5668
J. Am. Chem. Soc. 2000, 122, 5668-5669
Novel Aluminum-Based, Transition Metal-Free,
Catalytic Systems for Homo- and Copolymerization
of Alkenes
Jang Sub Kim, Louis M. Wojcinski II, Shengsheng Liu,
John C. Sworen, and Ayusman Sen*
Department of Chemistry
The PennsylVania State UniVersity
UniVersity Park, PennsylVania 16802
ReceiVed March 20, 2000
Since the initial reports by Ziegler1 on the “aufbau” reaction,
or stepwise insertion of ethene into the aluminum-carbon bond
of alkylaluminum compounds, it has been widely believed that
because of the accompanying displacement reaction the products
of this reaction were limited to oligoethenes, with few reports of
the preparation of high molecular weight polyethene at an
aluminum center. Martin has described the preparation of poly-
ethene by exposing ethene to a heptane solution of either bis-
(dichloroaluminum)ethane or trialkylaluminum over several days.2
Recently, Jordan and Gibson have reported the polymerization
of ethene using chelated alkylaluminum complexes activated by
a Lewis acid.3 On the other hand, the polymerization of higher
alkenes, such as propene, by an aluminum-based system has neWer
been reported. Herein, we report that high molecular weight, linear
homo- and copolymers of ethene and propene can be prepared,
in the absence of any transition metal species, via a catalyst system
consisting of simple alkylaluminum compounds activated by
Lewis acids.
The homo- and copolymerization of ethene and propene were
carried out in 125 mL glass-lined reactors and our results are
summarized in Tables 1-3. As can be seen, several systems based
on the combination of an alkylaluminum compound and a Lewis
acid are effective. Of note is the observation that methylalumi-
noxane (MAO) can act as either one of the two components
(presumably because of the presence of small amounts of
trimethylaluminum in the commercial sample). Several features
of these systems are of interest. First, the molecular weights are
high and 1H and 13C NMR spectroscopy indicates that the
polymers are highly linear. The linearity of the polyethene formed
was further supported by its high melting point (Tm > 135 °C).
The polypropene is atactic. For the ethene-propene copolymer,
the melting point was found to decrease with increasing propene
content in the copolymer.
Figure 1. 19F NMR spectra (C6D5Cl) of (a) AlEt3 + B(C6F5)3, (b) Al-
(C6F5)3, and (c) Al(C6F5)3 + 0.5AlEt3. The resonances marked with X
correspond to C6F5-Et. The resonances are referenced against CF3COOH
at -78.5 ppm.
A critically important issue that must be addressed for all
transition metal-free polymerization systems is whether trace
amounts of transition metal impurities are actually responsible
for the polymerization. For reasons given below, this appears to
be unlikely for the present systems. First, the two components
used in our systems were analyzed for Ti, Zr, and V by AA
spectroscopy and were found at levels less than the detection limit
(<20 ppm). Second, any one of the components, when used alone,
showed no polymerization activity (as anticipated slight polym-
erization activity was exhibited by MAO). Third, haloaluminum
alkyl compounds, such as AlEt2Cl and AlEtCl2, showed no activity
when used either alone or with an activator such as MAO or
B(C6F5)3. Were the alkylaluminum compound simply activating
a trace metal impurity, it seems likely that these compounds would
show some activity. Fourth, single-site catalysts do not result from
the combination of most transition metal salts and alkylaluminum
compounds.
To probe the nature of the active species, the reactions of
several trialkylaluminum compounds with B(C6F5)3 were moni-
tored by NMR spectroscopy (C6D5Cl).5 In the 1H NMR spectrum
of tri-n-octylaluminum, the R-CH2 attached to the aluminum
appeared as a triplet at 0.7 ppm, the â-CH2 appeared as a broad
multiplet at 1.7 ppm, the terminal CH3 appeared as a triplet at
1.1 ppm, and the remainder of the CH2 units appeared as a broad
resonance at 1.5 ppm. Following the addition of 1 equiv of
B(C6F5)3, the most significant change in the spectrum involved
the shift of the R-CH2 resonance from 0.7 to 1.8 ppm. The
spectrum now corresponded to that of tri-n-octylboron as con-
firmed by its independent synthesis from BH3‚THF and 1-octene!
Thus, as reported earlier,5 a near-quantitative exchange of organic
groups had occurred between aluminum and boron to form
trialkylboron and Al(C6F5)3. The same conclusion was also
reached from an examination of the 13C NMR spectrum of the
reaction mixture. Very similar results were also obtained with
triethylaluminum. Moreover, neither the combinations, triethyl-
boron and B(C6F5)3 (1:1) or triethylboron and triethylaluminum
(1:1), nor Al(C6F5)36 alone catalyzed the polymerization of ethene.
Narrow polydispersities (approximately 2) were observed for
the polyethene and polypropene formed, suggesting a single-site
catalyst. Finally, in the polymerization of ethene, the polymer
molecular weight was found to increase with increasing reaction
time (first entry in Table 1), indicating some degree of “living-
ness” to the system. This may be due to the lack of d-orbitals on
aluminum that are necessary for chain transfer through facile
â-hydrogen abstraction (â-hydrogen abstraction from neutral
aluminum alkyls occurs only at elevated temperatures). The
transfer of a growing polymer chain from a transition metal center
to aluminum to form a stable aluminum-terminated polymer has
been reported.4
(1) Ziegler, K.; Gellert, H.-G.; Zosel, K.; Holzkamp, E.; Schneider, J.; Soll,
M.; Kroll, W.-R. Justus Liebigs Ann. Chem. 1960, 121, 629.
(2) Martin, H.; Bretinger, H. Makromol. Chem. 1992, 193, 1283.
(3) (a) Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1997, 119, 8125. (b)
Cameron, P. A.; Gibson, V. C.; Redshaw, C.; Segal, J. A.; Bruce, M. D.;
White, A. J. P.; Williams, D. J. Chem. Commun. 1999, 1883.
(4) (a) Mogstad, A.-L.; Waymouth, R. M. Macromolecules 1992, 25, 2282.
(b) Chien, J. C. W.; Wang, B.-P. J. Polym. Sci., Polym. Chem. Ed. 1990, 28,
15. (c) Resconi, L.; Bossi, S.; Abis, L. Macromolecules 1990, 23, 4489.
(5) Leading references to previous reports on the interaction of trialkyl-
aluminum with borane derivatives: (a) Bochmann, M.; Sarsfield, M. J.
Organometallics 1998, 17, 5908. (b) Lee, C. H.; Lee, S. J.; Park, J. W.; Kim,
K. H.; Lee, B. Y.; Oh, J. S. J. Mol. Catal. A: Chem. 1998, 132, 231.
10.1021/ja0010960 CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/26/2000
Kim, Jang Sub
