Model studies of methacrylate chain transfer polymerization mediated by cationic zirconocene tert-butyl enolate

Article abstract of DOI:10.1021/om050119m

Enolizable ketones and thiols have been investigated as potential transfer agents for methyl methacrylate (MMA) polymerization mediated by the Cp ₂ZrMe₂/B(C₆F₅)₃ system. Addition of 1-10 equiv of acetophenone, acetone, or thiophenols inhibits polymerization, while the system tolerates the presence of tert-butylthiol (tBuSH). In this case, a moderate decrease in the molecular weight of the PMMAs is observed. The stoichiometric reactivity of these organic acids toward the cationic ester enolate complex [Cp₂Zr(THF)(O(tBuO)C=CMe ₂)]⁺[MeB-(C₆F₅)₃] ^- (1), which models the active species for MMA polymerization, has been investigated. Ketones undergo aldolization reactions with 1 to generate species that are inactive in MMA polymerization. Thiols readily cleave the Zr-O bond of 1 to give [Cp₂Zr(SR)(THF)]⁺ cations (R = tBu, 4; SC₆H₄-p-Cl, 6; SC₆H₄-o-OMe, 7). The crystal structure of 7 has been determined. In the presence of a tert-butyl ester R′CO₂tBu, thiolato complexes 4, 6, and 7 smoothly decompose into the corresponding cationic carboxylato complex [Cp ₂Zr(THF)(O₂CR′)]⁺ (R′ = iPr, 5; CH₃, 9) and thiol RSH, with release of isobutene. tert-Butylthiolato complex 4 and the in situ combination Cp₂Zr(StBu)Me/B(C ₆F₅)₃ polymerized quantitatively MMA in toluene to yield PMMAs with narrow dispersity (M_n/M_w = 1.26-1.48), but with molecular weight much higher than the expected M _n values, consistent with poor initiation efficiency and/or instability.

Full text of DOI:10.1021/om050119m

2466
Organometallics 2005, 24, 2466-2472
Model Studies of Methacrylate Chain Transfer
Polymerization Mediated by Cationic Zirconocene
tert-Butyl Enolate
Bing Lian,† Christian W. Lehmann,‡ Christophe Navarro,§ and
Jean-Franc¸ois Carpentier*,†
Organome´talliques et Catalyse, UMR 6509, Institut de Chimie de Rennes,
35042 Rennes Cedex, France, Max-Planck-Institut fu¨r Kohlenforschung, Chemical
Crystallography, Postfach 101353, 45466 Mu¨lheim an der Ruhr, Germany, and Arkema,
Lacq Research Center, PO Box 34, 64170 Lacq, France
Received February 18, 2005
Enolizable ketones and thiols have been investigated as potential transfer agents for methyl
methacrylate (MMA) polymerization mediated by the Cp2ZrMe2/B(C6F5)3 system. Addition
of 1-10 equiv of acetophenone, acetone, or thiophenols inhibits polymerization, while the
system tolerates the presence of tert-butylthiol (tBuSH). In this case, a moderate decrease
in the molecular weight of the PMMAs is observed. The stoichiometric reactivity of these
organic acids toward the cationic ester enolate complex [Cp2Zr(THF)(O(tBuO)CdCMe2)]+[MeB-
(C6F5)3]- (1), which models the active species for MMA polymerization, has been investigated.
Ketones undergo aldolization reactions with 1 to generate species that are inactive in MMA
polymerization. Thiols readily cleave the Zr-O bond of 1 to give [Cp2Zr(SR)(THF)]+ cations
(R ) tBu, 4; SC6H4-p-Cl, 6; SC6H4-o-OMe, 7). The crystal structure of 7 has been determined.
In the presence of a tert-butyl ester R′CO2tBu, thiolato complexes 4, 6, and 7 smoothly
decompose into the corresponding cationic carboxylato complex [Cp2Zr(THF)(O2CR′)]+ (R′ )
iPr, 5; CH3, 9) and thiol RSH, with release of isobutene. tert-Butylthiolato complex 4 and
the in situ combination Cp2Zr(StBu)Me/B(C6F5)3 polymerized quantitatively MMA in toluene
to yield PMMAs with narrow dispersity (Mn/Mw ) 1.26-1.48), but with molecular weight
much higher than the expected Mn values, consistent with poor initiation efficiency and/or
instability.
Introduction
ester enolate complexes that model the propagating
species, e.g., Cp2ZrMe[O(OtBu)CdCMe2],5 Cp2Zr+(THF)-
[O(OtBu)CdCMe2][Me(B(C6F5)3]-,4n and (rac-EBI)Zr+-
(THF)[O(OiPr)CdCMe2][Me(B(C6F5)3]-,2f were prepared
and their reactivity was fully investigated. A major
interest of these zirconocenes, when used in a suitable
initiating form, is the high degree of control they exhibit
over polymerization, i.e., the livingness and stereochem-
istry of polymerization. The use of a stoichiometric
amount of metal complex per macromolecular chain
Zirconocene-mediated polymerization of methacry-
lates has attracted much attention in recent years.1-4
Detailed studies by the groups of Collins, Chen, and
others established either bimetallic or monometallic
mechanisms, involving the rate-limiting intramolecular
Michael addition of a neutral or cationic zirconocene
enolate to activated or nonactivated methyl methacry-
late monomer (MMA), respectively.1b,d,2f,4g For this
purpose, a variety of neutral and cationic zirconocene
(4) (a) Soga, K.; Deng, H.; Yano, T.; Shiono, T. Macromolecules 1994,
27, 7938-7940. (b) Deng, H.; Shiono, T.; Soga, K. Macromolecules 1995,
28, 3067-3073. (c) Shiono, T.; Saito, T.; Saegusa, N.; Hagihara, H.;
Ikeda, T.; Deng, H.; Soga, K. Macromol. Chem. Phys. 1998, 199, 1573-
1579. (d) Bandermann, F.; Ferenz, M.; Sustmann, R.; Sicking, W.
Macromol. Symp. 2000, 161, 127-134. (e) Stuhldreier, T.; Keul, H.;
Ho¨cker, H. Macromol. Rapid Commun. 2000, 21, 1093-1098. (f)
Bandermann, F.; Ferenz, M.; Sustmann, R.; Sicking, W. Macromol.
Symp. 2001, 174, 247-253. (g) Frauenrath, H.; Keul, H.; Ho¨cker, H.
Macromolecules 2001, 34, 14-19. (h) Karanikolopoulos, G.; Batis, C.;
Pitsikalis, M.; Hadjichristidis, N. Macromolecules 2001, 34, 4697-4705.
(i) Ho¨lscher, M.; Keul, H.; Ho¨cker, H. Chem. Eur. J. 2001, 7, 5419-
5426. (j) Batis, C.; Karanikolopoulos, G.; Pitsikalis, M.; Hadjichristidis,
N. Macromolecules 2003, 36, 9763-9774. (k) Ferenz, M.; Bandermann,
F.; Sustmann, R.; Sicking, W. Macromol. Chem. Phys. 2004, 205, 1196-
1205. (l) Karanikolopoulos, G.; Batis, C.; Pitsikalis, M.; Hadjichristidis,
N. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 3761-3774. (m)
Strauch, J. W.; Faure´, J.-L.; Bredeau, S.; Wang, C.; Kehr, G.; Fro¨hlich,
R.; Luftmann, H.; Erker, G. J. Am. Chem. Soc. 2004, 126, 2089-2104.
