Modification of Catalytic Properties of Homogeneous Metallocene Catalytic Systems in Propylene Polymerization under Action of Triisobutylaluminum and Lewis Bases

Progress in Olefin Polymerization Catalysts and Polyolefin Polyolefin Materials T. Shiono, K. Nomura and M. Terano (Editors) © 2006 Elsevier B.V. All rights reserved.

77

13 Modification of Catalytic Properties of Homogeneous Metallocene Catalytic Systems in Propylene Polymerization under Action of Triisobutylaluminum and Lewis Bases N.M. Bravaya*, E.E. Faingol'd, E.A. Sanginov, A.N. Panin, O.N. Babkina, S.L. Saratovskikh, O.N. Chukanova, A.G. Ryabenko, E.N. Ushakov Institute of Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka Moscow Region, pr, akademika Sememova 5, Russia e-mail: [email protected]

Abstract Experimental results showing the unique possibilities of modification of catalytic properties of homogeneous metallocene IVB Group catalysts in propylene polymerization under the action of triisobutylaluminum (TIBA) or in the presence of small additives of Lewis bases (LB) are presented. It has been shown that the preactivation of zirconocene dichlorides at low MAO excess (~10z mol/mol) followed by the activation with TIBA also at low molar TIBA excess to over metallocene (-10 2 mol/mol) gives rise to very active homogeneous catalysts for olefin polymerization. The approach allows a 10100-fold decrease of MAO charges. Monomethylated zirconocene complexes with MAO as catalytic intermediates formed under preactivation conditions and the mixture of tetraisobutylalumoxane/polyisobutylalumoxane as cocatalysts formed at nearby stoichiometric amounts of TIBA and MAO were recognized as the key components of the analyzed catalytic systems. It has been shown for the first time that purposeful introduction of almost stoichiometric amounts of ternary amines to the L2MMe2/Ph3CB(C6Fs)4/TIBA catalytic systems (L2MMe2 = Ph2CCpFluMMe2 (M=Zr, Hf), mcMeiSilndiZrMea) can result in an extremely high increase in the catalyst activity, the effect being most pronounced for hafhoeenes in the presence of

78

N ,M, Bravaya et al.

NPh3. It was also shown that TTBA in these catalytic systems acts as the reagent reducing stereoselectivity of the catalyst leading to the formation of stereoblock polypropylenes for Cs-symmetry catalysts. The reasons for the effects observed are discussed. 1. INTRODUCTION One can find in literature a lot of examples of the active role of triisabutylaluminum (TIBA) in metalloeene-based catalytic systems. Being a poor activator for the most part of metalloeenes, it can effectively activate dimethylated 2-substituted bisindenylzirconocenes in ethylene and propylene polymerization [1,2]. Highly active catalytic systems for ethylene and propylene polymerization were obtained by using TIBA as the third component for several zirconocene dichlorides with Ph3CB(C6F5)4 (TB) [e.g., 3-5]. Partial replacement of MAO by TIBA is also used for modification of catalytic properties of homogeneous L2MCI2/MAO systems [e.g., 6-10] leading, under certain conditions, to enhanced catalyst activity, higher incorporation of comonomer, increased catalyst stability, solubility of the catalytic system in aliphatic solvents, etc. Nevertheless, in all listed examples, A1MAO/M molar ratios are sufficient for precatalyst activation without TIBA. There are some other examples of combined MAO/TIBA activation of L2MCI2 when MAO is used for catalyst preactivation at AIMAQ/M molar ratios about order of magnitude lower than those necessary for activation, and only introduction of TIBA makes the system active [11-13]. Therefore, it was of interest to find optimal conditions for metallocene activation in the latter systems, and to reveal catalytic intermediates, as well as to understand the role of TTBA. Analysis of reaction products of Ph2CCpFluZrCl2 (1) and rac-Me2Si(2-Me,4-PhInd)2ZrCl2 (2) with MAO under conditions of preactivation, those of MAO with TIBA, as well as the data on propylene polymerization under the activation of the catalysts with MAO/TIBA, will be presented in the first part of the paper. Highly electrophilic cationic metal-alkyl metallocene complexes of IVB Group, active species of olefm polymerization, are extremely sensitive to the action of nucleophilie substrates due to temporary or irreversible deactivation. However, one can find in literature the examples of the non-deactivating action of donor-like moieties causing either increase in the catalyst activity or changing the molecular-weight and microstructure characteristics of polymers formed [14-17]. Even rather strong Lewis bases (LB) do not necessarily lead to cationic complex decomposition being introduced to a reaction medium [18,19]. In the second part of the paper, exemplifying the catalytic systems L2MMe2/TB/TIBA (L2MMe2=racMe2SiInd2ZrMe2 (3Me) and Ph2CCpFluMMe2 (M=Zr (lMe), Hf (4Me)), we would like to demonstrate that simple introduction of NPhj in a reaction medium can lead to a very high increase in

