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-Me­Benz[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 me­tallocene 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

In recent years, versatile generations of Ziegler­Natta catalysts based upon MAO-activated metal­locenes have been developed.1

-3 Polypropylene can

be produced with high catalyst activity, isotacticity, and molecular weight. As a result of the uniform catalytically active sites ("single site catalyst"), it

is possible to control molecular weight, stereo­regularities, and comonomer incorporation without sacrificing the narrow molecular weight distribu­tions. In spite of these improvements in catalyst technology, there is still much to be learned about the elementary steps in olefin polymerization.

* To whom all correspondence should be addressed at Univ­ersitiit Freiburg, Institut fUr Makromolekulare Chemie, Stefan­Meier-Str. 31, 79104 Freiburg, Germany.

Small changes in the ligand structure of the metallocene can lead to polyolefins with greatly varied properties. Our research is aimed at better understanding of the main factors controlling the behavior of the two metallocenes rac-Me2Si (Benz[e]Indenyl}2ZrCI2 (BI) and rac-Me2Si(2-Me­Benz[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 molecular weight polymer, while MBI/MAO showed the opposite behavior, i.e., lower catalyst activity and higher polypropylene molecular weight.4

•5

RESULTS AND DISCUSSION

Propene polymerization was performed in a reactor at 0.5 to 6-bar propene pressure in toluene. The MAO Itoluene solution was injected into the reactor and saturated with propene. Polymerization was started by injecting a solution of the metallocene in diluted MAO Itoluene. The temperature was con­trolled within ±O.I°C, and pressure was maintained by feeding propene.

Influence of Propene Concentration

Table I shows the effect of propene concentration on the maximum catalyst activity at 40°C. Activities of both catalysts, measured as kgPP I (mol Zr X moll L Pr X h), increase strongly with propene concen­tration. Figures 1 and 2 show the time dependence of the catalyst activity for the two metallocenes for different propene pressures. For BI/MAO, catalyst activity increases within the first 10 to 20 min fol­lowed by a slow deactivation. The maxima of the activity I time curves shift to higher activities with increasing propene concentration. For MBI/MAO, maximum activity is reached shortly after injection of the metallocene IMAO solution. At propene pres­sures of 2 bar or less, almost no deactivation is de­tected. At higher pressures, the polymerization has to be quenched after 10 min due to stirring problems associated with extremely low bulk density of the resulting polypropylene.

Figure 3 shows the plot of the polymerization rates [kgPP I (mol Zr X h) ] that correspond to the activity maxima seen in Figures 1 and 2 vs. propene on a double logarithmic scale. The reaction order of the polymerization rate with respect to propene con­centration is 1.7 for both catalysts. This is signifi­cantly higher than expected for the model of mono­mer-metal7r-complex formation followed by the in­sertion as the rate-determining step.

Similar results concerning the relationship be­tween polymerization rate and monomer concentra­tion have been observed by Fink et al. for metallo­cene IMAO catalysts, e.g., a reaction order in pro­pene of 1.2 to 1.4 for Me2Si(IndhZrC12/MAO and Me2C(Cp)(Flu)ZrC12/MAO.6 This was attributed to additional reactions that were not specified in detail. Siedle et al. also reported higher orders of reaction for the system CP2ZrC12/MAO 11-hexene.7

Due to the heterogeneous character of the suspen­sion polymerization of propene in toluene, it is con­ceivable that mass transfer is the main factor con­trolling the activity of the system. Figure 4 shows the variation of the catalyst activity for MBI/MAO with propene concentration when the propene pressure is increased in steps from 1 bar to 1.5 and 2 bar, and decreased again to 1.5 and 1 bar. Catalyst activity in­creases and decreases parallel to propene concentra­tion. The catalyst activity at 2 bar and 80 min is almost identical to the activity at 80 min ofthe corresponding system (run #82) polymerizing propene with 2 bar propene over the entire reaction time. The reversibility of the activity Ipropene concentration dependence in­dicates that equilibria involving the active species are responsible for this effect rather than mass transfer, because the latter would be expected to depend on the previous history of the system.

