Catalyst-site-controlled coordination polymerization of polar vinyl monomers to highly syndiotactic polymers

Article abstract of DOI:10.1021/ja908818y

This contribution reports a combined synthetic, kinetic, mechanistic, and theoretical/computational study of the recently discovered catalyst-site- controlled coordination polymerization of polar vinyl monomers [such as methyl methacrylate (MMA) and N,N-dimethylacrylamide (DMAA)] into highly syndiotactic polymers. Among the 12 C_s-ligated ansa-cyclopentadienyl (Cp)-R ₂E(C,Si)-fluorenyl (Flu) group 4 metallocene catalyst systems examined-which varied in metal center, anion structure, bridging atom and substituents, and ligand substitution pattern-cationic ansa-metallocene ester enolate catalyst 6⁺[B(C₆F₅)₄] ^-, derived from the activation of the precatalyst [Ph ₂C(Cp)(2,7-^tBu₂-Flu)]Zr[OC(O ⁱPr)=CMe₂]₂ with [Ph₃C][B(C ₆F₅)₄], stood out as the best catalyst in all aspects of the MMA polymerization at room temperature, including the highest activity (1554 h^-1 TOF), efficiency (98% l*), syndiotacticity (94% rr), and control (predicted number-average molecular weight and 1.14 molecular weight distribution). Kinetic and mechanistic results are consistent with a catalyst-site-controlled, monometallic coordination-addition mechanism, involving fast intramolecular addition within the catalyst-monomer complex leading to the resting eight-membered ester enolate chelate, followed by the rate-limiting ring-opening of the chelate to regenerate the active species. This work has also uncovered several unique features of this polymerization system that are in marked contrast to the propylene polymerization by analogous C _s-ligated cationic alkyl catalysts: a constant syndiotacticity of PMMA produced over a wide polymerization temperature range (i.e., from 0°C, 94% rr to 25°C, 94% rrto 50°C, 93% rr); insensitivity of its high activity, degree of control, and stereoselectivity to solvent polarity and structure of weakly coordinating anions; and deviation from a pure site-control mechanism at high [MMA]/[catalyst] ratios. Computational results provide theoretical support for the proposed monomer-assisted, catalyst-site epimerization, after an enantiofacial mistake, to a thermodynamically more stable resting state, which accounts for the observed higher than expected [mr] contents based on a pure site-controlled mechanism. DFT calculations rationalize why the Ph₂C< bridged catalyst 6 exhibits higher stereoselectivity than other catalysts with the Me₂C< or Me ₂Si < bridge: the bridge rigidity pushes the η³- bound Flu ligand closer to the growing chain and the monomer, thereby increasing ΔE_stereo between the competing transition states for the addition of a monomer molecule to the opposite (correct and wrong) enantiofaces of the enolate growing chain. The relative polymerization activity of this catalyst series is shown to correlate with the relative energetics of the back-biting of the penultimate unit and ion-pair formation.

Full text of DOI:10.1021/ja908818y

Published on Web 02/03/2010
Catalyst-Site-Controlled Coordination Polymerization of Polar
Vinyl Monomers to Highly Syndiotactic Polymers
Yuetao Zhang,† Yalan Ning,† Lucia Caporaso,‡ Luigi Cavallo,*,‡ and
Eugene Y.-X. Chen*,†
Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523-1872, and
Dipartimento di Chimica, UniVersita` di Salerno, Via Ponte don Melillo,
I-84084 Fisciano (SA), Italy
Received October 15, 2009; E-mail: lcavallo@unisa.it; eugene.chen@colostate.edu
Abstract: This contribution reports a combined synthetic, kinetic, mechanistic, and theoretical/computational
study of the recently discovered catalyst-site-controlled coordination polymerization of polar vinyl monomers
[such as methyl methacrylate (MMA) and N,N-dimethylacrylamide (DMAA)] into highly syndiotactic polymers.
Among the 12 Cs-ligated ansa-cyclopentadienyl (Cp)-R2E(C,Si)-fluorenyl (Flu) group 4 metallocene catalyst
systems examinedswhich varied in metal center, anion structure, bridging atom and substituents, and
ligand substitution patternscationic ansa-metallocene ester enolate catalyst 6+[B(C6F5)4]-, derived from
the activation of the precatalyst [Ph2C(Cp)(2,7-tBu2-Flu)]Zr[OC(OiPr)dCMe2]2 with [Ph3C][B(C6F5)4], stood
out as the best catalyst in all aspects of the MMA polymerization at room temperature, including the highest
activity (1554 h-1 TOF), efficiency (98% I*), syndiotacticity (94% rr), and control (predicted number-average
molecular weight and 1.14 molecular weight distribution). Kinetic and mechanistic results are consistent
with a catalyst-site-controlled, monometallic coordination-addition mechanism, involving fast intramolecular
addition within the catalyst-monomer complex leading to the resting eight-membered ester enolate chelate,
followed by the rate-limiting ring-opening of the chelate to regenerate the active species. This work has
also uncovered several unique features of this polymerization system that are in marked contrast to the
propylene polymerization by analogous Cs-ligated cationic alkyl catalysts: a constant syndiotacticity of PMMA
produced over a wide polymerization temperature range (i.e., from 0 °C, 94% rr to 25 °C, 94% rr to 50 °C,
93% rr); insensitivity of its high activity, degree of control, and stereoselectivity to solvent polarity and structure
of weakly coordinating anions; and deviation from a pure site-control mechanism at high [MMA]/[catalyst]
ratios. Computational results provide theoretical support for the proposed monomer-assisted, catalyst-site
epimerization, after an enantiofacial mistake, to a thermodynamically more stable resting state, which
accounts for the observed higher than expected [mr] contents based on a pure site-controlled mechanism.
DFT calculations rationalize why the Ph2C< bridged catalyst 6 exhibits higher stereoselectivity than other
catalysts with the Me2C< or Me2Si< bridge: the bridge rigidity pushes the η3-bound Flu ligand closer to the
growing chain and the monomer, thereby increasing ∆Estereo between the competing transition states for
the addition of a monomer molecule to the opposite (correct and wrong) enantiofaces of the enolate growing
chain. The relative polymerization activity of this catalyst series is shown to correlate with the relative
energetics of the back-biting of the penultimate unit and ion-pair formation.
(R-olefins in particular),3 has attracted increasing interest due
Introduction
to its precision in catalyst-based stereochemical and architectural
Coordination polymerization of polar vinyl monomers by
single-site metal catalysts,1 especially those of cationic group
4 metallocene catalysts,2 which have been extensively investi-
gated for the (co)polymerization of nonpolar vinyl monomers
control as well as its ability to produce new classes of polymeric
(3) Selected books, reference works, or journal reviews: (a) StereoselectiVe
Polymerization with Single-Site Catalysts; Baugh, L. S., Canich,
J. A. M., Eds.; CRC Press: Boca Raton, FL, 2008. (b) Resconi, L.;
Chadwick, J. C.; Cavallo, L. Olefin Polymerizations with Group IV
Metal Catalysts In ComprehensiVe Organometallic Chemistry III;
Bochmann, M., Vol. Ed., Mingos, M. P., Crabtree, R. H., Chief Eds.;
Elsevier: Oxford, 2007; Vol4, pp 1005-1166. (c) Domski, G. J.; Rose,
J. M.; Coates, G. W.; Bolig, A. D.; Brookhart, M. Prog. Polym. Sci.
2007, 32, 30–92. (d) Marks, T. J., Ed. Proc. Natl. Acad. Sci. U.S.A.
2006, 103, 15288-15354 and contributions therein (issue on Polym-
erization Special Feature). (e) Gibson, V. C.; Spitzmesser, S. K. Chem.
ReV. 2003, 103, 283–315. (f) Gladysz, J. A., Ed Chem. ReV. 2000,
100, 1167-1681 and contributions therein (issue on Frontiers in Metal-
Catalyzed Polymerization). (g) Brintzinger, H. H.; Fischer, D.;
Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed.
1995, 34, 1143–1170.
† Colorado State University.
‡ Universita` di Salerno.
(1) Chen, E. Y.-X. Chem. ReV. 2009, 109, 5157–5214.
(2) Selected reference works and reviews: (a) Chen, E. Y.-X.; Rodriguez-
Delgado, A. Complexes of Zirconium and Hafnium in Oxidation State
IV In ComprehensiVe Organometallic Chemistry III; Bochmann, M.,
Vol. Ed., Mingos, M. P., Crabtree, R. H., Chief Eds.; Elsevier: Oxford,
2007; Vol. 4, pp 759-1004. (b) Cuenca, T. In Complexes of Titanium
in Oxidation State IV. In ComprehensiVe Organometallic Chemistry
III; Bochmann, M., Vol. Ed., Mingos, M. P., Crabtree, R. H., Chief
Eds.; Elsevier: Oxford, 2007; Vol4, pp 323-696, (c) Bochmann, M.
J. Chem. Soc., Dalton Trans. 1996, 255–270. (d) Jordan, R. F. AdV.
Organomet. Chem. 1991, 32, 325–387.
9
10.1021/ja908818y  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 2695–2709 2695

