An efficient method for controlled propylene oxide polymerization: The significance of bimetallic activation in aluminum Lewis acids

Article abstract of DOI:10.1002/anie.200390054

Combined strength: Only the combination of aluminate complexes with their neutral Lewis acids allows the controlled regioregular ring-opening polymerization of industrially important propylene oxide. The chain propagation proceeds at both aluminium center

Full text of DOI:10.1002/anie.200390054

Communications
In summary, the results of the measurements and the
simulations demonstrate that orientation selection affects
the DEER response. For az-1 it suggests that in solution more
conformations are accessible than in the crystal, an aspect
which is currently undergoing detailed analysis. With respect
to the determination of distances in other systems, usually
enough information about the relative g tensor orientation is
known to allow the distances to be determined with sufficient
accuracy. If the relative orientation is not known, the dipolar
frequency ndip will differ by a factor of 2 between the two
extreme angles qAB ¼ 08 and 908. The resulting difference in
rAB of 25% is therefore the maximum uncertainty expected
from orientation effects. Finally, the study demonstrates that
DEER can be applied to the determination of structures of
compounds which contain paramagnetic transition-metal
ions, and thus provides an additional method for structural
investigations on biological systems. This is particularly
important given the physiological significance of metal ions.
Propylene Oxide Polymerization
An Efficient Method for Controlled Propylene
Oxide Polymerization: The Significance of
Bimetallic Activation in Aluminum Lewis
Acids**
Wigand Braune and Jun Okuda*
Propylene oxide (PO) is with a worldwide production of 4.5
million tons a significant intermediate of the petrochemical
industry. Two-thirds of the propylene oxide is converted by
ring-opening polymerization into polypropylene glycols and
propoxylated products of polyurethane.[1] The classical ini-
tiator for this reaction is KOH, whose high basicity leads to
deprotonation of the methyl group, resulting in side reac-
tions.[2] Rapid and controlled PO polymerization has recently
become possible through so-called double-metal cyanide
compounds (DMC), whose structure and mechanism is,
however, still unclear.[3] Only a small number of structurally
characterized initiators for living coordination PO polymer-
ization has been published so far: aluminum complexes with
tri- or tetradentate ligands (porphyrins, phthalocyanine,
tetraazaannulene, salen-type ligands, diethylenetriamine)
polymerize PO by a mechanism based on chain growth at a
single metal center by reaction of the monomer with the
coordinated alkoxo ligand (in addition, cis migration, rear
attack, and participation of two metal centers are also
assumed).[4] The sterically demanding Lewis acidic organo-
aluminum complexes accelerate the coordination ring-open-
ing polymerization.[5] We report here the targeted synthesis of
new aluminate complexes and their polymerization proper-
ties in combination with their neutral Lewis acid precursors.
Our results prove for the first time that PO polymerization
does not occur at a single Lewis acidic metal center and
confirm the earlier proposal of bimetallic activation by Price
and Vandenberg.[6,7]
Experimental Section
Spectra were measured on a Bruker Elexsys E680 spectrometer with
modifications to the microwave bridge to introduce the second
microwave frequency analogous to those described by Pannier et al.[3]
using a HP synthesizer HP 83752B and a microwave amplifier
(Microwave Amplifiers AL 16-9 10-15). The four-pulse DEER
sequence[3] with pulse lengths of 16 ns for p/2, 32 ns for p pulses,
and t ¼ 896 ns was employed. Pulse powers were adjusted to equal
intensity at nobs and npump. The maximum frequency separation of nobs
and npump was estimated by detuning the measurement frequency from
the resonator frequency. At offsets of Æ 37.5mHz attenuation of the
signal was observed, leading to the choice of nobsÀnpump of 75 MHz. At
that separation, the echo is reduced by less than a factor of 2. The
excitation bandwidth of the pulses is smaller than in the experiments
described by Pannier et al., where a maximum of 25 MHz is given.[3]
In the Fourier transformation (by using the Origin program (Micro-
cal(TM) Northhampton, USA) of the modulation, a Gaussian
background was subtracted from the experimental data, the resulting
curve was filtered with a Hamming window, and filled with zeros to
1024 points to a total time of 3.2 ms.
