Gradient isotactic multiblock polylactides from aluminum complexes of chiral salalen ligands

Article abstract of DOI:10.1021/ja412798x

Aluminum complexes of enantiomerically pure aminomethylpyrrolidine-based salalen ligands and their application in the stereoselective polymerization of lactide are described. Poly(lactic acid) featuring the new gradient isotactic multiblock microstructure was synthesized by isoselective catalysts, which operate by a combination of enantiomorphic-site and chain-end control mechanisms.

Full text of DOI:10.1021/ja412798x

Communication
pubs.acs.org/JACS
Gradient Isotactic Multiblock Polylactides from Aluminum
Complexes of Chiral Salalen Ligands
Alessia Pilone,† Konstantin Press,‡ Israel Goldberg,‡ Moshe Kol,*,‡ Mina Mazzeo,†
and Marina Lamberti*,†
†Dipartimento di Chimica e Biologia, Universita
̀
di Salerno, via Giovanni Paolo II, 132, 84084 Fisciano, Italy
‡School of Chemistry, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel
S
* Supporting Information
hitherto unknown polymeric microstructure, viz., the gradient
isotactic multiblock PLA.9,10
ABSTRACT: Aluminum complexes of enantiomerically
pure aminomethylpyrrolidine-based salalen ligands and
their application in the stereoselective polymerization of
lactide are described. Poly(lactic acid) featuring the new
gradient isotactic multiblock microstructure was synthe-
sized by isoselective catalysts, which operate by a
combination of enantiomorphic-site and chain-end control
mechanisms.
Salalens11 are {ONNO}-type ligands including an imine- and
an amine neutral donors and two phenolate arms, whose
complexes have found use as catalysts for isospecific polymer-
ization of α-olefins12 and as asymmetric oxidation catalysts.13
They can be readily synthesized from the corresponding
diamines by a two-step condensation-substitution sequence.
Aluminum complexes of salalen ligands, recently described by
Jones, were found to polymerize rac-LA giving PLAs whose
microstructures ranged from medium-heterotactic to medium-
isotactic as a function of the salalen structure.14,15 We recently
found that aminomethylpyrrolidine based salalen ligands which
may be easily prepared in their enantiomerically pure form wrap
around group 4 metals in a highly diastereoselective manner
giving rise to enantiomerically pure stereogenic-at-metal
complexes.12a Accordingly, the four chiral salalen ligand
precursors described here were synthesized as single enantiomers
from aminomethylpyrrolidine. To address structure−activity
relationships, different combinations of bulky and electron-
withdrawing phenolate substituents were chosen. The salalen
aluminum complexes of the type [{ONNO}Al(OiPr)] were
prepared either by treatment of these ligand precursors with
equimolar amounts of AlMe3 and subsequent reaction with 2-
propanol or by direct reaction with Al(OiPr)3 (Scheme 1). For
characterization of the complexes, see the Supporting
Information (SI).
he synthesis of polyesters by the catalytic ring-opening
Tpolymerization (ROP) of cyclic esters is attracting a
considerable current interest. Poly(lactic acid) (PLA) prepared
by the ROP of lactide (LA)1,2 combines several attractive
properties: (1) It is an environmentally friendly material derived
from annually renewable resources which can be decomposed
postconsumption to nontoxic materials.3 (2) The presence of
two stereogenic centers in the LA monomer may give rise to
different polymer stereoregularities which affect properties such
as crystallinity and rate of hydrolysis.4 Polymerizations of the
homochiral L- or D-LA yield the respective isotactic polymers
PLLA and PDLA, whereas polymerizations of the racemic
monomer rac-LA may give rise to different stereoregularities
ranging from isotactic via atactic to heterotactic PLA depending
on the preference of the employed catalyst to elect a certain
monomer. Stereoselection may be achieved by either enantio-
morphic-site control mechanism (SCM; homochiral preference
by the catalyst chirality) or chain-end control mechanism (CEM;
homo- or heterochiral preference by the last inserted repeat
unit). The possible exchange of polymeryl chains between active
catalyst molecules may further diversify the microstructure.