A R T I C L E S
Zhang et al.
(EBI)Zr(THF)[OC(OiPr)dCMe2]}+[MeB(C6F5)3]-[EBI)C2H4(η5-
indenyl)2],4 that can readily produce highly isotactic (it-)
polymethacrylates (g95% mm)4,9 and polymethacrylamides
(>99% mm).10
Scheme 1. Elementary Steps Involved in Coordination-Addition
Polymerization of Methyl Methacrylatea
In marked contrast, attempts by at least five research
groups6c,9a,11 over the past decade to synthesize highly
syndiotactic polymethacrylates such as syndiotactic poly(m-
ethyl methacrylate) [st-PMMA, a technologically more
important material than its it-counterpart due to its much
higher (∼85 °C) glass-transition temperature (Tg)1], using
the prototype Cs-ligated cationic zirconocene methyl complex
[Me2C(Cp)(Flu)ZrMe]+ (Cp ) η5-cyclopentadienyl; Flu )
η3-fluorenyl), have been unsuccessful. Such Cs-ligated cata-
lysts are well-known for their ability to catalyze the syn-
diospecific polymerization of propylene,12 but the use of
[Me2C(Cp)(Flu)]ZrMe2, upon activation with various activa-
tors including B(C6F5)3,9a [nBu3NH][BPh4],6c [Ph3C][B(C6F5)4]/
Et2Zn (excess),11c and B(C12F9)3,11b as well as the use of the
isolated cationic species [Me2C(Cp)(Flu)]Zr(Me)MeB(C6F5)3
(1),11a yielded no polymer formation. Although this inactivity
issue was later solved by using an ester enolate derivative,
{Me2C(Cp)(Flu)Zr(THF)[OC(OiPr)dCMe2]}+[MeB(C6F5)3]-
(2),13 which simulates the proposed active propagating
species, the PMMA produced at room temperature is a
syndio-rich atactic polymer (64% rr, 32% mr, 4.0% mm)
formed via an apparent chain-end control mechanism [as a
methyl triad test using 4[mm][rr]/[mr]2 gave 1.0] rather than
the site-controlled mechanism expected on the basis of the
propylene polymerization precedent.
a Abbreviations: RS, resting state; CI, coordination intermediate; TS,
transition state; Ln, supporting ligand; M, metal; A, anion; and P, growing
polymer chain.
materials unattainable by other means of polymerization.1 In
this context, remarkable successes have been achieved in both
metal-catalyzed coordination-addition and coordination-insertion
polymerizations of polar vinyl monomers. On one hand,
coordination-addition polymerization of polar vinyl monomers
(Scheme 1) by single-site early metal and lanthanide catalysts
can generate a dazzling variety of stereomicrostructures, in
addition to its high activity and high degree of control over
polymerization characteristics. On the other hand,
coordination-insertion copolymerization of R-olefins with polar
vinyl monomers by late metal catalysts demonstrates tantalizing
versatility, incorporating a wide range of polar vinyl monomers
at controllable levels into polyolefins with diverse topologies.1
Mechanistic studies of the methyl methacrylate (MMA)
polymerization catalyzed by various types of group 4 cationic
metallocene complexes, including those supported by C2-,4 C2V-,5
C1-,6 and constrained geometry catalyst7 (Cs)-ligated catalysts,
have revealed important insights into polymerization kinetics,
fundamental steps (initiation, propagation, and termination/side
reactions), and stereocontrol events. Theoretical/computational
investigations8 provide a synergistic understanding of such
polymerization reactions, especially aspects of stereocontrol
mechanisms7a,8a,c in the MMA polymerization by chiral ansa-
zirconocenium complexes. Most notable in terms of a high
degree of control in both polymerization and stereoregulation
are chiral C2-ligated zirconocenium catalysts, such as {rac-
This quest has also been extended to lanthanide hydro-
carbyls or amides incorporating Cs-symmetric ansa-Cp-9-
Flu ligands, including Me2C< bridged yttrocene,
[Me2C(Cp)(Flu)]Y(THF)CH(SiMe3)2;14 Ph2C<bridgedlutetocene,
[Ph2C(Cp)(Flu)]LuN(SiMe3)2;15 Me2Si< bridged yttrocene,
(8) (a) Caporaso, L.; Cavallo, L. Macromolecules 2008, 41, 3439–3445.
(b) Tomasi, S.; Weiss, H.; Ziegler, T. Organometallics 2007, 26, 2157–
2166. (c) Caporaso, L.; Gracia-Budr`ıa, J.; Cavallo, L. J. Am. Chem.
Soc. 2006, 128, 16649–16654. (d) Tomasi, S.; Weiss, H.; Ziegler, T.
Organometallics 2006, 25, 3619–3630. (e) Ho¨lscher, M.; Keul, H.;
Ho¨cker, H. Macromolecules 2002, 35, 8194–8202. (f) Ho¨lscher, M.;
Keul, H.; Ho¨cker, H. Chem.sEur. J. 2001, 7, 5419–5426. (g)
Sustmann, R.; Sicking, W.; Bandermann, F.; Ferenz, M. Macromol-
ecules 1999, 32, 4204–4213.
(9) (a) Cameron, P. A.; Gibson, V.; Graham, A. J. Macromolecules 2000,
33, 4329–4335. (b) Deng, H.; Shiono, T.; Soga, K. Macromolecules
1995, 28, 3067–3073. (c) Collins, S.; Ward, D. G.; Suddaby, K. H.
Macromolecules 1994, 27, 7222–7224.
(4) (a) Rodriguez-Delgado, A.; Chen, E. Y.-X. Macromolecules 2005, 38,
2587–2594. (b) Bolig, A. D.; Chen, E. Y.-X. J. Am. Chem. Soc. 2004,
126, 4897–4906.
(10) (a) Miyake, G. M.; Caporaso, L.; Cavallo, L.; Chen, E. Y.-X.
Macromolecules 2009, 42, 1462–1471. (b) Miyake, G. M.; Chen,
E. Y.-X. Macromolecules 2008, 41, 3405–3416. (c) Miyake, G. M.;
Mariott, W. R.; Chen, E. Y.-X. J. Am. Chem. Soc. 2007, 129, 6724–
6725. (d) Mariott, W. R.; Chen, E. Y.-X. Macromolecules 2005, 38,
6822–6832. (e) Mariott, W. R.; Chen, E. Y.-X. Macromolecules 2004,
37, 4741–4743.
(5) (a) Ning, Y.; Cooney, M. J.; Chen, E. Y.-X. J. Organomet. Chem.
2005, 690, 6263–6270. (b) Stojcevic, G.; Kim, H.; Taylor, N. J.;
Marder, T. B.; Collins, S. Angew. Chem., Int. Ed. 2004, 43, 5523–
5526. (c) Lian, B.; Toupet, L.; Carpentier, J.-F. Chem.sEur. J. 2004,
10, 4301–4307. (d) Bandermann, F.; Ferenz, M.; Sustmann, R.;
Sicking, W. Macromol. Symp. 2001, 174, 247–253. (e) Li, Y.; Ward,
D. G.; Reddy, S. S.; Collins, S. Macromolecules 1997, 30, 1875–
1883.
(11) (a) Bolig, A. D.; Chen, E. Y.-X. J. Am. Chem. Soc. 2001, 123, 7943–
7944. (b) Chen, E. Y.-X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks,
T. J. J. Am. Chem. Soc. 1998, 120, 6287–6305. (c) Soga, K.; Deng,
H.; Yano, T.; Shiono, T. Macromolecules 1994, 27, 7938–7940.
(12) (a) Razavi, A.; Thewalt, U. J. Organomet. Chem. 1993, 445, 111–
114. (b) Razavi, A.; Ferrara, J. J. Organomet. Chem. 1992, 435, 299–
310. (c) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am.
Chem. Soc. 1988, 110, 6255–6256.
(6) (a) Strauch, J. W.; Faure´, J.-L.; Bredeau, S.; Wang, C.; Kehr, G.;
Fro¨hlich, R.; Luftmann, H.; Erker, G. J. Am. Chem. Soc. 2004, 126,
2089–2104. (b) Jin, J.; Mariott, W. R.; Chen, E. Y.-X. J. Polym. Chem.
Part A: Polym. Chem. 2003, 41, 3132–3142. (c) Frauenrath, H.; Keul,
H.; Ho¨cker, H. Macromolecules 2001, 34, 14–19.
(7) (a) Ning, Y.; Caporaso, L.; Correa, A.; Gustafson, L. O.; Cavallo, L.;
Chen, E. Y.-X. Macromolecules 2008, 41, 6910–6919. (b) Lian, B.;
Thomas, C. M.; Navarro, C.; Carpentier, J.-F. Organometallics 2007,
26, 187–195. (c) Rodriguez-Delgado, A.; Mariott, W. R.; Chen, E. Y.-
X. Macromolecules 2004, 37, 3092–3100. (d) Nguyen, H.; Jarvis,
A. P.; Lesley, M. J. G.; Kelly, W. M.; Reddy, S. S.; Taylor, N. J.;
Collins, S. Macromolecules 2000, 33, 1508–1510.
(13) Rodriguez-Delgado, A.; Mariott, W. R.; Chen, E. Y.-X. J. Organomet.
Chem. 2006, 691, 3490–3497.
(14) Kirillov, E.; Lehmann, C. W.; Razavi, A.; Carpentier, J.-F. Organo-
metallics 2004, 23, 2768–2777.
(15) Qian, C.; Nie, W.; Chen, Y.; Sun, J. J. Organomet. Chem. 2002, 645,
82–86.
9
2696 J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010

Coordination Polymerization of Polar Vinyl Monomers
A R T I C L E S
Scheme 2. Stereoerror Formation in MMA Polymerization by Cs-Ligated Catalysts
[Me2Si(Me4Cp)(Flu)]YN(SiMe3)2;16 and Me2Si< bridged dys-
proso- and erbocenes, [Me2Si(Cp)(Flu)]LnE(SiMe3)2 (Ln )
Dy, Er; E ) CH, N).17 Like the Cs-ligated ansa-group 4
metallocene catalysts mentioned above, these Cs-ligated ansa-
lanthanocene catalysts are even less active and syndioselec-
tive (∼60% rr at 20 °C or ∼82% rr at -95 °C) than the
unbridged, C2V-ligated Cp*2LnR(THF)-type catalysts. In
addition, their catalyzed MMA polymerizations are also
uncontrolled and give very low initiator efficiencies (I* )
3-14%). Furthermore, methyl triad distributions of the
PMMA prepared by these Cs-ligated ansa-lanthanocenes are
characteristic of chain-end control. The Me2Si< bridged
yttrocene, [Me2Si(Me4Cp)(Flu)]YN(SiMe3)2, even produced
iso-rich PMMA (58% mm at 25 °C, 56% mm at 0 °C) in
toluene, despite its Cs ligation.
The above-cited large number of Cs-ligated group 3 and 4
metallocene complexes that failed to lead to st-PMMA with
appreciable syndiotacticity in a catalyst-site-controlled mech-
anism highlights the significant challenge in this effort and the
complexity of Cs-ligated catalysts in coordination polymeriza-
tion. In propylene polymerization by Cs-ligated catalysts, several
pathways can introduce stereoerrors, including enantiofacial
misinsertion (mm type), site epimerization (m type), “back-side”
misinsertion (mr type), and chain epimerization (m or mm
type).18 In MMA polymerization by Cs-ligated catalysts, several
possible pathways could, in principle, lead to mm- and m-type
stereoerrors (Scheme 2). Starting with a chain where an
S-catalyst selects a si-enolate chain end, this correct addition
(C.A.) of MMA leads to an S chiral center in the chain and an
R catalyst site; this regular migratory addition sequence then
forms the syndiotactic polymer (rrrr..), case A. A misaddition
(M.A.) affords a kinetic product (the higher energy RS than
the RS after a correct addition); this stereomistake can be
corrected either by catalyst-site epimerization to the thermo-
dynamic RS followed by C.A. of MMA producing the isolated
m dyad stereoerror, or by subsequent C.A. of MMA generating
the mm type stereoerror (i.e., the enantiofacial misaddition error
by a site-control mechanism), case B. Alternatively, the m-type
stereoerror can also be generated either by the conventional site
epimerization (S.E.) route typically seen in the propylene
polymerization, or by back-side addition (B.A.) route, case C.
A rather successful example of MMA polymerization by the
Cs-ligated catalyst, in terms of activity and polymerization
control, involves the use of a CGC-type catalyst such as
{(CGC)Ti(THF)[OC(OiPr)dCMe2]}+[MeB(C6F5)3]- [CGC )
Me2Si(η5-(Me4C5)(tBuN)];7c however, it produces st-PMMA
with modest syndiotacticity (80% rr, 18% mr, 2.0% mm) at
room temperature with predominately isolated m meso dyad
stereoerrors (...rrrrmrrrr...), pointing to the apparent chain-end
control nature of this Cs-ligated catalysts.7c A combined
experimental and theoretical study shows the monomer addition
to be stereoselective and controlled by the chiral catalyst, while
MMA- or counterion-assisted catalyst-site epimerization after
a stereomistake, the process of which converts the kinetic
product after a stereomistake into a thermodynamically more
stable resting chelating intermediate, accounts for the formation
of the predominately isolated m stereoerrors (Scheme 2).7a It is
fascinating to point out here that, in the syndiospecific polym-
erization of propylene by Cs-ligated complexes, catalyst-site
epimerization generates m stereoerrors, whereas in the polym-
erization of MMA by Cs-ligated complexes, catalyst-site epimer-
ization corrects a stereomistake made in a previous enantiofacial
misaddition (Scheme 2).
In light of the above unsuccessful attempts toward the site-
controlled synthesis of highly syndiotactic PMMA, naturally it
came as a big surprise when we first discovered, in 2008, that
Ph2C< bridged, Cs-ligated ansa-zirconocene mono(ester enolate)
and bis(ester enolate) complexesswhich, upon activation with
B(C6F5)3 (as a THF adduct) and [Ph3C][B(C6F5)4], generate the
corresponding chiral cationic catalysts 3 and 4, respectively
(Chart 1)sproduce highly syndiotactic PMMA (94% rr) at 25
°C via a predominately site-controlled mechanism.19 Impres-
sively, this high level of syndiotacticity remains even at a
temperature of polymerization (Tp) of 50 °C (93% rr). This
exciting discovery brought forth the following four important
fundamental questions: (1) Why does this class of the Ph2C<
bridged Cs-symmetric catalysts produce highly syndiotactic
PMMA while others do not? (2) What is the role of the bridging
atom and its substituents (i.e., R2E<, R ) Me vs Ph; E ) C vs
Si)? (3) How do ligand substitution, metal, anion, concentration,
solvent, and temperature affect the polymerization characteris-
tics, especially stereoregulation? (4) What are the general
guidelines for devising a Cs-ligated catalyst with high activity
and syndiospecificity based on what we have learned so far?
Accordingly, the central objective of this study was to address
these important questions both through experiments involving
systematic variations of the catalyst or precatalyst structure
(16) Lee, M. H.; Hwang, J.-W.; Kim, Y.; Kim, J.; Han, Y.; Do, Y.
Organometallics 1999, 18, 5124–5129.
(17) (a) Nie, W.; Qian, C.; Chen, Y.; Sun, J. J. Organomet. Chem. 2002,
647, 114–122. (b) Qian, C.; Nie, W.; Sun, J. Organometallics 2000,
19, 4134–4140.
(18) Chen, M.-C.; Roberts, J. A. S.; Marks, T. J. J. Am. Chem. Soc. 2004,
126, 4605–4625.
(19) Ning, Y.; Chen, E. Y.-X. J. Am. Chem. Soc. 2008, 130, 2463–2465.
9
J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010 2697

