Yttrium and aluminium complexes bearing dithiodiolate ligands: Synthesis and application in cyclic ester polymerization

Article abstract of DOI:10.1039/c5dt02938d

Yttrium and aluminium complexes of two dithiodiolate ligands that feature different bridges (CF₃)₂C(OH)CH₂SRSCH₂C(OH)(CF₃)₂ (L¹-H₂, R = CH₂CH₂ and L²-H₂, R = C₆H₄) were synthesized in good yields by reacting tris(silylamide)yttrium or trimethylaluminium with one equivalent of the proligand. All complexes were characterized by NMR and elemental analysis, and single-crystal X-ray structural analysis was also performed for one of the yttrium complexes. The catalytic activities of the four complexes in the ring-opening polymerization of ε-caprolactone and rac-lactide have been investigated. Furthermore, DOSY experiment and DFT calculations have been carried out to determine the structure of the isopropoxo derivative of the complex L²Y amide.

Full text of DOI:10.1039/c5dt02938d

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DOI: 10.1039/C5DT02938D.
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ARTICLE
Yttrium and Aluminium Complexes bearing Dithiodiolate Ligands:
Synthesis and Application in Cyclic Esters Polymerization
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
b
b
a
a
A. Meduri, M. Cozzolino, S. Milione, K. Press, E. Sergeeva, C. Tedesco, M. Mazzeo and M.
c,
Lamberti *
DOI: 10.1039/x0xx00000x
www.rsc.org/
Yttrium and aluminium complexes of two dithiodiolate ligands that feature different bridges
1
2
(CF
3
)
2
C(OH)CH
2
SRSCH
2
C(OH)(CF
)
3 2
(L ‐H
2
, R = CH
2
CH
2
and L ‐H
2
, R = C
6
H
4
), were synthesized in good yields by reacting
tris(silylamide)yttrium or trimethylaluminium with one equivalent of the proligand. All complexes were characterized by
NMR and elemental analysis, single‐crystal X‐ray structural analysis was also performed for one of the yttrium complexes.
The catalytic activities of the four complexes in the ring‐opening polymerization of ε‐caprolactone and rac‐lactide have
been investigated. Furthermore, DOSY experiment and DFT calculations have been carried out to clarify the structure of
2
the isopropoxo derivative of the complex L Yamide.
the use of ancillary ligands bearing phenoxy groups as anionic
donors and nitrogen atoms as neutral donors has been
ubiquitous, alkoxide anionic donors and sulphur neutral
donors have been much less explored. Carpentier has
developed aluminium complexes bearing fluorinated
Introduction
Ring‐opening polymerization (ROP) of cyclic esters promoted
by metal catalysts has emerged as the most promising route
for the synthesis of aliphatic polyesters, such as poly‐
caprolactone and polylactide, with tuned properties. Taking
into account the importance of such class of polymers in
6
1
(di)alkoxy‐diimino ligands that show comparable efficiency as
7
ROP catalysts to their aryloxide analogues. A series of 1,ω‐
2
dithiaalkanediyl‐bridged bis(phenol)s (OSSO) have been
introduced by Okuda as ancillary ligands for third group metal
complexes. By variation of the ligand architecture, moderate
to high heteroselectivity for the ROP of rac‐LA has been
several fields (packaging, agricultural and biomedical areas )
numerous researches have focused on the exploration of new
metal complexes that are active in the ROP of cyclic esters.
Among the most successful catalysts, yttrium complexes have
emerged for their high activity and good polymerization
8
obtained. For these systems, the ligand fluxionality resulted
1,3
crucial in the mechanism of stereoselection. Following these
remarkable results we subsequently explored the behaviour of
a series of metal complexes bearing the unbridged bidentate
control, whereas aluminium complexes have played a special
role thanks to the high degree of stereocontrol which they are
4
able to induce, allowing, for example, the preparation of new
6c,9
5
phenoxythioether (OS) ligands in the ROP of cyclic esters.
polylactide (PLA) microstructures. Moreover, both these
metals are non toxic so that the synthesized polyesters can be
employed for any use, from the food industry to the
biomedical field, with no concern about possible catalyst
residue. The choice of the ancillary ligand is another crucial
aspect in the design of an efficient catalytic system. Numerous
new ligands have been prepared allowing detailed studies of
the effect of the coordinative environments on the catalytic
properties of the synthesized complexes. In particular, while
Some of us recently reported, for the first time, a new class of
ligands: the dithiodiolate ligands, bearing both alkoxide anionic
donors and sulphur neutral donors in the same skeleton.
Interestingly, dithiodiolate Group 4 complexes resulted highly
1
a
1
0
active catalysts for the production of heterotactic PLAs. In
this framework, we decided to explore the coordination
chemistry of dithiodiolate ligands around other metals,
focusing our attention on yttrium and aluminium, and to test
the synthesized complexes as catalysts in the ROP of
caprolactone and lactides.
‐
a.
Dipartimento di Chimica e Biologia, Università di Salerno, via Giovanni Paolo II
1
32, I‐84084, Fisciano, Salerno, Italy.
b.
c.
School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel
Aviv University, Ramat Aviv, Tel Aviv 69978, Israel
Dipartimento di Fisica “E. Caianiello”, Università di Salerno, via Giovanni Paolo II
Results and discussion
Synthesis and characterization of yttrium and aluminium
complexes 1‐4
1
32, I‐84084, Fi‐sciano, Salerno, Italy
Electronic Supplementary Information (ESI) available: NMR characterization,
kinetics data and tetrad distribution analysis. CCDC 1405712 contains the
crystallographic data. See DOI: 10.1039/x0xx00000x
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Yttrium complexes
1
and
2
were prepared by the amine In this case, the SCH
2
‐C(CF
3
)
2
protons were observed at 3.40
View Article Online
DOI: 10.1039/C5DT02938D
elimination reactions between the silyl amido precursor ppm (cfr with 3.08 ppm value observed for the same protons
1
2
{
Y[N(SiHMe ) ] (THF) } and the proligands L ‐H and L ‐H , (see of the corresponding diol proligand) and appeared as a narrow
2 2 3 2 2 2
Scheme 1) at room temperature in benzene solution. In both peak. The ‐CH protons of the THF molecule were observed
2
cases the corresponding monosilylamido complexes at 3.93 ppm.
