Subscriber access provided by RELIANCE INDUSTRIES LTD

Industrial & Engineering Chemistry Research is published by the AmericanChemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036

Article

Synthesis of Ultrahigh Molecular Weight Polyethylene UsingTraditional Heterogeneous Ziegler#Natta Catalyst Systems

Sudhakar Padmanabhan, Krishna R. Sarma, and Shashikant SharmaInd. Eng. Chem. Res., 2009, 48 (10), 4866-4871• Publication Date (Web): 14 April 2009

Downloaded from http://pubs.acs.org on May 18, 2009

More About This Article

Additional resources and features associated with this article are available within the HTML version:

• Supporting Information• Access to high resolution figures• Links to articles and content related to this article• Copyright permission to reproduce figures and/or text from this article

Synthesis of Ultrahigh Molecular Weight Polyethylene Using TraditionalHeterogeneous Ziegler-Natta Catalyst Systems

Sudhakar Padmanabhan,* Krishna R. Sarma, and Shashikant Sharma

Research Centre, Vadodara Manufacturing DiVision, Reliance Industries Limited, Vadodara, India, 391 346

Ultrahigh molecular weight polyethylene was synthesized from traditional Ziegler-Natta type catalysts (ZN),namely, TiCl4 anchored on MgCl2 support. This, upon activation with AlRR′2 (where R, R′ ) isoprenyl orisobutyl), gave precatalysts (C-2 to C-5) having 16, 21, 25, and 32% trivalent titanium, respectively. Thereduction in oxidation states also accompanies the reduction in particle size of the catalysts, which in turngets reflected in the resulting polymer properties under specified operating conditions. We have demonstratedthe effect of process conditions that can surmount the catalyst dependency over the polymer characteristics,and hence, it can result in polymer with consistent polymer properties, which is an important need of thepolymer industries. The polymer characteristics such as particle size distribution, average particle size, bulkdensity, reduced specific viscosity, and concentration of fine and coarse particles were determined and weredependent on various process parameters. Under identical reaction conditions, the polymerization with largerscale yield polymer with different characteristics. The fine-tuning of process conditions yielded polymer withconsistent quality.

Introduction

Ultrahigh molecular weight polyethylene (UHMWPE) be-longs to the specialty polymer grade, having unique propertiesand hence finding applications in areas requiring less abrasion,excellent impact strength, good chemical resistance, etc.1–3

UHMWPE has excellent wear resistance, outstanding impactstrength, and very good chemical resistance. Consequently, itfinds applications in diversified areas with unique requirements.3

More than two-thirds of the commercial processes involvedare based on Hostalen’s continuous stirred tank using conven-tional ZN catalysts.4 A couple of processes are also based onmetallocene catalyst systems with very limited capacities.5 Theinitial patents on catalysts relating to UHMWPE date back tothe early 1970s and still continue to dominate the scene, evenafter a span of 4 decades. The concept of anchoring TiCl4 onsupports like Mg(OR)2/MgCl2 followed by treatment withaluminum alkyls has been fully exploited through diverseprocess variations.6 Major players in this field arranged chro-nologically include Ruhrchemie, Hoechst, Himont, and Ticona.4

Petrobras aimed at improved morphology of the polymerthrough spherical catalyst systems involving supporting andspray-drying techniques.7 Phillips’ novelty was in the use ofmodified alumina and silica supports to immobilize metals likeTi, V, Cr, Zr, and Hf.8 Equistar derived their strengths throughthe use of quinolinoxy-containing single site catalysts througha nonalumoxane route.9 Besides, there are numerous examplesavailable in the literature pertaining to the use of homogeneoussingle-site catalysts involving metals like Ti, V, and Zr for thesynthesis of UHMWPE.10

Among the various grades of UHMWPE, the grade withmolecular weight 4-5 million g/mol is unique because of itsoptimum abrasion resistance, impact strength, chemical resis-tance, etc.3 Hence the 4-5 million molecular weight grade hasmaximum business volume. At higher molecular weights, thoughthe abrasion resistance was slightly better than that of the lowermolecular weight polymers, the impact strength dropped down

considerably. Considering this, it is imperative that we havespecial grades with unique properties for unique applications.2

