Alkaline earth metal complexes stabilized by amidine and guanidine ligands: Synthesis, structure and their catalytic activity towards polymerization of: Rac-lactide

Article abstract of DOI:10.1039/c8dt01434e

A series of amidine ligands RAm^DIPP (R is backbone substituent, R = 2-methylpyridine (HL¹), N,N,2-trimethylaniline (HL²), N,N-dimethylpropan-1-amine (HL³); DIPP (2,6-diisopropylphenyl) is N substituent) and a (Z

Full text of DOI:10.1039/c8dt01434e

View Article Online
View Journal
Dalton
Transactions
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: N. Liu, B. Liu and
D. Cui, Dalton Trans., 2018, DOI: 10.1039/C8DT01434E.
Volume 45 Number
1
7
January 2016 Pages 1–398
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Dalton
Transactions
An international journal of inorganic chemistry
www.rsc.org/dalton
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
rsc.li/dalton

Page 1 of 11
Plea sDe a dl t oo nn oT tr aa nd sj ua cs tt i om nasrgins
View Article Online
DOI: 10.1039/C8DT01434E
Journal Name
ARTICLE
Alkaline earth metal complexes stabilized by amidine and
guanidine ligands: synthesis, structure and their catalytic activity
towards polymerization of rac-lactide
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a
DOI: 10.1039/x0xx00000x
Na Liu, Bo Liu* and Dongmei Cui *
www.rsc.org/
DIPP
1
2
A series of amidine ligands RAm
(R is backbone substituent, R = 2-methylpyridine (HL ), N,N,2-trimethylaniline (HL ),
3
N,N-dimethylpropan-1-amine (HL) ; DIPP (2,6-diisopropylphenyl) is
N
substituent) and
a
(Z)-1,1-diethyl-2,3-bis(2-
](THF) or
∙(THF) , 3:
, 6: L Am CaN(SiMe ∙(THF)
). All the complexes were well-defined by NMR spectrum analyses and the
4
1
4
methoxyphenyl)guanidine ligand (HL ) have been synthesized. Reaction of HL -HL with Ca[N(SiHMe
2
)
2
1
DIPP
1
DIPP
Ca[N(SiMe
3
)
2
](THF)
2
, respectively, afforded complexes 1-8 (1: [L Am Ca(SiHMe
2
)
2
]
2
, 2: L Am CaN(SiMe
3
)
2
2
2
DIPP
2
DIPP
3
DIPP
3
DIPP
L Am CaN(SiHMe
2
)
2
∙(THF)
2
, 4: L Am CaN(SiMe
3
)
2
∙(THF), 5: L Am CaN(SiHMe
)
2 2
∙(THF)
2
3
)
2
2
,
4
4
7
: [L CaN(SiHMe
2
)
2
]
2
, 8: [L CaN(SiMe
3
)
2
]
2
molecular structures of 3 and 5-8 were further determined by single crystal X-ray diffraction analyses. In combination with
an excess of PhOH as the chain transfer agent, complexes 1-8 catalyzed immortal ring-opening polymerization of rac-
lactide in a controlled manner and exhibited moderate catalytic activity at room temperature. The end-group fidelity of
the resultant polymer was certified by NMR and MALDI-TOF mass spectra.
Introduction
The bidentate amidinate and guanidinate ligands have been used
to stabilize various metal ions across the periodic table of elements,
since they can be easily modified by attaching substituents of
Well-defined heteroleptic Ae complexes have displayed remarkable
catalysis in “immortal” ring-opening polymerization (iROP) of bio-
11
various steric and electronic properties. In addition, these anionic
ligands have a common characteristic that the negative charge is
delocalized to reduce their nucleophilicity, which effectively shields
the alkaline earth cations and increases stability of the resultant
1
2
resourced cyclic esters , hydroamination , terminal alkyne
3
4
coupling , hydrogenation and so on. However, compared with the
well-established transition metal based chemistry, the alkaline-
earth metal based chemistry is still an infant. The development was
impeded by the fact that the heteroleptic complexes Ae-Nu (Nu =
nucleophilic group) readily decompose during deleterious Schlenk-
complexes by avoiding reorganization and Schlenk equilibrium etc
5c,11a,b,12
side reactions.
Indeed, amidinate ligands RAmAr (R is
backbone substituent, Ar is N substituent) have successfully
stabilized calcium hydride complexes. In addition, a type of
tridentate amidinate ligand featuring a coordinating side-arm of N
type equilibria to generate the poorly reactive and ill-defined
-
homoleptic [{L
ligand) and [(AeNu
addressed by employing sterically demanding multi-dentate ligands,
n
X}
2
Ae] (where {L
) ] species. This issue has been preliminarily
2 n
n
X} is a monoanionic ancillary
13
substituent has been explored for calcium metal. While as far as
5
we known, the backbone substituent of amidinate ligands
containing coordinating heteroatoms have not been investigated
even for transition metals, which should provide a useful platform
for “tailoring” new versatile ligand systems due to its potential to
form cage coordination environment.
6
7
such as tris(pyrazolyl)borates,
aminotrop(on)iminates,
β-
2
b,8
9
diketiminates,
bis- or tris-(imidazolin-2-ylidene-1-yl)borate and
1
a,10
-
phenolate
to stabilize {L
. However, suitable {L
X}Ae-Nu are still very scarce.
n
X} ancillary ligands being able
n
Herein, we synthesized some novel amidine and guanidine
1
-4
ligands bearing different substituent groups on the backbone (HL
Scheme 1 and Chart 1) and through their reaction with homoleptic
calcium amide, Ca[N(SiHMe (THF) and Ca[N(SiMe (THF)
respectively, to prepare a series of heteroleptic calcium complexes
-8. The catalytic behavior of these complexes towards immortal
,
2
)
2
]
2
3
)
2
]
2
2
,
1
ring-opening polymerization of rac-lactide with an excess of phenol
as the chain transfer agent, for the first time, is presented.
Results and Discussion
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 2 of 11
View Article Online
DOI: 10.1039/C8DT01434E
ARTICLE
Journal Name
1
-4
Synthesis of amidine and guanidine ligands (HL
)
methoxyphenyl)methanediimine according to the literature
1
5
procedure. However, for the first step (Scheme 2), reaction of CS
2
with 2-methyloxy aniline and triethylamine in water always gave
In contrast to the classical route to synthesize N,N’-disubstituted
the corresponding thiourea (I) contaminated by the same amount
of triethylamine evidenced by the H NMR spectrum analysis, since
1
1a,14
1
amidines through imidoylchloride intermediates,
the N,N’-
disubstituted amidines with various backbone substitutents were
prepared via reaction of N,N’-disubstituted carbodiimide with
lithium alkyl species. Hence, HL was synthesized through the
the integral intensity ratio of resonances at 3.05 ppm, 1.35 ppm and
3
.77 ppm assigned to methylene and methyl protons of
triethylamine and methyl protons of methoxy group, respectively, is
:3:2 (Fig. S1). Treatment of the above product with mercuric oxide
1
following procedure (Scheme 1): 2-methylpyridine reacted with
butyl lithium in THF to afford 2-pyridylmethyllithium. Then a THF
solution of bis(2,6-diisopropylphenyl)carbodiimide was gradually
transferred into the above mixture. After 22 h, an aqueous solution
of ammonium chloride was added to terminate the reaction and
2
and magnesium sulfate in refluxing toluene solution afforded a pale
1
yellow solid. In the H NMR spectrum, resonances at 3.40 ppm and
1
.21 ppm belonged to methylene and methyl protons of
triethylamine appear again. To our surprise, the integration
intensity ratio of them to the singlet at 3.67 ppm assigned to methyl
protons of methoxy group is 5:3, suggesting that there are only two
ethyl groups (Fig. S2). X-ray diffraction analysis indicated that the
1
HL was isolated as white powder in 43%. The amino proton shows
1
a broad resonance at 7.68 ppm in the H NMR spectrum while a
doublet at 3.63 ppm is assigned to the methylene protons.
4
afforded product is a guanidine compound, HL (Chart 1, Fig. S3). It
exists exclusively in the Zanti configuration. There is unequivocal
difference of ca. 0.118 Å in the lengths of the C-N and the C=N
5
c,16
bonds, which is a typical bond distances of guanidine ligands.
4
The reason for the generation of HL is unclear since employing 2,6-
diisopropyl aniline as the reagent, the corresponding carbodiimide
was exclusively isolated even in the presence of triethylamine.
1
Scheme 1 Syntheses of HL .
2
Following the similar procedure, HL with a longer coordinating
side arm (Chart 1) was prepared through employing N,N’-dimethyl-
o-toluidine and bis(2,6-diisopropylphenyl)carbodiimide in a high
4
Scheme 2 Syntheses of HL .
2
yield (87%). The resonance belonged to methylene protons of HL
1
appears at 3.60 ppm comparable to that of HL . While the chemical
shift of the amino proton exhibits at a relatively up field 6.84 ppm.
