Isospecific living polymerization of 1-hexene by a readily available nonmetallocene C2-symmetrical zirconium catalyst [2]

Full text of DOI:10.1021/ja001219g

10706
J. Am. Chem. Soc. 2000, 122, 10706-10707
The Cs-symmetry of 1a allows olefin approach from the two
possible directions in each active position without preference, thus
the polymer obtained is atactic. Therefore, we aimed at complexes
of a different symmetry which may induce tactic polymerization,
that incorporate ligands having similar functional groups yet
having a different connectivity. Our approach is based on
replacing the “branched” mode of connectivity of donor atoms
with a sequential connectivity mode, namely diamine bis-
(phenolate) ligands.
Isospecific Living Polymerization of 1-Hexene by a
Readily Available Nonmetallocene C2-Symmetrical
Zirconium Catalyst
Edit Y. Tshuva, Israel Goldberg, and Moshe Kol*
School of Chemistry
Raymond and BeVerly Sackler Faculty of Exact Sciences
Tel AViV UniVersity, Tel AViV 69978, Israel
This new family of dianionic tetradentate chelating ligands is
easily synthesized by a one-pot Mannich condensation between
readily available di(secondary) amines, formaldehyde, and sub-
stituted phenols as demonstrated in eq 2. 2, a structural isomer
ReceiVed April 7, 2000
The search for new R-olefin polymerization catalysts based
on transition metal complexes is a field of major interest involving
many academic and industrial research groups. The ligands
surrounding the metal play a crucial role in determining the
activity as well as the stereospecifity of the catalyst, by affecting
the steric and electronic properties at the metal. Over the last
two decades, this field has been dominated by the metallocene
complexes of group IV metals. Especially, ansa-metallocenes of
C2 symmetry were found to induce isospecificity in the resulting
polymers.1 Recently, there has been a growing interest in the
development of non-cyclopentadienyl ligands for the polymeri-
zation of R-olefins.2 Most attention was drawn to chelating di-
(amido) ligands,3 some of whose group IV transition metal
complexes induce polymerization in a liVing manner,3a-c whereas
chelating di(alkoxo) ligands4 drew a more limited attention. The
number of nonmetallocene systems, which were found to induce
tacticity in the resulting polymer, is, however, quite small.5 In
this communication we introduce a novel family of di(alkoxo)
complexes, one member of which is the first nonmetallocene C2-
symmetrical complex, which, upon activation, leads to a highly
isospecific living polymerization of 1-hexene.
of 1, was synthesized by mixing N,N′-dimethyl-ethylenediamine,
2 equiv of formaldehyde, and 2 equiv of 2,4-di-tert-butyl-phenol
in methanol, and heating to reflux for 2 h. 2 precipitated as a
colorless solid and was isolated in 70% yield.
Upon reaction with tetra(benzyl) zirconium, a sequential
[ONNO]2- type ligand may wrap around the metal to afford
several possible isomers, as shown in Figure 1.
Recently, we introduced the amine bis(phenolate) family of
ligands to group IV transition metals.6 We found that the presence
of an extra donor group on a sidearm leads to octahedral LigMX2-
type complexes, in which the two labile X groups are forced into
a cis geometry.6a Catalysts derived from these complexes (e.g.
1a) lead to highly reactive 1-hexene polymerization catalysts.6b
Figure 1.
(1) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth,
R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
(2) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed.
Engl. 1999, 38, 428.
trans(Bn,Bn) isomers (Figure 1), which are commonly obtained
with salen-type ligands,7 are less desired as catalysts for R-olefin
polymerization. Two alternative chelating modes that feature two
cis-benzyl ligands are the cis(Bn,Bn)-trans(O,O) mode,8 which
(3) (a) Scollard, J. D.; McConville, D. H. J. Am. Chem. Soc. 1996, 118,
10008. (b) Baumann, R.; Davis, W. M.; Schrock, R. R. J. Am. Chem. Soc.
1997, 119, 3830. (c) Jeon, Y.-M.; Park, S. J.; Heo, J.; Kim, K. Organometallics
1998, 17, 3161. (d) Gade, L. H. Chem. Commun. 2000, 173. (e) Kempe, R.
Angew. Chem., Int. Ed. Engl. 2000, 39, 468. (f) Scollard, J. D.; McConville,
D. H.; Payne, N. C.; Vittal, J. J. Macromolecules 1996, 29, 5241. (g)
Aizenberg, M.; Turculet, L.; Davis, W. M.; Schattenmann, F.; Schrock, R. R.
Organometallics 1998, 17, 4795. (h) Liang, L.-C.; Schrock, R. R.; Davis, W.