1
2
L Y[N(SiHMe ) ](THF)
1
and L Y[N(SiHMe ) ](THF)
2
were X‐ray diffraction quality single crystals of
yellow prisms from hexane solutions at 253K. The X‐ray
silylamide molecular structure is shown in Figure 1, selected bond
Y[N(SiHMe ) ] (THF)} with one equiv of L ‐H was carried out distances and angles are listed in Table S1. The yttrium atom is
2 were obtained as
2
2
2 2
obtained in high yield (92% and 89%, respectively).
An NMR‐scale reaction of yttrium
1
{
2
2 3
2
in benzene‐d6 at room temperature. A quantitative com‐ six‐coordinate, adopting
a
distorted trigonal‐prismatic
1
plexation was observed after 4 h. Solution H NMR studies of geometry, where the two oxygen donors of the dithiodiolate
the reaction indicated that the neutral proligand was ligand are cis‐oriented, as indicated by O1‐Y1‐O2 angle of
deprotonated by two metal silylamide groups of 109.26(19)°, and also the nitrogen and THF oxygen atoms are
{
Y[N(SiHMe ) ] (THF)} with the release of 2 equiv of cis‐oriented (N1‐Y1‐O3 91.26(19)°). The same configuration
2 2 3
1
1
bis(dimethylsilyl)amine (Scheme 1).
was also found in yttrium complexes bearing a salen or a di‐
1
19
13
12
H, F and C NMR spectra at room temperature of complex
1
thiodiphenolate ligand. Y1‐O1 and Y‐O2 distances are
showed a single set of resonances indicative of the presence of respectively 2.153(5) Å and 2.122(5) Å, and Y‐S1 and Y‐S2
a single isomer on the NMR time scale. The sulphur distances are respectively 2.948(2) Å and 2.973(2) Å, similar
coordination binding to the central yttrium metal was values have been observed in analogous six‐coordinate Y
11,12
confirmed by the slight downfield shift of the SCH ‐C(CF )
complexes.
The Y atom results to be displaced by 1.05 Å
2
3 2
protons (2.9 ppm) compared to the corresponding diol from the mean plane (rmsd 0.195 Å) formed by O1, O2, S1 and
proligand (2.6 ppm). The silylamide group displayed a single S2 atoms of the chelating ligand, almost the same value is
doublet for the four equivalent methyl groups and one septet observed for the dithiophenolate Y complex (1.08 Å) and a
for Si‐H protons. The resonance relative to the ‐CH protons slightly lower value (0.95 Å) for the salen Y complex.
2
of the THF molecule (at 3.90 ppm) shifted downfield with Y1‐Si1 and Y1‐Si2 contacts are 3.229(3) Å and 3.480(3) Å, the
respect to the free THF (3.57 ppm) suggesting the coordination former distance displays a shorter value than those observed
8
b
of the THF at the metal centre. NMR data for complex 1 were for the analogous Y dithiophenolate complex (3.338 Å and
coherent with a structure consisting of one ligand, one amide 3.318 Å), where the presence of a
group and one THF molecule coordinated at the metal centre. was excluded.‡
(Si‐H) agostic interaction
For the equivalent protons of the two methylene groups CH ‐S To investigate the structures of complexes
1
and
2 in solution,
2
1
of the ethylene bridge, a single sharp signal was observed. variable‐temperature H NMR spectra in toluene‐d8 were
1
9
Analogously, F NMR spectrum contained only one sharp recorded. When tetradentate ligands, as the OSSO diolate
signal, indicating that all four CF groups in the complex are ligands, wrap around six coordinate metal centre in an
3
equivalent. This picture is indicative either of the presence of a octahedral fashion, three geometries are possible, designated
highly symmetric structure or the existence of a fast fluxional as mer‐mer (trans), fac‐fac (cis‐), or fac‐mer (cis‐β). Taking
process on the NMR time scale. Actually, the methylene into account that sulphur donors become stereogenic upon
1
protons of the SCH ‐C(CF ) groups appeared in the H NMR binding to the metal and that two different labile ligands
2
3 2
spectrum as a broad signal, suggesting the existence of an (amide and THF) are coordinated to the metal centre, all the
averaging process in solution. This broad resonance sharpened
on heating to 323 K and, at this temperature, the spectrum
showed all resonances as sharp signals. On the other hand, at
low temperature (203 K) the four protons of the SCH ‐C(CF )
2
3 2
groups resolved into two doublets. This aspect was carefully
studied by NMR variable temperature experiments (vide infra).
Analogously to complex
1, for complex 2 the NMR data
suggested the formation of a single isomer with a structure
consisting of one ligand, one amide group and one THF
molecule coordinated at the metal centre.
Y[N(SiHMe
2
)
2
]
3
2THF3
S
N
S
S
S
F C
CF
CF
3
3
F
3
C
CF
HO CF
3
3
Y
2 2
+ NH(SiHMe )
benzene
F
3
C
O
O
3
F C OH
THF
SiHSiH
2 2
Complex 1: SCH CH S
Complex 2: SC H S
6 4
Scheme 1. Synthetic pathway for the synthesis of complexes 1‐2
Figure 1. ORTEP drawing of compound 2. Hydrogen atoms have been omitted for
clarity. Ellipsoids are drawn at 30% probability level.
2
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possible isomers are C
isomers in which the sulphur atoms have opposite
configuration (Scheme 2).
1
symmetric, except for the trans
View Article Online
DOI: 10.1039/C5DT02938D
1
3
However, as already discussed, the broadness of the observed
signals at room temperature indicates a fluxional behaviour of
complexes
1 and 2 in solution.
The VT NMR analysis of complex
1
(Figure 2) showed that, at
sub‐ambient temperature, the two methylene protons S‐CH ‐
2
2
C(CF ) resolve into a diastereotopic AB pattern ( J = 13 Hz).
3
2
HH
The coalescence of the signals was observed at 283 K; below
this temperature the resonances became sharp and well
resolved. Kinetic parameters were calculated using line‐shape
1
analysis of the H NMR data over the temperature range (193 ‐
#
3
38 K). The activation enthalpy (

H = 1.84 Kcal/mol) and
#
entropy (S = ‐40.24 cal/mol K) values were evaluated from
1
8
Figure 2. Variable‐temperature H NMR spectra of 1 in toluene‐d .
the Eyring plot (Figure S9, Supporting Information). The
negative entropy of activation suggests that rearrangement of
the ligands occurs via a non‐dissociative mechanism, indicating
strong metal–sulphur bonds.
