Group 4 metal initiators for the controlled stereoselective polymerization of lactide monomers

Article abstract of DOI:10.1039/c1cc10149h

Group 4 metal initiators with a tetradentate bis(phenolato) ligand polymerized meso-lactide efficiently under ring-opening to give syndiotactic polylactide. l-Lactide was converted faster than rac-lactide and meso-lactide. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1cc10149h

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Group 4 metal initiators for the controlled stereoselective polymerization
of lactide monomersw
Jean-Charles Buffet and Jun Okuda*
Received 7th January 2011, Accepted 22nd February 2011
DOI: 10.1039/c1cc10149h
Group 4 metal initiators with a tetradentate bis(phenolato)
ligand polymerized meso-lactide efficiently under ring-opening
to give syndiotactic polylactide. L-Lactide was converted faster
than rac-lactide and meso-lactide.
Poly(lactide) (PLA) is currently attracting interest as a
biodegradable thermoplastic derived from ‘‘renewable’’ feed-
stocks.1 Isotactic poly(L-lactide) has been commercialized with
an annual production capacity exceeding 105 tons. The monomer
L-lactide (LA) is produced by thermolysis of oligo(L-lactide).2
As there are two stereogenic centers in one lactide molecule,
three stereoisomers (S,S)-LA, (R,R)-LA, and meso-LA can be
distinguished, the racemic mixture of (S,S)-LA and (R,R)-LA
Fig. 1 Group 4 metal complexes 1–5a as ROP initiators.
being referred to as rac-LA. Molecularly defined single-site
initiators are based on Lewis acidic di-,3 tri-4 and tetra-valent5
metal centers. Polymerization of L-lactide leads to isotactic
PLA, whereas polymerization of rac-lactide can lead to atactic,
heterotactic, stereoblock or stereocomplex PLA. meso-Lactide,
a by-product of the synthesis of L-lactide, can afford heterotactic
and syndiotactic PLA. To date, there are only few examples
for the efficient and stereoselective polymerization of meso-
lactide.6
We report here the use of group 4 metal complexes with an
1,o-dithia-alkanediyl-bridged bis(phenolato) ligand for the
efficient polymerization of meso-lactide in solution as well as
in melt.7 Group 4 metal initiators containing an 1,o-
dithia-alkanediyl-bridged bis(phenolato) ligand with bulky
groups at the ortho-substituent R1 are readily synthesized
from [M(OR)4ÀnCln] (M = Ti, Zr; R = iPr or tBu,
X = OR or Cl) and the bis(phenol) (Fig. 1).
Fig. 2 Molecular structure of 3. Hydrogen atoms, cumyl (CMe2Ph)
The molecular structure of complex 3 (Fig. 2) shows
groups and solvent molecule have been omitted for clarity. Selected
octahedral coordination geometry at the zirconium metal.
bond lenghts (A) and angles (1): Zr1–O1 2.047(2), Zr1–O2 2.034(2),
The complex adopts a C2-symmetrical configuration with
Zr1–O3 1.903(3), Zr–O4 1.914(2), Zr–S1 2.8767(10), Zr–S2 2.8370(10),
cis-arranged tert-butoxy (O3–Zr1–O4 = 105.60(11)1) and
thioether groups with zirconium–sulfur distances of Zr1–S1
O1–Zr1–O2 142.52(9), O1–Zr1–O3 98.90(10), O1–Zr1–O4 101.21(10),
O3–Zr1–O4 105.60(11), O1–Zr1–S1 70.71(7), O1–Zr1–S2 82.09(7),
2.877(1) A and Zr1–S2 2.837(1) A.
S1–Zr1–S2 77.07(3).
Complexes 1–5 polymerized meso-lactide at 100 1C in
Institut fur Anorganische Chemie, RWTH Aachen, Landoltweg 1,
¨
toluene (Table 1). When complexes 1, 4 and 5, all bearing
tert-butyl as ortho-substituents, are compared, the C3-linked
complex 5 gave the highest conversion under the same
polymerization conditions (38%, 8% and 71%, respectively,
Table 1, entries 1, 4 and 5) indicating the importance of a
flexible backbone.