Synthesis and characterization of complexes 1-8
Following the remarkable work reported by Anwander that the Si-
H∙∙∙Ln agostic interaction facilitates to stabilize the complexes based
1
7
10e,18
–
on rare-earth elements, Yann
has implemented N(SiHMe
(THF) (Ae = Ca, n = 1; Sr,
and has shown that Ae∙∙∙H secondary
interactions are very effective for the stabilization of electrophilic
2 2
)
amido groups to prepare Ae[N(SiHMe
2
)
2
]
2
n
1
0e
n = ⅔; Ba, n = 0)
2
+
Ae species. Thus we initially attempted to prepare heteroleptic
compounds by treatment of Ca[N(SiHMe (THF) with an
2 2 2
) ]
1
equimolar amount of HL through amine elimination reaction. A
white precipitate formed slowly upon stirring at room temperature,
which was collected by filtration after the suspension being stirred
2
3
4
Chart 1 Structures of HL , HL and HL .
1
overnight. In the H NMR spectrum recorded in C6D6 at room
3
Employing the above procedure to synthesize HL with flexible temperature, the Si-H resonance at δ 5.17 is noticeably deshielded
side arm N,N-(dimethylamino)propyl (Chart 1), encountered a as compared to that in the free amine (NH(SiHMe : δ 4.70 ppm).
problem that reaction of butyl lithium with N,N- The absence of resonances at 3.75 and 1.35 ppm indicates that
dimethylamino)propyl chloride can’t generate the expected lithium there is no coordination of THF molecule. Hence, the ortho proton
salt. Hence, instead of lithium salt, a Grignard reagent (N,N- of pyridyl group exhibits a signal at 8.20 ppm downfield shifting
dimethylamino)propyl)magnesium chloride was used to react with than that in the neutral ligand (8.17 ppm), suggesting that the
)
2 2
(
(
3
19
bis(2,6-diisopropylphenyl)carbodiimide to afford HL as light yellow pyridyl group may coordinate to the calcium center. The methyl
powder in a medium yield (67%). The resonance of amino proton protons of isopropyl groups show two doublets at 1.28 and 1.23
within HL shows at a much higher field (5.29 ppm) as compared to ppm, meaning the two asymmetric phenyl groups. The above
those of HL and HL . Meanwhile, the methylene protons gives a characterization precluded the possibility of complex 1 being
triplet at 2.28 ppm shifting upfield due to the absence of electron monomeric structure. Hence, we speculated that complex 1 is a
3
1
2
withdrawing effect of the phenyl group.
dimer with a center Ca N planar core where the two metal centers
2 2
are bridged by Npyridyl atoms (Scheme 3), leading to the poor
1
solubility of 1. While reaction of HL with Ca[N(SiMe ) ] (THF)
afforded complex 2 with good solubility even in hexane. H NMR
3
2 2
1
2
To introduce an additional coordination atom, we chose 2-
methyloxy
aniline
to
synthesize
N,N’-bis(2-
spectrum analysis indicated that there exists two coordinating THF
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 11
Plea sDe a dl t oo nn oT tr aa nd sj ua cs tt i om nasrgins
View Article Online
DOI: 10.1039/C8DT01434E
Journal Name
ARTICLE
molecules. The ortho proton of pyridyl group exhibits an upfield
shift at 6.09 ppm, suggesting the absence of pyridyl coordination to
the metal center, which corresponds to a monomeric species of
complex 2.
Ar
N
N
SiHMe
2
Me
2
HSi
HSi
Ar
N
Ca
N
Ca
N
N
Ca[N(SiHMe
2
)
2
]
2
(THF)
N
Ar
Toluene, r,t, overnight
SiHMe
2
Me
2
N
Ar
N
Ar
N
N
Ar
2 6 3
Ar = 2,6- Pr -C H
i
H
1
Ar = 2,6-
i
2 6 3
Pr -C H
Ar
N
N
HL
1
Ca[N(SiMe ) ] (THF)
2
O
3
2 2
Ca
SiMe
3
Toluene,r,t, overnight
Fig. 1 Crystal structure of complex 3. Thermal ellipsoids are drawn
at the 35% probability level. Hydrogen atoms except Si-H are
omitted for clarity. Selected bond lengths (Å) and bond angles (deg)
for complex 3: Ca1-N1 2.417, Ca1-N2 2.426, Ca1-O1 2.388, Ca1-O2
2.375, Ca1-N4 2.358, Ca1-H1 2.608, Ca1-Si1 3.151, Ca1-Si2 3.679,
Ca1-N4-Si1 101.19, Ca1-N4-Si2 130.94
N
N
O
Ar
SiMe3
i
2 6 3
Ar = 2,6-Pr -C H
2
Scheme 3 Syntheses of complexes 1 and 2.
2
3
HL with
Ca[N(SiHMe
a
longer coordinating side arm reacted with
HL
with
a
flexible coordination arm reacted with
(THF) to afford a stable monomeric complex 3 Ca[N(SiHMe
2
)
2
]
2
(THF) and Ca[N(SiMe (THF) to give the
3
)
2
]
2
2
2
)
2
]
2
confirmed by X-ray diffraction analysis (Fig. 1). The calcium center is corresponding complexes 5 and 6, both possess the similar
3
capped by the bidentate NCN ligand in a η -coordination fashion, monomeric structure determined by X-ray analyses (Fig. 2). While
1
two THF molecules and one amide group. Compound 3 also the
J(Si,H) coupling constant of 147.4 Hz in 5 falls in the range
presents an evident case of stabilization by Ca∙∙∙H–Si agostic indicative of moderate agostic Ca∙∙∙H–Si interactions (140-160 Hz)
2
0
interaction, as demonstrated by the asymmetry in the Ca– observed in lanthanide complexes . An amino moiety,
N(SiHMe
fragment, which is reflected by the discrepancy coordinating N,N-bidentate ligand, and two solvated THF molecules,
between the large Ca1-N4-Si2 and much more acute Ca1-N4-Si1 generate a distorted square pyramid environment around the
a
2 2
)
o
o
3
angles (130.94 and 101.19 ), corresponding to a much shorter calcium center. The ligand adopts a η -coordination fashion around
Ca1-Si1 distance (3.151 Å) with respect to Ca1-Si2 (3.679 Å). the center metal, while the N3 atom still doesn’t coordinate to the
Furthermore, the co-planar Ca1, N4, Si1 and H1A atoms also attest central metal, which might be attributed to the steric hindrance of
1
a
1
the agostic Ca1∙∙∙H1-Si1 interaction . The J(Si,H) coupling constant the isopropyl in phenyl ring.
of 163.3 Hz is further indicative of weak agostic Ca∙∙∙H–Si interaction,
which is in accordance with previous report for alkaline earth metal
1
a
2
Ar
N
complexes . While treatment of HL
Ca[N(SiMe (THF) generated complex 4 that contains only one
THF molecule, since the integration intensity ratio of resonances at
.89 ppm and 1.97 ppm assigned to CH and N(CH , respectively,
is 1:3. The chemical shift (1.97 ppm) of methyl protons of N(CH is
nearly the same with that found in complex 3, suggesting that N
with the bulky
N
)
2
]
2
2
THF
N
3
N
Ca
SiMe
SiMe
2
2
R
R
2 2 2 2
Ca[N(SiMe R) ] (THF)
N
3
2
3 2
)
Ar
N
N
Ar
Toluene,r,t, overnight
R = H, Me
H
THF
)
2
Ar
3
i
Ar = 2,6-Pr
2
-C
6
H
3
i
Ar = 2,6-Pr
2
-C
6
H
3
atom of N(CH
3
)
2
doesn’t coordinate to the calcium center.
HL3
5
: R = H
6: R = Me
Scheme 5 Syntheses of complexes 5 and 6.
Ar
N
(THF)x
N
N
Ca
SiMe2R
Ca[N(SiMe
2
R)
2
]
2
(THF)
2
N
N
Toluene, r,t, overnight
R = H, Me
Ar
N
N
Ar
H
Ar
Ar =2,6-
2
SiMe R
Ar = 2,6-
i
Pr
2
-C
6 3
H
i
2 6 3
Pr -C H
HL
2
3
4
: R =H, x = 2
: R= Me,x = 1
Scheme 4 Syntheses of complexes 3 and 4.
Complex 5
Complex 6
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 4 of 11
View Article Online
DOI: 10.1039/C8DT01434E
ARTICLE
Journal Name
Fig. 2 Crystal structures of complexes 5 and 6. Thermal ellipsoids 2.637, Ca1-N4 2.360, Ca1-Si1 3.527, Ca1-Si2 3.447, Ca1-N4-Si1
are drawn at the 35% probability level. Hydrogen atoms except Si-H 120.45, Ca1-N4-Si2 116.07
are omitted for clarity. Selected bond lengths (Å) and bond angles
(
deg) for complex 5: Ca1-N1 2.434, Ca1-N2 2.397, Ca1-O1 2.371,
Ring-opening polymerization of rac-LA initiated by calcium
Ca1-O2 2.389, Ca1-N4 2.326, Ca1-H1b 2.971, Ca1-Si2 3.212, Ca1-Si1
complexes 1-8
3
2
.564, Ca1-N4-Si2 105.68, Ca1-N4-Si1 125.63; complex 6: Ca1-N2
.454, Ca1-N1 2.405, Ca1-O1 2.406, Ca1-O2 2.389, Ca1-N4 2.332,
Ca1-Si1 3.506, Ca1-Si2 3.394, Ca1-N4-Si1 114.33, Ca1-N4-Si2 120.92
The highly efficient strategy of preparing biodegradable and
biocompatible polylactide (PLA) from the bio-resourced lactic acid
4
(
LA) has been established recently named “immortal ring-opening
When employing the guanidine compound HL as the ligand
21
polymerization”, which overcomes the vital drawback of low
catalytic efficiency for the conventional coordination
polymerization and provides PLA in a highly efficient mode of “one
metal-active species multiple polymer chains” with very low metal
scaffold, the corresponding heteroleptic calcium complexes 7 and 8
are C -symmetric dimers (Fig. 3). A NCN unit bridges two metal
2
centers via two separated N atoms. These two NCN planes locate in
trans configuration and are almost perpendicular to Ca1-Ca1' axial.