Most of the polyethylene produced based on the market needsare manufactured using traditional Ziegler-Natta catalysts,which typically comprise titanium halides (TiX4 where X isgenerally Cl) supported on magnesium chloride (MgCl2) throughvarious chemical modifications.6 Olefin polymerizations involv-ing such ZN catalysts involve a catalyst preactivation stepinvolving aluminum alkyls, aluminoxanes, or borate compounds(generally known as cocatalysts) wherein, apart from reductionof oxidation states of the titanium, there is also a vacantcoordination site created on the titanium. It is on this vacantsite that the olefin coordinates, and through a series oftransformations, the polymer chain grows. The activity of thesecatalysts not only depends on the total titanium present in thesystem but also depends on the percentage of the reducedtitanium. The production of UHMWPE using these catalystsystems is again a big task, taking in to account of the possibletermination reactions that can kill the propagating active species.The presence of excess aluminum alkyls can bring about thetermination via transfer of polymer chain to aluminum. Thiscan reduce the length/molecular weight of the polymer chainand also broaden the molecular weight distribution.11 Experi-ments on a slightly larger scale in a 5 L laboratory-scale reactorposes a vigorous threat because of the usage of less catalyst,which can easily be killed by the presence of a small amountof impurities in the reaction medium. Hence, process optimiza-tion studies play a bigger role in these reactions.

In this paper we have demonstrated the capability of usinghydrocarbon as a polymerization solvent for producing UHM-WPE having desired bulk density (BD), average molecularweight, and average particle size (APS) with controlled fine(<10 µm) and coarse (>250 µm) material and also developedlaboratory process for making UHMWPE of 4-6 million g/molmolecular weight with consistent productivities.

Experimetal Section

The required catalyst, C-1 with 20% titanium content (80%magnesium and chlorides), was synthesized by adopting a well-

* To whom correspondence should be addressed. Tel.: +91 265 6696260. Fax: +91 265 669 3934. E-mail: Sudhakar.padmanabhan@zmail.ril.com.

Ind. Eng. Chem. Res. 2009, 48, 4866–48714866

10.1021/ie802000n CCC: $40.75 2009 American Chemical SocietyPublished on Web 04/14/2009

known procedure:6 10 g of magnesium ethoxide is added to120 mL of varsol, a high boiling kerosene fraction, under anatmosphere of nitrogen and mechanical stirring. The temperatureis increased to 85 °C and maintained. Subsequently, about 35 g(20 mL) of TiCl4 is added to the magnesium ethoxide suspensionunder a gentle atmosphere of nitrogen slowly over a period of5-6 h. The molar ratio of Mg:Ti is about 1:2. After thecompletion of TiCl4 addition, the temperature is increased to120 °C and maintained for about 60 h to temper the catalyst.The solvent contains the precatalyst as a pale yellow to whitesuspension. The catalysts were stored under nitrogen atmosphereas a slurry in hexane. The slurry concentration was maintainedat 12-15% for easy handling. The slurry was homogenizedcompletely and was transferred using standard syringe tech-niques. The slurry concentration of the catalysts was determinedbefore each experiment to calculate the amount of the catalystadded. All manipulations like handling and transfer of catalystsand pyrophoric aluminum alkyls were carried out in a nitrogenglovebox/bag.

The actual catalyst for UHMWPE is prepared from C-1 (whitecatalyst) by reducing the same using AlRR′2. The molar ratioemployed between the titanium catalyst and the aluminum alkylvaried on the basis of the Ti3+ content intended. The aluminumalkyl is gently added at about 25 °C to the white catalyst slurryunder a stream of nitrogen and with mechanical agitation overa period of 3-5 h. The color of the slurry changes to grayishblack, and hence, the catalyst is also referred to as the “blackcatalyst”. Here the titanium is present as a mixture of quadriva-lent and trivalent titanium (predominantly) with traces ofdivalent titanium.

Polymerizations were carried out in laboratory Buchi reactorsof 1, 5, and 19 L capacity using well-established and validatedprocedures in hexane as the medium. The hexane used in theruns is dry distilled under a nitrogen atmosphere after refluxingit over sodium hydride as the desiccant, and the moisture contentwas around 5-8 ppm. The prereduced catalyst slurry in hexanewas homogenized and a suitable amount was transferred outfor a run. The agitation has been standardized around 500 rpmand the temperature was maintained at 75 °C over the periodof 2 h. Hydrogen dosing was done through a precalibrated bombhooked to the reactor for controlling the molecular weight. Thepolymer was then filtered, washed with acetone, and then driedin an air oven at about 75 °C. The weight of polymer wasrecorded to calculate the productivity of the catalyst in termsof grams of polymer/grams of catalyst and grams of polymer/millimole of Ti. The productivity was based on a 2 h period.