D-52056 Aachen, Germany. E-mail: jun.okuda@ac.rwth-aachen.de;
Fax: +49 241 8092644
w Electronic supplementary information (ESI) available: Experimental
preparations and characterizations of complexes 1–5a, polymerization
data and microstructure analysis. CCDC 807149. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c1cc10149h
c
4796 Chem. Commun., 2011, 47, 4796–4798
This journal is The Royal Society of Chemistry 2011

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Table 1 ROP of meso-lactide initiated by complexes 1–5
%
Entry Init. Conv.a
Mn,expb/g molÀ1 Mn,theoc/g molÀ1 Mw/Mn Psd
b
1
2
3
4
5
1
2
3
4
5
38
73
499
8
2250
6100
12 250
—
2739
5261
7134
577
1.27
1.13
1.57
—
0.63
0.73
0.71
—
71
7000
5146
1.09
0.70
Polymerization conditions: [mon]0/[initiator]0 = 100, 100 1C, 24 h,
toluene, 2 mL.a Conversion of monomer (([mon]0 À [mon]t)/[mon]0).
b
Measured by GPC, calibrated with PS standards in THF.
d
c
Calculated using Mn,theo = [mon]0/2[initiator]0  Mmon  conv. Ps
is the probability for the formation of a new s dyad, calculated from
1H{1H} NMR spectra.8
Fig. 3 Linear relationship between Mn and meso-lactide conversion
initiated by complex 5 ([mon]0/[initiator]0 = 100, T = 100 1C, toluene;
with Mw/Mn in parentheses).
Under similar polymerization conditions, conversion and
control over syndiotacticity are better when the ortho-position
is cumyl (1-phenylpropyl) (2, Table 1, entry 2) rather than tert-
butyl (1, entry 1). It is striking that complex 4 with a rigid
trans-1,2-cyclohexandiyl backbone gave only 8% conversion
after 24 h at 100 1C. At 120 1C in melt, complex 4 gave 93%
conversion with an initiator : monomer ratio of 1 : 100 after
66 h (Mn,exp = 4300 g molÀ1 and Mw/Mn of 1.21).
Table 3 ROP of meso-lactide initiated by complex 5
Mn,expb/ Mn,theo
c
/
b
Entry [mon]0/[initiator]0 t/h % Conv.a g molÀ1 g molÀ1 Mw/Mn
1
50
100
200
500
48 93
48 93
72 98
72 94
72 63
6250
10 000
14 750 14 081
25 000 33 762
46 750 45 112
3365
6723
1.12
1.17
1.15
1.11
1.09
29
3
49
59
A change in the central metal has a dramatic effect on meso-
lactide polymerization. With an initiator : monomer ratio of
1 : 100, polymerization using zirconium complex 3 is faster
(499%, entry 3) than using the homologous titanium complex
2 (73%, entry 2). The molecular weight of the polylactide
synthesized using complex 3 (Mn,exp = 12 250 g molÀ1) is two
times higher than that using complex 2 (Mn,exp = 6100 g molÀ1).
This indicates that the meso-lactide most likely inserts into one
of the two tert-butoxide groups in zirconium complex 3. In
contrast, both isopropoxide groups in the titanium complex 2
appear to initiate the polymerization. Interestingly, the degree of
syndiotacticity is similar (Ps of 0.73 for 2 and 0.71 for 3).
With an initiator 2a : monomer ratio of 1 : 50, at 100 1C in
C6D6, 71% conversion is attained after 24 h (Mn,exp of 6300 g
molÀ1, Mw/Mn of 1.06 and Ps of 0.62, Table S6, entry 7, ESIw).
Here, the theoretical molecular weight (5116 g molÀ1) was
calculated with one initiating alkoxide group. When using 5a,
94% conversion is attained after 24 h at 100 1C in C6D6
(Mn,exp of 6250 g molÀ1, Mw/Mn = 1.07, Ps = 0.71, Table S7,
entry 9, ESIw).