M.; McConville, D. H. J. Am. Chem. Soc. 1999, 121, 5797. (i) Tinkler, S.;
Deeth, R. J.; Duncalf, D. J.; McCamley, A. Chem. Commun. 1996, 2623. (j)
Gibson, V. C.; Kimberley, V. S.; White, A. J. P.; Williams, D. J.; Howard, P.
Chem. Commun. 1998, 313. (k) Male, N. A. H.; Thornton-Pett, M.; Bochmann,
M. J. Chem. Soc., Dalton. Trans. 1997, 2487. (l) Ziniuk, Z.; Goldberg, I.;
Kol, M. Inorg. Chem. Commun. 1999, 2, 549.
(4) (a) Tjaden, E. B.; Swenson, D. C.; Jordan, R. F.; Petersen, J. L.
Organometallics 1995, 14, 371. (b) Fokken, S.; Spaniol, T. P.; Okuda, J.;
Sernetz, F. G.; Mu¨lhaupt, R. Organometallics 1997, 16, 4240. (c) Matilainen,
L.; Klinga, M.; Leskela¨, M. J. Chem. Soc., Dalton Trans. 1996, 219.
(5) (a) van der Linden, A.; Schaverien, C. J.; Meijboom, N.; Ganter, C.;
Orpen, A. G. J. Am. Chem. Soc. 1995, 117, 3008. (b) Averbuj, C.; Tish, E.;
Eisen, M. S. J. Am. Chem. Soc. 1998, 120, 8640.
(6) (a) Tshuva, E. Y.; Versano, M.; Goldberg, I.; Kol, M.; Weitman, H.;
Goldschmidt, Z. Inorg. Chem. Commun. 1999, 2, 371. (b) Tshuva, E. Y.;
Goldberg, I.; Kol, M.; Weitman, H.; Goldschmidt, Z. Chem. Commun. 2000,
379.
(7) (a) Repo, T.; Klinga, M.; Pietikainen, P.; Leskela, M.; Uusitalo, A.-
M.; Pakkanen, T.; Hakala, K.; Altonen, P.; Lo¨fgren, B. Macromolecules 1997,
30, 171. (b) Coles, S. J.; Hursthouse, M. B.; Kelly, D. G.; Toner, A. J.; Walker,
N. M. J. Chem. Soc., Dalton. Trans. 1998, 3489. (c) Coles, S. J.; Hursthouse,
M. B.; Kelly, D. G.; Toner, A. J.; Walker, N. M. J. Organomet. Chem. 1999,
580, 304. (d) Floriani, C.; Solari, E.; Corazza, F.; Chiesi-Villa, A.; Guastini,
C. Angew. Chem., Int. Ed. Engl. 1989, 28, 64. (e) Mazzanti, M.; Rosset, J.-
M.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Chem. Soc., Dalton. Trans.
1989, 953.
(8) (a) Cross, R. J.; Farrugia, L. J.; Newman, P. D.; Peacock, R. D.; Stirling,
D. Inorg. Chem. 1999, 38, 1186. (b) Woodman, P.; Hitchcock, P. B.; Scott,
P. Chem. Commun. 1996, 2735. (c) Woodman, P. R.; Munslow, I. J.;
Hitchcock, P. B.; Scott, P. J. Chem. Soc., Dalton. Trans. 1999, 4069.
10.1021/ja001219g CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/14/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 43, 2000 10707
may result in either a C1- or a C2-symmetrical complex, and the
cis(Bn,Bn)-cis(O,O) mode,9 leading to C1-symmetrical complexes
(Figure 1). The preferred C2-symmetrical complex may induce
tacticity independently of polymeric chain rearrangement rate,10a
and may therefore be considered an ansa-metallocene analogue.1
2 reacted cleanly with 1 equiv of tetra(benzyl) zirconium at
65 °C yielding a single isomer of a dibenzyl zirconium complex
[ONNO]ZrBn2, 2a, quantitatively, as a yellow crystalline solid.
The spectral data of 2a indicated the formation of a C2-
symmetrical complex, evident from the symmetry-related phe-
nolate rings, the symmetry-related benzyl groups, and the three
AX spin systems which appear for three CH2 units. The
crystallographic analysis indicated that 2a adopts the desired
cis(Bn,Bn) C2-symmetrical structure (Figure 2).
Figure 3. Dependence of polymer weight average molecular weight (Mw)
on consumption of monomer (mg) using 32 µmol of 2a and 1.1 equiv of
B(C6F5)3 in 10 mL of neat 1-hexene at room temperature and PDI values.
A possible source of this high isotacticity is the bulk of the
t-Bu groups which direct the approaching olefin (Figure 2). To
evaluate the effect of steric bulk on the tacticity, we synthesized
the ligand precursor 3 in a similar manner (eq 1).
