Highly active, productive, and syndiospecific yttrium initiators for the polymerization of racemic β-butyrolactone

Article abstract of DOI:10.1002/anie.200600058

(Chemical Equation Presented) Alternate tacticities: The ring-opening polymerization of rac-β-butyrolactone (rac-BBL) using a six-coordinate yttrium complex bearing a dianionic amine-bis(phenolate) ligand as initiator proceeds readily at room temperature to give the corresponding poly(3-hydroxybutyrate) (PHB) with up to 94% syndiotacticity and narrow molecular-weight distributions (see scheme).

Full text of DOI:10.1002/anie.200600058

Communications
Polymer Chemistry
report a highly active and stereoselective initiator for the
polymerization of rac-BBL, generating highly syndiotactic
PHB.
DOI: 10.1002/anie.200600058
The high lactide polymerization activities observed with
the yttrium initiators 1a,b and 2a,b led us to investigate these
complexes for the ROP of rac-BBL (Scheme 1 and Sche-
me 2).[9b] The solution-state structures of 1a and 2a were
recently reported,[9b] and these structures are now supported
by the X-ray crystal structure of 1a (Figure 1).[10] Complex 1a
Highly Active, Productive, and Syndiospecific
Yttrium Initiators for the Polymerization of
Racemic b-Butyrolactone**
Abderramane Amgoune, Christophe M. Thomas,*
Simona Ilinca, Thierry Roisnel, and
Jean-François Carpentier*
Biodegradable polymers have recently gained
great attention as a replacement for conven-
tional synthetic materials.[1] Among the new
biodegradable polymers that have been devel-
oped during the last decade,[2] polyhydroxyal-
kanoates (PHAs) are of particular interest.[3]
Scheme 1. Synthesis of complexes 1 and 2.
These materials combine the film-barrier
properties of polyesters with the mechanical
performance properties of petroleum-based
polyethylene and polypropylene. The most common PHA is
poly(3-hydroxybutyrate) (PHB), which is an isotactic, highly
crystalline thermoplastic polyester produced by various
bacteria and algae. One of the most convenient and promising
synthetic routes to PHAs uses metal initiators to effect the
ring-opening polymerization (ROP) of b-butyrolactone
(BBL), where the relief of ring strain is the driving force for
polymerization.[2a] Highly isotactic (R)- or (S)-PHB can be
obtained when optically pure (R)- or (S)-BBL is involved,[4]
while use of a racemic (rac) mixture of BBL gives rise to
atactic PHB[5] and PHB enriched in isotactic[6] or syndiotac-
tic[7] diads. However, most initiators are extremely slow and
produce low-molecular-weight PHB. Although some initia-
tors polymerize BBL with excellent rates,[8] stereospecific
polymerization of rac-BBL remains a challenge. Recently, we
reported that well-defined single-site bis(phenolate) com-
plexes of Group 3 metals are highly active initiators in the
synthesis of heterotactic and syndiotactic poly(lactic acid)
from rac-lactide and meso-lactide, respectively.[9] Herein, we
Scheme 2. Synthesis of syndiotactic PHB.
Figure 1. Crystal structure of complex 1a; all hydrogen atoms and
solvent molecules are omitted for clarity.
[*] A. Amgoune, Dr. C. M. Thomas, S. Ilinca, Prof. Dr. J.-F. Carpentier
OrganomØtalliques et Catalyse
adopts a distorted octahedral coordination geometry about
the yttrium atom with all four donors of the bis(phenolate)
ligand bound to the metal center. One THF ligand and one
bis(dimethylsilyl)amido group occupy the remaining cis posi-
tions.
Initial polymerizations were performed using 0.5 mol% of
1a,b or in situ generated 2a,b with commercially available
rac-BBL. Complexes 1a,b and 2a,b are active initiators for
the controlled ROP of rac-BBL under mild conditions.