A R T I C L E S
Zhang et al.
Chart 1. Ansa-Zirconocene Complexes Incorporating Cs-Symmetric [R2E(Cp)(Flu)]-Based Ligands and Their Variants Investigated in This
Study
t
i
{[Ph2C(Cp)(2,7-Bu2-Flu)]Zr(THF)[OC(OPr)dCMe2]}+[MeB(C6F5)3]-
(Chart 1), activation methodology, and reaction conditions and
through theoretical/computational investigations.
{6a+[MeB(C6F5)3]-}. H NMR (C7D8, 23 °C): δ 7.90 (d, J ) 8.8
1
Hz, 1H, Flu), 7.85 (d, J ) 8.8 Hz, 1H, Flu), 7.74 (d, J ) 8.0 Hz,
1H, Ph), 7.68 (d, J ) 8.0 Hz, 1H, Ph), 7.30-6.94 (m, 10H, Flu,
Ph), 6.38 (br, s, 3H, Cp, Flu), 5.90 (s, 1H, Cp), 5.66 (s, 1H, Cp),
5.55 (s, 1H, Cp), 3.67 (sept, J ) 6.0 Hz, 1H, CHMe2), 3.05 (q, J
) 6.4 Hz, 2H, R-CH2, THF), 2.91 (q, J ) 6.0 Hz, 2H, R-CH2,
THF), 1.38 (s, 3H, dCMe), 1.30-1.22 (m, 4H, ꢀ-CH2, THF), 1.19
(s, 3H, dCMe), 0.98 (s, 9H, Bu), 0.95 (s, 9H, Bu), 0.93 (br, s,
3H, B-Me) (note the B-Me peak appeared at 0.49 ppm in CD2Cl2),
0.89-0.80 (m, 6H, CHMe2). 19F NMR (C7D8, 23 °C): δ -131.9
(d, JF-F ) 19.7 Hz, 6F, o-F), -164.4 (t, JF-F ) 20.9 Hz, 3F, p-F),
-166.9 (t, JF-F ) 18.8 Hz, 6F, m-F). 13C NMR (CD2Cl2, 23 °C):
δ 154.9 [OC(OiPr)d], 152.6, 151.8, 144.2, 143.9, 129.7, 129.6,
129.5, 129.3, 128.5, 127.7, 126.7, 126.4, 125.6, 125.0, 124.6, 123.6,
123.5, 122.9, 121.9, 121.4, 120.0, 119.6, 118.7, 117.8, 117.2, 108.0,
104.0, 89.85 (Flu, Ph, and Cp carbons; broad resonances for the
C6F5 groups due to C-F coupling omitted), 85.98 (dCMe), 78.34
(R-CH2, THF), 72.39 (CHMe2), 59.08 (CPh2), 35.46, 35.01
(C(CH3)3), 30.54, 30.48 (C(CH3)3), 25.88 (ꢀ-CH2, THF), 22.21,
21.66 (CHMe2), 18.71, 17.43 (dCMe), 11.64 (B-Me).
Experimental Section
[Ph2C(Cp)(Flu)]Ti[OC(OiPr)dCMe2]2 (5, M ) Ti). Anal.
Calcd for C45H48O4Ti: C, 77.13; H, 6.90. Found: C, 76.94; H, 6.71.
1H NMR (C7D8, 23 °C): δ 8.02 (d, J ) 7.8 Hz, 4H, Flu, Ph), 7.13
(t, J ) 7.2 Hz, 4H, Flu), 6.88-6.81 (m, 8H, Ph), 6.35 (q, J ) 2.7
Hz, 2H, Cp), 6.30 (d, J ) 8.1 Hz, 2H, Flu), 6.28 (q, J ) 2.7 Hz,
2H, Cp), 3.64 (sept, J ) 6.3 Hz, 2H, CHMe2), 1.51 (s, 6H, dCMe2),
1.32 (s, 6H, dCMe2), 0.98 (d, J ) 6.0 Hz, 6H, CHMe2), 0.88 (d,
J ) 6.3 Hz, 6H, CHMe2). 13C NMR (C7D8, 23 °C): δ 160.1
[OC(OiPr)d], 146.3, 138.2, 135.2, 131.7, 127.5, 126.9, 125.8, 125.3,
123.5, 121.3, 119.8, 115.2, 110.6, 74.27 (Flu, Ph, and Cp carbons),
89.80 (dCMe), 69.93 (CHMe2), 58.71 (CPh2), 22.49, 22.11
(CHMe2), 18.16, 17.92 (dCMe).
t
t
[Ph2C(Cp)(Flu)]Hf[OC(OiPr)dCMe2]2 (5, M ) Hf). Anal.
Calcd for C45H48O4Hf: C, 65.01; H, 5.82. Found: C, 64.74; H, 5.78.
1H NMR (C7D8, 23 °C): δ 8.05 (d, J ) 8.1 Hz, 2H, Flu), 7.84 (d,
J ) 8.1 Hz, 2H, Ph), 7.55 (d, J ) 8.1 Hz, 2H, Ph), 7.15-6.78 (m,
12H, Flu, Ph), 6.32 (t, J ) 2.6 Hz, 2H, Cp), 6.06 (t, J ) 2.6 Hz,
2H, Cp), 3.64 (sept, J ) 6.0 Hz, 2H, CHMe2), 1.63 (s, 6H, dCMe2),
1.44 (s, 6H, dCMe2), 1.04 (d, J ) 6.0 Hz, 6H, CHMe2), 0.92 (d,
J ) 6.0 Hz, 6H, CHMe2). 13C NMR (C7D8, 23 °C): δ 154.1
[OC(OiPr)d], 147.0, 130.5, 129.4, 129.2, 127.5, 127.4, 123.7, 123.1,
123.0, 122.8, 122.4, 120.8, 114.7, 103.0, 86.12 (Flu, Ph, and Cp
carbons), 87.30 (dCMe), 68.84 (CHMe2), 59.35 (CPh2), 22.56,
22.01 (CHMe2), 18.30, 18.19 (dCMe).
t
i
i
{[Ph2C(Cp)(2,7-Bu2-Flu)]Zr[OC(OPr)dCMeCH2C(Me2)C(OPr)dO]}+
[HB(C6F5)3]- {6b+[HB(C6F5)3]-}. H NMR (CD2Cl2, 23 °C): δ
1
8.26 (d, J ) 8.8 Hz, 1H, Flu), 8.09 (d, J ) 8.8 Hz, 1H, Flu), 8.02
(d, J ) 8.0 Hz, 1H, Ph), 7.98 (d, J ) 8.0 Hz, 1H, Ph), 7.93 (d, J
) 7.6 Hz, 2H, Ph), 7.62-7.39 (m, 8H, Flu, Ph), 6.62 (s, 1H, Flu),
6.54 (s, 1H, Flu), 6.54-6.52 (m, 1H, Cp), 5.97 (q, J ) 2.9 Hz, 1H,
Cp), 5.92 (q, J ) 2.8 Hz, 1H, Cp), 5.84 (q, J ) 2.8 Hz, 1H, Cp),
4.26 (sept, J ) 6.0 Hz, 1H, CHMe2), 3.31 (sept, J ) 6.2 Hz, 1H,
CHMe2), 2.44 (d, 2J ) 15.2 Hz, 1H, CH2), 1.58 (d, 2J ) 14.8 Hz,
1H, CH2), 1.44 (s, 3H, dCMe), 1.30 (s, 3H, CMe2), 1.24 (d, J )
6.4 Hz, 3H, CHMe2), 1.18 (s, 3H, CMe2), 1.10 (d, J ) 6.4 Hz, 3H,
[Ph2C(Cp)(2,7-tBu2-Flu)]Zr[OC(OiPr)dCMe2]2 (6). Anal. Cal-
cd for C53H64O4Zr: C, 74.34; H, 7.53. Found: C, 75.05; H, 7.61.
1H NMR (C6D6, 23 °C): δ 8.08 (d, J ) 8.8 Hz, 2H, Flu), 7.95 (d,
J ) 8.0 Hz, 2H, Ph), 7.54 (d, J ) 8.0 Hz, 2H, Ph), 7.33 (d, J ) 8.4
Hz, 2H, Flu), 7.16 (t, J ) 7.6 Hz, 2H, Ph), 7.06 (t, J ) 7.6 Hz, 2H,
Ph), 6.98 (t, J ) 7.2 Hz, 2H, Ph), 6.74 (s, 2H, Flu), 6.49 (m, 2H,
Cp), 6.25 (m, 2H, Cp), 3.93 (sept, J ) 6.0 Hz, 2H, CHMe2), 1.64
(s, 6H, dCMe2), 1.51 (s, 6H, dCMe2), 1.21 (s, 18H, tBu), 1.10 (d,
J ) 6.8 Hz, 6H, CHMe2), 1.08 (d, J ) 6.0 Hz, 6H, CHMe2). 13C
NMR (C6D6, 23 °C): δ 154.8 [OC(OiPr)d], 148.7, 146.4, 130.5,
128.8, 127.1, 126.7, 123.0, 122.4, 122.3, 121.6, 119.6, 118.5, 115.3,
103.8, 87.42 (Flu, Ph, and Cp carbons), 84.93 (dCMe2), 69.30
(CHMe2), 58.66 (CPh2), 35.22 (C(CH3)3), 31.11 (C(CH3)3), 22.55,
22.11 (CHMe2), 19.16, 17.83 (dCMe).
t
CHMe2), 1.07 (s, 18H, Bu), 1.04 (d, J ) 6.4 Hz, 3H, CHMe2),
0.94 (dd, 1J ) 6.0 Hz, 1H, BH), 0.84 (d, J ) 6.0 Hz, 3H, CHMe2).
19F NMR (CD2Cl2, 23 °C): δ -132.2 (d, JF-F ) 22.3 Hz, 6F, o-F),
-163.1 (t, JF-F ) 20.3 Hz, 3F, p-F), -165.9 (m, 6F, m-F). 13C
NMR (CD2Cl2, 23 °C): δ 190.7 [C(OiPr)dO], 155.80 [OC(OiPr)d],
153.5, 153.2, 144.0, 143.9, 129.7, 129.64, 129.59, 129.4, 128.0,
126.8, 126.7, 125.6, 124.2, 124.0, 123.7, 122.7, 121.8, 121.4, 120.7,
119.0, 117.6, 117.2, 115.2, 106.0, 102.8, 82.24 (Flu, Ph, C6F5, and
Cp carbons; broad resonances for the C6F5 groups due to C-F
coupling omitted), 86.52 (dCMe), 75.51, 68.87 (CHMe2), 59.25
9
2698 J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010