Incomplete heat transfer in the polymer I catalyst particles could be another reason for the observed

I':: • ::> 0 ~z

C':>l..Cc.oc-:l~

";<u:i~~~ a> a> a> a> a>

MO~LC~

~tt5t.OO~ LDll':llOc.oc.o ~ ~ 1""""1 ~,......t

l:t)C"Ir-IC"I-.::t' 1""""I-.::t'O':IOM cicicic-ic<i

"<f'oo a> a> ~~c--iM a> a> a> a>

00"''''''' c-i~LDcD "<f'"<f'"<f'"<f' 1""""1 r-4 r-4 r-4

OOC"l..-tC'J r-4~O':IO

oooc-i

1307

activity increase. MAO-activated metallocenes are known to show increasing activity with rising po­lymerization temperatures. Localized "overheating" at the catalyst could, thus, account for an increase of catalyst activity. This higher local temperature at the catalytic site should lead to lower polymer stereoregularity and reduced polymer melting points, as generally observed for polymers produced at 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 oftac­ticity on propene concentration has been recently reported for several isospecific metallocenes, es­pecially at low propene concentrations.s

As inhomogeneities of temperature or concentra­tions 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 heterogeneous 9 and homogeneous catalysts. lO Al­ternatively, propene might be involved in an equi­librium between dormant and active catalyst sites, thus increasing the concentration of currently active sites. The interaction of the active cationic metal­locene with its counterion could be such an equilib­rium, 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 pro­pene 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) n ktransfer + ktransfer monomer [M]

Equation 1 assumes that only one propene mol­ecule is involved in the propagation step, and that the observed higher reaction order of the rate of po­lymerization is caused by a change in the number of currently active sites and that the sites active for chain transfer and propagation are identical. Plot­ting 1 j P n vs. 1 j [ M] separates the relative rate con­stants for the (3-H -elimination with and without monomer participation. The relative rate constants 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 con­centrations: [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.

are 1.6 X 10-4 mol/L for the catalyst BI/MAO and 1.5 X 10-4 mol/L for MBI/MAO. The relative rate constants for chain transfer with monomer partic­ipation (ktransfer monomer! !lpropagation) are 1.3 X 10 -3 for BI/MAO and 1.4 X 10-4 for MBI/MAO. These data

agree well with the results found at 50°C for the same metallocenes.5 This explains the low molecular weight observed for the polypropylene produced by BI/MAO by the high rate of chain transfer to the monomer for the metallocene BI. The methyl groups in the 2-

100000

activity

~ BOOOO .r: )(

~

a. ...J

-::::. 80000 0 E )(

~

N

0 40000 .5-...... a. a. oo

20000 .)(.

6A~

j e

V

30

time [min]

40 50 eo

Figure 2. Catalyst activities of Me2Si(2-Me-Benz[ e Jlnd)2ZICI2/MAO at different propene concentrations: [ZrJ = 1 X 10-6 mol/L , [AIJ/[ZrJ = 20,000, toluene, 40°C, total pressure 0.5-6 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, (e) [Prj = 3.3 mol/L.

rate of polymerization

o

~ .D

M 100 .. o N

'0 a BI .... cr·

"-11. 11. .. 10

• ....... 0.·· WBI

0 ....

0.3 0.5 1 3 5

c(propene) [mol/L]

Figure 3. Rate of polymerization for MBI/MAO and BI/MAO at different propene concentrations; logarith­mic plot; MBI = Me2Si(2-Me-Benz[eJInd)2ZrCI2' BI = Me2Si(Benz[eJInd)2ZrCI2; [ZrJ = 1 X 10-6 mol/L, [AIJ/ [ZrJ = 20,000, toluene, 40°C, total pressure 0.5-6 bar; rp = k' X [propenel m

; log(rp) = m X log([propene]) + log(k'); m = 1.7; the maximum rate of polymerization correspond­ing to Figures 1 and 2 is used, the error bar covers the results of five polymerization runs, the largest uncertainty is due to the error in the metallocene concentration.

position at the benz [ e ] indenylligand in MBI seem to be able to suppress this type of transfer.

