Active Sites in Soluble Ziegler Polymerization ... - ACS Publications

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 10, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch040

40 Active Sites in Soluble Ziegler Polymerization Catalysts Generated from Titanocene Halides and Organoaluminum Lewis Acids J. J. Eisch and K. R. Caldwell Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902

By means of X-ray single-crystal data the Ti-C and Ti-Cl bond lengths in trimethylsilylmethyltitanocene chloride (11) have been de termined. The operation of σ-hyperconjugation in this compound leads to an unusual shortening of the Ti-C bond. In solution the observed shifts in the H NMR signals of the CH or the cyclopentadienyl (Cp) groups of 11 are attributed to varying degrees of coordinative solvation of the titanium center or hydrogen bonding at the chlorine center. Such inherent polarity of the Ti-Cl bond in 11 has been found to be sharply accentuated hy the introduction of a Lewis acid, AlCl R. By monitoring such reaction mixtures by H, C, and Al NMR spectroscopy, direct evidence is obtained for the gen eration of the alkyltitanocenium ion, Cp RTi , andAlCl -. Such com binations of 11 and Lewis acid are effective catalysts for the polym erization of ethylene, and Me Si fragments from 11 are found in the resulting polyethylene. These findings support the thesis that Cp RTi ions are the active polymerization sites in such Ziegler catalyst sys tems. 1

2

1

2

13

27

2

+

4

3

2

THE

+

N A T U R E O F O L E F I N P O L Y M E R I Z A T I O N CATALYSTS formed from com

binations of titanium halides and alkylaluminum halides has remained the subject of controversy (I) since the epoch-making discoveries of Ziegler et al. (2) and of Natta et al. (3) some 35 years ago. Much of the difficulty in 0065-2393/92/0230-0575$06.00/0 © 1992 American Chemical Society

In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

576

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

understanding the molecular basis for such Ziegler-Natta catalysis lies in the heterogeneous character of the subvalent titanium halides generated by reduction of T i C l with the aluminum alkyl (4). For this reason, mechanistic studies have often employed a soluble ethylene polymerization catalyst, which results from the interaction of titanocene dichloride and alkylaluminum halides (4). Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 10, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch040

4

First introduced by Breslow and Newburg (5), this system has been shown to contain tetravalent titanium in the active catalyst (6, 7), as well as cationic titanocene ions (8). It exhibits accelerated polymerization of ethylene in dipolar halocarbons, C H C l . , over that shown by aromatic hydrocarbons (9). Furthermore, equilibrium and kinetic (10) studies have demonstrated that the catalyst components, C p T i C l (1, C p is cyclopentadienyl) and R A l C l (2), are not themselves the active catalyst partners. Rather, they are converted into the active catalyst (4, eq 1), which to an undetectable extent is in equilibrium with a 1:1 complex (3) of 1 and 2. n

4

n

2

2

2

Cp TiCl 2

+

2

RA1C1

1

^ — »

2

« _ ^

3

4

( 1 )

2

What the structures of 3 and 4 might be was unknown. Hence, nothing certain could be concluded about the relative roles of the titanium and aluminum centers in such Ziegler polymerization. Our approach to clarifying the mechanism of Ziegler ethylene polym erization has been to attempt the isolation or chemical interception of in termediates 3 and 4 and to elucidate the structures of the crystalline products (11). Especially for 4, which exists at equilibrium in low concentrations, chemical trapping proved to be the only feasible way to identify its structure. With the assumption that 4 contained either a T i - C or an A l - C bond into which the ethylene units were inserted during polymerization, we searched for a surrogate for ethylene that would readily perform the first insertion but, because of steric hindrance, would not undergo a further insertion. Polymerization thus would not ensue so that the initial insertion product 6 might then be isolated and its structure determined. After considerable evaluation of potential ethylene surrogates (12), we found trimethylphenylethynylsilane (5) to be an eminently suitable trapping agent (eq 2). R - M 4

+

P h - C Ξ C — SiMe 5

P

»

f a s t

3

\

/

S1MC

C = C R

/

6

3

very s l o w

^

•

polymer

(2)

M

Thus, by admixing C p T i C l and M e A l C l (2a) in chloroform, we isolated crystalline 3a and showed by X-ray crystallography that it was a monochloro 2

2

2


40.

