Phosphasalen Indium Complexes Showing High Rates and Isoselectivities in rac-Lactide Polymerizations

Article abstract of DOI:10.1002/anie.201701745

Polylactide (PLA) is the leading bioderived polymer produced commercially by the metal-catalyzed ring-opening polymerization of lactide. Control over tacticity to produce stereoblock PLA, from rac-lactide improves thermal properties but is an outstanding

Full text of DOI:10.1002/anie.201701745

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Communications
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International Edition: DOI: 10.1002/anie.201701745
German Edition: DOI: 10.1002/ange.201701745
Polymerization
Phosphasalen Indium Complexes Showing High Rates and
Isoselectivities in rac-Lactide Polymerizations
Dominic Myers, Andrew J. P. White, Craig M. Forsyth, Mark Bown,* and Charlotte K. Williams*
Abstract: Polylactide (PLA) is the leading bioderived polymer
produced commercially by the metal-catalyzed ring-opening
polymerization of lactide. Control over tacticity to produce
stereoblock PLA, from rac-lactide improves thermal proper-
ties but is an outstanding challenge. Here, phosphasalen
indium catalysts feature high rates (30 ꢀ 3mꢁ1 minꢁ1, THF,
298 K), high control, low loadings (0.2 mol%), and isoselec-
tivity (Pi = 0.92, THF, 258 K). Furthermore, the phosphasalen
indium catalysts do not require any chiral additives.
earth, group 13, and late-transition metals.[5] So far, chiral
aluminium/salen complexes prepared from either binaphthyl
or cyclohexyl diamines show the best isoselectivity.[6] Nomura
and co-workers pioneered achiral aluminium/salen com-
plexes, some of which also showed very high isoselectivity.
The catalyst selectivity was highest using sterically hindered
ortho-phenolate substituents.[7] The leading catalysts showed
a probability of isotactic diad formation of Pi > 0.9 (343 K).
The catalysts showed high polymerization control and, in
some cases, showed narrow molecular weight distributions
despite operating by a polymer exchange mechanism.[6c] Yet,
the major limitation of the highest performing aluminium/
salen catalysts are the low rates and requirement for high
metal loadings (ꢂ 1 mol%), thus, more highly active iso-
selective catalysts are needed. One option is to replace
aluminium with its heavier congener indium, a strategy shown
to accelerate rates whilst maintaining control. Mehrkhoda-
vandi and co-workers have reported isoselective indium
complexes, coordinated by chiral salen or phenoxide diamine
ligands, with Pi = 0.77 (298 K).[8]
Considering the challenge of simultaneously improving
the catalytic rate and isoselectivity, the selection of the
ancillary ligand is important. So far, salen ligands have been
extensively optimized at the diimine linkers, phenyl ring
substituents, and even by changing the imine to an amine. We
have recently demonstrated the potential of the phosphasalen
ligand class, with yttrium and lutetium complexes affording
Pi values ranging from 0.81 to 0.84 (298 K).[5n–p,9] The phos-
phasalen ligand features iminophosphorane substituents
which deliver greater electron density at the metal center
compared to salen analogues, and thus accelerates rates. It is
important to emphasize that the phosphasalen ligands do not
contain any chiral diamines, that is, chiral at metal complexes
can form and stereocontrol is also feasible by chain-end
control mechanisms. Further, the phosphorus atom substitu-
ents provide steric directing effects at the active site since
directly analogous achiral salen yttrium complexes, which do
not benefit from such substituents, are not stereoselecti-
ve.[6c,10] Given the need for isoselective catalysts, the promis-
ing performance of indium salen catalysts was inspiring. Here,
the first examples of phosphasalen indium catalysts, featuring
ethylene diamine linkers, show high rates and isoselectivity
(Figure 1).