Influence of Temperature

Figure 7 shows the activity-time profile as a function of temperature for the two metallocene /MAO cat­alyst systems at constant propene pressure. At 60°C catalyst activity increases steeply within the first minutes of the polymerization, followed by a slow deactivation to roughly 50% over 3 h for both cat­alysts. At 40°C, only BI/MAO shows this type of behavior, while MBI/MAO deactivates much slower. At 20°C, both metallocenes polymerize with almost constant activity over several hours. Dou­bling the zirconocene concentration of MBI/MAO from 1 to 2 JLmol/L does not have any significant impact on catalyst activity, and polymer properties as can be seen in Table II.

MBI/MAO shows a very strong increase of ac­tivity with temperature, becoming faster than BI/ MAO at 60°C. This agrees with Spaleck's observa­tions that catalyst activity for MBI/MAO is higher than for BI/MAO at 70°C in liquid propene.4 The Arrhenius plot (Fig. 8) of the maximum activities

1309

yields E act = 54 kJ /mol for MBI/MAO and E act = 28 kJ /mol for BI/MAO. The higher activation energy for MBI/MAO is compensated by a more favorable entropy. The activation energy for MBI/MAO is surprisingly high in comparison to the values for BI/MAO and Cp2ZrCI2/MAO (Eact = 37 kJ /mol measured under similar conditions) .11

The Arrhenius plot for the polymer molecular weight is shown in Figure 9. In the case of MBI/ MAO, producing the higher molecular weight poly­propylene, one finds a higher difference Eact (Polym.) - E act (Termin.) = -40 kJ/mol than for BI/MAO with E act (Polym.) - E act (Termin.) = -16 kJ /mol. However, these values indicate the presence of dif­ferent types of chain transfer in comparison to the aspecific, oligomer-yielding Cp2ZrCI2/MAO with E act

(Polym.) - E act (Termin.) = - 51 kJ /molY

End Group Analysis

The typical polymer end groups, found in low mo­lecular polypropylenes produced by metallocene catalysts such as Cp2ZrCI2/MAO or Et(lndH4 )2 ZrCI2/MAO, are the vinylidene group resulting from {3-I-I-elimination and the n-propyl group formed by insertion of propene into the resulting Zr-hydrideP

Figure 10 shows the IH-NMR spectra of the ole­finic region for the polymers produced with BI/ MAO (a) and MBI/MAO (b). The vinylidene end group at 4.74 and 4.66 ppm is the only end group found for MBI/MAO (b). But for BI/MAO (a), in addition to the vinylidene end group signals, a more intense signal at 5.45 ppm is seen. For BI/ MAO, the 13C-NMR spectrum of the olefinic region ofthe polymer shows signals at 129.4 and 124.3 ppm. We assign these signals observed for polymer pro­duced with BI/MAO to a 2-butenyl end group (MeHC=CH-CH2-), which results from 2,1-propene insertion followed by {3-H -elimination.13

For run #89 (BI/MAO, 40°C, 0.91 mol/L pro­pene, Table I), one expects about 90% of the chain termination to occur with monomer participation, as estimated from the corresponding chain transfer rate constants determined above. The intensity of the IH-NMR olefinic proton signal at 5.45 ppm ac­counts for 60-70% of the total intensity observed for all olefinic end groups. This indicates that this new chain termination pathway is a {3-hydride transfer to a coordinated propene, following a 2,1-insertion, as shown in Scheme 2. The 2-methyl group in MBI/MAO seems to be able to suppress this pathway, explaining the higher molecular weight of polymer produced with MBI/MAO.

1310

30000 r------.-----.------,-----~------~----_.------~----~

',;i'

" 25000 .. II. ...:I ...... '0 S 20000

" .. ... '0 15000 ! ...... II.

; 10000

... -~ ... 5000

" ..

activity

,.------'1 I~ I propene conoentration I

o~----~----~----~------~----~----~----~~--~

2.0

loB

1.6

1.4

1.2

1.0

O.B

0.6

0.4

0.2

0.0 o 20 40 60 BO 100 120 140 160

time [min]

~ ...... '0 !

1'1 .2 ... .. .. ... 1'1 II.

