SUPPORTED ZIEGLER-NATTA CATALYSTS FOR ETHYLENE SLURRY POLYMERIZATION AND CONTROL OF MOLECULAR WEIGHT

Chinese Jourmal of Polymer Science Vol. 26, No. 5, (2008), 553-559Chinese Journal ofPolymer Science02008 World ScientificSUPPORTED ZIEGLER-NATTA CATALYSTS FOR ETHYLENE SLURRYPOLYMERIZATION AND CONTROL OF MOLECULAR WEIGHTDISTRIBUTION OF POLYETHYLENEVladimir Zakharov , Ludmila Echevskaya , Tatiana Mikenas, Mikhail Matsko, Andrey Tregubov,Marina Vanina and Marina NikolaevaBoreskov Institute of Catalysis, Novosibirsk 630090, RussiaAbstract The effect of chemical composition of highly active supported Zieglr-Natta catalysts with controlled morphologyon the MWD of PE has been studied. It was shown the variation of transition metal compound in the MgCl2 supportedcatalyst affect of MWD of PE produced in broad range: V anadium. magnesium catalyst (VMC) produce PE with broad andbimodal MWD (MwM。= 14 -21). MWD of PE, produced over titanium-magnesium catalyst (TMC) is nartow or mediumdepending on Ti content in the catalyst (Mw/M。= 3.1-4.8). The oxidation state of the initial titanium compounds in TMChas only slight effect on MWD of PE produced. Based on MWD data of PE heterogeneity of active centers of TMC andVMC was studied. The results of resolution of experimental GPC curves into Flory components indicated three Florycomponents are sufficient to describe MWD curve of PE, produced with TMC: six Flory components are required in the caseof VMC. In the case of copolymerization of ethylene with I-hexene over TMC the addition of I-hexene leads to decrease ofMW and to slight efect on Mw/Mn values. On the contrary the strong effect of 1-hexene on MWD of PE produced overVMC was found: the introduction of I-hexene results in considerable broadening of MWD due to the shifting of the mainMWD peak to low MW region. Al that comonomer does not affect the position of high molecular weight shoulder. Theresults indicate that some of active centers of VMC producing high MW polymer are not active in the reaction of chaintransfer with comonomer.Keywords: Polyethylene (PE); Vanadium magnesium catalyst (VMC); Titanium magnesium catalyst (TMC); Gel permeationchromatography (GPC); Molecular weight distribution (MWD).INTRODUCTIONZiegler-Natta (ZN) type catalysts are widely used in the industry for production of PE with various molecularweights (MWs) and molecular weight distributions (MWDs). There are lots of modifications of ZN catalystsvarying in the way of preparation of the support and differences in the active component compound structure andpresence of various modifying additives, for example, electron donor compounds used at support or catalystpreparation. The most famous supported ZN type catalysts are titanium-magnesium catalysts (TMC). The activecomponent of these catalysts is titanium chloride supported on the magnesium chloride. Titanium-magnesiumcatalysts produce PE with narrow and medium MWD: polydispersity values (M/M.) are usually within the rangeof 4-81.21. Vanadium-magnesium catalyst (VMC) containing vanadium chloride as active component supportedon the magnesium chloride produce PE with broad MWD; (MM.) values are usually within the range of10-25b]. One of the most important issues in ethylene polymerization over supported ZN catalysts is the controlof MWD of PE. It is supposed that the main reason of MWD broadening (Mw/M。