13, Modification of Homogeneous Metallocene Catalysts

79

the catalyst activity. The effect is most pronounced for hafhocenes. The effect of TIBA decreasing stereoselectivity of 4Me/TB/TlBA system accompanied by the formation of stereoblock syndio/atactic polypropylene will be demonstrated. 2. RESULTS AND DISCUSSION 2.1. Ternary catalytic systems (L2ZrCh+MAO)/TIBA Propylene polymerization. The results on propylene polymerization with ternary systems derived from precatalysts 1 and 2 under the conditions of preactivation with MAO at low molar ratio (AlMAo/Zr=2Q-30Q mol/mol) with further activation with TIBA also at low molar ratios (AlTiBA/Zr=100-1500 mol/mol) are presented in Table 1. Table 1. Propylene polymerization 8 with catalytic systems (1+MAQ)/TIBA and (2+ MAO)/TIBA. Entry

Cat.

solvent

A1 TIBA /Zr b

t,

Y,

(min)

(g)

Ae

l

d

toluene

100

30

0.67

834

2

l

d

heptane

100

32

0.72

845

3

1

heptane

100

20

0.77

1455

4

1

heptane

200

20

1.08

2010

5

1

heptane

300

20

1.32

2371

6

1

heptane

400

20

1.06

1995

7

1

heptane

600

17

0.88

1961

8

2

heptane

600

30

0.23

567

9

2

heptane

800

30

0.62

1510

1

10

2

heptane

1000

10

0.80

5738

11

2

toluene

1000

8

0J6

8700

12

2

heptane

1200

20

0.49

1804

13

2

heptane

1500

30

0.41

997

"Polymerization conditions: the catalysts were preaetivated by dissolving in M A O solution in toluene so that AlMiy}/Zr=3Q0 mol/mol if not specially specified; solvent = 30 mL; [Zr]=l-10" 4 mol/L (Cat. 1), 3-10"s mol/L (Cat. 2); propylene pressure 0.8 atm; 30 °C (Cat. 1), 20 °C (Cat. 2 ) . b Molar ratio in mol/mol; c Activity in (kg PP/(mol Zr h a t m ) ; d A1 MAO /Zr=100 mol/mol.

Both the catalysts show no activity at very low molar ratios A1MAO/Zr~20 mol/mol under the preactivation conditions followed by activation with TIBA in a wide range of AlTiBA/Zr molar ratios. One can see that (i) when preaetivated

N .M. Bravaya et al.

80

with MAO at AIMAO/ZI^OO mol/mol both the catalysts show an incremental increase in activity with an increase in the AITIBA/ZT molar ratio up to an highest values at AlTIBA/Zr=300 (entries3-5, Cat. 1) and 1000 (entries 8-10, Cat.2) followed by a decrease in the activity at higher concentrations of TIBA (entries 6,7 and 12,13, respectively); (ii) the ternary catalytic systems show similar (entries 1,2, Cat. 1) or comparable (entries 10,11, Cat. 2) activities in the medium of toluene and heptane. Comparison of activities of catalysts 1 (2) under their activation with MAO and MAO/TIBA under other optimized conditions is presented in Figure 1 (a, b, respectively). One can see that it is possible to reach much higher or comparable activity of the catalysts in ternary systems at much lower MAO charges. a)

1

AlMAO/Zr= 500 mol/mol 0

b)

15

2

AlMAO/Zr= 300 mol/mol AlTIBA/Zr= 300 mol/mol

A, kgPP/(mmol Zr h atm)

A, kgPP/(mmol Zr h atm)

2

10

AlMAO/Zr= 6000 mol/mol 5

AlMAO/Zr= 500 5C0 mol/mol moVmol AlTIBA/Zr= 500 mol/mol 5OOmol/mol

0

Fig. 1. Comparison of activities of catalysts 1 (a) and 2 (b) in propylene polymerization under their activation with MAO (left columns) and MAO/TIBA (right columns) under other similar conditions.