(n) Lian, B.; Toupet, L.; Carpentier, J.-F. Chem. Eur. J. 2004, 10,
4301-4307.
* To whom correspondence should be addressed. Fax: (+33)(0)223-
236-939. E-mail: jean-francois.carpentier@univ-rennes1.fr.
† Universite´ de Rennes 1.
‡ Max-Planck-Institut fu¨r Kohlenforschung.
§ Arkema.
(1) (a) Collins, S.; Ward, D. G. J. Am. Chem. Soc. 1992, 114, 5460-
5462. (b) Collins, S.; Ward, D. G.; Suddaby, K. H. Macromolecules 1994,
27, 7222-7224. (c) Li, Y.; Ward, D. G.; Reddy, S. S.; Collins, S.
Macromolecules 1997, 30, 1875-1883. (d) Nguyen, H.; Jarvis, A. P.;
Lesley, M. J. G.; Kelly, W. M.; Reddy, S. S.; Taylor, N. J.; Collins, S.
Macromolecules 2000, 33, 1508-1510. (e) Stojcevic, G.; Kim, H.; Taylor,
N. J.; Marder, T. B.; Collins, S. Angew. Chem., Int. Ed. 2004, 43, 5523-
5526.
(2) (a) Chen, E. Y.-X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks, T. J.
J. Am. Chem. Soc. 1998, 120, 6287-6305. (b) Bolig, A. D.; Chen, E.
Y.-X. J. Am. Chem. Soc. 2001, 123, 7943-7944. (c) Bolig, A. D.; Chen,
E. Y.-X. J. Am. Chem. Soc. 2002, 124, 5612-5613. (d) Chen, E. Y.-X.;
Cooney, M. J. J. Am. Chem. Soc. 2003, 125, 7150-7151. (e) Mariott,
W. R.; Chen, E. Y.-X. J. Am. Chem. Soc. 2003, 125, 15726-15727. (f)
Bolig, A. D.; Chen, E. Y.-X. J. Am. Chem. Soc. 2004, 126, 4897-4906.
(3) Cameron, P. A.; Gibson, V. C.; Graham, A. J. Macromolecules
2000, 33, 4329-4335.
(5) Stuhldreier, T.; Keul, H.; Ho¨cker, H.; Englert, U. Organometallics
2000, 19, 5231-5234.
10.1021/om050119m CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/13/2005

Methacrylate Chain Transfer Polymerization
Organometallics, Vol. 24, No. 10, 2005 2467
Scheme 1
polymerization (entries 2, 3); similar poisoning was also
observed with acetone and thiophenols (entries 7, 8).
With tBuSH, the polymerization was not inhibited for
tBuSH/Zr up to 5 equiv, but a significant although not
complete decrease in conversion was observed in the
presence of 10 and 20 equiv (entries 4-6). Promisingly,
the molecular weight of the obtained PMMAs (Mn,exp
)
was roughly half that in the absence of thiol, but still
much higher than the calculated molecular weight
(Mn,calc). Two possibilities can account for this decrease
of the Mn,exp values: (i) tBuSH may modify the mech-
anism to monometallic species, which promote MMA
polymerization still with moderate initiation efficiency
(f ) 49-67% for entries 4-6 in this hypothesis vs f )
73% in entry 1); (ii) tBuSH acts as a transfer agent with
moderate chain transfer efficiency.
To get a better insight in the reactions that actually
take place in the above polymerizations, we investigated
in more detail the stoichiometric reactivity of these
organic acids toward a model complex that mimics the
propagating species of the methacrylate polymerization
by the Cp2ZrMe2/B(C6F5)3 system. The cationic ester
enolate complex [Cp2Zr(THF)(O(tBuO)CdCMe2)]+[MeB-
(C6F5)3]- (1) that we reported recently4n was selected
for this purpose.
Table 1. MMA Polymerization Promoted by
Cp2ZrMe2/B(C6F5)3 in the Presence of Organic
Acidsa
b
c
org
yield Mn,calc
Mn,exp
acid/Zr (%) (g‚mol-1
)
(g‚mol-1
) Mw/Mn
c
entry
organic acid
none
1
2
3
4
5
6
7
8
98
<1
<1
97
48
43
38 800
54 500
1.97
PhCOMe
PhCOMe
tBuSH
tBuSH
tBuSH
p-ClC6H4SH
o-MeOC6H4SH
1
5
5
10
20
5
3880
960
430
29 900
19 800
15 700
2.85
1.38
2.21
<1
<1
5
Reactivity of Zr-Ester Enolate Cationic Species
1 toward Organic Acids. Complex 1-d8 reacts with 1
equiv of acetophenone in THF-d8 at room temperature
within a few minutes to give quantitatively the Zr-aldol
2 (Scheme 2). Cationic complex 2 was fully characterized
in solution by 1D and 2D NMR analyses. Key 1H NMR
resonances include a singlet for the equivalent Cp
protons (δ 6.46), a singlet for the Me at the chiral
(racemic) center (δ 1.78), and two singlets for the
diastereotopic C(CH3)2 (δ 1.19 and 1.07).8 This reactivity
evidences that acetophenone does not behave as an
organic acid toward 1-d8 in THF solution as anticipated,
but rather as an electrophile.9 In a separate experiment,
the Zr-aldol complex 2 was found not to initiate MMA
polymerization (THF or toluene, 20 °C, MMA/2 ) 200),
as expected for an alkoxy complex.6a These observations
are in direct line with the inhibition effect of 1-5 equiv
of acetophenone in the polymerization of MMA initiated
by the Cp2ZrMe2/B(C6F5)3 system in toluene (vide su-
pra). Complex 2 is not stable in THF-d8 at room
temperature and transforms over 5 days (52%, based
on tBu resonances) with release of isobutene (not
polymerized), to give a complex that could not be fully
characterized thus far.10
a Reaction time ) 3 h, T ) 20 °C, solvent ) toluene (2 mL);
MMA/Zr/B ) 200:1:1; Zr ) 0.050 mmol. b Calculated Mn,calc values
considering a bimetallic initiator (in the absence of organic acid)
or one polymer chain per organic acid. c Experimental Mn and Mw/
Mn values as determined by GPC in THF vs PMMA standards.
remains, however, a major limitation of initiating po-
lymerization systems. To transform zirconocenes from
initiators into true catalysts, the search for effective
chain transfer agents (CTAs) is of high industrial
relevance. In this process, two essential objectives must
be achieved: (i) CTAs must cleave the growing PMMA
chain, and (ii) the resulting new cationic species must
reinitiate polymerization of MMA (Scheme 1). Such an
approach that employs organic acids has been recently
reported for lanthanidocenes,6 which are isoelectronic
counterparts of cationic zirconocenes. In this note, we
report our preliminary studies on the use of various
organic acids as potential CTAs for zirconocene-medi-
ated polymerization of MMA. Polymerization data and
results from stoichiometric studies of model complexes
are described.