Each calcium center sits in one distorted trigonal bipyramid
geometry with two O atoms in the axial direction and three N
atoms in the equatorial plane. Thus each guanidinate ligand acts as
2
2
residue. Meanwhile, the molecular weight and the molecular
weight distribution of the resultant polymer are precisely controlled
according to the feed of the chain transfer agent. Noteworthy is
that the polymer chain ends are automatically functionalized by the
transfer agent. To date, the most suitable and widely investigated
chain transfer agents (CTAs) are small molecular alcohols or
1
1
1
1
a κ -O: κ -N/κ -N: κ -O tetradentate mode. This kind of coordination
mode is rare in alkaline earth metal complexes. The averaging bond
length between calcium and N atoms of NCN units in complex 7
2
3
polymeric alcohols. No reports have been related to a phenol as a
CTA because its acidity is much stronger than the alcohols, which
might abstract the ligand from the central metal. However, many
antimicrobial agents contain phenolic hydroxyl group rather than
alcoholic hydroxyl group, like chromene and its derivatives, which
(
2.3875 Å) is slightly shorter than those found in amidinate ligated
complexes 3 (2.4215Å) and 5 (2.4155 Å). While the Ca1-N4 (2.437 Å)
and Ca-O (2.5880 Å) bond distances are much longer compared to
those within 3 (Ca1-N4: 2.358 Å, Ca-O: 2.3815 Å) and 5 (Ca1-N4:
2
8
.326 Å, Ca-O: 2.3800 Å). The above trend is also observed between
and 6.
2
4
25
26
have been used for antimicrobial , antioxidant , anticancer ,
2
7
28
anthelmintic , congnitive functions enhanced
and anti-
2
9
inflammatory . Therefore, investigation of phenol as the chain
transfer agent of iROP is very attractive.
N
N
O
O
N
N
O
O
Ca[N(SiMe
2
R)
2
]
2
(THF)
2
RMe
RMe
2
Si
2
SiMe R
N
N
N
Ca
Ca
N
In the presence of 5 equiv. of phenol, the ability of heteroleptic
complexes 1-8 to promote the ring-opening polymerization of rac-
LA was examined (Table 1, entries 1-8). All the binary catalytic
systems exhibited moderate catalytic activities by giving
polylactides with relative narrow molecular weight distributions
H
Toluene,r,t, overnight
R = H,Me
2
Si
2
SiMe R
O
O
N
N
4
N
HL
7
8
: R =H
:R = Me
(
w n
M /M = 1.27-1.50). The observed molecular weights are in good
Scheme 6 Syntheses of complexes 7 and 8.
agreement with the calculated ones, attesting of efficient chain
transfer between the growing and resting (macro)alcohols, as
expected for effective iROP processes (Table 1, entries 1-8). On
contrary, the homoleptic calcium complexes Ca[N(SiHMe
and Ca[N(SiMe (THF) yielded PLAs with significantly broader
molecular weight distributions (M /M = 1.98-2.05) (Table 1,
2 2 2
) ] (THF)
3
)
2
]
2
2
w
n
entries 9 and 10), suggesting that the ligand wasn’t abstracted from
the metal center by the phenol. Independently of the nature of the
–
ancillary ligand, slower rates were observed with N(SiMe
3
)
2
bound
–
2 2
to the metal center than with N(SiMe H) . That was unexpected for
the immortal ROP of rac-LA performed in the presence of an excess
of transfer agent (PhOH) since following rapid protonolysis of the
Ca–Nu and/or the initial monomer insertion, all active species have
the same identity. The influence of the chain transfer agent was
then probed by using complex 2 (Table 1, entries 11-14). Under the
same monomer-to-Ca ratio (500:1), the molecular weight of the
Complex 7
Complex 8
4
4
resultant PLA decreased from 3.15 × 10 Da to 0.33 × 10 Da with
increasing the chain transfer agent loading from 1 to 20 equiv.
Fig. 3 Molecular structures of complexes 7 (left) and 8 (right).
Thermal ellipsoids are drawn at the 35% probability level. All
hydrogen atoms and the ethyl group of NEt for 7 are omitted for
clarity. All hydrogen atoms for 8 are omitted for clarity. Selected
bond lengths (Å) and bond angles (deg) for complex 7: Ca1-N3i
.377, Ca1-N2 2.398, Ca1-O2 2.629, Ca1-O1 2.547, Ca1-N1 2.437,
Ca1-Si2 3.208, Ca1-Si1 3.504, Ca1-N1-Si2 105.79, Ca1-N1-Si1 121.81;
complex 8: Ca1-N3 2.437, Ca1-N1 2.403, Ca1-O2 2.544, Ca1-O1
relative to [Ca]
the CAT-to-Ca ratio at
0
, consistent with the calculated one. While keeping
constant, the molecular weight
2
a
distributions of the afforded PLA became broader and the
discrepancies between calculated and observed molecular weights
became larger with increasing the rac-lactide loading from 200 to
2
1
0
000 equiv. relative to [Ca] (Table 1, entries 15 and 16). This might
be due to the serious transesterification. The PLA afforded under
4
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 11
Plea sDe a dl t oo nn oT tr aa nd sj ua cs tt i om nasrgins
View Article Online
DOI: 10.1039/C8DT01434E
Journal Name
ARTICLE
low monomer loading (Table 1, entry 16) was used to analyze the
end-group fidelity that is crucial to evaluate the efficiency of a
catalyst system for the iROP of cyclic esters. The polymer sample
1
was characterized by H NMR spectroscopy (Fig. 4). The polymer
chains are capped by the hydroxyl group at one end evidenced by
giving the resonance at 2.70 ppm and PhO from chain transfer
agent PhOH at the other end by giving the multiplets at 7.38 and
7
.08 ppm assignable, which is a typical feature of a polymer
obtained by coordination-insertion mechanism. This result was
further corroborated by matrix-assisted laser desorption/ionization
time-of-flight (MALDI-TOF) mass spectroscopy (Fig. 5). The MALDI-
TOF mass spectrum consists of one series of molecular ion peaks,
which presumably correspond to the linear PLA with PhO-/-OH as
the chain ends. The MALDI-TOF mass spectrum also shows that the
oligomers with a molecular weight interval of 72.06 arising from the
intermolecular transesterification.
1
o
3
Fig. 4 H NMR (400 MHz, CDCl , 25 C) spectrum of a low molecular
-1
w n
weight (Mn,GPC = 2 800 g∙mol , M /M = 1.17) prepared with the
a
Table 1 ROP of rac-LA catalyzed by calcium complexes 1-8/PhOH.
binary catalyst 2/PhOH (Table 1, entry 16).
b
c
d
Conv.
%)
Mn,calcd Mn,exp
d
entry cat. [M]
0
/[cat]
0
/[CTA]
0
w n
M /M
4
4
(
(10 ) (10 )
1
2
3
4
5
6
1
2
3
4
5
6
7
8
A
B
2
2
2
2
2
2
500/1/5
500/1/5
500/1/5
500/1/5
500/1/5
500/1/5
500/1/5
500/1/5
500/1/5
500/1/5
500/1/1
500/1/2
79
62
92
68
87
73
89
82
79
74
46
62
60
57
51
89
1.14
0.89
1.32
1.06
1.25
1.05
1.28
1.18
1.14
1.07
3.31
2.23
0.43
0.21
0.74
0.26
1.30
1.47
1.27
1.36
1.42
1.50
1.39
1.43
1.31
1.98
2.05
1.32
1.33
1.35
1.34
1.48
1.17
0.98
1.51
0.98
1.38
0.97
1.62
1.26
0.97
0.92
3.15
2.31
0.60
0.33
1.05
0.28
e
7
e
f
8
9
g
1
0
1
1
1
1
1
1
1
2
3
4
5
6
500/1/10
500/1/20
1000/1/10
200/1/10
+
Fig. 5 MALDI-TOF mass spectrum (Na ) of a PLA sample (Mn,GPC = 2
-
1
8
00 g∙mol , M /M = 1.17) prepared with the binary catalyst
w
n
2
/PhOH ([LA]
0
0 0 n
/[2] /[PhOH] = 200/1/10) (A = 144.13n + 22.99 +
-1
94.04, where n is the degree of polymerization, MNa = 22.99 g∙mol ,
M
-1
-1
LA = 144.13 g∙mol and MPhOH = 94.04 g∙mol ).
a
o
0
Cat.: 10 μmol, [M] : 1.0 M, Solvent: THF, Temperature: 25 C, Time:
b
1
c
Conclusions
1
1
2 h. Determined by H NMR spectrum.
