPS and mesitylene TPD. Finally, the activated catalyst was enclosed again in the HP cell and exposed to about 900 Torr of propylene for 14 h. The catalyst temperature was kept at 340 K during the polymerization. The polypropylene (PP) film produced on the model catalyst was analyzed with XPS and TPD in UHV and then retrieved to air for further analyses. The polypropylene films produced with model catalysts were analyzed with FTIR, atomic force microscopy (AFM), and scanning electron microscopy (SEM). IR spectra of the as-grown polymer films on the substrate were measured with an attenuated total reflection (ATR) mode. Contact-mode AFM images were taken for as-grown polymers and solvent-extracted polymers. The atactic and isotactic fractions of the PP sample were sequentially extracted with n-hexane and n-octane.18-20 Each extracted fraction was cast on a slide glass, dried in air, and characterized with AFM. A cantilever of a force constant of 0.05 N/m was used. III. Results Reactions of TiCl4 with Mg atoms or electrons produced thin films of model catalysts, TiClx/MgCl2 and TiCly, respectively. Figure 1 represents low-resolution XPS data confirming the presence of these films on the Au substrate. The TiClx/MgCl2 catalyst was composed of a TiClx layer on top of MgCl2 multilayers characteristic of the supported catalysts.14 The TiCly catalyst was composed of multilayers of mostly TiCl2 and some TiCl3, with Ti4+ species chemisorbed on it.15,17 The oxidation state distribution of titanium ions in the model catalysts could be controlled with deposition conditions and postdeposition treatments such as electron irradiation and annealing. More details of the catalyst film structures were described elsewhere.14,15,17 III.1. Stereospecificity in Propylene Polymerization. Once activated by reactions with ∼1 Torr of AlEt3 for 1 min, both TiClx/MgCl2 and TiCly model catalysts polymerized propylene,

J. Phys. Chem. B, Vol. 105, No. 18, 2001 3923

Figure 1. XPS spectra of (a) TiClx/MgCl2 and (b) TiCly catalyst films on Au. Deposition conditions: (a) T(Mg) ) 600 K, P(TiCl4) ) 2 × 10-7 Torr, time ) 10 min; (b) electron energy ) 500 eV, flux ) ∼1.5 × 1014 electrons/cm2 s, P(TiCl4) ) 1 × 10-7 Torr, time ) 10 min.

producing a thin film of polypropylene (PP) covering the catalyst surface.21,22 The atactic and isotactic fractions of the PP films produced in this way were separated into a “hexane-soluble” fraction and a “hexane-insoluble/octane-soluble” fraction, respectively,18-20 and then analyzed with AFM.23 After the n-octane extraction process, some fraction of polypropylene (octaneinsoluble fraction) still remained on the Au substrate.24 Parts a and b, respectively, of Figure 2 show AFM images of atactic and isotactic fractions of PP produced with TiClx/ MgCl2. At a low load (0.6 nN), both atactic and isotactic PP were imaged intact. However, at a high load (2 nN), the atactic PP was pushed out from the scan region, while the isotactic PP remained intact. This behavior was consistent with the material properties of atactic and isotactic PP, confirming the conformation of each extracted PP fraction. The isotactic PP was much harder than atactic PP,20,25,26 so it was able to sustain the pressure generated by the AFM tip at the high load condition (also seen in Figure 2d). In the case of the PP sample produced with the TiCly catalyst, Figure 2c clearly shows the absence of the atactic fraction. A small particle in Figure 2c was believed to be isotactic PP based on its hardness. Therefore, it could be concluded that the TiClx/ MgCl2 model catalyst produces both atactic and isotactic polypropylene, while the TiCly catalyst produces exclusively isotactic polypropylene. This was consistent with a typical polymerization behavior of the industrial counterparts of the Ziegler-Natta catalysts: in the absence of electron donors such as ethyl benzoate, the MgCl2-supported TiCl4 catalyst produces less isotactic polypropylene than the TiCl3-based catalyst.27 The ATR-IR analysis of the as-grown PP film, shown in Figure 3, supported the above conclusion. The isotactic index (I.I.) of polypropylene can be expressed by the intensity ratio of these two peaks, I(998 cm-1)/I(973 cm-1).20,28 The band at 998 cm-1 is associated with isotactic helix vibration, and the band at 973 cm-1 is an internal reference. The I.I. values calculated from the data in Figure 3 were ∼0.2 for PP produced with TiClx/MgCl2 and ∼0.5 for PP produced with TiCly. The I.I. value of the PP film produced with TiCly was much lower than expected from the solvent-extraction experiment shown

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Figure 3. ATR-IR spectra of as-grown polypropylene films on (a) TiClx/MgCl2 and (b) TiCly catalysts.

