C5Me5/ER-ligated samarium(II) complexes with the neutral 'C5Me5M' ligand (ER = OAr, SAr, NRR', or PHAr; M = K or Na): A unique catalytic system for polymerization and block-copolymerization of styrene and ethylene

Article abstract of DOI:10.1021/ja002305j

Reactions of (C₅Me₅)₂Sm(THF)₂ with 1 equiv of K(ER) in THF gave in high yields the Sm(II) complexes [(C₅Me₅)Sm(THF)(m)(ER)(μ-C₅Me₅)K(THF)(n)]∞ (m = 0 or 1; n = 1 or 2; ER = OC₆H₂(t)Bu₂-2,6-Me-4 (1a), OC₆H₃(i)Pr₂-2,6 (1b), SC₆H₂(i)Pr₃-2,4,6 (1c), NHC₆H₂(t)Bu₃-2,4,6 (1d), or N(SiMe₃)₂ (1e)), in which the '(C₅Me₅)K(THF)(n)' unit acts as a neutral coordination ligand bonding to the Sm(II) center with the 'C₅Me₅' part. The similar reaction of (C₅Me₅)₂Yb(THF)₂ with 1 equiv of KN(SiMe₃)₂ yielded the corresponding Yb(II) complex [(C₅Me₅)Yb(N(SiMe₃)₂)(μ-C₅Me₅)K(THF)₂]∞ (1f) in 90% yield. These complexes all adopt a similar polymeric structure via 'intermolecular' interactions between the K atom and a C₅Me₅ ligand. The analogous reaction of (C₅Me₅)₂Sm(THF)₂ with 1 equiv of KPHAr afforded [(C₅Me₅)Sm(THF)(μ-PHAr)K-(C₅Me₅)(THF)]∞ (1g, Ar = C₆H₂(t)Bu₃-2,4,6), in which the 'C₅Me₅K' unit is bonded to the phosphide site with its K atom. The reaction of the silylene-linked bis(tetramethylcyclopentadienyl) samarium(II) complex Me₂Si(C₅Me₄)₂Sm(THF)₂ with 1 equiv of KOAr in THF yielded [Me₂Si(C₅Me₄)(μ-C₅Me₄)K(THF)(n)Sm(OAr)]∞ (Ar = C₆H₂(t)Bu₂-2,6-Me-4 (1h, n = 2) or C₆H₃(t)Bu₂-2,6 (1i, n = 1)), which can be viewed as a C₅Me₄/OAr-ligated Sm(II) species coordinated by the silylene-linked, neutral 'C₅Me₄K' ligand. The use of NaN(SiMe₃)₂ in place of KN(SiMe₃)₂ in the reactions with (C₅Me₅)₂Ln(THF)₂ afforded the 'C₅Me₅Na(THF)₃'-coordinated, 'monomeric' Ln(II) complexes (C₅Me₅)Ln(N(SiMe₃)₂)(μ-C₅Me₅)Na(THF)₃ (Ln = Sm (1j) or Yb (1k)). Reactions of the polymeric complexes 1a,e with 2 equiv of HMPA (per Sm) in THF yielded the corresponding HMPA-coordinated, monomeric Sm(II) complexes (C₅Me₅)Sm(ER)(HMPA)₂ (ER = OC₆H₂(t)Bu₂-2,6-Me-4 (2a), N(SiMe₃)₂ (2e)). This type of C₅Me₅/ER-ligated Sm(II) complexes, particularly 1a-c, showed unique reactivity toward styrene and ethylene, which can not only polymerize styrene and ethylene but also copolymerize them into block styrene-ethylene copolymers under the presence of both monomers. The less reducing Yb(II) complex 1f or the silylene-linked cyclopentadienyl Sm(II) complex 1h did not show an activity for the polymerization of ethylene under the same conditions, suggesting that the polymerization reaction in the present systems is initiated by dissociation of the neutral 'C₅Me₅M' ligand (M = K or Na) from the Sm(II) center, followed one-electron transfer from the resultant C₅Me₅/ER-ligated Sm(II) species to an incoming monomer. As a leaving group, 'C₅Me₅K' seemed more suitable than 'C₅Me₅Na'. Among the ER ligands, the thiolate ligand SC₆H₂(i)Pr₃-2,4,6 (1c) showed the highest selectivity for the block copolymerization of styrene and ethylene, while the aryloxide OC₆H₂(t)Bu₂-2,6-Me-4 (1a) and the silylamide N(SiMe₃)₂ (1e) gave the highest activity for the polymerization of ethylene and that of styrene, respectively. Possible mechanisms for the polymerization and copolymerization reactions are proposed.

Full text of DOI:10.1021/ja002305j

J. Am. Chem. Soc. 2000, 122, 10533-10543
10533
C5Me5/ER-Ligated Samarium(II) Complexes with the Neutral
“C5Me5M” Ligand (ER ) OAr, SAr, NRR′, or PHAr; M ) K or
Na): A Unique Catalytic System for Polymerization and
Block-Copolymerization of Styrene and Ethylene
Zhaomin Hou,*,† Yugen Zhang,† Hiroaki Tezuka,‡ Peng Xie,† Oliver Tardif,†
Take-aki Koizumi,† Hiroshi Yamazaki,‡ and Yasuo Wakatsuki*,†
Contribution from the Organometallic Chemistry Laboratory, RIKEN (The Institute of Physical
and Chemical Research), Hirosawa 2-1, Wako, Saitama 351-0198, Japan, and Department of
Applied Chemistry, Chuo UniVersity, Kasuga 1-13-27, Bunkyo, Tokyo 112-0003, Japan
ReceiVed June 26, 2000
Abstract: Reactions of (C5Me5)2Sm(THF)2 with 1 equiv of K(ER) in THF gave in high yields the Sm(II)
t
complexes [(C5Me5)Sm(THF)m(ER)(µ-C5Me5)K(THF)n]∞ (m ) 0 or 1; n ) 1 or 2; ER ) OC6H2 Bu2-2,6-
i
i
t
Me-4 (1a), OC6H3 Pr2-2,6 (1b), SC6H2 Pr3-2,4,6 (1c), NHC6H2 Bu3-2,4,6 (1d), or N(SiMe3)2 (1e)), in which
the “(C5Me5)K(THF)n” unit acts as a neutral coordination ligand bonding to the Sm(II) center with the
“C5Me5” part. The similar reaction of (C5Me5)2Yb(THF)2 with 1 equiv of KN(SiMe3)2 yielded the corresponding
Yb(II) complex [(C5Me5)Yb(N(SiMe3)2)(µ-C5Me5)K(THF)2]∞ (1f) in 90% yield. These complexes all adopt a
similar polymeric structure via “intermolecular” interactions between the K atom and a C5Me5 ligand. The
analogous reaction of (C5Me5)2Sm(THF)2 with 1 equiv of KPHAr afforded [(C5Me5)Sm(THF)(µ-PHAr)K-
t
(C5Me5)(THF)]∞ (1g, Ar ) C6H2 Bu3-2,4,6), in which the “C5Me5K” unit is bonded to the phosphide site with
its K atom. The reaction of the silylene-linked bis(tetramethylcyclopentadienyl) samarium(II) complex
Me2Si(C5Me4)2Sm(THF)2 with 1 equiv of KOAr in THF yielded [Me2Si(C5Me4)(µ-C5Me4)K(THF)nSm(OAr)]∞
t
t
(Ar ) C6H2 Bu2-2,6-Me-4 (1h, n ) 2) or C6H3 Bu2-2,6 (1i, n ) 1)), which can be viewed as a C5Me4/OAr-
ligated Sm(II) species coordinated by the silylene-linked, neutral “C5Me4K” ligand. The use of NaN(SiMe3)2
in place of KN(SiMe3)2 in the reactions with (C5Me5)2Ln(THF)2 afforded the “C5Me5Na(THF)3”-coordinated,
“monomeric” Ln(II) complexes (C5Me5)Ln(N(SiMe3)2)(µ-C5Me5)Na(THF)3 (Ln ) Sm (1j) or Yb (1k)).
Reactions of the polymeric complexes 1a,e with 2 equiv of HMPA (per Sm) in THF yielded the corresponding
t
HMPA-coordinated, monomeric Sm(II) complexes (C5Me5)Sm(ER)(HMPA)2 (ER ) OC6H2 Bu2-2,6-Me-4 (2a),
N(SiMe3)2 (2e)). This type of C5Me5/ER-ligated Sm(II) complexes, particularly 1a-c, showed unique reactivity
toward styrene and ethylene, which can not only polymerize styrene and ethylene but also copolymerize them
into block styrene-ethylene copolymers under the presence of both monomers. The less reducing Yb(II) complex
1f or the silylene-linked cyclopentadienyl Sm(II) complex 1h did not show an activity for the polymerization
of ethylene under the same conditions, suggesting that the polymerization reaction in the present systems is
initiated by dissociation of the neutral “C5Me5M” ligand (M ) K or Na) from the Sm(II) center, followed
one-electron transfer from the resultant C5Me5/ER-ligated Sm(II) species to an incoming monomer. As a
leaving group, “C5Me5K” seemed more suitable than “C5Me5Na”. Among the ER ligands, the thiolate ligand
i
SC6H2 Pr3-2,4,6 (1c) showed the highest selectivity for the block copolymerization of styrene and ethylene,
t
while the aryloxide OC6H2 Bu2-2,6-Me-4 (1a) and the silylamide N(SiMe3)2 (1e) gave the highest activity for
the polymerization of ethylene and that of styrene, respectively. Possible mechanisms for the polymerization
and copolymerization reactions are proposed.