M
n,calcd = [M]
0
/[CTA]
0
×
d
44.13 × conv. (%). Determined by GPC against polystyrene
e
n
standard, M using a correcting factor for polylactides (0.58). cat.:
5
DIPP
1-3
(HL ) and a
f
g
A series of novel amidine ligands RAm
2 2 2 3 2 2 2
μmol. A = Ca[N(SiHMe ) ] (THF). B = Ca[N(SiMe ) ] (THF) .
4
guanidine ligand (HL ) have been prepared and characterized.
A large family of heteroleptic silylamido alkaline earth metal
complexes 1-8 supported by these novel amidinate ligands and
the guanidinate ligand have been developed. The solvation
and aggregation of the afforded complexes were related to the
flexibility of the substituent. Reaction of HL bearing rigid
1
2 2 2
substituent (R = 2-methylpyridine) with Ca[N(SiHMe ) ] (THF)
2
3
afforded an unsolvated dimer. While, treatment of HL and HL
containing flexible substituents (R = N,N,2-trimethylaniline or
N,N-dimethylpropan-1-amine) with Ca[N(SiMe
H, Me), respectively, gave THF-solvated monomeric
complexes. The guanidinate calcium complexes and are
2 2 2 2
X) ] (THF) (X =
7
8
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 6 of 11
View Article Online
DOI: 10.1039/C8DT01434E
ARTICLE
Journal Name
1
dimers bearing a rare κ -O:κ -N/κ -N:κ -O ligand coordination Crystal for X-ray analysis was obtained as described in the
1
1
1
mode, which is ascribed to the small steric hindrance. The
successful isolation of these complexes provides a new avenue
for the synthetic organometallic chemists to design stable
alkaline earth metal complexes. These complexes exhibit
moderate activities and an immortal manner toward the ROP
of rac-lactide in the presence of PhOH as the chain transfer
following preparations. The crystal was manipulated in a
glovebox. A suitable single crystal of complexes was sealed in a
thin-walled glass capillary, and the data collection was
o
performed at -88.5 C on a Bruker SMART APEX diffractometer
with a CCD area detector and graphite-monochromated Mo Kα
agent, for the first time. This provides a convenient approach radiation (λ = 0.71073 Å). The data collection for single crystal
4
to binding functional phenolic hydroxyl group to the of HL ligand was performed at 23 C on a Bruker SMART APEX
o
biocompatible polymers.
diffractometer with a CCD area detector and graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). The
determination of crystal class and unit-cell parameters was
Conflicts of interest
carried out by the SMART program package. The raw frame
There are no conflicts to declare.
data were processed using SAINT and SADABS to yield the
reflection data file. All calculations were carried out using the
Experiment section
SHELXL-2014/7 program. The molecular structure was resolved
by using the ORTEP program. The cif and checkcif files of
complexes are given as ESI.
Materials
All operations were carried out under a dry nitrogen
atmosphere using Schlenk techniques or in a nitrogen gas filled
MBraun glovebox. Solvents were reagent grade, dried by
A typical procedure for polymerization of rac-LA
A typical polymerization procedure (Table 1, entry 1) can be
standard methods and distilled under nitrogen prior to use. described as follows. Polymerizations of rac-LA were carried
Toluene was purified using an SPS Braun system. THF was out in a 25 mL flask under a nitrogen atmosphere. A 3 mL flask
dried by distillation over sodium with benzophenone as was charged with a solution of complex
indicator under a nitrogen atmosphere and was stored over transfer agent (CTA), prepared previously by addition of CTA to
freshly cut sodium in a glovebox. Phenol was purchased from complex
in 2 mL of THF. Next, the solution was added to a
1 and phenol as chain
1
o
Aldrich and dried over by calcium hydride prior to distillation. stirred solution of rac-LA in THF at 25 C. The polymerization
Calcium iodide was purchased from Aldrich and stored in a was stirred for 12 h and then quenched by adding several
glovebox. Ca[N(SiHMe
2 2 2 3 2 2 2
) ] (THF) or Ca[N(SiMe ) ] (THF) can be drops acidified ethanol. Then polymers were precipitated with
o
prepared via salt metathesis involving treatment of the metal abundant ethanol, collected, and dried at 45 C under a
halides with alkali metal amide, which is prepared by
3
0
vacuum to a constant weight.
adopting the metathetical procedure for the synthesis of
3 2 2 2
Ba[N(SiMe ) ] (THF)
. rac-LA was recrystallized with dry ethyl Synthesis of pro-ligands and representative complexes
acetate three times. Glassware and flasks using in the
o
polymerization were dried in an oven at 115 C overnight and
Synthesis
of
N,N'-bis(2,6-diisopropylphenyl)-2-(2-
1
methylpyridyl)acetimidamide (HL ). The DippN=C=NDipp was
exposed to a vacuum-nitrogen cycle three times.
1
synthesized according to a previous report. Under a nitrogen
n
atmosphere, BuLi (0.206 mmol) was added dropwise to a
5
Techniques
stirred solution of 2-methylpyridine (0.206 mmol) in THF at -30
o
Calcium complexes for NMR measurements were prepared in
1
C. During addition, the color of the solution changed to red
NMR tubes and sealed with paraffin film in the glovebox. H from colorless. The mixture was stirred 10h. Then
1
3
and C NMR spectra were recorded on a Bruker AV 400 (FT, DippN=C=NDipp was added to above solution and reacted for
1
13
o
4
00 MHz for H, 100 MHz for C) spectrometer. Elemental 2 h at -30 C and 22h at room temperature, then quenched
analyses were performed at the National Analytical Research with NH Cl aqueous solution. The insoluble white solid was
4
Centre of the Changchun Institute of Applied Chemistry
CIAC).The MALDI-TOF mass spectrum was obtained with a CH Cl twice, collected the organic phase, and dried with
removed by filtration, and the filtrate was extracted with
(
2
2
Bruker Daltonic MicroFlex LT at the National Analytic Research anhydrous MgSO4 overnight. Removing the solvent under
Center of the Changchun Institute of Applied Chemistry (CIAC). reduced pressure and recrystallization with hexane gave a
1
o
The number-molecular weight (M
n
3
) and weight distribution white powder (yield: 43 %). H NMR (CDCl , 400 MHz, 25 C): δ
(
PDI) of the polymers were measured using a TOSOH HLC 8220 8.57 (d, 1H, C H N), 7.68 (brs, 1H, NH), 7.63 (t, 1H, Ar-H), 7.22
5
4
o
GPC instrument at 40 C with THF as eluent with a flow rate of (m, 2H, Ar-H), 7.16 (m, 2H, Ar-H), 7.05 (d, 2H, Ar-H), 6.99 (m,
0
-1
.35 mL min . Note that the values were relative to 2H, Ar-H), 3.63 (d, 2H, CH ), 3.14 (sept, 2H, CH(CH ) ), 2.97
2
3
2
polystyrene standards.
3 2 3 2
(sept, 2H, CH(CH ) ), 1.30-1.21 (m, 5H, CH(CH ) ), 1.18 (d, 7H,
1
3
CH(CH
3
)
2
), 1.09 (d, 5H, CH(CH
3
)
2
), 1.01 (d, 7H, CH(CH
3
)
2
).
C
X-ray Crystallographic Study
o
, 100 MHz, 25 C): δ 156.61 (s, NCN), 152.68 (s,
NMR (CDCl
3
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 11
Plea sDe a dl t oo nn oT tr aa nd sj ua cs tt i om nasrgins
View Article Online
DOI: 10.1039/C8DT01434E
Journal Name
ARTICLE
C H N), 149.31 (s, Ar), 146.92 (s, Ar), 145.93 (s, Ar), 139.30 (s, layer was separated and dried over MgSO . After filtration, the
5
4
4
o
Ar), 137.12 (s, Ar), 134.55 (s, Ar), 127.44 (s, Ar), 124.18 (s, Ar), filtrate was concentrated and stored at -30 C to afford a white
1
1
3
23.05 (s, Ar), 122.72 (s, Ar), 122.32 (s, Ar), 122.04 (s, Ar), powder under reduced pressure (yield: 81%). H NMR (CDCl ,
o
8.33 (s, CH ), 28.77 (s, CH(CH ) ), 27.93 (s, CH(CH ) ), 23.89 (s, 400 MHz, 25 C): δ 8.05 (brs, 2H, NH), 7.98-7.96 (d, 2H, Ar-H),
3
2
3 2
3 2
CH(CH
3
)
2
), 23.47 (s, CH(CH
3
)
2
). Anal. Calcd for C31
H
41
N
3
: C, 7.14 (t, 2H, Ar-H), 6.95 (t, 2H, Ar-H), 6.89-6.87 (d, 2H, Ar-H),
3.77 (s, 6H, OCH ), 3.05 (q, 6H, CH CH ), 1.35 ppm (t, 9H,
CH CH ). C NMR (CDCl , 100 MHz, 25 C): δ 178.13 (C=S),
81.71; H, 9.07; N, 9.22. Found: C, 81.63; H, 8.99; N, 9.24.
