Transformation of Polymerization Sites into Hydrogen Dissociation

A gradual decrease of active sites concentration, increase of chain ... time suggesting that deactivation of polymerization sites could be associated ...
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Transformation of Polymerization Sites into Hydrogen Dissociation Sites on Propylene Polymerization Catalyst Induced by the Reaction with Al-Alkyl Cocatalyst Boping Liu, Namiko Murayama, and Minoru Terano* School of Material Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan

Hydrogen (H2) is the most important chain-transfer agent to control the molecular weight (MW) of polymer in all industrial processes using Ziegler-Natta catalysts. However, the specific mechanism of chain-transfer reaction by H2 is still open for discussion. Stopped-flow techniques were utilized to investigate the chain-transfer mechanism by H2 on TiCl4/EB/MgCl2 catalyst pretreated by triethylaluminum (TEA) cocatalyst within an ultrashort time (0-1.1 s). First, the results obtained in the absence of H2 demonstrated the first experimental evidence of plausible guard effect on the active sites by coordinating growing polymer chain in the initial stage of propylene polymerization. A gradual decrease of active sites concentration, increase of chain propagation rate constant and isospecificity of active sites were observed even after an ultrashort period pretreatment due to the preferential deactivation of aspecific sites, which was found not to be affected by the presence of H2. On the other hand, H2 could not decrease the MW of polymer except using catalyst with pretreatment. Moreover, the MW is gradually decreased with increase of pretreatment time and the extent of MW decrease was enhanced with the increase of pretreatment time suggesting that deactivation of polymerization sites could be associated with the formation of H2 dissociation sites for chain transfer. This was subsequently confirmed by H2-D2 exchange reaction combined with simultaneous pretreatment of the catalyst with TEA, which indicates a plausible transformation of polymerization sites into H2 dissociation sites for chain transfer on Ziegler-Natta catalysts induced by interaction with Al-alkyl cocatalyst. Introduction Ziegler-Natta catalyst is one of the most important discoveries in chemistry field in the last century for its successful synthesis of polyolefin at low pressure and temperature.1 Its polymerization activity has been drastically enhanced due to the discovery of MgCl2 as support in the 1960’s resulting in large-scale commercial production and application of numerous polyolefin materials.2 In developing MgCl2-supported catalyst for propylene polymerization, electron donor (ED) compound is crucial and indispensable for producing highly isotactic polypropylene (PP) with the highest commercial importance.3 Within the past several decades, highly isotactic PP has been achieved based on successful development of several generations of ED.3 However, ED also strongly affects the catalyst’s behavior regarding its response to H2,4 which has been used as chaintransfer agent for precise control of MW of the polymers in almost all industrial processes.3 A state-of-the-art design of the catalyst with an ideal balance between isospecificity and H2 response still remains as a challenging target due to poor understanding of the specific mechanism of chain transfer by H2. Some traditional kinetic investigations showed that MW of polymer decreases proportionally to square root of the partial pressure of H2 suggesting that H2 might act as chaintransfer agent solely in atomic state after dissociation on the catalysts.5-7 However, the specific mechanism relating to where and how H2 is dissociated into two H * To whom correspondence should be addressed. Tel: +81761-51-1620. Fax: +81-761-51-1625. E-mail: [email protected].

atoms has not been clarified yet. In traditional investigations, polymers obtained from long-period polymerization method have been taken as the main sources to study on the complicated mechanistic aspects in Ziegler-Natta catalysis.8-10 The disadvantage of these approaches is connected with the continuously varying states of active sites accompanied with various types of chain transfer (by either H2, Al-alkyl cocatalyst or monomer) and deactivation reactions during polymerization. The stopped-flow technique,11,12 by which the chain-transfer reactions by Al-alkyl cocatalyst or by monomer could be negligible, and thus a quasi-living polymerization can be realized within an extremely short period (ca. 0.2 s), should be one of the most powerful methods for studying the nature of active sites and elucidating the chain-transfer mechanism by H2. In our recent series of work on H2 effect, 13-15 it was first observed that H2 could not act as a chain-transfer agent for propylene polymerization (up to 0.2 s) using a MgCl2/ethyl benzoate(EB)/TiCl4 catalysts without pretreatment by TEA cocatalyst. Moreover, it has been also shown that H2 can act as a chain-transfer agent if the catalyst was pretreated with TEA for 5 min prior to polymerization.13 Moreover, further H2-D2 exchange experiments disclosed that H2 dissociation solely occurred on the 5 min pretreated catalyst.14 This is the first evidence demonstrating that pretreatment of catalyst by cocatalyst accompanied with deactivation of polymerization sites is directly correlated with the formation of dissociation sites of H2 for chain transfer. One of our concerns for the previous work is the problem of internal ED extraction,16-19 which might induce