However, as polymerizations of rac-BBL with 2a,b proceed
more rapidly than with 1a,b, we decided to explore this
reaction by using the former complexes (Table 1).[11] We
Institut de Chimie de Rennes
UniversitØ de Rennes 1
35042 Rennes (France)
Fax: (+33)2-2323-6939
E-mail: christophe.thomas@univ-rennes1.fr
jean-francois.carpentier@univ-rennes1.fr
Dr. T. Roisnel
Centre de DiffractomØtrie X
Institut de Chimie de Rennes
UniversitØ de Rennes 1
35042 Rennes (France)
[**] This research was financially supported in part by the Region
Bretagne (grant-in-aid COPOBIO to A.A.). We thank A. Bondon
(University of Rennes 1) for NMR experiments.
observed a strong influence of the solvent. For [BBL]/ACHTREUNG[2a] =
200, polymerization in toluene and chlorobenzene reached
97% conversion in less than 1 minute (Table 1, entries 1, 2),
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2782
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2782 –2784

Angewandte
Chemie
Table 1: Syndiospecific polymerization of rac-butyrolactone.[a]
Entry
Complex
[BBL]/[Y]
U
Solvent
t [min]
Yield [%][b]
Mn [gmolÀ1 [c]
]
PDI[c]
Pr[d]
Tm [8C][e]
1
2
3
4
5
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
200
200
200
400
400
400
400
600
1000
2000
400
400
400
2.44
2.44
2.44
2.44
1.00
4.80
1.00
2.44
neat
8.80
2.44
2.44
2.44
Toluene
C6H5Cl
THF
Toluene
Toluene
Toluene
Toluene
Toluene
–
Toluene
Toluene
Toluene
Toluene
1
1
120
1
10
4
720
5
98
95
98
98
98
95
98
98
95
87
94
95
95
22600
24600
27400
26100
37400
36100
37100
47200
48600
39900
27700
26400
33900
1.15
1.15
1.03
1.06
1.14
1.08
1.10
1.10
1.16
1.18
1.22
1.27
1.10
0.88
0.84
0.83
0.89
0.90
0.87
0.94
0.88
0.81
0.87
0.55
0.51
0.81
154
158
133
157
167
150
178
155
142
156
95
6
7[f]
8
9
5
10
11
12
13
18
4320
120
2
Y[N
“Y(OiPr)3”
2b
ACHTREUNG(SiHMe2)2]3ACHTRE(UGN THF)2
G
91
138
[a] All reactions were performed at 208C (except for Entry 7). [b] As determined by the integration of methine 1H NMR resonances of rac-BBL and PHB.
[c] Mn and Mw/Mn of polymer determined by size-exclusion chromatography in CHCl3 at room temperature using polystyrene standards. PDI=
polydispersity index. [d] Pr is the probability of racemic linkages between monomer units and is determined from the carbonyl region of the
13C{1H} NMR spectrum. [e] Determined by differential scanning calorimetry (DSC; second scan). [f] Reaction run at À208C.
while polymerization in THF led to 98% conversion within
2 h (Table 1, entry 3). Decreasing the concentration of
monomer resulted in a substantial decrease in the rate of
polymerization (Table 1, entries 4, 5). All other polymeri-
zation reactions were therefore preferably conducted in
toluene at a concentration of rac-BBL of 2.44m. Several
reactions were also performed by changing the monomer-to-
metal ratio. For instance, complete conversion of 400 equiv-
alents of rac-BBL was achieved within 1 minute at room
temperature (turnover frequency, TOF = 24000 hÀ1; Table 1,
entry 4). The activity of this system is thus more than one
order of magnitude higher than that of yttrium 2-methox-
yethoxide as previously reported by Spassky and co-workers
under similar conditions.[12] To confirm the influence of the
bis(phenolate) ligand on the activity, control experiments
of prior 13C NMR assignments of the carbonyl region of PHB,
the upfield and downfield signals in the carbonyl region
correspond to the meso (m) diad sequences, R–R and S–S, and
the racemic (r) diad sequences, R–S and S–R, respectively.[13]
Our PHBs feature a strong contribution correlating to r-
centered diads (d = 169.16 ppm), indicative of syndiotactic
PHB (Figure 2). Expansion of the methylene region shows
were carried out with Y[NACHTRUENG(SiHMe2)2]3ACHTRE(UGN THF)2 and “Y-
ACHTREUNG
12). The results show that 2a indeed displays a significantly
higher activity than the two reference systems when poly-
merizations are carried out in toluene. The yttrium alkoxide
2a can polymerize 1000 equivalents of monomer within
5 minutes at 208C in neat BBL resulting in TOF =
12000 hÀ1 (Table 1, entry 9), and 2000 equivalents at high
concentrations of monomer in less than 20 minutes (TOF =
6000 hÀ1; Table 1, entry 10).
Figure 2. Carbonyl and methylene regions of the 13C NMR spectrum
(125 MHz, CDCl3, 408C) of PHBprepared by polymerization of rac-
BBL using complex 2a (see Table 1, entry 7).