Catalyst characterization was carried out by measuringparameters like slurry concentration for the solid content;compositional analysis for Ti, Mg, and Cl by UV-vis spec-trophotometry and EDTA and argentometric titrations, respec-tively; oxidation states of Ti (quadrivalent, trivalent) bycerimetry; and the particle size distribution (PSD) for APS bya Malvern Mastersizer-E, a laser diffraction based particle sizeanalyzer. The viscosity-based average molecular weight wascalculated using Margolie’s equation. The reduced specificviscosity (RSV) was determined at 135 °C in decaline as solventin an Ubbelohde viscometer with constant ) 0.01 by measuring

the flow times for solvent and subsequently a 0.02% solutionof the polymer.

Results and Discussion

Titanium supported on MgCl2 (C-1) upon activation withAlRR′2 (an equal mixture of triisobutylaluminum and isopre-nylaluminum) and hydrogen as the molecular weight regulatoris being used for the generation of UHMWPE. There is enoughliterature precedence for the use of such catalysts with triethylaluminum (TEAL) as an activator for the production of highdensity polyethylene (HDPE).12 Tailoring such catalyst toproduce UHMWPE through process optimization in hydrocar-bon media meeting rigid polymer specifications has been achallenge in the industrial arena. The necessary precatalyst C-1has been prepared with 20% titanium loading on MgCl2 supportand activated with AlRR′

2, which yield catalysts with activetitanium center. The catalyst batches with different Ti 3+ contentswere synthesized by adjusting the AlRR′2 quantity and are C-2(16% Ti3+), C-3 (21% Ti3+), C-4 (25% Ti3+), and C-5 (32%Ti3+). The process overview is given in Scheme 1.

The role of aluminum alkyls in olefin polymerization is ofparamount importance and consequently today we have a diverserange of such Lewis acids, each with a unique role to play in apolymerization. The crux of the earlier statements is that thecorrect aluminum alkyl has to be primarily identified for apolymerization process and then subsequently its amount withrespect to the catalyst needs to be optimized to arrive at thedesired productivity and polymer characteristics, namely, mo-lecular weight, average particle size, bulk density, etc. The useof AlRR′2 as an activator yields the required polyethylene withultrahigh molecular weight. For a particular ethylene pressureand catalyst system (C-3), we carried out the Al/Ti optimizationexperiments and we observed that the optimum value of Al/Tiis around 7-8 under the specified operating conditions, namely,2 atm of ethylene pressure (PC2 2 atm). This exercise needs tobe optimized when the conditions are changed. Thus, at anethylene pressure above 5 atm we found the optimized Al/Tiratio was around 4-5. By operating at a different Al/Ti value,besides yield, the other polymer properties like BD and averagemolecular weight also change, thus providing a lever to alterthe polymer characteristics at the cost of yield. At 2 atm PC2,we have evaluated the polymerization with C-2 to C-5 and foundthat there is a close agreement between the polymer particle

Scheme 1. Process Overview

Figure 1. Comparison of PSD of polymer with catalyst nature.

Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 4867

size distribution (PSD) and catalyst PSD (Figure 1). As weincrease the % Ti3+, there is a reduction in catalyst PSD whichin turn gets reflected in the APS of the polymer obtained withincreased fines. The optimum value of Ti3+ content was foundto be 20-25% under PC2 2 atm conditions (Figure 1). Whenwe increased the pressure from 2 to 7.5 atm, we found there isnot much difference in PSD of the polymer obtained amongC-2 to C-5 catalyst systems, clearly revealing the importanceof process conditions over polymer properties.

Besides AlRR′2, we have also evaluated TEAL as theactivator12 for selected catalyst batches for obtaining UHMWPE.At 2 atm PC2 employed, though molecular weight between 4and 10 million g/mol could be achieved through H2 mediation,it was observed that BD was always around 0.25 g/cm3, andthe fines generated were also on the higher side. Extensiveprocess optimization studies need to be performed for betterpolymer characteristics.

During the course of our investigation with a view to generateUHMWPE with the desired characteristics (BD, PSD/APS,RSV), we have carried out polymerizations with ethylenepressures ranging from 2 to 7.5 atm. With typical catalyst andprocess conditions, we could achieve productivity of ∼2.5 (0.5 kg of UHMWPE/g of catalyst at 7.5 atm ethylene pressureover 2 h (Figure 2a). Nonetheless, besides productivity, the otherpolymer characteristics could be fine-tuned by playing with thepressure. BD improved considerably at enhanced pressures,which is highly desirable. Changing the ethylene pressure wasa convenient way to change the partial pressure of hydrogenduring molecular weight control experiments, thus providingleverage for producing UHMWPE with desired average mo-lecular weight.

