Highly isospecific polymerization of methyl methacrylate with a bis(pyrrolylaldiminato) samarium hydrocarbyl complex

Article abstract of DOI:10.1021/om030454f

New types of samarium alkyl complexes, L₂-SmMe (THF) (2) and L₂SmCH₂SiMe₃(THF) (3), supported by the pyrrolylaldiminato ligand L (L = [2-(2,6-iPr₂C₆H ₃N=CH)-5-tBuC₄H₂N]^-), were prepared from the corresponding chloride L₂SmCl(THF) (1) with the appropriate alkyllithium reagents; the corresponding yttrium alkyl L₂YCH ₂SiM₃ (4) was prepared similarly. The molecular structures of 1 and 3 were determined by X-ray single-crystal analyses. Compound 3 initializes the stereospecific polymerization of MMA to yield highly isotactic PMMA (mm triad 94.8%) with high molecular weight and narrow molecular distribution at room temperature while 2 is inactive. The possible initial step of the polymerization reaction was proposed.

Full text of DOI:10.1021/om030454f

Organometallics 2003, 22, 3357-3359
3357
High ly Isosp ecific P olym er iza tion of Meth yl
Meth a cr yla te w ith a Bis(p yr r olyla ld im in a to)sa m a r iu m
Hyd r oca r byl Com p lex
Chunming Cui, Alexandr Shafir, Craig L. Reeder, and J ohn Arnold*
Department of Chemistry, University of California at Berkeley,
Berkeley, California 94720-1460
Received J une 12, 2003
Summary: New types of samarium alkyl complexes, L2-
SmMe (THF) (2) and L2SmCH2SiMe3(THF) (3), sup-
ported by the pyrrolylaldiminato ligand L (L ) [2-(2,6-
iPr2C6H3NdCH)-5-tBuC4H2N]-), were prepared from the
corresponding chloride L2SmCl(THF) (1) with the ap-
propriate alkyllithium reagents; the corresponding yt-
trium alkyl L2YCH2SiMe3 (4) was prepared similarly.
The molecular structures of 1 and 3 were determined
by X-ray single-crystal analyses. Compound 3 initializes
the stereospecific polymerization of MMA to yield highly
isotactic PMMA (mm triad 94.8%) with high molecular
weight and narrow molecular distribution at room
temperature while 2 is inactive. The possible initial step
of the polymerization reaction was proposed.
of the known lanthanide systems show high stereoregu-
larity (mm or rr triad >90%) for MMA polymerization
at room temperature, which is a current strategic
objective.
We communicate herein that a non-metallocene, one-
component lanthanide system initializes polymerization
of MMA to generate highly isotactic PMMA with high
molecular weight even at elevated temperature. The
system is based on a bulky pyrroylaldiminato ligand16
and does not contain any chiral groups.
The samarium chloride L2SmCl(THF) (1) was ob-
tained by the reaction of KL (L ) [2-(CHdNC6H3-2,6-
iPr2)-5-tBuC4N]-) with SmCl3 in THF. The correspond-
ing hydrocarbyls L2SmR(THF) (R ) Me (2), CH2SiMe3
(3)) were prepared in high yield by the reaction of 1 with
the appropriate alkyllithium in toluene (Scheme 1). The
1H NMR spectra of the paramagnetic 2 and 3 include
the broad resonances of one coordinated THF molecule
and the R-protons for the alkyl moieties. The yttrium
compound L2YCH2SiMe3 (4) was prepared similarly.
Compound 4 is THF free, is monomeric (doublet of CH2-
SiMe3, δ 44.61, 44.17; 1J Y,C ) 44.2 Hz), and is produced
A substantial amount of work has been devoted to the
development of well-defined single-component catalytic
systems for polymerization of polar functional mono-
mers in a controlled fashion.1 Pioneering work by
Yasuda and Collins has shown that lanthanocenes2 and
cationic zirconocenes3 are excellent precursors for the
controlled polymerization of methyl methacrylate (MMA).
Recent studies have shown that ansa-metallocenes of
cationic group 4 enolates and multiple-component cata-
lytic systems can polymerize MMA stereospecifically
under certain conditions.3-5 In addition, a number of
lanthanocenes have been reported to generate syndio-
rich poly(methyl methacrylates) (PMMAs) under mild
conditions;6-15 while C1-symmetric ansa-bridged lan-
thanocenes were reported to induce isospecific polym-
erization of MMA at low temperature.8 However, none
1
as one diastereomer, as indicated by its H and 13C NMR
spectra. Attempts to prepare the methyl derivative in
this case have, thus far, been unsuccessful. The NMR
spectrum of compound 4 is unperturbed by addition of
several equivalents of THF in C6D6, and the solid
compound shows no evidence of coordinated solvent
when isolated in the presence of THF.