For [BBL]/ACHTREUNG[2a] < 1000, the resultant polymers reveal
narrow molecular-weight distributions and experimental
number-average molecular masses (Mn) close to the theoret-
ical ones. However, for higher ratios, experimental Mn values
do not correspond well with calculated Mn values. This
mismatch possibly arises from chain-transfer processes or
poor correlation between the polystyrene calibration of the
gel permeation chromatograph and the actual molecular
weights of the PHB chains, as already reported by Coates and
co-workers.[8]
four peaks that correspond to triad sensitivity (Figure 2). We
correlated the most intense resonance at d = 40.77 ppm to the
rr triad, and we attributed the resonances at d = 40.72 and
40.90 ppm to the mr and the rm triads, respectively. The
remaining mm triad at d = 40.85 ppm is almost negligible.[14]
Statistical analysis confirms a chain-end control mechanism
(See Supporting Information). We observed differences in the
microstructure of the PHBs depending on the nature of the
solvent. While polymerization of rac-BBL in toluene gave
PHB with nearly 90% of syndiotactic enchainment (Table 1,
entry 1), polymerizations in THF and chlorobenzene gave
Microstructural analysis by 13C NMR spectroscopy of the
different PHBs formed from rac-BBL revealed that com-
plexes 2 exert a significant influence at room temperature on
the tacticity of the polymer formed (Scheme 2). On the basis
Angew. Chem. Int. Ed. 2006, 45, 2782 –2784
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
2783

Communications
[5] a) N. C. Billingham, M. G. Proctor, J. D. Smith, J. Organomet.
Chem. 1988, 341, 83 – 93; b) M. Moeller, R. Kånge, J. L. Hedrick,
J. Polym. Sci. Part A 2000, 38, 2067 – 2074.
[6] a) B. Wu, R. W. Lenz, Macromolecules 1998, 31, 3473 – 3477;
b) S. Bloembergen, D. A. Holden, T. L. Bluhm, G. K. Hamer,
R. H. Marchessault, Macromolecules 1989, 22, 1656 – 1663.
[7] a) Y. Hori, T. Hagiwara, Int. J. Biol. Macromol. 1999, 25, 237 –
245; b) H. R. Kricheldorf, S. Eggerstedt, Macromolecules 1997,
30, 5693 – 5697.
[8] L. R. Rieth, D. R. Moore, E. B. Lobkovsky, G. W. Coates, J. Am.
Chem. Soc. 2002, 124, 15239 – 15248.
[9] a) C.-X. Cai, A. Amgoune, C. W. Lehmann, J.-F. Carpentier,
Chem. Commun. 2004, 330 – 331; b) A. Amgoune, C. M.
Thomas, T. Roisnel, J.-F. Carpentier, Chem. Eur. J. 2006, 12,
169 – 179.
[10] Diffraction data were collected at 100 K using an APEX 2 AXS-
Bruker diffractometer with graphite-monochromatized MoKa
radiation (l = 0.71073 Š). The value of Rint was found to be
greater than 0.15 because the crystal was weakly diffracting, so
that a large proportion of essentially “non-observed” reflections
were used in the refinement. Crystal data for 1a:
C61H77N2O4Si2Y·C7H8, Mr = 1139.47, white blocks, 0.25 ” 0.05 ”
0.05 mm3, monoclinic, space group P21/c, a = 15.4649(12), b =
20.1003(16), c = 19.7728(14) Š, a = 90, b = 98.426(3), g = 908,
V= 6080.0(8) Š3, Z = 4, 1calcd = 1.245 gcmÀ3, m = 1.048 mmÀ1, F-
83% and 84%, respectively, of syndiotacticity (Table 1,
entries 2 and 3). At À208C, polymerization proceeded
slowly but the resulting PHB revealed a higher degree of
syndiotacticity (Pr = 0.94; Table 1, entry 7). As already
observed for the polymerization of rac-lactide,[9b] complex
2b, which bears tert-butyl groups at the phenolate rings,
reveals a lower selectivity for syndiotactic PHB (Pr = 0.81;
Table 1, entry 13). This decrease in tacticity is consistent with
previous observations that ligand substituents are primordial
for stereochemical control through a chain-end control
mechanism.[15]
DSC measurements of selected PHB samples showed the
strong influence of the stereochemistry on the melting
temperature (Tm), which increases with the degree of
syndiotacticity (Table 1, entries 2, 7, 9, 10). For the most
syndiotactic polymer, Tm is around 1788C, that is, the highest
melting temperature reported so far for syndiotactic
PHB.[13,16] The DSC traces display a single, sharp transition
endotherm at high temperature, demonstrating the formation
of a uniform crystalline ordering in the solid state, which may
indicate a narrower dispersion of stereochemical chain
sequences.
