Salalen titanium complexes in the highly isospecific polymerization of 1-hexene and propylene

Article abstract of DOI:10.1002/anie.201007678

All lined up: C₁-symmetric octahedral titanium complexes (see structure, Ti dark gray, N blue, O red, I purple) whose labile positions reside in different electronic environments were designed using the readily available salalen ligands. With methylalumoxane as co-catalyst, highly active catalysts were obtained, which yielded high-molecular-weight polypropylene with ultra-high isotacticities (see ¹³C NMR spectrum) and melting transitions. Copyright

Full text of DOI:10.1002/anie.201007678

DOI: 10.1002/anie.201007678
Polymerization Catalysts
Salalen Titanium Complexes in the Highly Isospecific Polymerization
of 1-Hexene and Propylene**
Konstantin Press, Ad Cohen, Israel Goldberg, Vincenzo Venditto, Mina Mazzeo, and Moshe Kol*
Dedicated to Professor Adolfo Zambelli on the occasion of his 77th birthday
Isotactic polypropylene (iPP) is a thermoplastic material of
vast importance. The ever-increasing demand for it is derived
from its useful physical properties and the availability of its
feedstock, propylene. The most important microstructural
property of polypropylene is the degree of isotacticity, which,
combined with sufficiently high molecular weight, determines
its melting point (Tm) and thereby its possible applications.[1]
iPP produced by heterogeneous Ziegler–Natta catalysts has
typical Tm values not exceeding 1658C.[2–5] Homogeneous
catalysts of the metallocene,[6] and, more recently, nonmetal-
locene[7] families lead to polymers possessing narrower
molecular weight distributions. However, in spite of consid-
erable research efforts in the last 25 years, only a few such
systems were found to lead to iPP having Tm values
approaching those obtained by the heterogeneous cata-
lysts.[8,9] Herein, we describe a family of nonmetallocene
catalysts for olefin polymerization based on a new design
concept. Certain members of this family led to polypropylene
of exceptionally high isotacticities and Tm values.
C1-symmetric metallocenes bearing an overly crowded site
from which the polymeryl chain skips into the less crowded
and more directing site,[11,8d] as visualized in the appropriate
quadrant representation (Figure 1, left).[12] In designing the
current catalysts, we envisioned that a directional site
epimerization might also be promoted electronically,
namely, by placing donors of different trans influence trans
to the two coordination sites in an octahedral environment
(Figure 1, right).
Isospecific catalysts are capable of discriminating between
the two enantiotopic faces of an incoming olefin. This
differentiation is achieved through the different interactions
of these faces with the preferred conformation of the bound
polymeryl chain, which is oriented by its interactions with
substituents in the vicinity of the chiral metal environment.
C2-symmetric catalysts are relatively accessible, and their two
coordination sites are homotopic, so their induction of
isospecificity is independent of possible epimerization
events of the polymeryl chain. C1-symmetric complexes are
structurally more diverse, but the directing abilities of their
two diastereotopic sites are usually different. So, a successful
design of highly isospecific C1-symmetric catalysts should
include a directional polymeryl chain migration to the more
selective site.[10] This approach was previously developed for
Figure 1. Induction of directional polymeryl site epimerization by steric
pressure in C1-symmetric metallocenes (left) and electronic trans
influence in octahedral fac–mer complexes of tetradentate {OD1D2O}-
type ligands (right; D=donor). The bottom drawings are the corre-
sponding quadrant representations.
The tetradentate {ONNO}-type salan ligands tend to wrap
around Group 4 metals in the symmetric fac–fac mode, giving
octahedral complexes of the type [{ONNO}MX2] (X = O-iPr,
benzyl (Bn), etc.).[13] The ability of the dibenzyl complexes to
promote isospecific polymerization in the presence of coca-
talysts such as B(C6F5)3 or methylalumoxane (MAO) depends
on the steric bulk of the phenolate substituents and on the
metal.[14] Recently, we showed that C1-symmetric zirconium
complexes derived from nonsymmetrically substituted salan
ligands led to averaging of tacticities, thus implying that a
random polymeryl site epimerization was taking place.[15] To
promote a directional site epimerization, a tetradentate
ligand wrapping in a nonsymmetric manner would be
required. Thus, we turned to the salalens. Salalens are half-
salan/half-salen hybrid ligands, found to preferably wrap
around octahedral Group 4 metal centers so that the half-
salan O,N,N donors bind in a fac mode and the half-salen
O,N,N donors bind in a mer mode.[16] This fac–mer wrapping
places one coordination site trans to the neutral imine
N donor and the other site trans to the anionic phenoxy
O donor, thus satisfying the above requirement.[17] Complexes
[*] K. Press, A. Cohen, Prof. I. Goldberg, Prof. M. Kol
School of Chemistry, Tel Aviv University
Ramat Aviv, Tel Aviv 69978 (Israel)
Fax: (+972)3-640-7392
E-mail: moshekol@tau.ac.il
Prof. V. Venditto, Dr. M. Mazzeo
Dipartimento di Chimica, Universitꢀ di Salerno
Via Ponte don Melillo, 84084 Fisciano (Italy)
[**] We thank the Israel Science Foundation for financial support, Ms
Dvora Reshef (Tel Aviv) for technical assistance, Dr. Marina
Lamberti (Salerno) for valuable discussions, and Dr. Maria Grazia
Napoli (Salerno) for GPC analysis.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007678.