Aluminum complexes4,5 led to the highest stereocontrol in rac-
LA polymerization with important examples including Spassky’s
complexes6 of chiral salen ligands that gave highly isotactic
diblock PLA by an SCM controlled kinetic resolution, Nomura’s
complexes7 of achiral salen ligands that gave highly isotactic
multiblock PLA by the CEM control, and Gibson’s salan
complexes8 that gave PLAs of different tacticities as a
consequence of the phenolate substituents. Notably, all these
aluminum complexes included tetradentate-dianionic sequential
{ONNO}-type ligands. Here we describe the polymerization of
homochiral-LAs and rac-LA by aluminum complexes of chiral
salalen ligands. We demonstrate that SCM and CEM stereo-
control mechanisms act simultaneously, and lead to a proposed
1H NMR spectra of the salalen complexes indicated that they
were all obtained as single diastereomers despite the presence of
several sources of stereoisomerism, including the stereogeneity at
the amine-donor and the Al-center and the helicity of the ligand
wrapping. Thus, the ability of the aminomethylpyrrolidine
Scheme 1
Received: July 22, 2013
Published: February 12, 2014
© 2014 American Chemical Society
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Communication
backbone to induce chirality-at-metal is demonstrated around
this pentacoordinate aluminum center. In contrast, aluminum
complexes of salalen ligands having the trans-1,2-diaminocyclo-
hexane chiral backbone were obtained as mixtures of
diastereomers in solution.14c Single crystals of an ethyl-
aluminum complex of proligand (R)-3H2 were crystallized
from pentane at −35 °C and the molecular structure as
determined by X-ray diffraction is shown in Figure 1. The
between the ligand substitution pattern and the PLA stereo-
regularity. The isolated or in situ formed complex (S)-1b which
features halo substituents on the imine-side phenol and bulky
alkyl groups on the amine-side phenol gave PLAs of a
heterotactic nature (runs 1 and 2). In contrast, all complexes
with the opposite phenolate substitution pattern, viz., bulky alkyl
substituents on the imine-side phenol and halo groups on the
amine-side phenol gave isotactic PLAs (runs 3−6). A similar
inclination was reported for other salalen aluminum complexes.
However, in those cases the stereocontrol was lower and the
presence of chiral ligands on the aluminum center was
detrimental for activity.14c Complexes (S)-2b and (S)-3b,
which include either adamantyl or tert-butyl groups on the
imine-side phenol and chloro substituents on the amine-side
phenol, gave the highest isotacticities with identical values of Pm
= 82% (runs 3 and 4). Increasing the size of the halo group to
iodo resulted in substantial decrease of isotacticity (runs 4 and 5).
Employing the racemic version of catalyst 3b, prepared by mixing
equimolar quantities of the homochiral complexes, also resulted
in decrease of isotacticity (runs 4 and 6), invoking the
involvement of polymeryl exchange events.16,17
Figure 1. ORTEP representation of the structure of (R)-3-Al-Et.
Selected bond lengths (Å) and angles (°): O1-Al 1.794, O2-Al 1.816,
N1-Al 1.984, N2-Al 2.326, C25-Al 1.974, O2-Al-N2 160.1.
The availability of these pyrrolidine-based salalen ligands in
their enantiomerically pure form, their very highly diastereose-
lective wrapping around aluminum, and their tendency to give
relatively high degrees of isotacticity in rac-LA polymerization
prompted us to conduct further studies to elucidate the
stereocontrol mechanism. To avoid complications due to
polymeryl exchange, we employed only enatiomerically pure
complexes. First, we followed the kinetics of polymerization of
rac-, D-, and L-LA by complex (S)-3b in toluene-d8 at 70 °C by
aluminum adopts a geometry intermediate between trigonal-
bipyramidal (with the imine-side phenolate O-donor and the
pyrrolidine N-donor occupying the apical positions) and square
pyramidal (with the ethyl group at the axial vertex). The (R)-
configuration of the stereogenic center was established, and the
bond lengths and angles are close to those found for the
aluminum complexes of nonchiral salalen ligands.14a We propose
that this geometry is retained in solution.