Figure 2. AFM images of (a) hexane-soluble fraction and (b) hexaneinsoluble/octane-soluble fraction of polypropylene produced with the TiClx/MgCl2 catalyst and (c) hexane-soluble fraction and (d) hexaneinsoluble/octane-soluble fraction of polypropylene produced with the TiCly catalyst. The catalysts were preactivated with AlEt3. The propylene polymerization was carried out in the absence of gas-phase AlEt3 in the HP reaction cell (monomer pressure ) 900 Torr, reaction temperature ) 340 K).

in Figure 2c,d. This could be attributed to the presence of the amorphous phase in the as-grown PP film.29 Due to the small quantity of the PP sample produced with our small-surface-area model catalysts, other analytical techniques such as nuclear magnetic resonance (NMR) or gel permeation chromatography (GPC) were not applicable to the PP samples obtained in the current experiment. The XPS and TPD results reported in the next section were obtained with the catalysts that produced the polypropylene samples reported in this section. The comparison of these surface characterization data with the conformation of polypropylene provided insight into the origin of the stereospecific polymerization of the model catalysts. The results reported here were reproducible for other model catalysts prepared in this study. III.2. XPS and Mesitylene TPD Analyses of the Model Catalyst Surfaces. The titanium oxidation states of TiClx/MgCl2 and TiCly were measured with high-resolution XPS before and after the catalyst activation by AlEt3 and are reported in Figure 4. For both catalysts, the AlEt3 reaction resulted in reduction of the titanium oxidation state distribution. The TiCly catalyst showed a less prominent change in XPS due to some contribu-

Figure 4. Ti 2p3/2 XPS spectra of the TiClx/MgCl2 (left column) and TiCly (right column) catalysts that produced polypropylene of Figure 2. (a) and (c) before AlEt3 activation; (b) and (d) after the AlEt3 activation.

tion from the underlayers to the measured Ti XPS signal.17 Considering this underlayer contribution for TiCly, the oxidation state distributions of the activated surface species of these two catalysts appeared to be very close to each other. Therefore, it was unlikely that this small difference in the titanium oxidation state distribution caused such a drastic difference in the isotacticity of the PP film produced with these catalysts. It should also be noted that, in Figure 4, a decrease of the Ti4+ state, caused by the AlEt3 activation, was coupled with an increase of the Ti2+ state, which was consistent with a previous report.30 The change of the Ti3+ state was much smaller compared to the Ti2+ and Ti4+ states and not discernible within the fitting errors. These changes in the titanium oxidation distribution will be discussed in light of the active site nature in section IV.2. The adsorption site distribution on the model Ziegler-Natta catalyst surfaces was measured with TPD of mesitylene as a probe molecule.12,13 Though this method could not distinguish the metal composition and oxidation state due to chlorine termination of the catalyst surface, it could differentiate the

Catalyst Surface Structure and Polymer Tacticity

Figure 5. Mesitylene TPD profiles of the TiClx/MgCl2 (left column) and TiCly (right column) catalysts that produced polypropylene of Figure 2. (a) and (d) before AlEt3 activation; (b) and (e) after AlEt3 activation; (c) and (f) after propylene polymerization. Exposures: 0.2, 0.6, 1.0, and 1.4 L.

different adsorption sites depending on the heat of adsorption that, in turn, depended on the surface chlorine structure13 (for structural assignments, see section IV.1). The comparison of the mesitylene desorption profiles of the TiClx/MgCl2 and TiCly catalysts, reported in Figure 5a,d, clearly showed that the surface structure of TiClx/MgCl2 differed from that of TiCly. At an exposure of 1.4 langmuirs, two desorption peaks were observed at 202 and 246 K for the TiClx/ MgCl2 catalyst, while only one peak was observed at 248 K for the TiCly catalyst. There was no discernible peak at 200 K for the TiCly catalyst. For TiClx/MgCl2, the 200 K peak was always dominant. The repulsive lateral interactions between the adsorbed mesitylene molecules caused a slight shift of the desorption profile to lower temperature with increasing mesitylene exposure.12 The symmetric profile and narrow peak width (fwhm ) 30-40 K) suggested that each peak can be attributed to one type of surface adsorption site. The catalyst activation by reactions with the AlEt3 cocatalyst shifted the mesitylene desorption profiles to lower temperatures (Figure 5b,e). Both catalysts retained their original distribution of the surface adsorption sites. The activated TiClx/MgCl2 catalyst showed two mesitylene desorption peaks, one at 200 K and the other at 232 K, and the activated TiCly catalyst showed a single peak at 233 K. Since there was no significant difference in the titanium oxidation state distribution for the activated catalysts (Figure 4b,d), it could be concluded that the difference in the stereospecific polymerization of the model catalyst resulted from this difference in the surface structure. In the mesitylene desorption for the activated catalysts, the shifts in the peak temperatures of the high-temperature peaks were much larger (about 15 K) compared to that of the lowtemperature peaks (about 2 K). This was consistent with the effects of the aluminum alkyl reaction on the adsorption isotherm of allene, a molecule poisoning the catalytic activity, reported by Petts and Waugh.31 They observed a larger decrease in the

