Single-site anionic polymerization. Monomeric ester enolaluminate propagator synthesis, molecular structure, and polymerization mechanism

Article abstract of DOI:10.1021/ja044671z

The synthesis and molecular structure of the first examples of monomeric lithium ester enolaluminates that serve as structural models for single-site anionic propagating centers, as well as the mechanism of their polymerization of methacrylates catalyzed by conjugate organoaluminum Lewis acids, are reported. Reactions of isopropyl α-lithiolsobutyrate (2) with suitable deaggregating and stabilizing organoaluminum compounds such as MeAl(BHT)₂ (BHT = 2,6-di-tert-butyl-4-methylphenolate) in hydrocarbons cleanly generate lithium ester enolaluminate complexes such as Li⁺[Me₂C=C(O ⁱPr)OAIMe(BHT)₂]^- (3). Remarkably, complex 3 is isolable and exists as a monomer in both solid and solution states. Unlike the uncontrolled polymerization of methacrylates by the aggregating enolate 2, the methacrylate polymerization by the monomeric 3 is controlled but exhibits low activity. However, the well controlled and highly active polymerization can be achieved by using the 3/MeAl(BHT)₂ propagator/catalyst pair, which is conveniently generated by in situ mixing of 2 with 2 equiv of MeAl(BHT) ₂. The structure of the added organoaluminum compounds has marked effects on the degree of monomer activation, enolaluminate formation and reactivity, and polymerization control. Kinetics of the polymerization by the 3/MeAl(BHT)₂ pair suggest a bimolecular, activated-monomer anionic polymerization mechanism via single-site ester enolaluminate propagating centers. The molecular structures of activated monomer 1, aggregated initiator 2, and monomeric propagator 3 have been determined by X-ray diffraction studies.

Full text of DOI:10.1021/ja044671z

Published on Web 12/24/2004
Single-Site Anionic Polymerization. Monomeric Ester
Enolaluminate Propagator Synthesis, Molecular Structure, and
Polymerization Mechanism
Antonio Rodriguez-Delgado and Eugene Y.-X. Chen*
Contribution from the Department of Chemistry, Colorado State UniVersity,
Fort Collins, Colorado 80523-1872
Received September 2, 2004; E-mail: eychen@lamar.colostate.edu
Abstract: The synthesis and molecular structure of the first examples of monomeric lithium ester
enolaluminates that serve as structural models for single-site anionic propagating centers, as well as the
mechanism of their polymerization of methacrylates catalyzed by conjugate organoaluminum Lewis acids,
are reported. Reactions of isopropyl R-lithioisobutyrate (2) with suitable deaggregating and stabilizing
organoaluminum compounds such as MeAl(BHT)2 (BHT ) 2,6-di-tert-butyl-4-methylphenolate) in hydro-
carbons cleanly generate lithium ester enolaluminate complexes such as Li+[Me2CdC(OiPr)OAlMe(BHT)2]-
(3). Remarkably, complex 3 is isolable and exists as a monomer in both solid and solution states. Unlike
the uncontrolled polymerization of methacrylates by the aggregating enolate 2, the methacrylate
polymerization by the monomeric 3 is controlled but exhibits low activity. However, the well controlled and
highly active polymerization can be achieved by using the 3/MeAl(BHT)2 propagator/catalyst pair, which is
conveniently generated by in situ mixing of 2 with 2 equiv of MeAl(BHT)2. The structure of the added
organoaluminum compounds has marked effects on the degree of monomer activation, enolaluminate
formation and reactivity, and polymerization control. Kinetics of the polymerization by the 3/MeAl(BHT)2
pair suggest a bimolecular, activated-monomer anionic polymerization mechanism via single-site ester
enolaluminate propagating centers. The molecular structures of activated monomer 1, aggregated initiator
2, and monomeric propagator 3 have been determined by X-ray diffraction studies.
Introduction
unique size, charge density, and atomic orbitals available for
bonding by solvent molecules and chelating ligands, as well as
Single-site cationic transition-metal complexes have initiated
a new era in polymer synthesis, especially in the synthesis of
new polyolefins.1 In nearly a mirror-image picture, anionic
organometallic initiators such as organolithium compounds are
routinely used to polymerize polar vinyl monomers such as
(meth)acrylates to technologically important functionalized vinyl
polymers.2 Organolithium compounds are the most widely used
initiators in anionic polymerization reactions owing to their high
versatility and reactivity toward various types of monomers;
some noteworthy structural features of the common organo-
lithium initiators include aggregation to give dimers, tetramers
and higher oligomers, complexation of the Li metal center of
complexity and ambiguity in the nature of bonding (i.e., ionic
vs covalent).2 Specifically regarding applications of organo-
lithium initiators in anionic polymerization of alkyl (meth)-
acrylates, their aggregation nature becomes a major problem
of concern. Such anionic initiators as well as their derived
propagating species commonly exist as aggregates, both in solid
state and in solution. These anionic active species (initiators or
propagators) cannot control the polymer number average
molecular weight (Mn) and molecular weight distribution
(MWD) due to the coexistence of various aggregated species
that exhibit different reactivity and exchange slowly on, or
comparably to, the polymerization time scale.3 They can,
however, be characterized as multiple-site anionic active species.
Hence, an important challenge in anionic polymerization using
such initiators has been the extensive and continued quests for
effective additives or catalysts that can render these multiple-
site anionic active species to exhibit high polymerization
activities and degrees of polymerization control.4
(1) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283-315.
(b) Coates, G. W.; Hustad, P. D.; Reinartz, S. Angew. Chem., Int. Ed. 2002,
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Catalyst Design for Tailor-Made Polyolefins; Soga, K., Terano, M., Eds.;
Elsevier: Tokyo, 1994.
Several comprehensive reviews have surveyed substantial
strategies developed to achieve a high degree of polymerization
control.4 Among these strategies is a highly versatile approach
that attracted the most attention: combining alkyllithium
(2) Hsieh, H. L.; Quirk, R. P. Anionic Polymerization. Principles and Practical
Applications. Marcel Dekker: New York, 1996.
(3) Litvinenko, G.; Mu¨ller, A. H. E. Macromolecules 1997, 30, 1253-1266.
9
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J. AM. CHEM. SOC. 2005, 127, 961-974
961

A R T I C L E S
Rodriguez-Delgado and Chen
Scheme 1
initiators with common organoaluminum compounds, often
added in excess. This strategy was first developed by Hatada
et al.5 for the production of highly syndiotactic ([rr] g 90%)
poly(methyl methacrylate) (PMMA) with narrow MWDs (Mw/
Mn e 1.19) using a tBuLi/R3Al (e1/3) combination in toluene
t
at low temperatures (e-78 °C). By combining BuLi with a
bulky aluminum additive, bis(2,6-tert-butylphenoxy)methyl-
aluminum, in a 1:5 ratio, Hatada et al.6 also obtained heterotactic
([mr] ) 67.8%) PMMA with a narrow MWD (Mw/Mn ) 1.14).
Ballard et al.7 produced syndiotactic ([rr] g 70%) PMMA with
narrow MWDs (Mw/Mn ) 1.09-1.28) in toluene at elevated
t
temperatures (0-40 °C) using a combination of BuLi with
lithium ester enolates are unstable, even in the solid state, and
subject to decomposition to ketenes and lithium alkoxides and
â-keto ester enolates.11 Lithium ester enolates have a strong
tendency to aggregate in both crystalline12 and solution13 states,
which affect their reactivity as initiators and the resulting
polymer MWD. Scheme 1 summaries the above-mentioned
complexity and instability of typical lithium ester enolates. As
a result, anionic polymerizations by lithium ester enolates in
nonpolar solvents proceed largely in an uncontrolled manner,
leading to polymers with broad or multimodal MWDs.14 Again,
this uncontrolled polymerization behavior is attributed to the
coexistence of multiple active sites, which exhibit different
reactivity and exchange slowly on the polymerization time scale,
and to the side reactions of ester enolate propagating species.
A strategy to overcome the aggregation and side reaction
problems associated with lithium ester enolate initiators is the
same as that employed for the lithium alkyl initiators, that is,
combining the initiator with organoaluminum alkyls. Schlaad
and Mu¨ller15 proposed, on the basis of the 13C NMR spectro-
scopic evidence, that the bimetallic “ate” complex (B) is an
adequate model of the active center for the MMA polymerization
by ethyl R-lithioisobutyrate (EiBLi) in the presence of aluminum
alkyls. Complex B was detectable by 13C NMR up to 0 °C; at
iBu2Al(BHT) (BHT ) butylated hydroxytoluene, formally 2,6-
di-tert-butyl-4-methylphenolate). Schlaad and Mu¨ller8 reported
that the steric bulk and Lewis acidity of the added alkyl
aluminum compounds strongly influence the tacticity and MWD
t
of the PMMA produced by BuLi in toluene at -78 °C;
depending on the aluminum compound used, the MWD is in
the range 1.2 < Mw/Mn < 7, and the tacticity of the PMMA
can change from being highly syndiotactic to atactic, hetero-
tactic, or highly isotactic. Most recently, Hatada et al.9 reported
the living polymerization of primary alkyl acrylates using a
tBuLi/bis(2,6-tert-butylphenoxy)ethylaluminum combination.
Interestingly, replacing the ethyl group at Al with the smaller
methyl group drastically changed the polymerization behavior,
resulting in the formation of the polymer with a broad MWD
in low yield.
Lithium ester enolates should be, in principle, the ideal
initiators for the polymerization of alkyl (meth)acrylates because
the propagating centers (A) for the anionic polymerization of
(meth)acrylates initiated by organolithium compounds are the
lithium ester enolates.4 Rates of initiation and propagation should
be essentially the same for such initiators, and therefore
polymers with narrow MWDs should be formed. However, the
lithium ester enolate propagator A tends to stabilize through
aggregation (n ) 2-6), and the existence of various aggregated
ester enolates creates significant problems in controlling the
polymerization rate and the polymer MWD.10 Additionally,
ambient temperature, irreversible decomposition takes place. The
calculated structures for the complex of methyl R-lithioisobu-
tyrate (MiBLi) with AlEt3, however, reveal different degrees
of association, (MiBLi‚AlEt3)n (n ) 1, 2, 4), and different
stoichiometries, MiBLi‚xAlEt3 (x ) 1, 2).16 We found that the
addition of 2 equiv of the strongly Lewis acidic alane, Al(C6F5)3,
to the otherwise uncontrolled MMA polymerization by MiBLi,
(4) For comprehensive reviews, see: (a) Baskaran, D. Prog. Polym. Sci. 2003,
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1999, 32, 1731-1736. (c) Weiss, H.; Yakimansky, A. V.; Mu¨ller, A. H.
E. J. Am. Chem. Soc. 1996, 118, 8897-8903. (d) Krˇ´ızˇ, J.; Dybal, J.; Vlcˇek,
P.; Janata, M. Macromol. Chem. Phys. 1994, 195, 3039-3056. (e) Wang,
J. S.; Je´roˆme, R.; Warin, R.; Teyssie´, Ph. Macromolecules 1993, 26, 1402-
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9
962 J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005