Preliminary experiments revealed that all the aluminum
complexes of these chiral salalen ligands polymerized rac-LA
with activities typical of aluminum catalysts. Representative
polymerization results are summarized in Table 1. The molecular
1
monitoring the monomer consumption over time by H NMR
spectroscopy.18 The semilogarithmic plots are shown in Figure 2.
Table 1. Ring−Opening Polymerization of rac-LA Promoted
a
by Salalen Aluminum Complexes
d
calc
run
I
conv.
(%)
Mn
Mn
PDI
Pm
b
c
(kg/mol)
(kg/mol)
e
1
(S)-1a
(S)-1b
(S)-2b
(S)-3b
(R)-4b
rac-3b
73
75
42
78
91
85
10.5
10.8
6.1
9.1
8.5
6.8
9.0
9.7
9.2
1.09
1.07
1.05
1.07
1.07
1.05
0.23
0.24
0.82
0.82
0.59
0.70
2
3
4
5
6
11.2
13.1
12.3
Figure 2. Pseudofirst-order kinetic plots for ROP of lactides (L-LA; D-
LA; rac-LA) promoted by (S)-3b. [Al] = 1.0 × 10−2 M; [LA]/[Al] =
100; T = 70 °C; toluene-d8 as solvent. kL‑LA/kD‑LA = 10.
a
General conditions: initiator, 20 μmol; toluene, 2 mL; [rac-LA]0/
b
[Al], 100; temperature, 80 °C; time, 24 h. Calculated Mn of PLA (g/
mol) = 144.14 × ([LA]0/[Al]) × conversion (LA). Experimental Mn
values determined by GPC analysis in THF. Determined from the
methine region of the homonuclear decoupled H NMR spectrum.
Calculated from Bernoullian statistics. 20 μmol of iPrOH was added
to the initiator.
Two trends immediately emerge. (1) A first-order kinetics is
found for all monomers, viz., all the polymerizations obey the
following kinetic law: −d[LA]/dt = kapp[LA] and (2) a clear
preference for the polymerization of L-LA over that of D-LA with
a substantial rate ratio of σ = kapp(L‑LA)/kapp(D‑LA) = 10 is found.6
The close values of kapp(rac‑LA) and kapp(D‑LA) suggest that the
polymerization of D-LA is the rate determining step in the
polymerization of rac-LA by (S)-3b (and mirrored for (R)-3b).
We followed the consumption of rac-LA by (S)-3b with time and
found that the first order kinetics persisted at least until 90%
conversion (Figure S1). Moreover, the opposite enantiomeric
complex (R)-3b was found to exhibit the opposite rates in the
polymerization of L- and D-LA and the same rate toward rac-LA
(Figure S2), ruling out any technical sources for the rate
difference. These two trends are inconsistent with either a pure
c
d
1
e
weights of the obtained PLAs were in good agreement with the
calculated values, and the molecular weight distributions were
narrow (PDI < 1.1), supporting a well-controlled polymer-
ization. Addition of 2-propanol to the corresponding methyl
complex (S)-1a in the presence of rac-LA gave a catalyst whose
activity was practically identical to that of the isolated isopropoxo
complex (S)-1b (runs 1 and 2). The 1H NMR homodecoupled
spectra of the obtained PLAs revealed a clear relationship
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Communication
chain-end control mechanism for which the consumption of L-
and D-LA should be of identical rates, or with a pure
enantiomorphic site control (kinetic resolution) mechanism
for which the polymerization rate of rac-LA should decrease as
the faster of the two enantiomers is being consumed.19
Consistently, the obtained isotactic PLA cannot possess any of
the known microstructures of either multialternating D/L-blocks
of identical average lengths (CEM), or of a tapered diblock
copolymer (SCM). We hypothesize that the CEM and the SCM
mechanisms act simultaneously and the polymer obtained
exhibits the proposed novel microstructure of gradient isotactic
multiblock PLA, viz., consisting of D- and L-blocks of gradually
exchanging lengths.
inverted as the concentration ratio of the two enantiomers
changes.
