Neutral metallocene ester enolate and non-metallocene alkoxy complexes of zirconium for catalytic ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1021/om800602s

Four neutral zirconocene bis(ester enolate) and non-zirconocene bis(alkoxy) complexes have been employed for ring-opening polymerizations and chain transfer polymerizations of L-lactide (L-LA) and e-caprolactone (ε-CL). All C_2v-, C₂-, and C_s-ligated neutral zirconocene bis(ester enolate) complexes effectively polymerize ε-CL at 80°C with high (>90%) initiator efficiencies. The C_s-ligated complex Ph ₂C(Cp)(Flu)Zr[OC(OⁱPr)=CMe₂]₂ (1) also promotes highly efficient polymerization of L-LA and is at least 100 times more reactive than the C_2v.,- and C₂-ligated analogues. The L-LA polymerization by 1 exhibits living characteristics, producing PLA with quantitative isotacticity (no sign of monomer epimerization) and controlled molecular weight. This polymerization follows firstorder kinetics with respect to both [L-LA] and [1], consistent with a monometallic, coordinationanionic propagation mechanism. The structurally characterized non-zirconocene bis(alkoxy) complex [(2,6-ⁱPr₂C₆H ₄)N(CH₂)₃N(2,6-ⁱPr₂C ₆H₄)]Zr(OⁱPr)₂[O=C(NMe ₂)CHMe₂] (2) has been synthesized in quantitative yield from the reaction of the zirconium bis(amido) precursor with isopropyl isobutyrate. Complex 2 behaves as a single-site catalyst for polymerizations of both ε-CL and L-LA, unlike the comparative homoleptic zirconium tetraisopropoxy complex, which shows multisite behavior for the same processes. Both the zirconocene 1 and the non-zirconocene 2 catalyze efficient chain transfer polymerization in the presence of ⁱPrOH as a chain transfer reagent (CTR) for the catalytic production of PLA and PCL, respectively; however, the metallocene system is more robust (in terms of ligand stability and maintaining the polymerization rate) toward an excess of a protic CTR than the present non-metallocene system.

Full text of DOI:10.1021/om800602s

5632
Organometallics 2008, 27, 5632–5640
Neutral Metallocene Ester Enolate and Non-Metallocene Alkoxy
Complexes of Zirconium for Catalytic Ring-Opening Polymerization
of Cyclic Esters
Yalan Ning, Yuetao Zhang, Antonio Rodriguez-Delgado,† and Eugene Y.-X. Chen*
Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523-1872
ReceiVed June 28, 2008
Four neutral zirconocene bis(ester enolate) and non-zirconocene bis(alkoxy) complexes have been
employed for ring-opening polymerizations and chain transfer polymerizations of L-lactide (L-LA)
and ꢀ-caprolactone (ꢀ-CL). All C2V-, C2-, and Cs-ligated neutral zirconocene bis(ester enolate)
complexes effectively polymerize ꢀ-CL at 80 °C with high (>90%) initiator efficiencies. The Cs-
ligated complex Ph2C(Cp)(Flu)Zr[OC(OiPr)dCMe2]2 (1) also promotes highly efficient polymerization
of L-LA and is at least 100 times more reactive than the C2V- and C2-ligated analogues. The L-LA
polymerization by 1 exhibits living characteristics, producing PLA with quantitative isotacticity (no
sign of monomer epimerization) and controlled molecular weight. This polymerization follows first-
order kinetics with respect to both [L-LA] and [1], consistent with a monometallic, coordination–
anionic propagation mechanism. The structurally characterized non-zirconocene bis(alkoxy) complex
[(2,6-iPr2C6H4)N(CH2)3N(2,6-iPr2C6H4)]Zr(OiPr)2[OdC(NMe2)CHMe2] (2) has been synthesized in
quantitative yield from the reaction of the zirconium bis(amido) precursor with isopropyl isobutyrate.
Complex 2 behaves as a single-site catalyst for polymerizations of both ꢀ-CL and L-LA, unlike the
comparative homoleptic zirconium tetraisopropoxy complex, which shows multisite behavior for
the same processes. Both the zirconocene 1 and the non-zirconocene 2 catalyze efficient chain transfer
polymerization in the presence of iPrOH as a chain transfer reagent (CTR) for the catalytic production
of PLA and PCL, respectively; however, the metallocene system is more robust (in terms of ligand
stability and maintaining the polymerization rate) toward an excess of a protic CTR than the present
non-metallocene system.
elements, transition metals, and lanthanides, for controlled
(over tacticity or MW) ROP polymerization of lactides4 and
Introduction
Ring-opening polymerization (ROP) of heterocyclic mono-
mers via the coordination-anionic (also termed coordina-
tion-insertion) mechanism1 is a leading polymerization
technique to produce high-molecular-weight (MW) polymers
typical of step-growth products, yet through the chain-growth
mechanism (Scheme 1). A large number of metal complexes
of various types have been extensively investigated as
catalysts/initiators for the ROP of heterocyclic monomers,
particularly cyclic esters (lactides such as L-LA and rac-LA;
lactones such as ꢀ-caprolactone, ꢀ-CL),2 largely due to the
biodegradability and biocompatibility of the resulting lactide
and lactone polymers (e.g., poly(lactide), PLA, and poly(ꢀ-
CL), PCL).3 These catalysts/initiators have encompassed
organometallic or coordination complexes of main-group
(4) Selected recent examples of lactide polymerization: (a) Douglas,
A. F.; Patrick, B. O.; Mehrkhodavandi, P. Angew. Chem., Int. Ed. 2008,
47, 2290–2293. (b) Chisholm, M. H.; Gallucci, J. C.; Quisenberry, K. T.;
Zhou, Z. Inorg. Chem. 2008, 47, 2613–2624. (c) Pang, X.; Du, H.; Chen,
X.; Wang, X.; Jing, X. Chem. Eur. J. 2008, 14, 3126–3136. (d) Chmura,
A. J.; Chuck, C. J.; Davidson, M. G.; Jones, M. D.; Lunn, M. D.; Bull,
S. D.; Mahon, M. F. Angew. Chem., Int. Ed. 2007, 46, 2280–2283. (e)
Nomura, N.; Ishii, R.; Yamamoto, Y.; Kondo, T. Chem. Eur. J. 2007, 13,
4433–4451. (f) Darensbourg, D. J.; Choi, W.; Richers, C. P. Macromolecules
2007, 40, 3521–3523. (g) Tang, H.-Y.; Chen, H.-Y.; Huang, J.-H.; Lin,
C.-C. Macromolecules 2007, 40, 8855–8860. (h) Du, H.; Pang, X.; Yu, H.;
Zhuang, X.; Chen, X.; Cui, D.; Wang, X.; Jing, X. Macromolecules 2007,
40, 1904–1913. (i) Platel, R. H.; Hodgson, L. M.; White, A. J. P.; Williams,
C. K. Organometallics 2007, 26, 4955–4963. (j) Huang, C.-A.; Chen, C.-
C. Dalton Trans. 2007, 5561–5566. (k) Chisholm, M. H.; Patmore, N. J.;
Zhou, Z. Chem. Commun. 2005, 127–129. (l) Hsueh, M.-L.; Huang, B.-H.;
Wu, J.; Lin, C.-C. Macromolecules 2005, 38, 9482–9487. (m) Hormnirun,
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Chem. Soc. 2004, 126, 2688–2689. (n) Williams, C. K.; Breyfogle, L. E.;
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J. Am. Chem. Soc. 2003, 125, 11350–11359. (o) Cai, C.-X.; Amgoune, A.;
Lehmann, C. W.; Carpentier, J.-F. Chem. Commun. 2004, 330–331. (p)
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11298. (q) Chakraborty, D.; Chen, E. Y.-X. Organometallics 2003, 22, 769–
774. (r) Satoh, Y.; Ikitake, N.; Nakayama, Y.; Okuno, S.; Yasuda, H. J.