" 1'1 0

" II. 1'1 II. Po 0 .. Po

Figure 4. Catalyst activities of Me2Si(2-Me-Benz[ejInd)2ZICI2/MAO at different propene concentrations; step-wise change of propene pressure during one polymerization run: [Zrj = 1 X 10-6 mol/L , [Alj/[Zr j = 20,000, toluene, 40°C, total pressure 1, 1.5, 2 bar. After each propene pressure change activity measurement was omitted for about 15 min to allow for stabilization of the reaction conditions (dotted line).

The lH-NMR of polypropylene produced by BI/ MAO at low propene concentrations or higher tem­perature shows a third olefinic signal at 5.18 ppm that might be due to a Me2C=CH-CH- end

group. This group could arise from an isomerization of the vinylidene end group forming the thermo­dynamically favored vinylene group with internal double bond.14

100 .----------,---------,----------,---------, 165.0

99 162.5

9B 160.0

MBI 97 157.5

M ...... 96 S

155.0 E 9 95

S 94

93

92

91

152.5

150.0 BI

147.5

145.0

142.5

90 '--________ .J.-________ -"-________ ~ ________ -' 140.0

o 234 o(propene) [mol/L]

II ...

Figure 5. Dependence of polymer melting point and tacticity on propene con­centration for MBI/MAO and BI/MAO, MBI = Me2Si(2-Me-Benz[ejInd)2ZrCI2, BI = Me2Si(Benz[ejInd)2ZrCI2. [Zrj = 1 X 10-6 mol/L, [Alj/[Zrj = 20,000, toluene, 40°C, total pressure 0.5-6 bar.

JOOr--------.--------,---------r--------,

250

~ 200 '0 a ...... ~ 150

100

50 o BI

~ OL_ ______ -L ________ ~ ________ L_ ______ ~

o 2 J 4 o(propene) [mol/L]

Figure 6. Dependence of polymer molecular weight on propene concentration for MBI/MAO and BI/MAO, MBI = Me2Si(2-Me-Benz[ejInd)2ZrCI2' BI = Me2Si (Benz[ejInd)2ZrCI2' [Zrj = 1 X 10-6 mol/L , [AIll[Zrj = 20,000, toluene, 40°C, total pressure 0.5-6 bar.

Elimination following 2,1-insertion as the main chain termination pathway for BI/MAO requires that a significant part of the catalytic sites has a 2,1-unit as the last-inserted unit if one assumes chain transfer from a 2,1-unit to be of the same order as from an 1,2-unit. However 13C-NMR shows only

100000 IIBI-o

aoUnt,. / '''-'' ..... .... ,.... 80000 ~

... .. --"

1311

0.8% 2,1-units to be incorporated into the polymer. This would mean that the catalytic site is deacti­vated by an 2,1-insertion taking place. Such dormant sites are reactivated when a 1,2-insertion or chain transfer to propene occurs. Scheme 3 shows possible pathways for the reactivation of Zr-2,1-units.

For run #89 (BI/MAO, 40°C, 0.91 mol/L pro­pene, Table I) , one can estimate from the statistical chain length of the polymer and the amount of 2,1-insertions that chain transfer to propene from a Zr-2,1-unit is about 10 times slower than 1,2-insertion into a Zr-2,1-unit.

Polymerization in the Presence of Hydrogen

Formation of dormant sites after 2,1-insertions has been shown to be important for heterogeneous catalysts 15 as well as for zirconocene /MAO / propene systems 13.16 by the analysis of polymer end groups when hydrogen was added to the polymerization.

Table III shows the effect of addition of 0.35 bar H2 to the system for the metallocene catalysts BI/ MAO and MBI/MAO. Polymerization activity for BI/MAO is increased by 38%, while for MBI/MAO an activity increase of 17% is found. Adding H2 re­duces 2,1-units in the polymer to one-third for both metallocenes. This reduction of incorporated 2,1-units is paralleled by an increase of n-propyl- and

.. .. ~ 0..