S
S
AlMe
benzene
3
S
S
F
F
3
C
CF
CF
3
3
F
F3C
3
C
CF
CF
3
3
Al
+ CH
4
C
OH
HO
O
O
The whole picture demonstrates the presence in solution of a
symmetric isomer (C or C ), signifying either the formation of
3
2
S
Complex 3: SCH CH S
Complex 4: SC
2
2
the C ‐symmetric octahedral isomer with trans geometry (all
6
4
H S
S
the other possible octahedral isomers have lower symmetry),
or the presence of a species with a trigonal‐prismatic
geometry, i.e. the solid state structure is retained in solution. §
In both cases one single species is present in solution and the
observed fluxionality may be due to fast conformation change
Scheme 3. Synthetic pathway for the synthesis of complexes 3‐4.
NMR studies of the reactions performed in benzene‐d
6
indicated that the neutral proligands were deprotonated by
two alkyls of trimethylaluminium with the release of 2 equiv of
methane. The disappearance of the O‐H signal of the free
ligands and the appearance of a resonance for the protons of
of the six‐membered‐ring yttrium metallacycles.
1
At 298 K, the H NMR spectrum of complex
2 showed narrow
peaks. Upon lowering the temperature to 208 K, the S‐CH2‐
the methyl bound to the aluminium in the highfield region of
C(CF ) methylene protons were resolved into two sets of
1
3
2
the H NMR spectra (‐0.13 and ‐0.08 ppm for complexes
3 and
signals, at least. The main signal (70 %) appeared as a broad
signal, while minor signals appeared as an AB pattern
consistent with C2 or CS symmetry. These spectra were
reversibly reproduced over the temperature range examined,
indicating a reversible fluxional process between several
isomers.
4
, respectively) demonstrated the formation of the desired
complexes. In particular, for complex
protons S‐CH ‐C(CF ) resulted diastereotopic, appearing as an
AB‐system. This demonstrates both the coordination of the
ligand in a tetradentate fashion and the formation of a C ‐ or a
CS‐symmetric isomer.
On the other hand for complex
CH ‐C(CF ) protons appeared, at room temperature, as a
broad signal. In this case, a well resolved AB‐system for these
protons was observed in the H NMR spectrum acquired at 233
K (see the Supporting Information).
The higher rigidity of the phenylene group, with respect to that
of the ethylene group, can determine a stronger coordination
4 the two methylene
2
3 2
2
1
2
The reactions of the proligands L ‐H and L ‐H with 1 equiv of
2
2
3 the analogous methylene S‐
the aluminium precursor AlMe took place immediately on
3
2
3 2
addition at room temperature in benzene (Scheme 3). The
reaction mixtures were kept stirring for 1 h to afford the
1
aluminium methyl complexes
3 and 4 in good yields (73% and
8
7%, respectively).
of the sulphur atoms to the aluminium centre in complex
and so explain its lower fluxionality with respect to that of
complex
4,
3
F C
CF
3
N(SiHMe
3
)
2
3
F C
3.
S
O
F
3
C
19
S
S
Y
O
N
S
S
S
The F NMR spectrum of
3 displayed a broad signal. For
Y
Y
F
3
C
(Me
HSi)
THF
O
3
O
THF
CF
3
3
2
3 2
(Me HSi) N
THF
complex two sharp quartets of equal intensity were
4
CF
F
3
C
O
F
O
F3C
CF
3
CF
3
observed, indicating that the two CF groups within each
3
C
3
C(CF3)2 moiety are magnetically nonequivalent, coherently
with the symmetry deduced by the H NMR spectrum.
trans
-cis
-cis
1
Ring‐opening polymerization of ε‐caprolactone and rac‐lactide by
catalysts 1 and 2.
2 2
Scheme 2. Representative isomers for the octahedral (OSSO)Y[N(SiHMe ) ](THF)
complexes. All the other possible isomers can be obtained taking into account both the The prepared yttrium‐amide complexes were investigated in
chirality of the sulphur atoms and the interchange of the two labile ligands.
the ROP of ‐caprolactone.
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Representative data from a series of polymerizations con‐
Journal Name
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6 17
1
DOI: 10.1039/C5DT02938D
ducted in toluene are collected in Table 1. The polymers, Analogous behavior was observed for salalen and salen
precipitated from the reaction medium by addition of wet yttrium complexes, and for dithiaalkanediyl‐bridged lanthanide
8a
hexane, were analyzed by NMR spectroscopy and gel amide complexes. and was attributed to the dimeric nature
permeation chromatography (GPC).
of the rare‐earth metal alkoxides generated in situ by reaction
Under mild reaction conditions, complexes
1
and
2
alone with the alcohol. At higher temperature (70 °C), in the
exhibited extremely high activity with turnover frequency presence of isopropanol, good conversions were achieved
5
‐
among the highest reported in literature (TOF up to 1.2 10 h producing polymers whose molecular weights showed a good
1
)
. The high productivity was also preserved when a low agreement with the theoretical ones.
catalyst loading (2 mol) and high monomer/initiator molar The good control over the molecular weights was also
ratio were employed (run 3, Table 1). Under the reaction confirmed by the linear dependence of the M GPC values with

n
conditions explored, complex
2
showed higher catalytic the conversion (Figure S10, Supporting Information ). When an
additional equivalent of alcohol was used, the molecular
activity than complex (runs 1 and 2, Table 1).
1
The GPC analyses of the polymers showed monomodal weights were halved as a consequence of its role as chain
distributions of the molecular weights and polydispersity transfer agent (run 5, Table 1).
indexes relatively broad (1.60 ≤ M /M ≤ 2.1). However, the From these observations, we propose that alkoxides generated
w
n
number‐average molecular weights (M ,GPC, Table 1) were in situ may exist as a dimer in solution (see next paragraph). In
n
higher than those calculated on the assumption that a single the presence of the monomer, the increase of the
polymeric chain was produced per metal centre. This temperature favours the cleavage of the dimeric structure and
observation suggests that only a percentage of metal centres is promotes the propagation reaction.
involved in the catalytic process. One of the most common Kinetic studies were performed by monitoring the conversion
1
reason is a low initiation efficiency by the leaving group of ε‐caprolactone via
because of its low nucleophilicity.