Coordination Polymerization of Polar Vinyl Monomers
A R T I C L E S
(CPh2), 47.59 (CMe2), 40.58 (CH2), 35.59, 35.43 (C(CH3)3), 31.93
(CMe2), 30.82, 30.75 (C(CH3)3), 24.76 (CMe2), 22.46, 22.10, 21.82,
21.33 (CHMe2), 16.92 (dCMe). 11B NMR (CD2Cl2, 23 °C): δ -25.4
2H, Flu), 7.44-7.32 (m, 6H, Ph), 6.73 (s, br, 1H, Cp), 6.36 (s, 1H,
Flu), 6.35 (s, 1H, Flu), 6.03 (s, 2H, dCH2, MMA, Cp), 5.87 (s, br,
2H, Cp), 5.57 (s, 1H, dCH2, MMA), 4.31 (sept, J ) 6.0 Hz, 1H,
CHMe2), 3.65 (s, 3H, OMe, MMA), 3.60 (sept, J ) 6.0 Hz, 1H,
CHMe2), 3.36 (q, J ) 7.0 Hz, 8H, OCH2CH3), 2.06 (sept, J ) 6.3
Hz, 1H, CHMe2), 1.85 (s, 3H, dCMe, MMA), 1.20 (s, 3H, dCMe2),
1.16 (s, 3H, dCMe2), 1.14-1.12 (d, br, 3H, CHMe2), 1.09 (t, J )
7.0 Hz, 12H, OCH2CH3), 1.07-1.03 (t, J ) 5.6 Hz, 6H, CHMe2),
0.96 (s, 9H, Bu), 0.94 (s, 9H, Bu), 0.96-0.94 (m, 6H, CHMe2),
0.89 (d, J ) 6.9 Hz, 3H, CHMe2). 19F NMR (CD2Cl2, 282 MHz,
-80 °C): δ -131.8 (d, J ) 12.7 Hz, 8F, o-F of C6F5), -160.7 (t,
J ) 21.7 Hz, 4F, p-F of C6F5), -164.7 (t, J ) 18.9 Hz, 8F, m-F of
C6F5). 1H NMR (CD2Cl2, 300 MHz, 20 °C) for 6f+[B(C6F5)4]- and
PMMA (only major peaks are reported here): δ 4.53 (sept, J ) 6.0
Hz, 1H, CHMe2), 4.26 (pent, J ) 6.3 Hz, 1H, CHMe2), 3.61 (s, br,
OMe, PMMA), 2.93 (s, 3H, OMe), 2.42 (d, J ) 15 Hz, 1H, CH2),
2.24 (pent, J ) 6.9 Hz, 1H, CHMe2), 1.90 (s, br, CH2, PMMA),
1.59 (d, J ) 15 Hz, 1H, CH2). 19F NMR (CD2Cl2, 282 MHz, 20
°C): δ -131.4 (s, br, 8F, o-F of C6F5), -161.9 (t, J ) 20.3 Hz, 4F,
p-F of C6F5), -165.8 (t, J ) 18.2 Hz, 8F, m-F of C6F5).
1
(d, JB-H ) 93.7 Hz).
{[Ph2C(Cp)(2,7-tBu2-Flu)] Zr[OC(OiPr)dCMe2][OdC(OiPr)-
CMe2CH(CHdCH)2CdCPh2]}+[B(C6F5)4]-
{6c+[B(C6F5)4]-}
and {[Ph2C(Cp)(2,7-tBu2-Flu)]Zr[OC(OiPr)dCMeCH2C(Me2)-
C(OiPr)dO]}+[B(C6F5)4]- {6d+[B(C6F5)4]-}. 1H NMR (CD2Cl2,
300 MHz, -60 °C) for 6c+[B(C6F5)4]-: δ 6.60 (d, J ) 11.7 Hz,
1H, Ph2dC(CHdCH)2CH), 6.58 (s, 2H, Flu), 6.55 (d, J ) 12.6
Hz, 1H, Ph2dC(CHdCH)2CH), 6.32 (s, br, 1H, Cp), 6.00 (m, 1H,
Cp), 5.84 (m, 1H, Cp), 5.79 (m, 1H, Cp), 5.58 (d, J ) 9.6 Hz, 1H,
C(CHdCH)2CH), 5.02 (d, J ) 9.6 Hz, 1H, C(CHdCH)2CH), 4.80
(pent, J ) 6.0 Hz, 1H, CHMe2), 3.75 (pent, J ) 6.0 Hz, 1H,
CHMe2), 3.37 (s, br, 1H, C(CHdCH)2CH). 13C NMR (CD2Cl2, 300
MHz, -60 °C) for 6c+[B(C6F5)4]-: δ 190.8 [C(OiPr)dO], 156.9
[OC(OiPr)d], 151.7, 151.1, 144.6, 143.8, 141.3, 131.0, 129.9, 129.8,
129.6, 128.7, 128.4, 128.1, 128.0, 127.1, 126.8, 126.5, 126.2, 125.4,
125.0, 124.8, 123.5, 123.1, 122.5, 121.5, 120.6, 119.0, 118.4, 117.7,
116.8, 107.1, 103.8, 84.73 (Flu, Ph, C6F5, and Cp carbons; broad
resonances for the C6F5 groups due to C-F coupling omitted), 88.93
(dCMe2), 76.96, 74.70 (OCHMe2), 58.56 (CPh2), 49.22, 37.12
(CHMe2), 35.92, 35.43 (C(CH3)3), 30.66, 30.50 (C(CH3)3), 22.31,
21.90, 21.29, 21.04 (CHMe2), 18.39 (dCMe2), 17.63 (CHMe2). 1H
NMR (CD2Cl2, 300 MHz, -60 °C) for 6d+[B(C6F5)4]- and Ph3CH:
δ 6.49 (s, 1H, Flu), 6.41 (s, 1H, Flu), 6.32 (s, br, 1H, Cp), 5.95 (m,
1H, Cp), 5.84 (m, 1H, Cp), 5.79 (m, 1H, Cp), 5.56 (s, 1H, Ph3CH),
4.16 (pent, J ) 6.2 Hz, 1H, CHMe2), 3.20 (pent, J ) 6.0 Hz, 1H,
CHMe2), 2.31 (d, 2J ) 14.8 Hz, 1H, CH2), 1.50 (d, 2J ) 15.0 Hz,
1H, CH2). 13C NMR (CD2Cl2, 300 MHz, -60 °C) for
t
t
[Ph2C(Cp)(2-tBu-Flu)]Zr[OC(OiPr)dCMe2]2 (7). Anal. Calcd for
C49H56O4Zr: C, 73.55; H, 7.05. Found: C, 73.28; H, 7.04. Selected
crystal structural data for complex 7: C55H56O4Zr, T ) 100(2) K,
λ ) 0.71073 Å, monoclinic, space group P21/c, a ) 9.5340(9) Å,
b ) 11.8513(12) Å, c ) 41.303(4) Å, ꢀ ) 93.213(2)°, V )
4659.5(8) Å3, Z ) 4, Dcalcd ) 1.243 Mg/m3, GOF on F2 ) 0.986,
R1 ) 0.0555 and wR2 ) 0.1279 with I > 2σ(I). 1H NMR (C6D6, 23
°C): δ 8.10 (d, J ) 8.4 Hz, 1H, Flu), 8.09 (d, J ) 8.7 Hz, 1H,
Flu), 7.93 (d, J ) 8.1 Hz, 1H, Ph), 7.87 (d, J ) 8.1 Hz, 1H, Ph),
7.69 (d, J ) 8.1 Hz, 1H, Ph), 7.48 (d, J ) 7.8 Hz, 1H, Ph), 7.30
3
4
6d+[B(C6F5)4]- and Ph3CH:
δ
190.3 [C(OiPr)dO], 156.2
(dd, J ) 8.7 Hz, J ) 1.5 Hz, 1H, Ph), 7.18-6.83 (m, 9H, Flu,
Ph), 6.71 (d, 4J ) 1.5 Hz, 1H, Flu), 6.44 (q, J ) 2.4 Hz, 1H, Cp),
6.33 (q, J ) 2.4 Hz, 1H, Cp), 6.22 (q, J ) 2.7 Hz, 1H, Cp), 6.15
(q, J ) 2.7 Hz, 1H, Cp), 3.86 (sept, J ) 6.2 Hz, 1H, CHMe2), 3.76
(sept, J ) 6.2 Hz, 1H, CHMe2), 1.69 (s, 3H, dCMe2), 1.62 (s, 3H,
dCMe2), 1.50 (s, 3H, dCMe2), 1.48 (s, 3H, dCMe2), 1.21 (s, 9H,
tBu), 1.07 (d, J ) 6.3 Hz, 6H, CHMe2), 1.04 (d, J ) 6.3 Hz, 3H,
CHMe2), 0.96 (d, J ) 6.3 Hz, 3H, CHMe2). 13C NMR (C6D6, 23
°C): δ 154.8 [OC(OiPr)d], 153.9 [C(OiPr)dO], 149.3, 146.4, 146.2,
130.4, 129.9, 128.80, 128.76, 128.6, 127.8, 127.1, 126.8, 126.7,
126.6, 126.5, 123.2, 122.8, 122.7, 122.5, 122.3, 121.3, 120.8, 119.9,
118.2, 115.5, 115.4, 104.3, 103.7, 86.67 (Flu, Ph, and Cp carbons),
85.91, 85.28 (dCMe), 69.35, 68.13 (CHMe2), 58.78(CPh2), 35.18
(C(CH3)3), 31.02 (C(CH3)3), 22.54, 22.08, 21.99, 21.53 (CHMe2),
18.90, 17.93, 17.86, 17.70 (dCMe).
[OC(OiPr)d], 154.0, 153.7 (Flu), 106.5, 103.2, 82.7 (Cp), 86.97
(dCMe), 75.95, 69.30 (OCHMe2), 59.15 (CPh2), 57.46 (Ph3CH),
48.03 (CMe2), 41.00 (CH2), 36.01, 35.86 (C(CH3)3), 32.33 (CMe2),
31.03, 30.91 (C(CH3)3), 23.42 (CMe2), 22.82, 22.49, 22.22, 21.96
(CHMe2), 17.34 (dCMe). 19F NMR (CD2Cl2, 282 MHz, -60 °C)
for both 6c+[B(C6F5)4]- and 6d+[B(C6F5)4]-: δ -131.7 (d, J )
12.7 Hz, 8F, o-F of C6F5), -161.0 (t, J ) 21.0 Hz, 4F, p-F of
C6F5), -164.9 (t, J ) 18.9 Hz, 8F, m-F of C6F5).
{[Ph2C(Cp)(2,7-tBu2-Flu)]Zr[OC(OiPr)dCMe2][OdC(OiPr)-
CHMe2]}+[B(C6F5)4]- {6e+[B(C6F5)4]-} and {[Ph2C(Cp)(2,7-tBu2-Flu)]-
i
Zr[OC(OMe)dCMeCH2C(Me2)C(OPr)dO]}+[B(C6F5)4]- {6f+[B(C6F5)4]-}.
1H NMR (CD2Cl2, 300 MHz, -60 °C) for 6e+[B(C6F5)4]-: δ 8.14
(d, J ) 8.7 Hz, 1H, Flu), 8.10 (d, J ) 9.0 Hz, 1H, Flu), 7.97 (t, J
) 6.2 Hz, 2H, Ph), 7.82-7.78 (m, 2H, Ph), 7.55-7.48 (m, 2H,
Flu), 7.45-7.33 (m, 6H, Ph), 6.72 (q, J ) 2.4 Hz, 1H, Cp), 6.42
(s, 1H, Flu), 6.39 (s, 1H, Flu), 6.03 (q, J ) 2.4 Hz, 1H, Cp),
5.89-5.85 (m, 2H, Cp), 4.36 (sept, J ) 6.0 Hz, 1H, CHMe2), 3.63
(sept, J ) 6.0 Hz, 1H, CHMe2), 3.38 (q, J ) 7.2 Hz, 8H,
OCH2CH3), 2.07 (sept, J ) 6.6 Hz, 1H, CHMe2), 1.24 (s, 3H,
dCMe2), 1.19 (s, 3H, dCMe2), 1.14-1.12 (d, br, 3H, CHMe2),
1.11 (t, J ) 7.2 Hz, 12H, OCH2CH3), 1.08-1.04 (t, J ) 6.0 Hz,
[Ph2C(3-tBu-Cp)(Flu)]Zr[OC(OiPr)dCMe2]2 (8). Anal. Calcd
1
for C49H56O4Zr: C, 73.55; H, 7.05. Found: C, 73.51; H, 6.82. H
NMR (C6D6, 23 °C): δ 8.12 (d, J ) 8.0 Hz, 2H, Flu), 8.09 (d, J )
9.2 Hz, 1H, Ph), 7.96 (d, J ) 8.0 Hz, 1H, Ph), 7.60 (d, J ) 8.0 Hz,
1H, Ph), 7.55 (d, J ) 8.0 Hz, 1H, Ph), 7.20-6.80 (m, 12H, Flu,
Ph), 6.51 (t, J ) 2.8 Hz, 1H, Cp), 6.28 (t, J ) 3.0 Hz, 1H, Cp),
6.20 (t, J ) 2.6 Hz, 1H, Cp), 3.73 (sept, J ) 6.0 Hz, 2H, CHMe2),
1.62 (s, 6H, dCMe2), 1.50 (s, 3H, dCMe2), 1.42 (s, 3H, dCMe2),
t
t
6H, CHMe2), 0.98 (s, 9H, Bu), 0.97 (s, 9H, Bu), 0.98-0.97 (m,
6H, CHMe2), 0.92 (d, J ) 6.9 Hz, 3H, CHMe2). 19F NMR (CD2Cl2,
282 MHz, -60 °C): δ -131.7 (d, J ) 12.7 Hz, 8F, o-F of C6F5),
-161.0 (t, J ) 21.0 Hz, 4F, p-F of C6F5), -165.0 (t, J ) 18.2 Hz,
8F, m-F of C6F5). 13C NMR (CD2Cl2, -60 °C): δ 188.7
[C(OiPr)dO], 154.3 [OC(OiPr)d], 151.9, 150.8, 144.0, 143.6,
129.4, 129.3, 129.2, 129.0, 127.7, 126.5, 126.3, 124.7, 123.6, 122.9,
122.6, 122.2, 120.3, 119.2, 118.9, 118.0, 116.8, 116.7, 116.1, 105.6,
103.2, 84.62 (Flu, Ph, C6F5, and Cp carbons; broad resonances for
the C6F5 groups due to C-F coupling omitted), 89.25 (dCMe2),
75.56, 73.10 (OCHMe2), 65.98 (OCH2CH3), 58.11 (CPh2), 36.75,
36.52 (C(CH3)3), 35.14 (CHMe2), 30.40, 30.14 (C(CH3)3), 22.23,
22.02, 21.13, 21.05 (CHMe2), 19.40, 18.72 (dCMe2), 16.84
(CHMe2), 15.28 (OCH2CH3).
t
1.22 (s, 9H, Bu), 1.10 (d, J ) 6.0 Hz, 3H, CHMe2), 1.05 (d, J )
5.6 Hz, 3H, CHMe2), 1.04 (d, J ) 5.2 Hz, 3H, CHMe2), 0.92 (d, J
) 6.0 Hz, 3H, CHMe2). 13C NMR (C6D6, 23 °C): δ 155.0, 153.4
[OC(OiPr)d], 146.7, 146.3, 144.4, 130.3, 130.1, 128.8, 128.7, 128.6,
128.6, 127.8, 126.91, 126.86, 126.7, 126.3, 123.7, 123.10, 123.05,
122.83, 122.78, 122.6, 122.5, 122.3, 122.2, 122.1, 119.5, 112.6,
105.8, 102.1, 85.57 (Flu, Ph, and Cp carbons), 87.84, 87.19
(dCMe), 69.32, 68.47 (CHMe2), 58.30 (CPh2), 32.70 (C(CH3)3),
31.00 (C(CH3)3), 22.32, 22.19, 21.42 (CHMe2), 19.95, 18.95, 18.36,
18.22 (dCMe).
[Ph2C(Cp)(Oct-Flu)]Zr[OC(OiPr)dCMe2]2 (9). Anal. Calcd for
C61H76O4Zr: C, 75.96; H, 7.94. Found: C, 75.95; H, 7.92. 1H NMR
(C6D6, 23 °C): δ 8.33 (s, 2H, Flu), 7.94 (d, J ) 7.6 Hz, 2H, Ph),
7.50 (d, J ) 8.0 Hz, 2H, Ph), 7.16 (t, J ) 7.6 Hz, 2H, Ph), 7.09 (t,
J ) 7.6 Hz, 2H, Ph), 6.99 (t, J ) 7.2 Hz, 2H, Ph), 6.75 (s, 2H,
Flu), 6.38 (s, 2H, Cp), 6.13 (s, 2H, Cp), 3.87 (sept, J ) 6.0 Hz,
1H NMR (CD2Cl2, 300 MHz, -80 °C) for 6e+[B(C6F5)4]- and
MMA: δ 8.14 (d, J ) 8.7 Hz, 1H, Flu), 8.09 (d, J ) 8.7 Hz, 1H,
Flu), 7.96 (d, J ) 7.5 Hz, 2H, Ph), 7.78 (s, br, 2H, Ph), 7.50 (s, br,
9
J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010 2699