10.1021/ie0496129 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/11/2005

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significant change from the original catalyst due to ED extraction from the catalyst surface during the 5 min pretreatment. More recently, our investigation on the dynamic interaction between the MgCl2/EB/TiCl4 catalyst and TEA cocatalyst have shown that the extraction of EB initiated from ca. 10 s of interaction between catalyst and TEA.20 So after 5 min pretreatment to the TiCl4/EB/MgCl2 catalyst by TEA, most of the EB has been extracted from catalyst surface and thus significantly changed the nature of the catalyst system.21 In this study, a modified stopped-flow technique was utilized to control the pretreatment time precisely less than 10 s before polymerization (polymerization time was ca. 0.2 s) using the same catalyst in order to investigate the chain-transfer mechanism by H2 without internal ED extraction. The results were compared with those obtained through polymerization without pretreatment. The objective of this study was to investigate further on the specific relationship between the deactivation of polymerization sites induced by pretreatment and formation of H2 dissociation sites for chain transfer in the early stage of propylene polymerization using the TiCl4/EB/MgCl2 catalyst. Experimental Section Raw Materials. Propylene of research grade (donated by Mitsui Chemicals, Inc.) was used without further purification. Heptane (purchased from Wako Pure Chemical Industries, Ltd.) was purified by passing it through a 13× molecular sieve column. Ethyl benzoate (EB, purchased from Wako Pure Chemical Industries, Ltd.) was used as an internal electron donor after drying with 13× molecular sieves under a nitrogen atmosphere. Anhydrous MgCl2 (donated by Toho Catalyst Corp. Ltd.), TiCl4 (purchased from Wako Pure Chemical Industries, Ltd.), pure nitrogen (purchased from Uno Sanso Corp., 99.9995%), and TEA (donated by Tosoh Finechem Corp.) were used without further purification. TEA was used in heptane solution. H2 (Nippon Sanso Corp., 99.999%) and D2 (Nippon Sanso Corp., 99.9%) were used without further purification. Catalyst Preparation. A highly active MgCl2-supported Ziegler-Natta catalyst (TiCl4/EB/MgCl2) was prepared according to the following procedure. MgCl2 (36 g, 11 m2g-1) and EB (7.8 mL) were placed in a 1.2 L stainless steel vibration mill pot with 55 stainless balls (each ball with a diameter of 25 mm) under nitrogen and ground for 30 h at room temperature. The ground product was allowed to interact with TiCl4 (200 mL) in a 500 mL flask at 90 °C under stirring in nitrogen for 2 h. After the reaction, the catalyst was precipitated followed by a decantation. Then, the catalyst was washed with 150 mL of heptane 3 times at 70 °C, 5 times at 50 °C, and 5 times at room temperature, respectively. Each washing was performed for 5 min under stirring followed by precipitation and decantation. The catalyst was stored as a heptane slurry before polymerization. The titanium content of the catalyst was 0.4 mmol of Ti per gram of catalyst. Stopped-Flow Propylene Polymerization. A modified three-vessel-type stopped-flow apparatus (as shown in Figure 1), which can be used to precisely control both the pretreatment time and polymerization time within an ultra-short period, was utilized for propylene polymerization in the absence or presence of hydrogen. A, B, and C are glass vessels with water jacket. The polymerization procedure in the absence of hydrogen proceeded as follows. The first step of catalyst