J. Phys. Chem. B, Vol. 105, No. 18, 2001 3925 heat of allene adsorption for the high-temperature sites after the activation of a high-surface-area catalyst with trimethylaluminum. After propylene polymerization, both catalysts were completely covered with PP films produced. In XPS, only C 1s peak was observable. In the mesitylene TPD shown in Figure 5c,f, there was only single peak at 169 K. The desorption profile and temperature were the same as the mesitylene TPD for a PP film prepared by casting a polypropylene-cyclohexane solution on the Au substrate followed by vacuum annealing. III.3. Topographic Images of the As-Grown Polypropylene. Figure 6 shows the AFM images of the as-grown PP films for TiClx/MgCl2 and TiCly. The PP film on TiClx/MgCl2 (rootmean-square (rms) roughness ) 110 nm) was much smoother than that on TiCly (rms roughness ) 330 nm). The SEM images of these PP films (data not shown) revealed the same features that could be explained with the two-dimensional projection of Figure 6. This implied that the compression of a soft region by the AFM tip32 could not be the main cause for the rough film surface of the PP film produced by the TiCly catalyst. Therefore, we could rule out the presence of soft, low-molecular-weight regions that would result from the inhomogeneity of the active sites.33 The roughness of the PP film appeared to be due to the roughness of the catalyst film since the polymer particles tend to replicate the shape of the Ziegler-Natta catalyst.34,35 The film growth mode would be determined by the incoming rate of the deposition species to the surface and their diffusion rate at the surface as well as the thermodynamics of the deposited film. In the case of codeposition of Mg and TiCl4 onto the Au substrate at 300 K, the TiCl4 adsorption followed the Langmuir isotherm14 and the surface diffusion of the adsorbed TiCl4 was fast (at least much faster than the diffusion at 100 K). Thus, the TiClx/MgCl2 film was expected to grow layer by layer, with the most stable (001) basal plane exposed at the gas/film interface.36 However, in the case of electron-induced deposition of TiCl4 onto the substrate at 100 K, the surface concentration of TiCl4 was much higher, due to condensation at 100 K, and the surface diffusion rate was much lower, compared to the 300 K deposition.15 Under these conditions, it would be possible that the TiCly film grew randomly, not layer by layer, forming a rough film surface that contained no basal plane structure (Figure 5d). The polymer film grown from this rough surface would evolve a much rougher surface than that grown from the smooth surface. IV. Discussion IV.1. Surface Structure vs Stereospecificity in Propylene Polymerization. The most important finding of this study is the apparent difference in the adsorption site distribution of the Ziegler-Natta catalyst that could be correlated with the isotacticity of the polypropylene produced. This difference was detected directly by thermal desorption of the mesitylene probe molecule before polymerization, not by product analysis after polymerization as in other experimental studies. Our data confirmed experimentally the theoretical predictions that the structures of the active sites determine the stereospecificity of propylene polymerization.3,5,8-10 The surface of the TiClx/MgCl2 catalyst, producing both atactic and isotactic polypropylene, consisted of two types of adsorption site structures. In contrast, the TiCly catalyst consisted predominantly of one adsorption site structure and produced exclusively isotactic polypropylene. The absence of the other adsorption site, having a structure responsible for mesitylene desorption at ∼200 K (Figure 5d,e),

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Figure 6. AFM image of as-grown polypropylene films on (1) TiClx/MgCl2 and (b) TiCly. The load to the tip was 0.6 nN. The rms roughness of the polypropylene films was (a) 110 nm and (b) 330 nm, respectively.