Results and Discussion
The ubiquitous Cp2ZrMe2/B(C6F5)3 system3 was se-
lected for this preliminary study. The effectiveness of
several organic acids7 that were found suitable for
monocomponent lanthanidocene systems,6 such as eno-
lizable ketones and thiols, was first screened in MMA
polymerization. Representative results obtained with
PhCOMe, t-BuSH, p-ClC6H4SH, and o-MeOC6H4SH are
summarized in Table 1. In contrast to group 3 metal
initiators, poor performances were observed. Only 1
equiv of acetophenone suffices to inhibit completely
The reaction of complex 1-d8 with 1 equiv of acetone
in THF-d8 at room temperature proceeds similarly as
for acetophenone, giving quantitatively within a few
minutes the Zr-aldol 3 (Scheme 2).8 Complex 3 was
characterized in solution by 1D and 2D 1H and 13C
NMR. It also decomposes in THF solution at room
(8) 1H NMR in CD2Cl2 of complexes 2 and 3 generated in THF
solution instead of THF-d8 established that no THF molecule coordi-
nates the metal center in those species.
(9) Aldolization reactions between acetophenone derivatives and
neutral enolate complexes [{Ph2NC(CHR)O}ZrCp2Cl] (R ) H, Me) have
been described: Cozzi, P. G.; Veya, P.; Floriani, C.; Rotzinger, F. P.;
Chiesi-Villa, A.; Rizzoli, C. Organometallics 1995, 14, 4092-4100.
(10) Based on our previous decomposition studies of Zr-tert-butyl
enolate,4n the latter can be tentatively formulated “[Cp2Zr(µ2-O2CCMe2-
CMe(R)OH]+”.
(6) (a) Nodono, M.; Tokimitsu, T.; Tone, S.; Makino, T.; Yanagase,
A. Macromol. Chem. Phys. 2000, 201, 2282-2288. (b) Yanagase, A.;
Tone, S.; Tokimitsu, T.; Nodono, M. (Mitsubishi Rayon) Eur. Pat. Appl.
0919569A1, 1999.
(7) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456-463.

2468 Organometallics, Vol. 24, No. 10, 2005
Lian et al.
Scheme 2
Scheme 3
Scheme 4
and potential coordinating abilities thanks to the sub-
stituents on the aryl ring, were also investigated. A 1H
NMR monitoring showed that 1-d8 was consumed
within 10 min to give a 56:44 mixture of 5-d8 and the
thiophenate complexes 6-d8 and 7-d8, respectively.
Conversion of 6/7-d8 to 5-d8 was completed within 30
min, concomitant with the release of 1 equiv of isobutene
and the corresponding thiophenol (p-ClC6H4SH and
o-MeOC6H4SH, respectively) (Scheme 3). Thus, the more
acidic the thiol (i.e., thiophenols vs alkylthiol), the faster
the kinetics for both protonolysis of the enolato-Zr
complex 1-d8 and, more surprisingly, subsequent trans-
formation of the thiolato complex (4-d8, 6-d8, and 7-d8).
temperature with release of isobutene, somewhat faster
than the Zr-aldol 2 (72% based on tBu resonances after
5 days).
On the other hand, complex 1-d8 reacts in THF-d8 at
room temperature with tBuSH via protonolysis of the
Zr-O(enolate) bond to yield the thiolato complex 4-d8
with concomitant release of tert-butyl isobutyrate
(Scheme 3). This is the pathway expected to take place
while using tBuSH as a transfer agent in the polymer-
ization of MMA. However, species 4-d8 is not stable
under those conditions and decomposes to give isobu-
tyrato complex 5-d8, with release of isobutene and
regeneration of tBuSH.11,12 A 1H NMR monitoring of the
reaction of complex 1-d8 and 3 equiv of tBuSH showed
complete conversion of 1-d8 after 1 h to give a 70:30
mixture of 4-d8 and 5-d8.13 Complete conversion of 4-d8
to 5-d8 was observed after 8 h; at the same time, the
resonances for tert-butyl isobutyrate disappeared con-
comitantly with the appearance of those for isobutene
and increase of the singlet for tBuSH.
The cationic species 7 was independently synthesized
by the reaction of Cp2ZrMe2 with B(C6F5)3 in THF,
followed by the addition of o-MeOC6H4SH (Scheme 4).
This species is stable in THF solution for several weeks
at room temperature. Crystals of 7 suitable for an X-ray
The reactions in THF-d8 of 1-d8 with 1 equiv of
p-ClC6H4SH and o-MeOC6H4SH, which differ in acidity7
(11) The analogous complexes 4 and 5 were independently generated
in nondeuterated THF. 1H NMR spectroscopy in CD2Cl2 established
the coordination of one THF molecule in both complexes (see Experi-
mental Section).
(12) Slow decomposition of the parent complex [Cp2Zr(StBu)][BPh4]
at 20 °C in THF solution to the sulfide-bridged dimer [Cp2ZrS]2 and
organic products, presumably arising from the decomposition of
putative [Me3C][BPh4], i.e., Me2CdCH2, Me3CH, B(C6H5)3, and C6H6,
has been reported. This process (formation of [Cp2ZrS]2) was not
observed under our conditions. See: Piers, W. E.; Koch, L.; Ridge, D.
S.; MacGillivray, L. R.; Zaworotko, M. Organometallics 1992, 11, 3148-
3152.
Figure 1. Molecular structure of the cation [Cp2Zr(THF)-
(SC6H4-o-OMe)]+ of 7.
(13) Reactions of 1-d8 with 1 or 2 equiv of tBuSH were found to
proceed more slowly with the same selectivity.

Methacrylate Chain Transfer Polymerization
Organometallics, Vol. 24, No. 10, 2005 2469
Table 2. Crystal Data and Structure Refinement
for 7
Scheme 5
empirical formula
fw
temperature/K
wavelength/Å
cryst syst
space group
a/Å
C40H28BF15O2SZr
959.72 g‚mol-1
100
0.71073
monoclinic
P21/n (no. 14)
12.9296(2)
12.9776(2)
22.9040(3)
90
b/Å
c/Å
R/deg
â/deg
105.3940(10)
90
γ/deg
volume/Å3
Z
3705.30(9)
4
density (calcd) Mg‚m-3
absorp coeff/mm-1
F(000)
1.720
0.467
1920
cryst size/mm3
θ range for data
collection/deg
index ranges
0.04 × 0.04 × 0.02
3.14 to 26.40
generated in a two-step one-pot procedure: the reaction
of Cp2ZrMe2 with tBuSH (1 or 2 equiv) in benzene
affords Cp2Zr(StBu)Me (8), which was further reacted
with 1 equiv of B(C6F5)3 in THF-d8 at room temperature
to yield 4-d8. This species is as such stable at least for
2 days at room temperature; however, 4-d8 readily
reacts indeed with 1 equiv of tert-butyl isobutyrate
-16 e h e 16,
-16 e k e 16,
-28 e l e 28
56 591
7565 [Rint ) 0.0678]
5675
99.6%
semiempirical from equivalents
full-matrix least-squares on F2
7565/0/543
1.015
R1 ) 0.0349
wR2 ) 0.0623
R1 ) 0.0603
wR2 ) 0.0697
0.368 and -0.352
reflns collected
indep reflns
reflns with I >2σ(I)
completeness to θ ) 26.40°
absorp correction
refinement method
data/restraints/params
goodness-of-fit on F2
final R indices [I>2σ(I)]
1
(Scheme 5). A H NMR monitoring revealed that com-
plex 4-d8 was consumed over 8 h to form selectively
complex 5-d8 with release of 1 equiv of tBuSH and
isobutene (Figure 2).