Introduction
received much interest as homogeneous catalysts or precatalysts
for various useful transformations such as hydrogenation,7
hydrosilylation,8 and cyclization9 of olefins as well as polym-
erization and copolymerization of ethylene,4,5a,6a,10 butadiene,11
acrylates,2b,d,e,12 and lactones.13 Some of the lanthanide catalytic
Changing the ligand environment of a metal complex to
modify its properties is an important strategy for the develop-
ment of more efficient or selective catalysts. In the past two
decades, extensive studies have been carried out on lanthanide
complexes bearing two substituted or unsubstituted cyclopen-
tadienyl ligands, and a variety of such lanthanide metallocene
complexes have been synthesized and structurally character-
ized.1,2 Particularly, divalent samarium metallocene complexes
of types A3 and B4 (Figure 1) and trivalent lanthanide metal-
locene alkyl or hydride complexes of types C5 and D6 have
(1) For comprehensive reviews on organolanthanide chemistry, see: (a)
Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. ReV. 1995,
95, 865. (b) Edelmann, F. T. In ComprehensiVe Organometallic Chemistry
II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Lappert, M. F., Eds.;
Pergamon: Oxford, 1995; Vol. 4, p 11. (c) Schaverien, C. J. AdV.
Organomet. Chem. 1994, 36, 283. (d) Evans, W. J. Polyhedron 1987, 6,
803. (e) Evans, W. J. AdV. Organomet. Chem. 1985, 24, 131. (f) Marks, T.
J.; Ernst, R. D. In ComprehensiVe Organometallic Chemistry; Wilkinson,
G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 3,
p 173.
† RIKEN.
‡ Chuo University.
10.1021/ja002305j CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/14/2000

10534 J. Am. Chem. Soc., Vol. 122, No. 43, 2000
Hou et al.
Figure 1. Some representative lanthanide metallocene complexes.
systems display advantages over analogous d-block transition
metal-based catalysts such as extremely high activity and
stereoselectivity, good living character in polymerization, and
no requirement for a cocatalyst or activator. However, the olefin
polymerization efficiency of the lanthanide metallocene catalysts
has been largely limited to sterically unhindered monomers
because of the high degree of steric saturation required to stabi-
lize the large and highly reactive metal centers.14 Polymerization
of styrene by the lanthanide metallocene complexes has proved
to be more difficult.10c,15,16 The reaction of (C5Me5)2Sm with
an excess of styrene in toluene yielded a stable bimetallic
complex which was inert toward styrene.15a Polymerization of
ethylene by (C5Me5)2SmH in the presence of styrene incorpo-
rated, at maximum, no more than two molecules of styrene per
polyethylene chain while successive insertion of styrene was
not observed,10c owing to the steric hindrance of the two bulky
C5Me5 ligands. Linking the cyclopentadienyl rings with a
silylene group provided a more open ligand sphere (type D)6a
and enabled random incorporation of more styrene into ethylene
oligomers,17 but the activity as well as the molecular weight of
the resultant copolymers was low.
(2) For reviews on lanthanide metallocene catalysis in polymerization
and organic synthesis, see: (a) Hou, Z.; Wakatsuki, Y. In Science of
Synthesis; Noyori, R., Imamoto, T., Eds.; Thiem: Stuttgart, Vol. 2, in press.
(b) Yasuda, H. Top. Organomet. Chem. 1999, 2, 255. (c) Molander, G. A.,
Dowdy, E. C. Top. Organomet. Chem. 1999, 2, 119. (d) Yasuda, H.; Ihara,
E. Bull. Chem. Soc. Jpn. 1997, 70, 1745. (e) Boffa, L. S.; Novak, B. M.
Tetrahedron 1997, 53, 15367. (f) Anwander, R. In Applied Homogeneous
Catalysis with Organometallic Compounds; Cornils, B., Hermann, W. A.,
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G. W. Acc. Chem. Res. 1985, 18, 51.
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E.; Atwood, J. L. J. Am. Chem. Soc. 1985, 107, 941. (b) Evans, W. J.;
Hughes, L. A.; Hanusa, T. P. Organometallics 1986, 5, 1285.
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Yamagashira, M.; Hashimoto, H.; Kanehisa, N.; Kai, Y. Organometallics
1998, 17, 3945. (b) Ihara, E.; Nodono, M.; Katsura, K.; Yasuda, H.;
Kanehisa, N.; Kai, Y. Macromol. Chem. Phys. 1996, 197, 1909.
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H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091. (b) den Haan, K. H.;
der Boer, J. L.; Teuben, J. H.; Spek, A. L.; Kojic-Prodic, B.; Hays, G. R.;
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M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer,
W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203. (d) Heeres, H. J.;
Renkema, J.; Booij, M.; Meetsma, A.; Teuben, J. H. Organometallics 1988,
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tallics 1989, 8, 17.
(6) (a) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks,
T. J. J. Am. Chem. Soc. 1985, 107, 8103. (b) Coughlin, E. B.; Bercaw, J.
E., J. Am. Chem. Soc. 1992, 114, 7606. (c) Giardello, M. A.; Conticello,
V. P.; Brard, L.; Sabat, M.; Rheingold, A. L.; Stern, C. L.; Marks, T. J. J.
Am. Chem. Soc. 1994, 116, 10212. (d) Mitchell, J. P.; Hajela, S.; Brookhart,
S. K.; Hardcastle, K. I.; Henling, L. M.; Bercaw, J. E. J. Am. Chem. Soc.
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(7) (a) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T.
J. J. Am. Chem. Soc. 1985, 107, 8111. (b) Molander, G. A.; Hoberg, J. O.
J. Org. Chem. 1992, 57, 3266. (c) Giardello, M. A.; Conticello, V. P.; Brard,
L.; Gagne´, M. R.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10241.
(8) (a) Sakakura, T.; Lautenschlager, H.-J.; Tanaka, M., J. Chem. Soc.,
Chem. Commun. 1991, 40. (b) Molander, G. A.; Julius, M., J. Org. Chem.
1992, 57, 6347. (c) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem.
Soc. 1995, 117, 7157. (d) Molander, G. A.; Dowdy, E. D.; Noll, B. C.
Organometallics 1998, 17, 3754.
Complexes supported by mixed C5Me5/ER ligands (ER ) a
monodentate anionic ligand) (types E18 and F19) are anticipated
to offer a further more open ligand sphere and thus to show
(10) (a) Watson, P. L.; Herskovits, T. ACS Symp. Ser. 1983, 212, 459.
(b) Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. J. Am.
Chem. Soc. 1990, 112, 1566. (c) Evans, W. J.; DeCoster, D. M.; Greaves,
J. Macromolecules 1995, 28, 7929. (d) Evans, W. J.; Keyer, R. A.; Ziller,
J. W. J. Organomet. Chem. 1990, 394, 87. (e) Evans, W. J.; Ulibarri, T.
A.; Ziller, J. W. J. Am. Chem. Soc. 1990, 112, 2314.
(11) Kaita, K.; Hou, Z.; Wakatsuki, Y. Macromolecules 1999, 32, 9078.
(12) Giardello, M. A.; Yamamoto, Y.; Brard, L.; Marks, T. J. J. Am.
Chem. Soc. 1995, 117, 3276.
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Y. Macromolecules 1999, 32, 8245. (b) Boffa, L. S.; Novak, B. M.
Macromolecules 1997, 30, 3494. (c) Yamashita, M.; Takemoto, Y.; Ihara,
E.; Yasuda, H. Macromolecules 1996, 29, 1798. (d) Boffa, L. S.; Novak,
B. M. Macromolecules 1994, 27, 6993. (e) Evans, W. J.; Katsumata, H.
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molecules 1994, 27, 4011.
(14) Some examples of polymerization of R-olefins by modified lan-
thanide metallocene complexes were reported. See refs 4, 18, 19b.
(15) (a) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc.