Organomet. Chem. 2003, 667, 42–52. (s) Nomura, N.; Ishii, R.; Akakura,
M.; Aoi, K. J. Am. Chem. Soc. 2002, 124, 5938–5939. (t) Ovitt, T. M.;
Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 1316–
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Baker, G. L.; Smith, M. R. J. Am. Chem. Soc. 2000, 122, 1552–1553.
* To whom correspondence should be addressed. E-mail: eychen@
lamar.colostate.edu.
† Current address: Instituto de Investigaciones Qu´ımicas, CSIC, Av.
Ame´rico Vespucio 49, 41092 Sevilla, Spain.
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Polym. Sci. 1999, 147, 1–59. (b) Kuran, W. Prog. Polym. Sci. 1998, 23,
919–992.
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Chem. ReV. 2006, 250, 602–626. (b) Dechy-Cabaret, O.; Martin-Vaca, B.;
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10.1021/om800602s CCC: $40.75
2008 American Chemical Society
Publication on Web 10/09/2008

Complexes of Zr for Ring-Opening Polymerization
Organometallics, Vol. 27, No. 21, 2008 5633
Scheme 1
monomers with such highly electron-deficient group 4 metal-
locene complexes is less known,9 despite their established high
stereospecificity and degree of control for the polymerization
of methacrylates10 and acrylamides.11 On the other hand,
coordination-anionic ROP of cyclic esters using group 4 non-
metallocene complexes is well-known, thanks to extensive
studies in this area (the metal and monomer used in each of the
following examples selected for a brief overview are shown in
parentheses for clarity). These catalysts/initiators are typical of
those group 4 metal alkoxides (as initiating groups) supported
by chiral phenoxyimine (Zr; rac-LA),12 tris(phenoxy)amine (Zr,
Hf; rac-LA),13 bis(ꢀ-ketoamidate) (Ti, Zr; rac-LA, ꢀ-CL),14
bis(iminophenoxide) (“salen”) (Ti; rac-LA),15 N-heterocyclic
carbene (Ti; rac-LA),16 bis(phenoxy)amine (Ti, Zr, Hf; rac-
LA, L-LA, ꢀ-CL),17 pyrrolylamine (Zr, Hf; ꢀ-CL),18 tris(alkoxy
or aryloxy) (Ti; rac-LA, L-LA),19 tris(alkoxy or aryloxy)amine
(Ti; rac-LA, L-LA, ꢀ-CL),20 chalcogen-bridged chelating
bis(aryloxy) (Ti; L-LA, ꢀ-CL),21 and methylene-bridged bis(phe-
noxy) (Ti; ꢀ-CL)22 ligands. In addition to alkoxides, bis(ami-
do)titanium complexes supported by the chelating diaryloxy
ligands have also been used for the ROP of L-LA and ꢀ-CL.23
Other catalysts included homoleptic group 4 alkoxide (Ti, Zr;
L-LA, ꢀ-CL)24 and acetylacetonate (Zr; L-LA, ꢀ-CL)25 com-
plexes as well as the titanium-organic framework derived from
Ti(OiPr)4 and 1,4-butanediol (L-LA and ꢀ-CL).26
lactones.5 Relatively fewer reports have described studies on
the catalytic ROP of cyclic esters using metal complexes in
the presence of an excess of a chain transfer reagent (CTR).6
Group 4 non-metallocenes (non-Cp complexes) and metal-
locenes, typically in their cationic forms,7 are best known for
their remarkable success in the production of revolutionary
polyolefin materials through their catalyzed homogeneous,
single-site, (co)polymerization of nonpolar vinyl monomers (R-
olefins in particular).8 The polymerization of polar vinyl
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Sarazin, Y.; Schormann, M.; Bochmann, M. Organometallics 2004, 23,
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34, 6196–6201.
Fewer reports have described the polymerization of ꢀ-CL
using group 4 metallocene complexes. Titanocene and zir-
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3416. (b) Miyake, G. M.; Mariott, W. R.; Chen, E. Y.-X. J. Am. Chem.
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Marshall, E. L.; West, L. J. Organometallics 2007, 26, 316–320. (b)
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(16) Patel, D.; Liddle, S. T.; Mungur, S. A.; Rodden, M.; Blake, A.;
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5634 Organometallics, Vol. 27, No. 21, 2008
Ning et al.
Scheme 2
conocene alkyne complexes27 as well as their bimetallic
chain production (to point 3). In addition to addressing the above
three fundamentally important points, this study highlights the
sharp differences between group 4 metallocene and non-
metallocene catalysts in polymerization activity and, especially,
converse behavior in their catalyzed chain transfer polymeri-
zation upon addition of an excess of isopropyl alcohol as a CTR.
i
congeners formed with Bu2AlH28 were found active for the
polymerization of ꢀ-CL. Titanocene alkoxide species derived
from the Cp2TiCl-catalyzed radical ROP of epoxides can initiate
controlled polymerization of ꢀ-CL.29 Half-sandwich (half-
metallocene) dichlorotitanium alkoxides have been utilized to
mediate controlled polymerization of ꢀ-CL;30 substituting the
Cp ligand in half-titanocenes with the indenyl ligand resulted
in significant (greater than an order of magnitude) ꢀ-CL
polymerization rate enhancement.31 Cationic zirconocene com-
plexes such as [Cp2ZrMe]+[B(C6F5)4]- and Cp2ZrMe+MeB-
(C6F5)3-, which are derived from activation of the dimethyl
complex with Ph3CB(C6F5)4 and B(C6F5)3, respectively, are
known to promote living polymerization of ꢀ-CL; however, this
occurs through a noncoordination, cationic mechanism.32
The following three points readily become apparent on the
basis of the above overview. First, there has been no report, to
the best of our knowledge, on lactide polymerization using group
4 metallocenes, either in their neutral or cationic form. Second,
there has been no study on the polymerization of lactide or ꢀ-CL
using group 4 non-metallocene complexes supported by a
chelating diamide ligand. Third, as little has been studied on
the catalytic ROP of cyclic esters using chain transfer polym-
erization strategies, there is a need for more research effort in
this field devoted to the catalytic production of polymer chains.