The reaction of 4-d8 with tert-butyl acetate proceeds
similarly to give the cationic Zr-acetato complex 9-d8
with release of 1 equiv of tBuSH and isobutene (Scheme
5). The kinetics is significantly increased as compared
with tert-butyl isobutyrate (complete conversion within
4 h), presumably because of the lower bulkiness of this
ester.14 More importantly, no reaction between 4-d8 and
Me2CHCO2Me takes place, even for 12 h at 80 °C,
indicating this process is specific to tert-butyl esters1e,4n,15
and not relevant to MMA polymerization.
MMA Polymerization Initiated by Isolated [Cp2-
Zr(SR)(THF)][MeB(C6F5)3] Complexes and in Situ
Cp2Zr(StBu)Me/Activator Systems. The abilities for
MMA polymerization of the isolated [Cp2Zr(SR)(THF)]-
[MeB(C6F5)3] complexes 4 and 7 were next evaluated
(MMA/Zr ) 200:1). Because polymerizations are usually
conducted in an apolar solvent such as toluene, the
activity of [Cp2Zr(StBu)]+, i.e., the THF-free species
putatively generated by protonolysis of the polymer-
growing-chain zirconium intermediate by tBuSH (Scheme
1), was also investigated using in situ combinations of
the precursor Cp2Zr(StBu)Me (8) with a discrete methyl
abstractor. Representative results are summarized in
Table 4.
R indices (all data)
largest diff peak and
hole/e Å-3
Table 3. Selected Bond Lengths (Å) and Angles
(deg) for 7
Zr(1)-O(41)
Zr(1)-S(1)
Zr(1)-C(22)
Zr(1)-C(23)
Zr(1)-C(25)
Zr(1)-C(31)
2.2207(16)
2.4726(6)
2.499(2)
2.503(2)
2.523(2)
2.528(2)
Zr(1)-C(33)
Zr(1)-C(32)
Zr(1)-C(21)
Zr(1)-C(34)
Zr(1)-C(24)
Zr(1)-C(35)
2.470(2)
2.477(2)
2.503(2)
2.510(2)
2.527(2)
2.528(2)
C(11)-S(1)-Zr(1)
113.19(8) C(12)-C(11)-S(1)
119.02(19)
115.9(2)
C(16)-C(11)-S(1) 121.88(19) O(17)-C(16)-C(11)
C(16)-O(17)-C(18) 117.38(19) Cp(cent)-Zr-Cp′(cent)a 130.54
a Cp(cent) ) centroid of C(21)-C(25), Cp′(cent) ) centroid of
C(31)-C(35).
diffraction study were grown from a THF/toluene mix-
ture. The solid-state structure of 7 features a dissociated
-
ion pair with a normal MeB(C6F5)3 anion and a
tetragonal 16-electron cationic center with a coordinated
THF molecule and an uncoordinated methoxy group
(Figure 1). Important crystallographic data and bond
distances and angles are listed in Tables 2 and 3. The
Zr(1)-S(1) (2.4726(6) Å) and Zr(1)-O(41) (2.2207(16) Å)
bond distances are in agreement with those reported
for a related tert-butylthiolato-zirconocene cation (Zr-
S, 2.4618(13) Å; Zr-O, 2.2150(25) Å).12
In light of these observations, we assumed that
decomposition of the thiolato complexes 4-d8, 6-d8, and
7-d8 to 5-d8 proceeds by reaction with the released tert-
butyl isobutyrate (Scheme 3). To support this hypoth-
esis, the reaction of the latter ester with 4-d8 in the
absence of tBuSH was investigated in a separate experi-
ment. The cationic thiolato complex 4-d8 was cleanly
The thiophenato complex 7 does not show any sig-
nificant activity (Table 4, entry 1), consistent with the
(14) When the reaction of 4 with tert-butyl isobutyrate (1:1) was
carried out in CD2Cl2 instead of THF-d8, complete conversion to 5 with
release of 1 equiv of isobutene and HStBu was reached within 10 min
(vs 8 h in the THF-d8 solution). Increase of kinetics in a weakly
coordinating solvent such as CD2Cl2 is consistent with a decomposition
pathway involving a “base-free” cationic Zr species, as proposed in
Scheme 3; see refs 4n and 12.
(15) For examples of cationic zirconium complexes having a reactive
tert-butoxy group see: (a) Nguyen, H.; Jarvis, A. P.; Lesley, M. J. G.;
Kelly, W. M.; Reddy, S. S.; Taylor, N. J.; Collins, S. Macromolecules
2000, 33, 1508-1510. (b) Stoebenau, E. J., III; Jordan, R. F. J. Am.
Chem. Soc. 2003, 125, 3222-3223.

2470 Organometallics, Vol. 24, No. 10, 2005
Lian et al.
shown that thiols, in contrast to enolizable ketones, are
valuable candidates as chain transfer agents in these
polymerizations. tert-Butyl mercaptan and thiophenols
readily cleave under stoichiometric conditions the Zr-
O(enolate) bond of 1 to generate a cationic zirconocene-
thiolato species (4, 6, 7). The latter are stable at room
temperature in THF, except in the presence of tert-butyl
esters, which promote the formation of zirconocene-
carboxylato species. Of zirconocene-thiolato species 4,
6, and 7, only the tert-butyl thiolato complex 4 proved
able to reinitiate the polymerization of MMA. Cationic
complexes 6 and 7, which contain a thiophenate group,
are likely not nucleophilic enough for this purpose. As
revealed by the modest initiation efficiency of 4 and the
limited transfer efficiency of the tBuSH/Cp2ZrMe2/
B(C6F5)3 system in MMA polymerization, other organic
acids are still required to achieve effective chain transfer
and eventually render the polymerization process cata-
lytic; research toward this goal is currently underway.
Figure 2. 1H NMR monitoring (500 MHz, 20 °C, THF-d8)
of the 1:1 reaction of 4-d8 with Me2CHCO2tBu [after 3.5 h
at 20 °C]. Descriptors a, b, c, d, e, and s refer respectively
to 4-d8, Me2CHCO2tBu, 5-d8, Me2CdCH, tBuSH, and
residual solvent resonances.
Table 4. MMA Polymerization Promoted by
Isolated [Cp2Zr(SR)(THF)][MeB(C6F5)3] Complexes
and in Situ Cp2Zr(StBu)Me (8)/Activator Systemsa
b
c
yield Mn,calc
(%) (g‚mol-1
Mn,exp
(g‚mol-1
Mw/
Mn
Experimental Section
c
entry
initiator
)
)
General Procedures. All experiments were carried out
under argon using standard Schlenk techniques or a high-
performance glovebox (<1 ppm O2, 5 ppm H2O). Toluene and
pentane were distilled from Na/K alloy, THF was distilled from
Na/benzophenone, dichloromethane was distilled from calcium
hydride under nitrogen, and all were degassed by freeze-
thaw-vacuum cycles prior to use. Deuterated solvents were
purchased from Eurisotop and distilled over appropriate drying
agents before use. Methyl methacrylate (MMA, Acros) was
distilled twice under argon over CaH2. tert-Butylmercaptan
(tBuSH, Aldrich) was distilled before use. Thiophenols p-ClC6H4-
SH (Acros) and o-MeOC6H4SH (Acros), Cp2ZrMe2 (Aldrich),
[Ph3C][B(C6F5)4] (Boulder), and [HNMe2Ph][B(C6F5)4] (Boulder
Scientific Co.) were used as received. B(C6F5)3 (Boulder) was
sublimed twice before use. Complexes Cp2Zr(Me)(O(tBuO)Cd
CMe2)5 and [Cp2Zr(O(tBuO)CdCMe2)][(MeB(C6F5)3)]4n were
synthesized as previously reported.
