Dinuclear zinc-N-heterocyclic carbene complexes for either the controlled ring-opening polymerization of lactide or the controlled degradation of polylactide under mild conditions

Article abstract of DOI:10.1002/cctc.201301015

We describe the synthesis of the new Zn-N-heterocyclic carbene (NHC) alkoxide complexes [(S,C_NHC)ZnCl(OBn)]₂ (5) and [(O,C _NHC)ZnCl(OBn)]₂ (6) for use as ring-opening polymerization (ROP) initiators for lactide polymerization. Complexes 5 and 6 are readily available through an alcoholysis reaction between BnOH and the corresponding Zn-NHC ethyl species [(S,C_NHC)ZnCl(Et)] (3) and [(O,C _NHC)ZnCl(Et)] (4), and species 3 and 4 were obtained from the reaction of ZnEt₂ with the N-methyl-N'-ethylphenylsulfide (1HCl) and N-methyl-N'-ethylmethylether (2HCl) imidazolium salts, respectively. Both solution and solid-state structural data for Zn benzyloxide species 5 and 6 agree with dimeric structures under the studied conditions (reaction conditions: CH₂Cl₂ or THF, room temperature). A computational analysis of species 3 and 4 supports a dimeric structure in solution. The Zn^II alkoxide species 5 and 6 were found to mediate either the ROP of lactide (in an effective and controlled manner) to produce chain length-controlled polylactide (PLA) or, in the presence of an alcohol source such as MeOH, the controlled degradation of PLA through extensive transesterification reactions to afford methyl lactate as the major product. A thorough DFT computational analysis of the ROP of lactide initiated by complex 5 was performed, which revealed that the operating coordination-insertion mechanism was assisted by the second Zn center, leading to a lower-energy ROP process; this result may be of interest for the future design of well-defined and high-performance metal-based catalysts. Mild thing: Simple dinuclear zinc-N-heterocyclic carbene (NHC) alkyl/alkoxide complexes mediate, under mild conditions, either the ring-opening polymerization of lactide (in an effective and controlled manner) for the production of chain length-controlled polylactide or, in the presence of an alcohol source such as MeOH, the controlled depolymerization of polylactide through extensive transesterification reactions.

Full text of DOI:10.1002/cctc.201301015

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DOI: 10.1002/cctc.201301015
Dinuclear Zinc–N-Heterocyclic Carbene Complexes for
Either the Controlled Ring-Opening Polymerization of
Lactide or the Controlled Degradation of Polylactide
Under Mild Conditions
Christophe Fliedel,[a, b] Diogo Vila-ViÅosa,[c] Maria Josꢀ Calhorda,*[c] Samuel Dagorne,*[b] and
Teresa Avilꢀs*[a]
We describe the synthesis of the new Zn–N-heterocyclic car-
bene (NHC) alkoxide complexes [(S,CNHC)ZnCl(OBn)]2 (5) and
[(O,CNHC)ZnCl(OBn)]2 (6) for use as ring-opening polymerization
(ROP) initiators for lactide polymerization. Complexes 5 and 6
are readily available through an alcoholysis reaction between
BnOH and the corresponding Zn–NHC ethyl species
[(S,CNHC)ZnCl(Et)] (3) and [(O,CNHC)ZnCl(Et)] (4), and species 3
and 4 were obtained from the reaction of ZnEt2 with the
N-methyl-N’-ethylphenylsulfide (1·HCl) and N-methyl-N’-ethyl-
methylether (2·HCl) imidazolium salts, respectively. Both solu-
tion and solid-state structural data for Zn benzyloxide species
5 and 6 agree with dimeric structures under the studied condi-
tions (reaction conditions: CH2Cl2 or THF, room temperature). A
computational analysis of species 3 and 4 supports a dimeric
structure in solution. The ZnII alkoxide species 5 and 6 were
found to mediate either the ROP of lactide (in an effective and
controlled manner) to produce chain length-controlled polylac-
tide (PLA) or, in the presence of an alcohol source such as
MeOH, the controlled degradation of PLA through extensive
transesterification reactions to afford methyl lactate as the
major product. A thorough DFT computational analysis of the
ROP of lactide initiated by complex 5 was performed, which re-
vealed that the operating coordination–insertion mechanism
was assisted by the second Zn center, leading to a lower-
energy ROP process; this result may be of interest for the
future design of well-defined and high-performance metal-
based catalysts.
