Propene Polymerization Using Homogeneous MAO-Activated Metallocene Catalysts: Me2Si(Benz[e]lndenyI)2ZrClu'MAO vs. Me2Si(2-Me-Benz[e]lndenyI)2ZrClu'MAO STEPHAN JUNGLlNG,t ROLF MULHAUPT,b UDO STEHLlNG/ HANS-HERBERT BRINTZINGER/ DAVID FISCHER/ and FRANZ LANGHAUSER3 'Institut fOr Makromolekulare Chemie and Freiburger Materialforschungszentrum der Albert-Ludwigs-Universitat Freiburg, 0-79104 Freiburg, Germany; 2Fakultat fOr Chemie, Universitat Konstanz, 0-78434 Konstanz, Germany; and 3BASF AG Abteilung ZKP, 0-67056 Ludwigshafen, Germany SYNOPSIS Propene was polymerized at 40°C and 2-bar propene in toluene using methylalumoxane (MAO) activated rac-Me2Si(Benz[elIndenyl)2ZrCI2 (BI) and rac-Me2Si(2-Me­ Benz[elIndenyl)2ZrCl2 (MBI). Catalyst BI/MAO polymerizes propene with high activity to afford low molecular weight polypropylene, whereas MBI/MAO is less active and produces high molecular weight polypropylene. Variation of reaction conditions such as propene concentration, temperature, concentration of catalyst components, and addition of hydrogen reveals that the lower molecular weight polypropylene produced with BI/MAO results from chain transfer to propene monomer following a 2,1-insertion. A large fraction of both me­ tallocene catalyst systems is deactivated upon 2,1-insertion. Such dormant sites can be reactivated by H2-addition, which affords active metallocene hydrides. This effect of H2- addition is reflected by a decreasing content of head-to-head enchainment and the formation of polypropylene with n-butyl end groups. Both catalysts show a strong dependence of activity on propene concentration that indicates a formal reaction order of 1. 7 with respect to propene. MBI/MAO shows a much higher dependence of the activity on temperature than BI/MAO. At elevated temperatures, MBI/MAO polymerizes propene faster than BI/ MAO. Keywords: propene. polymerization. zirconocene • methylalumoxane • chain transfer· end groups· hydrogen. 2,1-insertion INTRODUCTION is possible to control molecular weight, stereo­ regularities, and comonomer incorporation without In recent years, versatile generations of Ziegler­ sacrificing the narrow molecular weight distribu­ Natta catalysts based upon MAO-activated metal­ tions. In spite of these improvements in catalyst 1 3 locenes have been developed. - Polypropylene can technology, there is still much to be learned about be produced with high catalyst activity, isotacticity, the elementary steps in olefin polymerization. and molecular weight. As a result of the uniform Small changes in the ligand structure of the catalytically active sites ("single site catalyst"), it metallocene can lead to polyolefins with greatly varied properties. Our research is aimed at better understanding of the main factors controlling * To whom all correspondence should be addressed at Univ­ the behavior of the two metallocenes rac-Me2Si ersitiit Freiburg, Institut fUr Makromolekulare Chemie, Stefan­ (Benz[e]Indenyl}2ZrCI2 (BI) and rac-Me2Si(2-Me­ Meier-Str. 31, 79104 Freiburg, Germany. Benz[e]IndenylhZrCI2 (MBI), see Scheme 1. The catalyst BI/MAO was previously shown to poly- 1305 1306 rac-M~Si(BeDz[e]Indcnyl~ (BI) rac-~Si(2-Me-Benz[e]IndenylhZrCl2' (MBI) Scheme 1. merize propene with high activity producing low Figure 3 shows the plot of the polymerization rates molecular weight polymer, while MBI/MAO showed [kgPP I (mol Zr X h) ] that correspond to the activity the opposite behavior, i.e., lower catalyst activity maxima seen in Figures 1 and 2 vs. propene on a 4 5 and higher polypropylene molecular weight. • double logarithmic scale. The reaction order of the polymerization rate with respect to propene con­ centration is 1.7 for both catalysts. This is signifi­ RESULTS AND DISCUSSION cantly higher than expected for the model of mono­ mer-metal7r-complex formation followed by the in­ sertion as the rate-determining step. Propene polymerization was performed in a reactor Similar results concerning the relationship be­ at 0.5 to 6-bar propene pressure in toluene. The tween polymerization rate and monomer concentra­ MAO Itoluene solution was injected into the reactor tion have been observed by Fink et al. for metallo­ and saturated with propene. Polymerization was cene IMAO catalysts, e.g., a reaction order in pro­ started by injecting a solution of the metallocene in pene of 1.2 to 1.4 for Me2Si(IndhZrC12/MAO and diluted MAO Itoluene. The temperature was con­ Me2C(Cp)(Flu)ZrC12/MAO.6 This was attributed trolled within ±O.I°C, and pressure was maintained to additional reactions that were not specified in by feeding propene. detail. Siedle et al. also reported higher orders of reaction for the system CP2ZrC12/MAO 11-hexene.7 Influence of Propene Concentration Due to the heterogeneous character of the suspen­ sion polymerization of propene in toluene, it is con­ Table I shows the effect of propene concentration ceivable that mass transfer is the main factor con­ on the maximum catalyst activity