Yttrium phosphasalen initiators for rac -lactide polymerization: Excellent rates and high iso-selectivities

Article abstract of DOI:10.1021/ja310003v

Highly active yttrium phosphasalen initiators for the stereocontrolled ring-opening polymerization of rac-lactide are reported. The initiators are coordinated by a new class of ancillary ligand: an iminophosphorane derivative of the popular "salen" ligand

Full text of DOI:10.1021/ja310003v

Communication
pubs.acs.org/JACS
Yttrium Phosphasalen Initiators for rac-Lactide Polymerization:
Excellent Rates and High Iso-Selectivities
Clare Bakewell,†,§ Thi-Phuong-Anh Cao,‡,§ Nicholas Long,† Xavier F. Le Goff,‡ Audrey Auffrant,*,‡
and Charlotte K. Williams*,†
†Department of Chemistry, Imperial College London, London SW7 2AZ, U.K.
‡Laboratoire Het
eroelements et Coordination, Ecole Polytechnique, CNRS, 9118 Palaiseau, France
́ ́ ́ ́
S
* Supporting Information
improved properties.3 Despite many reports of new initiators,
there remain only a few able to produce isotactic PLA from rac-
LA.2 Among the iso-selective initiators reported previously, the
majority of studies focused on Al−salen complexes and their
derivatives.4 Since the first reports, much attention has focused
on Al5 and other group 13 initiators.6 Although their optimized
iso-selectivities (Pi) exceed 0.9, these initiators are all marred by
low rates of polymerization (typically taking days to reach
completion), the need for high initiator loadings (typically ∼1
mol %) and a narrow range of operating conditions (temper-
atures >70 °C in toluene solution). Examples of iso-selective
initiators using elements other than those in group 13 remain
extremely rare.7 One very interesting example is the homochiral
yttrium phosphine oxide alkoxide complex reported by Arnold
and co-workers, for which Pi = 0.81 at 288 K.8
ABSTRACT: Highly active yttrium phosphasalen initia-
tors for the stereocontrolled ring-opening polymerization
of rac-lactide are reported. The initiators are coordinated
by a new class of ancillary ligand: an iminophosphorane
derivative of the popular “salen” ligand, termed “phospha-
salen”. Changing the phosphasalen structure enables access
to high iso-selectivities (Pi = 0.84) or hetero-selectivities
(Ps = 0.87). The initiators also show very high rates,
excellent polymerization control, and tolerance to low
loadings; furthermore, no chiral auxiliaries/ligands are
needed for the stereocontrol. The combination of such
high rates with high iso-selectivities is very unusual.
olylactide (PLA) is a leading bioderived polymer sold
We are interested in iminophosphorane (NP) ligands as
alternatives to conventional imine-based ligand systems.9
Concurrently, we have observed that yttrium- and lanthanide-
based LA polymerization initiators show significantly higher rates
with electron-donating ligands.10 Conventionally, increased
activity has been expected upon substitution of Lewis acidic
initiators with electron-withdrawing functionalities.2 To inves-
tigate further the electronic influence of the ancillary ligand, we
targeted iminophosphorane analogues of the ubiquitous salen
ligands. We have named these new ligands “phosphasalens”.
Earlier this year, we reported the first examples of yttrium
phosphasalen alkoxide complexes, which proved to be effective
LA polymerization initiators.11 Herein we report the new
phosphasalen initiators 1−3 (Figure 1), which operate with
very high rates and can exert high degrees of stereocontrol.
The proligands L1 and L2 were synthesized according to
modifications of published procedures [see Schemes S1 and S2 in
the Supporting Information (SI)].11 Initiators 1−3 were
prepared by addition of stoichiometric amounts of KN(SiMe3)2
to slurries of the corresponding proligands in tetrahydrofuran
(THF). This gave colorless suspensions after 4 h. The reactions
were monitored using 31P{1H} NMR spectroscopy, which
showed significant shifts in the singlet signals from 41.8 to 24.2
ppm for L1 and 40.1 to 20.3 ppm for L2 (R2 = OMe). This
indicated the clean formation of the anionic potassium salts.