...... ... - . ......... ..J ....... Ci E

" N Ci 5 ....... 0.. 0.. ... ~

60000

40000

20000

.. -----.--­...--------.. .-........... a....-.... .... '\

~

O~ ____ ~ ____ L-__ ~L_ __ ~ ____ ~ ____ ~ ____ -L ____ -L ____ ~~

o 20 40 60 80 100 120 140 160 180

time [min]

Figure 7. Activities of MBI/MAO and BI/MAO at different temperatures: [Zrj = 1 X 10-6 moljL , [Alj/[Zrj = 20,000, toluene, total pressure 2 bar. MBI = Me2Si(2-Me­Benz[ejInd)2ZrCI2, BI = Me2Si(Benz[ejlnd)2ZrCI2' (a) T = 20°C, (b) T = 40°C, (c) T = 60°C.

1312

:2 * ...

N

;3 - --­... -N 0 -5

.3

I': • ;:l 0 ..::Z

0000<0 <C"'<C <C.-<

~~c:.c ~t-t.- ~~r--: c-.i ~,....; c-.i""';""'; ~ ~!""'I

000 "" .... <0

000 000 r-oo <00 ....

""'"

000 "" .... <0

.-< .-< .-<

000 000 000 ""00 .... "" .... 00

000 "" .... <0

.-< .-< .-<

o

""

:<i' " 100 ...l "-'0 a " .. "" '0 ! "­Po. Po. 10 011 ..

Q ... 2.9 3.0 3.1 3.2 3.3 3.4 3.5

lOS l[ T-1 [K- 1]

Figure 8. Arrhenius plot for the activity of MBI/MAO and BI/MAO; log (activity) vs. l/temp [AI] = 20 X 10-3

mol/L, toluene, total pressure 2 bar. 0 = BI/MAO, [Zr] = 1 X 10-6 mol/L; 0 = MBI/MAO, [Zr] = 1 X 10-6 mol/ L; 0 = MBI/MAO, [Zr] = 2 X 10-6 mol/L.

n-butyl-polymer end groups as can be seen in Figure 11 for BIjMAO.

The n-propyl and n-butyl-polymer end groups have the same intensity, while isopropyl end groups could hardly be detected within the limits ofthe 13C_

NMR measurement.20 This shows that most metal­locenes have a 2,1-inserted propene as the last unit of the polymer chain, if one assumes the rates of chain transfer to H2 from a Zr-2,1-unit and a Zr-1,2-unit to be of the same magnitude. This seems to be realistic for the small hydrogen molecule.16

,17

The fraction of metallocenes deactivated by 2,1-units can be estimated by studying the effect of hy-

..-. '0 El

........ 110

1000 r---~---r--~~--~--~--~

~ 100

10 L-__ ~ __ ~ __ -L __ -L __ ~ __ ~

2.9 3.0 3.1 3.2 3.3 3.4 3.5

109 l[ T- 1 [K- 1

]

Figure 9. Arrhenius plot for polymer molecular weights for BIjMAO and MBI/MAO; log (activity) vs. l/temp, [Al] = 20 X 10-3 mol/L, toluene, total pressure 2 bar. 0 = BI/ MAO, [Zr] = 1 X 10-6 mol/L; 0 = MBI/MAO, [Zr] = 1 X 10-6 mol/L; 0 = MBI/MAO, [Zr] = 2 X 10-6 mol/L.

1313

* * a

* * b

(ppm)

Figure 10. Polymer IH-NMR-spectra for different metallocenes region of olefinic end groups; [AI] = 20 X 10-3 mol/L, toluene, 2 bar. (a) Me2Si(Benz[e]Ind)2ZrCI2/MAO, [Zr] = 1 X 10-6 mol/L, (b) Me2Si(2-Me-Benz[e]Ind)2ZrCI2/MAO, [Zr] = 2 X 10-6 mol/L. Signals marked with * are due to solvent C2D2C14 •

drogen on catalyst activity and polymer microstruc­ture. The chain transfer from a Zr-2,I-unit to H2 is about two times faster than 1,2-insertion of propene into this unit. This can be concluded from the con­centrations of the products of these steps. The in­tensities of signals in the I3C-NMR-spectra due to n-butyl-end groups are two times larger than those for 2,I-units in the polymer. Thus, the overall rate of deblocking 2,I-units either by 1,2-propene inser­tion or by transfer to hydrogen should be three times larger than without H 2-addition. This should lead