and the absence of the atactic PP fraction (Figure 2c) clearly suggested that the active sites of this structure are stereochemically nonspecific. The mesitylene TPD alone could not determine unambiguously the exact surface structure of the active site.13 However, one could interpret the mesitylene TPD data using the crystallographic data,37 surface structure,36 surface compositions,38,39 shape of microcrystals,40,41 and thermodynamics6,7,16 of the titanium chloride and magnesium chloride films or crystals obtained by other techniques. X-ray diffraction (XRD) found that these halide crystals have a layered structure, each layer consisting of two monatomic layers of chlorine ions octahedrally coordinated to intercalating metal ions.37 The low-energy electron diffraction (LEED) study of a MgCl2 film, deposited by thermal evaporation in UHV, showed that the film grows layer by layer and the film surface was composed of small domains of the (001) basal plane, the most stable crystallographic plane.36,6,7 The He+ ion scattering spectroscopy (ISS) studies of the model Ziegler-Natta catalyst films found that the surfaces of these films are terminated with chlorine ions and no metal ions are exposed to vacuum.16,38,39 The SEM pictures described that the TiCl3 microcrystallites, produced by a sublimation-recrystallization method, possess a very thin, hexagonal platelet shape.40,41 The hexagonal symmetry of the TiCl3 unit cell suggested that the platelet surface assumes the (001) basal plane structure and only one crystallographic plane is dominant for the sides of the crystals. This plane is most likely to be the (100) plane with some structural reconstruction, the second most stable plane of the crystal.6,7 Assuming this structural information is applicable to the model catalyst films, we could attribute the dominant mesitylene desorption peak at ∼200 K for TiClx/MgCl2 to a basal plane structure where the chlorine ions at the outermost layer are close packed and the metal ions under the chlorine layer are coordinated to six chlorine atoms. In this frame, the high temperature desorption peak at 246 K could tentatively be attributed to a nonbasal plane where the surface chlorine ions are not close packed and the metal ions under the chlorine layer are undercoordinated. The possible candidates would be the crystallographic dislocations on the basal plane or other crystallographic planes at the boundaries of the (001) basal plane (see ref 3, 6, 7, or 40 for crystallographic models). The absence of the mesitylene desorption at ∼200 K for TiCly catalyst implied that the TiCly film surface was predominantly composed of the nonbasal plane structure containing undercoordinated metal ions. The first-order desorption profile of mesitylene and its narrow peak width (fwhm ) 30-40 K), as shown in Figure 5d, could suggest that the catalyst surface assumes only one dominant structure. Even if there were more than one lateral plane on the model catalysts, the surface structures of these planes would be significantly different from their crystallographic structure in

the bulk crystal due to structural reconstruction to lower the surface energies.6,7 The reconstructed structures of lower surface energies may well be so similar to each other that they are undistinguishable. Busico et al. suggested the presence of the (100) and (110) planes on the small MgCl2 particles prepared by a ball-milling method.42 The MgCl2(100) surface has the Mg atoms coordinated to five chlorine atoms. The MgCl2(110) surface has the Mg atoms coordinated to four chlorine atoms. If these surfaces are present on the model catalyst films and retain their bulk-terminated crystallographic structure without any reconstruction, one would expect a higher heat of mesitylene desorption for the (110) plane than the (100) plane, giving rise to two desorption peaks for the coordinatively unsaturated sties, in addition to the main peak assigned to the basal plane structure. The possibility that mesitylene was not sensitive enough to distinguish various defect sites was quite unlikely. The mesitylene desorption profile shifted its desorption temperature in response to the surface reduction and alkylation (Figure 5). In another experiment, when the TiClx/MgCl2 surface was irradiated with electrons at 150 K, generating surface defects on the planar surface, the mesitylene desorption profile was changed in response to the newly generated defects sites.13 The mesitylene TPD method was also able to monitor the annealing process of these electron-induced surface defects that involved diffusion of bulk chlorine atoms to the surface.13 After the catalyst activation by reactions with the AlEt3 cocatalyst, the mesitylene desorption profiles were shifted to low temperatures. If one accepts the existing model for catalyst alkylation process,3c,43 the changes in the mesitylene desorption temperature could be explained as below. For the basal plane sites, the alkylation by AlEt3 would replace one Cl anion with a C2H5 group.3c,43 This would induce only a minor change in the mesitylene-surface interactions (only 2 K shift in the peak desorption temperature). For the undercoordinated sites of TiClx/ MgCl2 and TiCly, the alkylation by AlEt3 would occur by addition of one C2H5 group to the undercoordinated metal ions.3c,43 Thus, the alkylated nonbasal plane sites would assume a more close-packed structure that would have weaker interactions with mesitylene, causing a larger decrease (about 15 K) in the mesitylene desorption temperature. IV.2. Titanium Oxidation State vs Propylene Polymerization. We also would like to point out that (1) the reduction of the Ti4+ species by AlEt3 was accompanied by an increase in the Ti2+ component, not the Ti3+ component, in XPS and (2) the Ti2+ species thus formed were active in propylene polymerization. The titanium oxidation distribution observed in this study was consistent with other published data determined using XPS28,44 and electrochemical method,45 but not consistent with the data determined from a combination of wet redox titration and electron spin resonance (ESR).46-48 The latter approaches have reported that the Ti3+ species are a major product of the catalyst activation by AlEt3. Based on these ESR