We have observed that the temperature at which the polym-erization was performed had an effect on the average molecular

weight of UHMWPE, akin to what has been observed by othergroups.13 Thus, keeping all other parameters constant andcarrying out polymerizations at 70, 75, and 80 °C resulted inUHMWPE with progressive reduction in average molecular

Figure 2. (a) Effect of PC2 on polymer productivity. (b) Effect of Ti3+ content on molecular weight of the polymer obtained.

Figure 3. (a) H2 dosing bomb calibration. (b) Effect of H2 pressure on polymer RSV.

Table 1. Effect of H2 Pressure on Molecular Weight of UHMWPEat PC2 5 atma

run PH2 (atm) productivity (g PE/g cat.) BD (g/cm3) APS (µ)b Mηc

1 3 1442 0.36 94 4.02 2 1282 0.36 94 4.43 1 1359 0.35 97 4.34 0.5 1195 0.35 108 5.35 2 1049 0.33 97 4.66 2 1344 0.36 100 4.47 0.17 1282 0.36 103 5.18 0.34 1303 0.36 98 4.2

a General conditions: PC2 5 atm, Al/Ti ) 5, 75 °C, 500 rpm; 0.34 gof C-4; PH2 in 100 mL bomb. b Analyzed by both Malvern PSA andtraditional sieve shaker methods. c Viscosity-based average molecularweight (million g/mol) calculated using Margolie’s equation.

Table 2. Effect of H2 Pressure on Molecular Weight of UHMWPEat PC2 7.5 atma

run PH2 (atm) productivity (g PE/g cat.) BD (g/cm3) APS (µ)b Mηc

9 0 3235 0.41 124 8.310 3.0 3468 0.40 115 2.111 1.0 3439 0.41 120 3.112 0.65 3453 0.40 121 4.213 0.60 3147 0.41 116 4.314 0.55 3246 0.41 118 4.215 0.34 3235 0.42 114 4.516 0.08 3351 0.41 125 4.2

a General conditions: PC2 7.5 atm, Al/Ti ) 5, 75 °C, 500 rpm; 0.34 gof C-4; PH2 in 100 mL bomb. b Analyzed by both Malvern PSA andtraditional sieve shaker methods. c Viscosity-based average molecularweight (million g/mol) calculated using Margolie’s equation.

4868 Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009

weight, though not on a major scale because of the smalldifference in temperature. It has to be realized that carrying outpolymerization at much lower temperatures is not economicalfrom the commercial angle, since the reaction rate drops downdrastically for even a drop in temperature of about 10 °C.

Under more or less similar operating conditions (within limitsof experimental error) catalyst systems C-2, C-3, C-4, and C-5containing 16, 21, 25, and 32% Ti3+ were screened forUHMWPE polymerization. The trend when ethylene pressurewas 7.5 atm is shown in Figure 2b. We could see that theethylene pressure predominates over the trivalent Ti content inthe catalysts to alter the kinetics of the process.

Hydrogen has been in regular use as a molecular weightregulator in ethylene and propylene polymerizations. It isconvenient to use due to various practical reasons, since anextensive amount of data pertaining to its solubility in varioussolvents is available at different temperatures.14 Research groupshave also determined the Henry’s constant for hydrogen andethylene in hexane at different temperatures.15 We have alsostudied the solubility characteristics of ethylene, hydrogen, andtheir mixtures in solvents like varsol and hexane.16 Hydrogenis one such gas where its solubility increases with temperature,unlike the expected reverse trend. For UHMWPE systems, thiscan have far reaching implications since a proper combination

of solvent, temperature, and hydrogen partial pressure can resultin unique molecular weight control.

With an objective of controlling the molecular weight ofUHMWPE with hydrogen,17 we have calibrated the hydrogendosing bomb hooked to the Buchi reactor. The bomb waspressurized at ambient temperature, 30 °C, with hydrogen atdifferent pressures, and the volume of hydrogen was mea-sured using a gas flow meter. The results are given in Figure3a. This gave a method to measure the volume of hydrogendosed based on the pressure employed in the bomb. Thecalibration results are quite linear, with an excellent regres-sion constant of almost 1.

