Beyond Metallocenes - American Chemical Society

Chapter 4

Downloaded by COLUMBIA UNIV on September 2, 2012 | http://pubs.acs.org Publication Date: July 31, 2003 | doi: 10.1021/bk-2003-0857.ch004

Group 4 Octahedral Complexes Catalyzed the Stereoregular and Elastomeric Polymerization of Propylene 1

1

1

Victoria Volkis , Michal Shmulinson , Ella Shaviv , Anatoli Lisovskii , Dorit Plat , Olaf Kühl , Thomas Koch , Evamarie Hey-Hawkins , and Moris S. Eisen * 1

1

2

2

2

1,

1

Department of Chemistry and Institute of Catalysis Science and Technology, Technion-Israel Institute of Technology, Haifa 32000, Israel Institute für Anorganishe Chemie der Universität at Leipzig, D-04103 Leipzig, Germany 2

Some group 4 octahedral complexes with different ligations (benzamidinate, acetylacetonate, ß-diketoimidinate, phosphinoamide) have been synthesized. We demonstrate that early transition metals with a cis-octahedral geometry, when activated by methylalumoxane, catalyzed the stereoregular polymerization of propylene, which can be modulated by pressure. The mechanism of formation and structure of the new type of elastomeric polypropylene obtained is discussed.

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© 2003 American Chemical Society In Beyond Metallocenes; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction

During the last two decades a great advancement have been made in the synthesis and application of well-defined "single site" metallocene catalysts for the polymerization of α-olefins (7-5). Metallocene catalysts generally belong to the group 3 and 4 derivatives with two cyclopentadiene (Cp) rings, although some of these system (constrain geometry) contain one Cp and one pendant ligand (6,7). The structure of the ligands affect both electronic and sterically the properties of the metallocenes and thus predictably determined the stereospecificity of these catalysts (8,9). Recently, a great interest have been directed towards the synthesis of non-Cp complexes as potential catalysts for the polymerization of olefins as alternatives to metallocenes. Among these systems, attention have been devoted to complexes containing chelating dialkoxide (70), diamido (77) and amidinate (12-15) ancillary ligations. Group 4 chelating benzamidinate complexes are normally obtained as a mixture of racemic C -symmetry cw-octahedral structures. These complexes, when activated with methylalumoxane (MAO), catalyzed the polymerization of ethylene (16-18), propylene (16,19), styrene (77), the oligomerization of 1,5-hexadiene (16) and the isomerization of olefins (20). Currently, complexes with acetylacetonate (21) and phosphinoamide (22) ligations, were also found to be active in the polymerization of propylene. Here we report the synthesis of some group 4 octahedral benzamidinate, acetylacetonate, ketoimidinate and phosphinoamide complexes and their application for the polymerization of propylene, modulated by monomer concentration in the reaction mixture. This is a novel example of modulation of the stereoregularity of polymers by pressure, catalyzed by early transition metal octahedral complexes. The effects of pressure, temperature, nature of solvent in the polymerization process as well as the properties of the obtained polymers, are presented. A strategy for preparing a new type of an elastomeric polypropylene and the mechanism of its formation is also discussed. 2

Results and Discussions Synthesis of the Complexes and their Structural Features The lithium benzamidinate Li(TMEDA)[p-Me-C H C(NSiMe )2] was prepared through the reaction of p-toluonitrile and the lithium salt of hexamethyldisilazane. Reactions of MC1 with two equivalents of the ligand afforded the dichloride benzamidinates ( M = Zr, Ti) as crystalline solids. 6

4

3

4

In Beyond Metallocenes; Patil, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

48


TMS

(1) M = Zr (2) M = Ti

Alkylation of both compounds with two equivalents of M e L i ' L i B r yields the corresponding dimethyl complexes of Zr (1) and T i (2) (equation 1). X-ray studies of 1 (19) showed that the central Zr atom is octahedral surrounded by the two benzamidinate ligands and two terminal methyl groups (Figure 1). One C atom from the C H group (C(l)) and nitrogen atom of a chelate unit are in the axial positions, while the C(2) atom and the remaining nitrogen atoms occupy the equatorial positions, producing a C -symmetry complex. The two four-membering rings Z r C N are almost symmetric and perpendicular to each other. X-ray of the complex 2 shows that it is isostructural with the complex 1. 3