To address the above three points, we report in this contribution
the first efficient lactide polymerization system catalyzed by
group 4 metallocenes, especially Ph2C(Cp)(Flu)Zr[OC(OiPr)d
CMe2]2 (1) depicted in Scheme 2 (to point 1), the synthesis,
structural characterization, and cyclic ester polymerization
studies of the non-zirconocene bis(alkoxy) complex
(BAIP)Zr(OiPr)2[OdC(NMe2)CHMe2] (2, Scheme 2) supported
by the chelating bulky aryl diamido ligand [(2,6-iPr2C6H4)N-
(CH2)3N(2,6-iPr2C6H4)] (BAIP)33 (to point 2), and an investiga-
tion into the chain transfer polymerization characteristics of the
zirconocene and non-zirconocene systems for catalytic polymer
Experimental Section
Materials, Reagents, and Methods. All syntheses and manipu-
lations of air- and moisture-sensitive materials were carried out in
flamed Schlenk-type glassware on a dual-manifold Schlenk line,
on a high-vacuum line, or in an argon-filled glovebox. NMR-scale
reactions (typically on a 0.02 mmol scale) were conducted in Teflon-
valve-sealed J. Young type NMR tubes. HPLC-grade organic
solvents were first sparged extensively with nitrogen during filling
of 20 L solvent reservoirs and then dried by passage through
activated alumina (for Et2O, THF, and CH2Cl2) followed by passage
through Q-5 supported copper catalyst (for toluene and hexanes)
stainless steel columns. Benzene-d6 and toluene-d8 were dried over
sodium/potassium alloy and vacuum-distilled or filtered, whereas
CD2Cl2, and CDCl3 were dried over activated Davison 4 Å
molecular sieves. NMR spectra were recorded on either a Varian
1
Inova 300 (FT 300 MHz, H; 75 MHz, 13C; 282 MHz, 19F) or a
Varian Inova 400 spectrometer. Chemical shifts for 1H and 13C
spectra were referenced to internal solvent resonances and are
reported as parts per million relative to SiMe4, whereas 19F NMR
spectra were referenced to external CFCl3. Elemental analyses were
performed by Desert Analytics, Tucson, AZ.
ꢀ-Caprolactone (ꢀ-CL), L-lactide (L-LA), and butylated hydroxy-
toluene (BHT-H, 2,6-di-tert-butyl-4-methylphenol) were purchased
from Aldrich Chemical Co. ꢀ-CL was first degassed and dried over
CaH2 overnight, followed by vacuum distillation, while L-LA was
purified by sublimation. The purified monomers were stored in
brown bottles inside a -30 °C glovebox freezer. BHT-H was
recrystallized from hexanes prior to use. Isopropyl alcohol and
isopropyl isobutyrate were purchased from TCI America and dried
over CaH2, followed by vacuum distillation. Literature procedures
were employed for the preparation of the following ligand and
complexes: Cp2Zr[OC(OiPr)dCMe2]2 (Cp ) η5-cyclopentadi-
enyl),34 rac-C2H4(Ind)2Zr[OC(OiPr)dCMe2]2 (Ind ) η5-indenyl),10e
Ph2C(Cp)(Flu)Zr[OC(OiPr)dCMe2]2 (1, Flu ) η5-fluorenyl),10a
(2,6-iPr2C6H4)HN(CH2)3NH(2,6-iPr2C6H4) ((BAIP)H2),33 (BAIP)
(27) (a) Thomas, D.; Arndt, P.; Peulecke, N.; Spannenberg, A.; Kempe,
R.; Rosenthal, U. Eur. J. Inorg. Chem. 1998, 1351–1357. (b) Arndt, P.;
Thomas, D.; Rosenthal, U. Tetrahedron Lett. 1997, 38, 5467–5468.
(28) Arndt, P.; Spannenberg, A.; Baumann, W.; Rosenthal, U. Eur.
J. Inorg. Chem. 2001, 2885–2890.
(29) Asandei, A.; Saha, G. Macromol. Rapid Commun. 2005, 26, 626–
631.
(30) (a) Touris, A.; Kostakis, K.; Mourmouris, S.; Kotzabasakis, V.;
Pitsikalis, M.; Hadjichristidis, N. Macromolecules 2008, 41, 2426–2438.
(b) Okuda, J.; Rushkin, I. L. Macromolecules 1993, 26, 5530–5532.
(31) (a) Okuda, J.; Kleinhenn, T.; Ko¨nig, P.; Taden, I. Macromol. Symp.
1995, 95, 195–202. (b) Okuda, J.; Ko¨nig, P.; Rushkin, I. L.; Kang, H.-C.;
Massa, W. J. Organomet. Chem. 1995, 501, 37–39.
ZrX2(X
)
NMe2, Cl),33 [Zr(OiPr)4(iPrOH)]2 (3),35 and
Cp2Zr(OiPr)2.36
Synthesis of (BAIP)Zr(OiPr)2[OdC(NMe2)CHMe2] (2). In an
argon-filled glovebox, a 30 mL glass reactor was charged with
(BAIP)Zr(NMe2)2 (0.223 g, 0.39 mmol) and 20 mL of hexanes.
Neat Me2CHC(dO)OiPr (0.102 g, 0.12 mL, 0.78 mmol) was added
(32) (a) Kostakis, K.; Mourmouris, S.; Karanikolopoulos, G.; Pitsikalis,
M.; Hadjichristidis, N. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3524–
3537. (b) Hayakawa, M.; Mitani, M.; Yamada, T.; Mukaiyama, T.
Macromol. Chem. Phys. 1997, 198, 1305–1317. (c) Mukaiyama, T.;
Hayakawa, M.; Mitani, M.; Yamada, T. Chem. Lett. 1995, 737–738.
(33) (a) Scollard, J. D.; McConville, D. H.; Vittal, J. J. Organometallics
1997, 16, 4415–4420. (b) Scollard, J. D.; McConville, D. H.; Vittal, J. J.
Organometallics 1995, 14, 5478–5480.
(34) Ning, Y.; Zhu, H.; Chen, E. Y.-X. J. Organomet. Chem. 2007, 692,
4535–4544.
(35) Vaartstra, B. A.; Huffman, J. C.; Gradeff, P. S.; Hubert-Pfalzgraf,
L. G.; Daran, J.; Parraud, S.; Yunlu, K.; Caulton, K. Inorg. Chem. 1990,
29, 3126–3131.
(36) Gray, D. R.; Brubaker, C. H., Jr Inorg. Chem. 1971, 10, 2143–
2146.