3
2
3
o
1
3
2
3
3
2
Synthesis of HL
1
4
C
51.11 (Ar), 126.30 (Ar), 123.84 (Ar), 120.21 (Ar), 55.52 (OCH
5.68 (N(CH CH ), 8.42 (N(CH CH ). Anal. Calcd for
S: C, 64.75; H, 8.02; N, 10.79. Found: C, 64.83; H,
.17; N, 10.64.
3
),
2
3
)
3
2
3 3
)
2
HL was synthesized using the similar method as HL (yield: 87
1
21 31 3 2
H N O
1
o
%
). H NMR (CDCl
d, 1H, Ar-H), 7.17 (d, 1H, Ar-H), 7.13 (d, 2H, Ar-H), 7.08 (d, 2H,
Ar-H), 7.05-7.04 (m, 2H, Ar-H), 6.98 (m, 1H, Ar-H), 6.84 (brs,
3
, 400 MHz, 25 C): δ 7.23 (m, 1H, Ar-H), 7.19
8
(
4
Synthesis of HL . A mixture of the above thiourea compound (I)
1
2
1
H, NH), 3.60 (d, 2H, CH
H, CH(CH ), 2.73 (s, 6H, N(CH
.19 (d, 3H, CH(CH ), 1.15 (d, 3H, CH(CH
), 1.00 (d, 3H, CH(CH
2
), 3.21 (sept, 2H, CH(CH
), 1.33 (d, 8H, CH(CH
), 1.04 (d, 7H,
). C NMR (CDCl , 100 MHz, 25
3
)
2
), 3.10 (sept,
(
10 mmol), HgO (20 mmol) and anhydrous MgSO
4
(24 mmol) in
3
)
2
3
)
2
3 2
)
),
2
50 mL of toluene was refluxed overnight. After cooling to
3
)
2
3 2
)
room temperature, the reaction mixture was filtered and the
filtrate concentrated to dryness to afford a pale yellow solid
1
3
CH(CH
3
)
2
3
)
2
3
o
1
yield: 51%). H NMR (CDCl , 400 MHz, 25 C): δ 7.19 (d, 1H, Ar-
o
C): δ 155.01 (s, NCN), 152.83 (s, Ar), 146.95 (s, Ar), 146.38 (s,
(
3
Ar), 139.17 (s, Ar), 134.51 (s, Ar), 131.80 (s, Ar), 128.40 (s, Ar),
H), 7.04-6.96 (m, 1H, Ar-H), 6.82-6.79 (m, 4H, Ar-H), 6.77-6.75
m, 2H, Ar-H), 6.01 (s, 1H, NH), 3.66 (s, 6H, OCH ), 3.39 (q, 4H,
CH CH ), 1.21 (t, 6H, CH CH ). C NMR (CDCl , 100 MHz, 25
127.47 (s, Ar), 124.38 (s, Ar), 123.50 (s, Ar), 123.18 – 122.91
(
3
1
3
(m, Ar), 122.74 (s, Ar), 122.22 (s, Ar), 120.26 (s, Ar), 45.30 (s,
2
3
2
3
3
o
N(CH ) ), 33.70 (s, CH ), 28.68 (s, CH(CH ) ), 28.02 (s, CH(CH ) ),
3
2
2
3 2
3 2
C): δ 150.46 (S, NC(N)N), 120.93 (brs, Ar), 110.82 (s, Ar), 55.49
s, OCH ), 42.36 (s, N(CH CH ), 13.13 (s, N(CH CH ). Anal.
Calcd for C H N O : C, 69.70; H, 7.70; N, 12.83. Found: C,
2
4.26 (s, CH(CH
3
)
2
), 23.36 (s, CH(CH
3
)
2
). Anal. Calcd for
(
3
2
3
)
2
2
3 2
)
C
34
H
47
N
3
: C, 82.04; H, 9.52; N, 8.44. Found: C, 82.12; H, 9.41; N,
1
9
25
3
2
8.53.
6
9.89; H, 7.64; N, 12.71.
3
Synthesis of HL
Synthesis of complex 1
(
3,3-(dimethylamino)propyl)magnesium
chloride
was
Under a nitrogen atmosphere, to a stirred solution of
3
1
synthesized according to a previous report. The same
2 2 2
Ca[N(SiHMe ) ] (THF) (1.05 mmol) in 8 mL toluene was added
3
method to obtain the light yellow powder of HL (yield: 67 %).
1
1
the solution of ligand HL (1 mmol) in 10 mL of toluene at
room temperature. The reaction mixture was stirred at room
temperature for overnight and was concentrated to
approximately 1 mL, and was added several drops of hexane.
o
H NMR (CDCl
H, Ar-H), 7.03 (d, 1H, Ar-H), 5.29 (brs, 1H, NH), 3.15 (m, 4H,
CH(CH ), 2.28 (t, 2H, CH ), 2.16 (s, 6H, N(CH ), 2.05 (t, 2H,
CH ), 1.90 (quint, 2H, CH ), 1.31 (d, 6H, CH(CH ), 1.27 (d, 6H,
CH(CH ), 1.20 (d, 6H, CH(CH ), 1.01 (d, 6H, CH(CH
NMR (CDCl , 100 MHz, 25 C): δ 154.91 (s, NCN), 147.19 (s, Ar),
44.22 (s, Ar), 139.14 (s, Ar), 133.38 (s, Ar), 128.27 (s, Ar),
23.66 (s, Ar), 123.10 (s, Ar), 122.94 (s, Ar), 59.54 (s, CH ),
), 28.25 (s,
), 24.13 (s,
3
, 400 MHz, 25 C): δ 7.24 (d, 1H, Ar-H), 7.14 (q,
4
3
)
2
2
3 2
)
o
2
2
3 2
)
The residue was cooled to -30 C over several days to afford
1
3
1
o
3
)
2
3
)
2
3
)
2
).
C
yellow powders (yield: 53%). H NMR (C
.20 (d, 1H, C N), 7.12 (m, 5H, Ar-H), 7.13 (d, 1H, Ar-H), 6.65
t, 1H, C N), 6.37 (t, 1H, C N), 5.66 (d, 1H, C N), 5.17 (s,
JSi,H =147.5 Hz, 2H, SiH), 3.65 (s, 2H, CH ), 3.57 (sept, 4H,
6 6
D , 400 MHz, 25 C): δ
o
3
8
5 4
H
1
1
4
(
1
5
H
4
5
H
4
5 4
H
2
2
5.49 (s, N(CH
), 25.09 (s, CH(CH
), 23.09 (s, CH(CH ), 22.43 (s, CH(CH
C H N : C, 80.12; H, 10.53; N, 9.34. Found: C, 80.21; H, 10.39;
3
)
2
), 29.24 (s, CH
2
), 28.48 (s, CH(CH
3
)
2
CH(CH
3
)
2
), 1.28 (d, 12H, CH(CH
3
)
2
), 1.23 (d, 12H, CH(CH
3
)
2
),
). C NMR
C D , 100 MHz, 25 C): δ 171.18 (s, NCN), 148.46 (s, Ar),
1
3
CH(CH
CH
3
)
2
3
)
2
), 24.43 (s, CH(CH
3
)
2
0.17 (s, 10H, SiHMe
2
), 0.11 ppm (s, 2H, SiHMe
2
o
2
3
)
2
3 2
) ). Anal. Calcd for
(
6
6
3
0
47
3
146.05 (s, Ar), 143.09.60 (s, Ar), 134.80 (s, Ar), 124.85 (s, Ar),
N, 9.27.
1
2
3
8
2
23.83 (s, Ar), 123.49 (s, Ar), 120.30 (s, Ar), 40.03 (s, NCN-CH ),
8.49 (s, CH(CH ) ), 26.27 (s, CH(CH ) ), 23.52 (s, CH(CH ) ),
3
2
3 2
3 2
Synthesis of (Z)-1,1-diethyl-2,3-bis(2-
4
methoxyphenyl)guanidine (HL ).
.04 (s, SiHMe
2
). Anal. Calcd for C70
H108Ca
2
N
8
Si
4
: C, 67.04; H,
.68; N, 8.93. Found: C, 67.16; H, 8.73; N, 8.84.