A R T I C L E S
Zhang et al.
2H, CHMe2), 1.84 (m, 4H, CH2), 1.67 (s, 6H, dCMe2), 1.64-1.62
(m, 2H, CH2), 1.56 (s, 6H, dCMe2), 1.54-1.52 (m, 2H, CH2), 1.42
(s, 6H, CH3), 1.39 (s, 6H, CH3), 1.16 (s, 6H, CH3), 1.15 (s, 6H,
CH3), 1.08 (d, J ) 6.0 Hz, 6H, CHMe2), 1.05 (d, J ) 6.0 Hz, 6H,
CHMe2). 13C NMR (C6D6, 23 °C): δ 155.3 [OC(OiPr)d], 146.4,
144.9, 141.2, 130.7, 128.8, 128.7, 127.1, 126.6, 122.8, 122.0, 120.7,
120.3, 119.8, 115.6,103.7, 84.60 (Flu, Ph, and Cp carbons), 85.00
(dCMe), 68.24 (CHMe2), 58.70 (CPh2), 35.64, 35.41, 35.21, 35.04
(CH0 and CH2), 33.53, 33.08, 32.37, 30.82 (CH3), 22.60, 22.20
(CHMe2), 19.20, 17.98 (dCMe).
[(p-Et3SiPh)2C(Cp)(2,7-tBu2-Flu)]Zr[OC(OiPr)dCMe2]2 (10).
Anal. Calcd for C65H92O4Si2Zr: C, 71.97; H, 8.55. Found: C, 71.71;
H, 8.62. 1H NMR (C6D6, 23 °C): δ 8.12 (dd, 3J ) 7.8 Hz, 4J ) 1.2
Hz, 2H, Ph), 8.10 (d, J ) 8.8 Hz, 2H, Flu), 7.74 (dd, 3J ) 7.8 Hz,
4J ) 1.2 Hz, 2H, Ph), 7.50 (d, J ) 8.0 Hz, 2H, Ph), 7.41 (d, J )
8.0 Hz, 2H, Ph), 7.34 (d, J ) 8.4 Hz, 2H, Flu), 6.79 (s, 2H, Flu),
6.53 (t, J ) 2.6 Hz, 2H, Cp), 6.33 (t, J ) 2.6 Hz, 2H, Cp), 3.94
(sept, J ) 6.2 Hz, 2H, CHMe2), 1.64 (s, 6H, dCMe2), 1.51 (s, 6H,
Figure
1.
Molecular structure of [Ph2C(Cp)(2-tBu-Flu)]Zr-
[OC(OiPr)dCMe2]2 (7) with thermal ellipsoids drawn at the 50% probability.
Selected bond lengths (Å): Zr-O(1) ) 1.964(3), Zr-O(3) ) 1.981(3),
Zr-C(1) ) 2.546(4), Zr-C(2) ) 2.491(4), Zr-C(3) ) 2.490(4), Zr-C(4)
) 2.486(4), Zr-C(5) ) 2.539(4), Zr-C(6) ) 2.575(4), Zr-C(9) ) 2.571(4),
Zr-C(10) ) 2.491(4), Zr-C(7) ) 2.735(4), Zr-C(8) ) 2.727(4).
t
dCMe2), 1.23 (s, 18H, Bu), 1.10 (d, J ) 6.4 Hz, 6H, CHMe2),
1.08 (d, J ) 6.0 Hz, 6H, dCMe2), 0.93 (t, J ) 8.0 Hz, 18H,
CH3CH2Si), 0.69 (q, J ) 8.0 Hz, 12H, CH3CH2Si). 13C NMR (C6D6,
23 °C): δ 155.2 [OC(OiPr)d], 149.1, 147.2, 136.2, 135.3, 135.2,
130.3, 126.9, 123.4, 122.8, 122.7, 121.9, 120.0, 118.9, 115.7, 104.3,
87.79 (Flu, Ph, and Cp carbons), 85.34 (dCMe), 69.73 (CHMe2),
59.09 (CPh2), 35.65 (C(CH3)3), 31.61 (C(CH3)3), 22.92, 22.50
(CHMe2), 19.59, 18.22 (dCMe), 8.00 (CH3CH2Si), 4.03
(CH3CH2Si).
[Me2Si(Cp)(Flu)]Zr[OC(OiPr)dCMe2]2 (11). Selected crystal
structural data for complex 11: C34H44O4SiZr, T ) 120(2) K, λ )
j
0.71073 Å, triclinic, space group P1, a ) 10.8310(4) Å, b )
12.4790(4) Å, c ) 14.1278(5) Å, R ) 90.664(2)°, ꢀ ) 111.403(2)°,
γ ) 113.539(2)°, V ) 1601.96(10) Å3, Z ) 2, Dcalcd ) 1.319 Mg/
m3, GOF on F2 ) 1.030, R1 ) 0.0299 and wR2 ) 0.0737 with I >
Figure 2. Molecular structure of [Me2Si(Cp)(Flu)]Zr[OC(OiPr)dCMe2]2
(11) with thermal ellipsoids drawn at the 50% probability. Selected bond
lengths (Å): Zr-O(1) ) 1.9923(8), Zr-O(3) ) 1.9698(7), Zr-C(1) )
2.5308(10), Zr-C(2) ) 2.5416(11), Zr-C(3) ) 2.4922(11), Zr-C(4) )
2.5007(10), Zr-C(5) ) 2.4874(10), Zr-C(7) ) 2.6173(10), Zr-C(8) )
2.4975(10), Zr-C(9) ) 2.6117(10), Zr-C(6) ) 2.7378(10), Zr-C(10) )
2.7611(10), Zr-Cp(centroid) ) 2.201, Zr-Flu(centroid) ) 2.345.
1
2σ(I). H NMR (C6D6, 23 °C): δ 8.03 (d, J ) 7.8 Hz, 2H, Flu),
7.55 (d, J ) 7.8 Hz, 2H, Flu), 7.18-7.08 (m, 6H, Flu), 6.56 (t, J
) 2.4 Hz, 2H, Cp), 6.02 (t, J ) 2.4 Hz, 2H, Cp), 3.67 (sept, J )
6.2 Hz, 2H, CHMe2), 1.57 (br, s, 12H, dCMe2), 1.10 (d, J ) 6.3
Hz, 6H, CHMe2), 1.00 (d, J ) 6.3 Hz, 6H, CHMe2), 0.77 (s, 6H,
SiMe2). 13C NMR (C6D6, 23 °C): δ 154.5 [OC(OiPr))], 133.1,
130.1, 127.0, 123.5, 122.9, 122.5, 120.6, 113.6, 110.7 (Flu and Cp
carbons), 86.47 ()CMe), 68.35 (CHMe2), 22.16, 21.72 (CHMe2),
17.74 (dCMe), -1.18 (SiMe2).
the bond distances from Zr to the two wedge carbons of the
ring (C7 and C8 in 7; C6 and C10 in 11) are g2.73 Å for only
weak interactions. The methyl mono(ester enolate) complexes,
L2ZrMe[OC(OiPr)dCMe2], were synthesized using a three-step
approach we developed earlier,21 starting from L2ZrMe2 to
L2ZrMe(OTf) and last to L2ZrMe[OC(OiPr)dCMe2]2.
[Ph2Si(Cp)(Flu)]Zr[OC(OiPr)dCMe2]2 (12). Anal. Calcd for
C44H48O4SiZr: C, 69.52; H, 6.36. Found: C, 69.44; H, 6.24. 1H NMR
(C6D6, 23 °C): δ 8.08-8.05 (m, 6H, Flu, Ph), 7.27-7.10 (m, 10H,
Flu, Ph), 6.88 (t, J ) 7.6 Hz, 2H, Flu), 6.71 (m, 2H, Cp), 6.41 (m,
2H, Cp), 3.58 (sept, J ) 6.0 Hz, 2H, CHMe2), 1.63 (br, s, 6H,
dCMe2), 1.44 (br, s, 6H, dCMe2), 1.04 (d, J ) 6.0 Hz, 6H,
CHMe2), 0.91 (d, J ) 6.0 Hz, 6H, CHMe2). 13C NMR (C6D6, 23
°C): δ 155.0 [OC(OiPr)d], 135.4, 135.1, 132.9, 131.1, 130.5, 129.3,
127.6, 124.4, 124.0, 123.3, 121.6, 115.3, 107.2, 66.17 (Flu, Ph,
and Cp carbons), 87.13 (dCMe), 68.84 (CHMe2), 22.58, 22.00
(CHMe2), 18.39, 18.10 (dCMe).
The importance of employing cationic ester enolate complexes
for MMA polymerization has been well-documented.4,5 As such
complexes bypass the slow initiation step involved in the
polymerization by the typical alkyl complex and simulate the
structure of true active propagating species, MMA polymeri-
zation by cationic ester enolate catalysts is typically much more
active [by turnover frequency, TOF (h-1) ) moles of substrate
(monomer) converted to product per mole of catalyst per hour],
efficient (by initiator efficiency, I*), and controlled (by molecular
weight, MW, and molecular weight distribution, MWD) than
those mediated by the alkyl complexes. In this context, we
focused on the best-performing catalyst derived from complex
6 (vide infra) and explored four different ways to generate the
cationic ester enolate catalyst from the corresponding bis(ester
enolate) or methyl mono(ester enolate) complex, including (a)
methide abstraction of the Zr-Me bond, (b) vinylogous hydride
abstraction of the Zr-enolate moiety, (c) electrophilic addition
to the enolate moiety, and (d) protonolysis of the Zr-enolate
bond (Scheme 3).
Results and Discussion
Synthesis of Precatalysts and Generation of Cationic Ester
Enolate Catalysts. The synthesis of the herein-described bis(ester
enolate) complexes, L2Zr[OC(OiPr)dCMe2]2, was carried out
in a straightforward fashion by the reaction of the corresponding
dichloride precursors L2ZrCl2 and the isolated, structurally
characterized Me2CdC(OiPr)OLi.7a,20 Two of the Cs-ligated
zirconocene bis(ester enolate) complexes have been structurally
characterized (see Figures 1 and 2). The central five-membered
ring of the Flu moiety in both Ph2C< bridged 7 and Me2Si<
bridged 11 is best described as being η3-bonded to Zr because
(20) Rodriguez-Delgado, A.; Chen, E. Y.-X. J. Am. Chem. Soc. 2005, 127,
(21) Mariott, W. R.; Rodriguez-Delgado, A.; Chen, E. Y.-X. Macromol-
ecules 2006, 39, 1318–1327.
961–974.
9
2700 J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010

Coordination Polymerization of Polar Vinyl Monomers
A R T I C L E S
Scheme 3. Different Activation Mechanisms To Generate Cationic Ester Enolate Catalysts
First, methide abstraction of the methyl mono(ester enolate)
complex4,19 by THF·B(C6F5)3 at room temperature generates
the corresponding cationic ester enolate complex
6a+[MeB(C6F5)3]- (Scheme 3). The free [MeB(C6F5)3]- anion
ppm between the para- and meta-fluorines in the 19F NMR
spectrum, and a BH doublet at -25.4 ppm (1JB-H ) 93.7 Hz)
in the 11B NMR spectrum] and (b) the cationic eight-membered-
ring chelate4,26 [most notably the two diastereotopic CH2 protons
at 2.44 (d) and 1.58 ppm (d) as well as coexistence of both the
datively bound ester chain end (δ 4.26, sept. for OCHMe2 in
1H NMR and δ 190.7 for C(OiPr)dO in 13C NMR) and the
covalently bound ester enolate (δ 3.31, sept. for OCHMe2 and
δ 155.8 for OC(OiPr)d) entities].
1
is readily characterized by a broad singlet in H NMR at 0.93
ppm (C7D8, 23 °C) or 0.49 ppm (CD2Cl2, 23 °C) for B-Me22
and a small chemical shift difference of <3 ppm between the
para- and meta-fluorines in 19F NMR [∆(m,p-F) ) 2.5 ppm]
for B-C6F523 in the non-coordinated methyl borate anion, while
the ester enolate cation is characterized most notably by a septet
at 3.67 ppm for the OCHMe2 proton and 1H NMR patterns and
chemical shifts for the coordinated THF [3.05, 2.91 (q, 2H,
R-CH2), and 1.30-1.22 (m, 4H, ꢀ-CH2)], as well as by a 13C
NMR chemical shift at 154.9 ppm (C7D8, 23 °C) for OC(OiPr)d.
Worthy of mention is its solubility in hydrocarbons such as
toluene due to introduction of two tert-butyl groups to the Flu
ring, in contrast to the parent, unsubstituted complex and many
other THF-stabilized, separated metallocenium ion pairs which
typically require the use of polar, chlorinated solvents to
solubilize them.
Third, treatment of bis(ester enolate) complex 6 with
[Ph3C][B(C6F5)4] at -80 °C in CD2Cl2 cleanly affords a mixture
containing 6c+[B(C6F5)4]-, 6d+[B(C6F5)4]-, and Ph3CH (δ 5.56,
s, Ph3CH) in a 4:1:1 ratio (Scheme 3). The eight-membered
chelate 6d+[B(C6F5)4]- is stable from -80 °C to room temper-
ature, but complex 6c+[B(C6F5)4]- is unstable at temperatures
g -30 °C, undergoing decomposition and release of additional
Ph3CH upon warming. The formation of 6c+[B(C6F5)4]- is
proposed to follow the same pathway as we previously
elucidated for the reaction of silyl ketene acetal
Me2CdC(OMe)OSiMe3 with 0.5 equiv of [Ph3C][B(C6F5)4] at
-80 °C, which proceeds with electrophilic addition of Ph3C+,
via the para-carbon of Ph, to the acetal to cleanly form the
addition product [Ph2CdC(CHdCH)2CHCMe2C(OMe)d
O· · ·SiMe3][B(C6F5)4].27 Note that this adduct is identical to
complex 6c+[B(C6F5)4]-, except for a different cation (Me3Si+
vs Zr+). The formation of 6d+[B(C6F5)4]- and Ph3CH also
parallels our previous finding that vinylogous hydride abstraction
of the acetal by Ph3C+ at higher temperatures generates Me3Si+-
activated MMA, which is attacked by the remaining acetal
to form the corresponding Michael addition product
[Me3SiOC(OMe)dCMeCH2CMe2C(OMe)dO· · ·SiMe3][B(C6F5)4]
at -50 °C, with concomitant formation of Ph3CH. Hydride
Second, activation of the bis(ester enolate) complex by
B(C6F5)3 at room temperature affords cleanly the eight-membered-
ring chelate ion pair 6b+[HB(C6F5)3]- (Scheme 3). Without the
readily abstractable methyl group on Zr, the strongly Lewis
acidic borane undergoes vinylogous hydride abstraction from
the Me group of the enolate [OC(OiPr)dCMe2] moiety to form
[HB(C6F5)3]- and the resulting isopropyl methacrylate coordi-
nated to Zr; subsequent nucleophilic addition of the second
enolate ligand on Zr to this coordinated isopropyl methacrylate
(i.e., activated monomer) produces the cationic eight-membered-
ring chelate 6b+[HB(C6F5)3]-. This ion pair exhibits spectro-
scopic signatures (see Figures 3 and 4) for (a) the uncoordinated
[HB(C6F5)3]- anion24,25 [BH at 0.94 ppm (dd, J ) 6.0 Hz) in
1
abstraction
from
an
enolate
methyl
group
of
the 1H NMR spectrum, a small chemical shift difference of 2.50
(24) Garner, L. E.; Zhu, H.; Hlavinka, M. L.; Hagadorn, J. R.; Chen, E. Y.-
X. J. Am. Chem. Soc. 2006, 128, 14822–14823.
(22) Klosin, J.; Roof, G. R.; Chen, E. Y.-X.; Abboud, K. A. Organome-
tallics 2000, 19, 4684–4686.
(25) Yang, X.; Stern, C. L.; Marks, T. J. Angew. Chem., Int. Ed. 1992, 31,
1375–1377.
(23) Horton, A. D.; de With, J.; van der Linder, A. J.; van de Weg, H.
Organometallics 1996, 15, 2672–2674.
(26) Ning, Y.; Chen, E. Y.-X. Macromolecules 2006, 39, 7204–7215.
(27) Zhang, Y.; Chen, E. Y.-X. Macromolecules 2008, 41, 36–42.
9
J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010 2701