During the course of our investigation for regulating themolecular weight of UHMWPE in polymerization, we realizedthat there is a threshold limit for hydrogen using the specifiedbomb under the employed conditions. This is essentially thethreshold or saturated solubility of hydrogen in 3 L hexane atthe specified operating conditions based on the partial pressuresof hydrogen, ethylene, and hexane.18 We could not go down tolower hydrogen pressures than this due to the bomb limitingcapacity. The size of the dosing bomb was approximately 100mL at atmospheric pressure. The approach available to us wasto hook up another bomb of smaller size, say 50 mL in capacity,or to reduce the hydrogen partial pressure by significantly

Table 3. Scale up Studiesa

run reactor size (L) solvent vol (L) catalyst concn (mmol Ti) Al/Ti productivity (g PE/g cat.) Mηb

17 5 3 1.00 8 2760 10.018 1 0.5 0.2 8 3000 12.619 19 10 4.00 8 2250 5.320 19 10 4.00 5 1950 7.2

a General conditions: PC2 7.5 atm, PH2 0 atm, 75 °C, 500 rpm with C-4. b Viscosity-based average molecular weight (million g/mol) calculated usingMargolie’s equation.

Figure 4. SEM images of the polymers produced from (a) C-2, (b) C-3, (c) C-4, and (d) C-5.

Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 4869

increasing the ethylene pressure. We have used the secondapproach. Here also we could not go beyond 7.5 atm ethylenepressure due to system configurations. The results are depictedin Figure 3b.

It can be observed how effectively the partial pressure ofhydrogen is controlled at two different ethylene pressures, viz.,5 and 7.5 atm. Obviously, as expected the line at 5 atm ethylenepressure controls molecular weight regulation in a higher regionthan the 7.5 atm ethylene pressure, again verifying Henry’s lawfor the solubility of gases. We can observe from Figure 3b thatachieving an average molecular weight of ∼4.5 million isstatistically more favored at hydrogen pressures from 0.1 to 0.5atm, since the partial pressure of hydrogen is not lowered downsignificantly at these lower hydrogen pressures. Molecularweight control with hydrogen pressure of 1, 2, and 3 atm reflectsa linear response, with the molecular weight progressivelydropping down since the partial pressure of hydrogen nowbecomes significant (Figure 3b).

In case we wanted molecular weight control in a still higherregion compared to 5 atm ethylene pressure, the approach wouldbe to operate at lower ethylene pressures; this would lower thepartial pressure of hydrogen, thus increasing the molecularweight. In doing so, we might realize that other vantageproperties like productivity, BD and APS might get affected.In a nut shell, the overall game is optimization of all parameterssuch that we get all the desired properties.19

Thus, experiments at 5 atm ethylene pressure gave us goodproductivity, except that the bulk density was below 0.4 g/cm3

and APS was low. The results are shown in Table 1. Experi-ments at 7.5 atm ethylene pressure gave us most of the desiredpolymer properties. We found that it was an excellent recipefor making the 4.5 million molecular weight grade withenhanced productivity, desired BD, and PSD/APS (Table 2).UHMWPE produced using different catalyst batches withdifferent Ti3+ contents (Figure 4) hardly showed any variationin morphology. The SEM images of several other batches mimicthe same kind of images, confirming the consistent quality ofthe polymer obtained in different grades synthesized.7

After thorough examination of the 5 L scale laboratoryexperiments, we did scale up the same reaction to 19 L scale.The productivity and quality of the polymer in terms of otherpolymer characteristics were found to be comparable, but themolecular weight of the polymer obtained came down drastically(Table 3). This led us to do the reaction in smaller scale alsoand we indeed found that at 1 L scale the molecular weightwas higher. The experiment with lesser aluminum alkyl, i.e.Al/Ti ratio of 5 in 19 L, gave polymer with comparable yieldand increased molecular weight. It is worth noting that thealuminum alkyl, which is in excess, plays the role of a chainterminator, thereby reducing the molecular weight. The reactionin 1 L scale, having fewer aluminum alkyls available for chaintermination, gave higher molecular weight polymer.11 Thus,controlling the effective alkyl aluminum concentration is animportant parameter, especially while synthesizing polymershaving ultrahigh molecular weight.

Conclusion

The production of UHMWPE having molecular weight of4-6 million g/mol under specified operating conditions wasestablished on a scale of 5 L. The polymer obtained had definedproduct characteristics, which is highly desirable from anindustrial point of view. The study further emphasizes theimportance of the proper concentration of catalyst and cocatalyst

and other process conditions for achieving the desired polymercharacteristics.

