Polymerization of rac-Lactide Using Achiral Iron Complexes: Access to Thermally Stable Stereocomplexes

Article abstract of DOI:10.1002/anie.201903224

Enantiopure poly(lactic acid) (PLA) can form stereocomplexes when enantiomeric PLA chains are mixed in equivalent amounts. Such materials provide interesting features that might be suitable for numerous applications. Despite several advantages, the main d

Full text of DOI:10.1002/anie.201903224

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Accepted Article
Title: Polymerization of rac-Lactide Using Achiral Iron Complexes: A
New Access to Thermally Stable Stereocomplexes
Authors: Christophe M. Thomas, Paul Marin, Mathieu Tschan,
Florence Isnard, Carine Robert, Pierre Haquette, Xavier
Trivelli, Lise-Marie Chamoreau, Vincent Guérineau, Iker del
Rosal, Laurent Maron, and Vincenzo Venditto
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201903224
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10.1002/anie.201903224
Angewandte Chemie International Edition
Polymerization of rac-Lactide Using Achiral Iron Complexes: A
New Access to Thermally Stable Stereocomplexes
Paul Marin, Mathieu J.-L. Tschan, Florence Isnard, Carine Robert, Pierre Haquette, Xavier Trivelli,
Lise-Marie Chamoreau, Vincent Guérineau, Iker del Rosal, Laurent Maron, Vincenzo Venditto,* and
Christophe M. Thomas*
Abstract: Enantiopure poly(lactic acid) (PLA) can form
stereocomplexes when enantiomeric PLA chains are mixed in
equivalent amounts. Such materials provide interesting features that
might be suitable for numerous applications. Despite several
advantages, the main drawback of PLA is its narrow window of
processing, thus limiting its use for industrial applications. Herein, we
report achiral iron complexes, that are highly active, productive and
stereoselective in mild conditions for the ring-opening polymerization
of lactide. The corresponding catalytic systems enable the
production of stereoblock polymers with high molecular weights,
allowing the formation of thermally stable and industrially relevant
stereocomplexes.
fashion. It is now commonly accepted that the most efficient
approach for the production of well-controlled polyesters in terms
of molecular weight and microstructure is the ring-opening
polymerization (ROP) of lactide (LA) with metal initiators.[4]
Stereochemistry plays an important role in PLA, determining
its physical properties and ultimately the end use of the material.
Recent results indicated that the stereocontrol over polymer
sequences can be an effective strategy for tuning
macromolecular properties, and in particular crystallinity and
biodegradability.[5] For instance, enantiopure PLAs can form
stereocomplexes when enantiomeric PLA chains are mixed in
equivalent amounts.[6] Stereocomplexation results from
stereoselective interactions, mainly van der Waals forces,
between two opposite stereoregular polymers which interlock to
Considering a decreasing feedstock and increasing economic
and social pressures to reduce our dependence on petroleum,
development of biocompatible and/or biodegradable polymers
from renewable resources has become fertile research ground.[1]
Among the biobased polymers that have been developed during
the last two decades,[2] poly(lactic acid) (PLA) is of particular
interest.[3] This polymer, derived from 100% renewable
resources, has many properties similar or superior to traditional
olefin-based polymers with the added benefit of biodegradability.
The availability of monomers from renewable resources has
given PLA an increasing prominence in the market place. In this
context, there is significant interest in methods that allow for the
preparation of PLAs in a reproducible and (stereo)controlled
form
a new material with altered physical properties as
compared to the parent polymers. Stereocomplexation allows to
enhance the thermal resistance, mechanical properties as well
as the hydrolytic stability of PLA-based materials due to the
strong interaction between L-lactic acid and D-lactic acid
sequences. In particular, it has been demonstrated that
enantiopure semi-crystalline homopolymers poly(L-lactic acid)
(PLLA) and poly(D-lactic acid) (PDLA) form a stereocomplex
melting at 230°C, which is about 50°C higher than the
homochiral crystals of PLLA or PDLA.[7] The properties and
potential applications of poly(lactic acid) stereocomplexes have
been extensively investigated in the last years.[8] In particular,
such materials provide interesting features that are suitable for
biomedical applications (e.g., drug delivery, tissue engineering,
and nanostructured surfaces).[9] However, polymer blends of
high molecular weight PLLA and PDLA are likely to form
homochiral crystals of the respective polymers rather than the
stereocomplex crystals.[10] Therefore, despite the numerous
advantages of PLA, one major drawback remains to be solved,
namely, its narrow window of processing (i.e., the difference
between its melting temperature and its degradation
temperature).