1000
Polymerization conditions: 100 1C.a Conversion of monomer (([mon]0 À
b
[mon]t)/[mon]0). Measured by GPC, calibrated with PS standards in
c
THF. Calculated using Mn,theo = [mon]0/2[initiator]0  Mmon  conv.
are calculated taking into account that both isopropoxide
groups initiate the polymerization. Controlled polymerization
behavior, even at 100 1C, is demonstrated by the linear
relationship between the molecular weight and the conversion
of monomer (Fig. 3 and Fig. S1, ESIw). Variation of the initial
initiator 5 : monomer ratio from 50 to 1000 shows that the
molecular weights increase linearly at 100 1C. The polydisper-
sities vary between 1.09 and 1.17 (Table 3 and Fig. S2, ESIw).
Melt polymerization of meso-lactide using 5 at 100 1C
occurs at a higher rate (kobs = 0.160 hÀ1) than in solution
(kobs = 0.049 hÀ1) (Fig. S5, ESIw). After 8 h, 74% conversion
is attained (Mn,exp of 10 000 g molÀ1, Mw/Mn of 1.06, Table
S5, entry 4, ESIw), whilst only 16% conversion is observed in
toluene (Table 2, entry 2).
The poly(meso-lactide) was syndiotactic-biased with Ps
between 70–80% in melt and in solution. Fig. 4 shows the NMR
spectra for the methine region of a poly(meso-lactide) sample.
We previously reported that initiators based on group 3
bis(phenolate) complexes with a ‘‘temporary’’ chiral reaction
site were producing highly syndiotactic PLAs from meso-lactide
(Ps > 90%).10 Here, lower values for the syndiotacticities
indicate a minor influence of the chiral site on the control of
the polymerization of meso-lactide.
Using complex 5, the polymerization of rac-lactide11 at 100 1C
in toluene with an initiator : monomer ratio of 1 : 100 led to
96% conversion after 72 h (Mn,exp = 14 000 g molÀ1, Mw/Mn =
1.16, Table S3, entry 4, ESIw). The polymerization appears
controlled with polydispersity values below 1.25 (Fig. S3,
ESIw). The polylactides showed isotacticity with Pi values over
60% (Fig. S20–S23, ESIw). The fastest polymerization rate was
observed for the polymerization of L-lactide using complex 5 at
Data for the meso-lactide polymerization with initiator 5 in
toluene are compiled in Table 2. All polydispersities are below
1.2, suggesting a controlled polymerization. The experimental
molecular weights are close to those theoretical values, which
Table 2 ROP of meso-lactide initiated by complex 5
Entry t/h % Conv.a Mn,expb/g molÀ1 Mn,theoc/g molÀ1 Mw/Mn
b
1
2
3
4
5
6
4
8
7
16
1250
2000
4500
7000
7750
8500
519
1153
3690
5146
6508
6705
1.10
1.14
1.11
1.09
1.15
1.19
16 51
24 71
39 90
63 93
Polymerization conditions: [mon]0/[initiator]0 = 100, 100 1C, 2 mL
b
toluene.a Conversion of monomer (([mon]0 À [mon]t)/[mon]0). Measured
c
by GPC, calibrated with PS standards in THF. Calculated using
Mn,theo = [mon]0/2[initiator]0  Mmon  conv.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 4796–4798 4797

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References
1 (a) C. M. Thomas, Chem. Soc. Rev., 2010, 39, 165;
(b) R. H. Platel, L. M. Hodgson and C. K. Williams, Polym.
Rev., 2008, 48, 11; (c) A. Angoume, C. M. Thomas and
J.-F. Carpentier, Pure Appl. Chem., 2007, 79, 2013; (d) J. Wu,
T.-L. Yu, C.-T. Chen and C.-C. Lin, Coord. Chem. Rev., 2006,
250, 602; (e) O. Dechy-Cabaret, B. Martin-Vaca and
D. Bourissou, Chem. Rev., 2004, 104, 6; (f) B. J. O’Keefe,
M. A. Hillmyer and W. B. Tolman, J. Chem. Soc., Dalton Trans.,
2001, 2215.