Single-Site Anionic Polymerization
A R T I C L E S
1 equiv for generating the lithium ester enolaluminate propagat-
ing species and the other for activating monomer, brings about
a much faster and more controlled polymerization than that by
MiBLi alone, producing syndiotactic PMMA with [rr] ranging
from moderate 78% to high 94%, glass-transition temperatures
(Tg) from 127 to 138 °C, and MWDs as narrow as 1.08,
depending on polymerization temperature.14a A control run using
MiBLi alone in toluene at ambient temperature produced
isotactic ([mm] ) 67%) PMMA with a broad MWD (Mw/Mn )
18.8) and low polymerization activity. Holmes et al.17 investi-
gated ligand effects of organoaluminum amides iBuxAl(NRR′)3-x
(x ) 1, 2) and organoaluminum alkoxides iBuxAl(OR)3-x (x )
1-3) in the MMA polymerization initiated by EiBLi on polymer
tacticity and MWD; they concluded that both formations of the
“ate” complex (B-type structure) and monomer-organoalumi-
num complex are required to promote the controlled syndiose-
lective polymerization. Other additives used for the lithium ester
enolate-initiated (meth)acrylate polymerization, which have been
shown to promote moderate to high degrees of polymerization
control, include lithium tert-butoxide,18 Lewis bases,14b,19 and
tetraalkylammonium (or cesium) halide-trialkylaluminum com-
plexes.20
Thus, extensive research has been conducted on the strategies
for achieving living and/or controlled anionic polymerization
of (meth)acrylates using organolithium initiators; however, to
the best of our knowledge, monomeric anionic active species
(initiators and propagators) have not been unequivocally
established. Inspired by the above-overviewed work of many
research groups in the area of anionic polymerization of (meth)-
acrylates and by the tremendous success that the single-site
cationic transition-metal catalysts or initiators have achieved
in the area of olefin polymerization, we sought to establish the
single-site anionic polymerization for achieving a high degree
of polymerization control. To achieve this central objective of
the present study, we reasoned that, if single, uniform active
species in the form of monomeric ester enolaluminate com-
plexes, which model the structure of the active propagating
species B, can be generated, structurally characterized, and
employed to successfully control the polymerization, then the
“single-site active species” concept can also be realized in
anionic polymerizations by organolithium compounds. To this
end, we report here the synthesis and structural characterization
of the unprecedented monomeric lithium ester enolaluminates
as well as their use as single-site, anionic active species in the
polymerization of methacrylates. More specifically, the reactions
of lithium ester enolate aggregates with suitable deaggregating
and stabilizing organoaluminum compounds produce monomeric
lithium ester enolaluminate active species, the structures of
which have been characterized by spectroscopic, analytical, and
single-crystal X-ray diffraction studies. These active species
promote the rapid anionic polymerization of methacrylates with
a high degree of control, and the results of the polymerization
kinetics indicate a bimolecular, activated-monomer anionic
polymerization mechanism through the monomeric ester eno-
laluminate propagating center.
Experimental Section
Materials and Methods. All syntheses and manipulations of air-
and moisture-sensitive materials were carried out in flamed Schlenk-
type glassware on a dual-manifold Schlenk line, a high vacuum line
(typically 10-5 to 10-7 Torr), or in an argon-filled glovebox (typically
<1.0 ppm oxygen and moisture). NMR-scale reactions (typically in a
0.02 mmol scale) were conducted in Teflon-valve-sealed J. Young-
type NMR tubes. HPLC grade organic solvents were first saturated
with nitrogen during filling the solvent reservoir and then dried by
passage through activated alumina (for diethyl ether, THF, and
methylene chloride) followed by passage through Q-5-supported copper
catalyst (for toluene and hexanes) stainless steel columns. Benzene-d6,
toluene-d8, and THF-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
1
recorded on either a Varian Inova 300 (FT 300 MHz, H; 75 MHz,
13C; 282 MHz, 19F; 44 MHz, 6Li) 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
tetramethylsilane, whereas 19F NMR and 6Li NMR spectra were
referenced to external CFCl3 and LiCl in THF. Elemental analyses were
performed by Desert Analytics, Tucson, Arizona.
Isobutyric acid, thionyl chloride, n-BuLi (1.6 M in hexanes),
MeMgBr (3.0 M in diethyl ether), diisopropylamine, triethylamine, and
2,6-di-tert-butyl-4-methylphenol (butylated hydroxytoluene, BHT-H)
were purchased from Aldrich Chemical Co. and used as received, except
for the amines, which were degassed, dried over CaH2 overnight, and
then vacuum-distilled before use, and for BHT-H, which was recrystal-
lized from hexanes prior to use. Trimethylaluminum, triisobutylalu-
minum, and tri(n-octyl)aluminum were purchased from Strem Chemical
Co.
Methyl methacrylate (MMA) and n-butyl methacrylate (BMA) were
purchased from Aldrich Chemical Co.; both monomers were first
degassed and dried over CaH2 overnight, followed by vacuum distil-
lation. Final purification involved titration with neat tri(n-octyl)-
aluminum to a yellow end point21 followed by distillation under reduced
pressure. The purified monomers were stored in a -30 °C freezer inside
the glovebox.
Tris(pentafluorophenyl)borane, B(C6F5)3, was obtained as a research
gift from Boulder Scientific Co. and further purified by recrystallization
from hexanes at -30 °C. Tris(pentafluorophenyl)alane, Al(C6F5)3, as
a 0.5‚toluene adduct based on the elemental analysis for the vacuum-
dried sample, was prepared from the exchange reaction of B(C6F5)3
and AlMe3 in a 1:3 toluene/hexanes solvent mixture in quantitative
yield according to a literature procedure,22 which is the modified
synthesis of the alane first disclosed by Biagini et al.23 Extra caution
should be exercised when handling this material because of its thermal
and shock sensitiVity. Neutral organoaluminum compounds used for
the generation of the lithium ester enolaluminate complexes and as
catalysts for activating the monomer in polymerization, including
MeAl(BHT)2,24 iBuAl(BHT)2,24 ClAl(BHT)2,25 iBu2Al(BHT),26 and
Al(BHT)3,27 were prepared according to literature procedures.
(17) (a) Pe´ron, G. L. N.; Peace, R. J.; Holmes, A. B. J. Mater. Chem. 2001, 11,
2915-2918. (b) Peace, R. J.; Horton, M. J.; Pe´ron, G. L. N.; Holmes, A.
B. Macromolecules 2001, 34, 8409-8411.
(18) (a) Vlcˇek, P.; Otoupalova´, J.; Janata, M.; La´talova´, P.; Kurkova´, D.; Toman,
L.; Masa´r, B. Macromolecules 2004, 37, 344-351. (b) Lochmann, L.;
Rodova´, M.; Trekoval, J. J. Polym. Sci., Part A: Polym. Chem. 1974, 12,
2091-2094.
(19) Schlaad, H.; Schmitt, B.; Mu¨ller, A. H. E.; Ju¨ngling, S.; Weiss, H.
Macromolecules 1998, 31, 573-577.
(21) Allen, R. D.; Long, T. E.; McGrath, J. E. Polym. Bull. 1986, 15, 127-
134.
(20) (a) Schmitt, B.; Mu¨ller, A. H. E. Macromolecules 2001, 34, 2115-2120.
(b) Schmitt, B.; Stauf, W.; Mu¨ller, A. H. E. Macromolecules 2001, 34,
1551-1557. (c) Schmitt, B.; Schlaad, H.; Mu¨ller, A. H. E.; Mathiasch, B.;
Steiger, S.; Weiss, H. Macromolecules 2000, 33, 2887-2893. (d) Schlaad,
H.; Mu¨ller, A. H. E. Macromolecules 1998, 31, 7127-7132. (e) Schlaad,
H.; Schmitt, B.; Mu¨ller, A. H. E. Angew. Chem., Int. Ed. 1998, 37, 1389-
1391.
(22) Feng, S.; Roof, G. R.; Chen, E. Y.-X. Organometallics 2002, 21, 832-
839.
(23) (a) Biagini, P.; Lugli, G.; Abis, L.; Andreussi, P. U.S. Pat. 5,602,269, 1997.
(b) Lee, C. H.; Lee, S. J.; Park, J. W.; Kim, K. H.; Lee, B. Y.; Oh, J. S. J.
Mol. Catal., A: Chem. 1998, 132, 231-239.
(24) Shreve, A. P.; Mulhaupt, R.; Fultz, W.; Calabrese, J.; Robbins, W.; Ittel,
S. Organometallics 1988, 7, 409-416.
9
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A R T I C L E S
Rodriguez-Delgado and Chen
Preparation of MMA‚Al(C6F5)3 (1). The activated monomer adduct
114a was prepared by adding MMA to a toluene solution of the alane,
followed by removal of the volatiles in vacuo. Single crystals of 1
suitable for X-ray diffraction were grown from hexanes at -30 °C in
a freezer inside the glovebox.
1H NMR (C6D6, 23 °C): δ 5.80 and 4.92 (m, 1H each, CH2d),
3.05 (s, 3H, sOCH3), 1.22 (s, 3H, sCH3). 19F NMR (C6D6, 23 °C):
δ -123.38 (dd, 6F, o-F), -151.60 (t, 3F, p-F), -160.83 (m, 6F, m-F).
13C NMR (C6D6, 23 °C): δ 176.70 (CdO), 136.35 (CH2d), 133.01
[C(Me)d], 57.18 (OCH3), 16.64 (CH3). (Carbons for the C6F5 groups
omitted because of CsF coupling.)
Preparation of Me2CdC(OiPr)OLi (2). The precursor isopropyl
isobutyrate was prepared from the reaction of isobutyryl chloride and
dry 2-propanol in diethyl ether at room temperature, in the presence of
triethylamine, followed by a standard extractive workup and fractional
distillation. Literature procedures for the formation of tert-butyl
R-lithioisobutyrates28 were slightly modified to prepare isopropyl
R-lithioisobutyrate 2. The isolated lithium ester enolate was stored in
a freezer at -30 °C inside the glovebox; single crystals of 2 suitable
for X-ray diffraction were grown from hexanes at -30 °C inside the
glovebox.
1H NMR (C6D6, 23 °C) for 4: δ 4.37 (sept, 1H, sOCHMe2), 2.09
(sept, 3H, sCH2CHMe2), 1.61 (s, 3H, dCMe2), 1.44 (s, 3H, dCMe2),
1.21 (d, J ) 6.3 Hz, 18H, sCH2CHMe2), 1.07 (b, 6H, sOCHMe2),
0.07 (d, J ) 6.9 Hz, 6H, sCH2CHMe2). 13C NMR (C6D6, 23 °C): δ
147.95 [OC(OiPr)d], 92.02 (dCMe2), 74.05 (OCHMe2), 28.93 (CH2-
CHMe2), 26.05 (CH2CHMe2), 25.04 (CH2CHMe2), 22.15 (OCHMe2),
18.51(dCMe2), 17.46 (dCMe2). Anal. Calcd for C19H40AlLiO2: C,
68.22; H, 12.07. Found: C, 67.36; H, 11.51.
Synthesis of Li+(THF)2[Me2CdC(OiPr)OAl(C6F5)3]- (5). In an
argon-filled glovebox, a 20-mL glass reactor was equipped with a
magnetic stir bar and charged with 0.1 g (0.17 mmol) of Al(C6F5)3‚0.5
toluene in 5 mL of toluene. To this reactor, with vigorous stirring at
ambient temperature, was added a solution of 2 (0.024 g, 0.17 mmol)
in 5 mL of toluene and 0.2 mL of THF. After being stirred at ambient
temperature for 15 min, the resultant light-yellow clear solution was
evacuated to dryness. The yellow oily residue was extracted with 3 ×
3 mL of hexanes; the extract was filtered, and the light yellow filtrate
was concentrated to one-half of its volume. The concentrated filtrate
was left inside a freezer at -30 °C for several hours, yielding 0.084 g
(61%) of the spectroscopically pure product as a colorless microcrys-
talline solid after filtration and drying in vacuo. This complex is
thermally unstable, and several attempts to obtain single crystals suitable
for X-ray diffraction analysis were unsuccessful.
1H NMR (C6D6, 23 °C) for 2: δ 4.28 (sept, 1H, sOCHMe2), 1.86
(s, 3H, dCMe2), 1.76 (s, 3H, dCMe2), 1.24 (d, 6H, sCHMe2). 13C
NMR (C6D6, 23 °C): δ 156.15 (OC(OiPr)d), 78.01 (dCMe2), 72.21
(OCHMe2), 22.27 (OCHMe2), 18.07 (dCMe2), 17.71 (dCMe2).
Synthesis of Li+[Me2CdC(OiPr)OAlMe(BHT)2]- (3). In an argon-
filled glovebox, a 30-mL glass reactor was equipped with a magnetic
stir bar and charged with 0.19 g (0.46 mmol) of MeAl(BHT)2 in 10
mL of toluene. To this solution, with vigorous stirring at ambient
temperature, was added a solution of 2 (0.08 g, 0.58 mmol) in 10 mL
of toluene in one portion. The resulting clear solution turned to yellow
immediately, and the clear yellow solution gradually became a white
suspension after being stirred for 30 min. Stirring was continued for
an additional 30 min at ambient temperature, after which the solvent
was removed in vacuo. The solid residue was extracted with 3 × 5
mL hexanes; the extract was filtered through a pad of Celite, and the
light yellow filtrate was concentrated to one-half of its volume. The
concentrated filtrate was left inside a freezer at -30 °C, yielding 0.20
g (80%) of the pure product as a colorless microcrystalline solid after
filtration and drying in vacuo. Single crystals suitable for X-ray
diffraction analysis were obtained by slow recrystallization from toluene
layered with hexanes at -30 °C inside a freezer of the glovebox.
1H NMR (C6D6, 23 °C) for 3: δ 7.17 (s, 4H, Ar), 3.95 (sept, 1H,
sOCHMe2), 2.25 (s, 6H, ArsCH3), 1.87 (s, 3H, dCMe2), 1.54 (s,
3H, dCMe2), 1.49 (s, 36H, ArsCMe3), 0.78 (d, 6H, sCHMe2), 0.11
(s, 3H, AlsMe). 13C NMR (C6D6, 23 °C): δ 154.14 (OsCipso, BHT),
150.55 [OC(OiPr)d], 139.56 (p-CH, BHT), 126.50 (m-CH, BHT),
126.28 (o-CH, BHT), 87.59 (dCMe2), 74.99 (OCHMe2), 35.39 (CH3,
BHT), 31.99 (CMe3, BHT), 22.15 (OCHMe2), 21.10 (CMe3, BHT),
1H NMR (C6D6, 23 °C) for 5: δ 4.12 (sept, 1H, sOCHMe2), 3.30
(m, 8H, R-CH2, THF), 1.61 (s, 3H, dCMe2), 1.43 (s, 3H, dCMe2),
1.22 (m, 8H, â-CH2, THF), 0.89 (d, J ) 6.0 Hz, 6H, sCHMe2). 13C
NMR (C6D6, 23 °C): δ 151.80 [OC(OiPr)d], 148.85, 142.96, 138.66,
135.39 (C6F5), 92.01 (dCMe2), 71.93 (OCHMe2), 68.45 (R-CH2, THF),
25.45 (â-CH2, THF), 21.47 (OCHMe2), 20.82 (dCMe2), 17.83
3
(dCMe2). 19F NMR (C6D6, 23 °C): δ -124.89 (d, JF-F ) 19.5 Hz,
6F, o-F), -154.20 (t, 3JF-F ) 20.9 Hz, 3F, p-F), -161.76 (m, 6F, m-F).
X-ray Crystallographic Analyses of 1, 2, and 3. Single crystals
suitable for X-ray diffraction studies 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 liquors in the glovebox. The crystals
were then mounted on thin glass fibers and transferred into the cold
nitrogen steam of a Siemens SMART CCD diffractometer. The
structures were solved by direct methods and refined using the Siemens
SHELXTL program library.29 The structures were refined by full-
matrix-weighted 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. In 2, there is a hexane molecule in the lattice, and
half of the molecule is unique and present in the asymmetric unit due
to a crystallographically imposed inversion center. Selected crystal data
and structural refinement parameters are collected in Table 1.
Polymerization Procedures and Polymer Characterizations.
Polymerizations were performed either in 30-mL, oven- and flame-
dried vacuum flasks inside the glovebox for ambient-temperature
reactions or in 25-mL oven- and flame-dried Schlenk flasks interfaced
to the dual-manifold Schlenk line for lower temperature reactions. In
a typical procedure, a 2.5-mL stock solution of MeAl(BHT)2 (93.4
µmol) was mixed in a flask with a solution of the lithium ester enolate
2 in 2.5 mL of toluene (46.7 µmol) and stirred for 10 min to cleanly
generate the initiator/catalyst pair, 3/MeAl(BHT)2. MMA (1.00 mL,
9.35 mmol) or BMA (1.48 mL, 9.35 mmol) was quickly added via
pipette (for polymerizations in the glovebox) or gastight syringe (for
polymerizations on the Schlenk line), and the flask was sealed and
kept under vigorous stirring at the desired temperature. For block
copolymerizations, a second quantity of MMA or BMA was added
after the completion of the first block (1 h), and the polymerization
was continued. After the measured time interval, the polymerization
was quenched by the addition of 5 mL of 5% HCl-acidified methanol.
6
18.63 (dCMe2), 17.45 (dCMe2), -3.97 (AlMe). Li NMR (C6D6, 23
°C): δ -1.00. Anal. Calcd for C38H62AlLiO4: C, 73.99; H, 10.13.
Found: C, 73.28; H, 9.75.
Synthesis of Li+[Me2CdC(OiPr)OAl(iBu)3]- (4). The reaction of
2 and Al(iBu)3 was carried out in the same manner as for the synthesis
of 3 shown above, affording 4 (82%) as a colorless oil. The isolated,
spectroscopically pure 4 remained as an oily product after repeated
crystallization attempts in hexanes at low temperatures.
(25) Healy, M. D.; Ziller, J. W.; Barron, A. R. Organometallics 1992, 11, 3041-
3049.
(26) (a) Skowronska-Ptasinska, M.; Starowieyski, K. B.; Pasynkiewicz, S.;
Carewska, M. J. Organomet. Chem. 1978, 160, 403-409. (b) Starowieyski,
K. B.; Pasynkiewicz, S.; Skowronska-Ptasinska, M. J. Organomet. Chem.
1975, 90, C43-C44.
(27) Healy, M. D.; Barron, A. R. Angew. Chem., Int. Ed. Engl. 1992, 31, 921-
922.
(28) Kim, Y.-J.; Bernstein, M. P.; Galiano Roth, A. S.; Romesberg, F. E.;
Williard, P. G.; Fuller, D. J.; Harrison, A. T.; Collum, D. B. J. Org. Chem.
1991, 56, 4435-4439.
(29) SHELXTL, version 6.12; Bruker Analytical X-ray Solutions: Madison, WI,
2001.
9
964 J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005