Catalyst intermediates under preactivation conditions. Combined ! H NMR and UV-vis. studies allowed one to conclude what are catalytic intermediates under conditions of preactivation. 'H NMR analysis of the reaction products of 2 ([Zr]= 4-10"2 mol/1) with MAO showed the formation of monomethylated zirconocene (I) at AlMAo/Zr=40 mol/mol (Zr-Me, 0.87, 0.95 ppm). An increase in the AlMAff/Zr molar ratio to 150 led to the formation of the LaZrMeCl-MAO complex (II) (-1,13 ppm), as well as binuclear cationic complexes (III) ([L2ZrCl(|i-Me)MeZrL2]+[ClMAO]"5 -0.95, -1.66 ppm) and (IV) ([L2ZrMe(uMe)AlMe2]+[ClMAO]", -1.46 ppm). However, NMR experiments were conducted at high concentrations of reagents and can be irrelevant to reaction products formed at lower catalyst concentrations. Fig. 2a shows the UV-vis, absorption spectra of the 1/MAO system measured in toluene at different AlMAo/Zr molar ratios ([Zr]=8-10^* mol/1, and the AlMAo/Zr ratio was varied from 10 to 3000 mol/mol). The matrix consisted of these spectra was subjected to principal component analysis (PCA) [20]. The PCA results evidenced the

13, Modification of Homogeneous Metcdlocene Catalysts

81

formation of only three light-absorbing reaction products in this system: I, II, and either ([L2ZrMe]+[ClMAO] (V) or ([L2ZrMe(u-Me)AlMe2]+[ClMAO] (IV) [21], the latter formed at high AlMAo/Zr molar ratios. The spectra of individual reaction products (Fig. 2a, dash lines) were resolved using parameterized matrix self-modeling method [20]. PCA procedure makes it possible to represent each spectrum as a point in the basis of significant PCA vectors. Fig. 2b shows twodimensional representation of the experimental and theoretical spectra for the 1/MAO system in the basis of two main PCA vectors. The observed trajectory of "spectrum-point" describes the spectral transformations which take place on going from one to another reaction product. The fractional contributions of the reaction products along this trajectory can be estimated assuming equal molar absorption coefficients for all components (Fig 2c). So, only three reaction products are detectable at lower catalyst concentrations: monomethylated zirconocene I (the main product at low AlMAo/Zr molar ratios), cationic species IV or V formed at high A1MAO/& molar ratios, and intermediate polarized complexes of monomethylated zirconocene with MAO II formed under conditions ofpreactivation. For the 2/MAO catalytic system, PCA treatment of the UV-vis. absorption spectra measured at different AlMAo/Zr ratios led to the same conclusions.

0.0 AW

450

500

550

Wavelength / titn

Fig. 2. (a) Area-normalized UV-vis. spectra of the system 1/MAO at varied molar ratios of AIMAQ/ZT. Dash lines show the spectra of individual reaction products as calculated using parameterized matrix self-modeling method: /c - LjZrMeCl, 2c - LjZrMeCl-MAO, 3c ([LaZrMe]+[ClMAO] or ([L2ZrMe(p,-Me)AlMe2]+[ClMAO]; (b) Two-dimensional representation of the experimental and theoretical (indices c) spectra for the 1/MAO system in the basis of two main PCA vectors (p, and p2) ([Zr]=8.0'10"4 M, AWc/Zr mol/mol: 0 (1), 10 (2), 20 (3), 40 (4), 60 (5), 120 (6), 240 (7), 500 (8), 1000 (9), 1650 (10), 3000 (11)). (c) Fractional contributions of the reaction products lc, 2c, and 3c as a function of the AlMAo/Zr molar ratio.