1
2
3
4
5
7
2
98
95
11
<1
400
25 400 2.55
4-d8
19 600 373 200d 1.42d
19 000 234 000 1.26
8/B(C6F5)3
8/[HNMe2Ph][B(C6F5)4]
8/[Ph3C][B(C6F5)4]
2200
15 000 1.48
a Reaction time ) 1 h, T ) 20 °C, solvent ) toluene (2 mL);
MMA/Zr/B ) 200:1:1; Zr ) 0.050 mmol. b Calculated Mn,calc values
considering one polymer chain per Zr. c Experimental Mn and Mw/
Mn values as determined by GPC in THF vs PMMA standards.
d A second distribution that accounts for ca. 5% of the sample was
observed with Mn ) 12 300 and Mw/Mn ) 1.36.
aforementioned inhibition effect of 1-5 equiv of p-ClC6H4-
SH and o-MeOC6H4SH in the MMA polymerization
initiated by the Cp2ZrMe2/B(C6F5)3 system in toluene
(Table 1). This can be reasonably explained on the basis
of the poor nucleophilicity of the thiophenato moiety.
Conversely, complex 4, which has a relatively more
nucleophilic tBuS reactive group, and the in situ com-
bination of 8 and B(C6F5)3 gave quantitative conversions
of MMA (entries 2, 3). Although the resulting PMMAs
have relatively narrow molecular weight distribution
(Mn/Mw ) 1.26-1.48), the average number molecular
weights are ca. 1 order of magnitude higher than the
theoretical values. These results suggest that both the
THF adduct 4 and the corresponding base-free cationic
Zr-thiolato species are active in MMA polymerization,
but have poor initiation efficiency. Alternatively, they
may also be unstable in toluene solution at room
temperature and decompose to some extent12 before
MMA polymerization takes place. The very poor and low
MMA conversions obtained when using the virtually
noncoordinating activator [Ph3C][B(C6F5)4] and the
weakly stabilizing activator [HNMe2Ph][B(C6F5)4]16 (as
compared to the stabilization effect of the coordinating
anion MeB(C6F5)3-), respectively (entries 4, 5)), are in
line with this hypothesis.17
NMR spectra were recorded on Bruker AC-200, AC-300, and
AM-500 spectrometers in Teflon-valved NMR tubes at 20 °C
unless otherwise stated. 1H and 13C NMR chemical shifts were
determined using residual solvent resonances and are reported
vs TMS. 19F and 11B NMR chemical shifts are referenced to
external CFCl3 and external BF3‚Et2O, respectively. Assign-
ment of signals was made from 1H-1H COSY, 1H-13C HMQC,
and 1H-13C HMBC 2D NMR experiments. Coupling con-
stants are given in hertz. Cationic Zr complexes containing
-
MeB(C6F5)3 are totally dissociated in THF-d8 or CD2Cl2
solution, and the NMR resonances for this anion are almost
identical (see below). Elemental analyses (C, H, N) were
performed by the Microanalytical Laboratory at the Institute
of Chemistry of Rennes and are the average of two independent
determinations. Molecular weights of PMMA were determined
by gel permeation chromatography (GPC) at room temperature
on a Waters apparatus equipped with five PL gel columns
(Polymer Laboratories Ltd), a Waters WISP 717 autosampler,
and a Shimadzu RID 6A differential refractometer. THF was
used as eluent at a flow rate of 1.0 mL‚min-1. PMMA
standards were used for molecular weight calibration.
(16) Although NMR data of [Cp2Zr(StBu)]+ in THF-d8 indicated that
NMe2Ph is free and does not coordinate to the metal center, such a
coordination of NMe2Ph to [Cp2Zr(StBu)]+ is much more likely in a
weakly coordinating solvent such as toluene.
Conclusion
(17) Very similar results were obtained by using these combinations
in methylene chloride, confirming that these poor results reflect more
the instability of the produced species rather than their poor solubility
in toluene; 8/[HNMe2Ph][B(C6F5)4]: 18% yield, Mn,exp ) 68 000, Mw/
Mn ) 1.17; 8/[Ph3C][B(C6F5)4]: <1% yield (all other conditions as stated
in Table 4). We thank a reviewer for suggesting these experiments.
Using the cationic zirconocene ester enolate complex
[Cp2Zr(THF)(O(tBuO)CdCMe2)]+[MeB(C6F5)3]- (1), which
mimics the propagating species of methacrylate polym-
erization by cationic zirconocene complexes, we have

Methacrylate Chain Transfer Polymerization
Organometallics, Vol. 24, No. 10, 2005 2471
NMR Data for the Free Anion MeB(C6F5)3
.
- 18 1H NMR
mmol), and CH3COCH3 (5.5 mg, 0.094 mmol) in THF (1.0 mL).
The solvent was removed in a vacuum, and the residue was
washed with pentane (2 × 1 mL) and dried under vacuum,
1
(THF-d8): δ 0.50 (br s, 3H, BCH3). H NMR (CD2Cl2): δ 0.54
(br s, 3H, BCH3). 13C{1H} NMR (THF-d8): δ 148.5 (dm, JC-F
) 252, o-C6F5), 137.4 (dm, JC-F ) 247, p-C6F5), 136.2 (dm, JC-F
) 229, m-C6F5), 129.7 (Cipso), 9.7 (br, BCH3). 13C{1H} NMR
1
leaving 3 as a yellow solid (46 mg, 52%). H NMR in THF-d8
was as reported above. Anal. Calcd for C40H34BF15O3Zr
(949.71): C, 50.59; H, 3.61. Found: C, 51.16; H, 3.73. 1H NMR
(CD2Cl2): δ 6.36 (s, 10H, Cp), 1.67 (s, 9H, OC(CH3)3), 1.30 (s,
6H, C(CH3)2CO2tBu), 1.26 (s, 6H, (CH3)2COZr). 13C{1H} NMR
(CD2Cl2): δ 184.9 (CO(OtBu)), 115.0 (Cp), 90.7 (C(CH3)3), 83.2
(ZrOC(Me)2), 48.9 (C(Me)2CO2tBu), 28.1 (C(CH3)3CO2tBu), 27.3
(ZrOC(CH3)2), 21.6 (C(CH3)2CO2tBu). The NMR data for the
(CD2Cl2): δ 148.3 (dm, JC-F ) 238, o-C6F5), 137.4 (dm, JC-F
)
257, p-C6F5), 136.4 (dm, JC-F ) 245, m-C6F5), 128.9 (Cipso), 9.9
(br, BCH3). 11B NMR (THF-d8): δ -14.8 (s, BCH3). 11B NMR
(CD2Cl2): δ -14.9 (s, BCH3). 19F NMR (THF-d8): δ -134.5 (d,
3
3JF-F ) 21, 6F, o-F), -168.5 (t, JF-F ) 21, 3F, p-F), -170.6
(m, 3JF-F ) 18, 6F, m-F). 19F NMR (CD2Cl2): δ -133.6 (d, 3JF-F
) 18, 6F, o-F), -165.6 (t, 3JF-F ) 22, 3F, p-F), -168.2 (t, 3JF-F
) 22, 6F, m-F).