Addition of [YCl3(THF)3.5] gave products that showed just one
singlet in the 31P{1H} NMR spectra at 35.0 and 31.5 ppm,
P
worldwide for applications such as packaging and fiber
technology, and it is an important biomedical material for wound
healing, controlled release, and tissue regeneration.1 PLA is
produced from biomass and degrades to metabolites; therefore,
replacing petrochemicals with PLA can reduce environmental
impacts, in particular minimizing gas emissions and waste
pollution.1a PLA is produced by ring-opening polymerization
(ROP) of lactide (LA); commercially, this process is catalyzed
using tin(II) octanoate. New initiators with higher activities
(rates), greater degrees of polymerization control, and the ability
to control the PLA stereochemistry starting from racemic LA
(rac-LA) are much needed.2 The stereochemistry or tacticity of
PLA affects its physical and chemical properties, including its
melting point, mechanical strength, degradation rate, and
rheological and barrier properties.2b Thus, a range of polymer
microstructures can be produced starting from rac-LA. Most
initiators yield atactic or heterotactic PLA, which is useful but has
undesirable thermal and mechanical properties for some
applications. For example, amorphous atactic PLA has a glass
transition temperature (Tg) of ∼60 °C, and heterotactic PLA
undergoes very slow crystallization to yield a semicrystalline
polymer with a low melting temperature (Tm) of 120 °C.2b On
the other hand, isotactic PLA is crystalline, with Tm = 170 °C for
poly[(S)-LA]. Furthermore, polymerizing rac-LA with an iso-
selective initiator can yield stereoblock/gradient PLA, a block
copolymer with alternating isotactic sequences, for which Tm =
170−220 °C.2b This elevated Tm is due to an unusual
phenomenon in which sequences/blocks of a particular
enantiomer of PLA cocrystallize with the opposite enantiomer,
giving rise to a double-helical stereocomplex microstructure with
Received: October 10, 2012
Published: December 11, 2012
© 2012 American Chemical Society
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Table 1. Polymerization Data for Initiators 1−4
T
t
conv.
f
Mn
(kg/mol)
Mcnalc
(kg/mol)
g
h
(K) (min)
(%)
PDI
Pi
a
b
c
1
1
1
1
2
2
2
3
4
4
298
298
298
258
298
273
258
298
298
298
30
87
92
92
77
85
71
73
88
97
80
210
41.0
12.8
16.0
61.7
15.5
13.5
165
223
62.6
66.2
13.2
11.1
61.2
10.2
10.5
126.7
140
1.09
1.05
1.03
1.01
1.04
1.01
1.01
1.50
1.34
1.08
0.76
0.74
0.77
0.84
0.72
0.73
0.81
0.13
0.14
0.10
56
17
c
180
48
a
d
d
e
60
180
0.33
0.75
1.1
e
e
115
105
a
b
THF, [I]:[LA] = 1:500, [LA] = 1 M. Same as a except that 1 equiv
c
(vs 1) of iPrOH was added. THF, [I]:[iPrOH]:[LA] = 1:1:100, [LA]
= 0.5 M. THF, [I]:[LA] = 1:100, [LA] = 0.5 M. THF, [I]:[LA] =
1:1000, [LA] = 1 M. Determined by integration of the methyne
region of the H NMR spectrum (LA, 4.98−5.04 ppm; PLA, 5.08−
5.22 ppm). Determined by SEC-MALLS in THF. Determined by
analysis of all of the tetrad signals in the methyne region of the
Figure 1. Syntheses and structures of initiators 1−4. Conditions: (a) (i)
5 equiv of KN(SiMe3)2, THF, 4 h, 298 K; (ii) [YCl3(THF)3.5], THF, 4 h,
298 K; (iii) KOR1, THF, 7 h, 298 K. (b) (i) 4 equiv of KN(SiMe3)2,
THF, 4 h, 298 K; (ii) [YCl3(THF)3.5], THF, 4 h, 298 K; (iii) KOtBu,
THF, 4 h, 298 K.
d
e
f
1
g
h
1
respectively, indicating quantitative conversion to the yttrium
halide complexes. Salt metatheses using KOEt or KOtBu yielded
1−3, which also showed singlets in the 31P{1H} NMR spectra at
33.4 (1), 34.2 (2), and 31.6 (3) ppm. The new initiators were
homonuclear-decoupled H NMR spectrum.