2

2

Reaction of acetylacetone with BuLi afforded the lithium acetylacetonate ligand. Interaction of two equivalents of the ligand with MCI4 produced orange powder of the titanium complex (3) and light-yellow microcrystals of the zirconium complex (4) (equation 2). ch

3

(3) M = Ti

(4) M = Zr

In the absence of any X-ray data, stereoisomeric studies of acetylacetonate complexes have been conducted using N M R , vibration spectroscopy and dipole



Figure 1. X-ray of complex 1.


so

50

moment measurements. These complexes were characterized to be obtained with a c/s-octahedral configuration (25-29). The L i salt of ß-diketoimidinate ligand was obtained by the reaction between C H C N and LiCH(SiMe ) (30). Two equivalents of the ligand react with MC1 producing reddish-brown powder of the T i complex (5) and yellow crystals of the Zr complex (6) (equation 3). The T i complex 5 has an octahedral structure, whereas in complex 6 one of the ß-diketiminate ligands undergoes a retrobrook rearrangement to the amidinate isomer catalyzed by Z r C l (30). The homoleptic phosphinoamide complex [Zr(NPhPPh ) ] (7) was prepared by reacting ZrCU and four equivalents of LiNPhPPh (equation 4), whereas the bisamido complex [TiCl {N(PPh ) } ] (8) was obtained from TiCLj and two equivalents of LiN(PPh ) (equation 5). LiNPhPPh and LiN(PPh ) were prepared by deprotonation of the correspondent phosphinoamines Ph PN(H)Ph (23) and Ph PN(H)PPh (24) with butyllithium in hexane. At room temperature the P - N M R spectrum of the titanium complex 8 shows a major signal (90%) at 47.0 ppm which correspondent to the complex with tetrahedral symmetry, and a minor set (10%) of two small doublets at 5.0 and 10.0 ppm corresponding to the complex with a C -symmetry octahedral configuration. The 2D heterocosy spectrum shows that the two doublets are correlated to each other indicating that only one isomer was obtained among all the conceivable complexes that can be theretically formed. The complexes responsible for the signals at 47 and 5.0-10.0 ppm were found to be in equilibrium, as shown by the change in intensity of the signals by changing the temperature. The process is fully reversible by either cooling or warming the sample to room temperature. 6

5

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2

4

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2

2

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2

Propylene Polymerization Experiments The catalytic polymerization of propylene was studied using complexes 1-8 and M A O as a cocatalyst. Reactions at atmospheric pressure were carried out in a 100 ml flamed round-bottom flask. Experiments at higher pressures were performed either in 100 ml heavy-wall glass or in stainless steal reactors, equipped with a magnetic stirrer. Reactors were charged inside a glovebox with a catalyst and M A O and connected to a high vacuum line. After introducing certain amount of solvent (toluene or dichloromethane) the reactor was frozen at liquid nitrogen and pumped-down. For the reaction at atmospheric pressure, propylene was used to back filled the flask and maintained at 1.0 atm with a mercury manometer. In experiments at higher pressure propylene was vacuum transferred to the frozen reactor. After stirring for a certain period, the unreacted propylene was vented out and reaction was quenched by addition of HCI/CH3OH. Polymer was cleaned from the catalytic system residues by dissolving in a hot (120°C) 1,2,4-trichlorobenzene or decaline. After filtration of




(il


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the hot solution, the filtrate was poured into cold acetone to precipitate the polypropylene. The latter was then washed with acetone for removing traces of solvents and dried. Effect of the Complexes Structure on the Activity and Tacticity in the Polymerization of Propylene. In general, the tacticity of the polymers predictably differs with the structure of the catalysts. Complexes with C or C i symmetry are expected to produce isotactic polyolefins, while compounds with C symmetry produce atactic polymers (3, 31-35). However, when the polymerization of propylene with complex 1 is performed at atmospheric pressure, unexpectedly instead of an isotactic polymer, an oily atactic product is formed (Table I). A t the same time, when the reaction is carried out at high monomer concentration (in liquid propylene), the expected highly stereoregular polypropylene (mmmm > 99.5%, m.p. = 153°C) was obtained. This phenomena was rationalized by an intramolecular epimerization reaction of the growing polymer chain (P) at the last-inserted monomeric unit, (Scheme 1) (36-38). Apparently, a high propylene concentration allows faster insertion of the monomer and almost completely 2