2 R. E. Drumright, P. R. Gruber and D. E. Henton, Adv. Mater.,
2000, 12, 1841.
3 (a) M. H. Chisholm, J. C. Gallucci and K. Phomphrai, Inorg.
Chem., 2005, 44, 8004; (b) M. H. Chisholm, J. C. Gallucci and
K. Phomphrai, Inorg. Chem., 2004, 43, 6717; (c) M. G. Davidson,
C. T. O’Hara, M. D. Jones, C. G. Keir, M. F. Mahon and
G. Kociok-Kohn, Inorg. Chem., 2007, 46, 7686.
Fig. 4 1H{1H} (a) and 13C{1H} NMR (b) spectra of the methine
region of poly(meso-lactide) obtained at 100 1C with an initiator
(5) : monomer ratio of 1 : 100, Table 2, entry 4.
4 (a) N. Nomura, R. Ishii, Y. Yamamoto and T. Kondo, Chem.–Eur.
J., 2007, 13, 4433; (b) N. Nomura, R. Ishii, M. Akakura and
K. Aoi, J. Am. Chem. Soc., 2002, 124, 5938; (c) P. Hormnirum,
E. L. Marshall, V. C. Gibson, R. I. Pugh and A. J. P. White, Proc.
Natl. Acad. Sci. U. S. A., 2006, 103, 15343; (d) P. L. Arnold,
J.-C. Buffet, R. P. Blaudeck, S. Sujecki, A. Blake and C. Wilson,
Angew. Chem., Int. Ed., 2008, 47, 6033; (e) P. L. Arnold,
J.-C. Buffet, R. Blaudeck, S. Sujecki and C. Wilson, Chem.–Eur. J.,
2009, 15, 8241; (f) J.-C. Buffet, J. Okuda and P. L. Arnold, Inorg.
Chem., 2010, 49, 419; (g) H. Ma, T. P. Spaniol and J. Okuda,
Angew. Chem., Int. Ed., 2006, 45, 7818; (h) H. Ma, T. P. Spaniol
and J. Okuda, Inorg. Chem., 2008, 47, 3328; (i) L. M. Hodgson,
R. H. Platel, A. J. P. White and C. K. Williams, Macromolecules,
2008, 41, 8603; (j) R. H. Platel, A. J. P. White and C. K. Williams,
Chem. Commun., 2009, 4115; (k) A. D. Schwarz, Z. Y. Chu and
P. Mountford, Organometallics, 2010, 29, 1246.
Fig. 5 Kinetic measurements for the solution polymerization of L- (m),
5 (a) A. J. Chmura, M. G. Davidson, C. J. Frankis, M. D. Jones
and M. D. Lunn, Chem. Commun., 2008, 1437; (b) A. J. Chmura,
D. M. Cousins, M. G. Davidson, C. J. Frankis, M. D. Jones and
M. D. Lunn, Dalton Trans., 2008, 1293; (c) C. K. A. Gregson,
I. J. Blackmore, V. C. Gibson, N. J. Long, E. L. Marshall and A.
J. P. White, Dalton Trans., 2006, 3134; (d) S. Gendler, S. Segal,
I. Goldberg, Z. Goldschmidt and M. Kol, Inorg. Chem., 2006, 45,
4783; (e) C. Romain, L. Brelot, S. Bellemin-Laponnaz and
S. Dagorne, Organometallics, 2010, 29, 1191; (f) A. D. Schwarz,
A. L. Thompson and P. Mountford, Inorg. Chem., 2009, 48,
10442.