The molecular structures of 1 and 3, as determined
by single-crystal X-ray analyses,17 are shown in Figure
1, along with selected bond distances and angles. Both
1 and 3 adopt a distorted-octahedral geometry with the
THF oxygen located trans to one pyrrole nitrogen (O-
Sm-N angle 160.4(2)° in 1 and 160.37(9)° in 3). The
most striking features are the acute N-Sm-N angles
(average 70.3° in 1 and 69.12° in 3) of the chelating
ligand. Importantly, both compounds display molecular
C1 symmetry while the two pyrrolyaldiminato ligands
adopt an approximate C2 arrangement. The Sm-C bond
length in 3 is comparable to those found in a handful of
samarium alkyls characterized by X-ray diffraction
analysis, (C5Me5)2SmR(THF) (R ) Me, 2.484(14) Å;
R ) benzyl, 2.498(5) Å; R ) Ph, 2.511(5) Å),18-20 and
(1) Yasuda, H. J . Organomet. Chem. 2002, 647, 128-138.
(2) Yasuda, H.; Yamamoto, H.; Yokota, K.; Miyake, S.; Nakamura,
A. J . Am. Chem. Soc. 1992, 114, 4908-4910.
(3) Collins, S.; Ward, D. G. J . Am. Chem. Soc. 1992, 114, 5460-
5462.
(4) Bolig, A. D.; Chen, E. Y. X. J . Am. Chem. Soc. 2001, 123, 7943-
7944.
(5) Deng, H.; Shiono, T.; Soga, K. Macromolecules 1995, 28, 3067.
(6) Bala, M. D.; Huang, J . L.; Zhang, H.; Qian, Y. L.; Sun, J . Q.;
Liang, C. F. J . Organomet. Chem. 2002, 647, 105-113.
(7) Boffa, L. S.; Novak, B. M. J . Mol. Catal. A: Chem. 1998, 133,
123-130.
(8) Giardello, M. A.; Yamamoto, Y.; Brard, L.; Marks, T. J . J . Am.
Chem. Soc. 1995, 117, 3276-3277.
(9) Hou, Z. M.; Wakatsuki, Y. Coord. Chem. Rev. 2002, 231, 1-22.
(10) Ihara, E.; Taguchi, M.; Yasuda, H. Appl. Organomet. Chem.
1995, 9, 427-429.
(11) Li, F. F.; J in, Y. T.; Song, C. L.; Lin, Y. H.; Pei, F. K.; Wang, F.
S.; Hu, N. H. Appl. Organomet. Chem. 1996, 10, 761-771.
(12) Nodono, M.; Tokimitsu, T.; Tone, S.; Makino, T.; Yanagase, A.
Macromol. Chem. Phys. 2000, 201, 2282-2288.
(13) Yasuda, H.; Ihara, E. J . Synth. Org. Chem. J pn. 1993, 51, 931-
941.
(14) Ying, L. Q.; Ba, X. W.; Zhao, Y. Y.; Li, G.; Tang, T.; J in, Y. T.
Chin. J . Polym. Sci. 2001, 19, 85-87.
(15) Zi, G. F.; Li, H. W.; Xie, Z. W. Organometallics 2002, 21, 1136-
1145.
(16) Hao, H. J .; Bhandari, S.; Ding, Y. Q.; Roesky, H. W.; Magull,
J .; Schmidt, H. G.; Noltemeyer, M.; Cui, C. M. Eur. J . Inorg. Chem.
2002, 1060-1065. For transition metal chemistry with less hindered
examples, see: Matsui, S.; Spaniol, T. P.; Takagi, Y.; Yoshida, Y.;
Okuda, J . J . Chem. Soc., Dalton Trans. 2002, 4529, and refs therein.
(17) See the Supporting Information for X-ray crystal data for
complexes 1 and 2.