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t

Block-Copolymerization of Styrene and Ethylene
J. Am. Chem. Soc., Vol. 122, No. 43, 2000 10535
Scheme 1. Synthesis of the C5Me5/ER-Ligated Lanthanide(II) Complexes with the Neutral “C5Me5K” Ligand
higher activity toward sterically demanding monomers. How-
ever, only a few of this type of half-sandwich lanthanide
complexes have been synthesized and have had their reactivity
studied. One such a complex, [Me2Si(C5Me4)(NtBu)Y(µ-C6H13)-
(THF)x]2 (x < 1), has been recently reported to initiate styrene
polymerization to give atactic polystyrenes with narrow mo-
lecular weight distributions, but its activity remained low
because of the presence of the strongly coordinating THF ligand
which obstructs the access of a monomer to the metal center.19f
Divalent lanthanide complexes with mixed C5Me5/ER ligands
are more rare, owing to easy ligand redistribution to yield the
corresponding homoleptic compounds.20 It is apparent that to
create a C5Me5/ER-supported, highly reactive lanthanide system,
the use of an additional, easily dissociable stabilizing ligand is
Figure 2. Extended X-ray structure of 1b.
desired.
During our previous studies on lanthanide(II) aryloxide com-
t
i
i
ligand (ER ) OC6H2 Bu2-2,6-Me-4, OC6H3 Pr2-2,6, SC6H2 Pr3-
plexes,20,21 we serendipitously found that the “C5Me5K” unit
could act as a good neutral coordination ligand to stabilize
Sm(II) complexes with mixed C5Me5/OAr ligands (Ar )
t
t
2,4,6, NHC6H2 Bu3-2,4,6, N(SiMe3)2, or PHC6H2 Bu3-2,4,6; M
) K or Na). Insights into the mechanistic aspects of the
polymerization/copolymerization reactions are also described.
A portion of this work has been communicated previously.22,23
C6H2 Bu2-2,6-X-4; X ) H, Me, or tBu) (type G, ER ) OAr).20
t
This type of complexes showed high activity not only for
polymerization of styrene and ethylene but also for copolym-
erization of these two monomers, possibly as a result of
dissociation of the “C5Me5K” ligand from the C5Me5/OAr-
ligated Sm(II) center.22 To see if this unusual “C5Me5K” ligation
could be generally utilized to stabilize analogous lanthanide-
(II) complexes with other monodentate anionic ligands and to
probe the ligand effects on the reactivity of the mixed-ligand-
supported Ln(II) species, we have examined various monoden-
tate anionic ligands including aryloxides, thiolates, amides, and
phosphides. In this paper, we present a full account of the
synthesis, structural features, and styrene/ethylene polymeriza-
tion and copolymerization reactions of a series of C5Me5/ER-
supported lanthanide(II) complexes with the neutral “C5Me5M”
Results and Discussion
C5Me5/ER-Ligated Lanthanide(II) Complexes with the
“C5Me5K” Ligand. As previously reported,20 the reaction of
(C5Me5)2Sm(THF)2 with 1 equiv of KOAr in THF provided
the most efficient route to the C5Me5/OAr-ligated Sm(II)
complex with the neutral “C5Me5K(THF)2” ligand (1a, Ar )
t
C6H2 Bu2-2,6-Me-4) (Scheme 1). The use of the smaller
i
t
aryloxide KOC6H3 Pr2-2,6 in place of the bulkier KOC6H2 Bu2-
i
2,6-Me-4 in this reaction afforded the C5Me5/OC6H3 Pr3-2,6-
supported Sm(II) complex 1b, which also contains the
“C5Me5K(THF)2” unit as a stabilizing ligand. The overall struc-
ture of 1b is essentially the same as that of 1a, except that 1b
bears an extra THF ligand on the Sm(II) center owing to the
(19) (a) Piers, W. E.; Shapiro, P. J.; Bunel, E. E.; W. D.; Bercaw, J. E.
Synlett 1990, 74. (b) Shapiro, P. J.; Cotter, W. D.; Schaefer, W. P.; Labinger,
J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1994, 116, 4623. (c) Tian, S.;
Arrendondo, V. M.; Stern, C. L.; Marks, T. J. Organometallics 1999, 18,
2568. (d) Tian, S.; McDonald, F. E.; Stern, C. L.; Marks, T. J. J. Am. Chem.
Soc. 1999, 121, 3633. (e) Hultzsch, K. C.; Spaniol, T. S.; Okuda, J. Angew.
Chem., Int. Ed. 1999, 38, 227. (f) Hultzsh, K. C.; Voth, P.; Beckerle, K.;
Spaniol, T. P.; Okuda, J. Organometallics 2000, 19, 228.
i
smaller bulkiness of the OC6H3 Pr2-2,6 ligand (Figure 2). Upon
evacuation, two of the three THF ligands in 1b were removed
to give 1b′,24 as shown by elemental analyses. Analogous reac-
tion of (C5Me5)2Sm(THF)2 with 1 equiv of KSAr yielded the
corresponding C5Me5/SAr-supported Sm(II) complex 1c (Ar )
i
C6H2 Pr3-2,4,6), which bears one THF and the “C5Me5K(THF)”
(20) Hou, Z.; Zhang, Y.; Yoshimura, T.; Wakatsuki, Y. Organometallics
1997, 16, 2963.
unit as additional stabilizing ligands (Scheme 1 and Figure 3).
(21) (a) Hou, Z.; Fujita, A.; Zhang, Y.; Miyano, T.; Yamazaki, H.;
Wakatsuki, Y. J. Am. Chem. Soc. 1998, 120, 754. (b) Hou, Z.; Fujita, A.;
Yoshimura, T.; Jesorka, A.; Zhang, Y.; Yamazaki, H.; Wakatsuki, Y. Inorg.
Chem. 1996, 35, 7190. (c) Hou, Z.; Miyano, T.; Yamazaki, H.; Wakatsuki,
Y. J. Am. Chem. Soc. 1995, 117, 4421.
(22) Hou, Z.; Tezuka, H.; Zhang, Y.; Yamazaki, H.; Wakatsuki, Y.
Macromolecules 1998, 31, 8650.
One of the two THF ligands in 1c could also be removed under
(23) Part of this work was also presented at the 22nd Rare Earth Research
Conference, Argonne, July 10-15, 1999 (Abstract No. E-2). See also: Hou,
Z.; Wakatsuki, Y. J. Alloys Compd. 2000, 203-204, 75.
(24) The accurate structures of 1b′,c′,j′,k′ were not determined. Their
formulations were based on elemental analyses.

10536 J. Am. Chem. Soc., Vol. 122, No. 43, 2000
Hou et al.
unit is bonded to the Ln(II) center with the C5Me5 part, the
“C5Me5K” unit in 1g is bonded to the phosphide site with its K
atom (Figure 4),28 which is probably due to the stronger electron-
donating ability of the phosphide ligand. The Sm-C5Me5 bond
distances in 1g (av 2.862(9) and 2.923(8) Å) are comparable
with those found in 1a-e (2.83-2.98 Å) (Table 1), while the
Sm-P bond in 1g (3.234(2) Å) is significantly longer than those
found in samarium(II) bis(phosphide) complexes such as Sm-
(PAr2)2(THF)4 (3.139(3) Å for Ar ) Ph29a and 3.034(2) Å for
Ar ) C6H2Me3-2,4,6)29b and Sm(η1-PC12H8)(THF)4 (3.1908-
(6) Å),29c probably owing to the interaction between the
phosphide ligand and the K atom in 1g. The K-P bond distance
in 1g (3.350(3) Å) is comparable with those found in the
t
polymeric potassium phosphide compound [KPH(C6H2 Bu3-
2,4,6)]∞ (3.181(2)-3.357(2) Å).30
The “C5Me5K” ligation could also be applied to samarium-
(II) complexes with linked-cyclopentadienyl ligands. The reac-
tion of Me2Si(C5Me4)2Sm(THF)2 with 1 equiv of KOAr (Ar )
C6H2 Bu2-2,6-Me-4 or C6H3 Bu2-2,6) in THF yielded 1h or 1i,
respectively (eq 2). Complexes 1h,i can be viewed as a C5Me4/
Figure 3. Extended X-ray structure of 1c.