H
NMR spectroscopy. The
1
5,9a
To verify this hypothesis, semilogarithmic plot of ln[CL] /[CL]t versus time for the
0
additional polymerization reactions were performed in the polymerization initiated by
1
established a first‐order kinetics
presence of isopropanol. The addition of an external alcohol is in monomer concentration (Figure 3).
a common expedient to introduce highly reactive initiation The performance of the new yttrium complexes
1 and 2 was
groups such as alkoxides on the metal centre. This allows the also explored in the ring‐opening polymerization of racemic
production of polymeric chains of predictable molecular lactide (Table 2).
weights and narrow molecular weight distributions. After the Despite the high activity showed in the polymerization of
‐
addition of 1 equiv. of isopropanol as a co‐initiator, the caprolactone, both complexes resulted scarcely reactive to‐
catalytic activity of the amide precursors
decreased.
1
and
2
drastically ward rac‐lactide. At room temperature, after 24 h, no
conversion was observed.
After 30 minutes, at room temperature, the monomer At 70°C, after the same reaction time, a conversion of only
conversion was lower than 10%. Probably, the formation of 22% was achieved whereas, after the addition of one
dimeric yttrium alkoxo species was responsible for the drop of equivalent of isopropanol, an almost quantitative conversion
activity.
was obtained (runs 7 and 8 in Table 2). Kinetic study of LA
polymerization initiated by established the first‐order
kinetics in monomer concentration (Figure 4).
1
a
Table 1. Polymerization of ε‐caprolactone by catalysts 1‐2.
i
b
c
d
d
Run cat
PrOH
T
t
conv
M
n,th
M
n,GPC
w n
M /M
(
eq)
(°C) (min)
(%)
(kDa) (kDa)
3
2
2
.0
.5
.0
1
2
1
2
2
1
1
2
‐
25
25
25
70
70
70
2
0.25
5
82
47.7
57.1
92.4
55.9
25.6
39.0
209.4
181.6
328.9
40.2
1.60
2.11
1.82
1.27
1.10
1.14
‐
‐
100
81
e
3
4
1.5
1.0
f
1
2
1
30
30
20
98
g
0
.5
5
90
20.3
f
0.0
6
68
26.4
0
10
20
30
40
50
60
70
80
Time (s)
a
b
Polymerization conditions: catalyst: 5 μmol; [ε‐CL]/[Y] = 500; toluene = 4 mL.
Figure 3. Pseudofirst‐order kinetic plot for ROP of ‐CL promoted by 1. Pseudofirst‐
1
c
Determined by H NMR (CDCl
1
3
, RT). Theoretical molecular masses M
n
of PCL =
Molecular masses and their
‐2 ‐1
order rate constant is (3.83 ± 0.08) 10
T = 298 K; toluene = 4 ml as solvent.
s (R = 0,99642). [1]= 10 mol; [‐CL]/[1] = 500;
i
d
14.14 x ([ε‐CL]/[Y‐ OPr]) x conversion CL.
dispersities as determined by GPC (THF, 35°C) vs. polystyrene standards. The M
GPC values were corrected by 0.56 factor. Catalyst: 2 μmol, [ε‐CL]/[Y] = 1000.
n
14 e
4
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a
Table 2. Polymerization of rac‐lactide by catalysts 1‐2.
Information), at the tetrad level, with calculated values by
using Bernoullian statistics (Table S2, Supporting Information).
View Article Online
DOI: 10.1039/C5DT02938D
b
c
d,e
Conv
%)
M
n,th
M
n,GPC
(kDa)
d
Run
7
cat
1
solvent
M
w
/M
n
The good agreement observed
confirms that the
(
(kDa)
stereocontrol is due to a chain end control mechanism,
whereby the stereo‐chemistry of the last inserted LA in the
growing chain induces the selection of the incoming
enantiomeric monomer.
C
C
7
H
H
8
8
92
68
82
72
26.5
15.6
11.6
13.6
13.0
1.39
1.44
1.33
1.35
8
2
7
19.6
23.7
20.7
NMR and DFT studies on the isopropoxo derivative of complex 2
9
1
THF
THF
To acquire more insight into the mechanism of the
polymerization conducted in the presence of alcohol, the
f
10
1
reaction of
spectroscopy. When
2
with isopropanol was monitored by NMR
was treated with one equivalent of
2
a
i
Polymerization conditions: precatalyst: 10 μmol; PrOH: 10 μmol, [rac‐LA]/[Y]= isopropanol in CD Cl , a rapid evolution of HN(SiHMe ) was
2
2
2 2
b
1
2
3
00, solvent = 2mL, T = 70 °C, t = 24 h. Determined by H NMR (CDCl , RT).
observed together with the appearance of a new pattern of
c
‐1
i
Theoretical molecular masses M
n
of PLA (in gmol ) =144.13 x ([L‐LA]/[Y‐OPr]) x
2
resonances attributable to the alkoxo derivative L YOiPr(THF)
d
conversion LA Molecular masses and their dispersities as determined by GPC
THF, 35°C) vs. polystyrene standards. Corrected by 0.58 factor. T = 25 °C, t =
e
14 f
(2a). The isopropoxo group replacing the amide group gave
(
96 h.
rise to a doublet and a multiplet at 1.2 and 4.2 ppm,
respectively. The methylene groups SCH ‐C(CF ) appeared as a
2
3 2
1
sharp singlet slightly highfield shifted compared to that of 2. H
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
NMR monitoring of the reaction showed that the amine
elimination is fast and quantitative in few minutes.
It is well known that alkoxo‐group 3 metal complexes are
prone to aggregation to dimeric/oligomeric structures in which
2
the metal centres are linked by bridging

‐alcohoxide groups.
To assess the nuclearity of 2a, diffusion‐ordered NMR
measurements were performed (CD Cl , using TMSS as an
2
2
internal standard). The experimental self‐diffusion coefficient
‐
10
2 ‐1
of 2a was 9.83 x 10 m s providing a hydrodynamic volume
3
‐10
2 ‐
of 704 Å . An higher self‐diffusion coefficient (11.7 x 10 m s
0
2
4
6
8
10
12
1
)
was obtained for the amide precursor
2
which gave a smaller
hydrodynamic volume of 502 Å (Figure 5). The larger
molecular size observed for 2a (compared to ) suggested a
Time (h)
3
Figure 4. Pseudofirst‐order kinetic plot for ROP of rac‐LA promoted by 1. Pseudofirst‐
order rate constant is kapp = (7.13 ± 0.04 )10
LA]/[Al]/[PrOH] = 100:1:1; T = 298 K; toluene‐d
2
‐
2
‐1
h
(R = 0.99915). [1]= 0.2 mM;
i
dinuclear solution state structure for 2a. To propose a
[
8
(0.5 ml) as solvent.
molecular geometry and evaluate the relative van der Waals
The activity and the degree of control were not significantly molecular volume, density functional theory (DFT) calculations
dependent on the nature of the solvent: complex
showed were performed. The optimized structure of the dimer of 2a is
the same activity in toluene and in THF solution (cfr runs 7 and re‐ported in Figure 6, together with the structures of the
1
2
9
, Table 2).