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3529

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of salalen ligands were reported to catalyze various trans-
size of the halide substituent, with [mmmm] = 63% (Cl), 79%
(Br), and 94% (I). Replacing the tBu substituent in Lig3 with
either a larger substituent (Ad, Lig4) or a much smaller
substituent (H, Lig5) had little effect on the isotacticity of the
resulting poly(1-hexene) ([mmmm] = 89% and 87%, respec-
tively). In other words, only the substituents of the phenolate
proximal to the amine donor play a significant stereodirecting
role under these conditions. In com-
formations, including asymmetric oxidations[18] and epoxide–
CO2 polymerization,[19] but to date they have not been
employed in olefin polymerization catalysis.
Five salalen ligand precursors were screened initially
(Lig1–5H2, Scheme 1). They were designed to include alkyl
groups of different sizes (H, tert-butyl, adamantyl) on the
parison, for C1-symmetric salan
ligands that wrap in the fac–fac
mode, the substituents on both rings
affect the isotacticity of the resulting
poly(1-hexene).[15]
The salalen complexes [Lig1–
5TiBn2] were found to be suitable for
polymerization of liquid propylene
(cryogenically condensed in a stain-
less steel reactor, thawed, and let stir
for 14 h at 258C; see the Supporting
Scheme 1. Synthesis of salalen ligands and complexes.
phenolate ring close to the imine donor and halides of
different sizes (Cl, Br, I) on the phenolate ring close to the
amine donor to enable the evaluation of their individual
effects. The ligand precursors were all synthesized by a two-
step reaction sequence consisting of condensation of the
primary amine of N-methyldiaminoethane with a substituted
salicylaldehyde and subsequent nucleophilic attack of the
secondary amine on the bromomethyl derivative of the
corresponding phenol (see the Supporting Information).
The ligand precursors reacted with tetra(isopropoxide)tita-
nium and with tetrabenzyltitanium to give mononuclear
complexes of the types [Lig1–5Ti(O-iPr)2] and [Lig1–5TiBn2],
respectively, as single diastereomers of C1 symmetry.
We propose that the salalen ligands wrap diastereospecifi-
cally in the fac–mer mode in all complexes. Single crystals of
[Lig4Ti(O-iPr)2] were grown from cold toluene. Single-crystal
X-ray crystallographic analysis revealed the expected mono-
nuclear fac–mer complex. The difference in quadrant occu-
pancy between this complex and the seemingly related
complex of the salan ligand that features the same phenolate
rings but wraps in a fac–fac mode is clearly apparent
(Figure 2).[20] The bond lengths of [Lig4Ti(O-iPr)2] were
typical of the salan and salen structural motifs. A slightly
longer bond for the isopropoxo group trans to the phenoxy
group (1.825(2) ꢀ) relative to that trans to the imine donor
(1.807(2) ꢀ) may indicate the formerꢁs weaker bonding.
The three complexes of the salalen ligands with tert-butyl-
substituted phenolates ([Lig1–3TiBn2]) exhibited mild activ-
ities in neat 1-hexene polymerization after activation with
B(C6F5)3 (4.5–18 gmmolÀ1 hÀ1 for polymerization reactions of
2–4 h). Very narrow molecular weight distributions (with
polydispersity indices (PDIs) as low as 1.04) supported the
living nature of the polymerizations. However, the very high
polymer molecular weights (Mn = 360000–390000) relative to
the calculated values (Mcalcd = 9300–48000; obtained by
dividing the amount of polymer obtained by mole of
precatalyst employed) indicate partial precatalyst activation
under these conditions. 13C NMR spectroscopy revealed that
the degree of isotacticity in these polymers depended on the
Figure 2. Left: Crystal structure of fac–mer [Lig4Ti(O-iPr)2] with iPr
groups omitted for clarity. Selected bond lengths [ꢁ]: Ti–O2 2.003(2),
Ti–O3 1.889(2), Ti–N6 2.325(3), Ti–N7 2.194(2), Ti–O4 1.807(2), Ti–
O5 1.825(2). Right: Crystal structure of the corresponding fac–fac
[{salan}Ti(O-iPr)2] complex.