[*]
Dr. P. Marin, Dr. M. J.-L. Tschan, Dr. F. Isnard, Dr. C. Robert, Dr. P.
Haquette, Prof. Dr. C. M. Thomas
Chimie ParisTech
PSL University, CNRS, Institut de Recherche de Chimie Paris
75005 Paris, France
E-mail: christophe.thomas@chimie-paristech.fr
Dr. X. Trivelli
Univ. Lille, CNRS,
UMR 8576 - Unité de Glycobiologie Structurale et Fonctionnelle
F-59000 Lille, France
Among the variety of initiators developed for the
polymerization of lactide to poly(lactic acid), iron complexes are
notable due to their abundance, low toxicity and biocompatibility.
Inspired by the pioneering work of Hillmyer and Tolman,[11a-b]
only few iron catalysts were reported for ROP of LA and most of
these systems exhibit quite modest catalytic activities and
productivities and proceed only at high temperatures.[11] As a
consequence, their direct use in ROP reactions is energy
intensive and requires drastic conditions to drive the reaction
towards high conversion and to obtain high-molecular-weight
products. In this context, effective and stereoselective lactide
Dr. L.-M. Chamoreau
Sorbonne Université, CNRS
IPCM-UMR 8232, B.C. 229,
4 place Jussieu, 75252 Paris Cedex 05, France
V. Guérineau
Institut de Chimie des Substances Naturelles, CNRS UPR2301,
Université Paris-Sud, Université Paris-Saclay,
Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
Dr. I. del Rosal, Prof. Dr. L. Maron
1. Université de Toulouse; INSA, UPS; LPCNO (IRSAMC); 135
avenue de Rangueil, F-31077 Toulouse, France. 2. CNRS; UMR
5215 (IRSAMC); F-31077 Toulouse, France
Prof. Dr. V. Venditto
Department of Chemistry and Biology A. Zambelli,
INSTM Research Unit, University of Salerno,
Via Giovanni Paolo II 132, I-84084 Fisciano, SA, Italy
E-mail: vvenditto@unisa.it
polymerization under mild reaction conditions remains
a
challenge with iron complexes. As part of our ongoing effort to
develop new catalysts for the ring-opening polymerization of
cyclic esters,[12,13] we report herein the synthesis and reactivity of
a new class of iron complexes. Interestingly, the achiral iron
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201903224
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COMMUNICATION
structures, merge for the first time the three important features
of high activity and high molecular weight with control of the
polymer microstructure, affording the desired stereoblock PLAs.
Furthermore, all three factors can be influenced by catalyst
design, opening the way to a broad family of high molecular
weight thermally stable PLA materials.
The monophenolate pro-ligands 1-5 described in this study
were prepared in a straightforward manner, by reductive
amination, following a one-pot procedure using commercially
available or easily accessible starting materials (Figure 1).[14,15]
Such ligand architecture allows for steric and electronic
variations on the different donor fragments (Figure 1). The new
compounds 2, 3, 4, and 5 were isolated with good to excellent
yields. All compounds were obtained as white powders, and their
identity was confirmed by NMR and high-resolution mass
spectrometry or elemental analysis. All spectroscopic data of 1-
5 are given in the Experimental Section.
Scheme 1. Iron complexes 6-11.