Polylactide (PLA) is the leading bioderived polymer and is
a degradable replacement for petrochemical polymers in
packaging, fibers, and biomedical devices.[1] It is sourced from
starch and is commercially synthesized by the metal-catalyzed
ring-opening polymerization (ROP) of lactide (LA). Both the
applications and material performances of PLA are improved
by control of its stereochemistry. For example, atactic PLA is
an amorphous polymer suitable as packaging for products
having a short shelf life. In contrast, isotactic PLA is a semi-
crystalline polymer suitable for higher-strength and longer-
lifetime applications.[2] Moreover, co-crystallization of the
two enantiomeric homochiral PLA chains affords stereo-
block/stereocomplex PLA, with a significantly higher melting
temperature than isotactic PLA, a feature which facilitates
processing and may enable applications as an engineering
polymer.[3] Stereoblock/complex PLA can be made from
separate homochiral PLA chains.[4] An alternative route
involves rac-lactide with an isoselective catalyst to yield, in
one-step, stereoblock PLA. This approach is intrinsically
more attractive but is currently limited by the range and
performance of the catalysts. Despite the research on PLA
catalysis, there are remarkably few isoselective metal cata-
lysts, with leading examples including chiral complexes of rare
[*] D. Myers, Dr. A. J. P. White
Department of Chemistry, Imperial College London
London SW7 2AZ (UK)
Dr. C. M. Forsyth
School of Chemistry, Monash University
Clayton, VIC 3800 (Australia)
Dr. M. Bown
CSIRO Manufacturing
Bayview Avenue, Clayton, VIC 3168 (Australia)
E-mail: mark.bown@csiro.au
The most isoselective rare-earth catalysts were coordi-
nated by a pentadentate phosphasalen ligand featuring
a triamine linker with an NH donor group. Initially, this
same ligand was investigated for indium coordination chemis-
try but afforded a mixture of products, perhaps because of its
higher denticity and/or the tendency of indium alkoxide
complexes to dimerize. Using tetradentate phosphasalen
Prof. C. K. Williams
Department of Chemistry, Oxford University
12 Mansfield Road, Oxford, OX1 3TA (UK)
E-mail: charlotte.williams@chem.ox.ac.uk
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201701745.
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ligands, these are the initiating groups in catalysis and so do
not influence either rates of propagation or isoselectivity.
Both 1 and 2 showed singlets in the 31P{1H} NMR spectra,
at d = 39.6 and 40.3 ppm, respectively, thus indicating the
phosphorus atoms are in identical environments. The
1H NMR spectra showed the expected number and integra-
tion of signals for the phosphasalen ligands as well as for the
alkoxides. Indium coordination was confirmed by the appear-
ance of diastereotopic methylene environments for the
backbone protons. In terms of the ortho phenyl substituents,
2 showed two distinct sharp singlets for both the methyl
resonances, which were assigned to the ortho- and para-
phenyl groups.
In contrast, 1 showed two different methyl resonances for
the ortho-cumyl groups and both were asymmetrically
broadened, which was indicative of restricted rotation.
1H ROESY NMR spectra showed exchange processes
between the ethylene backbone signals and aromatic phenyl
groups (see Figure S11). These likely involve free rotation of
the phenyl groups and twisting of the ethylene backbone, and
1
is consistent with the H NMR data. Additional crosspeaks
were observed between the asymmetric methyl resonances of
the ortho-cumyl groups. VT-NMR studies revealed that the
ortho-cumyl resonances undergo signal coalescence at high
temperature (Tꢂ 318 K). A similar coalescence was also
observed for the ethylene backbone signals, changing from
two broad multiplets into a sharp doublet at 363 K (see
Figure S12–S15).
The solid-state structures were obtained by single-crystal
X-ray diffraction (Figure 1). The compound 1 is an ethoxide-
bridged dimer with each indium atom exhibiting octahedral
coordination geometry (see Figure S16 and Table S1). In
contrast, the solid-state structure of 2 is mononuclear and the
indium is pentacoordinate with a geometry intermediate
between trigonal bipyramid and square-based pyramidal (t =
0.33; see Figures S17 and S18, and Table S2). The distinct
nuclearities appear to be dominated by the nature of the
alkoxide, with the smaller ethoxide co-ligand (1) permitting
a bridging coordination geometry. Perhaps more unexpected
is the isolation of mononuclear 2, given the strong precedence
for dimeric [(k4-ligand)In(OR)]2 complexes in the litera-
ture.[13] In polymerization catalysis there is an excess of
monomer, which is a potential donor group, and THF
(tetrahydrofuran) is the solvent, thus establishing the nucle-
arity of the complexes under related conditions is important.
The complex 1 was analyzed by diffusion-ordered NMR
spectroscopy (DOSY) in both noncoordinating ([D6]benzene
and [D8]toluene) and coordinating solvents ([D8]THF), and
the diffusion coefficients and hydrodynamic radii (rH) were
compared.
Figure 1. Molecular representation of structures of the initiators 1 (top
left) and 2 (bottom right) obtained by X-ray diffraction techniques,
with hydrogen atoms and solvent molecules omitted for clarity (50%
probability ellipsoids).[21] General schematic representation of the
structures of 1 and 2 (top).
ligands, indium complexes featuring ethylene linkers were
isolated, and the ortho phenyl position was targeted for
substitution as it is close to the active site. The ligands were
synthesized using known or modified versions of the reported
routes (see Scheme S1 in the Supporting Information).[12] The
synthesis of the indium complexes 1 and 2 was achieved in
three steps and did not require any isolation of intermediates.