Information) with 500 equivalents of MAO as cocatalyst. All
the polymer samples were obtained as crystalline solids. The
complexes containing the tert-butyl-substituted phenolates
([Lig1–3TiBn2]) yielded highly isotactic polypropylene with
[mmmm] = 90, 96, and 96% according to 13C NMR spectros-
copy and melting points of 150, 157, and 1558C. A 2:2:1 ratio
of the [mmmr], [mmrr], and [mrrm] pentad peaks for those
samples was consistent with enantiomorphic site control of
stereoregularity.[21] No regioerror or chain-end peaks could be
detected in those spectra. The complex [Lig4TiBn2], featuring
the bulky adamantyl group, yielded polypropylene of
[mmmm] ꢀ 99% and a melting transition of 1648C, which is
one of the highest ever reported for a homogeneous titanium
catalyst.[8b,22] The stereoerrors are hardly observable for this
polymer, but they are also consistent with an enantiomorphic
site-control mechanism. In contrast, the sterically unhindered
complex [Lig5TiBn2] gave rise to mostly stereoirregular
polypropylene, as evident from its very low degree of
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Angew. Chem. Int. Ed. 2011, 50, 3529 –3532

crystallinity and its 13C NMR spectrum. All polymers had high
molecular weights and narrow molecular weight distributions,
as expected for homogeneous systems.
Notably, the ligand substituents have different effects on
stereospecificities in polymerization of the two monomers,
evident in particular for the adamantyl group. Most unusually,
the higher isotacticities of polypropylene relative to poly(1-
hexene) are opposite to the common trend of substantially
lower stereocontrol for the “slimmer” olefin.[14c] To test
whether the change of cocatalyst was responsible for this
opposite trend, we repeated the polymerizations of 1-hexene
with [Lig1–5TiBn2], this time employing 500 equivalents of
MAO as cocatalyst. Very high activities of up to
11700 gmmolÀ1 hÀ1 were found (boiling of the monomer
within a few seconds after the addition of MAO to the neat
monomer was observed), and, except for [Lig5TiBn2], all
precatalysts led to poly(1-hexene) of higher isotacticities
([mmmm] = 76–96%) relative to those obtained with B-
(C6F5)3 as cocatalyst. We found that as little as 50 equivalents
of MAO were sufficient to produce highly active catalysts
(For [Lig4TiBn2]: 600 gmmolÀ1 hÀ1; [mmmm] > 99%). The
narrow molecular weight distributions and high molecular
weights testify to the single-site nature of the catalysts and the
negligible chain transfer to MAO. We presume that the
enhancement of isospecificity (and activity) by employing
MAO rather than B(C6F5)3 as cocatalyst results from different
catalyst–counteranion interactions,[23] which have been pro-
posed to affect site epimerization rates.[24,25]
Figure 3. 13C NMR spectrum (C6D4Cl2) of polypropylene prepared with
[Lig6TiBn2]/neat propylene/MAO. Inset: Expansion of the methyl
region. The peaks in the vicinity of the mmmm peak are the 13
satellites. [mmmm]=99.6%.
C
catalysts and measured by a standard differential scanning
calorimetry (DSC) protocol (see the Supporting Informa-
tion).[5] It implies that the polypropylene obtained is of the
highest degrees of regio- and stereoregularity reported to
date.
In conclusion, we have introduced a new family of
octahedral C1-symmetric titanium catalysts for isospecific
polymerization, relying on the readily available and structur-
ally diverse salalen ligands. The ability to control the degree
of isotacticity, and, in particular, the synthesis of polyprop-
ylene samples with extraordinarily high isotacticities and
melting transitions indicates that the wealth of structural
motifs in nonmetallocenes is far from exhausted. Such
catalysts are expected to produce polymeric materials not
accessible with more traditional catalysts. Mechanistic studies
aimed at revealing the involvement of site epimerization,
further catalyst development based on the concepts intro-
duced herein, and application of these catalysts in other
polymerizations are underway.