> 2) of olefin polymerization"Coresponding author: Vladimir Zakharov, E -mail: zva@catalysis.ruLudmila Echevskaya, E- mail: echev@catalysis中国煤化工Invited lecture pesened at the Asian Polyolefn Workshop 2007, 2007:DHCNMHGReceived February 22, 2008; Revised April 2, 2008; Accepted April 3,554V. Zakharov et al.over supported ZN catalysts is the heterogeneity of the active centers of these catalysts (multisite catalysts).However, the nature of this heterogeneity remains unclear.It is important to use the catalyst with optimal morphology at the ethylene slurry polymerization over highlyactive supported catalysts (small particle size, narrow particle size distribution) to exclude the possible effect ofdiffusion restrictions on MWD of polymer produced. Only in this case it is possible to get reliable data on theeffect of catalysts composition on MWD of polymers. We used in this study the catalysts with cortolledmorphology (average particle size within the range of 5-10 μm and narrow particle size distribution) preparedaccording to'*. As example the photos of the catalyst particles and PE particles produced over this catalyst areshown in Fig. 1.l0μm10 pumFig. 1 Photos of the TMC (a) and respective PE (b) particlesIn the present paper the data on the effect of chemical composition of the supported ZN catalysts on theMWD of PE are presented. The effects of transition metal compound (Ti, V), Ti content and oxidation state oftitanium in TMC on MWD of PE characteristics are discussed.EXPERIMENTALCatalysts were synthesized via procedure described previously+T by supporting of titanium compounds ofdifferent composition (TMC) or VCl4 (VMC) on highly dispersed magnesium chloride with average particle size10 μum and narrow particle size distribution.Three catalysts with Ti content of 0.07 w% (TMC-0.07), 1.6 wt% (TMC-1.6) and 5 w% (TMC-5) havebeen used in this study.TMC with different titanium oxidation state compounds were obtained by deposition of Ti(I), Ti(II)nd Ti(IV) chlorides (f/-arene-Ti?*Al2Cls, Ti+Cl;nDBE (DBEdibutyl ether), and Ti*tCl4 compoundscorrespondingly) on highly dispersed magnesium chloride according to reference [5].Ethylene slurry polymerization was performed in a 0.8 L steel reactor, in heptane, at constant ethylenepressure 4 bar and polymerization temperature 80°C for 1 h; trisobutylaluminium (TIBA) was used as a .co-catalyst, its concentration being 2-10 mmol TIBAL, catalyst concentration was 0.04 g/L.MWD measurements were performed using a WATERS-150 C instrument in conjunction with DifferentialViscometer (Viscotek Model 100). Run conditions were as follows: temperature 140°C; 1,2,4 trichlorobenzene(TCB) was used as a solvent at a flow rate of 1 cm' /min. Four mixed bed TSK. gel columns (GMHXL-HT, TosohCorp.) were used. Calibration was made using narrow Polystyrene standards and PE standards.Copolymer composition (the content of CH; groups) was determined by IR spectroscopy (absorption band at1378 cm^ ) using an IR Fourier spectrometer Shimadzu FTIR 8400S according to reference [6]. .RESULTS AND DISCUSSIONEffect of Transition Melal CompoundTable 1 presents the data on the activity of TMC and VM(中国煤化工nd MWD of PEproduced. It is seen that VMC shows high activity similar to thai:YHCNMHG: data on ethyleneSuppored zN Catalysts: Control MWD ofPE555polymerization at different hydrogen concentration over TMC and VMC were published in our previouspapersts .Table 1. Data on the effect of transition metal compound on the activity of supported ZNcatalysts and MWD of PE (80°C, TIBA as cocatalyst)PE no.Catalyst Content ofTi (V)Activity (kg PE/MMwM. MJ M!type(wt%)gTi(V)xhx barCH4)__ x 10-3x10-3 x10-3 M。 M、TMC1.657210560 3.7 2.72bVMC1.831440180014 4.1"Ethylene pressure 4 bar; [H2]/IC2H4] = 0.25 in gas phase; "Ethylene pressure 7 bar; [H2]ICHJ] = 0.07 in gas phaseTable I and Fig. 2 present SEC data for PE synthesized with VMC and TMC. The great dffrence betweenPE produced over VMC and TMC is observed. Vanadium-magnesium catalysts produced PE with broad andbimodal MWD. Mw/M。