NMR spectra with the values predicted by Bernoullian statistics
(Figure 3 and Figure S4 in the SI).12 The resolution of the
1
also fully characterized using H and 13C{1H} NMR spectros-
copies and elemental analyses.
Crystals of compound 1 suitable for X-ray crystallography
were obtained by evaporation from pentane (Figure 2). 1 is
1
Figure 3. Methyne region of the homonuclear-decoupled H NMR
spectrum of isotactic PLA produced using initiator 1 ([1]:[iPrOH]:
[LA] = 1:1:100, [LA] = 0.5 M, THF, 258 K).
integrals for the peaks corresponding to the iis, iii, and isi tetrads
was improved using peak deconvolution methods. The Pi values
were determined for all five tetrads, and their average was used as
the Pi for the initiator (Table 1). We were very interested to
observe high stereoselectivities with all of the initiators and, in
particular, the high iso-selectivities of 1 and 2.
The initiator activities were very high. In fact, the initiators
were so effective that low loadings and high dilutions were
needed in order to monitor the polymerizations effectively.
Initiators 1 and 2 showed nearly complete conversion of 500
equiv of LA in ∼1 h. It is interesting to note the compounds’ high
activity in comparison with other reported yttrium complexes
featuring salens or closely related derivatives as ligands (Figure
4), all of which tend to be quite active polymerization initiators.2
The Y−salen initiator 5 required 14 h to reach complete
conversion at a higher initiator loading (1 mol %).4a Y−half-salen
complex 6 (1 mol %) required 3 days to reach complete
conversion.5d Qualitatively, initiators 1 and 2 have the same
activity as the recently reported Y−salen initiator 7,13 which was
notable for its high rates. It is important to note, however, that 7
Figure 2. Molecular structure of 1.
monomeric in the solid state, with coordination of the additional
NH donor to give a hexacoordinate Y center. The tert-butoxide
and one of the phenoxide O atoms occupy the axial positions; the
phosphasalen adopts a β-cis configuration. Selected bond lengths
and angles are given in Table S2 in the SI.
Compounds 1−3 were all highly active and controlled
initiators for ROP of rac-LA, and the ligands’ structures
influenced the reactivity (Table 1). The polymerizations were
all conducted in THF at T ≤ 298 K. The choice of solvent was
important, as the initiators were not tolerant of CH2Cl2 and a
homogeneous solution did not form in toluene at 298 K. Aliquots
1
were regularly removed and analyzed using H NMR spectros-
copy for % conversion and size-exclusion chromatography with
multiangle laser light scattering (SEC-MALLS) for number-
average molecular weight (Mn) and polydispersity index (PDI).
The PLA stereochemistry was assessed by comparison of the
1
tetrad resonances observed in the homonuclear-decoupled H
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All three initiators showed high degrees of stereocontrol, and
unexpectedly, they also showed a distinct difference in PLA
tacticity. Initiators 1 and 2 yielded highly isotactic PLA (Pi= 0.84
for 1 at 258 K; Figure 3), while initiators 3 and 4 yielded highly
heterotactic PLA (Ps > 0.87). At 298 K, 1 and 2 showed Pi values
of 0.74 and 0.72, respectively. It should be noted that initiators
capable of yielding such Pi values under ambient conditions
remain rare.6a,8b,15 To exploit their promising properties, the
polymerizations were conducted at a lower temperature (258 K),
and even higher Pi values (0.84 and 0.81, respectively) were
observed. Furthermore, the polymerization rates remained high,
and the polymerization control was excellent. In the case of 1, the
Figure 4. Structures of other high-activity yttrium initiators.