The molecular weight distributions of the PLAs were unimodal L YN(SiHMe
with polydispersity indexes M /M ≤ 1.44, but the degree of the dimer and the monomer for the alkoxo complexes were
monomeric alkoxo and amide complexes, i.e. L YOiPr(THF) and
2
2 2
) (THF). The van der Waals molecular volumes of
w
n
3
3
control over the molar masses was relatively low with Mn 858 Å and 507 Å respectively.
values significantly lower than those expected.
The ESI‐MS analysis revealed the formation of linear fragments
of the type H‐[OCH(CH )C(=O)] ‐OCH(CH )
and H‐
OCH(CH )C(=O)] ‐OH with non integer LA repeat units
3
n
3 2
[
3
n
indicating that intermolecular transesterifications occurred
extensively (Figure S11, Supporting Information).
1
Homodecoupled H NMR spectra showed that the PLA samples
obtained at 70 °C, both in toluene and THF, had atactic
microstructures. A weak heterotactic selectivity, with the
probability of racemic linkages between monomer units P =
r
0
.72, was observed when the polymerization reaction was
performed in THF at room temperature with complex (run
0, Table 2). The stereochemistry of rac‐lactide enchainment
was evaluated by comparing the integrals in the homonuclear
1
1
1
1
Figure 5. H DOSY spectrum of [L
2
YOiPr]
2
(2a) (CD
2
Cl
2
, rt, 400 MHz)
decoupled
H NMR spectrum (Figure S12, Supporting
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View Article Online
DOI: 10.1039/C5DT02938D
For comparative purpose the van der Waals molecular volume
of the monomeric amide complex was also evaluated (591
Å ), it was slightly larger than that of the monomeric alkoxo
derivative 2a. The molecular dimensions of 2a inferred from
diffusion NMR data proved to be close to the dimensions of
the dimeric species.
2
3
F3C
S
O
S
S
CF3
CF
F
3
C
3
O
O
O
O
F
3
C
S
S
- THF
THF
Y
Y
CF
CF
3
F
3
C
3
O
O
Y
O
O
THF
F
3
C
CF3
The dimerization process was further supported by the
theoretical calculation. The calculated free energy change (G)
CF
3
F
3
C
S
for the dimerization reaction, depicted in Scheme 4, indicated
that the process is energetically favoured by ‐14.2 kcal/mol. §§
It is worth noting that in the dimeric structure the yttrium
2 2
Scheme 4. Formation of the dimeric [L YOiPr] species.
atoms are ligated by two tetradentate OSSO ligands and two
2
The dimerization process was further supported by the
theoretical calculation. The calculated energy change ( E) for
the dimerization reaction, depicted in Scheme 4, indicated that
the process is energetically favoured by ‐2.6 kcal/mol.
ε‐Caprolactone Polymerizations by aluminium catalysts 3‐4
bridging

‐alkoxides, achieving octahedral environments at

each metal centre. Although higher coordination geometries
are possible, the THF was not included in the coordination
sphere of the metal centre. The absence of a tight interaction
between THF and the metal centre was suggested by a close
inspection of the DOSY spectrum of 2a which revealed for the The aluminium complexes
THF a diffusion coefficient different from that observed for 2a the ROP of ε‐CL and rac‐lactide. Both catalysts resulted
3 and 4 were tested as initiators in
‐
10
2 ‐1
‐10
2 ‐1
(
20.0 x 10 m s vs 9.83 x 10 m s , see Figure 5) and close inactive toward lactide and showed moderate activities toward
‐
10
2 ‐1
to the value of the free THF (26.5 x 10 m s ). In the case of ε‐CL. The polymerizations of ε‐CL were carried out in the
the monomeric precursor 2, the diffusion coefficient of THF is presence of one equivalent of iPrOH as activator and the main
‐
10
2 ‐1
very close to that of metal complex (14.7 x 10 m s vs 11.7 x results are summarized in Table 3. As well‐established for
‐
10
2 ‐1
1
0
m s ). These values suggest that the THF is reversibly aluminium complexes, the reactivity showed by
3 and 4 was
coordinated to the metal centre, according to the following much lower than that of parent yttrium complexes and
equilibria:
significant polymerization rates were achieved only at 70 °C,
with the almost quantitative conversion of 500 equivalents of
monomer after six hours (runs 11 and 12, Table 3). At room
temperature, the reactivity was notably reduced and
quantitative conversions were achieved only after 96 h (run
2
2
L YN(SiHMe ) + THF ↔ L YN(SiHMe ) (THF) (1)
2
2
2 2
2
2
[
L YOiPr] + THF ↔ [L YOiPr] (THF) (2)
2
2
1
4, Table 3).
Equilibrium 1 results shifted toward the product, whereas A good control over the molecular weights was observed as
equilibrium 2 is shifted toward the reagents.
evident by the agreement between the experimental number
average molecular weight values (M ) and the theoretical ones
n
calculated as the single‐site initiator. This was also confirmed
by a series of polymerizations conducted for various monomer
to initiator ratios, showing that the number average molecular
weight (M ) increased linearly as the [CL] /[Al] ratio was in‐
n
0
0
creased (Figure S13, Supporting Information).
a
Table 3. Polymerization of ε‐caprolactone by catalysts 3‐4.
b
c
d,e
n,GPC
[
ε‐CL]/
Al]
Conv
( %)
M
n,th
M
d
Run
cat.
3
w n
M /M
[
(kDa)
(kDa)
11
12
13
500
500
25
94
80
98
95
53.3
56.3
1.15
1.66
1.14
1.47
4
45.6
3.5
50.2
3.1
4
e
14
4
500
54.2
45.9
a
i
Polymerization conditions: precatalyst: 10 μmol; PrOH: 10 μmol; 10 μL of a 0.1
b
1
M toluene solution; toluene: 2 mL, T = 70 °C, t = 6 h. Determined by H NMR
c
d
(
CDCl
3
, RT).