Figure 1. Schematic illustration of the modified stopped-flow apparatus for TEA pretreatment within an ultrashort period followed by propylene polymerization using the TiCl4/EB/MgCl2 catalyst (A-C, vessels with water jacket; D, ethanol with HCl as quenching agent; X and Y, three-way valves). Polymerization conditions: catalyst ) 0.47 mmol [Ti], cocatalyst (AlEt3) ) 14 mmol [Al], solvent ) heptane, temperature ) 30 °C. When polymerization was carried out without pretreatment, propylene was saturated in vessel A, while cocatalyst and catalyst were put into vessels B and C, respectively. If hydrogen was used, it was further saturated in vessels B and C before polymerization. The polymerization time was set as 0.2 s by the length of Teflon tube from Y to Z. For polymerization using catalyst with pretreatment, propylene was saturated in vessel C, while catalyst and cocatalyst were put into vessels A and B, respectively. If hydrogen was used, it was further saturated in vessels A and B before polymerization. The pretreatment time was regulated from 0.05 to 1.1 s by the length of Teflon tube from X to Y. The polymerization time was set to 0.2 s by the length of Teflon tube from Y to Z.

pretreatment was carried out with TEA at Al/Ti molar ratio of 30 at 30 °C for different period of time ranging from 0.05 to 1.1 s followed by a second step of 0.2 s stopped-flow propylene polymerization which was performed with the catalyst (0.47 mmol of Ti) and 14 mmol of TEA (Al/Ti molar ratio ) 30) in heptane at 30 °C. As shown in Figure 1, the heptane slurry (100 mL) of the catalyst and TEA solution in heptane (100 mL) were placed in vessels A and B, respectively, while propylene (1 atm)-saturated heptane (100 mL) was placed in vessel C. The pretreatment of the catalyst with TEA was conducted in the Teflon tube from point X to Y, followed by the stopped-flow propylene polymerization in the part from point Y to Z. The pretreatment time can be accurately controlled between 0.05 and 1.1 s by changing the length of the Teflon tube from point X to Y. When polymerization was conducted using catalyst without pretreatment, the TEA solution in heptane saturated with propylene (1 atm) was placed in vessel C, while the catalyst slurry and heptane were placed in vessels A and B, respectively. All the vessels were stirred using a magnetic stirrer to maintain a homogeneous state. Propylene concentration in heptane at 30 °C was 0.60 mol/L. D in Figure 1 is a 1000 mL flask containing ethanol (400 mL) and HCl (10 mL) as quenching agents, which were agitated vigorously to stop the polymerization instantaneously. After the slurry and the solution achieved stable conditions and propylene reached saturation level, the mixtures were forced to flow simultaneously through Teflon tubes from vessels A, B, and C into flask D under nitrogen pressure. In the case of using hydrogen, the experimental conditions are almost the same except that the saturation of propylene or hydrogen for each vessel might be different. For the polymerization with pretreated catalyst, vessels A and B were saturated with hydrogen (1 atm), while vessel C was saturated with propylene (1 atm). In the case of using catalyst without pretreatment, vessel A was saturated with propylene (1 atm), while

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vessels B and C were saturated with hydrogen (1 atm) before polymerization. GPC Characterization of PP. MW and MWD of the PP obtained in this study were determined by GPC (Senshu SSC-7100) with polystyrene gel columns (Tosoh TSK-GEL G3000HHR and TSK-GEL G5000HHR) at 140 °C using o-dichlorobenzene (ODCB) as a solvent. 13C NMR Spectroscopy of PP. Stereoregularity of the PP obtained in this study was determined by 13C NMR spectra (Varian Gemini 300 spectrometer) with diluted solutions in 1,1,2,2-tetrachoroethane-d2 at 140 °C for internal lock and the central peak of 1,1,2,2tetrachoroethane-d2 (74.3 ppm) was used as an internal reference. Determination of Kinetic Parameters. Determination of the kinetic parameters is one of the useful methods to elucidate how the polymerization proceeds on the active sites formed by reaction between catalyst and cocatalyst. Compared with other methods for kinetic study on heterogeneous Ziegler-Natta catalysts, the stopped-flow method has been proven to be one of the most reliable and useful methods due to its quasiliving characteristic.12 The rate constants of propagation (kp), rate constants of chain transfer (ktr) and the concentration of active centers ([C*]) were determined by the following equations:

kp[M][C*]t M h n ) M0 [C*] + ktr[C*]t

(1)

Y ) kp[M]M0[C*]t

(2)