Complexes of Zr for Ring-Opening Polymerization
Organometallics, Vol. 27, No. 21, 2008 5635
to this precooled (-30 °C) and stirred solution via pipet. The
resulting solution was gradually warmed to room temperature and
stirred for 2 h, after which all volatiles were removed in vacuo to
give 0.280 g (quantitative yield) of spectroscopically pure complex
2 as a colorless crystalline solid. The use of toluene as the reaction
solvent gave a similar result. Anal. Calcd for C39H67N3O3Zr: C,
65.32; H, 9.42; N, 5.85. Found: C, 66.17; H, 9.23; N, 4.83.
Crystallization from hexanes at -30 °C inside the freezer of the
glovebox for 2 days afforded colorless single crystals, which were
subsequently characterized by X-ray diffraction.
(0.50 mL, [CL]0/[Zr]0 ) 200/1) was added in 4 mL of toluene. For
chain transfer polymerization, the monomer was premixed with a
i
predetermined amount of PrOH. Polymerization of L-LA was
performed in 25 mL Schlenk flasks on the Schlenk line at 80 °C.
A Schlenk flask was placed in the glovebox, charged with L-LA
(3.0 mmol) and 6 mL of toluene with or without 2-propanol, and
subsequently interfaced on the Schlenk line. After the desired
temperature was reached using an external temperature-controlled
bath, 4 mL of the catalyst solution in toluene (15 µmol, [L-LA]0/
[Zr]0 ) 200/1) prepared in the glovebox was injected into the flask
using a gastight syringe. The rest of the procedure is the same as
that described above for the zirconocene complex.
Polymerization Kinetics. Kinetic experiments were carried out
in a stirred glass reactor at ambient temperature (∼25 °C) inside
the glovebox for ꢀ-CL polymerization or in flasks at 80 °C on the
Schlenk line for L-LA polymerization using stock solutions of the
reagents and the procedures described in the literature.10d,38
Specifically, at appropriate time intervals, 0.2 mL aliquots were
withdrawn from the reaction mixture using a syringe and quickly
quenched into 1 mL vials containing 0.6 mL of undried “wet”
CDCl3 mixed with 250 ppm of BHT-H. The quenched aliquots were
analyzed by 1H NMR. For ꢀ-CL polymerization, the ratio of [ꢀ-CL]0
to [ꢀ-CL]t at a given time t, [CL]0/[CL]t, was determined by
integration of the peaks for ꢀ-CL (4.2 ppm for the OCH2 signal)
and PCL (4.0 ppm for the OCH2 signal) according to the equation
[ꢀ-CL]0/[ꢀ-CL]t ) (A4.2 + A4.0)/A4.2, where A4.2 is the integral for
the peak at 4.2 ppm and A4.0 is the integral for the peak at 4.0
ppm. For L-LA polymerization, the [L-LA]0/[L-LA]t ratio was
determined by integration of the peaks for L-LA (4.8 ppm for the
methine proton signal) and PLA (5.4 ppm for the methine proton
signal) according to the equation [LLA]0/[LLA]t ) (A4.8 + A5.4)/
A4.8. Apparent rate constants (kapp) were extracted from the slopes
of the best-fit lines to the plots of ln([M]0/[M]t) vs time.
1H NMR (C7D8, 23 °C): δ 7.18-7.10 (m, 6H, Ar), 4.26 (sept,
J ) 6.3 Hz, 2H, OCHMe2), 4.03 (sept, J ) 7.2 Hz, 4H, ArCHMe2),
3.56 (t, J ) 5.4 Hz, 4H, NCH2CH2), 2.68 (pent, J ) 5.4 Hz, 2H,
NCH2CH2), 2.40 (s, 3H, NMe2), 2.18 (sept, J ) 7.2 Hz, 1H,
CHMe2), 2.14 (s, 3H, NMe2), 1.41 (d, J ) 7.2 Hz, 24H, ArCHMe2),
1.03 (d, J ) 6.0 Hz, 12H, OCHMe2), 0.87 (d, J ) 7.2 Hz, 6H,
CHMe2). 13C{1H} NMR (C6D6, 23 °C): δ 176.85 (CdO), 145.50,
142.52, 128.33, 124.25, 123.83, and 123.71 (Ar), 71.40 (OCHMe2),
58.40 (NCH2CH2), 50.62 (NMe2), 36.30, 35.75, 32.51, 31.42, 30.58,
27.63, 27.07, 26.29, 25.28, 24.34, and 18.85 (NCH2CH2, CHMe2).
X-ray Crystallographic Analysis of 2. Single crystals of
complex 2 were quickly covered with a layer of Paratone-N oil
(Exxon, dried and degassed at 120 °C/10-6 Torr for 24 h) after
decanting the mother liquor. A crystal was then mounted on a thin
glass fiber and transferred into the cold nitrogen stream of a Bruker
SMART CCD diffractometer. The structure was solved by direct
methods and refined using the Bruker SHELXTL program library.37
The structure was refined by full-matrix least squares on F2 for all
reflections. All non-hydrogen atoms were refined with anisotropic
displacement parameters, whereas hydrogen atoms were included
in the structure factor calculations at idealized positions. Selected
crystallographic data for 2: C39H67N3O3Zr, monoclinic, space group
P21/n, a ) 18.422(2) Å, b ) 23.949(2) Å, c ) 18.896(2) Å, ꢀ )
101.982(1)°, V ) 8155.3(13) Å3, Z ) 8, Dcalcd ) 1.168 Mg/m3,
GOF ) 1.094, R1 ) 0.0498 (I > 2σ(I)), wR2 ) 0.1379.
Polymer Characterizations. The polymer number-average mo-
lecular weight Mn and polydispersity index (PDI ) Mw/Mn) were
measured by gel permeation chromatography (GPC) analyses
carried out at 40 °C and a flow rate of 1.0 mL/min, with CHCl3 as
the eluent on a Waters University 1500 GPC instrument equipped
with one PLgel 5 µm guard and three PLgel 5 µm mixed-C columns
(PolymerLaboratories;linearrangeofmolecularweight200-2 000 000).
The instrument was calibrated with 10 PMMA standards, and
chromatograms were processed with Waters Empower software
(version 2002). 1H NMR spectra for the analysis of PCL and PLA
microstructures39 were recorded in CDCl3.