Eiscii & C A L D W E L L

Active Sites in Ziegler Polymerization Catalysts

577

bridged complex. Similarly, the interaction of 1, 2a, and 5 in chloroform led to the precipitation of 6a. Γη

C

\

P

•

C

1

C H

\ / Al

T i

Cp' Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 10, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch040

1

/

3

Ph

^ « 6 % , C =

v

^ C l

C

M

l

e

^

C 89°

^145°^

3a

SiMe

T i C

3

+

P2

A1C1

2.92 in C H and 2.26 - » 2.70 ppm in C D C 1 ) . In both cases, a considerable proportion of C p T i C l was formed with time (eq 8). 2

3

6

6

3

2

SiMe

I

Cp Ti

.CH

2

2

3

SiMe M

e

A

1

Q

2

r

Cp Tt — C H 2

•

2

2

16

11

3

AlMeCl " 3

CH SiMe

Q

Cl

2

Cp Ti

.AlMeCl

2

3

(8)

cr Interconversions with the aluminum Dichloride System.

Titanocene Dichloride-MethylSpectroscopic monitoring (10) and

X-ray crystallography (11) have established that, in chloroform, 1 and 2a form a bridged complex (3a). By admixing M e A l C l with 1 with differ ent solvents and in different ratios, evidence was sought on whether other 1:1 complexes could be generated and whether the equilibrium suggested in eq 4 could be driven to the right for detection of C p T i M e C l . Further more, it was hoped that by admixing C p T i M e C l and A1C1 , the equilibrium hypothesized in eq 4 could be approached from the reverse direction. 2

2

2

3

Although 3a is the main complex formed between 1 and 2a in chloroform solution, the same two components in benzene solution show the presence of 75% of 3a, 20% of uncomplexed C p T i M e C l (17), and 5% of what may be C p T i M e A l C V (18) (eq 9). 2

2

+

In benzene the methyl protons of 3a, 17, and 18 occur at -0.01, and 1.20 ppm, respectively. The C p protons are exhibited at 5.89, (broad), and 5.68 ppm (20).


0.87, 5.89

586


Cp TiCl 2

+

2

MeAlCl

2

Cl

.AlMea,

P2 -.

C

T i

' '

3a

α

C

+

P2

"

T

i

\

( 9 )

17

«

Cp TiMe AlCl " +


2

4

18 From mixtures of 1 and 2a in benzene a brownish-red solid could be separated, which upon dissolution in C H C 1 yielded a * H N M R spectrum exhibiting the benzene peak (7.33 ppm) and the C p T i C l peak (6.51 ppm) in a 1:1 ratio. In addition, weak peaks in the M e - A l region (-0.57, -0.02, and -0.57 ppm) were observed. Thus it appears that 3a or an isomer (3a') forms a weak ιτ-complex with benzene. 3

2

+ / C l

2

Me

+

Cp Ti.

AlMeCV

2

*C H 6

Cp Ti .

o r

2

A

*C H

6

6

3a (solvate)

1C1 " 4

6

3a' (solvate)

Admixing methyltitanocene chloride and A1C1 in 1:1 ratio in C H C 1 gave mainly complex 3a. This product shows that the equilibrium in eq 4 lies preponderantly to the left (*H N M R signals in the product: C p , 6.89; Me, -0.31 ppm). Admixing methyltitanocene chloride (17) with 1 equiv of M e A l C l at 25 °C in C D C 1 gave as the principal product a titanocene derivative whose C H group had been shifted downfield from that in 17 (*H: 0 . 8 2 ^ 1.13; C : 5 0 . 3 ^ 59.5 ppm), as had its C p group ( H : 6.26 -> 6.71; C : 115.7 —» 117.6 ppm). These shifts are consistent with ion-pair formation (19) (eq 10). 3

2

3

3

3

1 3

!

1 3

Me Cp Ti '

M

2

c

A

1

C

l

2

»

Cp Ti '

ClAlMeCl -

2

2

Cl 17

C A

19 Cp TiCl 2

2

+ Me^AlCl

(10)

1 With time, again titanocene dichloride is a major product. Adding acetonitrile reforms 25% of 17, 44% of 1, and 31% of other cyclopentadienyltitanium products.


40.

EISCH & C A L D W E L L

587


Polymerization of Ethylene with Trimethylsilylmethyltitanocene Chloride and an Aluminum Chloride Cocatalyst. Compound 11, com bined with either aluminum chloride in 1,2-dichloroethane or methylaluminum chloride in chloroform, was found to be an effective catalyst system for the polymerization of ethylene at 0 °C and at pressures of 30 psig. Turnover numbers of 175 per mmol of titanium per hour were observed for A1C1 , and 245 per mmol of titanium per hour were observed for M e A l C l . Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 10, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch040

3

2

The resulting polyethylene had the following properties: • mp 135-139 °C; • infrared absorption bands consistent with those characteristic of high-density linear polyethylene: 2920, 2860, 1477, 1466, 1370, 1248, 728, and 717 (cm ); and 1

• mass spectrum (deep insertion at 70 eV) displaying fragments containing the M e S i end groups and between 22 and 25 eth ylene units. 3

Thus, highly linear polyethylene containing trimethylsilyl end groups is produced by catalysts employing compound 11.