-
free anion MeB(C6F5)3 were the same as those described
above. Compound 3 also decomposes at room temperature in
CD2Cl2 solution with release of isobutene and formation of a
thus far unidentified compound, tentatively formulated “[Cp2-
Zr(µ2-O2CCMe2CMe2OH]+”, which has the following key NMR
resonances: 1H NMR (CD2Cl2): δ 6.62 (s, Cp), 1.24 (s, O2CC-
(CH3)2), 1.13 (s, C(CH3)2OH). 13C{1H} NMR (CD2Cl2): δ 188.2
(O2CC(CH3)2), 115.7 (Cp), 82.8 (C(CH3)2OH), 48.7 (O2CC(CH3)2),
26.5 (C(CH3)2OH), 21.4 (O2CC(CH3)2).
Reaction of [Cp2Zr(THF-d8)(O(tBuO)CdCMe2)][MeB-
(C6F5)3] (1-d8) with PhCOCH3 in THF-d8. Generation of
[Cp2Zr(OC(Me)(Ph)C(Me)2CO2tBu)][MeB(C6F5)3] (2). To a
solution of Cp2Zr(Me)(O(tBuO)CdCMe2) (21 mg, 0.055 mmol)
in THF-d8 (ca. 0.5 mL) in a Teflon-valved NMR tube was added
at room temperature B(C6F5)3 (28 mg, 0.055 mmol). The
solution was left for 10 min at 20 °C to ensure the complete
formation of 1-d8, then PhCOMe (6.6 mg, 0.055 mmol) was
added via a microsyringe. Immediately, the pale brown solu-
tion changed to a colorless solution, and NMR spectra were
recorded. The conversion of 1-d8 to 2 was virtually quantitative
Reaction of [Cp2Zr(THF-d8)(O(tBuO)CdCMe2)][MeB-
(C6F5)3] (1-d8) with tBuSH in THF-d8: Generation of
[Cp2Zr(THF-d8)(StBu)][MeB(C6F5)3] (4-d8). B(C6F5)3 (25.8
mg, 0.05 mmol) was added at room temperature to a solution
of Cp2Zr(Me)(O(tBuO)CdCMe2) (19.1 mg, 0.05 mmol) in THF-
d8 (ca. 0.5 mL) in a Teflon-valved NMR tube. The solution was
left for 10 min at room temperature to ensure complete
formation of 1-d8, and then tBuSH (4.5 mg, 0.05 mmol) was
added via a microsyringe. No color change of the pale brown
solution was noticed. 1H NMR spectra were periodically
recorded and revealed that the reaction was finished within
1.5 h to give 4-d8 in 95% NMR yield. Over 8 h, the complex
4-d8 transformed cleanly to complex 5-d8 (98% NMR yield).
NMR data for 4-d8: 1H NMR (THF-d8): δ 6.68 (s, 10H, Cp),
1.61 (s, 9H, tBu). 13C{1H} NMR (THF-d8): δ 114.5 (Cp), 53.9
(C(CH3)3), 35.8 (C(CH3)3). NMR data for tert-butyl isobu-
tyrate: 1H NMR (THF-d8): δ 2.43 (sept, J ) 6.0, 1H,
CH(CH3)2), 1.45 (s, 9H, C(CH3)3), 1.11 (d, J ) 6.0, 6H, CH-
1
after 2 min. H NMR (THF-d8): δ 7.46 (m, 2H, Ph), 7.31 (m,
2H, Ph), 7.22 (m, 1H, Ph), 6.46 (s, 10H, Cp), 1.78 (s, 3H, ZrOC-
(CH3)(Ph)), 1.69 (s, 9H, OC(CH3)3), 1.19 (s, 3H, C(CH3)2), 1.07
(s, 3H, C(CH3)2). 13C{1H} NMR (THF-d8): δ 183.0 (CO(OtBu)),
142.8 (Ph-C1), 128.0 (Ph), 127.5 (Ph), 127.2 (Ph), 114.8 (Cp),
87.2 (C(CH3)3), 85.3 (ZrOC(Me)(Ph)), 49.1 (C(Me)2), 27.5
(C(CH3)3), 26.1 (ZrOC(CH3)(Ph)), 22.5 (C(CH3)2), 20.8 (C(CH3)2).
Over 5 days, decomposition of 2 (52%, based on tBu reso-
nances) ensues with formation of isobutene and a thus far
unidentified compound, tentatively formulated “[Cp2Zr(µ2-O2-
CCMe2CMe(Ph)OH]+”, which has the following key H NMR
1
resonances (THF-d8): δ 7.53-7.22 (m, Ph), 6.50 (s, 10H, Cp),
1.61 (s, 3H, CCH3(Ph)OH), 1.25 (s, 3H, C(CH3)2), 1.05 (s, 3H,
C(CH3)2). NMR data for isobutene: 1H NMR (THF-d8): δ 4.62
(m, 4J ) 1.0, 2H, CH2d), 1.69 (t, 4J ) 1.0, 6H, dC(CH3)2).
13C{1H} NMR (THF-d8): δ 142.0 (dC(CH3)2), 110.4 (CH2d), 23.6
(dC(CH3)2).
1
(CH3)2). NMR data for the cation 5-d8. H NMR (THF-d8): δ
6.50 (s, 10H, Cp), 2.53 (m, 3J ) 7.0, 1H, CH(CH3)2), 1.18 (d, 3J
) 7.0, 6H, CH(CH3)2). 13C{1H} NMR (THF-d8): δ 192.4 (O2C),
116.4 (Cp), 35.2 (CH(CH3)2), 16.7 (CH(CH3)2). The reactions
of 1-d8 with 2 and 3 equiv of tBuSH were also investigated
and found to proceed somewhat faster with the same selectiv-
ity.
Reaction of [Cp2Zr(THF-d8)(O(tBuO)CdCMe2)][MeB-
(C6F5)3] (1-d8) with Acetone in THF-d8. Generation of
[Cp2Zr(OC(Me)2C(Me)2CO2tBu)][MeB(C6F5)3] (3). This re-
action was conducted as described above starting from Cp2-
Zr(Me)(O(tBuO)CdCMe2) (17.8 mg, 0.047 mmol), B(C6F5)3 (24
mg, 0.047 mmol), and CH3COCH3 (2.7 mg, 0.047 mmol). The
conversion of 1-d8 to 3 was virtually quantitative after 2 min.