t
t
vacuum to give 1c′.24 Similarly, the reaction of (C5Me5)2Sm-
(THF)2 with 1 equiv of KNHAr or KN(SiMe3)2 in THF afforded
t
the C5Me5/NHAr-ligated Sm(II) complex 1d (Ar ) C6H2 Bu3-
2,4,6) or the C5Me5/N(SiMe3)2-ligated Sm(II) complex 1e,
respectively. In an analogous way the reaction of (C5Me5)2Yb-
(THF)2 with 1 equiv of KN(SiMe3)2 yielded the corresponding
Yb(II) complex 1f (Scheme 1).25 Complexes 1a-e all adopt a
similar polymeric structure via “intermolecular” interactions
between the K atom and a C5Me5 ligand. Some selected bond
lengths and angles are summarized in Table 1. These structural
data can be compared with those previously reported for the
related Sm(II) compounds, such as (C5Me5)2Sm(THF)2,3a Sm-
OAr-ligated Sm(II) species stabilized by the silylene-linked
“C5Me4K” unit. Similar to the unlinked analogue 1a, complexes
1h,i also form a polymeric structure via the K‚‚‚Cp′ interactions
(Figure 5). It is also noteworthy that the OAr ligands in 1h
(OAr)2(THF)3 (Ar ) C6H2 Bu2-2,6-Me-4),21b,26a [Sm(SAr)(µ-
t
SAr)(THF)3]2 (Ar ) C6H2 Pr3-2,4,6),26b Sm(N(SiMe3)2)2-
i
t
(THF)2,26c and [(C5Me5)Sm(OAr)(µ-C5Me5)K(THF)2]∞ (Ar )
( Sm-O-Ar: 110.3(3)°, Ar ) C6H2 Bu2-2,6-Me-4) and 1i
t
t
C6H3 Bu2-2,6).20 As far as we are aware, complex 1c represents
( Sm-O-Ar: 108(1)°, Ar ) C6H3 Bu2-2,6), as that in 1a
( Sm-O-Ar: 126.7(5)°, Ar ) C6H2 Bu2-2,6-Me-4),20 adopt
t
the first example of a lanthanide(II) complex bearing mixed
cyclopentadienide/thiolate ligands, and 1d-f are the first
examples of lanthanide(II) complexes with mixed cyclopenta-
dienide/amide ligands.
a bent structure, which is in contrast with the “linear” ones in
the analogous samarium(II) aryloxide complexes [(C5Me5)-
Sm(OAr)(µ-C5Me5)K(THF)2]∞ ( Sm-O-Ar: 175(1)°, Ar )
C6H3 Bu2-2,6)20 and 1b ( Sm-O-Ar: 165(1)°, Ar )
t
The analogous reaction of (C5Me5)2Sm(THF)2 with 1 equiv
of KPHAr afforded the C5Me5/PHAr-ligated Sm(II) complex
i
C6H3 Pr2-2,6), showing that the aryloxide ligands are of excellent
t
1g (Ar ) C6H2 Bu3-2,4,6) (eq 1).27 In contrast with the aryloxide,
flexibility.
Similar to 1a which yielded 2a upon reaction with hexam-
ethylphosphoric triamide (HMPA),20 the reaction of the poly-
meric 1e with 2 equiv of HMPA (per Sm) in THF afforded the
corresponding HMPA-coordinated, monomeric Sm(II) complex
2e (eq 3, Figure 6), indicating that the “C5Me5K” ligand in this
type of complexes could be replaced by an appropriate coor-
dinative ligand.
thiolate, and amide complexes 1a-f, in which the “C5Me5K”
(25) The cell parameters for 1f are almost the same as those for 1e (see
Table 5), although 1f could not be fully refined owing to poor quality of
the crystal: monoclinic, a ) 34.914(7) Å, b ) 10.155(6) Å, c ) 11.661(3)
Å, â ) 103.72(2)°, V ) 4017(3) Å3.
(26) (a) Qi, G.-Z.; Shen, Q.; Lin, Y.-H. Acta Crystallogr., Sect. C 1994,
50, 1456. (b) Mashima, K.; Nakayama, Y.; Fukumoto, H.; Kanehisa, N.;
Kai, Y.; Nakamura, A. J. Chem. Soc., Chem. Commun. 1994, 2523. (c)
Evans, W. J.; Drummond, D. K.; Zhang, H.; Atwood, J. L. Inorg. Chem.
1988, 27, 575.
(27) Although 1g in the polymeric solid state could also be viewed as a
“KPHAr” adduct of the samarocene species “(C5Me5)2Sm(THF)” via Sm‚
‚‚P bonding, its behavior in polymerization reactions (different from that
of “(C5Me5)2Sm”, see Tables 2 and 3) suggests that it should best be
considered as a “(C5Me5)Sm(PHAr)” species connected with a “C5Me5K”
unit through the P‚‚‚K interaction.
C5Me5/N(SiMe3)2-Ligated Lanthanide(II) Complexes with
the “C5Me5Na” Ligand. “C5Me5Na” could also act as a
(28) An interaction between the C(1) atom of the phosphide unit and
the K atom in 1g was also observed (C(1)-K(1): 3.124(6) Å).
(29) (a) Rabe, G.; Yap, G. P. A.; Rheingold, A. L. Inorg. Chem. 1995,
34, 4521. (b) Nief, F.; Ricard, L. J. Organomet. Chem. 1997, 529, 347. (c)
Nief, F.; Ricard, L. J. Organomet. Chem. 1994, 464, 149.
(30) Rabe, G.; Yap, G. P. A.; Rheingold, A. L. Inorg. Chem. 1997, 36,
1990.

Block-Copolymerization of Styrene and Ethylene
J. Am. Chem. Soc., Vol. 122, No. 43, 2000 10537
Table 1. Summary of Selected Bond Lengths (Å) and Angles (deg) for Cp*/ER-Ligated Lanthanide(II) Complexes of the Types of
“(Cp*1)Ln(ER)(Cp*2M)” (1a-k) and Cp*1Ln(ER)(HMPA)2 (2a,e) (cf. Scheme 1, eqs 1-4, and Figures 2-7)
1aa
1b
1c
1d
1e
1g
1h
1i
1j
1k
2ab
2e
Ln )
M )
ER )
Sm
Sm
K
Sm
K
Sm
K
Sm
K
Sm
K
Sm
K
Sm
K
Sm
Na
Yb
Na
Sm
Sm
K
OAr(tBu) OAr(iPr) SAr(iPr) NHAr(tBu) N(SiMe3)2 PHAr(tBu) OAr(tBu) OAr(tBu)′ N(SiMe3)2 N(SiMe3)2 OAr(tBu) N(SiMe3)2
Ln-Cp*1 (av)
Ln-Cp*2 (av.
Ln-ER
2.876(8) 2.92(2) 2.902(10) 2.87(3)
2.911(7) 2.95(2) 2.891(9) 2.90(3)
2.330(6) 2.37(1) 2.936(3) 2.48(2)
3.142(7) 3.14(2) 3.059(9) 3.11(3)
3.157(8) 3.21(2) 3.024(10) 3.14(3)
2.97(3)
2.83(3)
2.49(1)
3.08(2)
3.24(2)
2.862(9)
2.923(8)
3.234(2)
3.006(8)
2.84(2) 2.893(5) 2.86(3)
2.87(2) 2.852(4) 2.92(3)
2.37(1) 2.375(3) 2.50(2)
3.19(2) 3.134(4) 2.73(3)
3.20(2) 3.194(5)
2.77(3)
2.83(3)
2.41(2)
2.74(3)
2.860(7) 2.86(2)
2.345(4) 2.55(1)
M-Cp*2 (av)
M-Cp*1′ (av)
M-ER
3.350(3)
133.7(7)
102.0(5)
118.4(5)
123.9(2)
Cp*1-Ln-Cp*2 137.6(3) 131(1) 134.1(3) 138(1)
Cp*1-Ln-ER 101.9(3) 108(1) 103.1(3) 102(1)
Cp*2-Ln-ER 120.4(2) 114(1) 113.9(3) 118(1)
135(1)
109(1)
116(1)
120(1)
114(1)
118(1)
106(1)
136(1)
108(1)
117.8(2) 135(1)
135.1(2) 114(1)
107.1(2) 111(1)
110.3(3) 119(1)
112(1)
134(1)
115(1)
112(1)
120(1)
113(1)
116.1(2) 118.7(8)
Ln-E-R
126.7(5) 165(1) 124.9(3) 127(2)
163.6(4) 119.9(8)
115.7(7)
a Reference 20. b References 20, 21b.
Figure 4. Extended X-ray structure of 1g.
Figure 6. X-ray structure of 2e.
Figure 5. Extended X-ray structure of 1h.
stabilizing ligand for the C5Me5/N(SiMe3)2-ligated lanthanide-
(II) complexes. The use of NaN(SiMe3)2 in place of KN(SiMe3)2
in the reactions with (C5Me5)2Ln(THF)2 afforded the “C5Me5-
Na(THF)3”-coordinated Ln(II) complexes 1j (Ln ) Sm) and
1k (Ln ) Yb), respectively (eq 4). In contrast with the
Figure 7. X-ray structure of 1j.
“monomeric” form, owing to coordination of more THF ligands
to the Na atom (eq 4 and Figure 7), showing that the Na atom
in 1j,k prefers the coordination of THF rather than intermo-
lecular interaction with C5Me5. These results are in agreement
with the previous observation that C5Me5Na tends to form a
monomeric structure: C5Me5Na(Py)3, while C5Me5K prefers a
polymeric form: [C5Me5K(Py)2]∞.31 Upon evacuation, two of
the three THF ligands on the Na atom in 1j,k could be removed
to give 1j′,k′, respectively.24
Polymerization of Ethylene. To assess the ability of this
new class of lanthanide(II) complexes as a catalytic system for
“C5Me5K”-coordinated complexes 1a-f,h,i, which adopt a
polymeric structure through intermolecular K‚‚‚C5Me5 interac-
tions, the “C5Me5Na”-coordinated complexes 1j,k adopt a
(31) Rabe, G.; Roesky, H. W.; Stalke, D.; Pauer F.; Sheldrick, G. M. J.