Synthesis of thiourea compound (
I
). Carbon disulphide (50
Synthesis of complexes 2-7
mmol) was added dropwise to
a
stirred mixture of
1
DIPP
o-Anisidine (100 mmol) and NEt (100 mmol) in 100 mL of
3
3 2 2
Synthesis of L Am CaN(SiMe ) ∙(THF) (2). Following a similar
water at room temperature. The reaction mixture was stirred
o
for 1 h at room temperature and then heated to 90 C for 14 h.
procedure to that described for the preparation of complex
complex was isolated from the amine elimination reaction of
Ca[N(SiMe
yellow powder (yield: 47%). H NMR (C
1,
2
1
After re-cooling the reaction mixture to room temperature, it
3 2 2 2
) ] (THF) (1.05 mmol) and HL (1 mmol) as a pale
1
o
2 2
was poured into 100 mL of CH Cl , following which the organic
6 6
D , 500 MHz, 25 C): δ
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 8 of 11
View Article Online
DOI: 10.1039/C8DT01434E
ARTICLE
Journal Name
7
.23 (t, 2H, Ar-H), 7.17 (m, 3H, Ar-H), 7.14 (t, 2H, Ar-H), 6.62 analysis were obtained from a toluene/hexane solution at -30
1
N), 5.83 (t, 1H, Ar-H), 4.90 (s, ◦C after several days. H NMR (C
o
, 400 MHz, 25 C): δ 7.14
(m, 1H, Ar-H), 6.09 (d, 1H, C
5
H
4
6
D
6
1
), 3.61 (m, 8H, THF), 3.52-3.35 (m, 4H, (brs, 4H, Ar-H), 7.11-7.08 (m, 2H, Ar-H), 5.12 (brs, JSi,H =147.4
1H, CH
2
), 4.12 (s, 1H, CH
2
CH(CH ) ), 1.33 (m, 8H, THF), 1.28 (d, 11H, CH(CH ) ), 1.05 (m, Hz, 2H, SiH), 3.52 (sept, 4H, CH(CH ) ), 2.23 (m, 2H, CH ), 1.64
3
2
3 2
3 2
2
1
). C NMR (C
3
1
3H, CH(CH
3
)
2
), 0.38 (d, 18H, SiMe
3
6
D
6
, 126 MHz, (s, 6H, N(CH
3
)
2
), 1.58 (t, 2H, CH
2
3 2
), 1.38 (d, 12H, CH(CH ) ), 1.27
13
o
5 C): δ 160.37 (s, NCN), 157.93 (s, Ar), 147.65 (s, Ar), 145.43 (d, 14H, CH(CH
2
3
)
2
+ CH ), 0.15 ppm (s, 12H, SiHMe ). C NMR
2 2
o
s, Ar), 142.22 (s, Ar), 144.44 (s, Ar), 135.67 (s, Ar), 134.11 (s, (C D , 100 MHz, 25 C): δ 177.26 (s, NCN), 175.80 (s, Ar),
(
6 6
Ar), 124.63 (s, Ar), 124.39 (s, Ar), 123.96 (s, Ar), 121.61 (s, Ar), 147.05 (s, Ar), 146.58 (s, Ar), 142.39 (s, Ar), 123.67 (s, Ar),
09.06 (s, Ar), 75.94 (s, NCN-CH ), 69.30 (s, THF), 28.47 (s, 123.53 (s, Ar), 123.31 (s, Ar), 123.04 (s, Ar), 68.84 (s, THF),
), 25.32 (s, THF), 24.50 (s, 45.06 (s, NCN-CH CH CH N(CH ), 28.31 (s, NCN-
). Anal. Calcd for C45 Si : C, CH CH CH N(CH ), 25.83 (s, NCN-CH CH CH N(CH ), 25.51
(s, Ar-CH(CH ), 25.38 (s, THF), 24.16 (s, Ar-CH(CH ), 24.11 (s,
NCN-CH CH CH N(CH ), 5.03 ppm (s, SiHMe ). Anal. Calcd for
Si
1
2
CH(CH
CH(CH
3
)
)
2
2
), 28.24 (s, CH(CH
), 5.64 (s, SiMe
3
)
2
2
2
2
3 2
)
3
3
H74CaN
4
O
2
2
2
2
2
3
)
2
2
2
2
3 2
)
67.62; H, 9.33; N, 7.01. Found: C, 67.53; H, 9.47; N, 7.13.
3
)
2
3 2
)
2
2
2
3
)
2
2
2
DIPP
Synthesis of L Am CaN(SiHMe
similar procedure to that described for the preparation of
complex , complex was isolated from the amine elimination
reaction of Ca[N(SiHMe
2 2 2
) ∙(THF) (3). Following a
C
1
42
H
76CaN
4
O
2
2
: C, 65.83; H, 10.13; N, 7.31. Found: C, 65.91; H,
0.08; N, 7.26.
1
3
2
3
DIPP
2
)
2
]
2
(THF) (1.05 mmol) and HL (1 Synthesis of L Am CaN(SiMe
3
)
2
∙(THF)
2
(
6
). Following a similar
mmol) as a white solid (yield: 76%). Single crystals suitable for procedure to that described for the preparation of complex
1,
X-ray analysis were obtained from a toluene/hexane solution complex 6 was isolated from the amine elimination reaction of
1
at -30 ◦C after several days. H NMR (C
o
3
6 6 3 2 2 2
D , 400 MHz, 25 C): δ Ca[N(SiMe ) ] (THF) (1.05 mmol) and HL (1 mmol) as a pale
7
.68 (d, 1H, Ar-H), 7.11 (m, 4H, Ar -H), 7.03 (dd, 3H, Ar-H), 6.95 white solid (yield: 71%). Single crystals suitable for X-ray
1
t, 1H, Ar-H), 6.66 (d, 1H, Ar-H), 5.11 (m, JSi,H =163.3 Hz, 2H, analysis were obtained from a toluene/hexane solution at -30
(
1
o
2 3 2 6 6
SiH), 3.82 (s, 2H, CH ), 3.72 (sept, 4H, CH(CH ) ), 3.61 (m, 8H, ◦C after several days. H NMR (C D , 400 MHz, 25 C): δ 7.18 (d,
THF), 1.93 (s, 6H, N(CH ) ), 1.33 (m, 32H, CH(CH ) + THF), 0.42 1H, Ar-H), 7.16-7.15 (m, 3H, Ar-H), 7.11-7.07 (m, 2H, Ar-H),
3
2
3 2
(
d, 10H, SiHMe
2
), 0.20 (d, 1H, SiHMe
2
), 0.11 (d, 1H, SiHMe
2
). 3.66 (m, 6H, THF), 3.49 (sept, 4H, CH(CH
), 1.66 (s, 6H, N(CH ), 1.61 (m, 2H, CH
Ar), 146.47 (s, Ar), 142.55 (s, Ar), 133.34 (s, Ar), 126.58 (s, Ar), CH(CH ) ), 1.30 (m, 20H, CH(CH ) + CH + THF), 0.19 ppm (s,
3
)
2
), 2.22-2.18 (m, 2H,
1
3
o
C NMR (C
6
D
6
, 100 MHz, 25 C): δ 175.04 (s, NCN), 152.08 (s, CH
2
3
)
2
2
), 1.39 (d, 12H,
3
2
3 2
2
1
3
o
1
25.70 (s, Ar), 123.95 (s, Ar), 123.55 (s, Ar), 122.90 (s, Ar), 18H, SiMe
3 6 6
). C NMR (C D , 100 MHz, 25 C): δ 176.86 (s,
119.99 (s, Ar), 69.01 (s, THF), 44.43 (s, NMe ), 28.41 (s, NCN- NCN), 145.87 (s, Ar), 142.39 (s, Ar), 142.22 (s, Ar), 123.66 (s,
2
CH ), 26.44 (s, CH(CH ) ), 25.28 (d, THF), 23.92 (s, CH(CH ) ), Ar), 123.58 (s, Ar), 123.51 (s, Ar), 69.40 (s, THF), 59.98 (s, NCN-
2
3 2
3 2
5
.29 (s, SiHMe
2
). Anal. Calcd for C46
H
68CaN
4
O
2
Si
2
: C, 68.61; H, CH
2
CH
2
CH
2
N(CH
CH
), 23.63 (s, NCN-CH
Anal. Calcd for C44
3
)
2
), 45.11 (s, NCN-CH
2 2 2 3 2
CH CH N(CH ) ), 28.55
8.51; N, 6.96. Found: C, 68.53; H, 8.57; N, 7.04.
(s, NCN-CH
CH(CH
2
2
CH N(CH ), 25.80 (s, THF), 25.18 (s, Ar-
2
3 2
)
3
)
2
2
2
CH
2 2 3 2 3
CH N(CH ) ), 5.47 (s, SiMe ).
2
DIPP
Synthesis of L Am CaN(SiMe
procedure to that described for the preparation of complex
complex was isolated from the amine elimination reaction of
Ca[N(SiMe
3 2
) ∙(THF) (4). Following a similar
H80CaN
4
O
Si : C, 66.61; H, 10.16; N, 7.06.
2
1,
Found: C, 66.67; H, 10.11; N, 7.13.