1H NMR (THF-d8): δ 6.38 (s, 10H, Cp), 1.66 (s, 9H, OC(CH3)3),
1.27 (s, 6H, C(CH3)2CO2tBu), 1.23 (s, 6H, (CH3)2COZr). 13C{1H}
NMR (THF-d8): δ 182.8 (CO(OtBu)), 114.5 (Cp), 86.5 (C(CH3)3),
Reaction of [Cp2Zr(THF-d8)(O(tBuO)CdCMe2)][MeB-
(C6F5)3] (1-d8) with p-ClC6H4SH in THF-d8: Generation
of [Cp2Zr(THF-d8)(SC6H4-p-Cl)][MeB(C6F5)3] (6-d8). This
reaction was conducted as described above starting from [Cp2-
Zr(Me)(O(tBuO)CdCMe2)] (20.9 mg, 0.055 mmol), B(C6F5)3
(28.2 mg, 0.055 mmol), and p-ClC6H4SH (8.0 mg, 0.055 mmol).
The color changed from pale brown to bright yellow upon
addition of the thiophenol. 1H NMR spectra were periodically
recorded and revealed that the reaction was finished within
10 min to give 6-d8 in 95% NMR yield. NMR data for 6-d8:
1H NMR (THF-d8): δ 7.49-7.21 (m, 4H, Ph), 6.56 (s, 10H, Cp).
Over 30 min, complex 6-d8 decomposed cleanly to complex 5-d8
(98% NMR yield), isobutene, and p-ClC6H4SH. NMR data for
p-ClC6H4SH: 1H NMR (THF-d8): δ 7.49-7.21 (m, 4H, Ph),
4.41 (s, 1H, SH). The NMR data for 5-d8 and isobutene were
the same as those described above.
Reaction of [Cp2Zr(THF-d8)(O(tBuO)CdCMe2)][MeB-
(C6F5)3] (1-d8) with o-MeOC6H4SH in THF-d8: Generation
of [Cp2Zr(THF-d8)(SC6H4-o-Me)][MeB(C6F5)3] (7-d8). This
reaction was conducted as described above starting from Cp2-
Zr(Me)(O(tBuO)CdCMe2) (20.9 mg, 0.055 mmol), B(C6F5)3
(28.2 mg, 0.055 mmol), and o-MeOC6H4SH (7.8 mg, 0.055
mmol). The color changed from pale brown to bright yellow
upon addition of the thiophenol. 1H NMR spectra were
periodically recorded and revealed that the reaction was
finished within 10 min to give 7-d8 in 95% NMR yield. NMR
t
82.3 (ZrOC(Me)2), 48.8 (C(Me)2CO2 Bu), 27.4 (C(CH3)3), 26.2
(ZrOC(CH3)2), 21.1 (C(CH3)2CO2tBu). Over 5 days at room
temperature, 74% of compound 3 decomposed (based on tBu
resonances) with release of isobutene and formation of a thus
far unidentified compound, tentatively formulated “[Cp2Zr(µ2-
O2CCMe2CMe2OH]+”, which has the following key 1H NMR
resonances (THF-d8): δ 6.50 (s, 10H, Cp), 1.19 (s, 3H, O2CC-
(CH3)2), 1.15 (s, 3H, C(CH3)2OH).
Reaction of [Cp2Zr(THF)(O(tBuO)CdCMe2)][MeB(C6-
F5)3] (1‚THF) with Acetone. Synthesis of [Cp2Zr(OC-
(Me)2C(Me)2CO2tBu)][MeB(C6F5)3] (3). This reaction was
conducted as described above starting from Cp2Zr(Me)-
(O(tBuO)CdCMe2) (36 mg, 0.094 mmol), B(C6F5)3 (48 mg, 0.094
(18) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994,
116, 10015-10031. (b) Horton, A. D.; de With, J.; Linden, A. J. v. d.;
Weg, H. v. d. Organometallics 1996, 15, 2672-2674. (c) Bochmann,
M.; Green, M. L. H.; Powell, A. K.; Sassmannshausen, J.; Triller, M.
U.; Wocadlo, S. J. Chem. Soc., Dalton Trans. 1999, 43-49. (d)
Carpentier, J.-F.; Wu, Z.; Lee, C. W.; Stro¨mberg, S.; Christopher, J.
N.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 7750-7767.

2472 Organometallics, Vol. 24, No. 10, 2005
Lian et al.
data for 7-d8: 1H NMR (THF-d8): δ 7.28 (t, J ) 6, 1H, Ph),
7.20 (d, J ) 6, 1H, Ph), 7.12 (d, J ) 6, 1H, Ph), 6.98 (t, J ) 6,
1H, Ph), 6.51 (s, 10H, Cp), 3.85 (s, 3H, OCH3). 13C{1H} NMR
(THF-d8): δ 156.3 (Ph), 132.7 (Ph), 128.6 (Ph), 120.8 (Ph), 115.2
(Cp), 111.5 (Ph), 55.1 (OCH3). Over 30 min, complex 7-d8
transformed cleanly to complex 5-d8 (98% NMR yield),
isobutene, and o-MeOPhSH. NMR data for o-MeOPhSH: 1H
NMR (THF-d8): δ 7.19 (d, J ) 5, 1H, Ph), 7.04 (t, J ) 5, 1H,
Ph), 6.88 (d, J ) 5, 1H, Ph), 6.77 (t, J ) 5, 1H, Ph), 4.05 (s,
1H, SH), 3.83 (s, 3H, OCH3). 13C{1H} NMR (THF-d8): δ 153.7
(Ph), 127.3 (Ph), 124.0 (Ph), 118.9 (Ph), 108.8 (Cp), 52.7
(OCH3). The NMR data for the free anion MeB(C6F5)3-, 5-d8,
and isobutene were the same as those described above.
Synthesis of [Cp2Zr(THF)(SC6H4-o-OMe)][MeB(C6F5)3]
(7). In the glovebox, a flask was charged with Cp2ZrMe2 (65
mg, 0.26 mmol) and B(C6F5)3 (132 mg, 0.26 mmol). THF (5
mL) and o-MeOC6H4SH (37 mg, 0.26 mmol) were added
sequentially at room temperature. Evaporation of volatiles in
vacuo left a yellow powder that was recrystallized from
toluene/THF (3:1) at -34 °C to give yellow crystals of 7 (230
mg, 92%). A suitable crystal was selected for X-ray diffraction
analysis. Anal. Calcd for C40H28BF15O2SZr (959.72): C, 50.06;
H, 2.94. Found: C, 50.94; H, 2.68.
Reaction of [Cp2Zr(THF)(StBu)][MeB(C6F5)3] (4) with
(CH3)2CHCO2tBu in CD2Cl2. Generation of [Cp2Zr(THF)-
(O2CiPr)][MeB(C6F5)3] (5). An NMR tube containing a
solution of 4 in THF (generated from Cp2ZrMe2 (21.2 mg, 0.084
mmol), tBuSH (15.2 mg, 0.17 mmol), and B(C6F5)3 (43 mg,
0.084 mmol)) was dried in a vacuum, and CD2Cl2 (ca. 0.5 mL)
was vacuum transferred. Then, (CH3)2CHCO2tBu (12.2 mg,
1
0.084 mmol) was added via microsyringe. H NMR revealed
complete conversion of 4 to 5 within 10 min and release of 1
equiv of isobutene and tBuSH. NMR data for the cation of 4:
1H NMR (CD2Cl2): δ 6.60 (s, 10H, Cp), 3.83 (m, 8H, OCH2-
t
CH2), 1.94 (m, 8H, OCH2CH2), 1.66 (s, 9H, Bu). NMR data
for tBuSH: 1H NMR (CD2Cl2): δ 1.66 (s, 1H, SH), 1.46 (s, 9H,
C(CH3)3). 1H NMR for the cation of 5 (CD2Cl2): δ 6.42 (s, 10H,
Cp), 4.10 (m, 4H, OCH2CH2), 2.58 (m, 3J ) 7.0, 1H, CH(CH3)2),
2.17 (m, 4H, OCH2CH2), 1.24 (d, 3J ) 7.0, 6H, CH(CH3)2).