Theoretical molecular masses. Molecular masses and their
2
2
Figure 6. DFT‐optimized structures of L YN(SiHMe
2
)
2
(THF) (2) L YOiPr(THF) (2a’) and
dispersities as determined by GPC (THF, 35°C) vs. polystyrene standards and
2
[
L YOiPr]
2
(2a) with the relative van der Waals molecular volumes.
14 e
corrected by 0.56 factor. T = 25 °C, t = 96 h.
6
| J. Name., 2012, 00, 1‐3
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ARTICLE
The initiation mechanism was elucidated by end‐group
analysis. The H NMR spectrum (Figure 7) of a low molecular
View Article Online
DOI: 10.1039/C5DT02938D
1
weight chain (run 13, Table 3) clearly showed the presence of
isopropoxo group according to the resonances at 1.25 and 5.05
ppm, as a consequence of the exchange reaction of 2‐
propanol with the methyl group at the aluminium centre.
Kinetic studies of ε‐CL polymerization established the first‐
order kinetics in monomer concentration at different
temperatures (Figure 8). The relationship between ln kapp and
the reciprocal of polymerization temperature (1/T) is shown in
Figure 9. According to the Arrhenius equation, the activation
i
Figure 8. Kinetic plots for ROP of ε‐CL promoted by 4 + PrOH at different temperatures.
−
1
energy calculated for 4/ iPrOH is E = 67.8 kJ mol .
a
This value is quite high, coherently with the moderate
activities observed, however it is comparable with those ob‐
served for other aluminium complexes bearing two
1
8
monoanionic phenolate ligands.
Experimental
Materials and methods
All manipulations of air‐ and/or water‐sensitive compounds
were carried out under a dry nitrogen atmosphere using a Figure 9. Arrhenius plot for ROP of ε‐CL promoted by 4 + PrOH.
Braun Labmaster glove box or standard Schlenk‐line
techniques. Glassware and vials used in the polymerization
were dried in an oven at 120 °C overnight and exposed to
vacuum‐nitrogen cycles thrice.
i
NMR spectra of complexes and polymers were recorded on Bruker
Avance spectrometers; resonances are reported in ppm (δ) and the
coupling constants in Hz (J), and are referenced to the residual
solvent peak versus Si(CH ) : at δ 7.26 ( H) δ 77.16 ( C); in the case
of 19F, resonances were automatically referenced versus CF C H
6 5
1
13
Benzene, hexane, THF and toluene were distilled over sodium
benzophenone. CDCl , CD Cl , and C D were dried using
3 4
3
3
2
2
6 6
by the software. J.Young NMR tubes were employed for the spectra
of complexes and complexes reactivity.
molecular sieves. Lactide was purified by crystallization from
i
dry toluene. PrOH was purified by distillation over sodium. ‐
CL was dried with CaH for 24 h at room temperature and then Molecular masses (M and M ) and their dispersities (M /M ) were
2
n
w
w
n
distilled under reduced pressure. All other chemicals were measured by gel permeation chromatography (GPC). The
commercially available and used as received unless otherwise measurements were performed at 30 °C on a Waters 1525 binary
1
2
stated. Yttrium precursor and ligands L ‐H and L ‐H were system equipped with a Waters 2414 Refractive Index (RI) detector;
2
1
2
0,19
‐1
prepared according to published procedures.
Elemental analyses were performed in the microanalytical Styragel HR columns (7.8 x 300 mm; range 10 – 10 Å). Narrow
using THF as eluent (1.0 mL min ) and employing a system of four
3
6
laboratory of the Department of Chemistry and Biology.
polystyrene standards were used as reference and Waters Breeze
v3.30 software for data processing.
Crystallography
X‐ray diffraction quality single crystals of 2 were obtained as yellow
prisms from hexane solutions at 253 K. A suitable crystal (0.46 mm x
0
.39 mm x 0.30 mm) was selected and mounted on a cryoloop with
Paratone® N and measured at 100 K with a Rigaku AFC7S
2
diffractometer equipped with a Mercury CCD detector using
graphite monochromated MoK radiation ( = 0.71069 Å). Data
reduction was performed with the crystallographic package
2
0
CrystalClear. Data have been corrected for Lorentz, polarization
and absorption. The structure was solved by direct methods using
2
1
the program SIR2002 and refined by means of full matrix least‐
2
22
squares based on F using the program SHELXL97. For all
compounds non‐hydrogen atoms were refined anisotropically,
hydrogen atoms were positioned geometrically and included in
structure factors calculations but not refined. 389 refinable
parameters were finally considered, final disagreement indices are
1
Figure 7. H NMR (300 MHz, CDCl
3
, RT) spectrum of a low molecular weight poly(ε‐
caprolactone) by catalyst 4 (Table 3, run 13).
2
2
R = 0.0851 (5135 reflections F >2F ), wR2= 0.2440 (all 6894
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ARTICLE
Journal Name
1
independent reflections). Maximum and minimum residual density
δ NMR (62.9 MHz, C D , RT) 124.4 (4C, q, JCF 289.2, CF ),
were respectively +1.874 and ‐1.17 e Å . Crystal structures were 77.55 (2C, septuplet,
c 6 6 3
View Article Online
‐
3
2
JCF 29.4 Hz, C(CF ) ), 38.0 (2H, s,
DOI: 10.1039/C5DT02938D
3 2
SCH C(O)(CF ) ), 34.75 (2C, s, SCH CH S), ‐11.7 (1C, br s, Al‐
2
3
drawn using Mercury.
2
3 2
2
2
CH3).
Synthetic procedures
1
1
δ
F
(376.5 MHz, C
6 6 3
D , RT) ‐78.2 (12F, br s, CF ).
Synthesis of complex L YNR (1). A solution of ligand L ‐H (150
mg, 0.330 mmol) in benzene (5.5 mL) was added dropwise to a
solution of [Y(N(SiMe H) ](THF) (208 mg, 0.330 mmol) in
benzene (5.0 mL). The reaction mixture was stirred for 1 h at
room temperature, then the solvent was removed. The
obtained solid was dissolved in n‐hexane (1 mL) and dried in
vacuum to yield a white powder. Yield: 92 %.