Single-Site Anionic Polymerization
A R T I C L E S
Table 1. Crystal Data and Structure Refinements for 1, [2]3‚0.5 Hexane, and 3
1
[2]3
‚0.5 hexane
3
empirical formula
formula weight
temperature/K
wavelength/Å
crystal system
space group
a/Å
C23H8AlF15O2
628.27
173(2)
0.710 73
orthorhombic
Pbca
18.284(14)
12.623(10)
20.466(16)
90
90
90
4723(6)
8
1.767
C24H46Li3O6
451.43
169(2)
0.710 73
triclinic
P1h
11.939(3)
12.033(3)
12.538(3)
72.016(4)
61.720(4)
67.790(4)
1449.9(6)
2
C38H62AlLiO4
616.80
173(2)
0.710 73
monoclinic
P21/n
9.119(7)
24.277(17)
17.121(12)
90
93.528(16)
90
3783(5)
4
1.083
0.088
b/Å
c/Å
R/deg
â/deg
γ/deg
volume/Å3
Z
density (calcd)/mg/m3
abs coeff/mm-1
F(000)
1.034
0.070
494
0.226
2480
1352
crystal size/mm3
θ range for data
collection/deg
index ranges
0.10 × 0.20 × 0.30
0.42 × 0.38 × 0.35
0.38 × 0.38 × 0.10
1.99 to 23.33
1.85 to 20.81
2.06 to 20.81
-20 e h e 17,
-14 e k e 13,
-22 e l e 22
20837
3416 [Rint ) 0.0809]
3416/0/370
R1 ) 0.0344,
wR2 ) 0.0812
R1 ) 0.0636,
wR2 ) 0.0908
-11 e h e 11,
-12 e k e 12,
-12 e l e 12
7300
3027 [Rint ) 0.0532]
3027/0/284
R1 ) 0.0695,
wR2 ) 0.1877
R1 ) 0.0864,
wR2 ) 0.2018
0.073(9)
-9 e h e 9,
-24 e k e 24,
-17 e l e 17
18883
3955 [Rint ) 0.1117]
3955/0/398
R1 ) 0.0631,
wR2 ) 0.1522
R1 ) 0.0979,
wR2 ) 0.1695
0.0055(10)
reflections collected
independent reflections
data/restraints/parameters
final R indices
[I > 2σ(I)]
R indices (all data)
extinction coefficient
largest diff. peak
and hole/eÅ-3
0.289 and -0.224
0.442 and -0.378
0.272 and -0.268
Scheme 2
The quenched mixture was precipitated into 100 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.
Polymer molecular weights and molecular weight distributions were
measured by gel permeation chromatography (GPC) analyses carried
out at 40 °C, at a flow rate of 1.0 mL/min and with THF as the eluent,
on a Waters University 1500 GPC instrument. The instrument was
calibrated with 10 PMMA standards, and chromatograms were pro-
cessed with Waters Empower software. 1H NMR spectra for the analysis
of PMMA and PBMA microstructures were recorded in CDCl3 and
analyzed according to the literature.30
Polymerization Kinetics. As control experiments, the polymeriza-
tions were first monitored by NMR-scale reactions. Stock solutions of
MeAl(BHT)2 (20 mM) and 2 (10 mM) in toluene-d8 were prepared in
the glovebox. In a typical procedure, a small vial was charged with
0.4 mL of MeAl(BHT)2, 0.4 mL of 2, and 86 µL of MMA [MMA/
MeAl(BHT)2/2 ) 200:2:1], and the time was recorded immediately
after MMA was added to the solution mixture. The reaction mixture
was subsequently loaded into a J.-Young NMR tube and transferred to
an NMR spectrometer that was previously parametrized using MMA
in toluene-d8, for data collection.
in the region 3.2-3.6 ppm) and A5.2+6.1 is the total integrals for both
peaks at 5.2 and 6.1 ppm. Apparent rate constants (kapp) were extracted
by linearly fitting a line to the plot of ln([M]0/[M]t) vs time.
Kinetic experiments were also carried out in stirred Schlenk flasks
using the same procedure as already described for the polymerization,
except that, at appropriate time intervals, 0.2 mL aliquots were removed
from the reaction mixture using syringe and 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
determining the [M]0/[M]t ratio using the same procedure as described
above.
Data were acquired at 22 °C using one scan per time interval. The
ratio of [MMA]0 to [MMA]t at a given time t, [M]0/[M]t, was determined
by integration of the peaks for MMA (5.2 and 6.1 ppm for the vinyl
signals; 3.4 ppm for the OMe signal) and PMMA (centered at 3.4 ppm
for the OMe signals) according to [M]0/[M]t ) 2A3.4/3A5.2+6.1, where
A3.4 is the total integrals for the peaks centered at 3.4 ppm (typically
Results and Discussion
Synthesis of Monomeric Lithium Ester Enolaluminate
Complexes. The reaction of isopropyl R-lithioisobutyrate
[Me2CdC(OiPr)OLi, 2] with MeAl(BHT)2 in hexanes or toluene
cleanly produces the first example of an isolable lithium ester
enolaluminate complex, Li+[Me2CdC(OiPr)OAlMe(BHT)2]-
(3), Scheme 2. Significantly, complex 3 is stable at ambient
temperature under an inert atmosphere and has been isolated
as a crystalline solid; the single-crystal X-ray diffraction study
reveals 3 to be remarkably a monomeric structure (vide infra).
(30) (a) Rodriguez-Delgado, A.; Mariott, W. R.; Chen, E. Y.-X. Macromolecules
2004, 37, 3092-3100. (b) Bovey, F. A.; Mirau, P. A. NMR of Polymers;
Academic Press: San Diego, 1996. (c) Ferguson, R. C.; Ovenall, D. W.
Macromolecules 1987, 20, 1245-1248. (d) Ferguson, R. C.; Ovenall, D.
W. Polym. Prepr. (Am. Chem. Soc. DiV. Polym. Chem.) 1985, 26, 182-
183. (e) Subramanian, R.; Allen, R. D.; McGrath, J. E.; Ward, T. C. Polym.
Prepr. (Am. Chem. Soc., DiV. Polym. Chem.) 1985, 26, 238-240.
9
J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005 965