The coordination chemistry of the ligands was studied
towards an iron bisamide derivative.[16,17] Homoleptic metal
precursor Fe[N(SiMe3)2]2 was therefore reacted with one equiv
of the neutral mono(phenol)s HL 1-5 to give the corresponding
iron complexes {(L)Fe[N(SiMe3)2]} 6-10 in good yields (Scheme
1).[18,19] The products were easily isolated as solids by
evaporation of volatiles and washing of the residues with
pentane. The compounds are air- and moisture-sensitive, readily
soluble in aromatic hydrocarbons and slightly soluble in aliphatic
hydrocarbons. These complexes were characterized by mass
spectrometry, elemental analysis and X-ray diffraction studies for
11, which can also give the iron amido complex through a salt
metathesis route. Single crystals of the new trityl-substituted
complex 11 suitable for X-ray diffraction were obtained by slow
evaporation of a saturated toluene solution at room temperature
(Table S1). X-Ray crystallographic analysis of a single crystal of
complex 11 revealed the monomeric nature of the complex in the
solid state (Figure 2). Complex 11 features a five-coordinated
iron centre in a distorted bipyramidal trigonal geometry. The two
pyridyl groups per molecule are coordinated, together with the N
Figure 2. Molecular structure of 11 and optimized structure of complex A.
Hydrogen atoms and solvent molecule are omitted for clarity. Yellow iron, blue
N, red O, green Cl.
Amido complexes 6-10 are active initiators for the controlled
ROP of rac-lactide under mild conditions (Scheme 2).
Representative polymerization data are summarized in Table 1.
Homopolymerization of rac-lactide with the prepared metal
complexes proceeds rapidly at 20°C for monomer-to-initiator
ratios up to 800. Also, microstructural analysis of the different
PLAs formed from rac-LA revealed that the metal complexes 6-
10 exert at room temperature a significant influence on the
tacticity of the polymer formed, as the microstructure is
moderately to highly isotactic (up to 92% isotacticity).[20] All
values obtained from the 1H spectra were correlated to the ones
observed by 13C NMR (Figures S3-S10). The corresponding
statistical analysis confirmed a chain-end control mechanism
(Figures S3-S6, Tables S3 and S4). Thus, the stereogenic centre
from the last enchained monomer unit influences the
stereochemistry of monomer enchainment. This stereochemical
regulation was confirmed by the observation that stereochemical
errors are propagated during the polymerization process. To
examine the influence of the metal centre on the microstructures
of the produced PLAs, the lactide polymerization ability of
complex 6 was first studied in detail. This amido complex acts as
an efficient initiator for the ring-opening polymerization of
and
O atoms of the ligand, forming one six-membered
metallacycle and two five-membered metallacycles. Therefore,
the tripodal aminophenolate is actually tetradentate.
Figure 1. Synthesis of mono(phenolate) ligands 1-5.
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10.1002/anie.201903224
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racemic lactide under mild conditions to give isotactic PLA (Table
1, Entry 1). We assume that the environment around the iron
metal centre is hindered, with all donor atoms coordinated to the
metal centre.[21] As shown in the optimized structure of complex
A, the formation of five-coordinated Fe species seems to be
strongly favoured. Indeed, DFT investigations showed that an
isopropoxy iron complex A gives a stable five-coordinated iron
species able to ring-open LA, which confirmed the geometry
observed for the solid-state structure of complex 11 (Figures 2,
S11 and S12).
close to the theoretical ones (calculated on the assumption that
each isopropoxy group initiates the polymerization). These data
indicate that the polymerization proceeded in a living fashion as
confirmed by MALDI-TOF mass spectrometry (Figure S13). Also,
changing the amido initiator by an alkoxy initiator does not affect
the selectivity for isotactic PLA (Table 1, Entries 1 and 3), which
is consistent with initiation of polymerization by the amido or
isopropoxide group and then assistance of the ligand-metal
fragment on stereocontrol.
To study the influence of the ligand substituents on the
microstructures of the produced PLAs, the lactide polymerization
ability of complexes 6-10 having a series of substituents was
then examined. The factors investigated include steric and
possible electronic effects derived from the substituents on the
aromatic rings, as well as on the (potentially) pendant pyridine
arm.[12,13,22] Bulky groups were chosen as the ortho-phenolate
substituent R1 in order to favour mononuclear complex formation
and increase the influence of the stereogenic centre of the last
inserted monomer, which controls the stereochemistry of
propagation.[23] Complexes 7 and 8, which bears ortho-methoxy
or ortho-isopropyl groups at the phenolate rings, gave relatively
low selectivity for isotactic poly(lactic acid) (Table 1, Entries 5
and 6-7). This lack of tacticity is also consistent with previous
observations that bulky ligands are required for stereochemical
discrimination in the polymerization of rac-lactide via a chain-end
control mechanism.[23]
Scheme 2. Synthesis of PLA.