We found that polypropylene of even higher isotacticities
could be obtained by assembling the salalen ligands around
the chiral rigid aminomethylpyrrolidine backbone. For exam-
ple, Lig6H2, a salalen ligand precursor featuring the bulky
adamantylphenolate on the imine side arm and a dibromo-
phenolate on the amine side arm, led to [Lig6TiBn2] as a single
diastereomer (Scheme 2). Polymerization of liquid propylene
Received: December 7, 2010
Published online: February 21, 2011
Keywords: catalyst design · N,O ligands · polymerization ·
.
tacticity · titanium
Scheme 2. Structure and catalytic performance of [Lig6TiBn2].
[1] V. Busico, R. Cipullo, Prog. Polym. Sci. 2001, 26, 443 – 533.
[2] J. Severn, R. L. Jones, Jr., “Stereospecific a-Olefin Polymeri-
zation with Heterogeneous Catalysts” in Handbook of Transi-
tion Metal Polymerization Catalysts (Eds.: R. Hoff, R. T.
Mathers), Wiley, Hoboken, 2010, pp. 157 – 230.
[3] R. Paukkeri, A. Lehtinen, Polymer 1994, 35, 1673 – 1679.
[4] Tuning the polymerization conditions can lead to polypropylene
having a Tm value of 1688C by heterogeneous catalysts. See: P.
Viville, D. Daoust, A. M. Jonas, B. Nysten, R. Legras, M. Dupire,
J. Michel, G. Debras, Polymer 2001, 42, 1953 – 1967.
[5] Tm values of iPP exceeding 1708C were reported in the early
literature. A plausible explanation for the discrepancy with the
currently accepted Tm values of iPP is that the early measure-
ments were based on polarized light microscopy, whereas
modern Tm values are based on DSC measurements (second
yielded crystalline polypropylene with an extremely high
degree of stereoregularity ([mmmm] = 99.6%) and an even
higher degree of regioregularity (no observable peaks
between d = 30 and 45 ppm), as apparent from its 13C NMR
spectrum (Figure 3). Correspondingly, it exhibited a very high
Tm of 168.38C. For polymerization in toluene solution
(33.5 psigpropylene, 500 equiv MAO, RT), this catalyst exhibited
a very high activity exceeding 10000 gmmolÀ1 hÀ1, and the
isotactic polypropylene formed had a Tm of 169.98C. To our
knowledge, this is the highest melting transition reported for
“as prepared” (not extracted or annealed) isotactic polyprop-
ylene produced by either heterogeneous or homogeneous
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Communications
heating 108CminÀ1). See: G. Natta, I. Pasquon, A. Zambelli, G.
Gatti, J. Polym. Sci. 1961, 51, 387 – 398.
Kopilov, M. Lamberti, V. Venditto, M. Kol, Macromolecules
2010, 43, 1689 – 1691; d) V. Busico, R. Cipullo, R. Pellecchia, S.
Ronca, G. Roviello, G. Talarico, Proc. Natl. Acad. Sci. USA 2006,
103, 15321 – 15326; e) R. Cipullo, V. Busico, N. Fraldi, R.
Pellecchia, G. Talarico, Macromolecules 2009, 42, 3869 – 3872.
[15] a) A. Cohen, A. Yeori, J. Kopilov, I. Goldberg, M. Kol, Chem.
Commun. 2008, 2149 – 2151; b) A. Cohen, J. Kopilov, I. Gold-
berg, M. Kol, Organometallics 2009, 28, 1391 – 1405.
[16] A. Yeori, S. Gendler, S. Groysman, I. Goldberg, M. Kol, Inorg.
Chem. Commun. 2004, 7, 280 – 282.
[6] a) H. H. Brintzinger, D. Fischer, R. Mꢂlhaupt, B. Rieger, R. M.
Waymouth, Angew. Chem. 1995, 107, 1255 – 1283; Angew. Chem.
Int. Ed. Engl. 1995, 34, 1143 – 1170; b) L. Resconi, L. Cavallo, A.
Fait, F. Piemontesi, Chem. Rev. 2000, 100, 1253 – 1345.
[7] a) G. J. P. Britovsek, V. C. Gibson, D. F. Wass, Angew. Chem.
1999, 111, 448 – 468; Angew. Chem. Int. Ed. 1999, 38, 428 – 447;
b) V. C. Gibson, S. K. Spitzmesser, Chem. Rev. 2003, 103, 283 –
315.
[8] For selected metallocene-type catalysts leading to isotactic
polypropylene with high Tm values, see: a) H. Deng, H.