A R T I C L E S
Rodriguez-Delgado and Chen
The isolation, characterization, and structure of the ester
enolaluminate 3 are of great importance because it simulates
the structure of the monomeric active propagating center for
the anionic polymerization of (meth)acrylates by the combina-
tion of organolithium initiators with organoaluminum com-
pounds.
desired lithium ester enolaluminate complex, Li+[Me2CdC(OiPr)-
OAl(iBu)3]- (4), can be readily identified and characterized
based on the spectroscopic characteristics for the enolaluminate
moiety (see Experimental Section). However, this product exists
as a colorless oil and did not crystallize under various conditions,
hampering our structural characterization efforts.
The 1H NMR spectrum of 3 in C6D6 at 23 °C features sharp,
well-resolved peaks for all types of protons present in 3. Most
noticeable are the chemical shifts for the methine proton in
-OCHMe2 (3.95 ppm) and the methyl protons in -OCHMe2
(0.78 ppm), which are substantially upfield shifted by 0.33 and
0.46 ppm, respectively, as compared with those of the corre-
sponding signals for the -OCHMe2 (4.28 ppm) and -OCHMe2
(1.24 ppm) protons in the ester enolate complex 2. These upfield
shifts can be explained with the shielding effect imposed by
the BHT ligand on the -OCHMe2 group within the structure.
In contrast, the Al-Me peak (0.11 ppm) in 3 is downfield shifted
by 0.38 ppm from the Al-Me peak in the starting aluminum
compound MeAl(BHT)2. Furthermore, the 13C NMR spectra
show that the signal of C(O) [i.e., dC(O)OiPr] shifts upfield
from 156.2 ppm in 2 to 150.6 ppm in 3 (by 5.6 ppm) and that
the signal of C(R) (i.e., dCMe2) shifts downfield from 78.0
ppm in 2 to 87.6 ppm in 3 (by 9.6 ppm), reflecting the increased
double bond character of the C(R) and C(O) bond and also the
enhanced covalent character of the metal-oxygen bond in the
ester enolaluminate 3;15 this also implies that the complexation
of the lithium ester enolate with organoaluminum compounds
such as MeAl(BHT)2 reduces the nucleophilicity at R-C. Overall,
Numerous attempts to isolate the targeted ester enolaluminate
complex derived from the reaction of 2 with Al(C6F5)3 in
nonpolar hydrocarbons, such as toluene, at various temperatures
failed to obtain the clean product as it underwent various types
of decompositions. However, upon addition of a small amount
of donor solvents such as THF, the same reaction in toluene
produces the lithium ester enolaluminate complex,
Li+(THF)2[Me2CdC(OiPr)OAl(C6F5)3]- (5), which is isolable
but thermally unstable. Several attempts to obtain its crystal
structure were unsuccessful because of the low quality and
thermal instability of the crystals obtained; the proposed
structure of 5 shown above is based on the spectroscopic data
(see Experimental section) and the structure of 3. Nevertheless,
the formation of the aluminate entity within 5 is evident when
comparing its 19F NMR chemical shifts to those of MMA‚
Al(C6F5)3 (1) and our other structurally characterized alumi-
1
the sharp, well-resolved H and 13C NMR resonances and a
6
6
single, sharp Li NMR signal at -1.00 ppm (the Li NMR of
EiBLi exhibits two broad peaks at 0.31 and 0.43 ppm20c) for
complex 3 suggest that its monomeric structure is retained in
solution. However, the only observed one type of the BHT
ligand in the solution NMR spectra at ambient temperature
indicates a fluxional process in which the free and coordinated
(to the lithium cation) BHT groups are exchanged rapidly on
the NMR time scale.
nate complexes having a common structural fragment of
- 31
sOAl(C6F5)3
The 13C NMR spectroscopic data for the
.
enolate moiety are similar to those observed in 3, with
characteristic chemical shifts for the C(O) and C(R) carbons in
5 being 151.8 and 92.0 ppm, respectively.
Overall, these synthetic studies indicate that the BHT-type
ligand within the organoaluminum compound provides the
combination of the necessary steric bulkiness and electron-
donating ability to deaggregate the lithium ester enolate to its
monomer and stabilize it by forming the stable lithium ester
enolaluminate. Such aluminum compounds employed, however,
can be too bulky to generate the ester enolaluminate species
[e.g., iBuAl(BHT)2 and Al(BHT)3]; the change of the alkyl group
Thus, the employed MeAl(BHT)2 deaggregates the lithium
ester enolate 2 to its monomeric structure and stabilizes it by
forming the discrete lithium ester enolaluminate complex 3. The
addition of 2 equiv of MeAl(BHT)2 to 2 also cleanly generates
3 with 1 equiv of MeAl(BHT)2 left unreacted, indicating the
excess of MeAl(BHT)2 neither reacts further with nor decom-
poses 3. This fact considerably simplifies the activated-monomer
polymerization procedures and kinetic experiments, in terms of
the ability to readily generate the lithium ester enolaluminate
active species (initiator or propagator) and the organoaluminum
catalyst by in situ mixing of the lithium enolate 2 with suitable
organoaluminum compounds in a predetermined ratio. The
formation of the “ate” complex (i.e., the enolaluminate structure)
between iBu2Al(BHT) and methyl R-lithioisobutylate has been
previously observed spectroscopically;17b however, we did not
observe the formation of the anticipated enolaluminate species
i
i
from Me in MeAl(BHT)2 to Bu in BuAl(BHT)2 results in a
completely different reaction pattern toward the lithium ester
enolate and polymerization characteristics (vide infra). For those
sterically bulky, non-BHT-carrying organoaluminum compounds
such as Al(iBu)3 and Al(C6F5)3, the corresponding enolaluminate
complexes can be generated and isolated; however, their
structural characterizations were hampered by the noncrystal-
lizability [Al(iBu)3] or the need to use the external stabilizing
reagent [Al(C6F5)3].
i
from the reactions of 2 with BuAl(BHT)2 and Al(BHT)3,
Monomer Activation by Organoaluminum Lewis Acids.
Activation of monomer with sterically crowded, strongly Lewis
acidic organoaluminum alkyls incorporating bulky phenoxide
ligands greatly accelerates the polymerization by aluminum
presumably due to steric reasons.
To examine whether the presence of the sterically crowded,
electron-donating BHT ligand at the aluminum moiety is
required or not in terms of stability and isolability of the
resulting ester enolaluminate complexes, the bulky aluminum
alkyl, Al(iBu)3, was used to react with 2. The formation of the
(31) (a) Bolig, A. D.; Chen, E. Y.-X. J. Am. Chem. Soc. 2004, 126, 4897-
4906. (b) Chakraborty, D.; Chen, E. Y.-X. Organometallics 2003, 22, 207-
210.
9
966 J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005