Finally, we conducted tetrad analysis of the homonuclear
decoupled 1H NMR spectra, to elucidate the relative roles of the
CEM and SCM mechanisms (Figure 3). Second in intensity to
To obtain further insight on the formation of the hypothesized
gradient multiblock PLA chains, polymerization runs of rac-LA
for different times and consequent different conversions were
1
carried out with complex (S)-3b (Table 2). The H NMR
Table 2. Polymerization of rac-Lactide at Different
a
Conversions by Complex (S)-3b
b
c
1
25
run
time
(h)
conv.
(%)
Mn
PDI
Pm
[α]D
Figure 3. Methine region of the homonuclear-decoupled H NMR
b
spectra of isotactic PLAs at different conversions produced using
initiator (S)-3b (runs 7, 8, and 4 in Table 2).
(kg/mol)
d
e
f
7
5
10
15
24
30
26
48
60
78
86
4.0
n.d.
0.79
0.80
0.82
0.82
0.82
−30
−25
−20
−16
−9
8
5.1
7.1
9.0
9.5
1.12
1.08
1.07
1.08
9
the isotactic mmm tetrad are the tetrads corresponding to the
stereoblock −SSSSRRRR-type sequences, viz., the mmr, mrm,
and rmm tetrads. However, the occurrence of the
−SSSSRRSSSS-sequence corresponding to a single misinsertion
of the opposite enantiomer is also evident by the presence of the
rmr tetrad. We found that the relative ratio of the rmr tetrad
diminishes in respect to both the mmm and the rmm tetrads as a
function of conversion. Namely, the CEM control becomes more
dominant as a function of conversion. For the polymeric samples
obtained in runs 7 (26% conversion) and 4 (78% conversion), we
calculated the tetrad probabilities based on both Bernoullian
(CEM) and SCM statistics, and these values were compared with
the experimental values obtained by the NMR analysis (Tables
S1, S2). At 26% conversion, the experimental values deviated
significantly from the theoretical values calculated by both
statistical treatments, but for the 78% conversion, a good
agreement was observed with the tetrad probabilities based on
the Bernoullian statistics.
These results indicate that CEM is the prevailing mechanism
of sterocontrol operating in the polymerization experiments
carried out with this class of isospecific chiral catalysts, although
at low conversions the contribution of the SCM is apparent.25
Consequently, the polymeric chains are not symmetrical,
including longer blocks of the faster reacting enantiomer having
occasional misinsertions close to the isopropoxo terminus, and
longer blocks of the slower reacting enantiomer free of
misinsertions close to the hydroxo terminus.
In conclusion, we introduced well-defined single-diastereo-
meric aluminum complexes of enantiomerically pure amino-
methylpyrrolidine-based salalen ligands that acted as active
catalysts in lactide polymerization, with stereoselection derived
from their specific substitution pattern. Enantiomerically pure
isoselective catalysts were found to act by a combination of
enantiomorphic-site and chain-end control mechanisms, and led
to PLA of a proposed new microstructure, viz., the gradient
isotactic multiblock PLA. We are currently investigating the
properties of this architecture,26,27 and extending the polymer-
ization studies of these catalysts to other polymerizations.
4
10
a
General conditions: initiator, 20 μmol; toluene, 2 mL; [rac-LA]0/
[Al], 100; temperature, 80 °C. Experimental Mn values were
determined by GPC analysis in THF. Determined from the methine
region of the homonuclear decoupled H NMR spectrum. Initiator,
30 μmol. Determined from H NMR. n.d. = not determined.