Organomet. Chem. 1991, 403, 11.

10538 J. Am. Chem. Soc., Vol. 122, No. 43, 2000
Hou et al.
Table 2. Polymerization of Ethylene by Lanthanide(II)
Table 3. Polymerization of Styrene by Lanthanide(II) Complexesa
Complexesa
yield
Mn
b
b
yield
(g)
0.42c
0
Mn
run
cat
time
24 h
24 h
(%) (×10-4
)
Mw/Mn
b
b
run
cat
time
1 hc
1 h
(×10-4
)
Mw/Mn
2.28d
-
1
2
3
4
5
6
7
8
9
(C5Me5)2Sm(THF)2
Sm(OAr)2(THF)3
Sm(N(SiMe3)2)2(THF)2 24 h
0
0
0
-
-
1
2
3
4
5
6
7
8
9
(C5Me5)2Sm(THF)c
Sm(OAr)2(THF)3
<2.50d
-
-
-
-
-
1a
1b′
1c′
1d
1e
1f
1g
10 min 1.62
10 min 1.00
10 min 0.99
10 min 0.48
30 min 0.55
43.4
33.0
58.0
49.7
309.9
-
2.22
2.49
1.79
2.90
2.83
-
1a
1a
1b′
1c′
1d
1e
30 min
89
34.7
34.0
24.5
17.1
14.4
8.2
-
1.73
1.76
1.93
1.45
2.16
2.45
-
40 min
30 min
2.5 h
20 min
10 min
24 h
100
100
79
100
100
0
trace
36
59
1 h
10 min 1.36
1 h
30 min 1.15
0
81.8
-
1.64
-
10 1f
11 1g
12 1g
13 1h
14 1h
15 1j′
16 2a
17 [(C5Me5)Sm(µ-OAr)]2
10 1h
11 1j′
12 2a
0
24 h
-
-
3.5 hc
30 min
2 h
2.7
32.5
32.2
8.2
-
1.51
1.59
1.97
2.28
-
86.5
-
2.30
-
1 h
0
0
13 [(C5Me5)Sm(µ-OAr)]2 1 h
-
-
100
23
0
e
14 [(C5Me5)Sm(µ-OAr)]2 10 min 0.27
32.6
2.37
2 h
24 h
24 h
a Reaction conditions: cat., 0.05 mmol; ethylene, 1 atm; toluene,
15 mL, 25 °C (water bath). b Determined at 135 °C against polystyrene
standard by GPC. c 0.083 mmol of the catalyst was used. These data
were taken from ref 10d. d See ref 4. e THF (5 equiv per Sm) was also
added.
0
-
-
a Conditions: cat., 0.05 mmol; styrene, 4 mL (35 mmol); toluene,
10 mL; 25 °C (water bath). b Determined at 135 °C against polystyrene
standard by GPC. c 50 °C.
polymerization reactions, their reactivity toward ethylene was
first examined. As shown in Table 2, the C5Me5/ER-ligated
Sm(II) complexes 1a,b′,c′,d,e,g,j′, which are stabilized by the
“C5Me5M” ligand (M ) K or Na), all showed high activity for
the polymerization of ethylene at 25 °C under 1 atm, yielding
linear polyethylene with Mn up to 3 × 106 (based on polystyrene
standard). Compared to the samarocene(II) complexes (C5Me5)2-
Sm(THF)n (n ) 0-2) (run 1, Table 2),4,10a,d,e these mixed-ligand-
supported complexes are more active and afford polymers with
much higher molecular weight. Moreover, the reactivity of this
type of complexes appeared to be ER-ligand dependent. The
4-Me-2,6-tBu2C6H2O-ligated complex 1a showed the highest
activity (run 3, Table 2), while the (Me3Si)2N-ligated complex
1e gave the highest molecular weight of polyethylene (run 7,
Table 2).
Table 2), suggesting that one-electron transfer from the Ln(II)
center to an ethylene monomer plays a critically important role
in the initiation of the polymerization reactions.2e,4,10c
Polymerization of Styrene. The C5Me5/ER-ligated Sm(II)
complexes 1a,b′,c′,d,e showed high activity also for the po-
lymerization of styrene at room temperature (runs 4-9, Table
3), in striking contrast with the homoleptic type Sm(II)
complexes (C5Me5)2Sm(THF)n (n ) 0 or 2),10c,15 Sm(OAr)2-
t
(THF)3 (Ar ) C6H2 Bu2-2,6-Me-4), and Sm(N(SiMe3)2)2(THF)2,
which were inactive for styrene polymerization under the same
conditions. In most cases, a quantitative conversion was
achieved in less than 1 h, affording atactic polystyrenes with
Mn ranging from 8.2 × 104 to 3.5 × 105 and Mw/Mn ) 1.45-
2.45. The ER-ligand dependence of the activity was also
observed in this case. Complex 1b′, which bears the less
sterically demanding 2,6-iPr2C6H3O ligand, was more active than
the 4-Me-2,6-tBu2C6H2O-ligated complex 1a (run 4 vs run 6,
Table 3). The more electron-donating amide complexes 1d,e
showed higher activity than the aryloxide and thiolate complexes
1a,b′,c′ (runs 4-9, Table 3).
In contrast with the highly reactive 1a, the analogous complex
1h, which bears the silylene-linked cyclopentadienyl ligands,
was inactive for the polymerization of ethylene (run 10, Table
2). This is probably due to the silylene bridge in 1h, which
could prevent complete dissociation of the “C5Me4K” unit from
the Sm(II) center and thus hamper the access of an ethylene
monomer to the metal center. In consistence with this result,
complex 2a was also inert toward ethylene because of the
strongly coordinating HMPA ligands (run 12, Table 2).32
Similarly, the dimeric complex [(C5Me5)Sm(µ-OAr)]2 (Ar )
Compared to ethylene polymerization, the polymerization of
styrene appeared to be more susceptible to the changes in the
ligand environment of a complex. In contrast with the active
aryloxide, thiolate, and amide complexes 1a,b′,c′,d,e, the
phosphide complex 1g was almost inactive for styrene polym-
erization at room temperature (run 11, Table 3), although it
showed high activity for ethylene polymerization. The lower
activity of 1g than those of 1a,b′,c′,d,e toward styrene could
probably be due to the strong interaction between the phos-
phide ligand and the “C5Me5K” unit in 1g (cf. eq 1 and Fig-
ure 4).28 In the case of 1g, dissociation of the “C5Me5K” unit
from the Sm(II) center would lead to formation of a species
such as “(C5Me5)Sm(µ-PHAr)K(C5Me5)”, which is apparently
more crowded than the analogous species “(C5Me5)Sm(ER)”
generated in the case of 1a,b′,c′,d,e, and could therefore re-
tard the access of the sterically demanding styrene monomer
to the central samarium atom. The “C5Me5M” ligand also
showed much influence on the polymerization of styrene. The
“C5Me5Na”-coordinated complex 1j′ (run 13, Table 3) showed
a much lower activity than the analogous “C5Me5K”-coordi-
nated complex 1e (run 9, Table 3). This could be due to the
difference in structural preference between C5Me5K and
C5Me5Na.31
t
C6H2 Bu2-2,6-Me-4) did not show an activity for ethylene
polymerization in pure toluene probably owing to the strong
and bulky µ-OAr-bridges (run 13, Table 2).20 Upon addition of
a small amount of THF, however, this dimeric complex became
active, yielding polyethylene with Mn ) 3.26 × 105 and Mw/
Mn ) 2.37 (run 14, Table 2), possibly as a result of the forma-
tion of a THF-coordinated, monomeric Sm(II) species such as
“(C5Me5)Sm(OAr)(THF)x”. All of these results strongly suggest
that dissociation of the neutral “C5Me5M” ligand to generate a
sterically unsaturated, C5Me5/ER-ligated Sm(II) species must
be an essential step in the polymerization reactions promoted
by complexes 1a,b′,c′,d,e,g,j′. On the other hand, the analogous,
less reducing Yb(II) complex 1f showed no activity for the
polymerization of ethylene under the same conditions (run 6,
(32) It is well-known that HMPA usually forms a very strong coordina-
tion bond with lanthanide ions. For examples, see: (a) Hou, Z.; Zhang, Y.;
Wakatsuki, Y. Bull. Chem. Soc. Jpn. 1997, 70, 149. (b) Hou, Z.; Wakatsuki,
Y. J. Chem. Soc., Chem. Commun. 1994, 1205. (c) Hou, Z.; Kobayashi,
K.; Yamazaki, H. Chem. Lett. 1991, 265.