M0 ktrt 1 1 ) ) + P hn M h n kp[M]t kp[M]t

(3)

where M0, M h n, t, Y, and [M] are molecular weight of monomer, number-average molecular weight, polymerization time, polymer yield, and monomer concentration, respectively. Rearrangement of eq 1 affords eq 3; therefore, a plot of 1/P h n versus 1/t should be linear. From the tangent and the intercept of the plot of 1/P h n versus 1/t, kp and ktr were obtained. When the value of kp was substituted, [C*] could be calculated from eq 2. H2-D2 Exchange Reaction. The pretreatment of the catalyst with TEA and H2-D2 exchange reaction were carried out in the absence of monomer using the two-vessel-type stopped-flow apparatus as illustrated in Figure 2. A and B are glass vessels with water jacket. Reaction conditions were as follows: the catalyst (0.47 mmol of Ti), 14 mmol of TEA, Al/Ti molar ratio 30, temperature 30 °C, pretreatment time 0.08, 0.1, and 0.31 s, respectively. As shown in Figure 2, the heptane slurry (100 mL) of the catalyst and TEA solution in heptane (100 mL) were placed in vessels A and B, respectively. Vessels A (1 atm) and B (1 atm) were saturated with H2 and D2, respectively. After the slurry and the solution achieved stable conditions and H2 and D2 reached saturation level, the mixtures were forced to flow simultaneously through Teflon tubes from vessels A, B into bottle C under nitrogen pressure. The pretreatment of the catalyst with TEA and H2-D2 exchange reaction occurred simultaneously from point p to point q. C is a 1 L bottle containing 300 mL of ethanol as a quenching agent, which was stirred vigorously to stop the reactions immediately. After the reaction was quenched at point q, 2 mL of the resulted

Figure 2. Schematic illustration of the basic stopped-flow apparatus for TEA pretreatment within an ultrashort period simultaneously accompanied by H2-D2 exchange reaction using the TiCl4/EB/MgCl2 catalyst (A and B, vessels with water jacket; C, ethanol as quenching agent; p, three-way valve). Reaction conditions: catalyst ) 0.47 mmol [Ti], cocatalyst (AlEt3) ) 14 mmol [Al], solvent ) heptane, temperature ) 30 °C. For using catalyst without pretreatment, catalyst and cocatalyst were put into vessels A and B, respectively. H2 and D2 were saturated into vessel A, while propylene was saturated into vessel B. When using catalyst with pretreatment, catalyst and cocatalyst were put into vessels A and B, respectively, while H2 and D2 were saturated into vessel A and vessel B, respectively. The H2-D2 exchange reaction time was controlled from o to 0.31 s through the length of Teflon tube from p to q.

gas in bottle C was withdrawn from the gas phase in bottle C using a syringe. The amounts of H2, D2, and HD in the gas sample were detected by GC methods according to our previous report.14 For the investigation of H2-D2 exchange reaction using catalyst without pretreatment, propylene (1 atm) was saturated in vessel A, while H2 (0.5atm) and D2 (0.5atm) were saturated in vessel B. The H2-D2 exchange reaction was performed from point p to q simultaneously accompanied by polymerization for 0.2 s with other conditions similar to those using pretreated catalyst. Results and Discussion Deactivation of Polymerization Sites by Pretreatment. A preliminary investigation using TiCl4/EB/ MgCl2 catalyst pretreated for 0 to 1.1 s followed by stopped-flow propylene polymerization in the absence of hydrogen was carried out for a basic understanding of the deactivation behavior induced by ultrashort time interaction between the catalyst and TEA cocatalyst. Figure 3 shows the dependence of polymer yield on pretreatment time from 0 to 1.1 s. The polymer yield was found to decrease gradually with increase of pretreatment time, which indicates that the degree of deactivation of active sites by TEA increases with increase of pretreatment time when monomer is absent in the system during pretreatment. Deactivation of activated Ti species can be regarded to occur even within such an ultrashort pretreatment period.

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Figure 3. Dependence of polymer yield on pretreatment time from 0.0 to 1.1s and polymerization time was set at 0. 2s (the specific polymerization conditions are shown in Figure 1).