[{Al(L)Cl}2] (1: L ¼ mbmp, 2: mmcp), easily obtained by
reaction of AlEt2Cl with 2,2’-methylenebis(6-tert-butyl-4-
methylphenol) (mbmpH2) or 2,2’-methylenebis(4-methyl-6-
(1-methylcyclohexyl)phenol) (mmcpH2), as well as the iso-
propanolato complexes [{Al(L)(m-OiPr)}2] (3: L ¼ mbmp, 4:
mmcp), obtainable from trimethylaluminum in a two-step
procedure, did at first appear to be suitable for ring-opening
polymerization of PO (Scheme 1).[8,9] With these initiators we
could, however, only observe slow (> 24 h) and regioirregular
oligomerization of PO.[10,11] We suspect that the ring opening
Received: June 20, 2002 [Z19574]
[1] Distance Measurements in Biological Systems by EPR, (Eds.: L. J.
Berliner, S. S. Eaton, G. R. Eaton), Kluver Academic, New York,
2000.
[2] P. P. Borbat, A. J. Costa-Filho, K. A. Earle, J. K. Moscicki, J. H.
Freed, Science 2001, 291, 266 269.
[3] M. Pannier, S. Veit, A. Godt, G. Jeschke, H. W. Spiess, J. Magn.
Res. 2000, 142, 331 340.
[4] I. M. C. van Amsterdam, M. Ubbink, O. Einsle, A. Messer-
schmidt, A. Merli, D. Cavazzini, G. L. Rossi, G. W. Canters, Nat.
Struct. Biol. 2002, 9, 48 52.
[5] I. M. C. van Amsterdam, M. Ubbink, L. J. C. Jeuken, M. P.
Verbeet, O. Einsle, A. Messerschmidt, G. W. Canters, Chem.
Eur. J. 2001, 7, 2398 2406.
[6] R. Codd, A. V. Astashkin, A. Pacheco, A. M. Raitsimring, J. H.
Enemark, J. Biol. Inorg. Chem. 2002, 7, 338 350.
[7] R. G. Larsen, D. J. Singel, J. Chem. Phys. 1993, 98 5134 5146.
[8] J. A. Coremans, O. G. Poluektov, E. J. Groenen, G. W. Canters, H.
Nar, A. Messerschmidt, J. Am. Chem. Soc. 1994, 116, 3097 3101.
[*] Prof. Dr. J. Okuda, Dipl.-Chem. W. Braune
Institut f¸r Anorganische Chemie und Analytische Chemie
Johannes Gutenberg-Universit‰t Mainz
Duesbergweg 10 14, 55099 Mainz (Germany)
Fax: (þ49)6131 39 25605
E-mail: okuda@mail.uni-mainz.de
[**] This work has been supported by Bayer AG and the Fonds der
Chemischen Industrie. We are grateful to Dr. J. Hofmann for helpful
discussions, Dr. T. P. Spaniol for the crystallographic studies, and
Prof. Dr. M. Schmidt for the MALDI-TOF mass spectra.
64
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Figure 1. Polymerization of PO (0.21 mL) initiated by 3 and 5 ([3]0/[5]0/
[PO]0 ¼1:1:200) in CDCl3 (0.45 mL) at room temperature.
5 8 were not reactive towards ring-opening polymerization
of PO.