10.1021/om030454f CCC: $25.00 © 2003 American Chemical Society
Publication on Web 07/17/2003

3358 Organometallics, Vol. 22, No. 17, 2003
Communications
Sch em e 1
Ta ble 1. Su m m a r y of P olym er iza tion Resu lts a n d P olym er P r op er ties w ith Com p lexes 3 a n d 4
c
c
initiator entry MMA/3 (mol/mol) time (h) T (°C) yield (%) 10-4Mn Mw/Mn Tge (°C) [mm]d (%) [mr] (%) [rr] (%) 2[rr]/[mr]
3
3
3
3
3
3
3
3
3
3
4
4
4
1a
500
500
1
2
1
1
2
2
3
10
4
0.5
40
0
100
100
65
60
60
30
75
80
100
80
3.27
5.92
0.75
1.35
15.8
4.47
10.6
20.2
16
141
1.49
1.48
1.24
1.65
1.86
1.98
1.47
1.63
1.57
1.40
1.31
1.72
1.46
1.48
93.8
97.3
94.5
94.7
94.8
91.3
94.9
94.9
97.8
91.8
81.9
84.9
87.2
5.2
2.1
4.7
4.3
4.1
6.7
3.9
4.3
1.7
6.1
12.3
10.2
8.7
1.0
0.6
0.8
1.0
1.1
2.0
1.2
0.8
0.5
2.1
5.8
4.8
4.1
0.38
0.57
0.33
0.47
0.54
0.60
0.62
0.37
0.58
0.69
0.94
0.94
0.94
2a
56.4
3a
1000
2000
3000
1000
1000
1000
1000
1000
1000
1000
1000
23
23
23
65
23
23
0
23
23
0
4a
5a
59.6
47.2
55.4
57.5
6a
7a
8a
9a
10b
11
12
13
2
2
8
100
100
60
0.92
6.23
-30
a
b
d
Polymerization in toluene solution. Bulk polymerization. c From GPC relative to polystyrene standards. From 1H NMR in CDCl3.
e From DSC.
the ate complex Li(THF)2[Sm(OAr)3(CH2SiMe3)2] (2.451-
ticity of the PMMA samples obtained declined slightly
with increasing reaction temperature. At 65 °C, the
PMMA yield is relatively low, presumably due to ther-
mally induced chain termination steps. The high iso-
tacticity is observed even in the bulk polymerization
process, although the mm triad content (91.8%) is
slightly low due to high exothermicity of the polymer-
ization reaction.
1
(10) Å, Ar ) 2,6-iPr2C6H3).21 The H NMR spectrum of
the paramagnetic 3 in toluene-d8 shows two broad
singlets (δ 10.9, 12.4 ppm) for the diastereotopic protons
of the methylene group in CH2SiMe3, consistent with
the solid-state C1-symmetric structure.
The alkyl compounds 2-4 have been investigated as
initiators for the polymerization of methyl methacrylate
(MMA). Compounds 3 and 4 were found to be highly
active and isospecific for the MMA polymerization in
toluene (Table 1, entries 1-9) and in bulk MMA (entry
10), as shown by the triad contents obtained from the
1H NMR spectra22,23 (Table 1). The samarium species 3
showed excellent stereoregularity in a wide range of
temperatures (-40 to +65 °C, mm triad content ranging
from 91.3 to 97.8%). This feature sets compound 3 apart
from known lanthanocenes, which generated syndio-rich
or atactic PMMA at normal temperature. The isotac-
Remarkably, the bulk polymerization with 3 gener-
ates very high molecular weight isotactic PMMA (Mn )
1.41 × 106, PDI ) 1.31), which, to the best of our
knowledge, has not been obtained with known catalytic
systems. As can be seen in Table 1, the yttrium complex
4 shows relatively low selectivity. The polydispersity of
the PMMA is relatively narrow (1.24-1.98), indicating
a single-site catalytic system. (For comparison, chiral
lanthanocene systems that yield isotactic PMMA show
PDIs in the range 1.8-7.9.8) The PDI is slightly higher
with an increase in the MMA/3 ratio and reaction
temperature. The Tg values are consistent with the
reported values for isotactic PMMA.4 The 1H NMR
spectra of the PMMA samples showed characteristic
traces of the SiMe3 group (δ 0.04 ppm), suggesting that
the initial step of polymerization proceeds by a coordi-
nation insertion mechanism (Scheme 2). Furthermore,
MS analysis of a hydrolyzed sample of the reaction
mixture from 3 and MMA (1:5 molar ratio) exhibited
ions of the formula HC(Me)(CO2Me)CH2[C(Me)(CO2Me)-
(18) Evans, W. J .; Bloom, I.; Hunter, W. E.; Atwood, J . L. Organo-
metallics 1985, 4, 112-119.
(19) Evans, W. J .; Chamberlain, L. R.; Ulibarri, T. A.; Ziller, J . W.
J . Am. Chem. Soc. 1988, 110, 6423-6432.
(20) Evans, W. J .; Ulibarri, T. A.; Ziller, J . W. Organometallics 1991,
10, 134-142.