Discussion The relative shortening of the titanium-methylene bond length in angstroms in 11, compared with the t i t a n i u m - η carbon distance in C p T i ^ - C p ) , can be attributed to σ-bond hyperconjugation (20b-20c) operative in the crystalline state. The polar titanium-chlorine bond would further enhance this effect (20a). 1

SiMe

SiMe CH

P 2

20a

SiMe 3

2

CH

2

Q

-

2

+

3

I +/ C Ti

1

2

M

•

C Ti

Cp->Ti

//

P 2

" \ 20b C l

CHo " c

f

20c

Even in solution, the changes in the H N M R chemical shifts as a l

function of the donor or polar character of the solvent indicate that the electron-deficient titanium (20a) may undergo coordination with stronger n- or ir-donors dimethyl sulfoxide and hexamethylphosphoramide or toluene and mesitylene, as shown in Table I, thereby increasing the electron density about titanium and shifting the C H signal to higher magnetic field. The effects of n- and ττ-donors on the chemical shift of the C p protons seems to be opposite: n-donors deshield while ιτ-donors shield such protons. A 2


3

588


Stuart-Briegleb model of 11 (Figure 1) shows that such solvent coordination would have to occur from the flank of the T i - C l bond anti to the C H S i M e group (20e). With such relatively weakly coordinating solvents as the haloalkanes, the downfield chemical shift exhibited by the C H signal does not correlate with dipole moment but is the greatest for chloroform. Because of the known acidity of H - C C 1 , this result suggests that hydrogen bonding may be operative in enhancing the T i - C l bond polarization (20a) and hence in deshielding the C H protons (20f). 2

3

2


3

2

From the crystal structure of the adduct, C p T i C l · M e A l C l , it is known that the Lewis acidic M e A l C l coordinates with the already polarized T i - C l bond and stretches it by a further 0.16 Â. A similar effect would be expected upon treating methyltitanocene chloride (9) or its trimethylsilyl derivative (11) with such Lewis acids as A1C1 or M e A l C l . In these cases, the stretching of the T i - C l bond by the T i — C l — A 1 C 1 R ' interaction should be even greater because the C H or C H S i M e group can release electron density to the developing positive charge on titanium (21a). 2

2

2

2

3

2

2

3

2

3

CH R 2

C

p

2

T

i

.

y

κ δ-

«

~· , R = SiMe

p

•

χ

CH

δ+ 2

SiMe

3

Cp Ti 2

\

3

Cl-«-AlCl R'

δCl-.-AlCl R

2

2

21a

21b

Again, by σ-bond hyperconjugation the trimethylsilyl derivative (21b) should be better able to sustain such developing positive charge. Such in teractions should lead to downfield shifts in the H N M R signals of the C H or C H groups, and indeed, shifts in the magnitude of +0.8 to +2.0 ppm have been observed. l

3

2

The further crucial aspect of such interactions is whether the T i - C l bond is completely severed by such Lewis acids and whether a titanoeenium cation is thereby formed. Although significant downfield shifts in H signals of the C H group in 9 and of C H group in 11 are caused by Lewis acids, it cannot be concluded whether such shifts arise from an adjacent positively polarized titanium (22a) or from a titanocene cation (22b). l

3

2


40.

EISCH & C A L D W E L L CH SiMe 2

Cp Ti

/

CH SiMe

3

2

or

2

CI —

3

CI4AI

AICI3

22b

22a


589


However, when the interaction of trimethylsilyl derivative 11 with A1C1 at -23 °C in 1,2-dichloroethane was monitored by A l N M R spectroscopy, the major A l signal initially was at 103.3 ppm. This value is exactly that previously reported for the tetrachloroaluminate anion (19). Thus, for 11 there is no doubt that the T i - C l bond is ruptured completely by the elec trophilic AICI3 and that the titanocenium ion pair 22b is generated. Having thus established that such cations are generated in this case, one can readily trace the reason for such cation stabilization to the same σ-bond hyperconjugation already evident in the neutral trimethylsilyl derivative itself (20a-20c).