to a threefold catalyst activity increase for the sys­tems with hydrogen, if the major fraction of the me­tallocenes is blocked by 2,1-units. The observed cat­alyst activity increase by H2 is much lower (38% for BI/MAO, 17% for MBI/MAO). This phenomenon could be explained either by impurities introduced by H 2-addition or by questioning the assumption of the same HTtransfer rate for Zr-2,1- and Zr-l,2-units or by a lower rate of insertion at the beginning of the polymer chain. IS Using the three times higher rate of transfer of 2,I-units into 1,2-units in the

CHs I

CH3 CH3

CHs CH2-CH-P \ /

CHs I

I I

CH-CH-CH -CH-P .,/'" I 2

M H \ \

~C~ CH2 CH3

CH=CH I \

M H , , , , CH2 ... CH

\ CHs

H3C-CH=CH-CH2 -CH-P

+

Scheme 2. j3-hydrogen transfer from a Zr-2,1-unit to propene.

1314

CHs CHs CHs I I I

M-CH2 -CH-CH-CH2-CH2-CH-P

Scheme 3. Pathways of reactivation of a dormant Zr-2,1-unit.

presence of H2 as determined above and the observed catalyst activity increase, one can estimate the por­tion of Zr-2,1-units to be ca. 40% for BIjMAO and

ca. 20% for MBIjMAO under the assumption of a rate of insertion independent of chain length. This fits to the two times higher probability for 2,1-inser-

82

23

m mm

mm

22 21

cPS I P2 P1

mrr Pol-C-C-C-C

CB6 I B2 B1

Pol-C-C-C-C-C

mrrm m-2,i m-2,i

86 P2 mrrm P5

m-2,1 m-2,1

20 19 18

(ppm) 17

m-2.1 m-2.1 C C C I I I

Po I-C-C-C-C-C-C-P 0 I

C r-2.1 C I I

Po I-C-C-C-C-C-C-P 0 I I C r-2.1

r-2,i r-2,i Pi

Pi 81

16 15 14

Figure 11. l3C NMR-spectra of polymers prepared in the absence (a) and presence (b) of hydrogen, region of methyl carbons; for Me2Si(Benz[eJlnd)2ZrCl2/MAO; [ZrJ = 1 X 10-6

moljL, [AlJ/[Zr] = 20,000, toluene, 40°C, 2 bar propene. (a) Without hydrogen, run #89; (b) with 0.35 bar hydrogen, run #103. PI, P2, P5: n-propyl end group signals; B1, B2, B6: n-butyl end group signals; m-2,1: signals of meso-2,1-units; r-2,1: signals of rac-2,1-units.

>:: • ::> 0 ~z

00 00 00 010 MM

00 00 00 00 <!:! .... <!:!

1315

tion for BI/MAO compared to MBI/MAO. But this leaves the problem that there are almost no isopropyl end group signals detectable in the 13C_NMR.20

CONCLUSION

Propene concentration has been shown to be an im­portant parameter for MAO-activated homogeneous catalysts based on the metallocenes BI and MBI, influencing catalyst activity, polymer molecular weight, polymer end groups, and even polymer tac­ticity and melting temperatures. For BI/MAO, chain transfer to propene from a Zr-2,1-unit controls the molecular weight of the polymer, leading to MeHC=CH-CH2- end groups.

Addition of hydrogen to polymerizations with BI/ MAO and MBI/MAO reduces the incorporation of 2,1-inserted propene units into the polymer and leads to n-butyl and n-propyl end groups. This in­dicates that a substantial part of the metallocene is blocked by 2,1-units.

The introduction of a methyl group in the 2-po­sition on the benz [e 1 indenylligand in MBI/MAO leads to a higher energy of activation for the poly­merization and to a more favorable entropy of ac­tivation than that of BI/MAO. The difference be­tween the energies of activation for chain propa­gation and termination is higher for MBI/MAO than for BI/MAO, as can be expected from the high molecular weight polymer produced by MBI/MAO.