Reaction products of MAO with TIBA (real activators). Reactions of MAO with TIBA were monitored by *H NMR and allowed one to conclude that an increase in the TIBA concentration leads, as illustrated by Fig.3, to the successive: (i) formation of modified MAO (MMAO) due to alkyl groups

82

N ,M. Bravaya et al.

exchange with a simultaneous decrease in the association degree of MAO (spectrum b); (ii) formation of tetraisobutylalumoxane (TIBAO) as the main product at about 30 mol % of TIBA (spectrum c), and (iii) formation of a mixture of TIBAO and polyisobutylalumoxane (IBAO) at equimolar ratios of reagents (spectrum d). In the latter case, the molar ratios of AIMAO/AITIBA are very similar to those providing a high activity of ternary catalytic systems. The results allow one to conclude that IBAO and TIBAO are activators in the examined ternary catalytic systems. Me

MAO MA O

O (0

a

Me

A l —)-O O— A l Al

\

Jnn

Me

A i

M e

'B Bu

—O O

MMAO M MA O

A ll — C O o

Al

i

2.0

Bu

i 'B u ^ B u

\

1.6

1

b

O

u

—AC

i

B u

c

Al

iB

u

'B u

A"

1.2 1.2

B u

Al i'B B

Al

TIBAO TIB AO

2.4

i

O

n

O

R

iB

Al^—O Al O n

A Al ( R R

i

u

+

•

i

)BB u"

B uu^ B

;BBu^ u

Al

O

Al

i

IBAO IB A O

iB u ^'Bu

d

TIBAO TIB AO A.

0.8

0.4

-0.0

-0.4

-0.8

(ppm (PPm))

Fig. 3. Fragments of *H NMR spectra of MAO (a) and reaction mixtures of MAO + TIBA (b-d) at varying molar ratios of AIXIWAIMAO (mol %): (b) - 6, (c) - 30, (d) -100.

2.2, Promoting effect of Lewis bases on the catalytic properties in propylene polymerization

of

For the 3Me/Ph3CB(C6Fs)4 (TB)/TIBA catalytic system in propylene polymerization it has been shown for the first time that the activity of the system can multiply be increased by introduction of nucleophilic reagents as a co-solution with zirconocene at their stoichiometric amounts. One can see from the Fig.4a that about 30-fold increase in the catalyst activity can be reached in the presence of aniline-type amines (Fig.4a), while trialkylamines, phosphines, and diphenyl ether deactivate the catalyst. The observed "amine effect" can result in an increased efficiency of active sites initiation at complexation with sterically demanded LB or with LB-TIBA complexes due to a decrease in the energy barrier for counterion displacement [19]. It is probable that weakening of ion-counterion interaction is achieved via delocalization of a charge either on the anion or cation as in [22].

13, Modification of Homogeneous Metcdlocene Catalysts

83

Fig. 4. Effect of LBs on the activity in propylene polymerization of (a) 3Me/TB/TIBA and (b) 4Me/X (X=MAO or TB/TIBA) systems (toluene, 30 DC, propylene pressure 6 atm).

The activity of syndiospecific zirconocene 3Me is only slightly increased in the presence of NPh3 under activation with TB/TIBA; however, about twice increase in activity was observed under MAO activation (Fig.4b). The most pronounced effect of NPh3 was observed in propylene polymerization with hafnocene 4Me (Table 2). The catalyst activated with MAO gives rise only to trace amounts of PP (entry 1). Introduction of Ph3N makes the promoting effect on the catalyst productivity under activation with MAO (entry 2). Combined activator TB/TTBA makes the catalyst much more active (entry 3). However, less stereoregular sPP is formed in this case compared to that with MAO-activated system. Catalytic system 4Me/Me2NHPhB(C6Fs)4 (DMAB)/TIBA gives rise to high-molecular-weight sPP of higher stereoregularity but with low activity (entry 4), probably, due to catalyst deactivation in the presence of MeaNPh eliminated in the reaction of cationic active species formation. Similar activity was observed when (Ph2CCpFluHfMe2+Me2NPh) was activated with TB/TIBA (entry 5). Highly active catalyst was obtained under introduction of NPh3 (entries 6-9). Five-fold increase in the Ph3N amount leads to about threefold decrease in the catalyst activity, lowering of Mw and PDI (entry 9). Increase of AlTiBA/Zr molar ratio in amine-free catalytic systems is accompanied by a linear decrease in the activity and lowering of PDI values (entries 10,11 vs. 3). The diversity of active species formed in the absence of amine (e.g., entry 10) and at different ways of amine introduction (as solution with hafnocene (entry 6), TB (entry 7), or that with TIBA (entry 8)) should be mentioned. The catalytic systems show different rates of chain propagation and chain transfer under other similar conditions. One can find the evidences for different effects of coordinating substrates on the properties of operating active species by comparison of GPC curves (Fig.5), activities, and stereoregularities (Table 2) of sPPs obtained in the absence of amine (entry 10) and those in the presence of NPh3 introduced either as complex with hafnocene (entries 6, 9) or as the complex with TIBA (entry 8).