b
c
d
1
e
f
1
spectrum of the sample at low conversion after hydrolytic
workup (run 7) showed the existence of HOCH(CH3)CO- and
-OCH(CH3)2 as exclusive chain end groups. In addition, linear
increase of Mn with conversion, and narrow Mw/Mn ratios at all
conversions were found. These findings point to the aluminum-
isopropoxide group as the sole initiating anchor, supporting the
integrity of the salalen-aluminum framework during the
polymerization. Chain growth proceeds by consecutive
insertions of the lactide acyl-oxygen bond to the aluminum-
alkoxo bond of the growing polymeryl chain in a highly living
manner. The obtained polymers were analyzed by optical
rotation measurements in chloroform. All samples had negative
25
[α]D values confirming that the L-LA is preferentially
polymerized by (S)-3b.20,21 However, the low enantiomeric
excess of L-LA units in the obtained polymers (maximum value:
21% for the sample of run 7), determined by comparison of the
optical rotation of the polymeric samples with the optical
rotation of PLLA samples having the same molecular weights,
indicated that both lactide enantiomers are consumed even at
low conversions. The selectivity factor (s), defined as the ratio of
the polymerization rates of the faster reacting monomer over that
of the slower reacting monomer, can be determined from the %
ee and conversion according to Kagan’s equation,22−24 and was
calculated to be 1.6 in favor of L-LA in the polymerization of rac-
LA by (S)-3b. This value represents a low preference for the
faster enantiomer,23b and is in line with the hypothesis of a
multiblock copolymerization of the two enantiomers in which at
low conversion L-LA is preferentially consumed by (S)-3b, giving
L-LA blocks longer than D-LA blocks. This trend is gradually
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(14) (a) Whitelaw, E. L.; Loraine, G.; Mahon, M. F.; Jones, M. D.
Dalton Trans. 2011, 40, 11469. (b) dos Santos Vieira, I.; Whitelaw, E. L.;
Jones, M. D.; Herres-Pawlis, S. Chem.Eur. J. 2013, 19, 4712.
(c) Hancock, S. L.; Mahon, M. F.; Jones, M. D. Dalton Trans. 2013,
42, 9279.
(15) For salalen complexes of other metals employed for lactide
polymerization see: (a) Whitelaw, E. L.; Jones, M. D.; Mahon, M. F.
Inorg. Chem. 2010, 49, 7176. (b) Whitelaw, E. L.; Davidson, M. G.;
Jones, M. D. Chem. Commun. 2011, 47, 10004. (c) Nie, K.; Gu, W.; Yao,
Y.; Zhang, Y.; Shen, Q. Organometallics 2013, 32, 2608.
ASSOCIATED CONTENT
* Supporting Information
Detailed experiments protocols, kinetics data and tetrad
distribution analysis; CCDC 950722 with crystallographic data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
■
Corresponding Authors
moshekol@post.tau.ac.il
mlamberti@unisa.it
(16) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 1316.
(17) Polymerization of meso-LA with (R)-3b and with rac-3b led to
syndiotactically and heterotactically inclined PLA’s, respectively (SI).
The formation of syndiotactic-PLA is governed by homochiral
preference of an SCM control mechanism, and is consistent with the
prevalence of this mechanism at the early stages of rac-LA polymer-
ization. Polymeryl exhange between propagating species is degenerate
for an enantiomerically pure catalyst. This degeneracy is lifted for the
racemic catalyst. The observed reversal of tacticity to heterotactic is an
unequivocal support for polymeryl exchange between enantiomorphous
metal centers.16
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Dedicated to Prof. Adolfo Zambelli on the occasion of his 80th
birthday. We thank Ilaria D’Auria for GPC measurements,
Patrizia Oliva for NMR technical assistance, Fabia Grisi for GC
measurements and Vincenzo Venditto for PLA characterization.
M.K. thanks the Israel Science Foundation and “Shikun & Binui”
for support. We thank Purac for a generous gift of lactides.
(18) Homodecoupled 1H NMR spectra of the PLA samples obtained
from enantiopure monomers (D- and L-LA) were perfectly isotactic
testifying that no epimerization reactions occur.
(19) Du, H.; Velders, A. H.; Dijkstra, P. J.; Sun, J.; Zhong, Z.; Chen, X.;
Feijen, J. Chem.Eur. J. 2009, 15, 9836.
REFERENCES
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the same conditions of run 10 (conversion 90%) showed an [α]D25 of +8
indicating that this catalyst preferentially polymerizes the D-LA
enantiomer.
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