AHCTRE(UNG 000) = 2424, T= 100(2) K, R1 = 0.0975, wR2 = 0.3101, 5286
It is well established that the ring opening of BBL by
metal alkoxides can proceed by a coordination–insertion
mechanism or an anionic mechanism.[8] To determine the
route involved in the ROP of BBL with 2a, reactions were
conducted with [BBL]/[Y] ratios of less than 10. The
independent reflections [2q ꢀ 51.008] and 695 parameters.
CCDC-294486 (1a) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
1
[11] Polymerizations with complex 2a were found to proceed five
times faster than with complex 1a.
[12] A. Le Borgne, C. Pluta, N. Spassky, Macromol. Rapid Commun.
1994, 15, 955 – 960.
[13] J. E. Kemnitzer, S. P. McCarthy, R. A. Gross, Macromolecules
1993, 26, 1221 – 1229.
[14] For the methyl and methine carbon signals, diad and triad
sequences, respectively, were assigned and were in good agree-
ment with the carbonyl and methylene carbon signals. See
Supporting Information.
[15] a) B. M. Chamberlain, M. Cheng, D. R. Moore, E. B. Lobkovsky,
T. M. Ovitt, G. W. Coates, J. Am. Chem. Soc. 2001, 123, 3229 –
3238; b) M. Mitani, R. Furuyama, J.-I. Mohri, J. Saito, S. Ishii, H.
Terao, T. Nakano, H. Tanaka, T. Fujita, J. Am. Chem. Soc. 2003,
125, 4293 – 4305; c) B. L. Small, M. Brookhart, Macromolecules
1999, 32, 2120 – 2130.
unambiguous identification by H NMR spectroscopy of the
-OCHACHTREUNG(CH3)2 end group at d = 5.00 ppm confirmed that the
BBL ring is cleaved at the acyl–oxygen bond and inserted into
the metal–isopropoxide bond. Furthermore, trans-crotonate
(and carboxy) groups were not observed in the 1H NMR
spectrum, confirming the coordination–insertion mechanism.
Indeed an anionic mechanism leads generally to a high
concentration of crotonate.[8]
In conclusion, the yttrium alkoxide complex 2a exhibits
the highest activity and greatest stereoselectivity for syndio-
specific polymerization of rac-BBL to date. These results
suggest a number of new avenues for biodegradable polymers.
Received: January 6, 2006
Published online: March 20, 2006
[16] a) H. R. Kricheldorf, S.-R. Lee, N. Scharnagi, Macromolecules
1994, 27, 3139 – 3146; b) R. A. Gross, Y. Zhang, G. Konrad,
R. W. Lenz, Macromolecules 1988, 21, 2657 – 2668.
Keywords: homogeneous catalysis · polymers ·
.
ring-opening polymerization · tacticity · yttrium
[1] R. E. Drumright, P. R. Gruber, D. E. Henton, Adv. Mater. 2000,
12, 1841 – 1846.
[2] a) M. Okada, Prog. Polym. Sci. 2002, 27, 87 – 133; b) O. Dechy-
Cabaret, B. Martin-Vaca, D. Bourissou, Chem. Rev. 2004, 104,
6147 – 6176; c) J. Wu, T.-L. Yu, C.-T. Chen, C.-C. Lin, Coord.
Chem. Rev. 2006, 250, 602 – 626.
[3] a) K. E. Ulrich, S. M. Cannizzaro, R. S. Langer, K. M. Shake-
sheff, Chem. Rev. 1999, 99, 3181 – 3198; b) H.-M. Müller, D.
Seebach, Angew. Chem. 1993, 105, 483 – 509; Angew. Chem. Int.
Ed. Engl. 1993, 32, 477 – 502; c) K. Sudesh, H. Abe, Y. Doi,
Progr. Polym. Sci. 2000, 25, 1503 – 1555.
[4] Y. Yori, M. Suzuki, A. Yamaguchi, T. Nishishita, Macromole-
cules 1993, 26, 5533 – 5534.
2784 www.angewandte.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2782 –2784