With these results in hand, we were then interested in
evaluating the catalytic reactivity of iron-based systems by
varying different parameters (Table 1, Entries 2-5). Addition of
isopropanol to the polymerization reaction resulted in better
initiation efficiency and then affected the activity of the iron
complexes. All the polymers produced with these in-situ
generated alkoxide derivatives have narrow molecular weight
distributions and number-average molecular masses (Mn) values
Table 1. Polymerization of rac-lactide.[a]
[e]
[e]
Entry
1
Complex
[M]/[I]/[iPrOH]
200/1/0
200/1/0
200/1/1
800/1/1
200/1/1
800/1/1
800/1/1
400/1/1
200/1/1
100/1/1
100/1/1
Time [min]
Yield[b][%]
Mn[c] [kDa]
112.5
73.0
PDI[c]
1.32
1.43
1.04
1.08
1.20
1.14
1.38
1.09
1.14
1.16
1.16
[mm][d][%]
0.84
ΔHm
Tm
6
6
45
90
20
120
1
83
96
82
65
75
67
82
48
93
84
43
ND
ND
29
19
ND
21
14
27
1
ND
ND
2[g]
3
0.78
6
24.8
0.84
173
162
ND
4
6
61.5
0.83
5
7
16.7
0.79
6
8
1
51.9
0.82
152
142
155
136
193
195
7
8
1.5
0.5
7
79.9
0.79
8
9
29.9
0.83
9
10
6
21.3
0.68
10[h]
11[i]
105
180
13.5
0.91
42
48
6
6.7
0.92
[a] [a] All reactions performed at RT in toluene at [lactide] = 1 M. [b] As determined by the integration of 1H NMR methine resonances of lactide and PLA.
[c] Mn and Mw/Mn of polymer determined by SEC-RI in THF at RT using polystyrene standards and Mn corrected by the Mark-Houwink parameter (0.58).
[d] Pm is the probability of forming a new m-triad. [e] Melting temperature and enthalpy of first DSC run. Values are given in °C and J/g, respectively. [f]
Not Determined. [g] Polymerization performed in THF. [h] Polymerization performed at 0 °C. [i] Polymerization performed at -10 °C
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10.1002/anie.201903224
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The polymerization data also indicate that, when the steric
hindrance is sufficient, the ortho-substituents on the ligand do not
significantly affect the ability of the initiator to control monomer
enchainment; for instance, changing the ligand ortho-substituents
from trityl to sec-butyl groups results in almost no change in
isotacticity (Table 1, Entries 4 and 8). However, by substituting a
tetradentate ligand with a tridentate ligand (complex 10, Table 1,
Entry 9), we evidenced a decrease of the polymerization
stereoselectivity (Pm = 0.68). This influence may be due to the
vacant coordination site, which leads to additional monomer
addition site and consequently affect the catalyst selectivity and
the polymer tacticity. Finally, it appears that the temperature of
the reaction can lead to a pronounced isotacticity enhancement
of the resulting polymerization. Indeed, the PLAs produced by 6
at 0 °C or -10 °C is highly isotactic with Pm values of 0.91 and 0.92
respectively (Table 1, Entries 10-11).
We were then interested in studying the main thermophysical
properties (glass transition temperature Tg, melting temperature
Tm, melting enthalpy ΔHm, crystallization temperature Tc, and
crystallization enthalpy ΔHc) of selected PLA samples to evaluate
the influence of the polymer isotacticity. Representative values for
both first and second DSC heating runs are reported in Table S5.
Thermograms of the first heating runs of all samples are reported
in Figure S15.
Figure 4. Plot of Tg values (from second DSC heating runs) vs. isotacticity
degree for PLAs produced via ROP of rac-LA. Selected samples are the ones
from Table S5, Entries 1-9.