4
2
4
3
)
2
]
2
(THF)
2
(1.05 mmol) and HL (1 mmol) as a pale Synthesis of [L CaN(SiHMe
2
)
2
]
2
(7). Following a similar
1
o
white solid (yield: 68%). H NMR (C D , 400 MHz, 25 C): δ procedure to that described for the preparation of complex 1,
6
6
7
7
2
2
.51-7.48 (m, 1H, Ar-H), 7.24 (brs, 1H, Ar-H), 7.18 (d, 1H, Ar-H), complex
.13 (s, 1H, Ar-H), 7.16 (s, 2H, Ar-H) 7.11(d, 1H, Ar-H), 7.03 (m, Ca[N(SiHMe
H, Ar-H), 6.73-6.70 (m, 1H, Ar-H), 3.89 (m, 4H, THF), 3.85 (s, solid (yield: 48%). Single crystals suitable for X-ray analysis
H, CH ), 3.63 (sept, 4H, CH(CH ), 1.97 (s, 6H, N(CH ), 1.38 were obtained from a toluene/hexane solution at -30 ◦C after
+ THF), 0.28 (s, 18H, SiMe
00 MHz, 25 C): δ 175.05 (s, NCN), 146.64 (s, Ar), 142.43 (s, H), 6.82 (t, 2H, Ar-H), 6.62 (t, 2H, Ar-H), 6.19 (d, 2H, Ar-H), 5.18
7 was isolated from the amine elimination reaction of
4
2 2 2
) ] (THF) (1.05 mmol) and HL (1 mmol) as a white
2
3
)
2
3 2
)
1
3
1
o
(q, 28H, CH(CH
3
)
2
3
). C NMR (C
6
D
6
,
6 6
several days. H NMR (C D , 400 MHz, 25 C): δ 7.04 (d, 2H, Ar-
o
1
1
Ar), 133.33 (s, Ar), 126.58 (s, Ar), 123.54 (s, Ar), 122.91 (s, Ar), (m, JSi,H =167.9 Hz, 1H, SiH), 5.06 (m, 1H, SiH), 3.48 (s, 2H,
19.99 (s, Ar), 68.99 (s, THF), 44.35 (s, NMe ), 28.41 (s, NCN- OCH ), 3.34 (s, 4H, OCH ), 3.11 (sept, 1H, CH CH ), 2.85 (sept,
1
2
3
3
2
3
CH ), 26.43 (s, CH(CH ) ), 25.26 (d, THF), 23.89 (s, CH(CH ) ), 3H, CH CH ), 0.89 (t, 4H, CH CH ), 0.65 (t, 2H, CH CH ), 0.46 (d,
3
2
3 2
3 2
2
3
2
3
2
5
.27 ppm (s, SiMe
3
4 2 2 2
). Anal. Calcd for C44H72CaN OSi : C, 68.69; H, 4H, SiHMe ), 0.40 (brs, 4H, SiHMe ), 0.20 ppm (brs, 4H,
1
3
o
9.43; N, 7.28. Found: C, 68.73; H, 9.37; N, 7.23.
SiHMe
59.01 (s, Ar), 151.60 (s, Ar), 151.31 (s, Ar), 139.68 (s, Ar),
139.57 (s, Ar), 138.21 (s, Ar), 122.94 (s, Ar), 121.85 (s, Ar),
2 6 6
). C NMR (C D , 100 MHz, 25 C): δ 164.54 (s, NCN),
1
3
DIPP
Synthesis of L Am CaN(SiHMe ) ∙(THF) (
5
). Following a
similar procedure to that described for the preparation of
complex , complex was isolated from the amine elimination
reaction of Ca[N(SiHMe
as a white solid (yield: 74%). Single crystals suitable for X-ray
2
2
2
1
1
4
21.32 (s, Ar), 121.02 (s, Ar), 120.21 (s, Ar), 118.63 (s, Ar),
1
5
10.44 (s, Ar), 110.08 (s, Ar), 56.15 (s, OCH
3
), 55.74 (s, OCH
3
),
),
3
2 2 2
) ] (THF) (1.05 mmol) and HL (1 mmol)
2
3
3
2
3 3
2.11 (s, N(CH CH ) ), 12.11 (s, N(CH CH ) ), 4.93 (s, SiHMe
2
8
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 11
Plea sDe a dl t oo nn oT tr aa nd sj ua cs tt i om nasrgins
View Article Online
DOI: 10.1039/C8DT01434E
Journal Name
ARTICLE
4
.66 (s, SiHMe2), 4.22 (s, SiHMe2). Anal. Calcd for
8
(a) M. R. Crimmin, I. J. Casely and M. S. Hill, J. Am. Chem.
Soc., 2005, 127, 2042; (b) A. G. Avent, M. R. Crimmin, M. S.
Hill and P. B. Hitchcock, Dalton Trans., 2005, 278; (c) S.
Harder and J. Brettar, Angew. Chem. Int. Ed., 2006, 45, 3474;
C
46
H76Ca
2
N
8
O
4
Si : C, 55.38; H, 7.68; N, 11.23. Found: C, 55.41;
4
H, 7.64; N, 11.13.
(
Harder and H. W. Roesky, J. Am. Chem. Soc., 2006, 128,
15000; (e) S. P. Sarish, H. W. Roesky, M. John, A. Ringe and J.
Magull, Chem. Commun., 2009, 2390.
(a) M. Arrowsmith, M. S. Hill and G. Kociok-Kohn,
Organometallics, 2009, 28, 1730; (b) M. Arrowsmith, A.
Heath, M. S. Hill, P. B. Hitchcock and G. Kociok-Köhn,
Organometallics, 2009, 28, 4550.
d) C. Ruspic, S. Nembenna, A. Hofmeister, J. Magull, S.
4
Synthesis of [L CaN(SiMe ) ] (
8
). Following a similar procedure
, complex
was isolated from the amine elimination reaction of
3
2 2
to that described for the preparation of complex
1
8
9
4
3 2 2
Ca[N(SiMe ) ] (THF) (1.05 mmol) and HL (1 mmol) as a white
solid (yield: 53%). Single crystals suitable for X-ray analysis
were obtained from a toluene/hexane solution at -30 ◦C after
1
o
10 (a) D. J. Darensbourg, W. Choi, P. Ganguly and C. P. Richers,
Macromolecules, 2006, 39, 4374; (b) D. J. Darensbourg, W.
Choi and C. P. Richers, Macromolecules, 2007, 40, 3521; (c)
D. J. Darensbourg, W. Choi, O. Karroonnirun and N.
Bhuvanesh, Macromolecules, 2008, 41, 3493; (d) V. Poirier,
T. Roisnel, J. F. Carpentier and Y. Sarazin, Dalton Trans, 2009,
several days. H NMR (C
H), 6.83 (t, 2H, Ar-H), 6.60 (t, 2H, Ar-H), 6.17 (d, 2H, Ar-H), 3.37
s, 6H, OCH ), 3.09 (m, 2H, N(CH CH ), 2.97 (m, 2H,
N(CH CH ), 0.84 (t, 6H, N(CH CH ), 0.26 ppm (s, 18H,
SiMe3). C NMR (C D , 100 MHz, 25 C): δ 176.04 (s, NCN),
6 6
D , 400 MHz, 25 C): δ 6.95 (d, 2H, Ar-
(
3
2
3 3
)
2
3
)
3
2
3 3
)
o
1
3
6
6
9
820; (e) Y. Sarazin, D. Rosca, V. Poirier, T. Roisnel, A.
1
53.01 (s, Ar), 144.92 (s, Ar), 142.29 (s, Ar), 123.58 (s, Ar),
23.43 (s, Ar), 120.17 (s, Ar), 69.66 (s, -OCH ), 44.28 (s,
Silvestru, L. Maron and J. F. Carpentier, Organometallics,
2010, 29, 6569.
1 (a) J. Barker and M. Kilner, Coord. Chem. Rev., 1994, 133, 219;
1
3
1
1
N(CH CH ) ), 23.25 (s, N(CH CH ) ), 5.42 ppm (s, SiMe ). Anal.
3
2
3 3
2
3 3
(
b) F. T. Edelmann, Coord. Chem. Rev., 1994, 137, 403; (c) S.
Calcd for C64
2 8 4 4
H84Ca N O Si : C, 62.91; H, 6.93; N, 9.17. Found: C,
P. Green, C. Jones and A. Stasch, Science, 2007, 318, 1754.
2 (a) S. P. Sarish, S. Nembenna, S. Nagendran and H. W.
Roesky, Acc. Chem. Res., 2011, 44, 157; (b) F. T. Edelmann,
Adv. Organomet. Chem., 2008, 57, 183 (c) C. Glock, C. Loh, H.
Gorls, S. Krieck and M. Westerhausen, Eur. J. Inorg. Chem.,
6
2.87; H, 6.84; N, 9.23.
2
013, 3261.
1
1
1
3 H. F. Hu, C. M. Cui, Organometallics 2012, 31, 1208.
4 F. Qian, K. Y. Liu, H. Y. Ma, Dalton Trans., 2010, 39, 8071.
5 M. Findlater, N. J. Hill and A. H. Cowley, Dalton Transactions,
Acknowledgements
We are thankful for the financial support from the National
Natural Science Foundation of China for the project Nos.
2
008, 4419.
6 M. P. Coles, Dalton Trans. 2006, 985.
1
1
21274143, 21774119 and 21574125.
7 (a) W. A. Herrmann, J. Eppinger, M. Spiegler, O. Runte and R.
1813; (b) R.
Anwander, Organometallics, 1997, 16,
Anwander, O. Runte, J. Eppinger, G. Gerstberger, E.
Notes and references
Herdtweck and M. Spiegler, J. Chem. Soc., Dalton Trans.,
1
Anwander, Eur. J. Inorg. Chem., 1999, 1405; (d) C.