Acknowledgment

We thank Mr. Viral Kumar Patel for his technical andanalytical assistance throughout the course of the work. Sincerethanks are due to Dr. R. Char and his team for the pilot plantstudies. We also sincerely thank Dr. Ajit Mathur and Dr. RakhV. Jasra for their continuous encouragement to carry out thiswork.

Supporting Information Available: The detailed procedurefor estimating total titanium content and different oxidationstates present in the catalyst systems is given in detail. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited

(1) Kurtz, S. M. UHMWPE Handbook, 1st ed.; Elsevier: New York,2004.

(2) For more information visit www.dsm.com/en_US/html/dep/stamylanuh.htm.

(3) (a) Kurtz, S. M.; Muratoglu, O. K.; Evans, M.; Edidin, A. A.Advances in the processing, sterilization, and crosslinking of ultra-highmolecular weight polyethylene for total joint arthroplasty. Biomaterials 1999,20, 1659. (b) Rose, R. M.; Cimino, W. R. Exploratory investigations onthe structure dependence of the wear resistance of polyethylene. Wear 1982,77, 89. (c) Weightman, B.; Light, D. A comparison of RCH 1000 and Hi-Fax 1900 ultra-high molecular weight polyethylenes. Biomaterials 1985,6, 177. (d) Nakayama, K.; Furumiya, A.; Okamot, T.; Yag, K.; Kaito, A.;Choe, C. R.; Wu, L.; Zhang, G.; Xiu, L.; Liu, D.; Masuda, T.; Nakajima,A. Structure and mechanical properties of ultra-high molecular weightpolyethylene deformed near melting temperature. Pure Appl. Chem. 1991,63, 1793.

(4) Patent search results related to UHMWPE: (a) Siegfried, L.;Birnkraut, H. W.; Moser, H. Process for the polymerization of alpha olefins.US Patent 3,910,870, 1975. (b) Heinrich, A.; Bohm, L.; Scholz, H. A.Process for the preparation of ethylene (co)polymers. US Patent 5,292,8371994. (c) Ehlers, J.; Walter, J. Process for the preparation of ultrahighmolecular polyethylene having high bulk density. US Patent 5,587,440,1996. (d) Bilda, D.; Boehm, L. Process for the preparation of a polymer-ization and copolymerization of ethylene to give ultra high molecular weightethylene polymers. US Patent 6,114,271, 2000. (e) Payer, W.; Ehlers, J.Method for the production of olefin polymers and selected catalysts. USPatent 7,157,532, 2007. (f) Ehlers, J.; Haftka, S.; Wang, L. Method forproducing a polymer. US Patent 7,141,636, 2006.

(5) (a) Honma, S.; Tominari, K.; Kurisu, M. Injection molding polyolefincomposition. US Patent 5,019,627, 1991. (b) Liu, J. C. Olefin polymerizationwith pyridine moiety-containing single-site catalysts. US Patent 6,767,975,2004.

(6) For synthesis of the catalyst recipes MgOEt2 + TiCl4, see: Berthold,J.; Diedrich, B.; Franke, R.; Hartlapp, J.; Schafer, W.; Strobel, W. Processfor the preparation of a polyolefin, and a catalyst for this process. US Patent4,447,587, 1984; Process for the preparation of a polyolefin, and a catalystfor this process. US Patent 4,448,944, 1984.

(7) Da Silva, J. C.; De Figueiredo, M. O. Spherical ultra high molecularweight polyethylene. US Patent 5,807,950, 1998.

(8) (a) Martin, J. I.; Secora, S. J. Benham, E. A.; McDaniel, M. P.;Hsieh, E.; Johnson, T. W. Olefin polymerization process and productsthereof. US Patent 6,034,186, 2000. (b) Martin, J. I.; Bergmeister, J. J.;Hsieh, E.; McDaniel, M. P.; Benham, E. A.; Secora, S. J. Olefinpolymerization process and products thereof. US Patent 6,657,034, 2003.

(9) Liu, J. C.; Mack, M. P.; Lee, C. C. Preparation of ultra high molecularweight polyethylene. US Patent 6,265,504, 2001.

(10) Novel catalysts reported in the scholarly literature demonstratedto produce ultrahigh molecular weight polyethylenes:(a) Tamm, M.; Randoll,S.; Herdtweck, E.; Kleigrewe, N.; Kehr, G.; Erker, G.; Rieger, B. Imidazolin-2-iminato titanium complexes: Synthesis, structure and use in ethylenepolymerization catalysis. Dalton Trans. 2006, 459. (b) Starzewski, K. A. O.;Xin, B. S.; Steinhauser, N.; Schweer, J.; Benet-Buchholz, J. Donor-acceptormetallocene catalysts for the production of UHMW-PE: Pushing theselectivity for chain growth to its limits. Angew. Chem. 2006, 118, 1831.(c) Karam, A.; Casas, E.; Catarı, E.; Pekerar, S.; Albornoz, A.; Mendez, B.