A R T I C L E S
Zhang et al.
Figure 3. 1H NMR spectra of precatalyst 6 (top, C6D6, 23 °C) as well as cationic ester enolate catalysts 6b+[HB(C6F5)3]- (middle, CD2Cl2, 23 °C) and
6e+[B(C6F5)4]- (bottom, CD2Cl2, -60 °C). Only the most diagnostic peaks are assigned here (see Experimental Section for complete assignments).
Figure 4. 11B NMR (CD2Cl2) (top, 23 °C) and 19F NMR (middle, 23 °C) spectra of 6b+[HB(C6F5)3]- (top, 23 °C) and 19F NMR (CD2Cl2) spectrum of
6e+[B(C6F5)4]- (bottom, -60 °C).
Cp2ZrMe[OC(OtBu)dCMe2] by Ph3C+ was also reported to lead
to the formation of a zirconium carboxylate dication after subse-
quent elimination of methane and isobutene.5c
at δ 188.7 and 154.3 for the ester C(OiPr)dO and enolate
OC(OiPr)d carbons, respectively. Addition of MMA to this
cationic solution at g -60 °C led to the formation of the eight-
membered chelate 6f+[B(C6F5)4]- and subsequent PMMA
formation. From this set of variable-temperature NMR experi-
ments, we accomplished the detection and characterization of
all key species involved in the catalyst formation and polym-
erization initiation and propagationsfrom the precatalyst to the
cationic active species to the resting intermediate and finally to
the polymersthereby providing critical insight into the polym-
erization mechanism (vide infra).
Characteristics of Polymerization by Catalysts Derived
from Complexes 3-12. The above understanding of how the
ester enolate precatalysts are activated into active cationic
species also gave us important perspectives as to what activators
to use for polymerizations. Over the course of this study, we
found that the catalyst systems derived from the activation using
[Ph3C][B(C6F5)4] at ambient temperature using the in-reactor
Fourth, through protonolysis of the Zr-enolate bond with a
strong acid,7d the reaction of bis(ester enolate) complex 6 with
[H(Et2O)2][B(C6F5)4] at -80 °C cleanly generates cationic ester
enolate complex 6e+[B(C6F5)4]- (Scheme 3), which is stabilized
by isopropyl isobutyrate, the protonolysis coproduct, and is
stable at -60 °C. Spectral changes correlate with the transfor-
mation of the Cs-symmetric neutral bis(ester enolate) 6 to the
C1-symmetric cationic ester enolate complex 6e+[B(C6F5)4]-,
and the spectral features of the cation are characteristic of the
isopropyl isobutyrate-stabilized cationic zirconocene ester eno-
1
late, including key chemical shifts in H NMR (CD2Cl2, -60
°C) at δ 4.36 (sept, OCHMe2, enolate), 3.63 (sept, OCHMe2,
isobutyrate), and 2.07 (sept, CHMe2, isobutyrate) for the CHMe2
group present in the covalently bound ester enolate ligand and
the datively bound isobutyrate ligand, as well as in 13C NMR
9
2702 J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010

Coordination Polymerization of Polar Vinyl Monomers
A R T I C L E S
Table 1. Results of MMA Polymerization by Catalysts Based on Complexes 3-12a
run no.
catalyst
solvent
time (min)
convb (%)
TOF (h-1
)
Mnc (×10-3 g/mol)
PDIc (Mw/Mn)
I*d (%)
[rr]b (%)
[mr]b (%)
[mm]b(%)
1
2
3
4
5
6
7
8
3
4
4
DCM
DCM
TOL
DCM
DCM
TOL
DCM
TOL
TOL
TOL
TOL
TOL
15
15
15
1440
1440
15
23
1440
60
15
1440
1440
50.0
44.8
30.6
0
<0.5
97.1
71.6
0
800
717
490
41.5
44.3
28.5
1.48
1.32
1.56
48.6
41.1
43.9
94.2
93.5
93.7
4.0
4.5
4.3
1.8
2.0
2.0
5(Ti)
5(Hf)
6
7
8
92.9
93.7
92.3
4.6
4.9
6.0
2.4
1.4
1.7
1554
747
0
105
1557
0
40.0
45.5
1.14
1.19
97.8
63.5
9
9
26.2
97.3
0
34.8
1.23
30.9
86.2
94.0
11.4
4.2
2.3
1.8
10
11
12
10
11
12
0
0
a Carried out at ambient temperature (∼25 °C) in 10 mL of toluene (TOL) or CH2Cl2 (DCM) + MMA solutions, where [MMA]0 ) 0.935 M and
[precatalyst]0 ) [activator]0 {[Ph3C][B(C6F5)4]} ) 2.34 mM for a [MMA]/[catalyst] ratio of 400/1. b Monomer conversions and PMMA methyl triad
distributions measured by H NMR. c Mn and PDI determined by gel permeation chromatography relative to PMMA standards. d Initiator efficiency (I*)
1
) Mn(calcd)/Mn(exptl), where Mn(calcd) ) MW(MMA) × [MMA]0/[catalyst]0 × conversion% + MW of chain-end groups.
activation methodology (see Experimental Section), which
generates the corresponding ester enolate cation paired with the
unassociated anion [B(C6F5)4]-, give the highest catalyst activity
(TOF), polymerization efficiency (I*), and polymer syndiotac-
ticity (%rr). An exception here is the methyl mono(ester enolate)
precatalyst, which is best activated cleanly with THF ·B(C6F5)3
to the corresponding cationic catalyst (cf., complex 3). Accord-
ingly, in our study examining the relative TOF, I*, %rr, and
degree of polymerization control [number-average molecular
weight (Mn) and polydispersity index (PDI)] of catalysts based
on complexes 3-12, we fixed the [MMA]/[catalyst] ratio to be
400/1 (and the MMA and catalyst concentrations) and the
activator to be [Ph3C][B(C6F5)4] (except for complex 3);
additionally all polymerizations were carried out at ambient
temperature. The results of this comparative study are sum-
marized in Table 1.
substitution on the Flu ring made its cationic species it soluble
in hydrocarbon solvents so that the polymerization can be readily
carried out in less toxic solvents such as toluene, instead of
CH2Cl2.
Removing one of the two tBu groups on the Flu ring produced
a less active and less efficient catalyst system, 7. Interestingly,
despite its C1-ligation due to the uneven substitution on the Flu
ring, this catalyst still affords highly syndiotactic PMMA (92%
rr) at room temperature (run 7). However, placing a tBu group
on the Cp ring resulted in an inactive catalyst system, 8 (run
8). Intriguingly, catalyst 9, based on the sterically expanded Oct-
Flu ligation that furnished one of the most syndiospecific
propylene polymerization catalysts,28 is not only much less
active (105 h-1 TOF) but also considerably less syndioselective
(86% rr, run 9) than the parent catalyst 3 or 4; its comparison
with catalyst 6 is even more extreme.
It can be seen from the table that, while the parent zirconocene
catalysts 3 and 4 (no substitutions to either the Cp or Flu ring)
produce st-PMMA with a remarkably high syndiotacticity at
room temperature (94% rr),19 they failed to achieve high
monomer conversions (regardless of the reaction time), with
low I* of <50% and relatively broad MWD of >1.3 (runs 1-3).
It should be mentioned that at a low [MMA]/[catalyst] ratio of
100/1, high quantitative monomer conversions can be achieved,
and the resulting polymers exhibit narrower MWDs.19 Further-
more, catalysts 3 polymerized 200 equiv of n-butyl methacrylate
(BMA) in 1 h, achieving a 90% monomer conversion and
producing st-PBMA with a high syndiotacticity of 94% rr; it
also polymerized 200 equiv of DMAA in 30 min, achieving a
97% monomer conversion and producing highly syndiotactic
PDMAA with Tm of 283 °C. On the other hand, their titanium
and hafnium derivatives 5 exhibit either no activity (Ti, run 4))
or negligible activity (Hf, run 5) for MMA polymerization,
although the Hf catalyst produced st-PMMA with a high
syndiotacticity of 93% rr.
The catalyst derived from the di-tert-butyl-substituted Flu
complex 6 is most remarkable in all aspects of the polymeri-
zation (run 6): TOF up to 1554 h-1 (thus highly actiVe), nearly
quantitative I* of 98% (thus highly efficient), syndiotacticity of
94% rr (thus highly syndiospecific), and predicted Mn of 4.00
× 10-4 and narrow MWD of 1.14 (thus well-controlled). This
catalyst system is also robust, producing highly syndiotactic
PMMA at higher temperatures (e.g., 93% rr at 50 °C). PBMA
produced by this catalyst is also highly syndiotactic (94% rr).
Furthermore, from a polymerization process point of view, this
catalyst offers an additional advantage because the di-tert-butyl
By examining the electronic effect of the bridging aryl groups
we found that catalyst 10, with the Et3Si group substituted on
the para-Ph ring, performs nearly identically to catalyst 6 in
terms of both polymerization activity and polymer syndiotac-
ticity (run 10 vs 6). As a matter of fact, the apparent rate
constants (kapp) of the two catalysts, obtained from the slopes
of the best-fit lines to the first-order kinetic plots of ln([MMA]0/
[MMA]t) vs time (Figure 5), are identical: kapp ) 0.31 min-1
.
Finally, the Me2Si< and Ph2Si< bridged Cs-catalysts 11 and 12
are inactive for MMA polymerization under the current reaction
conditions. While the catalyst resulting from the activation of
12 with B(C6F5)3 exhibits marginal activity (TOF ∼2 h-1), the
PMMA produced has a low syndiotacticity of 63% rr.
Kinetics and Mechanism of Polymerization by Catalyst
6+[B(C6F5)4]-. Having established catalyst 6+[B(C6F5)4]-, de-
rived from activation of precatalyst 6 with [Ph3C][B(C6F5)4] by
the in-reactor activation method, as the best catalyst system in
all aspects for the MMA polymerization, we subsequently
examined the MMA polymerization by this catalyst in more
detail, specifically concerning its degree of control and kinetics.
Table 2 summarizes the kinetic results of the MMA polym-
erization with two different [MMA]/[Zr] ratios at 25 °C in
toluene: a low ratio of 200 and a high ratio of 1000. As can be
seen from this table, the MMA polymerization with a [MMA]0/
[Zr]0 ratio of 200 is rapid, achieving a quantitative monomer
conversion in 10 min. The initiator efficiencies I* were modest
in the low monomer conversion regime (49-86%, most likely
(28) Miller, S. A.; Bercaw, J. E. Organometallisc 2004, 23, 1777–1789.
9
J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010 2703