When neutral complexes 1, 3, and 4 were combined with
ate complexes 5 8 to form initiator systems [Al(L)(X)]2/
[Al(L)(X)2]À (L ¼ mbmp, mmcp; X ¼ Cl, OiPr), ring-opening
polymerization of PO occurred with pronounced speed and
1
control at room temperature. Figure 1 shows the H NMR
spectroscopically observed conversion of PO (0.21 mL) in
CDCl3 (0.45 mL) at room temperature with the initiator
system formed from equimolar quantities of 3 and 5, and with
a monomer/initiator ratio of 50:1 in accordance to the
mechanism discussed below. In Table 1, the polymerization
conversions at room temperature obtained with Lewis acid
precursor (LA) to aluminate (AT) ratios of 1.0 and 1.5 are
compared.
Scheme 1. Compounds 1 8.
is, at least partly, induced by the chelating bisphenolato
ligands.
Polymerization initiated by the isopropanolato systems is
clearly faster than that initiated by the corresponding chloro
systems. The most active initiator system was the combination
of 4 and 6 (Experiment 3, Table 1) with a conversion of 77%
after 180 min. The 1-methylcyclohexyl substituent of the
bisphenolato ligand mmcp in 4 may cause a significant shift of
the monomer dimer equilibrium in favor of the Lewis acidic
monomeric species.[8a] The cesium aluminate 7 in combination
with 3 did not initiate polymerization, addition of crown ether
([18]crown-6), however, allowed an activity comparable to
By reaction of [NEt4][OiPr] with 3 or 4 at À208C, the
aluminate [NEt4][Al(L)(OiPr)2] (5: L ¼ mbmp; 6: mmcp)
could be isolated in yields from 63 to 74% (Scheme 1). The
1H NMR spectra of these complexes in CDCl3 at room
temperature showed the presence of Cs symmetry, with two
sharp doublets for the methyl protons for each of the syn- and
3
anti-isopropoxy ligands (5: d ¼ 1.07, 1.15 ppm with JH,H
¼
3
5.9 Hz; 6: d ¼ 1.15, 1.12 ppm with JH,H ¼ 5.9 Hz). A crystal
structure determination of the alu-
minate (ate) complexes confirmed
Table 1: Polymerization of PO (2 mL) in CH2Cl2 (2 mL) with various systems composed of neutral Lewis
the expected tetrahedral coordina-
tion of the aluminum centers within
the anion, which has merely an
electrostatic interaction with the
cation.[12] To exclude the possibility
of participation of the cation in the
polymerization, the cesium alumi-
nate Cs[Al(mbmp)(OiPr)2] (7) was
prepared by reaction of 3 with
CsOiPr. We obtained the related di-
chloroaluminate [NEt4][Al(L)Cl2]
(8) quantitatively by direct reaction
between 1 and NEt4Cl. Like the
acid precursors (LA) and ate complexes (AT).
Exp. LATA [L]/TA[]A
t [min] Ratio
Conversion Mn
[gmolÀ1
Mtheor.
Mw/Mn
PO/active centers[a] [%]
]
[gmolÀ1
]
1
2
3
4
5
6
7
8
1
3
4
3
3
3
3
4
8
5
6
7
1.0
1.0
1.0
1.0
180
180
180
180
180
180
180
180
100
100
100
100
100
100
100
100
22
49
77
0
1190
2500
3580
1280
2850
4470
1.09
1.18
1.22
7[b] 1.0
7[c] 1.0
42
33
49
65
2270
1680
2320
3090
2440
1920
2850
3780
1.19
1.12
1.20
1.22
5
6
1.5
1.5
[a] The active centers are the alkoxo and chloro ligands of the initiator system. [b] With addition of two
neutral compounds 1 4, aluminates molar equivalents of [18]crown-6. [c] With addition of one molar equivalent of [18]crown-6.