4870 Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009

Effect of the alkoxyl ligands on ethylene polymerization by TpTiCl2(OR)complexes. J. Mol. Catal. A: Chem. 2005, 238, 233. (d) Michiue, K.; Jordan,R. F. Synthesis, structures, and olefin polymerization behavior of stericallycrowded tris(pyrazoyl) borate zirconium and hafnium complexes. Orga-nometallics 2004, 23, 460. (e) Ionkin, A. S.; Marshall, W. J. Ortho-5-methylfuran and benzofuran-substituted η3-allyl(adiimine)nickel(II)com-plexes: Syntheses, structural characterization, and the first polymerizationresults. Organometallics 2004, 23, 3276. (f) Fujita, M.; Seki, Y.; Miyatake,T. Synthesis of ultra-highmolecular-weight poly(R-olefin)s by thiobis(phe-noxy)titanium/MMAO system. J. Polym. Sci. A: Polym. Chem. 2004, 42,1107. (g) Makio, H.; Kashiwa, N.; Fujita, T. A new family of highperformance catalysts for olefin polymerization. AdV. Synth. Catal. 2002,344, 477. (h) Mori, H.; Ohnishi, K.; Terano, M. The heterogeneous modifiedpolypropylene-supported Ziegler catalyst/MMAO system for producingUHMWPE and poly(ethane-co-hex-1-ene) with a homogeneous comonomerdistribution. Macromol. Chem. Phys. 1999, 200, 2320. (i) Kageyama, K.;Tamazawa, J.; Aida, T. Extrusion polymerization: Catalyzed synthesis ofcrystalline linear polyethylene nanofibers within a mesoporous silica. Science1999, 285, 2113. (j) Peucker, U.; Heitz, W. Vinylic polymerization byhomogeneous chromium(III) catalysts. Macromol. Rapid Commun. 1998,19, 159. (k) Chen, Y. -X.; Stern, C. L.; Marks, T. J. Very large counteranionmodulation of cationic metallocene polymerization activity and stereoregu-lation by a sterically congested (perfluoroaryl)fluoroaluminate. J. Am. Chem.Soc. 1997, 119, 2582. (l) Sano, A.; Iwanami, Y.; Matsuura, K.; Yokoyama,S.; Kanamoto, T. Ultradrawing of ultrahigh molecular weight polyethylenereactor powders prepared by highly active catalyst system. Polymer 2001,42, 5859. (m) Nomura, K. Design of new generation vanadium complexcatalysts offering new possibilities for controlled olefin polymerization. NewDeVelopments in Catalysis Research; Bevy, L. P., Ed.; Nova SciencePublishers: New York, 2005; p 199. (n) Wang, W.; Nomura, K. Notableeffects of aluminum alkyls and solvents for highly efficient ethylene(co)polymerizations catalyzed by (arylimido)(aryloxo)vanadium complexes.AdV. Synth. Catal. 2006, 348, 743. (o) Wang, W.; Nomura, K. Remarkableeffects of Al cocatalyst and comonomer in ethylene copolymerizationscatalyzed by (arylimido)(aryloxo)vanadium complexes: Efficient synthesisof high molecular weight ethylene/norbornene copolymer. Macromolecules2005, 38, 5905.

(11) (a) Zakharov, V. A.; Bukatov, G. D.; Yermakov, Y. I. The role oforganometallic co-catalysts in catalytic Ziegler-Natta systems. Die Mak-romol. Chem. 1975, 176, 1959. (b) Mejzlik, J.; Lesna, M.; Kratochvila, J.Determination of the number of active centers in Ziegler-Natta polymeriza-tions of olefins. Chem. Mater. Sci. 1986, 81, 83–120. (c) Soga, K.; Shiono,K. Ziegler-Natta catalysts for olefin polymerizations. Prog. Polym. Sci.1997, 22, 1503.

(12) (a) Bohm, L. L. The ethylene polymerization with Ziegler catalysts:Fifty years after the discovery. Angew. Chem., Int. Ed. 2003, 42, 5010. (b)Bohm, L. L. High mileage Ziegler-catalysts: Excellent tools for polyethyleneproduction. Macromol. Symp. 2001, 173, 53. (c) Koranyi, T. I.; Magni, E.;Somorjai, G. A. Surface science approach to the preparation and charac-terization of model Ziegler-Natta heterogeneous polymerization catalysts.Top. Catal. 1999, 7, 179.