Single-Site Anionic Polymerization
A R T I C L E S
Table 2. Selected 13C NMR Spectroscopic Data (C6D6, ppm) for
MMA‚[Al] Adducts
of activity for the MMA polymerization using the combination
of 2 and Al(BHT)3 (vide infra). On the other hand, the MMA‚
Al(C6F5)3 adduct experiences the largest ∆δ values for all three
types of carbons [i.e., C(O), CH2d, and CH3O], arguing for
the highest degree of monomer activator by Al(C6F5)3 within
this series; this conclusion again correlates well with its observed
highest polymerization activity under activated monomer po-
lymerization conditions (vide infra).
δ
C(O)
δ
(
CH2d
∆δ a
δ
CH3O
a
a
adducts
(∆δ
)
)
(∆δ )
MMA
167.2
124.9
51.2
MMA‚Al(BHT)3
168.1 (+0.9)
170.9 (+3.7)
171.1 (+3.9)
172.7 (+5.5)
174.2 (+7.0)
176.7 (+9.5)
125.0 (+0.1)
135.0 (+10.1)
136.0 (+11.1)
134.5 (+9.6)
136.2 (+11.3)
136.4 (+11.5)
51.1 (-0.1)
54.0 (+2.8)
54.1 (+2.9)
55.6 (+4.4)
57.1 (+5.9)
57.2 (+6.0)
MMA‚AliBu3
MMA‚AliBu(BHT)2
MMA‚AliBu2(BHT)
MMA‚AlMe(BHT)2
MMA‚Al(C6F5)3
b
For those monomer-activating adducts, the relative magnitude
of the ∆δ values is, however, different for C(O), CH2d, and
CH3O carbons. Consistently for each adduct, the CH2d carbon
has the largest ∆δ value, followed by C(O), whereas the CH3O
carbon is downfield shifted to the least extent. The order of the
magnitude of the ∆δ values suggests that the relative contribu-
tion of each resonance form to the overall structure of all adducts
follows this order: D > E > F; this order is an ideal scenario
in terms of the activated monomer polymerization, because the
resonance structure D represents the most active form of the
activated monomer toward anionic ester enolaluminate active
species in the rate-determining propagating step (vide infra).
Molecular Structures of Activated Monomer 1, Lithium
Ester Enolate Initiator 2, and Lithium Ester Enolaluminate
Propagator 3. Special techniques at low temperatures are
required for generation and isolation of suitable single crystals
of lithium alkyl ester enolates11 because of their thermal
instability. Owing to this reason, only a handful of lithium ester
enolate structures have been reported, but all of which are
aggregates (dimer, tetramer, hexamer) either solvated with THF
or TMEDA12 or stabilized by intramolecular Li-N coordination
in heteroatom-substituted, lithium R- or â-amino ester enolates.34
We were not aware of any reported structures of the nondonor-
solvated, non-N-substituted lithium alkyl ester enolates, although
the unsolvated lithium ketone enolates are known.35 Because
the current polymerizations were carried out in toluene, it is of
great interest to obtain the crystal structure of the unsolvated
lithium ester enolate. Additionally, lithium ester enolaluminate
propagating species are structurally unknown; none of such
complexes have been previously isolated and structurally
characterized. Furthermore, the structures of both the activated
monomer and the lithium ester enolaluminate propagator are
of importance to the understanding of the fundamental propagat-
ing step. Our intense efforts toward these significant fronts have
finally yielded X-ray crystal structures of all three key com-
ponents in the activated-monomer polymerization: activated
monomer 1, initiator 2, and propagator 3, shown in Figures 1,
2, and 3, respectively; important bond distances and angles for
these three complexes are tabulated in Tables 3-5.
a ∆δ relative to MMA. b Data taken from ref 33.
porphyrin initiators in Inoue’s high-speed living polymerization
of methacrylic esters.32 The activated monomer polymerization
mechanism is also believed to be operative in Hatada’s
stereospecific living methacrylate polymerization by the tBuLi/
bulky aluminum phenoxide combination.6a Likewise, the acti-
vated monomer polymerization mechanism is anticipated for
the current polymerization system by the lithium ester enola-
luminate (e.g., 3) in combination with the conjugate bulky
aluminum Lewis acid [e.g., MeAl(BHT)2]. To provide the
necessary background knowledge for the future discussion on
polymerization and mechanism sections, interactions of the
monomer, MMA, with six bulky organoaluminum Lewis acids
of current interest, including MeAl(BHT)2, iBuAl(BHT)2, iBu2-
Al(BHT), AliBu3, Al(C6F5)3, and Al(BHT)3, were investigated
by way of forming MMA‚[Al] adduct complexes from the 1:1
molar ratio reactions and then comparing their 13C NMR spectral
features; additionally, the molecular structure of the adduct
MMA‚Al(C6F5)3 (1) was also determined by X-ray diffraction.
There are four possible resonance forms (C-F) associated
with the structure of the MMA‚[Al] adducts; the relative
contribution of each resonance form to the overall structure can
be estimated by comparing the relative magnitude of the
chemical shift difference (∆δ) for each functional group, using
MMA as a reference.33 Based on this argument, the selected
diagnostic 13C NMR data for CH2d, C(O), CH3O carbon
chemical shifts related to resonance forms D, E, and F,
respectively, are compiled in Table 2.
It can be clearly seen from the table that the 13C NMR
resonances of C(O), CH2d, and CH3O carbons of MMA in the
adduct forms are shifted significantly downfield, as compared
with those in the free MMA, for all the bulky aluminum Lewis
acids investigated, except for the most bulky aluminum phe-
noxide Al(BHT)3 in the series, which shows minimal to virtually
no changes in chemical shifts. This observation implies no
activation toward MMA by Al(BHT)3; this inactivity toward
the monomer activation correlates well with the observed lack
The MMA molecule in complex 1 adopts a s-cis conformation
as to the CdC and CdO double bonds about the C(19)sC(21)
bond and also a s-cis conformation as to the CdO and C(20)s
O(2) bonds about the C(19)sO(2) bond (Scheme 3). These
preferred conformations in the solid-state structure are identical
to those observed in the MMA‚AlMe2(BHT) adduct33 and also
consistent with the solution conformations for the molecular
(34) (a) McNeil, A. J.; Toombes, G. E. S.; Chandramouli, S. V.; Vanasse, B.
J.; Ayers, T. A.; O’Brien, M. K.; Lobkovsky, E.; Gruner, S. M.; Marohn,
J. A.; Collum, D. B. J. Am. Chem. Soc. 2004, 126, 5838-5939. (b)
Jastrzebski, J. T. B. H.; van Koten, G.; van de Mieroop, W. F. Inorg. Chim.
Acta 1988, 142, 169-171.
(35) (a) Williard, P. G.; Carpenter, G. B. J. Am. Chem. Soc. 1986, 108, 462-
468. (b) Williard, P. G.; Carpenter, G. B. J. Am. Chem. Soc. 1985, 107,
3345-3346.
(32) (a) Sugimoto, H.; Inoue, S. AdV. Polym. Sci. 1999, 146, 39-119. (b)
Sugimoto, H.; Kuroki, M.; Watanabe, T.; Kawamura, C.; Aida, T.; Inoue,
S. Macromolecules 1993, 26, 3403-3410. (c) Kuroki, M.; Watanabe, T.;
Aida, T.; Inoue, S. J. Am. Chem. Soc. 1991, 113, 5903-5904.
(33) Akakura, M.; Yamamoto, H.; Bott, S.; Barron, A. R. Polyhedron 1997,
16, 4389-4392.
9
J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005 967