3 was obtained by mixing N,N′-dimethyl-ethylenediamine, 2
equiv of formaldehyde, and 2 equiv of 2,4-dimethylphenol in
methanol, and heating to reflux for 2 h. 3 precipitated as a
colorless solid and was isolated in 50% yield.
3 also reacted cleanly with 1 equiv of tetra(benzyl) zirconium
at 65 °C yielding the dibenzyl complex [ONNO]ZrBn2, 3a,
quantitatively as a yellow crystalline solid. The spectral data of
3a indicated the formation of a single isomer having analogous
symmetry to 2a.
Upon activation with tris(pentafluorophenyl)borane, 3a exhibits
a somewhat higher activity in the polymerization of 1-hexene
relative to 2a of 35 g mmolcat-1 h-1, leading to a polymer having
a molecular weight of 23 000 and a PDI of Mw/Mn ) 1.57. In
contrast to the polymer obtained from 2a, the polymer obtained
from 3a was atactic according to 13C NMR.11 Thus, the size of
the substituents was shown to have a significant influence on the
tacticity of the poly(1-hexene) produced using these C2-sym-
metrical [ONNO]Zr-type catalysts.
Figure 2. Molecular structure of 2a exhibiting the C2 symmetry and
cis(Bn,Bn) geometry (one of two molecules in the asymmetric unit shown;
H atoms and phenyl groups omitted for clarity). See Supporting
Information for complete molecular structure.
The asymmetric unit contains two homochiral molecules of
the slightly distorted octahedral complex, and three molecules of
toluene. The two nitrogen donors are in a cis configuration, as
expected, and the two oxygen atoms of the phenolate rings are
in a trans configuration. The cis configuration between the two
benzyl groups is evident from the C-Zr-C angle of 110.5°.
To the best of our knowledge, this novel [ONNO] system is
the first nonmetallocene C2-symmetrical system which is active
in isospecific and living polymerization of high-olefins.10,12
Furthermore, this is a rare example of living polymerization of
high-olefins at room temperature. The extreme ease of synthesis
of a variety of ligand precursors and the resulting metal complexes
make this new family of catalysts a potential alternative to the
well-established ansa-metallocene family. We are currently
studying the parameters responsible for the catalysts polymeri-
zation activity, as well as looking for further applications of this
ligand family.
Upon activation with tris(pentafluorophenyl)borane, 2a was
found to be an active 1-hexene polymerization catalyst. The
addition of 32 µmol of 2a and 1.1 equiv of B(C6F5)3 to neat
1-hexene at room temperature yielded 300 mg of poly(1-hexene)
after 30 min, corresponding to an activity of 18 g mmolcat-1 h-1
.
1
Olefinic termination groups were not observed in the H NMR
spectra, and the polymer obtained had a molecular weight of Mw
) 12 000 and a narrow PDI of Mw/Mn ) 1.15. The narrow PDI
and the linear dependence of the polymer molecular weight on
the consumption of the monomer shown in Figure 3 suggest that
the polymerization system is living. Furthermore, six narrow
singlets in the 13C NMR indicate an isospecific 1-hexene
polymerization, affording >95% isotactic poly(1-hexene).11
Acknowledgment. This research was supported in part by the Israel
Science Foundation founded by the Israel Academy of Sciences and
Humanities.
Supporting Information Available: Synthesis and characterization
of ligand precursors 2 and 3, complexes 2a and 3a, and poly(1-hexene);
crystal data, atomic coordinates and bond lengths and angles for 2a (PDF).
This material is available free of charge via the Internet at http://pubs.acs.org.
(9) (a) Corden, J. P.; Errington, W.; Moore, P.; Wallbridge, M. G. H. Chem.
Commun. 1999, 323. (b) Corazza, F.; Solari, E.; Floriani, C.; Chiesi-Villa,
A.; Guastini, C. J. Chem. Soc., Dalton. Trans. 1990, 1335. (c) Fairhurst, S.
A.; Hughes, D. L.; Kleinkes, U.; Leigh, G. J.; Sanders, J. R.; Weisner, J. J.
Chem. Soc., Dalton. Trans. 1995, 321.
(10) (a) Jayaratne, K. C.; Sita, L. R. J. Am. Chem. Soc. 2000, 122, 958.
(b) Fukui, Y.; Murata, M.; Soga, K. Macromol. Rapid. Commun. 1999, 20,
637.
JA001219G
(11) See 13C NMR spectra of isotactic poly(1-hexene) derived from 2a and
atactic poly(1-hexene) derived from 3a in Supporting Information.
(12) 1-Octene was also polymerized by 2a/B(C6F5)3, yielding a highly
isotactic poly(1-octene).