Angew. Chem. Int. Ed. 2003, 42, No. 1
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65

Communications
We propose that the ring-opening polymerization proceeds as
shown in Scheme 2 under the synergic interaction of a
phenolatoaluminum complex with the corresponding ate
complex. The first step consists of the reaction of the dimeric
neutral complex with PO to form a labile adduct (step 1 in
Scheme 2).[8a,c,18] Ring opening of the epoxide activated by the
monomeric complex proceeds by transfer of an alkoxy group
from the corresponding ate complex; this is accompanied by
simultaneous regeneration of the aluminate (steps 2 and 3 in
Scheme 2). The activity of the initiator system based on the
cesium alkoxide with added crown ether shows that separa-
Figure 2. 13C NMR spectrum (CDCl3) of the polyether obtained with ini-
tiators 3 and 5 ([3]0/[5]0/[PO]0 ¼1:1:400, 49% conversion).
that of Experiment 2 to be achieved. GPC measurements of
the polyether showed polydispersities between 1.09 and 1.22.
To examine the polymer end groups more closely, we
separated the initiators obtained from equimolar combina-
tions of either 1 with 8 or 3 with 5 from their polymers with
molecular weights of 2.4 î 103 gmolÀ1 and 2.5 î 103 gmolÀ1,
respectively, by column chromatography. Figure 2 shows the
13C NMR spectrum of the polyether obtained with isopropa-
nolato complexes 3 and 5 in CDCl3.
The signals of the CH3- (a), CH2- (b), and CH groups (c)
appeared at d ¼ 17.2, 17.4 (a), 72.8, 73.2 (b), and 75.0, 75.2, 75.4
(c) ppm, and showed a chiral polymer with exclusive head-to-
tail bonding, with the four possible head-to-tail triads
appearing with the same frequencies in a purely statistical
distribution.[13,7a] The weaksignals arose from the end groups:
the isopropoxy group appeared at d ¼ 21.9, 22.0 (CH(CH3)2
(d)), and 71.8 ppm (CH(CH3)2 (e)), and the terminal methine
carbon atom CHOH (f) appeared at d ¼ 65.4 and
67.1 ppm.[5b,14] The resonance signal of the terminal methylene
carbon atom CH2OiPr (g) was found at d ¼ 72.0 ppm.[15] The
relatively strong resonance signals at d ¼ 18 ppm therefore
stem not only from the methyl groups CH3 (h) at the ends of
the regularly obtained polymer, but also from those at the
ends of the very low-molecular-weight oligomers (Mn <
600 gmolÀ1) resulting from the bisphenolato ligands.[16] The
13C NMR spectrum of the polyether obtained with chloro
complexes 1 and 8 gave the same statistical distribution of
triads and the same resonance signals corresponding to the
end groups (CH2Cl: d ¼ 47.4 ppm; CHOH: d ¼ 65.4,
67.1 ppm). The MALDI-TOF mass spectra (dithranol, K,
THF) of the polymers examined showed a monomodal weight
distribution. The mass difference was 58 in each case, and the
detected masses confirmed the end groups determined from
the 13C NMR spectra.[17]
Scheme 2. The concept of coordination anionic polymerization with
chain transfer.
tion of the ion pairs is a requirement for efficient transfer of
the alkoxy group. When the initiator consisted of a combi-
nation of 1 and 8, the chloro complexes underwent a prior
reaction with PO to form the corresponding 1-chloro-2-
propanolato complexes. During polymerization, the neutral
phenolato aluminum complexes bear one growing polymer
chain, and the aluminate two (step 3 in Scheme 2). The
average initiator efficiency (number of polymer chains
initiated per aluminum atom) should therefore depend on
the ratio of neutral to anionic complexes. To checkthe
proposed mechanism, we carried out the polymerization of
PO with four different initiator efficiencies until complete
conversion had taken place (Figure 3).