EXPERIMENT AL

The time dependence of the polymerization activity was determined by the pressure drop in a thermo­stated propene storage vessel when keeping the pro­pene pressure constant in the thermostated 2-L­Biichi-glass polymerization reactor, as described previously.11

All catalyst components were handled and stored under argon. 99.99% argon (Messer Griesheim GmbH) was purified by passing through BTS-cat­alyst (BASF AG) and 0.4 nm molecular sieve. Po­lymerization grade propene (BASF AG) was stored over Al(i-Buh/toluene prior to use. 99.999% hy­drogen (Messer Griesheim GmbH) was used without further purification. Toluene p.a. (Roth AG) was purified by passing through a column with acidic Al20 3 (Merck AG) and distillation over LiAIH4 fol­lowed by at least 12 h refluxing over Na/K-alloy and fresh distillation prior to use.

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The synthesis of the zirconocenes BI and MBI has been described previously.5

The autoclave and pressure buret were washed with a ca. 1 wt % Al(i-Buh/toluene solution at 60°C. A 10 wt % solution of MAO (8.7 mL) in tol­uene (WitcoAG Mn = ca. 800 g/mol) was added to 700 mL oftoluene. This solution (600 mL) was can­nulated into the evacuated autoclave followed by saturation with propene at the polymerization tem­perature. The above solution (30 mL) was used to dissolve 1.23 jlmol of the zirconocene. Twenty min­utes later, 15 mL of this solution was injected into the thermostated, pressurized reactor via a pressure buret to start the reaction. The concentration is, thus, 1jlmol/L for the zirconocene, 20 mmol/L Al for MAO, with a ratio AI/Zr = 20000 (total Zr = 6.15 X 10 -7 mol, total Al = 1.23 X 10 -2 mol).

The reaction was stopped by venting off the pro­pene and precipitating the polymer in 1 L methanol with 40 mL 10% aq. HCI and 0.5 g 2,6-di-t-butyl-4-methyl-phenol (BHT). After stirring the mixture overnight, the polymer was filtered off and washed with methanol and dried at 60°C at reduced pressure to constant weight.

NMR spectra were recorded from solutions of 40 to 110 mg of polymer in 0.4 mL C2D2Cl4 at 400 K by a Bruker ARX 300. IH-NMR spectra were re­corded at 300 MHz with 128 scans, and 13C-NMR spectra at 75.4 MHz with a 30° pulse angle, 3.5 s pulse repetition and 11,111 Hz spectra width, and at least 5,000 scans. For the polymers produced in the presence of hydrogen, 20,000 scans were re­corded. For the spectrum of the olefinic carbons of the polypropylene from BI/MAO, 50,000 scans were recorded at 380 K, with the reduced spectra width of 4167 Hz and a pulse repetition of 6 sand 30° pulse angle. Signals were assigned according to the literature.12,13

Melting points were determined by DSC from the melting endotherm at a heating rate of 20 K/min after previous heating to 185°C and cooling to 50°C by 10 K/min.

Polymer molecular weight and molecular weight distribution were determined by size exclusion chromatography and viscosity measurements at the polymer research division of BASF AG.

This work was supported by the Bundesminister fiir Forschung und Technologie (project #03M40719) and by BASF AG. We thank Witco AG for providing sam­ples of MAO and Dr. Lilge, BASF AG, for analytical assistance.

REFERENCES AND NOTES

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8. Busico et. al. and Resconi et. al. recently reported a similar dependence of polypropylene tacticity on pro­pene concentration for BI/MAO and Et(IndH4h ZrCldMAO. (a) V. Busico, lecture #1119, Milano, STEPOL 1994; (b) G. Balbontin, A. Fait, F. Piemon­tesi, L. Resconi and H. Rychlicki, poster #40, Milano, STEPOL 1994.

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20. The smallest detectable intensity is ca. 0.05% of the mmmm-signal under the described experimental conditions. For BIjMAO (run #103), a concentra­tion of isopropyl end groups less than ca. 10% of the n-butyl group concentration is, thus, not de­tectable. For MBIjMAO (run #104), isopropyl sig­nals with intensity of less than 25% of the n-butyl signals are not detectable due to the higher molec­ular weight of this polymer.