General Procedures for Polymerizations Using Zirconocene
Complexes. Polymerizations were performed either in 25 mL flame-
dried Schlenk flasks interfaced to the dual-manifold Schlenk line
for runs using an external temperature bath or in 20 mL glass
reactors inside the argon-filled glovebox for ambient-temperature
(ca. 25 °C) runs. A predetermined amount of a zirconocene complex
such as 1 was first dissolved in 2 mL of toluene inside a glovebox,
and the polymerization was started by rapid addition of the above
solution via gastight syringe to a solution of L-LA (4.5 mmol) or
ꢀ-CL (4.5 mmol) in 3 mL of toluene with vigorous stirring at the
pre-equilibrated bath temperature (on a Schlenk line). The amounts
of monomers used were the same for all polymerizations, whereas
the amount of the zirconocene was adjusted according to the
[M]/[Zr] ratio specified in the text. For chain transfer polymeriza-
Results and Discussion
Polymerizations of L-LA and E-CL by Zirconocene
Bis(ester enolate) Complexes. In this study we chose L-LA
over rac-LA for a simple reason: high-purity (99.9%) L-LA is
currently produced on a large industrial scale from L-lactic acid
derived from fermentation of biorenewable raw materials such
as corn40 and the prices for L-LA and rac-LA are about the
same (a rare case where a racemic substance is not any cheaper
than its enantiopure isomer), thus possessing no economical
advantage for polymerization of rac-LA over L-LA as far as
the production of isotactic PLA is concerned.
i
tions, a predetermined amount of PrOH was mixed with the
monomer in solution before addition of the catalyst. After the mea-
sured time interval, a 0.2 mL aliquot was taken from the reaction
mixture via syringe and quickly quenched into a 4 mL vial
containing 0.6 mL of undried “wet” CDCl3 stabilized by 250 ppm
of BHT-H; the quenched aliquots were later analyzed by 1H NMR
to obtain the monomer conversion data. The polymerization was
immediately quenched after the removal of the last aliquot by
addition of 1 mL of methanol, and the solvents were removed in
vacuo. The polymer product was precipitated into 50 mL of
methanol, stirred for 1 h, filtered, washed with methanol, and dried
in a vacuum oven at 50 °C overnight to a constant weight.
(38) Rodriguez-Delgado, A.; Chen, E. Y.-X. J. Am. Chem. Soc. 2005,
127, 961–974.
(39) (a) Chisholm, M. H.; Iyer, S. S.; McCollum, D. G.; Pagel, M.;
Werner-Zwanziger, U. Macromolecules 1999, 32, 963–973. (b) Thakur,
K. A. M.; Kean, R. T.; Hall, E. S.; Kolstad, J. J.; Lindgren, T. A.; Doscotch,
M. A.; Siepmann, J. I.; Munson, E. J. Macromolecules 1997, 30, 2422–
2428.
General Procedures for Polymerization Using Non-Zirconocene
Complexes. In a typical ꢀ-CL polymerization procedure, 22.5 µmol
of the complex was first dissolved in 6 mL of toluene and ꢀ-CL
(40) (a) Mecking, S. Angew. Chem., Int. Ed. 2004, 43, 1078–1085. (b)
Drumright, R. E.; Gruber, P. R.; Henton, D. E. AdV. Mater. 2000, 12, 1841–
1846.
(37) SHELXTL, Version 6.12; Bruker Analytical X-ray Solutions,
Madison, WI, 2001.

5636 Organometallics, Vol. 27, No. 21, 2008
Ning et al.
Table 1. L-LA Polymerization Results by Zirconocene Bis(ester
enolate) 1a
We initially screened the L-LA polymerization using cationic
zirconocene alkyl or alkoxy complexes of various ligation,
-
including
Cp2ZrMe+MeE(C6F5)3
(E
)
B,
Al),
b
[Zr]
[L-LA]0/
[Zr]0
time
conversn 103Mn
PDIb
I*c
Cp2Zr(OMe)+MeB(C6F5)3-, rac-Me2Si(Ind)2ZrMe+ MeE(C6F5)3-,
and rac-Me2Si(Ind)2Zr2+[MeAl(C6F5)3-]2, and found that all were
inactive. Next, we turned our attention to neutral metallocene
bis(ester enolate) complexes, as we reasoned that the ester
enolate ligand, being a strong nucleophile, could initiate the
lactide polymerization. Indeed, the C2V-ligated achiral zir-
conocene bis(ester enolate) complex Cp2Zr[OC(OiPr)dCMe2]2
exhibits some activity for L-LA polymerization with a [L-LA]/
[Zr] ratio of 200 in toluene at 80 °C, although achieving only
7% monomer conversion in 26 h and producing PLA with a
low MW of Mn ) 5.29 × 103 and narrow MW distribution
(MWD) of PDI ) 1.12 (the calculated initiator efficiency, I*,
is 41%). Similarly, C2-ligated chiral rac-C2H4(Ind)2-
Zr[OC(OiPr)dCMe2]2 is only marginally active for L-LA
polymerization ([L-LA]/[Zr] ) 200) in toluene at 80 °C,
achieving only 7% monomer conversion in 18 h; the resulting
PLA also has a low MW of Mn ) 4.38 × 103 and narrow MWD
of PDI ) 1.15 (I* ) 49%). Excitingly, the Cs-ligated complex
Ph2C(Cp)(Flu)Zr[OC(OiPr)dCMe2]2 (1) is substantially more
active (with >100-fold rate enhancement) under the same
conditions (toluene, 80 °C) and can also achieve high monomer
conversions. This interesting finding prompted us to investigate
the L-LA polymerization by complex 1 in more detail, the results
of which are summarized in Table 1.
(mmol/L)
(min)
(%)
(g/mol) (Mw/Mn) (%)
18.0
50
11
15
20
25
35
50
80
105
31
36
46
53
64
76
86
92
2.58
2.82
3.38
3.73
4.34
5.07
5.76
5.86
1.18
1.21
1.22
1.23
1.23
1.24
1.24
1.25
92
97
102
106
109
110
110
115
12.9
70
10
15
20
30
45
60
100
151
19
27
34
45
58
68
83
91
2.03
2.96
3.31
4.03
4.64
6.12
7.17
7.93
1.26
1.27
1.26
1.33
1.32
1.23
1.28
1.22
101
97
107
116
128
114
118
117
9.02
6.68
100
135
15
20
35
45
65
24
30
47
56
68
77
85
3.07
4.21
6.38
6.51
7.95
9.14
9.90
1.28
1.25
1.22
1.26
1.22
1.21
1.28
117
106
108
126
125
123
125
90
130
5
10
20
30
50
10
21
31
48
60
77
84
90
93
2.61
4.38
5.99
9.36
9.97
1.21
1.25
1.26
1.23
1.21
1.21
1.18
1.15
1.12
80
96
103
101
118
117
111
115
108
The polymerization in a low [L-LA]/[Zr] ratio of 50 exhibits
living characteristics. A plot of the PLA Mn vs monomer
conversion gave a linear relationship (R2 ) 0.997), whereas the
MWD remains narrow for all conversions (PDI ) 1.18-1.25)
(Figure 1). The calculated initiator efficiency I* values (based
on one chain produced per Zr center) range from 92% to 102%
for conversions below 46%, implying that only one of the two
enolate ligands is used for chain initiation. As monomer
conversion gets higher, the I* value goes higher (up to 115%
at 92% conversion), suggesting either a small degree of chain
transfer occurring at the late stage of polymerization or the
possibility of a small percentage of initiation by the second
enolate ligand at Zr.