2
2
2
2
Synthesis of complex L AlMe (4). A solution of ligand L ‐H
2
(
100 mg, 0.199 mmol) in benzene (2.0 mL) was added
2
2
)
3
2
3
dropwise to a solution of AlMe (14.8 mg, 0.205 mmol) in
benzene (2.5 mL). The reaction mixture was stirred for 1 h at
room temperature, then concentrated at reduced pressure.
The obtained solid was dissolved in n‐hexane and dried in
vacuum to yield a white powder. Yield: 87 %.
Elemental analysis calcd (%) for C18
.06; N, 1.88; S, 8.60. Found: C, 28.82; H, 4.13; N, 1.83; S, 8.67
(300 MHz, C , RT) 4.94 (2H, br s, SiH), 3.90 (4H, br s,
OCH ), 2.80 (4H, br s, SCH C(O)(CF ), 1.98 (4H, br s,
SCH CH S), 1.24 (4H, br s, OCH
SiH(CH ).
(75.5 MHz, C
m, C(CF ), 71.1 (2C, s, OCH
3.45 (2C, s, SCH CH S), 25.15 (2C, s, OCH
SiH(CH ).
(376.5 MHz, C
Synthesis of complex L YNR
30 3 2 2
H F12NO S Si Y: C, 28.99; H,
2 2
Elemental analysis calcd (%) for C15H11AlF12O S : C, 33.22; H,
4
2
δ
.04; S, 11.82. Found: C, 33.35; H, 2.16; S, 11.94.
(400 MHz, C , RT) 6.70–6.60 (2H, m, Ar‐H), 6.60–6.50 (2H,
δ
H
6 6
D
H
6 6
D
2
2
3 2
)
2
2
m, Ar‐H), 3.29 (2H, d, J
14.2, SCH ), 3.04 (2H, d, J
HaHb
14.2,
HaHb
2
2
2
2
CH
2
), 0.36 (12H, d, JHH 3.0,
SCH
(62.9 MHz, C
H), 130.7 (2C, s, ArC‐H), 123.8 (4C, 2q, JCF 289.8, CF
2
), ‐0.08 (3H, s, Al‐CH
3
).
3 2
)
1
δ
C
6 6
D
, RT) 134.5 (2C, s, ArC‐S), 132.5 (2C, s, ArC‐
δ
C
6
D
6
, RT) 126.95 (4C, q, JCF 290.3, CF
3
), 82.7 (2C,
C(O)(CF ),
), 3.2 (4C, s,
1
3
), 77.1 (2C,
septuplet, J 30.1, C(CF ) ), 43.1 (2C, s, S‐CH ), ‐11.0 (1C, br s,
3
)
2
2
), 39.45 (2C, s, SCH
2
3 2
)
2
CF
3 2
2
3
2
2
2
CH
2
Al‐CH
3
).
3 2
)
4
δ
J
F
(376.5 MHz, C D , RT)‐78.25 (6F, q, JFF 8.5, CF ), ‐78.4 (6F, q,
FF 8.5, CF
6
6
3
δ
F
6 6 3
D , RT) ‐78.9 (12F, s, CF ).
4
2
2
3
).
2
(2). A solution of ligand L ‐H
2
(150 mg,
Polymerization reactions
0
.330 mmol) in benzene (5.5 mL) was added dropwise to a solution
ε‐Caprolactone polymerizations by catalysts 1‐2. In a typical
experiment (Table 1, run 1), in glovebox, a 4 mL vial was
sequentially charged with a solution of catalyst (5 μmol) in
toluene (1 mL), a solution of ε‐caprolactone (2.5 mmol) in
toluene (1 mL). The reaction mixture was stirred for the
desired time and then quenched by adding wet n‐hexane.
Precipitated solid was filtered and dried in a vacuum oven
overnight at 40 °C.
of [Y(N(SiMe H) ](THF) (188 mg, 0.298 mmol) in benzene (5.0
2
2
)
3
2
mL). The reaction mixture was stirred for 1 h at room temperature,
then concentrated at reduced pressure. The obtained solid was
dissolved in n‐hexane (1 mL) and dried in vacuum to yield a white
powder. Yield: 89 %.
Elemental analysis calcd (%) for C22
.81; N, 1.76; S, 8.08. Found: C, 33.97; H, 3.77; N, 1.66; S, 8.12.
(400 MHz, C , RT) 6.90–6.85 (2H, m, Ar‐H), 6.75–6.67 (2H, m,
Ar‐H), 4.76 (2H, br s, SiH), 3.93 (4H, m, OCH ), 3.40 (4H, br s, SCH ),
.31 (4H, br s, OCH CH ), 0.29 (12H, d, JHH 2.7, SiH(CH ).
(62.9 MHz, C
30 3 2 2
H F12NO S Si Y: C, 33.29; H,
3
δ
H
6 6
D
2
2
rac‐Lactide polymerizations by catalysts 1‐2. In a typical
experiment (Table 2, run 2), in glovebox, a 10 mL Schlenk tube
was sequentially charged with rac‐lactide (2.0 mmol), toluene
1
2
2
3 2
)
δ
C
6
D
6
, RT) 137.55 (2C, s, ArC‐S), 134.0 (2C, s, ArC‐H);
1
1
30.1 (2C, s, ArC‐H); 125.3 (4C, q,
J
CF 291.3, CF
), 44.15 (2C, s,
S), 3.22 (4C, s, SiH(CH ).
, RT) ‐77.9 (12F, br s, CF ).
Crystallographic data of 2: Molecular formula YC22
MW 793.68, triclinic, P‐1, a = 10.584(9) Å, b = 10.661(9) Å, c =
4.958(11) Å,  = 91.476(18)°,  = 95.111(19)°,  = 104.21(3)°,
3
), 82.5 (2C,
2
(
1 mL), a solution of catalyst (10 μmol) and isopropanol (10
septuplet, JCF 28.5 Hz, C(CF
SCH
δ
3
)
2
), 71.0 (2C, s, OCH
), 25.65 (2C, s, SCH CH
(376.5 MHz, C
2
μmol, 100 μL of a 0.1 M toluene solution) in toluene (1 mL).
The tube was put in an oil bath, preheated and thermostated
at 70 °C. The reaction mixture was stirred for the desired time
and then quenched by adding wet n‐hexane. Precipitated solid
was filtered and dried in a vacuum oven overnight at 40 °C.