Figure 7. Dependence of isotacticity of PP on pretreatment time with or without hydrogen. The pretreatment time was adjusted from 0.0 to 1.1 s, and polymerization time was set at 0.2 s (the specific polymerization conditions are shown in Figure 1).

Figure 4. Dependence of concentration of active centers [C*] on pretreatment time. The pretreatment time was adjusted from 0.0 to 1.1 s, and polymerization time was set at 0.2 s (the specific polymerization conditions are shown in Figure 1).

Figure 8. Dependence of molecular weight of PP on pretreatment time with or without hydrogen. The pretreatment time was adjusted from 0.0 to 1.1 s, and polymerization time was set at 0.2 s (the specific polymerization conditions are shown in Figure 1).

Figure 5. Dependence of propagation rate constant on pretreatment time. The pretreatment time was adjusted from 0.0 to 1.1 s, and polymerization time was set at 0.2 s (the specific polymerization conditions are shown in Figure 1).

Figure 6. Dependence of polymer yield on pretreatment time with or without hydrogen. The pretreatment time was adjusted from 0.0 to 1.1 s, and polymerization time was set at 0.2 s (the specific polymerization conditions are shown in Figure 1).

Figure 4 shows the relationship between pretreatment time and concentration of active center (C*). The C* was found to decrease gradually with increase of

pretreatment time. The tendency of C* decrease with increase of pretreatment time confirmed the deactivation of active Ti species due to TEA pretreatment. It is generally accepted that the formation of active sites on heterogeneous Ziegler-Natta catalysts is accomplished through reduction with alkylation by the interaction of supported Ti species with TEA.22-24 When monomer is absent in the polymerization system during pretreatment, TEA can easily access again to the activated Ti species resulting in deactivation through over-reduction of the active Ti species. However, when stopped-flow propylene polymerization was carried out within an ultrashort period (∼0.2 s) using the same catalyst without pretreatment, a completely different tendency was observed according to our previous reports.12 Linear relations of polymer yield and molecular weight versus polymerization time through the original point are observed up to ca. 0.2 s. The quasi-living polymerization within 0.2 s using the stopped-flow method indicates the deactivation of active sites in the early stage of polymerization can be neglected for more than 0.2 s. However, in the absence of monomer, as shown in Figures 3 and 4, deactivation can be clearly observed even within 0.1 s. According to our previous report, after reaction between the catalyst and TEA in the absence of monomer, the average binding energy of Ti species measured by XPS gradually decreased with increase of reaction time, indicating that the Ti species on the catalyst could be reduced by TEA compound.25 The deactivation could be attributed to over-reduction of activated Ti species by the TEA compound. The absence or presence of monomer leads to the absence or presence of the coordinating growing polymer chain, respectively, on the active sites during the interaction between catalyst and TEA. This makes the difference in terms of the existence of either Ti-Et

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Figure 9. Dependence of GC chart of the gas product after the H2-D2 exchange reaction on pretreatment time, pretreatment time: (a) 0.0 s, (b) 0.08 s, (c) 0.1 s, (d) 0.31 s (the specific reaction conditions are shown in Figure 2).

bond or Ti-growing polymer chain. When no pretreatment was done, the coordinating growing polymer chain always exists on the active Ti species during the polymerization to prevent it from further reaction with TEA compound and thus protect it from deactivation. The higher stability of the Ti-growing polymer chain might be derived from the steric hindrance of the much bulky chain and/or its intrinsic stability compared with Ti-ethyl bond. This guarded the active sites from deactivation through preventing further attack and possible over-reduction by TEA. However, if pretreatment using TEA was carried out before introduction of the monomer, the activated Ti species in terms of Tiethyl bond formed by the first reaction between catalyst and TEA might be easily subject to deactivation through over-reduction by TEA due to the absence of coordinat-