Firstly, the phosphasalen ligands were deprotonated, by
reaction with excess NaH, and the disodium salts were then
reacted with InCl3. The indium chloride complexes were
reacted with either KOEt or KOtBu, and the solutions were
filtered to remove the salt by-products, thus allowing isolation
of the pure indium alkoxide complexes 1 and 2 in 31 and 44%
yields, respectively (Figure 1; see Figures S1–S10). The reac-
tions were all monitored using in situ 31P{1H} NMR spectros-
copy, and at each stage, products showed characteristic
singlets at different chemical shifts. It is relevant to note
that although the complexes feature different alkoxide co-
Whilst such analysis is only semi-quantitative, the rH
values in THF are approximately half those obtained in
8
6
benzene/toluene
(rH½D ꢃTHF = 4.7 ꢀ,
rH½D ꢃbenzene = 6.8 ꢀ,
rH½D ꢃtoluene = 7.0 ꢀ, rH
dimer = 8.1 ꢀ; see Figures S19–
solid-state
8
S21). The complex 2 showed a mononuclear solid-state
structure and there was close agreement with hydrodynamic
values in THF, and is thus strongly indicative of it adopting
solid-
a mononuclear solution structure (rH½D ꢃTHF = 6.2 ꢀ, rH
8
state = 5.9 ꢀ; see Figure S22). Therefore, it is reasonable to
2
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expect that both initiators are mononuclear under the
polymerization conditions.
Both 1 and 2 were highly active for the ROP of rac-LA
and showed good polymerization control (Table 1; see
Table 1: Polymerization data using 1 and 2 in THF, [LA]=1m.
Conv.[l]
[%]
t
Mn
Mn
ꢀ[m]
Pi[n]
calc
Entry Ini.
T
[K]
[min] [kDa][m] [kDa]
1[a]
2[b]
3[c]
4[d]
5[e]
6[f]
1
1
1
1
1
1
1
1
2
2
2
298 96
298 97
298 98
298 98
298 88
298 90
258 96
403 56
298 80
298 93
298 97
298 80
60
48
40
28
30
90
985
10
153
106
55
65.3
53.4
45.6
29.9
12.5
22.2
69.8
46.2
50.8
52.0
39.4
105
69.2
55.3
42.4
28.2
12.7
25.9
69.2
40.4
57.7
46.9
35.0
115
1.15 0.87
1.17 0.85
1.20 0.85
1.18 0.85
1.22
1.38
–
–
7[g]
8[h]
9[a]
10[i]
11[j]
1.14 0.92
2.29 0.66
1.19 0.75
1.25 0.72
Figure 2. Overlay of semi-logarithmic first-order plots for the polymer-
ization of rac-LA initiated by 1. Reaction conditions: [LA]/[1]=500
(black); [LA]/[1]=400 (red); [LA]/[1]=300 (blue); [LA]/[1]=200
(green), [LA]=1m, THF, 298 K. Inset: Plot of kobs vs. [1] for calculation
of the propagation rate constant, kp.
1.17
–
Y-Phos[k]
70 s
1.08 0.10
[a] 500 equiv. [b] 400 equiv. [c] 300 equiv. [d] 200 equiv. [e] 100 equiv,
0.5 M. [f] Reaction mixture for [e] with an additional 100 equiv of rac-LA
added. [g] 500 equiv, 0.75 M. [h] 500 equiv, neat. [i] 350 equiv.
[j] 250 equiv. [k] 1000 equiv, 1m, 1 equiv of iPrOH added. [l] Determined
by integration of the methine region of the 1H NMR spectrum (LA, 4.96–
5.04 ppm; PLA, 5.10–5.22 ppm). [m] For initiator 1: determined by SEC
analysis against polystyrene standards in CHCl3 using a Mark–Houwink
correction factor of 0.58;[11] for initiator 2: determined by SEC-MALLS in
THF. [n] Determined by integration of PLA methine tetrads in the
1H{1H} NMR spectrum using values predicted according to Bernoullian
statistics.
Figure S48). Further evidence of control was obtained from
a sequential monomer-addition experiment, in which an
additional 100 equivalents of lactide were added to an
experiment conducted under conditions as per entries 5 and
6 in Table 1. This set of conditions led to complete conversion
and an increase in the molecular weight of the polymer from
12.5 to 22.2 kgmolꢁ1 (see Figure S49).