70
12.9
113
150
210
14.9
15.4
16.9
4.51
200
5
10
20
30
50
5
8
1.97
1.05
1.14
1.28
1.26
1.15
1.14
1.14
1.14
1.14
80
97
2.50
3.90
4.80
17
24
49
61
71
75
84
129
147
128
112
132
129
118
11.0
70
15.8
15.6
16.8
20.5
110
150
285
Variations of the [L-LA]/[Zr] ratio to 70, 100, 135, and 200
still afforded generally well-behaved polymerization (Table 1),
although in some cases the I* values become somewhat higher
due to longer reaction times. The PLA produced from all the
[L-LA]/[Zr] ratios is 100% isotactic (Figure 2), and there is no
sign of epimerization at the NMR detection limit.
a All polymerizations were carried out in flame-dried Schlenk flasks
(5 mL toluene solution) on
a Schlenk line using an external
temperature-control bath set at 80 °C. b Number-average molecular
weight (Mn) and polydispersity index (PDI) determined by GPC relative
to PMMA standards. c Initiator efficiency (I*) ) Mn(calcd)/Mn(exptl),
where Mn(calcd)
MW(chain-end groups).
)
MW(M)
×
[M]/[I]
×
conversion (%) +
The polymerization by 1 follows strictly first-order kinetics
with respect to monomer concentration for all five [L-LA]/[Zr]
ratios investigated (Figure 3). A double-logarithm plot (Figure
4) of the apparent rate constants (kapp), obtained from the slopes
of the best-fit lines to the plots of ln([L-LA]0/[L-LA]t) vs time,
as a function of ln [Zr], was fit to a straight line (R2 ) 0.995)
of slope 0.9(1). The kinetic order with respect to [Zr], given by
this slope, reveals that propagation is also approximately first
orderin[Zr], thusconsistentwiththe monometallic, coordination-
anionic propagation mechanism depicted in Scheme 1.
calculated I* is 91%, suggesting again that only one of the two
ester enolate groups was used to initiate the polymerization.
The C2-ligated chiral rac-C2H4(Ind)2Zr[OC(OiPr)dCMe2]2 is
quite active even at ambient temperature, achieving 98%
monomer conversion in 9 h with an [ꢀ-CL]/[Zr] ratio of 100.
The same polymerization but with an [ꢀ-CL]/[Zr] ratio of 200
in toluene at 80 °C gelled in less than 30 min and gave 100%
monomer conversion upon quenching in 7 h (Mn ) 2.43 × 104,
PDI ) 1.48, I* ) 95%, run 4). The Cs-ligated complex 1 is
more active than Cp2Zr[OC(OiPr)dCMe2]2 but less active than
rac-C2H4(Ind)2Zr[OC(OiPr)dCMe2]2, achieving 88% monomer
conversion at ambient temperature after 19 h (run 5 vs runs 1
and 3). This activity trend in ꢀ-CL polymerization is different
from that observed for the L-LA polymerization, where the Cs-
ligated complex 1 is at least 100 times more reactive than the
other two (vide supra). It is important to note here that all three
zirconocene bis(ester enolate) complexes employed for poly-
We also investigated characteristics of ꢀ-CL polymerization
by these three metallocene bis(ester enolate) complexes, the
selected results of which are summarized in Table 2.
Cp2Zr[OC(OiPr)dCMe2]2 exhibits no activity for ꢀ-CL polym-
erization in toluene at ambient temperature but good activity in
toluene at 80 °C with an [ꢀ-CL]/[Zr] ratio of 200, achieving
89% monomer conversion in 8 h and producing PCL with a
medium MW of Mn ) 2.26 × 104 and PDI ) 1.48 (run 2). The

Complexes of Zr for Ring-Opening Polymerization
Organometallics, Vol. 27, No. 21, 2008 5637
Figure 1. Plot of Mn and PDI of PLA by 1 vs monomer conversion.
Conditions: [MMA]0/[1]0 ) 50, toluene, 80 °C.
Figure 4. Plot of ln kapp vs ln [Zr] for the L-LA polymerization by
complex 1 in toluene at 80 °C.
polymerization of L-LA and ꢀ-CL require the synthesis of non-
zirconocene bis(ester enolate) complexes. Attempts to synthesize
an ester enolate complex of zirconium supported by the diamido
ligand BAIP33 (point 2, vide supra) involved reactions of
zirconium precursors, including (BAIP)ZrCl2, (BAIP)Zr(OTf)2,
and (BAIP)ZrMe(OTf), with Me2CdC(OiPr)OLi, all of which
led to either decomposition of the starting reagents or formation
of unidentifiable product mixtures. Interestingly, treatment of
(BAIP)Zr(NMe2)2 with isopropyl isobutyrate (IPIB) at ambient
temperature led to a facile reaction, affording a single, crystalline
product in quantitative yield. The spectroscopic data (see the
Experimental Section) indicate that the product is a zirconium
bis(isopropoxy) complex, (BAIP)Zr(OiPr)2[OdC(NMe2)-
CHMe2] (2; Scheme 3). Two possible paths leading to the
formation of 2 are depicted in Scheme 3. Path A follows a
classic mechanism of ester nucleophilic substitution catalyzed
by the Lewis acidic Zr center, while path B proceeds with a
plausible aminolysis which leads to an unstable bis(ester enolate)
intermediate, followed by its decomposition involving isopro-
poxide abstraction by the electron-deficient zirconium center,
with concomitant elimination of a ketene which is subsequently
trapped by the amine eliminated in the prior aminolysis step,
forming 2 equiv of N,N-dimethyl isobutyramide (IBA, 1 equiv
is free and subsequently removed during workup, while the other
1 equiv is coordinated to Zr). As all experiments intended to
detect the bis(enolate) and other transient species associated with
path B even at low temperatures or by using bulky tert-butyl
isobutyrate and the more stable amide enolate precursor IBA
did not yield any positive evidence, we concluded that the
reaction proceeds most likely via path A.
Figure 2. Homonuclear decoupled 1H NMR of the methine region
of isotactic-PLA produced by 1.
The single-crystal structure of (BAIP)Zr(OiPr)2-
[OdC(NMe2)CHMe2] (2) is shown in Figure 5. This complex
features a five-coordinate zirconium center in a distorted-
trigonal-bipyramidal geometry, with one amido nitrogen and
the isobutyramide oxygen atoms occupying approximately axial
positions (N(1)-Zr-O(3) ) 175.45(7)°) and the other amido
nitrogen and two isopropoxy oxygen atoms occupying ap-
proximately equatorial positions (the sum of the angles around
the Zr in a trigonal arrangement for the three equatorial ligands
is 358.60°). The two Zr-N(amido) bond lengths are identical
(Zr-N(1) ) 2.101(2) Å, Zr-N(2) ) 2.102(2) Å) and are the
same as those found in a related five-coordinate chelating
diamido zirconium dimethyl complex.41 On the other hand, the
dative Zr-O bond (Zr-O(3) ) 2.273(2) Å) is considerably
Figure 3. Semilogarithmic plots of ln{[L-LA]0/[L-LA]t]} vs time
for the polymerization of L-LA by 1 in toluene at 80 °C. Conditions:
[L-LA]0 ) 0.90 M, [1]0 ) 18.0 mM (0), 12.9 mM (9), 9.02 mM
(4), 6.68 mM (2), 4.51 mM (]).
merizations here are stable (with no evidence for decomposition
by NMR) in toluene solutions at 80 °C up to 20 h.