ing growing polymer chain. This is the first experimental evidence giving an indication of guard effect on the active sites by the coordinating growing polymer chain during the polymerization. As shown in Figure 5, the propagation rate constant (kp) was observed to increase gradually with increase of pretreatment time. This can be rationalized well according to our previous reports.20,26,27 The preferential deactivation of aspecific sites and slight transformation of aspecific sites into isospecific sites through Al-Ti bimetallic complexing within ultra-short interaction between catalyst and TEA account for this gradual increase of kp value.20 It is known that the propagation rate constant of different types of active sites increases with increase of isospecificity of the active sites.28 Formation of Hydrogen Dissociation Sites by Pretreatment. In this stage, investigation using the same catalyst pretreated by TEA for 0-1.1 s followed by stopped-flow propylene polymerization in the presence of hydrogen was carried out. H2-D2 exchange reaction was also investigated by stopped-flow methods using the same catalyst pretreated by TEA. The objective is to further elucidate a possible correlation between deactivation of polymerization sites and formation of hydrogen dissociation sites for chain transfer. Figure 6 shows the dependence of polymer yield on pretreatment time with or without hydrogen. In both cases, the polymer yield was found to decrease gradually with increase of pretreatment time indicating that the presence of hydrogen has almost no effect on the deactivation of active sites. This is consistent with our former results obtained from stopped-flow propylene polymerization using the same catalyst pretreated by TEA for 5 min.13 These results suggested that the formation of dormant sites from 2,1-insertion can be negligible in the very initial stage of propylene polymerization by stopped-flow method, which accounts for the no observation of activity enhancement in the presence of hydrogen. Such activity enhancement by hydrogen can be usually observed for normal long-term propylene polymerization due to reactivation of dormant sites.29-34 The dependence of the isotacticity of PP on pretreatment time was shown in Figure 7. The isotacticity of obtained polymer directly reflects the stereospecificity of active sites on the catalyst. The results showed the same dependence of isotacticity on pretreatment time for absence or presence of hydrogen. The increase of

Figure 10. “Island Model”: An “island” of monolayer multinuclear Ti species and the states of the internal donor on the surface of the heterogeneous Ziegler-Natta catalyst.35

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isotacticity with pretreatment time shown in Figure 7 is directly corresponding to the increase of kp shown in Figure 5. This can be rationalized well as follows. The preferential deactivation of aspecific sites and slight transformation of active sites with low isospecificity into highly isospecific sites through Al-Ti bimetallic complexing within ultra-short time interaction between catalyst and TEA account for this gradual increase of isotacticity of PP with increase of pretreatment time.20 Aspecific active sites are supposed to have less steric hindrance and thus much more easily subject to deactivation and transformation in comparison with isospecific active sites.27 It is also reasonable to find no effect of hydrogen on the isospecificity of active sites within the ultra-short period of propylene polymerization. Figure 8 shows the dependence of molecular weight on pretreatment time with or without H2. In the case of without H2, molecular weight is increased with pretreatment time. This result can be explained well by the tendency of increase of kp value with pretreatment time for the case of without H2 shown in Figure 5. On the other hand, in the case of using H2, molecular weight is gradually decreased with increase of pretreatment time. Therefore, it is clear that H2 becomes effective as a chain-transfer agent within a short polymerization period. Moreover, the extent of MW decrease was enhanced with the increase of pretreatment time suggesting that deactivation of active Ti species for propylene polymerization could be plausibly associated with the formation of H2 dissociation sites for chain transfer. To further confirm this speculation, the pretreatment of the catalyst with TEA and H2-D2 exchange reaction within an ultrashort period were carried out in the absence of monomer using the twovessel-type stopped-flow method. It was utilized as a model reaction to investigate the possible relationship between deactivation of polymerization sites by pretreatment and formation of hydrogen dissociation sites. As shown in Figure 9, the HD product derived from simultaneous dissociation of H2 and D2 cannot be found from the model reaction system using catalyst without pretreatment but can be detected increasingly with increase of pretreatment time. This is the first direct experimental evidence demonstrating the direct correlation between the deactivation of polymerization sites and formation of H2 dissociation sites for chain-transfer reaction after an ultrashort period reaction between the catalyst and cocatalyst. These results also bring us a question of how the H atoms derived from the H2 dissociation sites (presumably on deactivated Ti species) diffuse to the polymerization sites namely active Ti species on the catalyst surface. The “spillover” of hydrogen atoms has never been reported to occur on the surface of metal halide like MgCl2, and it would be generally much more reasonable to consider that the hydrogen dissociation sites presumably deactivated Ti species exist in the instant neighborhood of the polymerization sites namely active Ti species on the catalyst surface, which could be most plausible when the real nature of surface Ti species exists in terms of monolayer multinuclear Ti species corresponding to the “island model” (Figure 10) as previously proposed on heterogeneous Ziegler-Natta catalyst.35 Only in this case the H atoms derived from the H2 dissociation sites can easily access to the polymerization sites and act as a chain-transfer agent on heterogeneous Ziegler-Natta catalysts.