value for PE synthesized with VMC is equal to 14, in contrast to 3.7 for PE synthesizedwith TMC. The MWD curve for PE produced with VMC demonstrates explicit shoulder within the high-MWregion.0.6-屋0.42M20.2-0.0,-lgMFig.2 MWD curves of PE samples, produced over TMC (curve 1) and VMC(curve 2) (polymerization conditions see in Table 1)Experimental data on the effect of catalyst composition on the MWD of PE was used for analysis ofheterogeneity of active centers of TMC and VMC. For that analysis we used the approach based on resolution ofexperimental GPC curves into Flory components according to Ref. [7]. Two PE samples (from Table 1)0.8F~一Experimental---- Calculated--- Calculated0.4-2售X30.0Le，5 4.0 4.55.0 5.5 6.0 6.Fig. 3 MWD curve of PE sample produced with TMCFi中国煤化工VMC(Tablel,(Table I exp. 1) and it's resolution into Flory componentsex-nponentsYHCNMHG556V. Zakharov et al.produced over TMC and VMC were analyzed. The results of resolution of GPC curves into Flory componentsare presented in Figs. 3, 4 and Table 2. The data indicate three Flory components are sufficient to describe MWDcurve of PE, produced with TMC (Fig. 3). This result is in agreement with our previous experimental dataobtained at analysis of heterogeneity of active centers of TMC by fractionation of PE into fractions with narrowMWD with M/M.≤28. It was found that three components corresponding to three groups of active centers aresufficient for proper ftting of experimental MWD curve of PE obtained over TMC with different Ti content.Analysis the MWD of PE, produced with VMC is more complicated. Six Flory components are required in thiscase (Fig. 4, Table 2).Table 2. Data on calculated MWD parametersof PEPE nOo. aalystFlory componentbPortion of Flory component (%6)M. x 10-333.458243.9160TMC323.7475.94.612.527VMC24.424028.8990PE numbers are corresponds to those in Table 1 and Fig. 2;"Flory components numbers and MWD curves are shown on Figs. 3, 4.Control of MWD of PE via Variation of Chemical Composition of Ti Based CatalystsEffect of the supporn (MgCl2) and Ti content in supported catalysts on MWD of PE producedThe data on the effect of Ti content on the activity of TMC and MWD of PE are presented in Table 3. Theactivity of TMC-0.07 is about ten times higher then that of TMC-5. It was shown24 that the reason of higheractivity of TMC with low Ti content is the higher number of the active centers. The similar Kp values at ethylenepolymerization over TMC with different Ti content (0.1 wt% and 0.9 wt% of Ti) were foundl9!.Table 3. Data on the efect of Ti content on the catalyst's activity and MWD of PETi contentActivity (kg PE/CatalystPE no.M。x 10-M.x103 MJ/M。(wt%)gTi xh x bar C2H4)TMC (0.07)0.07460571803.1TMC (5)“5.0482804.88-TiCl°.0402706.8"Polymerization conditions: 80°C, TIBA, ethylene pressure 4 bar. (H2|/[C:H4| = 0.25;'The numbers correspond to the content of Ti;“Commercial sample of Solvay type nonsupported catalystThe increase of titanium content in TMC results in the increase of Mw value of PE (Mw increases from180x 10' up to 280x 10) and increase of MJ/Mn value from 3.1 to 4.8 (exp. 1 and 2 in Table 3). Mn valueschange slightly and remain within the range of 57x 10'-58x 10'. It is seen that the MWD broadening arisesfrom the high-molecular-weight part of PE (see curves 1 and 2 in Fig. 5).Data on the polymer obtained at ethylene polymerization over non-supported 8-TiCl; catalyst are presentedin Table 3 and Fig. 5 as well. It is seen that δ-TiCl catalyst produces PE with broader MWD in comparison withsupported TMC catalysts. This broadening is caused by formation of both low-molecular and high-molecularparts of PE (compare curves 1 and 3, Fig. 5).中国煤化工MHCNMHGSupporled ZN Catalysts: Conrol MWD ofPE5571.0p0.80.6-害0.4-0.2-IgMFig.5 MWD curves of PE produced over TMC with different Ti content and 8-TiCI3 catalystCurves numbers correspond to PE numbers in Table 3.Effect of the oxidation state ofTi in TMC composition on the activity and MWD of PE producedIt is supposed that the main reason of MWD broadening (MJM > 2) is heterogeneity of the active centers of ZNcatalysts (multisite catalysts). Some authorsl10. 