Block-Copolymerization of Styrene and Ethylene
J. Am. Chem. Soc., Vol. 122, No. 43, 2000 10539
Scheme 2. A Flowchart for Separation of Block
Styrene-Ethylene Copolymers from Atactic
Homopolystyrenes and Homopolyethylenes
increased almost linearly as the feeding amount of styrene was
raised (Figure 9). In each styrene feeding, the styrene incorpora-
tion increased in the order of 1b′ > 1a > 1c′ (Figure 9),
reflecting the activity trend of these complexes shown for styrene
homopolymerization reactions (see also Table 3).
In contrast with the aryloxide and thiolate complexes 1a,b′,c′,
the amide complexes 1d,e, which had shown much higher
activity for styrene polymerization (cf. Table 3), preferred the
homopolymerization of styrene, yielding homopolystyrene as
the only or a major product under the presence of both
monomers (runs 14 and 15, Table 4). On the other hand, the
phosphide complex 1g and the “C5Me5Na”-coordinated complex
1j′, which had shown much lower activity for styrene polym-
erization (cf. Table 3), afforded only homopolyethylene under
the similar conditions (runs 16 and 17, Table 4). These results
again show that the present copolymerization reactions are
closely related to the activity of a complex shown for the
homopolymerization reactions and are strongly influenced by
the ligand environment around the central metal atom.
13C NMR analyses have revealed that the copolymer products
obtained in the present reactions are block styrene-ethylene
copolymers rather than random or alternating ones (for an
example see Figure 10). To further confirm that the copolymer
products do not contain homopolymers, an artificial mixture of
homopolyethylene (Mn ) 5.7 × 104, Mw/Mn ) 1.11), ho-
mopolystyrene (Mn ) 24.5 × 104, Mw/Mn ) 1.93, obtained in
run 6, Table 3), and a copolymer (Mn ) 15.1 × 104, Mw/Mn )
1.92, PS content ) 68 mol %, obtained in run 7, Table 4) was
extracted in the same way as the crude product was done (cf.
Scheme 2). Each of these three polymers was thus recovered
quantitatively, which suggests that this copolymer does not
contain homopolystyrenes with Mn e 24.5 × 104 or homopoly-
ethylenes with Mn g 5.7 × 104.34 Since the GPC curve of the
copolymer showed a unimodal narrow molecular weight dis-
tribution and that of its mixture with the homopolystyrene (Mn
) 24.5 × 104) or the homopolyethylene (Mn ) 5.7 × 104) was
bimodal, the content of homopolystyrenes with Mn > 24.5 ×
104 or homopolyethylenes with Mn < 5.7 × 104 in this
copolymer should also be negligible. The TEM (transmission
electron microscopy) images of the copolymer products showed
good uniform phases, which are in sharp contrast with that of
a mixture of the immiscible homopolystyrene and homopoly-
ethylene. Viscoelasticity studies further confirmed that the
copolymers are mechanically much stronger and more flex-
ible than the mixtures of homopolystyrenes and homopolyeth-
ylenes. All of these results clearly show that the copolymer
products obtained in the present reactions are true block sty-
rene-ethylene copolymers rather than mixtures of the ho-
mopolymers.
As in the case of ethylene, the Yb(II) complex 1f again did
not show an activity for styrene polymerization. Neither did
the HMPA-coordinated complex 2a or the dimeric complex
t
[(C5Me5)Sm(µ-OAr)]2 (Ar ) C6H2 Bu2-2,6-Me-4) in toluene or
toluene/THF. Unexpectedly, the silylene-linked cyclopentadienyl
complex 1h initiated styrene polymerization at room temperature
(runs 13,14, Table 3), although its activity was lower than that
of the unlinked analogue 1a.33
Block Copolymerization of Styrene and Ethylene. Since
complexes 1a,b′,c′,d,e,g,j′ were active for the polymerization
of both ethylene and styrene, copolymerization of the two
monomers by these complexes was investigated. The copolym-
erization reactions were carried out in toluene at 25 °C under
an atmosphere of ethylene with varying amounts of styrene
monomer. The crude polymer products, precipitated by addition
of MeOH, were first repeatedly washed with THF at room
temperature to remove homopolystyrene, and then extracted with
toluene at 100-108 °C to collect the copolymers (Scheme 2).
Since the homopolystyrenes formed in the present reactions are
atactic and very soluble in THF at room temperature while the
homopolyethylenes are insoluble in refluxing toluene (110 °C),
the styrene-ethylene copolymers, which are soluble in hot
toluene, can be easily separated from the homopolymers by the
above extractions. Some representative results are summarized
in Table 4 and Figures 8 and 9.
The aryloxide and thiolate complexes 1a,b′,c′ showed good
activity and selectivity for the copolymerization reactions under
the presence of both monomers, affording styrene-ethylene
copolymers with Mn around 2 × 105 and Mw/Mn < 2 in most
cases (runs 1-8,10-13, Table 4). Besides the major copolymer
products, homopolystyrene was also obtained as a byproduct
while homopolyethylene was almost not observed, except when
a very small amount of styrene monomer was fed, which led to
formation of homopolyethylene as a dominant product (cf. run
9, Table 4). Among these complexes, the thiolate complex 1c′
showed the highest selectivity for the copolymerization reaction,
which could reach up to as high as 96% (run 10, Table 4). As
the feeding amount of styrene monomer was raised under 1 atm
of ethylene, the selectivity for styrene-ethylene copolymers
generally decreased for all these three complexes owing to
increased formation of homopolystyrene (Table 4 and Figure
8). The polystyrene content in the copolymers, however,
The selective formation of block styrene-ethylene copoly-
mers in the present reactions is in striking contrast with what
was observed in the case of the silylene-linked bis(cyclopen-
tadienyl) lanthanide complexes Me2Si(C5Me4)2LnCH(SiMe3)2
(Ln ) Nd or Sm)17a and in group 4 metal-catalyzed styrene-
ethylene copolymerization reactions.35 The latter two systems
always yielded random or alternating styrene-ethylene copoly-
mers under the presence of both monomers. As far as we are
aware, selective formation of block styrene-ethylene copoly-
mers under the presence of both monomers is unprecedented.
These reactions are not only of great fundamental interest, but
(34) In fact, atactic polystyrenes with Mn > 24.5 × 104 are also soluble
in THF at room temperature and can therefore be removed by the extraction
with THF. The solubility of polyethylene, however, decreases greatly as
the molecular weight increases.
(33) The mechanism of styrene polymerization by 1h was not clear. It
might proceed to some extent through an “anionic” process. However, the
polymerization did not take place in a THF solution.

10540 J. Am. Chem. Soc., Vol. 122, No. 43, 2000
Hou et al.
Table 4. Block-Copolymerization of Styrene and Ethylene by Samarium(II) Complexes Bearing Mixed C5Me5/ER Ligandsa
yield (g)
THF-sol
tol-sol (<108 °C)
(PSE)d
tol-Insol (108 °C)
(PE)d
PSE selectivity
(wt %)
PS cont
c
c
run
cat.
styrene (mL)
(PS)d
(mol %)b
Mn (×10-4
)
Mw/Mn
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1a
1a
1a
1a
3
5
7
10
3
5
7
10
2
3
5
7
10
3
5
7
0.65
1.18
1.54
1.81
0.46
1.29
1.37
2.40
trace
0.06
0.23
0.27
0.34
2.36
4.55
trace
trace
1.59
1.62
2.04
2.12
2.14
2.83
2.94
3.99
0.29
1.31
2.55
2.74
2.82
0.78
trace
trace
trace
trace
trace
trace
trace
trace
trace
trace
trace
0.19
trace
trace
trace
trace
trace
trace
0.51
0.17
71
58
57
54
82
69
68
62
-
31
44
61
79
34
48
68
81
<2
13
37
43
60
15
-
20.3
20.6
24.0
27.6
15.9
14.6
15.1
13.1
13.6
11.3
10.7
13.7
14.6
11.3
-
1.60
1.69
1.74
1.68
1.97
1.82
1.92
1.84
2.49
2.21
2.01
1.73
1.66
1.99
-
1b′
1b′
1b′
1b′
1c′
1c′
1c′
1c′
1c′
1d
1e
96
92
91
89
25
-
1g
1j′
-
-
-
-
-
-
-
-
5
a Conditions: cat., 0.05 mmol; ethylene, 1 atm; the total volume of styrene and toluene, 25 mL, 25 °C (water bath), 30 min. b Polystyrene
content in the copolymers, determined by 13C NMR in o-dichlorobenzene/CDCl2CDCl2 at 125 °C. c Determined at 135 °C against polystyrene
standard by GPC. d PS ) atactic polystyrene, PSE ) styrene-ethylene block copolymer, PE ) polyethylene.