3

2 7

2 7

Such titanocenium cations have been postulated to be the active catalyst in the Ziegler polymerization of ethylene in homogeneous media. Therefore, one would expect combinations of the trimethylsilyl derivative and A l C l to cause polymerization of ethylene. Further, M e S i C H end groups should appear in the resulting polyethylene. Both expectations were fulfilled by experiment. Compound 11 by itself was inactive in polymerization but im mediately became active when A l C l was added. Any donor solvent or Lewis base (ethers or amine) that vitiated the action of the Lewis acid also destroyed the catalytic activity. As to the resulting high-density linear polyethylene produced, mass spectral measurements of the solid by direct insertion into the ionization chamber readily revealed the presence of the M e S i group and its fragmentation peaks. 3

3

2

3

3

A remaining significant question for the titanocenium cations detected in this study is the nature of their ion-pairing and solvation. This matter is under active investigation.

Acknowledgments This chapter is part 46 of the series, "Organometallic Compounds of Group III". Part 45 was "Hydroalumination of C = C and C C Linkages", by Eisch, J. J., in Comprehensive Organic Synthesis, Trost, Β . M . ; Fleming, I., Eds.; Pergamon: Oxford, 1991. The authors are indebted to the National Science Foundation for support of this research under Grant CHE-87-14911. The X-ray structure deter minations of compounds 9 and 11 were carried out by Carl Kruger and Stefan Werner at the Max-Planck-Institut fur Kohlenforschung, Mulheim (Ruhr), Germany. Complete details will be presented in a separate publication. The


590


N S F International Travel Grant 88-13722 aided the principal investigator (J. J. Eisch) in his work at the Max Planck Institut.


References

1. Boor, J., Jr. Ziegler-Natta Catalysis and Polymerization; Academic: New York, 1979; p 670. 2. Ziegler, K.; Holzkamp, E.; Breil, H.; Martin, H. Angew. Chem. 1955, 67, 541. 3. Natta, G.; Pino, P.; Carradini, P.; Danusso, F.; Mantica, E.; Mazzanti, G.; Moraglio, G. J. Am. Chem. Soc. 1955, 77, 1708. 4. Eisch, J. J.; Boleslawski, M. P.; Piotrowski, A. M. Transition Metals and Or ganometallics as Catalysts for Olefin Polymerization; Kaminsky, W.; Sinn, H Eds.; Springer-Verlag: Berlin, 1988; p 371. 5. Breslow, D. S.; Newburg, N. R. J. Am. Chem. Soc. 1957, 79, 5072. 6. Long, W. P.J.Am. Chem. Soc. 1959, 81, 5312. 7. Breslow, D. S.; Newburg, N. R. J. Am. Chem. Soc. 1959, 81, 81. 8. Dyachkovskii, F. S. Visn. Mol. Soedin 1965, 7, 114. 9. Eisch, J. J.; Galle, J. Ε.; Piotrowski, A. M. Transition Metal Catalyzed Polym erizations: Alkenes and Dienes; Quirk, R. P., Ed.; Harwood Academic: New York, 1983; Part Β, p 799. 10. Fink, G.; Zoller, W. Makromol. Chem. 1981, 182, 3265. 11. Eisch, J. J.; Piotrowski, A. M.; Brownstein, S. K.; Gabe, E. J.; Lee, F. L. J. Am. Chem. Soc. 1985, 107, 7219. 12. Eisch, J. J.; Manfre, R. J.; Komar, D. A. J. Organomet. Chem. 1978, 159, C13. 13. Hanstein, W.; Berwin, H. J.; Trayler, J. G. J. Am. Chem. Soc. 1970, 92, 829. 14. Eisch, J. J.; King, R. B. Organometallic Syntheses; Academic: New York, 1981; Part I.A. 15. Brown, H. C. Organic Synthesis via Boranes; Wiley: New York, 1975; Chapter 9. 16. Jeffery, J.; Lappert, M. F.; Luong-Thi, Ν. T.; Webb, M.; Atwood, J. L.; Hunter, W. E.J.Chem. Soc., Dalton Trans. 1981, 1593. 17. Waters, J. Α.; Mortimer, G. A. J. Organomet. Chem. 1970, 22, 417. 18. Krüger, C.; Werner, S. Max-Planck-Institut für Kohlenforschung, Mülheim (Ruhr), Germany, unpublished results. 19. Nöth, H.; Rürlander, R.; Wolfgardt, P. Z. Naturforsch. 1982, 37b, 29. 20. Eisch, J. J.; Pombrik, S. I. State University of New York at Binghamton, unpublished results, 1990. RECEIVED for review October 19, 1990. A C C E P T E D revised manuscript May 29, 1991.


Active Sites in Soluble Ziegler Polymerization ... - ACS Publications

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