Synthesis and Structure of the Non-Zirconocene
Bis(isopropoxide) 2. Comparative studies of metallocene vs
non-metallocene zirconium bis(ester enolate) complexes in the

5638 Organometallics, Vol. 27, No. 21, 2008
Ning et al.
Table 2. Selected Results of E-CL Polymerization by Zirconocene Bis(ester enolate) Complexesa
run no.
cat. (ligand)
[ꢀ-CL]/[Zr]
temp (°C)
time (h)
conversn (%)
104Mn (g/mol)
PDI (Mw/Mn)
I* (%)
1
2
Cp2
Cp2
100
200
25
80
8
8
0
89
2.26
1.48
91
3
4
rac-EBI
rac-EBI
100
200
25
80
9
7
98
100
2.43
1.64
1.48
1.67
95
5
Ph2C(Cp)(Flu)
200
25
19
88
122
a Carried out in 5 mL of toluene at the indicated temperatures. See the footnotes in Table1 for definitions of the abbreviations.
Scheme 3
longer than the other two covalent Zr-O bonds (Zr-O(1) )
1.945(2) Å, Zr-O(2) ) 1.971(2) Å) in 2. The sums of the angles
around the N(1), N(2), and N(3) nitrogens are 359.8, 360.0,
and 360.0°, respectively, for the sp2-hybridized, trigonal-planar
nitrogen centers.
Polymerization of L-LA and E-CL by the Non-Zirconocene
Bis(alkoxy) Complex 2. Complex 2 exhibits negligible activity
for L-LA polymerization in toluene at 25 °C; however, at 80
°C with a [L-LA]/[Zr] ratio of 200, 76% monomer conversion
was achieved in 6 h, producing isotactic PLA (no epimerization)
with Mn ) 2.09 × 104 and PDI ) 1.35; the calculated I* is
105%, suggesting only one of two isopropoxy groups was used
to initiate the polymerization. The comparable zirconium
tetraisopropoxy complex [Zr(OiPr)4(iPrOH)]2 (3) is also quite
active for L-LA polymerization, even at 25 °C, achieving 95%
monomer conversion in 4 h with a [L-LA]/[Zr] ratio of 200.
However, the resulting PLA exhibits a bimodal MWD and an
initiator efficiency of greater than 250%, indicative of the
multisite nature of complex 3. These results highlight the
importance of supporting ligands around the metal center for
rendering its single-site nature in this polymerization catalysis.
Polymerization of ꢀ-CL by 2 proceeds rapidly in toluene at
25 °C in an [ꢀ-CL]0/[2]0 ratio of 200, which showed first-order
kinetics with respect to monomer concentration (Figure 6) and
achieved quantitative monomer conversion in 2 h. All polymers
produced at various monomer conversions exhibit unimodal and
Figure 5. Molecular structure of (BAIP)Zr(OiPr)2-
[OdC(NMe2)CHMe2] (2). Selected bond lengths (Å) and angles
(deg): Zr-N(1) ) 2.101(2), Zr-N(2) ) 2.102(2), Zr-O(1) )
1.945(2),Zr-O(2))1.971(2),Zr-O(3))2.273(2);N(1)-Zr-O(3)
) 175.45(7), N(1)-Zr-N(2) ) 90.66(7), O(1)-Zr-O(2) )
125.42(8), O(2)-Zr-O(3) ) 87.24(7), N(1)-Zr-O(1) )
93.71(7).
Figure 6. First-order plots of ln([M]0/[M]t) (M ) ꢀ-CL) vs time
for the polymerization of ꢀ-CL by 2 in 10 mL of solvent at 25 °C.
Conditions: [ꢀ-CL]0 ) 0.450 M, [2]0 ) 2.25 mM, with a [ꢀ-CL]0/
[2]0 ratio of 200, in toluene (2) and CH2Cl2 (4).

Complexes of Zr for Ring-Opening Polymerization
Organometallics, Vol. 27, No. 21, 2008 5639
Scheme 4
narrow MWDs in the range of PDI ) 1.05-1.30, indicative of
the single-site nature of this catalyst in ꢀ-CL polymerization.
Unexpectedly, the same polymerization in the polar, noncoor-
dinating solvent CH2Cl2 is approximately 4.5 times slower than
that in toluene. As complex 2 is stable in CH2Cl2 (no
spectroscopic changes observed for a CD2Cl2 solution of 2 at
room temperature for at least 3 days), the reduced polymeri-
zation rate in CH2Cl2 may suggest that this polar solvent
stabilizes the ground-state structure more than the transition-
state structure associated with this ring-opening polymerization
reaction.
The comparative zirconium isopropoxy complex 3 exhibits
much lower activity than complex 2 for ꢀ-CL polymerization
in toluene at 25 °C, achieving 100% monomer conversion only
after 24 h (vs 2 h by complex 2) with a [ꢀ-CL]/[Zr] ratio of
200. More important, the resulting PCL by 3 shows a trimodal
MWD, and the polymerization gave an initiator efficiency I*
of greater than 450%, again indicative of the multisite nature
of complex 3.
Figure 7. Plots of 1/Mn vs [iPrOH]/[M] for chain transfer polym-
erization of L-LA by complex 1 (2) and of ꢀ-CL by complex 2
(4).
However, two major differences exist between the two polym-
erization processes. First, for the L-LA polymerization by
complex 1, addition of the CTR did not noticeably affect the
polymerization rate (runs 1-3), implying that the relative rate
constants for propagation (kp) and reinitiation (kri) by the
Zr-OiPr moiety (derived from the Zr-PLA chain cleavage by
iPrOH) is about the same (kp ≈ kri); thus, the observed decrease
in Mn while the same polymerization rate is more or less
maintained upon addition of the CTR can be characterized as a
“normal chain transfer” polymerization.42 On the other hand,
the ꢀ-CL polymerization by complex 2 experienced a large
Chain Transfer Polymerization Catalyzed by Zirconocene
Complex 1 and Non-Zirconocene Complex 2. To render a
catalytic production of polymer chains in the present coordina-
tion-anionic polymerization catalyzed by zirconium complexes,
a suitable CTR added externally must effectively cleave the
growing polymer chain from the active metal center, and the
resulting new species containing the nucleophilic part of the
CTR moiety (typically in its deprotonated, anionic form) must
efficiently reinitiate the polymerization (Scheme 4). The control
experiment with Cp2Zr(OiPr)2, which showed that it exhibits
activity for L-LA polymerization in toluene at 80 °C similar to
i
decrease in the polymerization rate as the amount of PrOH
added increases (runs 4-7). This observation may be indicative
of ineffective reinitiation (i.e., kri < kp), but this appears unlikely,
because the Zr-OiPr moiety resulting from chain transfer is
essentially identical with the Zr-OR (Zr-PCL) propagation
species. Alternatively, catalyst deactivation by iPrOH is respon-
sible for the drop in the polymerization rate, which was verified
i
by the NMR-scale reaction showing that an excess of PrOH
indeed decomposes catalyst 2 to the neutral ligand and
[Zr(OiPr)4(iPrOH)]2 (3). As complex 3 exhibits substantially
lower activity than 2 in ꢀ-CL polymerization and also produces
i
that of Cp2Zr[OC(OiPr))CMe2]2, suggests that PrOH be a
i
suitable CTR. Accordingly, to ascertain if PrOH is indeed a
i
suitable CTR for promoting efficient chain transfer, we varied
the [iPrOH]/[monomer] ratio and examined the changes in
polymerization activity and the resulting polymer Mn for the
L-LA polymerization by complex 1 and for the ꢀ-CL polym-
erization by complex 2. Table 3 summarizes the selected results.