Meermann, G. Gerstberger, M. Spiegler, K. W. Tornoos and
R. Anwander, Eur. J. Inorg. Chem., 2008, 2014.
8 (a) A. R. Faro, P. Carpentier, G. Jonasson, G. Pompidor, D.
Arcizett, I. Demachy and D. Bourgeois, J. Am. Chem. Soc.,
2011, 133, 16362; (b) B. Liu, T. Roisnel and Y. Sarazin, Inorg.
Chim. Acta, 2012, 380, 2.
998, 847; (c) I. Nagl, W. Scherer, M. Tafipolsky and R.
1
(a) B. Liu, T. Roisnel, J. P. Guegan, J. F. Carpentier and Y.
Sarazin, Chemistry-a European Journal, 2012, 18, 6289; (b) Y.
Sarazin, B. Liu, T. Roisnel, L. Maron and J. F. Carpentier, J.
Am. Chem. Soc., 2011, 133, 9069.
1
2
(a) J. Jenter, R. Koppe and P. W. Roesky, Organometallics,
2
011, 30, 1404; (b) A. G. M. Barrett, I. J. Casely, M. R.
Crimmin, M. S. Hill, J. R. Lachs, M. F. Mahon and P. A.
Procopiou, Inorg. Chem., 2009, 48, 4445; (c) B. Liu, T. Roisnel,
J. F. Carpentier and Y. Sarazin, Angew. Chem. Int. Ed., 2012,
1
2
9 Y. P. Pan, W. F. Rong, Z. B. Jian and D. M. Cui,
Macromolecules, 2012, 45, 1248.
51, 4943; (d) B. Liu, T. Roisnel, J. F. Carpentier and Y. Sarazin,
0 (a) W. A. Herrmann, J. Eppinger, M. Spiegler, O. Runte, and R.
Anwander, Organometallics, 1997, 16, 1813; (b) R. Anwander,
O. Runte, J. Eppinger, G. Gerstberger, E. Herdtweck and M.
Spiegler, J. Chem. Soc., Dalton Trans., 1998, 847; (c) W.
Hieringer, J. Eppinger, R. Anwander and W. A. Herrmann, J.
Am. Chem. Soc, 2000, 122, 11983; (d) C. Meermann, G.
Gerstberger, M. Spiegler, K. W. Tӧrnroos and R. Anwander,
Eur. J. Inorg.Chem., 2008, 2014; (e) H. J. Yuen and T. J. Marks,
Organometallics, 2008, 27, 155.
Chemistry-a European Journal, 2013, 19, 13445.
A. G. Barrett, M. R. Crimmin, M. S. Hill, P. B. Hitchcock, S. L.
Lomas, P. A. Procopiou and K. Suntharalingam, Chem.
Commun. (Camb), 2009, 2299.
3
4
5
J. Spielmann, F. Buch and S. Harder, Angew. Chem. Int. Ed.,
2
008, 47, 9434.
(a) S. Range, D. F. J. Piesik and S. Harder, Eur. J. Inorg. Chem.,
2
008, 3442; (b) D. L. Liu, Y. L. Wang, C. F. Pi and W. J. Zheng,
Organometallics, 2014, 33, 6013; (c) G. J. Moxey, A. J. Blake,
W. Lewis and D. L. Kays, Eur. J. Inorg. Chem., 2015, 5892.
(a) M. H. Chisholm, J. Gallucci and K. Phomphrai, Chem.
Commun., 2003, 48; (b) Chisholm, M. H.; Gallucci, J. C.;
Phomphrai, K. Inorg. Chem. 2004, 43, 6717 M. H. Chisholm, J.
C. Gallucci and K. Phomphrai, Inorg. Chem., 2004, 43, 6717;
2
2
1 S. Asano, T. Aida and S. Inoue, J Chem Soc Chem Comm,
1
985, 1148.
2 (a) T. Aida and S. Inoue, Accounts of Chemical Research,
996, 29, 39; (b) M. Akatsuka, T. Aida and S. Inoue,
6
7
1
Macromolecules, 1994, 27, 2820; (c) S. Inoue, J Polym Sci Pol
Chem, 2000, 38, 2861.
(
c) M. H. Chisholm, J. C. Gallucci, G. Yaman and T. Young,
2
3 (a) A. Amgoune, C. M. Thomas and J. F. Carpentier,
Macromol. Rapid Commun., 2007, 28, 693; (b) L. Clark, M. G.
Cushion, H. E. Dyer, A. D. Schwarz, R. Duchateau and P.
Mountford, Chem. Commun., 2010, 46, 273; (c) Y. Wang, W.
Zhao, X. Liu, D. Cui and E. Y. X. Chen, Macromolecules, 2012,
Chem. Commun., 2009, 1828; (d) M. H. Chisholm, J. C.
Gallucci and G. Yaman, Dalton Transactions, 2009, 368..
(a) S. Datta, P. W. Roesky and S. Blechert, Organometallics,
2007, 26, 4392; (b) S. Datta, M. T. Gamer and P. W. Roesky,
Organometallics, 2008, 27, 1207.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Please do not adjust margins
Dalton Transactions
Page 10 of 11
View Article Online
DOI: 10.1039/C8DT01434E
ARTICLE
, 6957; (d) W. Zhao, Y. Wang, X. Liu and D. Cui, Chem
Journal Name
45
Commun (Camb), 2012, 48, 4588; (e) W. Zhao, Y. Wang, X.
Liu, X. Chen, D. Cui and E. Y. Chen, Chem Commun (Camb),
2
Cui, Chem Asian J, 2012, , 2403; (g) W. Zhao, D. M. Cui, X. L.
012, 48, 6375; (f) W. Zhao, Y. Wang, X. Liu, X. Chen and D.
7
Liu and X. S. Chen, Macromolecules, 2010, 43, 6678; (h) W.
Zhao, C. Y. Li, B. Liu, X. Wang, P. Li, Y. Wang, C. J. Wu, C. G.
Yao, T. Tang, X. L. Liu and D. M. Cui, Macromolecules, 2014,
47, 5586; (i) N. Liu, B. Liu, C. Yao and D. Cui, Polymers, 2016,
8, 17.
2
4 (a) M. J. Matos, S. Vazquez-Rodriguez, L. Santana, E. Uriarte,
C. Fuentes-Edfuf, Y. Santos and A. Munoz-Crego, Medicinal
Chemistry, 2012, 8, 1140; (b) F. Chimenti, B. Bizzarri, A.
Bolasco, D. Secci, P. Chimenti, A. Granese, S. Carradori, D.
Rivanera, A. Zicari, M. M. Scaltrito and F. Sisto, Bioorg. Med.
Chem. Lett., 2010, 20, 4922.
2
2
5 I. Kostova, S. Bhatia, P. Grigorov, S. Balkansky, V. S. Parmar,
A. K. Prasad and L. Saso, Current Medicinal Chemistry, 2011,
18
, 3929.
6 (a) F. Belluti, G. Fontana, L. Dal Bo, N. Carenini, C.
Giommarelli and F. Zunino, Biorg. Med. Chem., 2010, 18
543; (b) S. Serra, A. Chicca, G. Delogu, S. Vazquez-
,
3
Rodriguez, L. Santana, E. Uriarte, L. Casu and J. Gertsch,
Bioorg. Med. Chem. Lett., 2012, 22, 5791.
2
2
7 B. Sandhya, D. Giles, V. Mathew, G. Basavarajaswamy and R.
Abraham, European Journal of Medicinal Chemistry, 2011,
46, 4696.
8 J. D. William Kemnitzer, Songchun Jiang, Hong Zhang,
Candace Crogan-Grundy, Denis Labreque, Monica Bubenick,
Giorgio Attardo, Real Denis, Serge Lamothe, Henriette
Gourdeau, Ben Tseng, Shailaja Kasibhatla, and Sui Xiong Cai,
J. Med. Chem., 2008, 51, 417.
2
9 J. M. Timonen, R. M. Nieminen, O. Sareila, A. Goulas, L. J.
Moilanen, M. Haukka, P. Vainiotalo, E. Moilanen and P. H.
Aulaskari, European Journal of Medicinal Chemistry, 2011,
46
, 3845.
0 M. Westerhausen, Inorg. Chem., 1991, 30, 96.
Helvetica Chimica Acta 1969, 52(25), 262.
3
3
1
,
1
0 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 11 of 11
Dalton Transactions
View Article Online
DOI: 10.1039/C8DT01434E
Alkaline earth metal complexes stabilized by amidine and guanidine ligands:
synthesis, structure and their catalytic activity towards polymerization of
rac-lactide
a,b
*a
*a
Na Liu, Bo Liu, and Dongmei Cui
a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Science, 5625 Renmin street, Changchun 130022, China. Fax:
+
86 431 85262774; Tel: +86 431 85262773; E-mail: dmcui@ciac.ac.cn; liubo@ciac.ac.cn.
b
University of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China.
Amidine ligands with coordination backbone substitutes were synthesized, for the
first time, and useful to stabilize heteroleptic calcium complexes. The rigidity of the
substitutes has an influence on the solvation and aggregation of the corresponding
calcium complexes. Delightedly, in the presence of PhOH as chain transfer agent, the
afforded complexes could unprecedentedly catalyze “immortal” ring-opening
polymerization of lactide.