Catalyst Surface Structure and Polymer Tacticity studies and other kinetic studies, it has long been suggested and believed that only the Ti3+ species, formed by the AlEt3 activation, can polymerize propylene.49 However, it should be noted that the Ti2+ and Ti4+ ions are not ESR active and more than 80% of total Ti3+ ions in the Ziegler-Natta catalysts are ESR silent due to interactions with the adjacent Ti3+ ions.46,47 In situ measurements of both XPS and ESR for the same catalyst sample would provide an answer to this discrepancy. V. Conclusions The mesitylene TPD from the model Ziegler-Natta catalysts revealed an apparent difference in the distribution of the surface adsorption site structures that was folded into the isotacticity of propylene produced with them. The TiClx/MgCl2 catalyst, produced by codeposition of Mg and TiCl4, had two distinct adsorption sitessmostly the basal plane structure and some minor fraction of a nonbasal plane structuresand produced a mixture of atactic and isotactic polypropylene. In contrast, the TiCly catalyst, produced by electron-induced deposition of TiCl4, had only the nonbasal plane structure and produced exclusively the isotactic polypropylene. Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Material Science Division, of the U.S. Department of energy under Contract No. DE-AC03-76SF00098. The authors also acknowledge support from Montell USA, Inc. References and Notes (1) Soga, K.; Shiono, T. Prog. Polym. Sci. 1997, 22, 1503. (2) Dusseault, J. J. A.; Hsu, C. C. J. Macromol. Sci.sReV. Macromol. Chem. Phys. 1993, C33, 103. (3) Cossee, P. J. J. Catal. 1964, 3, 80. (b) Arlman, E. J. J. Catal. 1964, 3, 89. (c) Arlman, E.; Cossee, P. J. J. Catal. 1964, 3, 99. (4) Fujimoto, H.; Koga, N.; Hataue, J. J. Phys. Chem. 1984, 88, 3539. (5) Guerra, G.; Pucciariello, R.; Villani, V.; Corradini, P. Polym. Commun. 1987, 28, 100. (b) Venditto, V.; Guerra, G.; Corradini, P. Eur. Polym. J. 1991, 27, 45. (c) Cavallo, L.; Guerra, G.; Corradini, P. J. Am. Chem. Soc. 1998, 120, 2428. (6) Lin, J. S.; Catlow, C. R. A. J. Mater. Chem. 1993, 3, 1217. (7) Colbourn, E. A.; Cox. P. A.; Carruthers, B.; Jones, P. J. V. J. Mater. Chem. 1994, 4, 805. (8) Shiga, A.; Kawamura-Kuribayashi, H.; Sasaki, T. J. Mol. Catal. 1994, 87, 243. (b) Shiga, A.; Kawamura-Kuribayashi, H.; Sasaki, T. J. Mol. Catal. 1995, 98, 15. (9) Puhakka, E.; Pakkanen, T. T.; Pakkanen, T. A. Surf. Sci. 1995, 334, 289. (b) Puhakka, E.; Pakkanen, T. T.; Pakkanen, T. A. J. Mol. Catal. A 1997, 120, 143. (10) Boero, M.; Parrinello, M.; Terakura, K. J. Am. Chem. Soc. 1998, 120, 2746. (b) Boero, M.; Parrinello, M.; Terakura, K. Surf. Sci. 1999, 438, 1. (c) Boero, M.; Parrinello, M.; Huffer, S.; Terakura, K. J. Am. Chem. Soc. 2000, 122, 501. (11) Potapov, A. G.; Kriventsov, V. V.; Kochubey, D.; Bukatov, G. D.; Zakharov, V. A. Macromol. Chem. Phys. 1997, 198, 3477.

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