13C{1H} NMR (CD2Cl2): δ 193.0 (O2C), 116.1 (Cp), 75.3(OCH2-
CH2), 35.4 (CH(CH3)2), 25.2 (OCH2CH2), 17.2 (CH(CH3)2). The
-
NMR data for the free anion MeB(C6F5)3 were the same as
those described above.
Crystal Structure Determination of Complex 7. A
suitable single crystal of 7 was mounted onto glass fibers using
the “oil-drop” method. Diffraction data were collected at 100
K using a NONIUS Kappa CCD diffractometer with graphite-
monochromatized Mo KR radiation (λ ) 0.71073 Å). A combi-
nation of ω- and æ-scans was carried out to obtain at least a
unique data set. Crystal structures were solved by means of
the Patterson method, and remaining atoms were located from
difference Fourier synthesis, followed by full-matrix least-
squares refinement based on F2 (programs SHELXS-97 and
SHELXL-97).19 Many hydrogen atoms could be found from the
Fourier difference. Carbon-bound hydrogen atoms were placed
at calculated positions and forced to ride on the attached
carbon atom. The hydrogen atom contributions were calculated
but not refined. All non-hydrogen atoms were refined with
anisotropic displacement parameters. The locations of the
largest peaks in the final difference Fourier map calculation
as well as the magnitude of the residual electron densities were
of no chemical significance. Crystal data and details of data
collection and structure refinement are given in Table 2.
Crystallographic data for 7 are also available as a cif file (see
Supporting Information).
Generation of Cp2Zr(Me)(StBu) (8). Cp2ZrMe2 (18.3 mg,
0.074 mmol) and tBuSH (13.3 mg, 0.15 mmol, 2 equiv) were
charged in a Teflon-valved NMR tube, and C6D6 (ca. 0.5 mL)
was vacuum transferred. The tube was sealed and kept at 80
°C for 6 h. NMR was recorded which revealed the quantitative
formation of 8, together with excess (1 equiv) tBuSH. 1H NMR
(C6D6): δ 5.80 (s, 10H, Cp), 1.51 (s, 9H, C(CH3)3), 0.13 (s, 3H,
1
ZrCH3). H NMR (THF-d8): δ 6.13 (s, 10H, Cp), 1.38 (s, 9H,
C(CH3)3), -0.15 (s, 3H, ZrCH3). 13C{1H} NMR (C6D6): δ 110.0
(Cp), 47.1 (C(CH3)3), 36.2 (C(CH3)3), 26.9 (ZrCH3).
Reaction of Cp2Zr(Me)(StBu) (8) and B(C6F5)3 in THF.
Synthesis of [Cp2Zr(THF)(StBu)][MeB(C6F5)3] (4). A C6D6
solution of 8 was prepared as described above starting from
Cp2ZrMe2 (51 mg, 0.203 mmol) and tBuSH (36 mg, 0.41 mmol,
1
2 equiv). After monitoring by H NMR to ensure completion
of the reaction, the solution was concentrated under vacuum
to remove excess tBuSH and THF (1.0 mL) was added. In the
glovebox, B(C6F5)3 (104 mg, 0.205 mmol) was added and the
solution was stirred for 15 min at room temperature. Volatiles
were removed under vacuum, and the residue was washed
with pentane (3 × 1 mL) and dried under vacuum to leave 4
as a yellow powder (150 mg, 81%). The NMR data for 4 were
the same as those described above. Anal. Calcd for C37H30BF15-
OSZr (909.71): C, 48.85; H, 3.32. Found: C, 49.24; H, 3.42.
Reaction of [Cp2Zr(THF-d8)(StBu)][MeB(C6F5)3] (4-d8)
with (CH3)2CHCO2tBu in THF-d8. Generation of [Cp2Zr-
(THF-d8)(O2CiPr)][MeB(C6F5)3] (5-d8). To the above NMR
tube (freshly prepared), (CH3)2CHCO2tBu (10.1 mg, 0.074
mmol) was added via microsyringe. The tube was sealed and
1H NMR was recorded periodically. Complete conversion of
4-d8 to 5-d8 and release of 1 equiv of isobutene and tBuSH
was observed over 8 h. The NMR data for 5-d8 were the same
as those described above.
Typical Procedure for MMA Polymerization. In the
glovebox, to a 10 mL vial equipped with a magnetic stirrer
and containing a solution of the Zr catalyst (0.05 mmol) in
toluene or THF (1.0 mL), was added the activator (B(C6F5)3,
[HNMe2Ph][B(C6F5)4], or [Ph3C][B(C6F5)4] (0.05 mmol) in
toluene or THF (1.0 mL). The solution was stirred, and MMA
(10.0 mmol) (or a mixture of MMA and CTA) was added by
syringe within 20 s. The polymerization was carried out for
1-3 h at 20 °C and quenched by addition of acidified methanol
(3% HCl, 200 mL). The polymer was filtered and dried
overnight under vacuum at 60 °C. The microstructure of the
1
PMMAs was determined by H NMR in CDCl3 and found in
all cases to be in the range 63-65% rr, 5-8% mm, and 29-
33% mr.
Acknowledgment. We thank Total and Arkema Co.
(formerly AtoFina) (grant to B.L.) and the Centre
National de la Recherche Scientifique for financial
support of this work.
Reaction of [Cp2Zr(THF-d8)(StBu)][MeB(C6F5)3] (4-d8)
with CH3CO2tBu in THF-d8. Generation of [Cp2Zr(THF-
d8)(O2CCH3)][MeB(C6F5)3] (9-d8). To an NMR tube contain-
ing a solution of 4-d8 in THF-d8 (generated from Cp2ZrMe2
(16.8 mg, 0.067 mmol), tBuSH (12 mg, 0.13 mmol), and
B(C6F5)3 (34 mg, 0.067 mmol)) was added CH3CO2tBu (7.7 mg,
0.067 mmol) via microsyringe. The tube was sealed and 1H
NMR was recorded periodically. Complete conversion of 4-d8
to [Cp2Zr(THF-d8)(O2CCH3)][MeB(C6F5)3] and release of 1
equiv of isobutene and tBuSH was observed over 4 h. NMR
data for 9-d8: 1H NMR (THF-d8): δ 6.50 (s, 10H, Cp), 2.07 (s,
3H, CH3). 13C{1H} NMR (THF-d8): δ 185.9 (CH3CO2), 114.5
Supporting Information Available: Crystallographic
data for 7 as a CIF file. This material is available free of charge
via the Internet at http://pubs.acs.org
OM050119M
(19) (a) Sheldrick, G. M. SHELXS-97, Program for the Determina-
tion of Crystal Structures; University of Goettingen: Germany, 1997.
(b) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structures; University of Goettingen: Germany, 1997.
-
(Cp), 20.0 (CH3). The NMR data for the free anion MeB(C6F5)3
and isobutene were the same as those described above.