Both catalysts induced high levels of stereocontrol with
a strong isotactic bias in resultant polymers, and 1 afforded
highly isotactic PLA (Pi = 0.85–0.87, 298 K; see Figures S50-
S53). The analysis of defect tetrad resonances in the 1H-
{1H} NMR spectrum indicate a chain-end control mechanism
for the isoselectivity,[7a,15] that is, the last inserted lactide unit
dictates the selectivity between the two lactide enantiomers.
In addition to homonuclear decoupled NMR spectroscopy,
differential scanning calorimetry (DSC) confirmed the for-
mation of semi-crystalline PLA showing a Tm of 1658C (see
Figure S54), and is consistent with recent literature detailing
the variation of Tm against Pi.[5c] Given the good rates of
polymerizations, it was also feasible to run the polymeri-
zations at low temperature (entry 7, Table 1), and this led to
increased isoselectivity with Pi = 0.92 at 258 K (see Fig-
ure S55). In this case, the DSC analysis showed an increased
Tm of up to 1798C, thus confirming the formation of stereo-
block PLA (Figure 3). A polymerization was also run at
403 K, and at this higher temperature there was a reduction in
isoselectivity, although the resultant PLA was moderately
isotactic (Table 1, entry 8; see Figure S56). The catalyst 2
showed lower isoselectivity under the standard reaction
conditions with Pi = 0.75 (see Figures S57 and S58). Thus, it
is clear that the bulky ortho cumyl substituents in 1 are
important to steric protection and direction at the metal
center. It is relevant to note that ortho-phenyl substituents
were also important in mediating high isoselectivity for
aluminium salen complexes, and thus may be considered
a useful site for further ligand development studies.[7b,16]
Another unexpected feature of 2 is that the same ligand
coordinated to yttrium leads to a catalyst that is hetero-
Figures S23–S43). The typical polymerization conditions
were in THF solutions, at 298 K, and used [LA]0 = 1m and
[catalyst]0 = 2 mm. By using 1, polymerization proceeded
rapidly, reaching near-quantitative conversion in 1 hour
(Table 1, entry 1). Whilst still rapid in the broader context
of this catalysis,[14] using 2, under identical reaction conditions
resulted in reactions that were more than four times slower.
Linear semi-logarithmic plots for the ROP of rac-LA by 1 and
2 confirmed that the rates were first-order with respect to
monomer concentration (Figure 2; see Figures S44–S46). The
rate coefficients were determined as kobs = 8.27 ꢀ 0.48 ꢁ
10ꢁ4 sꢁ1 and 1.94 ꢀ 0.04 ꢁ 10ꢁ4 sꢁ1, respectively. Polymeri-
zations for 1 and 2 also showed first-order rate dependences
on initiator concentrations, as observed by linear fits to kobs vs.
[1] (Figure 2; see Figure S47). Thus the overall rate laws are
second-order and the propagation rate constants are kp = 30 ꢀ
3mꢁ1 minꢁ1 (1) and 20 ꢀ 1mꢁ1 minꢁ1 (2).
Both initiators displayed high degrees of polymerization
control as demonstrated by predictable molecular weights
(Mn), narrow dispersities, and linear evolutions of Mn values
with conversion. By using 2, slight broadening of distributions
(ꢀ = 1.20) was observed at higher conversions, and in contrast
to using 1, the distributions remained narrow (ꢀ < 1.10)
throughout. The analysis of the end groups was conducted
using MALDI-ToF mass spectrometry (catalyst 1), and
showed a single set of peaks assigned to chains with ethoxide
end groups. The peaks are separated by 144 amu, which
confirms the absence of transesterification side reactions (see
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nation geometry closer to square-based pyramidal (t = 0.08)
ꢁ
and, as expected, shows longer M N bond lengths [Y–N =
2.324(2) ꢀ and 2.375(2) ꢀ vs. In–N = 2.203(5) ꢀ and 2.187-
(5) ꢀ].[12] One explanation for the different selectivity may be
that the yttrium is more tightly coordinated by the ligand,
thereby resulting in the less steric hindrance and thus
formation of heterotactic PLA. Comparing the performance
of 1 against other known isoselective catalysts, particularly of
group 13, the best chiral indium salen catalysts show Pi = 0.77
(298 K).[8a] The catalyst 1 is significantly more isoselective and
obviates the use of chiral diamines whilst maintaining
approximately equivalent rates (Figure 4). Similarly, indium
complexes derived upon fluorinated dialkoxy-diimino salen-
like ligands have afforded moderate Pi values of up to 0.69.[17]
The best aluminium salen complexes showed Pi = 0.97
(368 K).[7b] However, the rates were slow with kp =
0.054mꢁ1 minꢁ1, which is 500 times slower than 1 (kp =
30.6mꢁ1 minꢁ1).