(21) Clark, D. L.; Gordon, J . C.; Huffman, J . C.; Watkin, J . G.; Zwick,
B. D. Organometallics 1994, 13, 4266-4270.
(22) Chujo, R. H., K.; Kitamura, R.; Kitayama, T.; Sato, H.; Tanaka,
Y. Polym. J . 1987, 413-424.
(23) Subramanian, R. A., R. D.; McGrath, J . E.; Ward, T. C. Polym.
Prepr., Am. Chem. Soc., Div. Polym. Chem. 1985, 26, 238-240.

Communications
Organometallics, Vol. 22, No. 17, 2003 3359
Sch em e 2
eluded characterization to date. One possible explana-
tion is that 2 does not form an active enolate species
with MMA, due to a slower rate of dissociation of the
coordinated THF. Instead, the methyl group may di-
rectly attack the MMA carbonyl to deactivate the
initiator. Consistent with this notion, we saw no evi-
dence for polymerization when the same procedures
were conducted in THF or in toluene/THF (15:1 v:v)
solutions of 3 and 4. Further experiments to probe the
mechanism are in progress.
The triad and pendant content tests of the PMMA
samples obtained from initiator 3 indicate that the
stereocontrol mechanism conforms to neither enantio-
morphic site control nor chain end control, while those
from 4 are produced by an enantiomorphic site control
mechanism (2[rr]/[mr] close to 1).24 Although the de-
tailed stereocontrol mechanism is not available at this
stage, it is assumed that the steric environment around
the metal center may hinder isomerization of lanthanide
enolate species and thus MMA coordination always
takes place at a fixed site (Scheme 2). This mechanism
has been proposed previously for a chiral lanthanocene
catalytic system.8
F igu r e 1. ORTEP drawings of 1 (top) and 3 (bottom) with
thermal ellipsoids at 50% probability. Hydrogen atoms
have been omitted for clarity. Selected bond lengths (Å)
and angles (deg) for 1: Sm1-Cl1 ) 2.625(2), Sm1-O1 )
2.497(6), Sm1-N1 ) 2.426(7), Sm1-N2 ) 2.429(1), Sm1-
N3 ) 2.537(7), Sm1-N4 2.570(7); ) N1-Sm1-N3 ) 69.9-
(3), N2-Sm-N4 ) 70.7(3), O1-Sm1-N3 ) 160.4(2), Cl1-
Sm1-N4 ) 156.0(2), N1-Sm1-N2 ) 141.6. Selected bond
lengths (Å) and angles (deg) for 3: Sm1-C29 ) 2.455(4),
Sm1-O1 ) 2.578(3), Sm1-N3 ) 2.464(3), Sm1-N4 )
2.467(3), Sm1-N5 ) 2.588(3), Sm1-N6 ) 2.626(3); N3-
Sm1-N5 ) 69.13(10), N4-Sm1-N6 ) 69.11(9), O1-Sm1-
N5 ) 160.37(9), N3-Sm1-N4 ) 141.70(10), N6-Sm1-C29
) 155.6(1).
In summary, we have developed a new type of
lanthanide system incorporating a bulky pyrroylaldi-
minato ligand, which exhibits unprecedented stereo-
regularity for the polymerization of MMA at ambient
temperatures. The polymerization proceeds initially by
a coordination insertion mechanism. Further studies on
kinetics and mechanistic details of the polymerization
as well as on other applications of these type of com-
plexes are in progress.
CH2]nCH2SiMe3 (n ) 0-6), corresponding to the oligo-
mers resulting from the initial insertion of the CH2-
SiMe3 group into the MMA molecules.
Ack n ow led gm en t. We thank the NSF and DOE for
support of this work. We are grateful to the reviewers
for their insightful comments.
The reaction of 3 and 4 with 1 equiv of MMA in C6D6
only led to the consumption of ca. 20% of 3 and 4,
respectively. 1H NMR spectra revealed that even when
a large excess (40 equiv) of MMA was employed, a trace
of the initiator was always observed, indicating that the
first step of the reaction (MMA insertion into the M-C
bond to form lanthanide enolate2) is much slower than
chain propagation.
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving crystallographic data for complexes 1 and 3, experi-
mental details for the synthesis and characterization of 1-4,
and details of the polymerization process. This material is
available free of charge via the Internet at http://pubs.acs.org.
In contrast, compound 2 is inactive for MMA polym-
erization. Instead, it reacts completely with 1 equiv of
MMA immediately in C6D6. The 1H NMR spectrum
indicates the presence of multiple compounds that have
OM030454F
(24) Sogah, D. Y.; Hertler, W. R.: Webster, O. W.; Cohen, G. M.
Macromolecules 1987, 20, 1473-1488.