A R T I C L E S
Rodriguez-Delgado and Chen
Figure 3. Molecular structure of the propagator Li+[Me2CdC(OiPr)-
OAlMe(BHT)2]- (3).
Figure 1. Molecular structure of the activated monomer MMA‚Al(C6F5)3
(1).
Table 3. Selected Bond Distances (Å) and Bond Angles (deg)
for 1
Al(1)-O(1)
Al(1)-C(7)
O(1)-C(19)
O(2)-C(20)
C(21)-C(22)
1.811(2)
1.990(3)
1.255(3)
1.446(4)
1.329(5)
Al(1)-C(1)
Al(1)-C(13)
O(2)-C(19)
C(19)-C(21)
C(21)-C(23)
1.987(3)
1.993(3)
1.299(4)
1.450(5)
1.483(5)
O(1)-Al(1)-C(1)
O(1)-Al(1)-C(13)
C(1)-Al(1)-C(13)
Al(1)-O(1)-C(19)
O(1)-C(19)-O(2)
O(2)-C(19)-C(21)
108.68(11) O(1)-Al(1)-C(7)
105.44(12) C(1)-Al(1)-C(7)
117.31(12) C(7)-Al(1)-C(13)
107.47(11)
106.44(12)
111.12(12)
118.1(3)
148.2(2)
118.9(3)
115.5(3)
C(19)-O(2)-C(20)
O(1)-C(19)-C(21)
125.6(3)
C(22)-C(21)-C(19) 117.3(3)
C(19)-C(21)-C(23) 117.5(4)
C(22)-C(21)-C(23) 125.2(4)
Table 4. Selected Bond Distances (Å) and Bond Angles (deg)
for 2
Li(1)-O(2)
Li(1)-O(4)
Li(2)-O(3)
Li(2)-O(2)#1
Li(3)-O(4)
Li(3)-O(3)#1
C(1)-O(1)
1.862(7)
1.983(7)
1.860(6)
1.979(6)
1.864(6)
1.980(6)
1.422(4)
1.325(5)
1.412(4)
1.320(4)
Li(1)-O(3)
Li(1)-O(1)
Li(2)-O(4)
Li(2)-O(5)#1
Li(3)-O(2)#1
Li(3)-O(6)#1
C(1)-O(4)
C(11)-O(2)
C(11)-C(12)
C(15)-O(6)
1.976(7)
2.039(6)
1.971(6)
2.057(6)
1.971(6)
2.059(6)
1.327(4)
1.327(4)
1.322(4)
1.421(4)
1.322(5)
Figure 2. Molecular structure of the initiator [Me2CdC(OiPr)OLi]6 {[2]6}.
complexes between MMA and Lewis acids derived by 1H NMR
studies.36 These conformations, however, are different from
those found in the crystallographically characterized, noncoor-
dinated diphenylmethyl methacrylate37 and η2-CdC coordinated
MMA to the Ni{P(C6H11)3}2 fragment,38 both of which adopt
s-trans, s-cis conformations about the CsC and CsO bonds,
respectively. As expected, the sum of the angles around both
C(19) and C(21) carbons is 360.0° for sp2-hybridized, trigonal-
planar carbon centers. When compared with the free diphenyl-
methyl methacrylate, the activated MMA molecule in complex
1 has nearly the same CdC double bond [1.329(5) Å] length,
but shorter internal CsC bond [1.450(5) Å] and CsO bond
[1.299(4) Å] lengths by 0.04 Å and a longer CdO bond [1.255-
(3) Å] length by 0.06 Å. A similar trend is observed when
C(1)-C(2)
C(11)-O(5)
C(15)-O(3)
C(15)-C(16)
O(1)-Li(1)-O(2)
O(1)-Li(1)-O(4)
O(2)-Li(1)-O(4)
Li(1)-O(4)-Li(3)
O(1)-C(1)-C(2)
C(1)-C(2)-C(3)
C(3)-C(2)-C(4)
142.8(4)
68.0(2)
122.5(3)
114.2(3)
121.4(3)
120.3(4)
O(1)-Li(1)-O(3)
O(2)-Li(1)-O(3)
O(3)-Li(1)-O(4)
O(1)-C(1)-O(4)
C(2)-C(1)-O(4)
C(1)-C(2)-C(4)
118.2(3)
97.7(3)
93.4(3)
109.7(3)
128.5(4)
123.7(4)
116.0(4)
comparing the activated MMA molecular in complex 1 with
the free MMA structures (s-trans or s-cis about the C-C bond)
derived from gas electron diffraction studies.39 The results of
these comparative analyses are consistent with the afore-depicted
contributions from the resonance forms D, E, and F to the
overall structure of the activated MMA molecular 1.
(36) (a) Furukawa, J.; Kobayashi, E.; Nagata, S. J. Polym. Sci., Polym. Chem.
Ed. 1976, 13, 237-255. (b) Allen, P. E. M.; Bateup, B. O. Eur. Polym. J.
1973, 9, 1283-1288.
(37) Kageyama, H.; Miki, K.; Tanaka, N.; Kasai, N.; Okamoto, Y.; Hatada, K.
Yuki, H. Makromol. Chem. 1982, 183, 2863-2870.
(38) Jarvis, A. P.; Haddleton, D. M.; Segal, J. A.; McCamley, A. J. Chem. Soc.,
Dalton Trans. 1995, 2033-2040.
(39) Tsuji, T.; Ito, H.; Takeuchi, H.; Konaka, S. J. Mol. Struct. 1999, 475, 55-
63.
9
968 J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005

Single-Site Anionic Polymerization
A R T I C L E S
Scheme 3
Table 5. Selected Bond Distances (Å) and Bond Angles (deg)
for 3
of 2 are very similar, Scheme 4 depicts only one of three
enolates with bond lengths, which is discussed herein. As can
be seen from Scheme 4, each of the anionic enolate oxygen
Li-O(1)
2.002(9)
2.256(8)
2.456(8)
2.536(9)
1.734(3)
1.820(3)
1.343(5)
1.325(6)
Li-O(4)
1.954(8)
2.417(9)
2.488(9)
2.547(9)
1.779(3)
1.955(5)
1.416(5)
1.357(5)
Li-C(24)
Li-C(29)
Li-C(28)
Al-O(2)
Li-C(25)
Li-C(26)
Li-C(27)
Al-O(3)
Scheme 4
Al-O(1)
Al-C(1)
C(17)-O(4)
C(24)-O(3)
C(17)-O(1)
C(17)-C(21)
O(1)-Li-O(4)
O(2)-Al-O(3)
O(3)-Al-O(1)
O(3)-Al-C(1)
O(1)-C(17)-O(4)
O(4)-C(17)-C(21)
68.5(3)
Al(-O(3)-C(24)
128.7(3)
112.63(15) O(2)-Al-O(1)
96.66(15)
115.31(18) O(1)-Al-C(1)
107.7(4)
122.6(4)
111.14(15)
105.96(18)
115.21(18)
129.7(4)
O(2)-Al-C(1)
O(1)-C(17)-C(21)
C(17)-C(21)-C(22) 122.1(4)
C(22)-C(21)-C(23) 116.2(4)
C(17)-C(21)-C(23) 121.7(4)
The Al-O distance [1.811(2) Å] in adduct 1 is noticeably
shorter than that in the MMA‚AlMe2(BHT) adduct [1.867(8)
Å], whereas the Al-O-C vector angle [148.2(2)°] is ∼12.5°
smaller than that in the MMA‚AlMe2(BHT) adduct, indicative
of a stronger Al-O σ bond in 1. The geometry around aluminum
is a distorted tetrahedron with a sum of the C-Al-C angles of
334.9°, and the average Al-C(aryl) distance (1.990 Å) compares
well to those in other Al(C6F5)3 complexes with metallocene
alkyl,40 imidazole,41 toluene,42 THF,43 water,31b and methanol.31b
The lithium ester enolate 2 crystallizes in hexanes as a
hexamer (Figure 2), but only half of it is unique and present in
the asymmetric unit due to a crystallographically imposed
inversion center; this situation resembles the structure of the
unsolvated lithium pinacolone enolate.35 The Li6O6 core can
be described as a hexagonal prism with the hexagonal face
defined by alternate Li and O atoms and the edges by Li-O
bonds; such a hexameric aggregated structure of prismatic type
for methyl R-lithioisobutyrate in nonpolar solvents was also
predicted by DFT calculations.13b The Li3O3 hexagonal faces
are slightly twisted out of planarity with the maximum deviation
being <0.08 Å. The hexagonal Li3O3 ring is distored, with the
average intraring angle at Li (123.5°) being somewhat larger
than that at O (116.2°).
atoms is bonded to three lithium atoms, but each Li is
tetrahedrally coordinated to four O atoms because of additional
coordination of Li to the alkoxy O atom; this latter coordination
mode differs from the previously determined lithium ester
enolate structures.12,34 The four Li-O bonds at each Li center
have bond lengths ranging from 1.862 to 2.039 Å, with one of
the Li-anionic enolate O bonds being the shortest (Li1-O2 or
Li3-O4) and the Li-alkoxy O being the longest (Li1-O1).
The chelating effect of the ester enolate ligand by way of Li
coordination to the two O centers of the same ester enolate anion
molecule results in an acute O1-Li1-O4 angle of only 68.0-
(2)°, whereas the O1-Li1-O2 angle is strikingly large
[142.8(4)°], both substantially deviating from a tetrahedral angle.
The sum of the angles around the C(R) and C(O) carbons in
2 is 360.0° and 359.6°, respectively, for sp2-hybridized, trigonal-
planar carbon centers. A bond length of C1sC2 ) 1.325(5) Å
is nearly identical to that of the CdC double bond observed in
the activated MMA molecular in complex 1, for a typical Cd
C double bond; this double-bond length is somewhat shorter
than that in the TMEDA-solvated dimer [Me2CdC(OtBu)OLi‚
TMEDA]2 [1.348(6) Å].12 The C1sO4 (anionic enolate O) bond
length [1.327(4) Å] is considerably longer than a typical
carbonyl CdO bond by ∼0.13 Å, somewhat shorter than that
of the unsolvated lithium pinacolone enolate [1.341(5) Å],35 and
longer than that of [Me2CdC(OtBu)OLi‚TMEDA]2 [1.304(5)
Å].12 All crystallographical data are consistent with the depicted
lithium ester enolate structure.
Because the metric parameters for the three independent ester
enolate anions present in the crystallographic asymmetric unit
(40) (a) Liu, Z.; Somsook, E.; Landis, C. R. J. Am. Chem. Soc. 2001, 123, 2915-
2916. (b) Chen, E. Y.-X.; Kruper, W. J.; Roof, G.; Wilson, D. R. J. Am.
Chem. Soc. 2001, 123, 745-746.
(41) LaPointe, R. E.; Roof, G. R.; Abboud, K. A.; Klosin, J. J. Am. Chem. Soc.
2000, 122, 9560-9561.
(42) Hair, G. S.; Cowley, A. H.; Jones, R. A.; McBurnett, B. G.; Voigt, A. J.
Am. Chem. Soc. 1999, 121, 4922-4923.
(43) Belgardt, T.; Storre, J.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G.
Inorg. Chem. 1995, 34, 3821-3822.
9
J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005 969