(13) Joo, Y. I.; Han, O. H.; Lee, H.-K.; Song, J. K. Characterization ofultra high molecular weight polyethyelene nascent reactor powders by X-raydiffraction and solid state NMR. Polymer 2000, 41, 1355–1368.

(14) Morsi, B. I. Gas-liquid mass transfer in a slurry reactor operatingunder olefinic polymerization process conditions. Chem. Eng. Sci. 1996,51, 549–559.

(15) (a) Waters, J. A.; Mortimer, G. A.; Clements, H. E. Solubility ofsome light hydrocarbons and hydrogen in some organic solvents. J. Chem.Eng. Data 1970, 15, 174–176. (b) Ohgaki, K.; Sano, F.; Katayama, T.Solubilities of hydrogen and nitrogen in alcohols and n-hexane. J. Chem.Eng. Data 1976, 21, 194–196. (c) Gao, W.; Robinson, R. L.; Gasem,K. A. M. Solubilities of hydrogen in hexane and of carbon monoxide incyclohexane at temperatures from 344.3 to 410.9 K and pressures to 15MPa. J. Chem. Eng. Data 2001, 46, 609–612.

(16) Sivalingam, G.; Natarajan, V.; Sarma, K. R.; Parasuveera, U.Solubility of ethylene in the presence of hydrogen in process solvents underpolymerization conditions. Ind. Eng. Chem. Res. 2008, 47, 8940–8946.

(17) For homogeneous systems: (a) Toyota, A.; Tsutsui, T.; Kashiwa, N.J. Mol. Catal. 1989, 56, 237. (b) Peng, K.; Xiao, S. J. Mol. Catal. 1994, 90,201. (c) Reddy, S. S.; Sivaram, S. Homogeneous metallocene-MAO catalystsystems for ethylene polymerization. Prog. Polym. Sci. 1995, 20, 309. (d)Huang, J.; Rempel, G. L. Ziegler-Natta catalysts for olefin polymerization:Mechanistic insights from metallocene systems. Prog. Polym. Sci. 1995, 20,459. (e) Imanishi, Y.; Naga, N. Recent developments in olefin polymerizationswith transition metal catalysts. Prog. Polym. Sci. 2001, 26, 1147 Forheterogeneous systems. (f) Kissin, Y. V. Multicenter nature of titanium-basedZiegler-Natta catalysts: Comparison of ethylene and propylene polymerizationreactions. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1475. (g) Chu, K. J.;Soares, J. B. P.; Penlidis, A. Effect of hydrogen on ethylene polymerizationusing in-situ supported metallocene catalysts. Macromol. Chem. Phys. 2000,201, 552. (h) Kissin, Y. V.; Sivek, A. J. Modification mechanism in olefinpolymerization catalysts TiCl4/MgCl2-aromatic ester-Al(C2H5)3. J. Polym. Sci.,Polym. Chem. 1984, 22, 3747. (i) Kissin, Y. V. Main kinetic features ofethylene polymerization reactions with heterogeneous Ziegler-Natta catalystsin the light of a multicenter reaction mechanism. J. Polym. Sci., Part A: Polym.Chem. 2001, 39, 1681. (j) Kissin, Y V.; Mink, R. I.; Nowlin, T. E. Ethylenepolymerization reactions with Ziegler-Natta catalysts. I. Ethylene polymeri-zation kinetics and kinetic mechanism. J. Polym. Sci., Part A: Polym. Chem.1999, 37, 4255.

(18) Khare, N. P.; Seavey, K. C.; Liu, Y. A.; Ramanathan, S.; Lingard,S.; Chen, C. Steady-state and dynamic modeling of gas-phase polypropyleneprocesses using stirred-bed reactors. Ind. Eng. Chem. Res. 2002, 41, 5601.

(19) (a) Bohm, L. L. Ethylene polymerization process with a highlyactive Ziegler-Natta catalyst: 1. Kinetics. Polymer 1978, 19, 553. (b) Bohm,L. L. Ethylene polymerization process with a highly active Ziegler-Nattacatalyst: 1. Molecular weight regulation. Polymer 1978, 19, 562.

ReceiVed for reView December 29, 2008ReVised manuscript receiVed February 25, 2009

Accepted March 17, 2009

IE802000N

Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009 4871