As noted earlier, PLA performances in terms of durability are
usually limited by thermal decomposition. However, in our case,
thermal analysis revealed decomposition temperatures of up to
350°C, similar to the ones of aromatic terephthalate polyesters.[30]
Degradation temperatures for all PLA samples are listed in Table
S5. The significant rise in degradation temperature supports the
hypothesis that the thermal stability of these isotactic PLA
stereoblocks is strongly dependent on molecular weight (Figures
5 and S14). In marked contrast with what was observed for other
PLA stereocomplexes, these results clearly show a strong
correlation between the molecular weight and the degradation
temperature and almost no correlation between the degree of
stereoregularity and the degradation temperature. This
dependence can be explained by an unzipping depolymerization
mechanism.[24]
Except for the non-stereoregular PLA (Table S5, Entry 1), all
samples are crystalline (Table S5, Entries 2-9), and present a
single transition endotherm at high temperature (Table S6, Figure
S15). Wide-angle X-ray diffraction (WAXD) analysis reveals that
all polymers with Pm ≥ 0.64 have the typical pattern of PLLA-PDLA
stereocomplex crystal form, resulting from the co-crystallization of
isotactic blocks of opposite configuration. The corresponding
WAXD patterns are reported in Figure S17. Crystallinity,
estimated by comparing the sample melting enthalpies of first
DSC heating runs to melting enthalpy of crystal of infinite size
(142-146 J/g),[24] ranged from 10 to 20% for samples with Pm
ranging from 0.78 to 0.84, respectively (Table S5, Entries 2-7).
Although all DSC analysed samples do not crystallize from melt,
samples with high isotacticity present a small exothermic peak,
which is indicative of a weak crystallization phenomenon, in the
second DSC heating run. The Tm values of crystalline samples of
Table S5 increase linearly with the degree of isotacticity (Figure
S16). A similar behaviour for PLA stereoblocks obtained from rac-
LA had already been observed by Nomura (whose data are also
shown in Figure S16), Du, Chen and Kol.[25]
We also observed a correlation between the degree of
stereoregularity and the glass transition temperature. Indeed, a
linear relationship between Tg and Pm is apparent in Figure 4.[26]
An influence of polymer tacticity on Tg was already shown for
PLAs,[27] and Tg values ranging from 34°C and 49°C for
syndiotactic[28] and hetereotactic PLAs,[29] respectively, were
described. However, for the first time, a linear dependence
between Tg and the degree of stereoregularity of PLAs is clearly
evidenced.
Figure 5. Representative thermal degradation curves of selected samples with
increasing Mn values. Selected samples are the ones from Table S5, Entry 4
(Mn = 61 kDa, Pm = 0.83), Table S5, Entry 5 (Mn = 21 kDa, Pm = 0.80), Table S5,
Entry 6 (Mn = 80 kDa, Pm = 0.78), Table S5, Entry 9 (Mn = 23 kDa, Pm = 0.64).
These high molecular weight stereoblock PLAs may be useful
as materials with high thermal stability for various application
fields. Indeed, the main limit of industrial use of PLA
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10.1002/anie.201903224
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COMMUNICATION
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Y. T. Tam, C. Huang, M. Poellmann, G. S. Kwon, ACS Nano, 2018, 12,
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stereocomplex crystal form is the low degradation temperature of
samples obtained from equimolar mixtures of PLLA and PDLA,
which, coupled with the high melting temperature of the
stereocomplex crystal form, reduce the processing temperature
range of this polymer. For this reason, getting highly stereoregular
PLA stereoblocks having high melting and degradation
temperatures represents a highly relevant industrial interest result.
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We have reported a new series of iron complexes supported
by readily available tripodal ligands. These iron catalysts are
highly active and stereoselective in mild conditions for the ROP of
lactide. The corresponding versatile systems enable the
production of stereoblock polymers with high molecular weights,
allowing the formation of thermally stable stereocomplexes.
These results suggest a number of new avenues for industrial
applications of PLA stereocomplexes.
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Acknowledgements
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CNRS, ENSCP, ANR (grant ANR-10-PDOC-010-01), Fondation
Pierre-Gilles de Gennes, and French Ministry of Research and
Higher Education are thanked for financial support of this work.
Financial support from the TGIR-RMN-THC Fr3050 CNRS for
conducting the research is gratefully acknowledged. The authors
would like to thank Purac for a generous loan of rac-lactide. CMT
is grateful to the Institut Universitaire de France.
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Conflict of interest
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The authors declare no conflict of interest.
Keywords: iron • homogeneous catalysis • tacticity • polymer
stereocomplex • stereoblock
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