A R T I C L E S
Zhang et al.
Figure 5. Plots of the first-order kinetics of ln([MMA]0/[MMA]t) vs time
(min) for the polymerization of MMA by 6 (pink 9) and 10 (blue 2)
activated with equimolar [Ph3C][B(C6F5)4] in toluene at 25 °C. Conditions:
[MMA]0 ) 0.935 M, [Zr]0 ) 2.34 mM.
Figure 6. Plots of Mn and PDI of PMMA vs MMA conversion in toluene
at 25 °C: [MMA]0 ) 0.935 M, [6]0 ) [Ph3C][B(C6F5)4]0 ) 4.67 mM with
a [MMA]0/[catalyst]0 ratio of 200/1.
Table 2. Selected Results for MMA Polymerization by Catalyst
6+[B(C6F5)4]- in Toluene at 25 °C
run no.
time (min)
conv (%)
Mn (×10-3 g/mol)
PDI (Mw/Mn)
I* (%)
[6] ) 4.67 mmol/L, [MAA]0/[6]0 ) 200
1
2
3
4
5
6
7
8
0.5
1.0
1.5
2.0
3.0
4.0
6.0
15.4
49.1
61.5
76.7
91.5
96.0
98.9
6.90
13.1
1.13
1.22
1.16
1.17
1.15
1.15
1.15
1.15
48.5
77.3
85.7
91.7
92.9
95.3
94.7
95.5
14.7
17.0
20.0
20.4
21.2
21.2
10
100
[6] ) 0.935 mmol/L, [MAA]0/[6]0 ) 1000
9
1.5
3.0
5.0
7.0
15.1
33.6
51.1
63.6
71.7
79.8
88.8
93.9
98.9
16.0
29.3
40.2
47.1
53.3
58.9
66.8
67.1
70.0
1.15
1.16
1.17
1.19
1.18
1.20
1.20
1.25
1.26
96.2
115
128
136
135
136
134
141
142
10
11
12
13
14
15
16
17
10
15
25
40
120
Figure 7. Plots of Mn and PDI of PMMA vs MMA conversion in toluene
at 25 °C: [MMA]0 ) 0.935 M, [6]0 ) [Ph3C][B(C6F5)4]0 ) 0.935 mM with
a [MMA]0/[catalyst]0 ratio of 1000/1.
due to errors associated with Mn measurements for low-MW
polymer samples), but the I* values were high (∼95%) in the
high monomer conversion regime with typical low PDI values
∼1.15, consistent with a monometallic propagation mechanism
(i.e., one polymer chain per metal center). This observation,
together with the observed linear increase of Mn vs MMA
conversion (with a nonzero intercept) while maintaining nearly
constant low PDI values (Figure 6), demonstrates that the
polymerization is well-controlled under the current conditions.
The polymerization with a high [MMA]/[Zr] ratio of 1000 is
also controlled, as judged by a linear increase of Mn vs MMA
conversion with low PDI values (Figure 7). However, the degree
of control is less than the polymerization at lower [MMA]/[Zr]
ratios, as evidenced by slowly increasing PDI and I* values
with increase in monomer conversion (Table 2, Figure 7). The
I* values of over 100% (128 -142%) with conversions >50%
are indicative of chain-transfer reactions at later stages of the
polymerization with this high [MMA]/[Zr] ratio of 1000.
Nonetheless, the catalyst is not deactivated by such reactions,
as the polymerization can proceed to nearly quantitative
conversion in 2 h (run 17).
Kinetic experiments employed [MMA]0/[Zr]0 ratios ranging
from 100 to 1000, clearly showing first-order dependence on
[MMA] for all the ratios (Figure 8). Furthermore, a double
logarithm plot (Figure 9) of the apparent rate constants (kapp),
obtained from the slopes of the best-fit lines to the plots of
ln([MMA]0/[MMA]t) vs time as a function of ln[Zr]0, was fit
to a straight line (R2 ) 0.99) with a slope of 1.05. Thus, the
kinetic order with respect to [Zr], given by the slope of 1, reveals
that the propagation is also first-order in catalyst concentration,
indicating that the MMA polymerization follows the monome-
tallic, intramolecular coordination-addition mechanism, similar
to that undergone by ansa-C2-ligated catalysts.4
Overall, the above kinetic results, coupled with mechanistic
insight obtained through monitoring the polymerization and
identification of intermediates (vide supra), led to the proposed
polymerization mechanism outlined in Scheme 4. Hence, the
propagation “catalysis” cycle involves fast intramolecular
Michael addition within catalyst-monomer complex D, leading
9
2704 J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010

Coordination Polymerization of Polar Vinyl Monomers
A R T I C L E S
coordinated ester group in E by the incoming monomer to
regenerate E (by going through enantiomeric structures of D
and E). In this scheme, catalyst-monomer complex D and
catalyst resting intermediate E are simulated by complexes
6e+[B(C6F5)4]- and 6f+[B(C6F5)4]- (Schemes 3, 4, and Experi-
mental Section), respectively.
Effects of Polymerization Conditions on Syndiospecificity. To
understand the polymerization stereoregulation, we examined
the effects of polymerization conditions on syndiospecificity of
the MMA polymerization by catalyst 6+. The polymerization
conditions examined include monomer-to-catalyst ratio (con-
centration), temperature, solvent polarity, and activator (anion
structure).
First, polymerization in toluene at 25 °C using catalyst
6+[B(C6F5)4]-, derived from activation of precatalyst 6 with
[Ph3C][B(C6F5)4] by the in-reactor activation method, afforded
st-PMMA with a constant syndiotacticity (93-94% rr) as the
[MMA]/[catalyst] ratio was varied 20-fold from 100/1 to 2000/1
(runs 1-7, Table 3). However, close inspection of the methyl
triad-level stereomicrostructure of PMMA revealed an important
insight: at a low [MMA]/[catalyst] ratio of 100, a triad test of
2[mm]/[mr] gave 1.00 (run 1), characteristic of a perfect site-
controlled mechanism, but at higher [MMA]/[catalyst] ratios
(lower catalyst or higher monomer concentrations) the test
deviated from unity, ranging from 0.77 to 0.52, due to either
reduced enantiofacial misaddition errors (...rrrrmmrrrr...) or
enhanced catalyst-site epimerization errors (...rrrrmrrrr...), or
both (runs 2-7). This observation is intriguing because the trend
for the isolated m meso dyad stereoerrors as a function of the
[monomer]/[catalyst] ratio is opposite that observed for syn-
diospecific propylene polymerization by Cs-ligated catalysts.29
This unique behavior in the MMA polymerization by Cs-ligated
catalysts is further discussed in the next section.
Figure 8. Plots of the first-order kinetics of ln([MMA]0/[MMA]t) vs time
(min) for the polymerization of MMA by 6 activated with equimolar
[Ph3C][B(C6F5)4] in toluene at 25 °C. Conditions: [MMA]0 ) 0.935 M;
[Zr]0 ) 9.35 (*), 4.67 ([), 2.34 (9), 1.56 (2), and 0.935 (b) mM.
Second, it is remarkable to see the syndiotacticity of PMMA
remain constant (93-94% rr) as the polymerization temperature
is varied from 0 (run 8) to 50 °C (run 9), showing the robustness
of this catalyst system. However, the polymerization temperature
does affect the Tm of the resulting st-PMMA: Tm ) 156.3 °C
for polymerization at 25 °C by 6+[B(C6F5)4]-, and Tm ) 151.9
and 163.5 °C for polymerizations by the parent catalyst
4+[B(C6F5)4]- at 50 and 0 °C, respectively (Figure 10).
Figure 9. Plot of ln(kapp) vs ln[Zr] for MMA polymerization by 6 in toluene
at 25 °C.
Scheme 4. Proposed Mechanism of Methacrylate Polymerization
Catalyzed by 6+[B(C6F5)4]-
Third, this polymerization is also insensitive to solvent
polarity, again in marked contrast to the propylene polymeri-
zation. Thus, switching the solvent from toluene (ε ) 2.38) to
CH2Cl2 (ε ) 8.93) had little effect on polymerization charac-
teristics (run 10), with only a slight drop of the syndiotacticity
to 92% rr, due to the enhanced [mr] stereodefect (6.1%).
Fourth, we examined the potential anion (i.e., ion-pairing
strength) effects by varying activator structures (and thus the
resulting anion structure), the results of which are summarized
in Table 4. As can be readily seen from the table, the
polymerization results with catalysts 6(c,d)+[B(C6F5)4]-,
6e+[B(C6F5)4]-, and 6a+[MeB(C6F5)3]-, derived from the in-
reactor activation of the bis(ester enolate)
6
with
[Ph3C][B(C6F5)4] and [H(OEt2)2][B(C6F5)4] and activation of the
methyl mono(ester enolate) derivative with THF ·B(C6F5)3,
respectively, are remarkably similar in all polymerization aspects
(runs 1-3). Polymerization by the catalyst derived from
activation with [Ph3C][TRISPHAT], which introduces the bulky,
racemic, chiral anion [TRISPHAT]- to the cation, is somewhat
to the eight-membered cyclic ester enolate chelate (resting active
intermediate E), followed by the slower, rate-limiting ring-
opening of the chelate via frontside displacement of the
(29) Busico, V.; Cipullo, R. Prog. Polym. Sci. 2001, 26, 443–533.
9
J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010 2705

A R T I C L E S
Zhang et al.
Table 3. Selected Results of MMA Polymerization by Catalyst 6+[B(C6F5)4]-
run no.
solvent
temp (°C)
[M]/ [Zr]
time (min)
conv (%)
Mn (10-3 g/mol)
PDI (Mw/Mn)
I* (%)
[rr] (%)
[mr] (%)
[mm] (%)
2[mm]/[mr]
1
2
3
4
5
6
7
8
9
10
TOL
TOL
TOL
TOL
TOL
TOL
TOL
TOL
TOL
DCM
25
25
25
25
25
25
25
0
100
200
400
600
800
1000
2000
400
400
200
4
10
45
60
90
120
2880
120
30
100
100
100
98.7
97.4
98.9
67.5
92.4
91.9
100
14.7
21.9
42.2
60.4
77.5
71.2
141
1.15
1.14
1.14
1.16
1.31
1.32
1.53
70.1
92.2
95.4
98.6
101
92.8
92.8
93.7
93.7
94.2
93.5
94.1
94.2
92.9
92.1
4.8
5.2
4.9
4.8
4.6
5.1
4.4
4.3
5.3
6.1
2.4
2.0
1.4
1.5
1.2
1.4
1.5
1.5
1.8
1.8
1.00
0.77
0.57
0.63
0.52
0.55
0.68
0.70
0.68
0.59
139
96.4
50
25
30
25.7
1.12
78.9
Table 4. Results of MMA Polymerization with 6 and Different Activatorsa
run no.
catalyst system
time (min)
conv (%)
Mn (10-3 g/mol)
PDI (Mw/Mn)
[rr] (%)
[mr] (%)
[mm] (%)
1
2
3
4
6(c,d)+[B(C6F5)4]-
6e+[B(C6F5)4]-
15
15
15
60
97.1
95.3
94.6
99.3
40.0
42.5
51.7
39.6
1.14
1.10
1.14
1.15
93.7
94.6
94.6
93.2
4.9
4.1
4.1
5.0
1.4
1.3
1.3
1.8
6a+[MeB(C6F5)3]-
6(c,d)+[TRISPHAT]-
a Carried out at 25 °C in 10 mL of toluene + MMA solutions, where [MMA]0 ) 0.935 M and [6]0 ) [activator]0 ) 2.34 mM, for a [MMA]/
[catalyst] ratio of 400/1.
Figure 10. Differential scanning calorimetry traces of st-PMMA produced by 6+[B(C6F5)4]- at 25 °C (left, ∆H ) 6.6 J/g) and by 4+[B(C6F5)4]- at 0 °C
(right, ∆H ) 4.9 J/g).
slower (run 4), but it is nonetheless still highly efficient (100%
I*) and syndiospecific (93% rr).
Figure 11. Transition states for MMA addition to the enolate growing
chain in Ph2C< bridged 3 (distances in Å and ∆Estereo in kcal/mol): (a) TS
generating a stereomistake; (b) favored TS.
Stereoregulation and Error Formation. In this section we
provide a computational rationalization of the experimental
observations, especially concerning stereochemical outcomes.
Experimentally we observed a high syndiotacticity of 94% rr
produced by the Ph2C< bridged catalyst 3. The calculated ∆Estereo
for this catalyst system was 2.3 kcal/mol, which is consistent
with the experimental result. The competing transition states
for the addition of a MMA molecule to the opposite enantiofaces
of the enolate growing chain are shown in Figure 11. Both
transition states show a relative trans disposition of the methoxy
groups of the growing chain and of the monomer. For a complex
with an S configuration at the metal, the transition state
corresponding to MMA attack to the re-face of the growing
chain is higher and disfavored by steric interaction between the
methoxy group of the growing chain and the Flu moiety;8c see
the short distances in Figure 11a. The preferred transition state
for the same S complex, which corresponds to MMA attack to
the si-face of the growing chain, is lower in energy because the
methoxy group of the monomer interacts less severely with the
Flu moiety (Figure 11b). In both transition states the Flu moiety
is pushed toward an η3-hapticity; see the much larger Zr-C(ꢀ)
distances relative to the Zr-C(R) distances in the case of the
Flu moiety, compared to the rather similar Zr-C(R) and
Zr-C(ꢀ) distances the in case of the Cp moiety, in Figure 11.
9
2706 J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010