Introduction
Polylactide (PLA), a biodegradable thermoplastic polyester de-
rived from lactic acid, which is a renewable resource, is cur-
rently attracting attention for various applications, ranging
from biomedical to food packaging and device applications.[1]
It is also considered a promising alternative to petrochemical-
based plastics.[2] Because of rapidly increasing production ca-
pacities (150000 tons/year currently), PLA is predicted to be
a high-volume commodity material in the coming years.[3] PLA
is best produced in a controlled manner through the ring-
opening polymerization (ROP) of lactide initiated by molecular-
ly well-defined initiators. Notably, commercial PLA is currently
manufactured through the ROP of l-lactide with SnII octa-
noate.[4] Over the past 15 years, intensive research efforts have
been directed toward the design and synthesis of ligand-sup-
ported metal-based initiators for the stereoselective ROP of
rac-lactide (rac-LA) to access stereoregular PLA with improved
properties, and, at present, various metal-based initiators effec-
tively and stereoselectively mediate the ROP of rac-LA.[5,6] The
organocatalysts for the ROP of lactide were also developed in
the last decade.[7]
[a] Dr. C. Fliedel, Prof. T. Avilꢀs
In view of the growing need of PLA, the recycling of PLA
through degradation–depolymerization into its starting com-
ponent (i.e., lactic acid) for reuse and/or for conversion into
valuable materials (such as lactate esters, which have applica-
tions in cosmetics, fragrances, and food additives) is of current
interest.[8] Although the enzymatic PLA degradation (through
hydrolysis) has been established,[9] the chemical depolymeriza-
tion of PLA typically requires drastic reaction conditions, which
are detrimental to product selectivity and impose a high
energy input.[10] A few representative reports in this area in-
clude the following: 1) PLA breakage under harsh acidic or
basic conditions (NaOH or HCl),[11] 2) the use of 4-dimethylami-
nopyridine for PLA degradation (1358C, 24 h) via ester cleav-
age,[12] and 3) PLA depolymerization through transesterification
REQUIMTE, Departamento de Quꢁmica, Faculdade de CiÞncias e Tecnologia
Universidade Nova de Lisboa
2829-516 Caparica (Portugal)
E-mail: teresa.aviles@fct.unl.pt
[b] Dr. C. Fliedel, Dr. S. Dagorne
Institut de Chimie de Strasbourg
CNRS-Universitꢀ de Strasbourg
1 rue Blaise Pascal, 67000 Strasbourg (France)
E-mail: dagorne@unistra.fr
[c] D. Vila-ViÅosa, Prof. M. J. Calhorda
Departamento de Quꢁmica e Bioquꢁmica, CQB
Faculdade de CiÞncias, Universidade de Lisboa
Campo Grande, Ed. C8, 1749-016 Lisboa (Portugal)
E-mail: mjc@fc.ul.pt
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201301015.
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with SnII octanoate (1208C, 24 h).[13] In contrast, al-
though widely studied for the controlled production
of PLA, discrete initiators that can mediate the degra-
dation–depolymerization reaction in
a controlled
manner and under mild conditions are rare and, to
our knowledge, are restricted to the following:
1) group 4 metal complexes bearing salalen-type lig-
ands found to depolymerize PLA at room tempera-
ture (through transesterification) in the presence of
MeOH[14] and 2) degradation of PLA with a triazabicy-
clodecene–alcohol mixture.[15]
Zn-based alkoxides have been widely investigated
as initiators for the ROP of lactide because such de-
rivatives typically demonstrate good to excellent ROP
performances.[16,17] In addition, Zn is cheap, less toxic,
and biocompatible, which is important for the subse-
quent use of the produced PLA in biomedicine. Yet,
well-defined Zn alkoxide/amido species frequently
decompose in the presence of protic sources, which
may significantly hamper their ROP performances
and limit their utility.