ε‐Caprolactone polymerizations by catalysts 3‐4. In a typical
experiment (Table 2, run 1), in glovebox, a 10 mL Schlenk tube
was sequentially charged with a solution of ε‐caprolactone (5.0
mmol) in toluene (1 mL), a solution of catalyst (10 μmol) and
isopropanol (10 μmol, 100 μL of a 0.1 M toluene solution) in
toluene (1 mL). The tube was put in an oil bath, preheated and
thermostated at 70 °C. The reaction mixture was stirred for the
desired time and then quenched by adding wet n‐hexane.
Precipitated solid was filtered and dried in a vacuum oven
overnight at 40 °C.
2
C(O)(CF
3
)
2
2
2
3 2
)
F
6
D
6
3
30 3 2 2
H F12NO S Si ,
1
3
3
‐1
V=1628(2) Å , Z = 2, D
Synthesis of complex L AlMe (3). A solution of AlMe
.416 mmol) in benzene (1 mL) was added dropwise to a
c
= 1.619 g/cm , F(000) = 800,  = 2.088 mm .
1
3
(30 mg,
0
1
2
solution of L ‐H (189 mg, 0.416 mmol) in benzene (4 mL). The
reaction mixture was stirred for 30 min at room temperature,
then concentrated at reduced pressure. The obtained solid
was dissolved in n‐pentane (1 mL) and dried in vacuum to yield
a white powder. Yield: 73 %
2 2
Elemental analysis calcd (%) for C11H11AlF12O S : C, 26.73; H,
2
δ
.24; S, 12.97. Found: C, 26.61; H, 2.17; S, 13.06.
(400 MHz, C , RT) 3.15‐2.00 (4H, br m, SCH
C(O)(CF ), ‐0.13 (3H, br s, Al‐CH ).
Kinetic studies. In a typical experiment, in glovebox, a J.Young
NMR tube was sequentially charged with a solution of catalyst
H
6
D
6
2 2
CH S), 1.66
(
4H, br s, SCH
2
3
)
2
3
8
| J. Name., 2012, 00, 1‐3
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5 μmol) in toluene‐d
ARTICLE
(0.1 mL), isopropanol (5 μmol, 50 μL of a and DFT calculations suggested the formation of a dimeric
(
8
View Article Online
0
.1 M toluene solution), monomer (1.00 mmol, 200 equiv), 0.4 structure for the isopropoxo derivative
DO
in
I:
w
10
hi
.1
0
ch39/
t
C
h
5
e
DT
y
0
t
2
tr
to adjust the total volume to 0.5 mL. The atoms result bound to two tetradentate OSSO ligands and two
8
93
i
u
8D
m
mL of toluene‐d
2
NMR tube was placed into the NMR Bruker spectrometer, bridging
 ‐alkoxides while the monomeric species is favoured
for the yt‐trium amide complex. The different steric
encumberance of the metal centres in the two discussed
species could explain the lower activities of the isopropoxo
species with respect to the amide species in the
polymerization reactions.
preheated and thermostated at the desired temperature, and
the multi_zgvd (variable delay) function was employed to
record spectra at fixed intervals of time. Conversion values
were calculated from polymer and monomer integrals, and the
spectrum recording time was taken into account for the kinetic
plot.
Aluminium complexes showed moderate activities toward ε‐
CL while resulted inactive with respect to rac‐lactide.
Investigation on catalytic species
Synthesis of complex L2YOiPr(THF) (2a). (NMR Tube Reaction) In a
glovebox, isopropanol (1.1 µL, 0.015 mmol) dissolved in CD
mL) was added to a solution of complex 2 (11.5 mg, 0.015 mmol) in
2
Cl
2
(0.3 Acknowledgements
The authors thank Moshe Kol (Tel Aviv University) for useful
discussion, Dr. Ilaria D’Auria for GPC measurements, Dr.
Patrizia Iannece for elemental analysis and Dr. Patrizia Oliva
CD Cl (0.3 mL) at room temperature. The resulting pale yellow
2
2
solution was transferred to a NMR tube and analyzed.
(400 MHz, CD Cl
.25 (1H, m, OCH(CH ) ), 3.88 (4H, br s, OCH ), 3.59 (4H, br s, SCH ), Italian Ministry of University and Research (PRIN 2010–2011:
δ
H
2
2
, RT) 7.64 (2H, m, Ar‐H), 7.41 (2H, m, Ar‐H), for NMR technical assistance. This work was supported by the
4
1
3
2
2
2
.88 (4H, m, OCH
2
CH
2
), 1.22 (6H, d, OCH(CH
3
)
2
).
nanostructured polymeric materials with tailored molecular
and crystalline structures, for advanced technologies and for
the environment)
DOSY experiments. The measurement of diffusion has been carried
out by observing the attenuation of the NMR signals during a
pulsed field gradient experiment using the double stimulated echo
2
0
pulse sequence. In particular, 2D DOSY PGSE NMR spectra were
performed on a Bruker Avance 400 spectrometer at 298 K without
spinning. Tetrakistrimethylsilylsilane (TMSS) was added as the
internal standard. The parameters δ and Δ were kept constant
during the experiments, whereas G varied from 2 to 95% in 32
steps. The value of δ was 500 s. (see Supporting Information for of pentacoordinate species by dissociation of the THF. In our
further details). The diffusion coefficients remained substantial case we rule out this possibility taking into account both the
unchanged over the range of concentration 30 ‐ 3 mM.
Notes and references
‡
Indeed in this case the H atom bonded to Si1 atom could be
located in the difference Fourier map (at difference with the H
atom bonded to the Si2 atom).
§
12
For similar complexes, some authors invoked the formation
1
chemical shift of the THF in the H NMR and the DOSY
experiment results (vide infra).
Computational details. Density functional theory (DFT) calculations §§ DFT calculations indicated that the rupture of the dimer by CL
2
1
were performed with the program suite Gaussian 09. All or LA is not energetically favoured
geometries were optimization without constraints at the BP86 level,
.
1
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2
2
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2
3
Hay and Wadt (LANL2DZ) for Y and of the 6‐31G(d) basis sets for
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2
3
4
4
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(
Conclusions
,
Two new yttrium and two new aluminium complexes, bearing
dithiodiolate ligands, have been synthesized, fully
characterized and tested as initiators in the ring‐opening
polymerization of ε‐CL and rac‐LA. Yttrium complexes
promoted controlled polymerization of ε‐CL and rac‐LA in the
presence of isopropyl alcohol, whereas in the absence of the
alcohol these complexes resulted highly active in the
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2
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