lacks any tacticity control in rac-LA ROP. Analysis of the
conversion versus time data for 1 and 2 indicated a first-order
dependence of the polymerization rate on the LA concentration,
and pseudo-first-order rate constants kobs = 6.9 × 10−4 and 7.9 ×
10−4 s−1 (0.2 mol %, [LA] = 1 M, THF) were obtained (Figure
S2). Initiators 3 and 4, on the other hand, exhibited even greater
activity; it took just seconds to reach complete conversion even at
0.1 mol % loading. Thus, these phosphasalen initiators show
some exceptionally high activities and significantly higher rates
than analogous Y−salen initiators. As iminophosphoranes are
strong σ and π donors, phosphasalen ligands are expected to be
more electron-donating than their salen counterparts.10b,d,11 It is
interesting that the more electron-donating ligands accelerate the
polymerization rate. This can be rationalized by considering that
reducing the Lewis acidity of the Y center may labilize the Y−
alkoxide bond. It is this latter bond that is central to efficient
polymerization, as it is the propagating species in the
coordination−insertion mechanism. Thus, electron-donating
ligands may accelerate the reaction by increasing the rate of LA
insertion. A related mechanistic hypothesis was elegantly
investigated by Tolman and Hillmyer very recently.14 They
used it to rationalize the activity of a series of phenolate
aluminum initiators of ε-caprolactone polymerization. Our
findings illustrate the potential for other new ligands and metal
centers substituted with electron-donating substituents to
accelerate polymerization rates.
i
addition of PrOH that was necessary to improve the polymer-
ization control had a negligible influence on Pi; this is interesting,
as it shows that high degrees of stereocontrol are also possible in
the presence of a chain-transfer agent. Polymerizations using 3
resulted in highly heterotactic PLA (Ps = 0.87), similar to the
value observed previously for 4 (Ps = 0.9).11 Thus, it is clear that
modification of the electronic properties of 3 relative to 4 did not
diminish the high tacticity control. The mechanism for
heteroselectivity involves chain-end control (CEC), where the
stereochemistry of the last inserted LA enantiomer dictates the
coordination/insertion of the subsequent enantiomer.
To understand the unusual iso-selectivity exhibited by 1 and 2,
we analyzed the relative intensities of the stereoerror tetrad
signals in the PLA produced by them (Figure 3). The sii:iis:isi
signal intensity ratio was 1:1:1, in line with a CEC mechanism
and synthesis of stereoblock PLA. Analysis of the polymer by
differential scanning calorimetry showed a Tm value of 178 °C,
further confirming the formation of a stereoblock polymer.16
Most iso-selective initiators operate by a site-control mechanism,
wherein chiral (or racemic) initiators select for particular
enantiomers of LA. There are a few examples of Al initiators
that are iso-selective as a result of a CEC mechanism, most
notably the Al−salen systems reported by Nomura5h,n and
others5b,f,17 and the Al−salan complexes reported by Gibson.5i,k
In an attempt to understand the contrasting tactity control of
initiators 1 and 2 versus 3 and 4, the initiator structures were
investigated. The solid-state structure of 4 was reported
previously; it crystallizes as a mononuclear complex in which
the Y center exhibits a square-pyramidal geometry with the
phosphasalen ligand occupying the basal plane and the alkoxide
the apical position.11 All of the NMR spectra of 3 and 4 are
closely related, and several signals are indicative of a
mononuclear solution structure, most notably the observation
of just one singlet signal in the 31P{1H} NMR spectrum and the
equivalence of the quaternary phenolate carbon signals in the
13C{1H} NMR spectrum. Variable-temperature (VT) 1H NMR
measurements on solutions of 3 indicated significant fluxionality
on the NMR time scale that could not be sufficiently resolved
even at 188 K. The rotational Overhauser effect spectroscopy
(ROESY) NMR spectrum showed exchange between the phenyl
substituents on the iminophosphorane functionalities, also
indicative of solution fluxionality.