In the same study, aluminium/salen complexes, featuring
the same ortho substituents and an ethylene linker, showed an
isoselectivity (Pi = 0.85) equivalent to that of 1, and this direct
comparison highlights the potential to deliver both signifi-
cantly higher rates and isoselectivity relative to those
obtained with aluminium/salen catalysts. Aluminium/salalen
catalysts incorporating a chiral pyrrolidine ligand showed Pi =
0.82 (toluene, 353 K).[18] Related aluminium/salalen catalysts,
Figure 3. The methine region of the 1H{1H} NMR spectrum of PLA in
CDCl3. Reaction conditions: [LA]/[1]=500, [LA]=0.75m, THF, 258 K
(Pi =0.92). Inset: Heat flow vs. temperature curve for the aforemen-
tioned PLA sample. The curve shows three distinct transitions: Tg, cold
crystallization, and Tm.
selective (Y-Phos; Table 1). This selectivity is surprising
because in general, a particular ligand scaffold will induce
the same tacticity when coordinated to different metals.[5n,o,12]
Analysis of the catalyst structures determined by X-ray
crystallography shows that the yttrium complex has a coordi-
Figure 4. Isoselective initiators for the synthesis of PLA, including this work (bottom right).
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[4] T. Rosen, I. Goldberg, V. Venditto, M. Kol, J. Am. Chem. Soc.
incorporating 2-aminopiperidine, showed Pi = 0.83 (CH2Cl2,
298 K) although the rates were slow.[19] Thus, 1 shows
substantially improved isoselectivity compared to indium/
salen catalysts and much higher rates whilst maintaining
competitive isoselectivity compared to aluminium/salen cata-
lysts. Considering other metals, by using chiral zinc catalysts,
the most selective of which features trityl ortho phenyl
substituents, gave a maximum value Pi = 0.81 (298 K, THF),
although the most selective catalyst is approximately four
times slower than 1.[5d,20] Chiral zinc/amido-oxazolinate
catalysts showed Pi = 0.91, but are more than 700 times
slower with kp = 0.04mꢁ1 sꢁ1 (toluene, 296 K).[5e] Zirconium/
salalen catalysts showed Pi = 0.86, but are much slower than
1.[5l] Diimino pyrrolide copper catalysts showed lower iso-
selectivity (Pi = 0.70) and lower rates.[5g] The only comparable
catalyst was reported very recently, and is an yttrium
bis(phenolate) catalyst showing high rates (kobs = 4.25 ꢁ
10ꢁ3 sꢁ1, toluene, 298 K, [LA] = 2.5m, [catalyst] = 10 mm)
and isoselectivity (Pi = 0.84 at 298 K; Pi = 0.90 at 258 K).[5q]
Overall, in comparison with other isoselective catalysts,
1 shows leading performance both in terms of isoselectivity
and rate. Furthermore, the catalyst is very well controlled and
shows minimal transesterification reactions.
In conclusion, indium phosphasalen catalysts show very
good performance and demonstrate the potential for the
ligand class and indium in this field of catalysis. The two new
catalysts yielded polymers with a strong isotactic bias (Pi =
0.87; 298 K) and the incorporation of bulky ortho cumyl
groups leads to a marked enhancement in isoselectivity.
Given the importance of salen-type ligands in stereoselective
catalysis, these phosphasalen ligands and complexes merit
exploration as they allow control in stereochemistry without
the need for chiral additives/ligands.
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Acknowledgments
The EPSRC (EP/L017393/1; EP/H046380/1), Imperial Col-
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Conflict of interest
The authors declare no conflict of interest.
Keywords: indium · isoselectivity · polymerization ·
structure elucidation · synthetic methods
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Manuscript received: February 16, 2017
Final Article published: && &&, &&&&
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These are not the final page numbers!

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Communications
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Communications
Polymerization
New on the block: Phosphasalen indium
complexes are fast and stereoselective
catalysts allowing production of stereo-
block polylactide from racemic lactide
(rac-LA; Pi =0.92). The catalysts do not
feature any chiral additives and operate
by chain-end control mechanisms.
D. Myers, A. J. P. White, C. M. Forsyth,
M. Bown,*
C. K. Williams*
&&&& — &&&&
Phosphasalen Indium Complexes
Showing High Rates and Isoselectivities
in rac-Lactide Polymerizations
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ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!