multimodal PCL in the presence or absence of PrOH, it is a
reasonable assumption that the polymer product (all with
i
unimodal MWD) for the current 2 + PrOH chain transfer
polymerization system contains no to negligible contribution
from 3, the catalyst 2 decomposition product in the presence of
iPrOH. In comparison, the metallocene system is more robust
in the presence of a protic CTR.
i
Indeed, addition of PrOH effected chain transfer polymeri-
zation for both polymerization systems, as evidenced by a
gradual decrease in Mn (thus increased initiator efficiency of
>100% for the catalytic production of polymer chains) as an
The second major difference between these two polymeri-
zation systems is in their chain transfer constants. A plot of
1/Mn vs the [iPrOH]/[M] ratio gives a linear relationship for
i
increase in the amount of PrOH added (runs 1-7, Table 3).
i
Table 3. Chain Transfer Polymerization Results Using PrOH as a Chain Transfer Reagenta
c
run no. cat. (amt (mM)) monomer (M) [iPrOH]/[Zr] [iPrOH]/[M] time (h) conversnb (%) 103Mn (g/mol) PDIc (Mw/Mn) I*d (%)
1
2
3
1 (4.51)
1 (4.51)
1 (4.51)
L-LA
L-LA
L-LA
0
10
50
0
0.05
0.25
5
6
6
84
100
100
20.5
9.59
3.69
1.14
1.30
1.30
118
302
785
4
5
6
7
2 (2.25)
2 (2.25)
2 (2.25)
2 (2.25)
ꢀ-CL
ꢀ-CL
ꢀ-CL
ꢀ-CL
0
1
5
0
2
5
11
48
100
100
99
24.2
15.2
7.10
3.10
1.29
1.42
1.11
1.22
95
151
320
694
0.005
0.025
0.10
20
94
a
L-LA polymerizations were carried out in 5 mL of toluene at 80 °C, and ꢀ-CL polymerizations were carried out in 10 mL of toluene at 25 °C; [M]/
[Zr] ) 200 for all runs. b Monomer conversion determined by 1H NMR. c Mn and PDI determined by GPC relative to PMMA standards. d Initiator
efficiency (I*) as defined in Table1.

5640 Organometallics, Vol. 27, No. 21, 2008
Ning et al.
both systems (Figure 7), the slope of which corresponds to the
chain transfer constant (Ctr): that is, the ratio of rate constants
of chain transfer to propagation, Ctr ) ktr/kp.42 Accordingly, Ctr
(×104) values for the L-LA polymerization by complex 1 and
the ꢀ-CL polymerization by complex 2 are estimated to be 8.74
and 27.4, respectively. These results indicate that chain transfer
processes are much more efficient for the ꢀ-CL polymerization
by complex 2 than for the L-LA polymerization by complex 1,
while the ratio kp/ktr is much smaller for the former system (3.65
× 102) than for the latter (1.14 × 103).
reaction and is unique in terms of its synthetic utility and high-
yielding synthesis. The bis(isopropoxy) complex 2 supported
by the chelating bulky aryl amido ligand behaves as a single-
site catalyst for polymerizations of both ꢀ-CL and L-LA, whereas
the comparative homoleptic zirconium tetraisopropoxy complex
3 shows multisite behavior for the same polymerizations.
Both the zirconocene 1 and non-zirconocene 2 catalyze
efficient chain transfer polymerization in the presence of iPrOH
as CTR for the catalytic production of PLA and PCL,
respectively, but they do exhibit two major differences. First,
while complex 1 catalyzes the “normal chain transfer” L-LA
polymerization without loss in the polymerization rate upon
increasing [CTR], complex 2 experiences a large decrease in
the ꢀ-CL polymerization rate as [CTR] is increased, reflecting
a certain instability of the non-Cp-based ligand present in
complex 2 under the protic reaction conditions. Second, the
chain transfer process is much more efficient for the ꢀ-CL
polymerization by 2 than for the L-LA polymerization by 1, as
reflected by their estimated chain transfer constants Ctr (×104)
of 27.4 and 8.74, respectively.
Conclusions
Neutral C2V-, C2-, and Cs-ligated zirconocene bis(ester enolate)
complexes effectively polymerize ꢀ-CL in toluene at 80 °C with
high initiator efficiencies based on one chain per metal initiation.
In the polymerization of L-LA, there are large activity differ-
ences among these three types of zirconocenes, with the Cs-
ligated complex 1 exhibiting markedly higher activity (greater
than an order of magnitude) than the C2V- and C2-ligated
complexes. Additionally, the L-LA polymerization by 1 is well-
behaved and characteristic of living. The PLA produced is 100%
isotactic, and there is no sign of epimerization. This polymer-
ization follows first-order kinetics with respect to both monomer
and catalyst concentrations. Overall, all evidence obtained from
the current study points to a conclusion that the polymerization
of L-LA by the zirconocene bis(ester enolate) 1 is well-behaved
and proceeds with a monometallic, coordination-anionic propa-
gation mechanism.
Acknowledgment. This work was supported by the
National Science Foundation (Grant No. NSF-0718061). We
thank Susie Miller for the single-crystal X-ray diffraction
analysis of complex 2.
Supporting Information Available: A CIF file giving crystal-
lographic data for 2. This material is available free of charge via
the Internet at http://pubs.acs.org.
Attempts to synthesize non-zirconocene bis(ester enolate)
complexes for comparative polymerization studies led to isola-
tion of the structurally characterized zirconium bis(isopropoxy)
complex 2 in quantitative yield from the reaction of the
zirconium bis(amido) precursor with isopropyl isobutyrate. This
route utilizes a Zr-catalyzed ester nucleophilic substitution
OM800602S
(41) Gue´rin, F.; McConville, D. H.; Vittal, J. J. Organometallics 1996,
15, 5586–5590.
(42) Odian, G. Principles of Polymerization, 4th ed.; Wiley: Hoboken,
NJ, 2004; pp 238-255.