The ratio of monomer to the total number of centers
active for polymerization was deliberately kept constant (a:
50; b: 100). In good agreement with the hypothesis, the
molecular weights obtained in the experimental series hardly
66
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Experimental Section
5: At 708C, sodium (46 mg, 2.0 mmol) was dissolved with generation
of hydrogen gas in isopropyl alcohol (10 mL). This solution was added
dropwise at À208C to a solution of NEt4Cl (331 mg, 2.0 mmol) in
isopropyl alcohol (3 mL); this resulted in precipitation of NaCl. To
reduce the solubility of the NaCl, diethyl ether (20 mL) was added at
À208C. In another reaction flask, 3 (849 mg, 1.0 mmol) was suspend-
ed in diethyl ether (5 mL) cooled to À208C, and isopropyl alcohol
(5 mL) was added. The precooled solution of the tetraethylammo-
nium isopropoxide was added dropwise to this suspension; the
reaction mixture dissolved as a result. The solution was stirred at
below 08C for 45 min, and then the solvent was removed in vacuo.
The residue was dissolved in dichloromethane (3 mL) and filtered.
After the addition of diethyl ether (15 mL), the aluminate was
recrystallized at À308C. Yield: 910 mg (74%) of 5 as colorless
crystals. Complex 6 was prepared similarly.
3
1H NMR (400 MHz, CDCl3, 258C): d ¼ 0.85 (t, JH,H ¼ 7.4 Hz,
3
12H; NCH2CH3), 1.07 (d, JH,H ¼ 5.9 Hz, 6H; CH(CH3)2), 1.15 (d,
3JH,H ¼ 5.9 Hz, 6H; CH(CH3)2), 1.37 (s, 18H; 6-C(CH3)3), 2.13 (s, 6H;
3
2
4-CH3), 2.46 (q, JH,H ¼ 7.4 Hz ; NCH2CH3), 3.00 (d, JH,H ¼ 13.7 Hz,
3
1H; 2-CH2), 4.19 (septet, JH,H ¼ 5.9 Hz; CH(CH3)2), 4.36 (septet,
3JH,H ¼ 5.9 Hz; CH(CH3)2), 3.75 (d, JH,H ¼ 13.7 Hz, 1H; 2-CH2), 6.72
2
4
4
(d, JH,H ¼ 2.0 Hz, 2H; 5-H), 6.83 ppm (d, JH,H ¼ 2.0 Hz, 2H; 3-H);
13C NMR (100.6 MHz, CDCl3, 258C): d ¼ 7.3 (NCH2CH3), 20.9 (4-
CH3), 28.0 (CH(CH3)2), 28.2 (CH(CH3)2), 30.1 (6-C(CH3)3), 33.8 (6-
C(CH3)3), 34.9 (2-CH2), 52.0 (NCH2CH3), 62.4 (CH(CH3)2), 62.6
(CH(CH3)2), 122.7, 125.1, 128.6, 131.4, 138.0 (phenyl 2-C to 6-C),
156.2 ppm (ipso-phenyl-C); elemental analysis (%) calcd for
C37H64AlNO4 (613.91): C 72.39, H 10.51, N 2.28; found: C 72.30 H
10.64 N 2.71.
Received: July 9, 2002
Revised: September 19, 2002 [Z19702]
[1] a) X. Zuwei, Z. Ning, S. Yu, Science 2001, 292, 1139 1141; b) K.
Weissermel, H.-J. Arpe, Industrielle Organische Chemie, 4th ed.,
Wiley-VCH, Weinheim, 1994, chap. 11.
[2] S. D. Gagnon, Encyclopedia of Polymer Science and Engineer-
ing, Vol. 6, 2nd ed., Wiley-VCH, Weinheim, 1994, pp. 275 307.
[3] The polymerization of alkylene oxides by double and multimetal
cyanide compounds has been comprehensively patented. Exam-
ples: a) P. Ooms, J. Hofmann, C. Steinlein, S. Ehlers, PCT Int.
Appl. WO 0134297 2001; b) T. Ostrowski, K. Harre, P. Zehner, J.
M¸ller, D. St¸tzer, G. H. Grosch, J. Winkler, PCT Int. Appl. WO
0162826 2001.
[4] For living coordination polymerization of PO, see: a) T. Aida, R.