Scheme 1. Synthesis of [(D,CNHC)ZnClEt] (3 and 4) and [(D,CNHC)ZnCl(OBn)]2 (5 and 6)
(D=S and O, respectively) complexes from the corresponding imidazolium salts. Reaction
conditions: i) 1 equiv. of ZnEt2, THF or CH2Cl2, T=À408C to RT, overnight; ii) 2 equiv. of
ZnEt2, THF or CH2Cl2, T=À408C to RT, overnight; iii) 1 equiv. of BnOH, THF or CH2Cl2,
T=À408C to RT, t=2 h.
We are interested in the synthesis of robust N-het-
erocyclic carbene (NHC)–ZnII alkoxide species for use
in the ROP of lactide, a class of species little explored
thus far.[18] The exceptional ligating properties of NHCs should
ensure the formation of robust Zn–NHC alkoxide species with
improved stability. In addition, such Zn derivatives may be
available through straightforward and high-yield syntheses.
More specifically, we aimed at the synthesis of new Zn alkoxide
complexes of neutral S,CNHC ligands (in which S represents an
N-thioether functionality) of the type [(D,CNHC)ZnX(OR)] (X=hal-
ogen, alkyl, aryl, amide; R=alkyl, aryl; D=neutral donor) for
use in the ROP of lactide. The association of such an additional
labile soft donor with the NHC ligand may lead to an improve-
ment in the catalytic performance of the system.[19] As de-
scribed herein, the synthesized Zn complexes [(D,CNHC)ZnX(OR)]
were found to mediate in a controlled manner and under mild
conditions either the ROP of lactide (for the production of PLA)
or, in the presence of MeOH, the degradation–depolymeriza-
tion of PLA through transesterification reactions. Theoretical
mechanistic investigations of the ROP of lactide initiated by
these Zn systems along with the controlled character of both
the polymerization and the depolymerization catalysis are also
discussed here.
Zn–NHC adducts [(S,CNHC)ZnCl(Et)] (3) and [(O,CNHC)ZnCl(Et)] (4),
respectively (Scheme 1, route ii). Both species 3 and 4 were iso-
lated in quantitative yield as light brown oils. The use of
excess ZnEt2 (i.e., 2 equiv. in the present case) seems to be re-
quired for the quantitative formation of monocarbene Zn–NHC
species 3 and 4, at least in the case of 3. Thus, the reaction of
1·HCl/ZnEt2 in a 1:1 ratio resulted in a mixture of the monocar-
bene adduct 3 and the biscarbene [(S,CNHC)2ZnCl2] (3’) complex
in a 30:70 ratio (Scheme 1, route i), which indicated a fairly
basic ZnÀEt group in species 3.
The identity of Zn species 3 and 4 was determined from
NMR analysis. Notably, the 1H NMR spectra for both species
demonstrate no imidazolium NCHN resonance but contain sig-
nals corresponding to the ZnÀEt moiety in the high-field
region (ꢀ0.2 (CH2) and 1.1 (CH3) ppm). The 13C{1H} NMR spec-
tra of both complexes contain a characteristic Ccarbene reso-
nance at d 175.65 (3) and 175.40 (4) ppm. The corresponding
Zn alkoxide complexes [(S,CNHC)ZnCl(OBn)]2 (5) and
[(O,CNHC)ZnCl(OBn)]2 (6) could be synthesized in quantitative
yield through an alcoholysis reaction upon mixing complexes
3 and 4, respectively, with 1 equiv. of BnOH (reaction condi-
tions: CH2Cl2, room temperature). Species 5 and 6 were isolat-
ed as colorless solids, which were stable under N2 in solution
for several days and, surprisingly, air stable for hours in the
Results and Discussion
Synthesis and characterization
1
solid state. The H NMR data agree with the proposed formula-
The desired ZnÀNHC alkoxide complexes [(D,CNHC)ZnX(OR)]
(D=S or O) were prepared through the alcoholysis of the cor-
responding zinc ethyl complexes [(D,CNHC)ZnClEt], and a prior
synthesis of these complexes was thus required. The protonol-
ysis reaction (at the C2 position) of the N-methyl-N’-ethylphe-
nylsulfide (1·HCl) or N-methyl-N’-ethylmethylether (2·HCl) imi-
dazolium chlorides, both readily accessible from reported
methods,[19e,20] with 2 equiv. of ZnEt2 yielded the corresponding
tion, effective Cs-symmetric structures, and with no coordina-
tion of the (thio)ether moiety to the Zn center. The Ccarbene res-
onances for complexes 5 and 6 at d 171.82 and 171.86 ppm,
respectively, are in the expected range. In solution, the pro-
posed dimeric forms for the ZnÀOBn complexes 5 and 6 were
deduced from diffusion-ordered NMR spectroscopy data col-
lected for complex 5, which yielded a hydrodynamic radius of
6.2 ꢂ and thus a calculated molecular volume of 1010 ꢂ3. The
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latter value was compared with the volume of 5 estimated
from its molecular dimensions in its dimeric solid-state struc-
ture (14.06ꢃ12.57ꢃ6.15=1087 ꢂ3; see below for the molecu-
lar structure of 5 determined by X-ray crystallography stud-
ies).[21] The two estimated values are comparable; thus, it is
probable that the solid-state dimeric BnO-bridged structure of
5 is retained in solution (reaction conditions: CH2Cl2, room
temperature).