84

N.M. Bravaya et at.

Table 2., Results on propylene polymerizations*1 with 4Me:in combination with different activators, such as MAO, TB/TIBA, and DMAB/TIBA, as well as in the presence ofPh3N. Entry

Activator

t,

Molar ratio Al:Hf:B:N

(min)

Ab

Y,

te)

rr (%)

-

-

-

n.d.

n.d.

87

13.5

1123000

3.50

76

2.S

0.9

1538000

2.18

83

0.5

1.3

n.d.

D-d.

n.d.

1

MAO

1340:1:0:0

30

Traces

2

MAO

1300:1:0:1

63

1.1

0.9

3

TB

90:1:1:0

30

14.3

4

DMAB

90:1:1:0

72

5

C

TB

40:1:1:1

20

6

TB

7

MJMn

-

90:1:1:1

4.5

17.3

105.0

917000

5.40

60

d

100:1:1:1

6.0

11.5

57.0

1580000

2.36

70

B

TB

S

TB

130:1:1:1

8.5

6.7

37.5

2169000

1.69

73

9

TB

100:1:1:5

7.5

7.0

29.0

727000

2.50

75

10

TB

150:1:1:0

60

13.5

7.6

1281000

2.30

65

11

TB

200:1:1:0

30

2.3

3.5

n.d.

n.d.

60

"Polymerization conditions: the catalyst was prepared as toluene solution with PhsN if not specially notified, 100 ml of toluene, 30 °C, propylene pressure 6 atm; b Activity in kg PP/(mol Hf min atm);G The catalyst was introduced as co-solution with MeaNPh;d The order of reagents loading was: toluene+TTBA+toluene solution of 4Me+(toluene solution of TB with PhaN); e The order of reagents loading was: toluene+(toluene solution of TIBA with PhjNJ+toluene solution of 4Me+toluene solution of TB.

Thus both productivities and stereoselectivities of the catalyst in entries 8,9 are of about similar values, Mw of generated sPP being about 3 times higher when amine was introduced as the NPhj-TIBA complex. In an excess of TIBA in the absence of amine (entry 10), sPP with much lower activity and stereoregularity is formed compared to that with the NPhs-TIBA system (entry 10 vs. 8), Mw of sPP formed in this case being about twice lower than that in entry 8 is about twice higher than that in entry 9, Increase of Al-nBA/Hf molar ratio in the absence of amine is accompanied by a decrease in the activity. The most surprising effect was a sharp decrease in stereoregularity at sPP in the increased A1TIHA/Hf molar ratios. Stereoblock syndio/atactic PP is formed in excess of TIBA (entries 10, 11). Reversible coordination of TIBA multiply perturbing the geometry of the catalyst during chain growth [2] was proposed as a working hypothesis to explain the effect. One can see that the (4Me+NPh3)/TB/TIBA system shows very high activity and produce low stereoregular sPP with high PDI at AlT1BA/Hf=9G mol/mol and N/Hf=l mol/mol (entry 6).