Initiator 1, which has a tert-butoxide coligand, showed very
high rates but rather less impressive polymerization control. The
experimentally observed Mn (200 kg/mol) far exceeded that
predicted on the basis of the reaction stoichiometry (63 kg/mol),
likely because of relatively slower initiation (vs propagation)
using the sterically hindered tert-butoxide coligand. To overcome
i
this, we added 1 equiv of PrOH, which would be expected to
undergo rapid and reversible alcoholysis reactions, thereby
enabling efficient initiation and controlled polymerization.
i
Indeed, the addition of PrOH greatly improved the polymer-
ization control, giving Mn values (41.0 and 12.8 kg/mol) close to
those predicted on the basis of the initiator loading (66.2 and
13.2 kg/mol, respectively). In every case, the PDI was narrow
(<1.10), also indicative of good control. Initiator 2, which has an
ethoxide coligand, showed excellent polymerization control
without the need for any exogeneous alcohol, again supporting
the notion that less sterically hindered alkoxide coligands enable
more rapid initiation. Thus, 500 equiv of LA was polymerized
using 2 to yield PLA with Mn = 61.7 kg/mol (Mn calc = 61.2 kg/
mol) and PDI = 1.04. When the reaction temperature was
lowered, the polymerizations remained very well controlled,
yielding PLA with Mn = 15 kg/mol and PDI = 1.01 even at 258 K.
Figure S3 illustrates the linear evolution of Mn versus conversion
In contrast, in the solid-state structure of 1, Y has a distorted
octahedral geometry and is coordinated by the additional NH
substituent. The N1−O1−N2−O2 dihedral angle is 166.38°,
indicating a significant deviation from planarity compared with
the pentacoordinated complex 3. Diffusion-ordered spectrosco-
py (DOSY) NMR experiments using 1 in THF solution provided
an estimated hydrodynamic radius (Rh) of 4.5 Å, in reasonable
agreement with the value estimated from the X-ray crystal
structure of 1 (6.0 Å) (Figure 2). Crystals of 2 could not be
i
for both initiators 1 (with 1 equiv of PrOH) and 2, providing
further evidence of their good polymerization control. Initiators
3 and 4 showed slightly broader PDI values, in line with the
exceptionally high rates and the tert-butoxide initiating group.
The polymerization control could again be improved by addition
of 1 equiv of iPrOH.
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Journal of the American Chemical Society
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species is a dimer wherein the two Y cores are bridged by both an
alkoxide and a hydroxyl coligand, the latter presumably arising
from exposure to adventitious water. Also, the additional NH
donor in the ligand backbone is coordinated to each Y center.
The coordination geometry at each Y center is monocapped
trigonal-prismatic. From the X-ray structure of the partially
hydrolysed dimer of 2, an Rh of 8.9 Å was obtained (see the SI).
These findings indicate that 1 and 2 are likely to adopt
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1
mononuclear structures in THF solution. VT H NMR studies
using 1 also indicated some degree of fluxionality, but the
ROESY spectrum showed no exchange between the iminophos-
phorane substituents, consistent with a more constrained
structure.
In any case, the coordination geometries of 1 and 2 are quite
different from those of 3 and 4, with the former containing a
hexacoordinate Y and the latter a pentacoordinate Y. Thus, we
tentatively attribute the iso-selectivity observed with 1 and 2 to
their more sterically congested and constrained active sites
compared with 3 and 4. Such a finding is in line with Nomura’s
hypothesis regarding CEC observed with Al−salen complexes.5h
Further studies aimed at understanding the detailed mechanisms
of iso-selectivity are underway.
In conclusion, we have reported the first example of a highly
iso-selective and highly active yttrium phosphasalen initiator.
These initiators show very high rates, excellent polymerization
control, tolerance to low loadings, and high iso-selectivities.
When the phosphasalen ancillary ligand was changed, the high
rates were maintained, but the stereocontrol changed to highly
heterotactic. The new initiators do not require any chiral ancillary
ligands or complexes and operate by CEC mechanisms.
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ASSOCIATED CONTENT
* Supporting Information
Detailed experimental protocols, X-ray data (CIF), polymer-
ization data and figures, and DOSY calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
audrey.auffrant@polytechnique.edu; c.k.williams@imperial.ac.
■
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Author Contributions
§C.B. and T.-P.-A.C. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The EPSRC, CNRS, and Ecole Polytechnique are thanked for
financial support.
■
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