川relate a broad MWD of PE produced with TMC to theformation of various active sites containing T(I), T(II) and Ti(IV) compounds. In this study, we investigatedthe influence of oxidation state of titanium in titanium compounds on the catalytic properties of MgCl2 supportedcatalysts in ethylene polymerization and on MWD of PE produced. Soluble compounds of titanium in differentoxidation states (n"-arene-Tit*Al2Cl, Tit*Cl3 mDBE, and Ti*Cls) were synthesized according to reference [5],analyzed by ESR and "C-NMR methods, and adsorbed on the highly dispersed magnesium chloride (Table 4).Table 4. Effect of the oxidation state of Ti in TMC on catalyt's activityt and MWD of PECatalystIITitanium compound[n"BenzeneTiAl2ClJ] TiCl nDBETiCl4Ti content (Wt%)0.70.9Ti oxidation state in Ti compoundTi(I)T(II)Ti(IV)Possible oxidation state of active centerTi2+Ti3+, Ti2+Ttit, Tit, Ti?+Activity (kg PE/g Ti h)30170280Mwx10310MIM.5.24.7"Polymerization conditions: heptane, 80°C, TIBA as cocatalyst, ethylene pressure 4 bar,hydrogen pessure I bar, for ihTMC obtained by deposition of Ti(I), T(II) and Ti(IV) chlorides on a highly dispersed magnesiumchloride (catalysts I, II, II in Table 4) were tested in ethylene polymerization. Table 4 presents the data on thecatalyst activity and MWD of PE produced.Catalyst I containing divalent titanium shows the activity in ethylene polymerization in the absence oforgano-aluminum co-catalyst (11 kg/g Ti h bar CzH4) (80°C). This result confirms that catalyst I actually containsa divalent titanium compound as the active component, since according to the literature data, divalent titaniumcompounds are active in ethylene polymerization without organo- aluminum co-catalyst!12. '3l Note that activity ofthe supported catalyst obtained in our study (11 kg/g Ti h bar C2H4) is approximately two orders of magnitudehigher as compared to the activity of bulk TiCl2 (0.16 kg/g Ti h bar C2Hy)31. The data of Table 4 show thatcatalyst I containing a Ti(I) compound shows a maximum activity. Presumably, this relates to a more dispersedstate of the active component in this catalyst due to formation of titanium dichloride complexes with aluminumchlorides. Catalyst II of TiC:nDBE/MgCl2 composition is less active as compared to conventional catalyst IlI ofTiCl/MgCl2 composition. This may be caused by partial blocki中国煤化Ireviously. it wasshown in reference [14] that calysts TiCl/MgCl2 and TiClyMg)se activities.TYHCNMHG558V. Zakharov et al.As shown from Table 4, catalysts I, II and II, containing titanium chlorides in different oxidation states, inthe presence of TIBA as cocatalyst produce PE with a close MW and MWD values (Mw/M。= 4.7-5.5).Thus catalysts I, II, Il produce PE with the similar (broad) MWD (MJ/Mn = 3.9-5.2). It means possibleformation of active centers in the different oxidation states of Ti is not the real reason of the broad MWD of PEproduced over TMC.Effect of Comonomer (I-Hexene) on the MWD of Polymers Produced over TMC and VMCEarlier we have studied copolymerization of ethylene with a-olefins over supported ZN catalysts of differentcomposition'. It was shown VMC have much higher copolymerizing reactivity in comparison with TMC. Herewe present the data on the efect of l-hexene on MWD of copolymers prepared with TMC and VMC.Table 5 and Figs. 6, 7 present the data on ethylene-l-hexene copolymerization initiated by TMC and VMC.data indicate very different effect of l-hexene on MWD of copolymers produced. In the case ofcopolymerization over TMC, adition of I-hexene proceeds to decrease of MW and to slight effect on the Mw/Mnvalue. In the case of VMC the data indicate on the strong effect on MWD: the introduction of 1-hexene results inconsiderable broadening of MWD: while the main MWD peak is shifted to low MW region, the high MWshoulder stays at the same position (Fig. 7). As a result, M.JM。