2 v

(?J = polymer

Scheme 1. Intramolecular mechanism for the epimerization of polypropylene. Thus, the stereoregularity of polypropylene with the system "1+ M A O " can be generalized as described in equation 6. A n additional corroboration that the mechanism shown in Scheme 1 is responsible for the stereodefects of polypropylene was obtained by transformations of 1-octene in the presence of 1 and M A O (19-21). The polymerization of 1-octene in these conditions is extremely low. Hence, a ß-hydrogen elimination can take place either from each of the C H groups at the α-position in the intermediate A (Scheme 1), or from the C H group of the 1-octene molecule attached to the complex. In the first case no changes in the alkene are expected, but in the second mechanism, an isomerization of the double bond will be induced. It was shown that 1-octene almost completely 3

2




in

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Table L Data for the Polymerization of Propylene Catalyzed by Benzamidinate Complexes 1-8 Activated by Methylalumoxane*

Run

C"

Al:M

e

A

Mri

mwcß

mmmm %

aim 1

1

Τ

250

1.0

8.1

2

1

Τ

250

9.2

1.1'

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 a

1 1 2 2 2 2 3 3 3 4 4 4 5 5 7 8

D D T D T D T T D T T D T D T T

250 1000 1000 1000 1000 1000 430 430 400 480 480 480 400 800 500 500

b

9.2 9.2 1.0 1.0 9.2 9.2 1.0 9.2 9.2 1.0 9.2 9.2 9.2 9.2 9.2 9.2

Type"

oil 1.69 2.35

2.2 7.9

261 36 11 58

-

i

2.49 1.42

11 90 98

oil i i

-

-

-

-

1.9 1.9

51 55

1.83 1.51

21 18

e

-

-

-

-

-

1.9 7.3

129 35

1.92 3.32

15 22

e e

e

-

-

-

-

-

0.9 0.2 1.1 0.2 0.010 0.016

53 71 596

1.24 1.82 2.14

99 28 27

-

-

-

48 88

2.40 1.65

i e e oil e e

c

d

e

- 25°C; - catalyst; - solvent (T = toluene, D = dichloromethane), - pressure, - A = activity χ 10", g PP/mol cat-h; - Μη χ 10" gr/mol, - molecular weight distribution; i = isotactic polymer, e = elastomeric polymer, - mixture of isotactic (50%) and atactic (50%) fractions; - isotacticity of isotactic fraction. 5

f

3

8

1

]


h

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isomerized to a mixture of octenes, strongly supporting the formation of intermediate A . As compared to toluene, larger activities and stereoregularities of the polymers are achieved when the polymerization is carried out in C H C 1 (entries 2 and 3). Plausibly, this polar solvent causes charge separation between the cationic complex and the M A O counter-anion, thus encouraging the insertion of the monomer. For toluene, insertion of the monomer as well as termination of the polymer chain is inhibited by bounding of the ring to the cationic center (39-41), resulting in a decrease of the polymerization rate and an increase of polymer's molecular weight. In contrary to 1, complex 2 is inactive in the polymerization at atmospheric pressure in both solvents (entries 5 and 6). The solvent-metal interaction seems to be stronger in T i than in Zr because of the effective charge at the metal center, resulting in an inhibition of the monomer insertion for complex 2. At higher pressure, complex 2 is an active precatalyst in both solvents producing an elastomeric polymer. Thus, group 4 ^^(benzamidinate) complexes activated by M A O can be considered as a new catalytic system for the polymerization of propylene, when the stereoregularity of the polymers can be modulated by pressure (from atactic to isotactic through elastomers) (19). This fact raises a conceptual question regards whether simple octahedral or even tetrahedral complexes having a dynamic Lewis-base pendant group donating an electron pair to the electrophilic metal center, are suitable for the synthesis of a unique product - an elastomeric polypropylene. As is shown in Scheme 2, a dynamic equilibrium occurs between tetrahedral and cw-octahedral configurations (X = halide, Ε = donor group with a electron lone pair, R = C, Ν, Ρ or other anionic bridging group). A plausible /r

Beyond Metallocenes - American Chemical Society

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