6 (a) H. Du, A. H. Velders, P. J. Dijkstra, Z. Zhong, X. Chen and
J. Feijen, Macromolecules, 2009, 42, 1058; (b) M. H. Chisholm,
J. C. Gallucci, K. T. Quisenberry and Z. Zhou, Inorg. Chem., 2008,
47, 2613; (c) A. P. Dove, H. Li, R. C. Pratt, B. G. G. Lohmeijer,
D. A. Culkin, R. M. Waymouth and J. L. Hedrick, Chem.
Commun., 2006, 2881; (d) A. Amgoune, C. M. Thomas,
T. Roisnel and J.-F. Carpentier, Chem.–Eur. J., 2006, 12, 169;
(e) T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 2002, 124,
1316; (f) B. M. Chamberlain, M. Cheng, D. R. Moore, T. M. Ovitt,
E. B. Lobkovsky and G. W. Coates, J. Am. Chem. Soc., 2001, 123,
3229; (g) M. H. Chisholm, N. W. Eilerts, J. C. Huffman, S. S. Iyer,
M. Pacold and K. Phomphrai, J. Am. Chem. Soc., 2000, 122,
11845; (h) T. M. Ovitt and G. W. Coates, J. Am. Chem. Soc., 1999,
121, 4072; (i) A. Pietrangelo, S. C. Knight, A. K. Gupta, L. J. Yao,
M. A. Hillmyer and W. B. Tolman, J. Am. Chem. Soc., 2010, 132,
11649.
7 For use of group 4 metal complexes for L- and rac-lactide
polymerization, see: E. Sergeeva, J. Kopilov, I. Goldberg and
M. Kol, Inorg. Chem., 2010, 49, 3977.
8 Probability of tetrad sequences in PLA based on Bernoullian
statistics: [sss] = Ps2+PsPi/2, [sis] = (Pi2+PiPs)/2, [ssi] = [iss] =
(PsPi)/2 and [isi] = Pi2/2.
9 Tg values for representative example of polylactides in Table 3:
entry 2, 43.1 1C, entry 4, 43.6 1C and entry 5, 42.8 1C.
10 J.-C. Buffet, A. Kapelski and J. Okuda, Macromolecules, 2010, 43,
10201.
11 No kinetic resolution has been observed; the GPC traces are
monomodal during the entire conversion.
rac- (~) and meso-lactides (K) using complex 5 (initiator : monomer
ratio of 1 : 100, T = 100 1C, in toluene). Rate constants: L-LA (kobs
=
0.107 hÀ1), rac-LA (kobs = 0.045 hÀ1), and meso-LA (kobs = 0.049 hÀ1).
100 1C. After 24 h, 94% conversion was attained in toluene
(Mn,exp of 11 500 g molÀ1, Mw/Mn of 1.29, Table S4, entry 3,
Fig. S4, ESIw). No epimerization occurred even when L-lactide
was polymerized at 100 1C in solution or in melt (Fig. S24
and S25, ESIw).
The observed propagation rates kobs were determined by
analysis of a semi-logarithmic plot of ln([LA]0/[LA]t) vs. time,
where [LA]0 = 0.520 mol LÀ1 with an initiator : monomer ratio
of 1 : 100 (Fig. 5). The pseudo-first order rate constant for
L-lactide was 0.107 hÀ1, rac-lactide 0.045 hÀ1 and meso-lactide
0.049 hÀ1. The kinetic data for the polymerization by complex
5a showed a 4 h induction period (kobs = 0.027 hÀ1, Fig. S7,
ESIw) before the rate of propagation increased to kobs = 0.105 hÀ1
(polymerization using complex 5 gave kobs = 0.049 hÀ1).
The fact that L-lactide is polymerized faster than rac- and
meso-lactides may be due to the ‘‘temporary’’ chirality at the
metal center which favors the insertion of the next monomer with
the same configuration in agreement with the chain-end control
mechanism. Similar results have been observed previously.12
In summary, we reported here structurally defined initiators
based on group 4 metal bis(phenolate) complexes which
catalyzed the syndioselective polymerization of meso-lactide
efficiently and in a controlled fashion. L-Lactide was converted
more rapidly than meso-lactide and rac-lactide indicating a
reaction site with a preference for L-lactide.
We thank the Fonds der Chemischen Industrie for financial
support, Inna Schuller for experimental assistance, and Uhde
Inventa-Fischer for a gift of meso-lactide.
12 A. F. Douglas, B. O. Patrick and P. Mehrkhodavandi, Angew.
Chem., Int. Ed., 2008, 47, 2290.
c
4798 Chem. Commun., 2011, 47, 4796–4798
This journal is The Royal Society of Chemistry 2011