Figure 8. Yields (g) of PSE and PS vs styrene monomer feed (mL) for styrene-ethylene copolymerization reactions with (a) 1a, (b) 1b′, or (c)
1c′ as a precatalyst (see also Table 4).
could also be of practical usefulness as a convenient route to a
new class of important polymer materials.36,37
(35) For examples see: (a) Xu, G. Macromolecules 1998, 31, 2395. (b)
Xu, G.; Lin, S. Macromolecules 1997, 30, 685. (c) Arai, T.; Ohtsu, T.;
Suzuki, S. Macromol. Rapid Commun. 1998, 19, 327. (d) Sernetz, F. G.;
Mu¨lhaupt, R.; Fokken, S.; Okuda, J. Macromolecules 1997, 30, 1562. (e)
Sernetz, F. G.; Mu¨lhaupt, R.; Waymouth, R. M. Macromol. Chem. Phys.
1996, 197, 1071. (f) Oliva, L.; Mazza, S.; Longo, P. Macromol. Chem.
Phys. 1996, 197, 3115. (g) Pellecchia, C.; Pappalarrdo, D.; D’Arco, M.;
Zambelli, A. Macromolecules 1996, 29, 1158. (h) Aaltonen, P.; Seppala,
J.; Matilainen, L.; Leskela, M. Macromolecules 1994, 27, 3136. (i) Miyatake,
T.; Mizunuma, K.; Kagugo, M. Makromol. Chem., Macromol. Symp. 1993,
66, 203. (j) Longo, P.; Grassi, A.; Oliva, L. Makromol. Chem. 1990, 191,
2387.
(36) Generally, block copolymers have uniquely different chemical,
physical, and mechanical properties compared to their random or alternating
copolymeric analogues, and particularly show great emulsifying or com-
patibilizing effect in the blending of the corresponding homopolymers. Since
polystyrene and polyethylene are among the most widely used polymer
materials, the effective blending of these two immiscible homopolymers
could result in various commercial applications. For a recent review on
compatibilization of polymer blends, see: Koning, C.; Van Duin, M.;
Pagnoulle, C.; Jerome, R. Prog. Polym. Sci. 1998, 23, 707.
Figure 9. Polystyrene content (mol %) in styrene-ethylene copolymers
vs styrene monomer feed (mL) for reactions with 1a,b′,c′ as precatalysts
(see also Table 4).
Polymerization Mechanisms. (i) Homopolymerization Re-
actions. It is well-known that the polymerization of ethylene
by the samarocene(II) complexes (C5Me5)2Sm(THF)n (n ) 0-2)
is initiated by a one-electron transfer from the Sm(II) center to
an ethylene monomer to give a Sm(III) alkyl species.2e,4,10c In
the present systems, a similar mechanism could also be
operative. Since ER-ligand dependence of the polymerization
reactions was observed in all cases, a bonding interaction
(37) Conventional sequential “living” polymerization methods are not
suitable for the synthesis of styrene-ethylene block copolymers, since there
is no “living system” which is able to polymerize both styrene and ethylene
owing to the different natures of these two monomers. Attempts to obtain
a block styrene-ethylene copolymer by sequential copolymerization of
styrene with ethylene by group 4 metal-based catalysts produced the
corresponding homopolymers as major products. See: (a) Inoue, N.; Jinno,
M.; Shiomura, T. JP 0687937, 1994; Chem. Abstr. 1994, 121, 109890h.
(b) Naganuma, A.; Tazaki, T.; Machida, S. JP 04130114, 1992; Chem. Abstr.
1993, 118, 7536t.

Block-Copolymerization of Styrene and Ethylene
J. Am. Chem. Soc., Vol. 122, No. 43, 2000 10541
Scheme 3. A Possible Mechanism for Ethylene Polymerization by the C5Me5/ER-Ligated Sm(II) Complexes with the Neutral
“C5Me5M” Ligand
(ii) Copolymerization Reactions. For complexes active
toward both ethylene and styrene, the polymerization of ethylene
and that of styrene could start independently in the presence of
both monomers. The selective formation of block rather than
random styrene-ethylene copolymers in the present systems
strongly suggests that the reactivity of the propagation site of
polyethylene unit and that of polystyrene unit are critically
different from each other, one being able to incorporate both
ethylene and styrene, while the other only the identical
monomer. To gain more information on the copolymerization
reactions, sequential polymerization reactions of styrene with
ethylene were carried out by use of 1b′ as a precatalyst. The
reaction of 1 mL of styrene (0.9 g) with 0.05 mmol of 1b′ was
first carried out at 25 °C in toluene (10 mL) for 1 h. Quenching
this reaction mixture with MeOH yielded 0.88 g (98% yield)
of polystyrene with Mn ) 8.8 × 104 and Mw/Mn ) 1.34, showing
that the polymerization reaction was almost completed under
the present conditions. Addition of another 1 mL of styrene to
this reaction mixture, followed by further stirring of the resultant
mixture for 1 h, afforded 1.75 g of polystyrene whose Mn
increased to 13.9 × 104 while Mw/Mn (1.37) remained almost
unchanged, which shows that the completed styrene-polymer-
ization solution was still “living” and able to incorporate and
polymerize styrene. Exposure of this “living” styrene-polym-
erization solution to an atmosphere of ethylene with stirring
for 30 min, however, did not give a styrene-ethylene copoly-
mer, but instead yielded 0.85 g of polystyrene (Mn ) 8.6 ×
104, Mw/Mn ) 1.35) and 0.76 g of polyethylene (Mn ) 43.2 ×
104, Mw/Mn ) 1.67) after being quenched with MeOH and
separated by THF/toluene extractions (cf. Scheme 2). These
results suggest that the propagation site of the polystyrene unit
in the present system does not allow insertion of ethylene,
although it can generate a species (possibly a Sm(III) hydride)
active for ethylene homopolymerization. In contrast, when the
introduction of ethylene to this styrene polymerization mixture
was immediately followed by addition of 7 mL of styrene, a
block styrene-ethylene copolymer product (0.42 g, Mn ) 8.7
Figure 10. Aliphatic region of the 13C NMR spectrum of a block
styrene-ethylene copolymer (run 7, Table 4) in ortho-dichlorobenzene/
CDCl2CDCl2 at 125 °C.
between the ER ligand and the Sm atom in a catalytic species
must remain during the polymerization reactions. A possible
mechanism for the polymerization of ethylene by the present
systems is shown in Scheme 3. Dissociation of the “C5Me5M”
unit from the Sm(II) center in 1 (either dissociative or associa-
tive) followed by coordination of an ethylene molecule would
yield H. Rapid electron transfer from the Sm(II) center to the
ethylene ligand in H and the subsequent dimerization of the
resultant radical species I would afford the Sm(III) alkyl species
J. Successive insertion of ethylene into the Sm-alkyl bond in J
and the following â-hydrogen elimination from the resulting
polymeric compound K could release polyethylene and the
hydride species L. The C5Me5/ER-ligated Sm(III) hydride
species L is probably a true catalytic species in the present
systems, which could constitute a catalytic cycle by the sub-
sequent ethylene insertion/â-H elimination reactions (Scheme
3). The polymerization of styrene could take place via an ana-
logous mechanism. The more open ligand sphere provided by
the mixed C5Me5/ER ligands could explain why the present
systems are more active than the metallocene complexes
(C5Me5)2Sm(THF)n (n ) 0-2).38
(38) The inactivity of the Sm(II) complexes with two ER ligands such
as Sm(OAr)2(THF)3 or Sm(N(SiMe3)2)2(THF)2 for the polymerization of
ethylene or styrene is probably due to the weak electron-donating ability
of the ER ligands,40 with which the Sm(II) center is not able to transfer an
electron to an olefin monomer to initiate the polymerization reaction,
although these complexes are less sterically demanding than (C5Me5)2Sm-
(THF)n.

10542 J. Am. Chem. Soc., Vol. 122, No. 43, 2000
Hou et al.
Scheme 4. A Possible Mechanism for Block Copolymerization of Styrene and Ethylene
× 104, Mw/Mn ) 1.54, PS content ) 48 mol %) together with
1.11 g of homopolystyrene (Mn ) 9.4 × 104, Mw/Mn ) 1.34)
was obtained after the resultant mixture was stirred under the
ethylene atmosphere for 30 min, whereas homopolyethylene was
almost not observed. All these results strongly suggest that the
present copolymerization reactions must be initiated by polym-
erization of ethylene followed by successive incorporation of
styrene. Since the copolymerization reactions were also ER-
ligand dependent and the molecular weight distributions of the
resulting copolymers were relatively narrow (see also Table 4),
the possibility that the copolymerization reactions proceeded
via a radical process could be excluded. A possible mechanism
for reactions starting with the C5Me5/ER-ligated Sm(III) hydride
species L is shown in Scheme 4.39
the difference in ancillary ligands between these two types of
complexes. Since the ER ligand is less electron donating than
“C5Me4”,40 the metal center supported by the mixed C5Me5/
ER ligands would be electron-poorer than that supported by
the two silylene-linked-“C5Me4” ligands. This could be a reason
the C5Me5/ER-supported polystyrene-propagation site distin-
guished styrene from ethylene and thus led to formation of block
styrene-ethylene copolymers under the presence of both
monomers.41 These results well demonstrate that replacement
of one of the two cyclopentadienyl ligands in an ordinary
lanthanide metallocene complex with an appropriate monoden-
tate anionic ligand can effectively modify the steric and
electronic properties of the metal center and thus create a brand-
new catalytic system.