Coordination Polymerization of Polar Vinyl Monomers
A R T I C L E S
Figure 12. Transition states generating a stereomistake in MMA addition to the enolate growing chain with 2 (a), 6 (b), and 11 (c). Distances in Å and
∆Estereo in kcal/mol.
Scheme 5. Catalyst-Site Epimerization Assisted by MMA after a Stereomistake
Of course, the syndioregularity of the obtained PMMA can be
easily rationalized in the framework of the chain migratory
mechanism.
Finally, to understand the role of the bridging atom, we
focused on system 11, which corresponds to replacement of the
Ph2C< bridge of 3 with the Me2Si< bridge. This modification
results in a sharp decrease of ∆Estereo by 1.0 kcal/mol (see Figure
12c vs Figure 11a), for a ∆Estereo of only 1.3 kcal/mol, consistent
with the poor stereoregularity of the PMMA produced by R2Si<
bridged catalysts relative to the R2C< analogue. The reason for
this rather different behavior can be found in the rather longer
Zr-Flu distances in the Me2Si< bridged 11 relative to R2C<
bridged 2 and 3. Of course, a slightly looser coordination of
the Flu ligand results in an increased distance between the
methoxy group of the chain and the Flu ligand (see Figure
12a,c). In the framework of our mechanism, these distances are
the keys to the transfer of steric stress from the catalyst to the
reactants, from which stereoselectivity originates, and longer
such distances are consistent with reduced stereoinduction.
Concerning the nature of the stereoerrors (i.e., the relative
amount of the mm and mr triads in the obtained st-PPMA), one
must recall that, in the framework of a regular chain-migratory
mechanism, only mm stereoerrors should be observed. Our
previous contribution7a attributed the high content of mr triads
of the PMMA produced by CGC systems, apparently consistent
with a chain-end stereocontrol, to catalyst-site epimerization
occurring after a stereomistake (Scheme 2). The driving force
for this site epimerization was related to a different stability of
the diastereomeric complexes corresponding to a given (fixed)
configuration of the growing chain coupled with the (flexible)
configuration at the metal. In fact, calculations indicated that
the diastereomeric insertion kinetic product corresponding to
the collapse of the transition state yielding a stereomistake was
of higher energy relative to that resulting from a correct
stereoselectivity. According to our calculations, coordination of
a free MMA molecule could assist the epimerization reaction
through the intermediates shown in Scheme 5.
Moving to the Me2C< bridged catalyst system 2, replacement
of the Ph2C< bridge with the more flexible Me2C< bridge results
in a small decrease in ∆Estereo by 0.4 kcal/mol relative to that
of catalyst 3 (see Figure 12 vs Figure 11). The trend is consistent
with the decreased PMMA %rr triads on moving from 3 to 2.
It is rather difficult to rationalize this small energy difference,
but comparing the two transition states leading to a stereomistake
in 2 and 3, depicted in Figures 11a and 12a, respectively, it is
clear that slightly longer distances are found between the
methoxy group of the growing chain and the Flu ligand in 2
relative to 3, which results in a slightly decreased steric stress
in 2. This change can be reasonably ascribed to a higher rigidity
of the metallocene skeleton in 3, a consequence of the bulkier
Ph2C< bridge, which gives rise to a small decrease of the
Flu-R2C-Cp angle on going from 2 (56.7°) to 3 (55.9°).
Nevertheless, this small angle decrease results in slightly shorter
distances of coordination of the Flu moiety to Zr (compare
Figure 11a with Figure 12a). This analysis suggests that the
presence of the Ph2C< bridge pushes the Flu ligand slightly
toward the growing chain and the monomer, thereby explaining
the higher ∆Estereo calculated for 3.
Moving to the di-tert-butyl-substituted Flu system 6, intro-
duction of tBu groups on the Flu moiety has virtually no effect
on ∆Estereo, which is only 0.1 kcal/mol greater than that
calculated for 3 (see Figure 12b vs Figure 11a). This value is
consistent with the experimentally observed similar amounts
of the %rr triads in the st-PPMA produced by catalyst systems
based on 3 and 6. Visual inspection of the transition states
leading to a stereomistake in 3 and 6 rationalizes the minor
t
t
role of the Bu groups in 6, since these Bu groups are more
than 4 Å away from the growing chain, while all the other
relevant distances are rather similar in 3 and 6.
The results show that for systems 2, 3, and 6, as in the case
of the CGC system, the eight-membered product corresponding
9
J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010 2707

A R T I C L E S
Zhang et al.
Scheme 6. Energetics of Back-Biting (EBB) and Ion-Pair Formation
(EIP) in 2, 3, and 6 in CH2Cl2
coordinating anions, and deviation from a pure site-control
mechanism at high [MMA]/[catalyst] ratios (as opposed to low
ratios in the case of propylene polymerization), all of which
argue in favor of what we proposed: bimolecular site epimer-
ization involving MMA.
Figure 13. Energy profiles corresponding to the epimerization reaction
with systems 2, 3, and 6 in CH2Cl2.
to the favored transition state (S-metal/si-chain) is favored
relative to the product corresponding to the stereomistake (S-
metal/re-chain; see Figure 13). This result implies that, after a
stereomistake has occurred, the eight-membered product has a
driving force to epimerize. This driving force originates from
steric repulsion between the methoxy group of the growing chain
and the Flu moiety that already destabilize the S-metal/re-chain
transition state, as shown in Figures 11 and 12. Indeed, the more
stereoselective the catalyst, the larger the energy difference
between the eight-membered products corresponding to correct
and wrong enantiofacial addition. Nevertheless, the systems
show a different propensity to accommodate both the growing
chain and an additional MMA molecule at the metal center,
namely the intermediate structures in Figure 13. Again, the
higher the stereoselectivity, the higher the energy of the
intermediates with a coordinated MMA molecule. This trend is
particularly evident in the case of the very bulky system 6. We
postulate that the high energy of the intermediates with a
coordinated MMA molecule can be related to a slower epimer-
ization rate, which thus should decrease in the order 2 > 3 > 6.
This very simple scheme would rationalize the higher amount
of mr triads in st-PMMA produced by 2 relative to those
produced by 3. In short, the main message here is that lower
stereoselectivity and faster epimerization rates correspond to
poorly hindered catalysts, because the reactive pockets of these
systems have enough space to accommodate both the transition
state corresponding to a stereomistake and the intermediates of
the epimerization reaction. Even with bulky catalyst 6, evidence
showed that the polymerization at high [MMA]/[catalyst] ratios
deviated from a pure site-control mechanism due to higher than
expected [mr] contents (Table 3), a phenomenon attributable
to the herein-described monomer-assisted, catalyst-site epimer-
ization after an enantiofacial mistake.
Finally, the energy profiles of Figure 13 suggest that the
ability of the various catalysts to coordinate different ligands is
quite different, with obvious consequences on their activity. To
exploit this point we investigated the energetics of the back-
biting of the penultimate unit, EBB, and the energetics of ion-
pair formation, EIP, as shown in Scheme 6. The dissociation
energy of the CdO group of the last inserted unit in the three
representative systems 2, 3, and 6 shows a decreasing trend that
is in qualitative agreement with the experimental activity, and
the same trend is observed for the ion-pair dissociation energy
(Scheme 6). Basically, the above-discussed increased rigidity
of 3 relative to 2 also results in a reduced ability of 3 to interact
with the counterion, as indicated by the EIP of 3 that is 2.7 kcal/
mol lower than that of 2. This effect is much more pronounced
for catalyst system 6, the EIP of which is 5.8 kcal/mol lower
than that of 3. These results suggest that higher activity in these
strictly related catalysts can be reasonably related to the ability
of the metallocene ligand to prevent tight binding of the bulky
counterion.
Conclusions
In summary, this contribution presented a full account of our
investigations into the recently discovered catalyst-site-controlled
coordination polymerization of polar vinyl monomers into highly
syndiotactic polymers. Our combined synthetic, kinetic, mecha-
nistic, and theoretical/computational study has uncovered, and
subsequently gained a fundamental understanding of, several
unique features of this polymerization system catalyzed by Cs-
ligated cationic ansa-metallocene ester enolate catalysts.
First, four different activation pathways can be employed to
convert the precatalyst to the cationic ester enolate catalyst.
Methyl mono(ester enolate) precatalysts are best activated with
THF·B(C6F5)3 at room temperature via methide abstraction,
cleanly generating the corresponding cationic ester enolate
complexes such as 3 and 6a+[MeB(C6F5)3]-. For bis(ester
enolate) precatalysts, activation by B(C6F5)3 at room temperature
affords cleanly the eight-membered chelate 6b+[HB(C6F5)3]-
through vinylogous hydride abstraction from the enolate moiety.
Activation of the bis(ester enolate) complex with
[Ph3C][B(C6F5)4] at -80 °C gives a mixture of 6c+[B(C6F5)4]-
and chelate 6d+[B(C6F5)4]- as a result of electrophilic addition
of Ph3C+ to the enolate and of vinylogous hydride abstraction
of the enolate group by Ph3C+, respectively. Although
6c+[B(C6F5)4]- is stable only at low temperatures, in the
presence of monomer it can be rapidly converted to the resting
chelate 6d+[B(C6F5)4]-; this feature ensures the success seen
As described earlier, a propylene-polymerization-like site
epimerization followed by a correct MMA addition would also
generate the m-type stereoerror (Scheme 2). However, in
propylene polymerization, site epimerization is a unimolecular
process and is favored at low monomer concentration.18 On the
other hand, in MMA polymerization, site epimerization can be
either a unimolecular process (by a dissociative pathway, e.g.,
assisted by the pairing anion or coordinating solvent) or a
bimolecular process (assisted by monomer). Our calculations
on the CGC catalyst system indicated that the site epimerization
assisted by MMA is more facile than that assisted by anion.7a
Furthermore, our experiments based on the current Cs-ligated
catalysts showed a constant syndiotacticity of PMMA produced
over a wide polymerization temperature range, insensitivity of
its stereoselectivity to solvent polarity and structures of weakly
9
2708 J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010

Coordination Polymerization of Polar Vinyl Monomers
A R T I C L E S
for the in-reactor activation methodology operating at low,
ambient, or higher temperatures. The protonolysis approach
involves the cleavage of the Zr-enolate bond with
[H(Et2O)2][B(C6F5)4], which occurs instantaneously even at
-80 °C, cleanly generating the cationic ester enolate
6e+[B(C6F5)4]-. This structure simulates the catalyst-monomer
complex in a propagation catalysis cycle and is stable only at
low temperatures; however, we can “watch” it consume MMA,
leading to the eight-membered chelate 6f+[B(C6F5)4]- and
subsequent PMMA formation.
Second, among the 12 catalyst systems we examined in this
studyswhich varied in metal center, anion structure, bridging
atom and substituents, and ligand substitution patternsthe Ph2C<
bridged, di-tert-butyl-substituted Flu catalyst system 6, by in-
reactor activation with [Ph3C][B(C6F5)4] at room temperature,
is the best in all aspects of the polymerization, having the highest
observed activity (1554 h-1 TOF), efficiency (98% I*), syn-
diotacticity (94% rr), and control (predicted Mn and 1.14 MWD).
This catalyst system is also remarkably robust, producing highly
syndiotactic PMMA with a constant syndiotacticity over a wide
polymerization temperature range, from 0 °C, 94% rr to 25 °C,
94% rr to 50 °C, 93% rr. Its polymerization characteristics are
insensitive to solvent polarity. Such temperature and solvent
insensitivities are in marked contrast to the propylene polym-
erization by analogous Cs-ligated cationic alkyl catalysts.
Additionally, the polymerization characteristics of catalyst
system 6 are insensitive to anion structure of weekly coordinat-
ing anions, such as [MeB(C6F5)3]-, [B(C6F5)4]-, and
[TRISPHAT]-. Furthermore, this cationic catalyst is soluble in
toluene, adding an additional advantage to the polymerization
process.
the Me2Si< bridge). Installation of the bulky Ph2C< bridge
decreases the Flu-C-Cp angle and thus pushes the η3-bound
Flu ligand closer to the growing chain and the monomer, thereby
increasing ∆Estereo between the competing transition states for
the addition of a MMA molecule to the opposite (correct and
wrong) enantiofaces of the growing enolate chain. Computa-
tional results also provided theoretical explanations for the
observed deviation from a pure site-control mechanism at high
[MMA]/[catalyst] ratios due to higher than expected [mr]
contents, a phenomenon attributable to the herein-described
monomer-assisted, catalyst-site epimerization, after an enantio-
facial mistake, to a thermodynamically more stable resting state.
Furthermore, the relative polymerization activity of this strictly
related catalyst series was shown to correlate with the relative
energetics of the back-biting of the penultimate unit and ion-
pair formation.
Finally, on the basis of what we have learned so far and in
the interest of creating more active, efficient, and stereoselective
Cs-ligated catalysts for coordination polymerization of polar
vinyl monomers, future efforts should perhaps be directed at
designing catalyst structures that can provide higher rigidity of
the Cs-metallocene skeleton, achievable by using appropriate
bridging atoms and associated substituents as well as substitu-
tions at suitable positions of the Flu ring. An ideal outcome
should be a balanced stereo-interplay between the catalyst and
the reactant for both high reactivity and stereoselectivity.
Stronger coordination of the Flu ligand should more effectively
transfer steric stress from the catalyst site to the reactants.
Research toward these directions is currently underway.
Acknowledgment. This work was supported by the National
Science Foundation (NSF-0718061) for the work carried out at
Colorado State University and by INSTM (Cineca Grant Key
Project) for the work carried out at the University of Salerno. L.
Cavallo thanks the HPC team of Enea (www.enea.it) for use of
the ENEA-GRID and the HPC facilities CRESCO (www.cresco.e-
nea.it) in Portici, Italy. We thank Susie Miller for help with the
X-ray diffraction analyses of complexes 7 and 11, and Boulder
Scientific Co. for the research gifts of B(C6F5)3, [Ph3C][B(C6F5)4],
and [Ph2C(Cp)(Flu)]ZrCl2.
Third, kinetic experiments show that MMA polymerization
by 6+[B(C6F5)4]- in toluene at room temperature follows first-
order kinetics in both monomer and catalyst concentrations,
consistent with a monometallic, intramolecular coordination-
addition mechanism. Other mechanistic insights gained from
examining fundamental reaction steps and polymerization
processes provide collaborative evidence to support this mech-
anism that involves in the propagation “catalysis” cycle the fast
intramolecular Michael addition within the catalyst-monomer
complex, leading to the resting eight-membered ester enolate
chelate, followed by the slower, rate-limiting ring-opening of
the chelate to regenerate the active species.
Supporting Information Available: Experimental details and
crystallographic data for 7 and 11 (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Fourth, on the theoretical side, DFT calculations rationalized
why the Ph2C< bridged catalyst system 6 exhibits higher
stereoselectivity than 2 (with the Me2C< bridge) or 11 (with
JA908818Y
9
J. AM. CHEM. SOC. VOL. 132, NO. 8, 2010 2709