Conclusions Stopped-flow techniques were utilized to investigate the chain-transfer mechanism by H2 using TiCl4/EB/ MgCl2 as catalyst and TEA as cocatalyst. The results using the catalyst pretreated for 0-1.1 s followed by stopped-flow propylene polymerization (0.2 s) in the absence of H2 demonstrated the first experimental evidence of guard effect on the active sites by coordinating growing polymer chain during the polymerization. A gradual decrease of active sites concentration and increases of average propagation rate constant and isospecificity of active sites were observed even within an ultrashort period of pretreatment plausibly due to the preferential deactivation of aspecific sites, which was found not to be affected by the presence of H2. Without pretreatment, no effect of H2 on MW of PP was observed. However, after pretreatment of the catalyst by TEA, MW is gradually decreased by H2 with increase of pretreatment time. The extent of MW decrease was enhanced with the increase of pretreatment time suggesting that deactivation of active Ti species for propylene polymerization could be associated well with the formation of H2 dissociation sites for chain transfer. This was subsequently confirmed by H2-D2 exchange reaction combined with simultaneous pretreatment of the catalyst with TEA within an ultra-short period, which indicates a plausible transformation of polymerization sites into H2 dissociation sites for chain transfer on Ziegler-Natta catalysts induced by interaction with Alalkyl cocatalyst. Acknowledgment We thank Toho Catalyst Co., Ltd., Japan Polychem. Co., Mitsui Chemicals, Inc., and Tosoh Finechem Co. for their support and donations to our laboratory Literature Cited (1) Natta, G.; Pino, P.; Corradini, P.; Danusso, F.; Mantica, E.; Mazzanti, G.; Moraglio, G. Crystalline high polymers of R-olefins. J. Am. Chem. Soc. 1955, 77, 1708. (2) Boor, J., Jr. Ziegler-Natta Catalysts and Polymerizations; Academic Press: New York, 1979. (3) Moore, E. P., Jr. The Rebirth of Polypropylene: Supported Catalysts; Hanser Publishers: Munich, 1998. (4) Mori, H.; Endou, M.; Terano, M. Deviation of hydrogen response during propene polymerization with various ZieglerNatta catalysts. J. Mol. Catal. A: Chem. 1999, 145, 211. (5) Keii, T.; Doi, Y.; Suzuki, E.; Tamura, M.; Murata, M.; Soga, K. Propene polymerization with a magnesium chloride-supported Ziegler catalyst 2. Molecular weight distribution. Makromol. Chem. 1984, 185, 1537. (6) Soga, K.; Shiono, T. Effect of hydrogen on the molecular weight of polypropylene with Ziegler-Natta catalysts. Polym. Bull. 1982, 8, 261. (7) Chien, J. C. W.; Nozaki, T. High activity magnesium chloride supported catalysts for olefin polymerization. XXIX. Molecular basis of hydrogen activation of magnesium chloride supported Ziegler-Natta catalysts. J. Polym. Sci. Polym. Chem. Ed. 1991, 29, 505. (8) Chadwick, J. C.; Morini, G.; Balbontin, G.; Camurati, I.; Heere, J. J. R.; Mingozzi, I.; Testoni, F. Effects of internal and external donors on the regio- and stereoselectivity of active species in MgCl2-supported catalysts for propene polymerization. Macromol. Chem. Phys. 2001, 202, 1995. (9) Doi, Y. Structure and stereochemistry of atactic polypropenes. Statistical model of chain propagation. Makromol. Chem., Rapid Commun. 1982, 3, 635. (10) Xu, J.; Feng, L.; Yang, S. Formation mechanism of stereoblocks in polypropylene produced by supported Ziegler-Natta catalysts. Macromolecules 1997, 30, 2539.

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Received for review May 10, 2004 Revised manuscript received November 26, 2004 Accepted December 2, 2004 IE0496129