Winkelbach, K. Taeji, W. Kaminsky, K. Soga, Macromolecules
1996, 29, 6371 – 6376; b) J. A. Ewen, A. Zambelli, P. Longo, J. M.
Sullivan, Macromol. Rapid Commun. 1998, 19, 71 – 73; c) J. A.
Ewen, M. J. Elder, R. L. Jones, A. L. Rheinegold, L. M. Liable-
Sands, R. D. Sommer, J. Am. Chem. Soc. 2001, 123, 4763 – 4773;
d) S. A. Miller, J. E. Bercaw, Organometallics 2006, 25, 3576 –
3592.
[9] For nonmetallocene catalysts leading to isotactic polypropylene
with high Tm values, see: a) A. V. Prasad, H. Makio, J. Saito, M.
Onda, T. Fujita, Chem. Lett. 2004, 33, 250 – 251; b) T. R. Boussie,
O. Brummer, G. M. Diamond, A. L. LaPointe, M. K. Leclerc, C.
Micklatcher, P. Sun, X. Bei, U.S. Patent, US 7,241,714 B2, 2007;
c) E. T. Kiesewetter, S. Randoll, M. Radlauer, R. M. Waymouth,
J. Am. Chem. Soc. 2010, 132, 5566 – 5567; d) R. L. Jones, N.
Seidel, M. J. Elder, J. A. Ewen, U.S. Patent, US 7,544,749 B2,
2009.
[17] While a change of wrapping tendency of the salalen ligands upon
formation of the cationic active species is not excluded, the high
activity (consistent with a cis relationship between active sites),
and the very high stereoregularity (consistent with non-averag-
ing of tacticities, see below) both support the preservation of the
fac–mer wrapping.
[18] K. Matsumoto, B. Saito, T. Katsuki, Chem. Commun. 2007,
3619 – 3627.
[19] K. Nakano, M. Nakamura, K. Nozaki, Macromolecules 2009, 42,
6972 – 6980.
[20] CCDC 802925 and 802926 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[21] G. W. Coates, Chem. Rev. 2000, 100, 1223 – 1252.
[22] A dibenzyltitanium complex of a related salalen ligand (R1 =
adamantyl, R2 = Me, R3 = Cl) yielded polypropylene having a
melting point of 1668C ([mmmm] > 99%, Mn = 430000, Mw =
787000), which is the highest reported for isotactic polypropy-
lene derived from homogeneous titanium catalysts, to our
knowledge. It implies that combining the bulky iodo and
adamantyl groups in [Lig4TiBn2] induces quadrant overcrowd-
ing, which reduces stereospecificity.
[10] For 13C NMR spectroscopy evidence for the C1 symmetry of
active species in MgCl2-supported Ziegler–Natta catalysts, see:
V. Busico, R. Cipullo, G. Talarico, A. L. Segre, J. C. Chadwick,
Macromolecules 1997, 30, 4786 – 4790.
[11] a) J. A. Ewen, M. J. Elder, R. L. Jones, L. Haspeslagh, J. L.
Atwood, S. G. Bott, K. Robinson, Makromol. Chem. Macromol.
Symp. 1991, 48–9, 253 – 295; b) A. Razavi, U. Thewalt, Coord.
Chem. Rev. 2006, 250, 155 – 169.
[23] J. A. S. Roberts, M.-C. Chen, A. M. Seyam, L. Li, C. Zuccaccia,
N. G. Stahl, T. J. Marks, J. Am. Chem. Soc. 2007, 129, 12713 –
12733.
[12] A. Poater, L. Cavallo, Dalton Trans. 2009, 8878 – 8883.
[13] E. Y. Tshuva, I. Goldberg, M. Kol, J. Am. Chem. Soc. 2000, 122,
10706 – 10707.
[14] a) S. Segal, I. Goldberg, M. Kol, Organometallics 2005, 24, 200 –
202; b) S. Gendler, A. L. Zelikoff, J. Kopilov, I. Goldberg, M.
Kol, J. Am. Chem. Soc. 2008, 130, 2144 – 2145; c) A. Cohen, J.
[24] a) M. Bochmann, J. Organomet. Chem. 2004, 689, 3982 – 3998;
b) G. Bellachioma, G. Ciancaleoni, C. Zuccaccia, D. Zuccaccia,
A. Macchioni, Coord. Chem. Rev. 2008, 252, 2224 – 2238.
[25] It should be noted, though, that aluminum cocatalysts are known
to react with imine functionalities of chelating ligands and affect
stereoselectivity. See Ref. [9a].
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