and M/Mw values are increased (Table 5).Table 5. Data on ethylene-I-hexene copolymerization' over ZN catalysts with different compositionActivityMI(5)C6HI2°Polymer no.CatalystCH2/C2H"__(kg/gTi x hx bar)(g/10 min)M.x10~3 MJ/Mn(mol%)TMC0.0704503103.3.610200.412203..9TMC-5330.113704.2.60.5.5VMC260.15440140.4056_1.743020.5.2"Polymerization conditions: 80°C, TIBA as cocatalyst, ethylene pressure = 2 bar in exps.1-4 and 7 bar in exps.5, 6;the ratio H2/C2Hs= 0.13 in exps.1-4 and 0.07 in exps.5, 6; .Comonomer concentration ratio in heptanes;“1-Hexene content in copolymer.6厂.8-0.4-0.6-s 0.4-吉.2+.0L4~56lgMIgMFig.6 Effect of I-hexene on MWD of PE producedFig.7 Efft of l-hexene on MWD of PE producedover TMC-5 (curves 1 and 2 are correspond to exps. 3over VMC (curves 1 and 2 are correspond to exps. 5and 4 Table 5)and 6 Table 5)The results indicate that some of active centers of VMC producing high MW polymer are not active in thereaction of chain transfer with comonomer.In our previous paper we proved the heterogeneity of VM(of chain transferwith hydrogen'). Data presented here indicates heterogeneity中国煤化工ecion ahaihTYHCNMHGSupported ZN Catalysts: Control MWD ofPE559transfer with I-hexene. Similar to the hydrogen effect, the VMC contains two types of active centers, one ofthem is active in the chain transfer reaction with I-hexene. These centers produce polymer with lower MW atcopolymerization with I-hexene. Another type of active centers is not active in the chain transfer reaction with1-hexene. These centers produce high MW PE (Mw= 1 x 106- 3 x 10) and comonomer doesn't affect theposition of bigh molecular weight shoulder (Fig.7).CONCLUSIONSTransition metal compound in the MgCl-supported ZN catalyst affect of MWD of PE produced in broad range:VMC produce PE with broad and bimodal MWD (MwMn= 14-21). MWD of PE. produced over TMC is narrowor medium depending on Ti content in the catalyst (M./Mn = 3.1-4.8). Non supported 8-TiCl3 catalyst producesPE with broader MWD (MJM。= 6.8) in comparison with supported TMC catalysts.Heterogeneity of active centers of TMC and VMC was studied based on detailed analysis of MWD data ofPE produced. The results of resolution of GPC curves into Flory components indicated three Flory componentsare sufficient to describe MWD curve of PE, produced with TMC. Six Flory components are required in the caseof VMC. It indicates on higher heterogeneity of active centers VMC in comparison with TMC.TMC containing the initial titanium compounds with different titanium oxidation state (Ti(II), Ti(II),TI(IV)) produce PE with a close Mw and MWD values (MJM,= 4.7- -5.2). Probably possible formation of activecenters in the different oxidation states of Ti is not the real reason of the broad MWD of PE produced over TMC.Completely dfferent effect of comonomer (I-hexene) on the MWD of polymer produced over TMC andVMC have been found. In the case of copolymerization of ethylene with 1-hexene over TMC the addition ofI-hexene leads to decrease of MW and to slight effect on MwMn values. In contrary the introduction of Il-hexeneat ethylene polymerization over VMC results in considerable broadening of MWD due to the shifting of the mainMWD peak to low MW region. At the same time comonomer doesn't affect the position of high molecularweight shoulder. The results indicate that some of active centers of VMC producing high MW polymer are notactive in the reaction of chain transfer with comonomer.REFERENCES1 Zucchini, U. and Cecchin, G.. Adv. Polym. Sci, 1983. 51: 103? Echevskaya, L.G.. Matsko, M.A.. Mikenas, T.B.. Nikitin, V.E. and Zakharov, V.A.. J. Appl. Polym. Sci.. 2006, 102:54363 Echevskaya, L.G.. Matsko, M.A.. Mikenas. T.B. and Zakharov, V.A.. Polym. Int, 2006, 55: 165Nikitin, V.E.. Mikenas, T.B. and Zakharov, V.A.,. 2005. Rus. Pat., 22572635 Mikenas, T.B.. 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Echevskaya, L.G. and Zakharov, V.A., "Transition Metals and Organomallics as Catalysts for OlefinPolymerization", ed. by Kaminsky, W. and Sinn, H, Springr-Verla中国煤化工TYHCNMHG