at 40°C. Activities trolling the activity of the system. Figure 4 shows the of both catalysts, measured as kgPP I (mol Zr X moll variation of the catalyst activity for MBI/MAO with L Pr X h), increase strongly with propene concen­ propene concentration when the propene pressure is tration. Figures 1 and 2 show the time dependence increased in steps from 1 bar to 1.5 and 2 bar, and of the catalyst activity for the two metallocenes for decreased again to 1.5 and 1 bar. Catalyst activity in­ different propene pressures. For BI/MAO, catalyst creases and decreases parallel to propene concentra­ activity increases within the first 10 to 20 min fol­ tion. The catalyst activity at 2 bar and 80 min is almost lowed by a slow deactivation. The maxima of the identical to the activity at 80 min ofthe corresponding activity I time curves shift to higher activities with system (run #82) polymerizing propene with 2 bar increasing propene concentration. For MBI/MAO, propene over the entire reaction time. The reversibility maximum activity is reached shortly after injection of the activity Ipropene concentration dependence in­ of the metallocene IMAO solution. At propene pres­ dicates that equilibria involving the active species are sures of 2 bar or less, almost no deactivation is de­ responsible for this effect rather than mass transfer, tected. At higher pressures, the polymerization has because the latter would be expected to depend on the to be quenched after 10 min due to stirring problems previous history of the system. associated with extremely low bulk density of the Incomplete heat transfer in the polymer I catalyst resulting polypropylene. particles could be another reason for the observed 1307 activity increase. MAO-activated metallocenes are known to show increasing activity with rising po­ lymerization temperatures. Localized "overheating" C':>l..Cc.oc-:l~ "<f'oo a> a> at the catalyst could, thus, account for an increase ";<u:i~~~ ~~c--iM a> a> a> a> a> a> a> a> a> of catalyst activity. This higher local temperature at the catalytic site should lead to lower polymer stereoregularity and reduced polymer melting points, MO~LC~ ~tt5t.OO~ 00"'''''''c-i~LDcD as generally observed for polymers produced at LDll':llOc.oc.o "<f'"<f'"<f'"<f' ~ ~ 1""""1 ~,......t 1""""1 r-4 r-4 r-4 higher temperatures. However Figure 5 shows the opposite behavior, i.e., increasing stereoregularities and melting points with rising propene pressure, thus ruling out this possibility. A dependence oftac­ ticity on propene concentration has been recently reported for several isospecific metallocenes, es­ pecially at low propene concentrations.s As inhomogeneities of temperature or concentra­ tions cannot explain the observed order of reaction of 1. 7 relative to the propene concentration, one can assume two propene molecules to participate in the rate-determining step as proposed for some lO heterogeneous 9 and homogeneous catalysts. Al­ ternatively, propene might be involved in an equi­ librium between dormant and active catalyst sites, thus increasing the concentration of currently active sites. The interaction of the active cationic metal­ locene with its counterion could be such an equilib­ rium, or propene might be involved in the release of dormant sites that could arise from misinsertions. Figure 6 shows the influence of propene pressure on the molecular weight of polypropylene produced with BljMAO and MBljMAO. For MBljMAO, we find an increasing chain length with increasing pro­ pene concentration, whereas in the case of BljMAO, hardly any increase is detected. This and the less than proportional molecular weight increase with propene concentration for MBljMAO in Figure 6 indicate that propene is involved in chain termination. The degree of polymerization can be described by eq. (1). p = kpropagation [m] ( 1) l:t)C"Ir-IC"I-.::t' OOC"l..-tC'J 1""""I-.::t'O':IOM r-4~O':IO n ktransfer + ktransfer monomer [M] cicicic-ic<i oooc-i Equation 1 assumes that only one propene mol­ ecule is involved in the propagation step, and that the observed higher reaction order of the rate of po­ lymerization is caused by a change in the number of currently active sites and that the sites active for chain transfer and propagation are identical. Plot­ ting 1 j P n vs. 1 j [ M] separates the relative rate con­ stants for the (3- H -elimination with and without I':: • monomer participation. The relative rate constants ::> 0 ~z without monomer paticipation (ktransferj kpropagation) 1308 activity 80000 x ~ a. 60000 .........J o E x ~ N 40000 o E ~ ...... a. a. ...0> 20000 od-____L- ____L- __~ ____~ ____~ _____L ____ _L ____ ~ ____~_J o 20 40 80 80 100 120 140 180 180 time [min] Figure 1. Catalyst activities of Me2Si(Benz[eJlndhZrCI2/MAO at different propene con­ centrations: [ZrJ = 1 X 10-6 mol/L , [AI]/[ZrJ = 20,000, toluene, 40°C, total pressure 0.5- 4 bar. (a) [Prj = 0.18 mol/L, (b) [Prj = 0.42 mol/L, (c) [Prj = 0.91 mol/L, (d) [Prj = 2.02 mol/L.