Mizuta, Y. Yoshida, S. Inoue, Makromol. Chem. 1981, 182, 1073
1079; b) T. Aida, S. Inoue, Macromolecules 1981, 14, 1162 1166;
c) T. Aida, S. Inoue, Macromolecules 1981, 14, 1166 1169; d) T.
Aida, Y. Maekawa, S. Asano, S. Inoue, Macromolecules 1988, 21,
1195 1202; e) T. Aida, K. Wada, S. Inoue, Macromolecules 1987,
20, 237 241; f) V. Vincens, A. Le Borgne, N. Spassky, Makro-
mol. Chem. Macromol. Symp. 1991, 47, 285 291; g) A. Le B-
orgne, V. Vincens, M. Jouglard, N. Spassky, Makromol. Chem.
Macromol. Symp. 1993, 73, 37 46; h) D. A. Atwood, J. A.
Jegier, D. Rutherford, Inorg. Chem. 1996, 35, 63 70; i) N. Emig,
H. Nguyen, H. Krautscheid, R. Rÿau, J.-B. Cazaux, G. Bertrand,
Organometallics 1998, 17, 3599 3608. For the proposed mech-
anisms, see h), i) and: K. Shimasaki, T. Aida, S. Inoue,
Macromolecules 1987, 20, 3076 3080.
Figure 3. Polymerization of with PO (2 mL) a) variable ratios [1]0/[8]0
and constant ratio [PO]0/[active centers]0 ¼50:1 (calculated according
to a coordination anionic mechanism with chain transfer) in CH2Cl2
(1 mL) at room temperature and b) variable ratios [3]0/[5]0 and con-
stant ratio [PO]0/[active centers]0 ¼100:1 in CH2Cl2 (2 mL) at room
*
*
temperature. Plotted are Mn ( ), Mw/Mn ( ), the theoretical course
when a coordination anionic mechanism with chain transfer is as-
sumed (- - - -), and the theoretical course when a simple anionic mecha-
*
*
nism is assumed (
).
differed from each other. With a simple anionic mechanism
without simultaneous chain growth at all metal centers, the
molecular weight would have been linearly dependent on the
number of ate complexes or neutral phenolatoaluminum
complexes used.
In summary, we have confirmed here that ring-opening
polymerization of PO cannot occur at simple Lewis acidic
centers, but that nucleophilic ate complexes must be present
at the same time. The fundamentally new mechanism
reported herein should have far-reaching consequences for
understanding PO polymerization, and should allow the
design of new, structurally characterized initiators, also for
the stereoselective polymerization of PO.[19]
[5] a) H. Sugimoto, C. Kawamura, M. Kuroki, T. Aida, S. Inoue,
Macromolecules 1994, 27, 2013 2018; b) M. Akatsuka, T. Aida,
S. Inoue, Macromolecules 1994, 27, 2820 2825; c) T. Aida, S.
Inoue, Acc. Chem. Res. 1996, 29, 39 48; d) H. Sugimoto, S.
Inoue, Adv. Polym. Sci. 1999, 146, 39 119.
Angew. Chem. Int. Ed. 2003, 42, No. 1
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67

Communications
[6] a) C. C. Price, R. Spector, J. Am. Chem. Soc. 1965, 87, 2069
2070; b) E. J. Vandenberg, J. Polym. Sci. 1960, 47, 486 489;
c) E. J. Vandenberg, J. Polym. Sci. Polym. Chem. Ed. 1969, 7,
525 567.
[7] Chisholm©s microstructure analysis of the oligomers obtained by
phenolato complexes is indicative of the necessity of two metal
centers during the polymerization process. The inversion of the
chiral carbon atoms in the head-to-head linkages partially found
can only be explained by this, see: a) B. Antelmann, M. H.
Chisholm, S. S. Iyer, J. C. Huffman, D. Navarro-Llobet, M.