Thus, the reactions initiated by styrene polymerization would
afford only homopolystyrene (PS), while those by ethylene
polymerization could end up with the formation of block
styrene-ethylene copolymers (PSE) through the intermediates
M, N, and O, if there is a sufficient amount of styrene monomer
in the reaction systems (Scheme 4). These reaction paths could
explain why homopolystyrene was always obtained as a
byproduct while homopolyethylene was almost not observed
in most of the copolymerization reactions by 1a,b′,c′ (Table
4). When a very small amount of styrene monomer was
employed, the initiation of styrene polymerization and the
incorporation of styrene into the polyethylene unit in M would
be significantly slowed, and homopolyethylene (PE) thus formed
as a dominant product (cf. run 9, Table 4). Obviously, a complex
must show a well-balanced activity toward each monomer to
selectively achieve the copolymerization reaction under the
presence of both monomers. If the activity of a complex for
styrene polymerization is much higher than that for ethylene,
the formation of homopolystyrene would become dominant and
Vice Versa (cf. 1d,e and 1g,j′).
Concluding Remarks
By use of “C5Me5M” (M ) K or Na) as a neutral stabilization
ligand, a new series of C5Me5/ER-supported lanthanide(II)
complexes have been successfully synthesized and structurally
characterized. The “C5Me5K”-coordinated Ln(II) complexes,
which bear one or two THF ligands on the K atom, prefer
formation of a polymeric structure through intermolecular
K‚‚‚C5Me5 interactions when ER is an aryloxide, thiolate, or
amide ligand (1a-f,h,i). In contrast, the analogous “C5Me5-
Na”-coordinated complexes (1j,k) adopt a “monomeric” form,
owing to the coordination of more (three) THF ligands to the
Na atom. When ER ) PHAr (1g), the “C5Me5K” unit prefers
to bond to the phosphide ligand with the K atom, and a
polymeric structure through intermolecular C5Me5‚‚‚Sm interac-
tions is formed. Dissociation of the “C5Me5M” ligand from the
central lanthanide metal can yield a sterically unsaturated
Ln(II) species which is supported by the mixed C5Me5/ER
ligands. As a dissociable stabilization ligand, “C5Me5K” is
unique and more suitable than “C5Me5Na”, possibly owing to
its tendency to adopt a polymeric, insoluble form. Since the
The difference in selectivity for styrene-ethylene copolym-
erization reactions observed between the present C5Me5/ER-
supported systems and the lanthanidocene complexes Me2Si-
(C5Me4)2LnCH(SiMe3)2 (Ln ) Nd or Sm)17a must result from
(40) Using the “neutral ligand formalism”, an ER ligand (ER ) OAr,
SAr, NR2, etc.) can donate, at most, three electrons to the central metal,
two electrons fewer than a pentahapto cyclopentadienyl ligand.19a
(41) The propagation site of the polystyrene unit in the present case might
be to some extent “anionic” and thus more reactive toward styrene than
toward ethylene. Further studies are, however, required to understand the
details.
(39) Reactions starting with the Sm(II) species would follow a similar
mechanism and end up with the formation of the Sm(III) hydride species
L as in the case of the homopolyerization reactions (cf. Scheme 3).

Block-Copolymerization of Styrene and Ethylene
J. Am. Chem. Soc., Vol. 122, No. 43, 2000 10543
Table 5. Summary of Crystallographic Data
compound
1b
1c
1d
1e
1 g‚C6H6
formula
C44H71KO4Sm
853.54
monoclinic
P21/n (No. 14)
14.867(6)
18.269(2)
17.362(6)
C43H69KO2SSm
839.59
triclinic
P-1 (No. 2)
10.139(6)
14.180(6)
17.465(5)
105.79(3)
93.83(3)
70.70(4)
2279(2)
2
C46H76KNO2Sm
864.61
monoclinic
C2/c (No. 15)
42.125(6)
10.238(9)
27.706(6)
C34H64KNO2Si2Sm
764.56
monoclinic
C2 (No. 5)
35.013(10)
10.306(5)
C49H79KO2PSm
920.63
formula weight
crystal system
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
triclinic
P-1 (No. 2)
14.043(5)
18.638(5)
10.291(1)
92.05(1)
99.46(2)
106.92(2)
2531(1)
2
1.21
13.09
10392
8547
11.684(4)
103.190(7)
125.17(1)
104.00(3)
4591(2)
4
1.24
14.08
8415
4800
411
9767(8)
8
1.18
13.23
7362
3409
460
4091(3)
4
1.24
16.25
5187
4479
369
Z
Dc (g cm-1
)
1.22
14.64
11410
7299
µ (cm-1
)
no. of reflns collcd
no. of reflns with Io > 3σ(Io)
no. of variables
433
487
Rint
R
Rw
0.051
0.055
0.074
0.055
0.065
0.083
0.096
0.120
0.109
0.122
0.061
0.082
compound
1h
1i
1j
1k
2e
formula
formula weight
crystal system
space group
a (Å)
C43H69KO3SiSm
851.60
monoclinic
P21/c (No. 14)
14.222(3)
C38H59KO2SiSm
765.47
monoclinic
P21/n (No. 14)
15.080(4)
C38H72NNaO3Si2Sm
820.56
monoclinic
P21/n (No. 14)
17.716(6)
C38H72NNaO3Si2Yb
843.20
monoclinic
P21/n (No. 14)
17.627(4)
C28H69N7O2P2Si2Sm
804.43
monoclinic
P21/n (No. 14)
19.205(4)
b (Å)
18.04(1)
15.566(2)
18.188(5)
18.130(3)
21.877(4)
c (Å)
17.978(2)
17.295(9)
14.379(4)
14.300(2)
10.283(2)
R (deg)
â (deg)
γ (deg)
V (Å3)
102.317(6)
105.40(4)q
94.13(3)
94.32(1)
92.89(1)
4506(2)
4
1.26
14.58
9154
4934 (x ) 5)
442
3913(2)
4
1.30
16.69
8179
6477 (x ) 3)
388
4621(2)
4
1.18
13.62
6573
3849 (x ) 3)
415
4556(1)
4
1.23
21.36
6321
3437 (x ) 3)
415
4315(1)
4
1.24
15.17
10801
4744 (x ) 3)
379
Z
Dc (g cm-1
)
µ (cm-1
)
no. of reflns collcd
no. of reflns with Io > xσ(Io)
no. of variables
Rint
R
Rw
0.066
0.078
0.094
0.062
0.060
0.087
0.058
0.064
0.094
0.097
0.135
0.043
0.080
ER ligand is less sterically demanding and less electron-donating
than the C5Me5 ligand, the mixed C5Me5/ER ligand system
provides a sterically and electronically unique environment for
the lanthanide metal center. This mixed ligand system is
particularly effective for the Sm(II) species, which constitutes
the first lanthanide catalytic system which is active not only
for polymerization of styrene and ethylene but also for copo-
lymerization of these two monomers. Furthermore, the subtle
interplay between the steric and electronic effects of the mixed
C5Me5/ER ligands has enabled the metal center to discern
styrene from ethylene in reactivity, and thus resulted in
formation of block styrene-ethylene copolymers under the
presence of both monomers, a unique reaction which has never
been reported for any other kind of catalytic systems. The
reactivity difference observed among the C5Me5/ER-ligated
Sm(II) complexes 1a,b′,c′,d,e,g,j′ and that between these mixed-
ligand-supported complexes and the ordinary lanthanide met-
allocene complexes clearly show that the reactivity of a lan-
thanide complex can be fine-tuned by changing the ancillary
ligands.
Experimental Section
See Supporting Information for experimental details. The crystal data
of complexes 1b-e, g-k, 2e are summarized in Table 5.
Acknowledgment. We are grateful to Mr. Nobuo Oi of
Sumitomo Chemical Co., Ltd. for the TEM and viscoelasticity
measurements of the polymer products. This work was partly
supported by the President’s Special Research Grant of RIKEN
and a grant-in-aid from the Ministry of Education, Japan.
Supporting Information Available: Experimental details,
tables of atomic coordinates, thermal parameters, and bond
distances and angles for complexes 1b-e, g-k, 2e, X-ray
structures of complexes 1d, e, i, k, and TEM images, GPC
profiles, and hysteresis curves of some typical polymer materials
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA002305J