A R T I C L E S
Scheme 5
Rodriguez-Delgado and Chen
η6-coordination of one of the BHT ligands to the Li cation
results in two striking angles associated with the coordinated
BHT ligand: the smallest O1sAlsO3 angle [98.7(2)°] as
compared with the rest of the angles at the Al center in 3 and
the significantly smaller AlsO3sC(ipso) angle [128.7(3)°] for
the coordinated BHT ligand than that for the noncoordinated
BHT ligand [150.1(3)°] in 3. The aryl ring in the coordinated
BHT ligand is more planar than is the noncoordinated BHT
ligand at Al; the maximum deviations of the carbon atoms from
planes defined by C24 to C29 and C2 to C7 atoms are <0.083
and <0.12 Å, respectively. To the best of our knowledge, the
structure of 3 represents the first example of crystallographically
characterized monomeric lithium ester enolates or enolalumi-
nates.
Polymerization of Methacrylates by Lithium Ester Eno-
laluminate Active Species. Table 6 summarizes the results of
MMA and BMA polymerizations by the lithium ester enolate
2 and enolaluminate 3 with and without the catalyst, MeAl-
(BHT)2. As anticipated, the MMA polymerization in toluene
by the aggregating initiator 2 produces isotactic PMMA {[mm]
) 75.5%} with a broad MWD (PDI ) 6.8) and a low initiator
efficiency of I* ) 43% (run 1, Table 6). In sharp contrast, the
enolaluminate 3 produces syndiotactic PMMA {[rr] ) 72.9%}
with a much narrower MWD (PDI ) 1.20, run 2). The polymer
yield, however, is reduced by one-fourth as compared with that
by 2, reflecting the lower reactivity of lithium enolaluminates
vs lithium enolates as previously alluded to on the basis of the
spectroscopic data. In the presence of 1 equiv of the conjugate
Lewis acid catalyst MeAl(BHT)2, the polymerization by 3
becomes highly active, producing PMMA in quantitative yield
and with a low PDI value of 1.12 (run 3), providing evidence
for the activated-monomer polymerization. As discussed earlier,
the second equivalent of MeAl(BHT)2 does not further react
with 3 but forms the stable activated monomer-aluminum
complex. Lowering the polymerization temperature to 0 °C
increases the syndiotacticity to 79.3% and the initiator efficiency
to 81%, but the PDI value is kept the same (run 4 vs 3). Further
lowering of the polymerization temperature to -40 °C qua-
druples the time for achieving a quantitative polymer yield;
however, four other important polymer and polymerization
characteristics, including Mn, PDI, I*, and [rr], are virtually
unaffected (run 5 vs 4), indicating the controlled behavior of
this polymerization over a broad temperature range.
The lithium ester enolaluminate 3 crystallizes in the mono-
clinic space group P21/n from a mixture of toluene and hexanes
at -30 °C. The most remarkable structural feature of 3 is its
monomeric structure (Figure 3), which can be attributed to
stabilization via intramolecular coordination of the lithium cation
to both the ester enolate ligand, which serves as a chelating
ligand, and one of the bulky, electron-donating BHT ligands at
Al, which serves as an η6-ligand. As can be seen from Scheme
5, the anionic enolate oxygen atom is bonded to both the lithium
cation and the aluminum center, whereas the alkoxy O atom is
coordinated to only the Li atom. In addition to the coordination
to the chelating ester enolate ligand [note again the acute
O-Li-O angle of only 68.5(3)°], the Li cation is also
coordinated to one of the BHT ligands in an η6-fashion. The
Li-C bond length, however, spans a wide range, with the Li-
C(ipso) bond being the shortest [2.256(8) Å], the Li-C(para)
bond the longest [2.547(9) Å], and the Li-C(ortho) and Li-
C(meta) bonds between [from to 2.417(9) Å to 2.536(9) Å].
Using a working criterion for coordination to the metal cations
that the metal-atom distance not be greater than the sum of
the van der Waals radii of the metal and atom as listed by
Pauling, the mean Li-C length is 2.259 Å, with minimum and
maximum Li-C lengths being 2.041 and 2.557 Å, respec-
tively.44 Hence, the observed distances between Li and the BHT-
ring carbons are within the defined bonding range.
The BMA polymerization by the propagator/catalyst pair,
3/MeAl(BHT)2, is as effective as the MMA polymerization,
producing PBMA with a narrow MWD and moderately high
syndiotacticity of 86.6% (run 6). The PBMA-b-PMMA block
copolymer with a narrow MWD (PDI ) 1.12) has also been
synthesized with virtually a quantitative initiator efficiency (run
7); the molar composition of two monomer units in the block
The CdC double bond length in 3 is identical to that in 2,
and other metric parameters associated with the ester enolate
moiety are remarkably similar to those observed in 2 as well,
except for a somewhat lengthened CsO1 (enolate anion O)
bond and shortened CsO4 (alkoxy O) bond in 3. The geometry
around the four-coordinate aluminum center is that of a distorted
tetrahedron with a sum of the OsAlsO angles of 320.4°. The
Al-C bond in 3 [1.955(5) Å] is somewhat longer than that found
in the neutral MeAl(BHT)2 [1.927(3) Å],24 as predicted on
changing from a planar to tetrahedral geometry in terms of the
increased p character in the AlsC bond. The AlsO (BHT)
bonds in 3 [1.779(3), 1.734(3) Å] are also longer than those
found in the neutral MeAl(BHT)2 [1.687(2), 1.685(2) Å].24 On
the other hand, the change in geometry adapted for the
1
copolymer obtained from the H NMR analysis is the same as
the monomer molar feed ratio (i.e., 1:1), and the syndiotacticity
of the block copolymer is collectively ∼84%, an overall value
accounting for both blocks.30a Thus, several lines of evidence,
including the observed narrow MWD (PDI < 1.18) of PMMA
and PBMA formed, the production of the well-defined block
copolymer, and the observed linear increase of the polymer Mn
vs polymer yield, which is coupled with the small, nearly
constant PDI values (Figure 4), suggest living characteristics
of the 3/MeAl(BHT)2 system.
(44) (a) Williard, P. G. ComprehensiVe Organic Synthesis; Pergamon: New
York, 1991; Vol. 1, pp 1-47. (b) Setzer, W. H.; Schleyer, P. V. R. AdV.
Organomet. Chem. 1985, 24, 353-451.
9
970 J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005

Single-Site Anionic Polymerization
A R T I C L E S
Table 6. MMA and BMA Polymerization Results by Lithium Enolate 2 and Enolaluminate 3a
b
run
no.
initiator/catalyst/
monomer
Tp
C)
tp
yield
(%)
104 Mn
PDIb
I* c
[rr]d
(%)
[mr]d
[mm]d
(%)
(
°
(h)
(g/mol)
(Mw/Mn)
(%)
(%)
1
2/MMA
23
1
80
3.75
6.8
43
8.1
16.4
75.5
2
3
4
5
6
3/MMA
23
23
0
1
1
1
4
1
22
>99
>99
>99
>99
2.24
3.21
2.46
2.45
3.38
1.20
1.12
1.12
1.09
1.18
20
62
81
82
84
72.9
71.3
79.3
79.5
86.7
24.6
24.7
19.9
19.6
12.6
2.5
4.0
0.8
0.9
0.7
3/MeAl(BHT)2/MMA
3/MeAl(BHT)2/MMA
3/MeAl(BHT)2/MMA
3/MeAl(BHT)2/
BMA
-40
0
7
3/MeAl(BHT)2/
BMA/MMA
0
1/1
>99
4.69
1.12
102
83.6
16.4
0.0
a [Monomer]0/[initiator]0 ) 200, 5 mL of toluene. 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). d Tacticity (methyl triad distributions) determined by 1H NMR spectroscopy in CDCl3.
Al(BHT)3. In the case of iBuAl(BHT)2, considerable monomer
activation, at least to the same level as that by Al(iBu)3, has
been observed (vide supra); however, it is its inability to form
the observable enolaluminate propagating species that renders
its ill-behaved polymerization with only marginal activity. In
agreement with the finding of the related systems investigated
by Holmes and co-workers,17 the results obtained from the
current system show that, to achieve a high degree of polym-
erization control and the syndiotacticity of polymer, organoalu-
minum compounds must have the structures capable of forming
both monomeric ester enolaluminate and activated-monomer
complexes.
The alane, Al(C6F5)3, is a unique activator or catalyst because
the three C6F5 groups provide both adequate steric protection
of the aluminum center, rendering its monomeric form,42 and
high electron-withdrawing capacity, rendering its remarkably
high Lewis acidity.40b Thus, it is of great interest to use this
Figure 4. Plot of Mn and PDI of PMMA by 3/MeAl(BHT)2 at 0 °C vs
alane, in combination with the initiator 2, for MMA polymer-
ization. While the 1:1 mixture of 2 and Al(C6F5)3 (to generate
the enolaluminate in situ) at 23 °C is inactive, the polymerization
using a 1:2 ratio (to generate both the enolaluminate propagator
and the activated monomer) is highly active, producing syn-
diotactic {[rr] ) 73.5%} PMMA with quantitative yield and a
narrow MWD (PDI ) 1.14, run 15, Table 7). When the same
polymerization is carried out at 0 °C, syndiotactic {[rr] )
82.5%} PMMA with PDI as small as 1.04 is obtained (run 16).
The instability of the enolaluminate derived from Al(C6F5)3 in
nonpolar solvents (vide supra) presumably causes the observed
much-higher-than-calculated Mn (i.e., low I* values); however,
this problem can be largely overcome by changing the reagent
addition sequence in the polymerization procedures as we have
demonstrated that such an enolaluminate is stable in donor
solvents. Hence, instead of preforming the enolaluminate by
premixing 2 and Al(C6F5)3 in toluene, the addition of an alane
solution in toluene to MMA, followed by addition of 2 to start
the polymerization, allowed for a drastically increased initiator
efficiency (I* ) 72%) but the same narrow MWD (PDI ) 1.04,
run 17).
polymer yield.
The results from the investigation of structural effects of five
additional bulky Lewis acidic aluminum catalysts and the
corresponding enolaluminates derived from them on the be-
havior of MMA polymerization are summarized in Table 7. The
lithium enolaluminate generated from 2 and Al(iBu)3 produces
essentially atactic PMMA with relatively large PDI values of
>1.3 (runs 1 and 2, Table 7). Consistent with the activated-
monomer polymerization, when using 2 equiv of Al(iBu)3 vs 2
(to form a 1/1 ratio of enolaluminate/catalyst, vide supra), the
same polymerization becomes not only syndioselective but also
well-behaved with PDI < 1.17 (runs 3 and 4). A gradual
increase of the steric bulkiness of the organoaluminum com-
i
pounds by sequential substitution of the Bu ligand with the
BHT ligand results in pronounced changes in the MMA
polymerization behavior. Thus, as compared with Al(iBu)3, the
i
polymerization using Bu2Al(BHT) is noticeably more syndi-
oselective (runs 5-7 vs 1-4); the syndioselectivity of the MMA
polymerization by the sterically bulkier iBuAl(BHT)2 is further
enhanced, but with substantially reduced activity and an even
bimodal MWD (runs 8-10). Most strikingly, the polymerization
is shut down with the sterically bulkiest Al(BHT)3 in this series
(runs 11-13). The small amount of PMMA formed from a run
with an extended polymerization time shows to be isotactic (run
13), indicative of the polymer formed by the lithium enolate
alone17 (i.e., no participation of the activated monomer or the
lithium enolaluminate); this observation is consistent with the
previously discussed finding of no activation of MMA by Al-
(BHT)3 and no enolaluminate formation when mixing 2 and
Polymerization Kinetics and Mechanism. Scheme 6 il-
lustrates the proposed bimolecular chain propagation for the
MMA polymerization by the lithium ester enolaluminate and
organoaluminum catalyst combination. This mechanism involves
Michael addition of enolaluminate propagator G to Al-activated
monomer H, followed by the release of the coordinated
aluminum catalyst to the ester group of the polymer chain by
MMA to regenerate the propagating species G and the activated
9
J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005 971