Pagel, W. J. Simonsick, W. Zhong, Macromolecules 2001, 34,
3159 3175; b) M. H. Chisholm, D. Navarro-Llobet, W. J. Si-
monsick, Macromolecules 2001, 34, 8851 8857.
20.181(1), c ¼ 22.539(2) ä, V¼ 8854(1) ä3; orthorhombic, Pbca,
Z ¼ 8, T¼ 193 K, 2qmax. ¼ 468; 32347 reflections collected, of
which 6400 independent (Rint ¼ 0.1843), Structure solution with
direct methods, refinement against F2, 482 refined parameters,
R1 ¼ 0.0666, wR2 ¼ 0.1529 (observed data). CCDC-193556 con-
tains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.
ac.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12, Union Road, Cambridge CB21EZ,
UK; fax: (þ 44)1223-336-033; or deposit@ccdc.cam.ac.uk).
[13] For a detailed study of the microstructure of atactic polypropyl-
ene oxides by 13C NMR spectroscopy, see: F. C. Schilling, A. E.
Tonelli, Macromolecules 1986, 19, 1337 1343.
[8] a) I. Taden, H.-C. Kang, W. Massa, T. P. Spaniol, J. Okuda, Eur. J.
Inorg. Chem. 2000, 441 445; b) Y.-C. Liu, B.-T. Ko, C.-C. Lin,
Macromolecules 2001, 34, 6196 6201; c) H.-L. Chen, B.-T. Ko,
B.-H. Huang, C.-C. Lin, Organometallics 2001, 20, 5076 5083.
[9] mmcpH2 was kindly donated by Baerlocher GmbH.
[10] Polymerization example with 1: A molar ratio [PO]0/[Al]0 of 200/
1 for a reaction time of five days at room temperature in bulk
gave a regioirregular oligomer in 15% yield (Mn ¼ 940, Mw/Mn ¼
1.33).
[11] A system obtained by reacting AlEt2Cl with 25,27-dimethoxy-
26,28-dihydroxy-p-tert-butyl-calix[4]arene shows similarity with
the bisphenolato compound described herein. These authors
report a regioregular PO polymerization that proceeds only at
elevated temperatures and a reaction time of several days and
resulted in only low conversion: W. Kuran, T. Listos, M.
Abramczyk, A. Dawidek, J. Macromol. Sci. Pure Appl. Chem.
1998, A35, 427 437.
[14] The assignments of the signals according to the literature was
checked by comparison of both polymer spectra and by DEPT-
135 experiments.
[15] The weaksignals at d ¼ 30 ppm are assigned to the tert-butyl
carbon atoms C(CH3)3 of the bisphenolato ligands, which despite
its chelate structure evidently polymerized PO under ring-
opening to a minor extent. Assignment according to the
13C NMR spectrum of mbmpH2: d ¼ 29.8 (C(CH3)3).
[16] The MALDI-TOF mass spectrum of the polymer showed above
m/z 600 exclusively masses separated by m/z 58 without any
intermediate masses.
[17] Number of monomer units N ¼ (MdetÀMþÀMend)/MPO, with
Mdet: detected mass; Mþ: mass of cations; Mend: mass of the
end group; MPO: 58.08 (mass of PO).
[18] a) C.-H. Lin, L.-F. Yan, F.-C. Wang, Y.-L. Sun, C.-C. Lin, J.
Organomet. Chem. 1999, 587, 151 159; b) B.-T. Ko, C.-C. Wu,
C.-C. Lin, Organometallics 2000, 19, 1864 1869.
[12] Crystallographic data of 6: Colorless needles were obtained at
À308C from a mixture of dichloromethane and diethyl ether,
Bruker-AXS diffractometer, l ¼ 0.71073 ä, a ¼ 19.466(2), b ¼
[19] T. Tsuruta, J. Polym. Sci. Part D 1972, 6, 179 250, and references
therein.
68
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