13, Modification of Homogeneous Metallocene Catalysts

85

At the same time, multiple Gauss fitting of the curve shows the presence of all the peaks observed for sPPs of entries 8-10 with additional low-molecular peak, probably, arising due to a high value of heat evolution at the initial stage of the polymerization process. 1.4

8

1.2

9 10

d w t/d(lo g (MW))

1.0

0.8

6 0.6

0.4

0.2

0.0 7.0

6.5

6.0

5.5

5.0

4.5

4.0

log(MW)

Fig. S, GPC curves of sPPs obtained with the PhzCCpFluHfMe3/TB/TIBA system in the absence and presence of PhjN (curve numbers correspond to those in Table 2),

3. CONCLUSIONS Thus, the above-discussed experimental data show that both TIBA and Ph3N can be used for modification of metallocene-derived catalytic systems with MAO and TB activators. It should be mentioned that almost all present-day approaches to controlling the activity and stereoselectivity of metallocene catalytic systems are primarily based on the use of either new metallocene complexes or new activators. Considering the results of this study, we would like to attract the attention to poorly investigated possibilities of controlling the activity and stereoselectivity of metallocene systems that are offered by the addition of TIBA or LBs. Referencei [1] A. N. Panin, Z. M. Dzhabieva, P. M. Nedorezova, V. I. Tsvetkova, S. L. Saratovskikh, O. N. Babkina, N. M. Bravaya, J. Polym. Sci. A: Polym. Chem. 39 (2001) 1915-1930. [2] O. N. Babkina, N. M. Bravaya, P.M. Nedorezova, S. L. Saratovskikh, V. I. Tsvetkova, Kinet. Catal. 43 (2002) 341-350.

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[3] Y. X. Chen, M. D. Rausch, J. C. W. Chien, J. Polym. Sci. A: Polym. Chem. 33 (1995) 2093-2108. [4] F. Q. Song, M. D. Hannant, R. D. Cannon, M. Bochmann, Maeromol. Symp. 213(2004)173-185. [5] C. Gotz, A. Rau, G. Luft, Maeromol. Symp. 178 (2002) 93-107. [6] W. Michiels, A. Munozescalona, Maeromol. Symp. 97 (1995) 171-183. [7] R. Kleinsehmidt, Y. Leek, M, Reffke, G. Fink, J. Mol. Catal. A: Chem. 148 (1999)29-41. [8] M. L. Britto, G. B. Galland, J. Henrique, Z. Dos Santos, M. C. Forte, Polym. 42 (2001) 6355-6361. [9] W. Wang, Z. Q. Fan, Y. B. Zhu, Y. H. Zhang, L. X, Feng, Europ. Polym. J. 38(2002)1551-1558. [10] M. Lahelin, E. Kokko, P. Lehmus, P. Pitkanen, B. Lofgren, J. Seppala, Maeromol. Chem. Phys. 204 (2003) 1323-1337. [11] O. M. Khukanova, O. N. Babkina, L. A. Rishina, P. M. Nedorezova, N. M. Bravaya, Polymery 45 (2000) 328-332. [12] P. M. Nedorezova, V. I. Tsvetkova, A. M. Aladyshev, D. V. Savinov, T. L. Dubnikova, V. A. Optov, D. A. Lemenovskii, Polymery 45 (2000) 333-338. [13] M. A. Esteruelas, A. M. Lopez, L. Mendez, M. Olivan, E. Onate, Orrganometallics, 22 (2003) 395-406. [14] K. Musikabhumma, T. Uozumi, T. Sano, K. Soga, Maeromol. Rapid. Common. 21 (2000) 675-679. [15] J. C. Flores, J. C. W. Chien, M. D. Rauseh, Organometallies 13 (1994) 4140-4142. [16] O. Kuhl, T. Koch, F. B. Somoza, P. C. Junk, E. Hey-Hawkins, D. Plat, M. S. Eisen, J. Organomet, Chem. 604 (2000) 116-125, [17] P. Mehrkhodavandi, R. P. Schrock, L. L. Pryor, Organometallies 22 (2003) 4569-4583. [18] P. G. Belelli, M. L. Ferreira, D. E. Damiani, Maeromol. Chem. Phys. 201 (2000) 1458-1465. [19] F. Schaper, A. Geyer, H. H. Brintzinger, Organometallies 21 (2002) 473483. [20] A. G. Ryabenko, E. E. Faingol'd, E. N. Ushakov, N. M. Bravaya, Russ. Chem. Bull, (in press). [21] J.-N. Pideutour, K. Radhakrishnan, H. Cramail, A. Deffieux, . Mol. Catal. A: Chem. 185 (2002) 119-125. [22] J. Zhou, S. J. Lancaster, D. A. Walker, S. Beck, M. Thorton-Pett, M. Bochmann, J. Am. Chem. Soe. 123 (2001) 223-237.