A R T I C L E S
Rodriguez-Delgado and Chen
Table 7. Effect of Enolaluminate and Aluminum Catalyst Structures on MMA Polymerizationa
b
run
no.
initiator/
activator
Tp
C)
tp
yield
(%)
104 Mn
PDIb
[rr]c
(%)
[mr]c
(%)
[mm]c
(%)
(
°
(h)
(g/mol)
(Mw/Mn)
1
2/Al(iBu)3
23
0
23
0
1
40
3.49
1.76
3.24
2.83
1.32
1.31
1.08
1.17
27.6
37.2
57.8
63.0
47.6
42.0
35.2
32.9
24.8
20.8
7.0
2
3
4
2/Al(iBu)3
2/2Al(iBu)3
2/2Al(iBu)3
4
1
4
>99
>99
>99
4.1
5
6
7
2/Al(iBu)2(BHT)
2/2Al(iBu)2(BHT)
2/2Al(iBu)2(BHT)
23
23
0
1
1
4
22
89
>99
1.49
3.38
3.71
1.16
1.51
1.16
49.0
64.5
69.0
38.3
31.8
27.4
12.7
3.7
3.7
8
9
10
2/Al(iBu)(BHT)2
2/2Al(iBu)(BHT)2
2/2Al(iBu)(BHT)2
23
23
0
1
1
4
4
6
12
72.7
75.4
23.0
20.0
4.3
4.6
11, 0.8
2.9, 1.1
11
12
13
2/Al(BHT)3
2/2Al(BHT)3
2/2Al(BHT)3
23
23
23
1
1
4
trace
trace
5
9.5
25.7
64.8
14
15
2/Al(C6F5)3
2/2Al(C6F5)3
2/2Al(C6F5)3
2/2Al(C6F5)3
23
23
0
1
1
0.5
1
trace
>99
>99
>99
11.9
11.0
6.92
1.14
1.04
1.04
73.5
82.5
79.6
24.6
15.9
19.4
1.9
1.6
1.0
16
17d
0
a [MMA]0/[2]0 ) 200, 5 mL of toluene. b Number-average molecular weight (Mn) and polydispersity index (PDI) determined by GPC relative to PMMA
standards. c Tacticity (methyl triad distributions) determined by 1H NMR spectroscopy in CDCl3. d Conditions: [MMA]0/[2]0 ) 500, 10 mL of toluene.
Addition sequence: Al(C6F5)3 in toluene, MMA, and 2.
Scheme 6
Figure 5. First-order time-conversion plot of the MMA polymerization in
toluene-d8 at 22 °C. Conditions: [MMA]0 ) 1.0 M, [3]0 ) 5.0 mM, [MeAl-
(BHT)2]0 ) 5.0 mM.
monomer H; repeated Michael additions of G to H produce
the polymer in a controlled fashion.
If the release of the catalyst from the ester group of the
polymer to the incoming monomer (i.e., I + MMA f G + H)
is fast relative to the addition step (i.e., G + H f I) and the
equilibrium favors the formation of activated monomer H,
application of the steady-state approximation of the intermediate
I results in first-order kinetics with respect to active species
[G] (i.e., the enolaluminate propagator concentration [Al-]) and
activated monomer [H] (i.e., the aluminum catalyst concentra-
tion [Al]0) but zero-order kinetics in monomer concentration
[M]:
The monitoring of the polymerization by the enolaluminate/
organoaluminum combination [i.e., the 3/MeAl(BHT)2 pair]
using the NMR techniques clearly shows, however, the first-
order dependence of monomer concentration (Figure 5); this
implies that coordination of the aluminum catalyst to the ester
group of the polymer chain and to the monomer is competitive:
K
I + MMA y
\
z G + H
(3)
Schlaad and Mu¨ller showed that the equilibrium constants
for complexation of Al(iBu)3 with MMA and methyl pivalate
(serving as a model for the ester group of the polymer chain)
are approximately the same,15 giving for eq 3 K ≈ 1. Because
of this coordination competing process, the concentration of the
activated monomer [H] now depends on monomer conversion:
d[M]
-
) kp[G][H] ) kp[Al-][Al]0
(1)
(2)
dt
[M]0 - [M]
t ) xp(conversion) ) kapp
t
[M]0
This type of polymerization kinetics has been observed for
the MMA polymerizations by the nonbridged zirconocene
enolate/zirconocenium cation45 and enamine/Lewis acid46 com-
binations.
(45) (a) Bandermann, F.; Ferenz, M.; Sustmann, R.; Sicking, W. Macromol.
Symp. 2001, 174, 247-253. (b) Li, Y.; Ward, D. G.; Reddy, S. S.; Collins,
S. Macromolecules 1997, 30, 1875-1883.
(46) Miyamoto M.; Kanetaka, S.-Y. J. Polym. Sci., Part A: Polym. Chem. 1999,
37, 3671-3679.
9
972 J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005

Single-Site Anionic Polymerization
A R T I C L E S
[M]
[H] ) (1 - xp)[H]o )
t[Al]0
[M]0
(4)
Assuming that MeAl(BHT)2 behaves similarly to Al(iBu)3 in
terms of complexation with MMA and the ester group of the
polymer and using a rate expression worked out by Schlaad
and Mu¨ller,15a we have a first-order dependence:
[M]
d[M]
dt
-
) kp[G][H] ) kp[Al-]
t[Al]0
(5)
(6)
(
)
[M]0
[M]0
kp[Al-][Al]0
ln
)
t ) kapp
t
(
)
[M]t
[M]0
Hence, the polymerization should follow first-order kinetics with
respect to the active species, aluminum catalyst, and monomer
concentrations. To investigate the rate order of the aluminum
catalyst in the current MMA polymerization by the 3/MeAl-
(BHT)2 pair, polymerization kinetics with varied concentrations
of MeAl(BHT)2 at a constant concentration of 3 were carried
out in stirred glass reactors. In each [MeAl(BHT)2]0 (note that
this is the concentration of the “free” aluminum catalyst after
the formation of 3), first-order kinetics in [MMA] were observed
up to high monomer conversions (g90%), Figure 6. A double
logarithm plot (Figure 7) of the apparent rate constants (kapp),
which were obtained from the slopes of the best-fit lines to the
plots of ln([M]0/[M]t) versus time, as a function of [MeAl-
(BHT)2]0 was fit to a straight line of slope ) 1.1(1). This rate
order of 1.1(1) in [MeAl(BHT)2]0 is the same as that obtained
by the in situ monitoring of the polymerization using NMR
techniques, although the polymerizations in the NMR tubes were
considerably slower (due to the lack of stirring) than those
carried out in the stirred reactors. These results indicate that
the rate order in [MeAl(BHT)2]0 is approximately unity, and
this polymerization can be described as the activated-monomer
polymerization.
The syndiotacticity of the resulting polymers produced via
the activated monomer propagation can be understood from
Scheme 7, which shows the propagating step of methacrylate
polymerization involving the Michael addition of the enolalu-
minate active species to the activated monomer in the s-cis
conformation; for steric considerations, ester groups of the
monomer and the last added monomer unit are placed on the
opposite side, and the resulting polymer linkage is syndiotactic.
Noteworthy is the observed, nearly constant syndiotacticity
{[rr] ≈ 75%} for the MMA polymerizations at all the [MeAl-
(BHT)2]0/[3] ratios (1, 2, 3, and 4) and at all conversions,
indicating the activated-monomer polymerization occurs through-
out the course of the polymerization. It should be reminded that
the combined spectroscopic and polymerization studies con-
cluded that lithium ester enolaluminates are less reactive than
lithium ester enolates. For example, the enolaluminate species,
either isolated or generated in situ, have low or no activity
toward MMA polymerization. However, under the activated-
monomer polymerization conditions (i.e., in the presence of the
conjugate aluminum catalysts), these less reactive enolaluminate
propagating species become selective and preferentially add to
the activated monomer. Therefore, both formations of the
enolaluminate and the activated monomer complex are necessary
for achieving syndiotacticity and polymerization control.
Figure 6. Semilogarithmic plots of ln{[M]0/[M]t} versus time for the
polymerization of MMA by 3 and MeAl(BHT)2 in toluene at 23 °C.
Conditions: [MMA]0 ) 1.87 M, [3]0 ) 9.34 mM, [MeAl(BHT)2]0 ) 9.34
mM (2), 18.7 mM (4); 28.0 mM ([), 37.4 mM (]).
Figure 7. Plot of ln(kapp) versus ln{[MeAl(BHT)2]0} for the MMA
polymerization by 3 and MeAl(BHT)2 in toluene at 23 °C.
Scheme 7
Summary
In conclusion, the combined synthetic, structural, polymer-
ization, and kinetic study has established the monomeric lithium
ester enolaluminate as the single-site anionic propagating center
and the bimolecular, activated-monomer mechanism for the
controlled polymerization of methacrylates in the presence of
conjugate Lewis acidic, bulky organoaluminum catalysts. With
suitable structures, the organoaluminum compounds function
as deaggregating and stabilizing agents for lithium ester enolates
by forming the stable, monomeric ester enolaluminate active
species and as catalysts for monomer activation, when used in
excess for methacrylate polymerization; these attributes account
for the often-observed controlled polymerization by the com-
9
J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005 973

A R T I C L E S
Rodriguez-Delgado and Chen
bination of lithium ester enolates with organoaluminum com-
pounds. Because of the competitive coordination of the orga-
noaluminum catalyst to the monomer and the ester group of
the polymer chain, the activated-monomer polymerization of
the present system follows first-order kinetics with respect to
both monomer and catalyst concentrations.
control, the organoaluminum compounds added to the polym-
erization initiated by lithium ester enolates are required to have
structures capable of forming both monomeric ester enolalu-
minates and activated-monomer complexes. The results obtained
from the current investigation of the single-site anionic polym-
erization, coupled with the previous findings from other related
systems,5-7,15-17 establish the general guidance for the design
and synthesis of more efficient and controlled anionic initiators
and catalysts.
The fully characterized monomeric lithium ester enolalumi-
nate 3, which serves as the structural model for the single-site
anionic propagating center, exhibits the unique mode of in-
tramolecular coordination of the lithium cation to both oxygen
atoms of the ester enolate moiety, acting as a chelating ligand,
and to one of the BHT ligands at the aluminate center in an
η6-fashion; these structural characteristics account for the
isolability and monomeric feature of this complex. The structural
variations of the employed organoaluminum compounds have
substantial influence on the degree of monomer activation,
enolaluminate formation and reactivity, and polymerization
control. Ester enolaluminates are less reactive than ester enolates
but more selective with preferential addition to the activated
monomer in a syndioselective fashion. Hence, to achieve the
syndiotacticity of polymer and a high degree of polymerization
Acknowledgment. This work was supported by the National
Science Foundation and Colorado State University. We thank
Susie M. Miller for determination of the crystal structures and
Boulder Scientific Co. for the gift of B(C6F5)3. E.Y.C. acknowl-
edges an Alfred P. Sloan Research Fellowship.
Supporting Information Available: Crystallographic data for
1, 2, and 3 (CIF) and representative GPC traces (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA044671Z
9
974 J. AM. CHEM. SOC. VOL. 127, NO. 3, 2005