The solid-state molecular structures of complexes 5 and 6
were determined by X-ray crystallography studies, and they
are depicted in Figure 1. Both complexes crystallized in the
P21/c space group and consist of centrosymmetric BnO-
bridged dimeric structures. In each structure, the Zn metal
center adopts a distorted tetrahedral geometry composed of
one NHC ligand, one Cl atom, and two OBn moieties forming
a Zn2O2 core, a common structural feature in Zn aryloxide/alk-
oxide chemistry.[18,22] The ZnÀCcarbene bond lengths—C1ÀZn1=
2.015(4) (5) and 2.0255(17) ꢂ (6)—are rather short but remain
in the range observed for reported Zn–NHC complexes.[18,23]
Notably, for both complexes, the side-arm donor function
(thioether for 5 and ether for 6) does not interact with the
metal center, which, along with the absence of steric hin-
drance, allows the NHC ligand to coordinate perpendicularly
with the Zn2O2 core).
DFT[24] calculations (using Gaussian 09[25] and PBE0[26]) were
performed on the dimers 3–6, starting from the experimental
X-ray data described above, and on their monomers (3m–6m);
the DFT-calculated structures of 5m and 6m are shown in
Figure 2. In the most stable geometry of the monomers 5m
and 6m (Figure 2), the metal is three coordinated in a planar
environment (sum of the three angles is ꢀ3608). The S or O
donors of the NHC side arm are found at a long distance from
Zn (i.e., no bond). Tetrahedral coordination around Zn was
never observed.
The calculated structures of the dimeric complexes are close
to the experimentally determined structures (Figure 1), with
C1ÀZn1=2.040 ꢂ, Cl1ÀZn1=2.250 ꢂ, Zn1ÀO1=2.002 ꢂ, O1’À
Zn1=2.004 ꢂ in 5 and C1ÀZn1=2.038 ꢂ, Cl1ÀZn1=2.252 ꢂ,
O2ÀZn1=1.998 ꢂ, and O2’ÀZn1=2.008 ꢂ in 6.
To evaluate the dimer versus monomer preference in solu-
tion, the dimerization energy was calculated (as the difference
between the energy of the dimer and twice the energy of the
monomer). This energy was À35.3 kcalmolÀ1 for 5 in the gas
phase, which increased to À61.8 kcalmolÀ1 in THF and with
a higher quality basis set (see the Computational methods sec-
Figure 1. Solid-state molecular structure of a) complex 5 and b) complex 6.
H atoms are omitted for clarity. Selected bond lengths (ꢂ) for 5: C1À
Zn1=2.015(4), Cl1ÀZn1=2.2412(10), Zn1ÀO1=1.983(2), O1’ÀZn1=1.994(2);
for 6: C1ÀZn1=2.0255(17), Cl1ÀZn1=2.2637(5), O2ÀZn1=2.0149(13), O2’À
Zn1=2.0243(14). Bond angles (deg) for 5: C1ÀZn1ÀCl1=117.47(11), O1À
Zn1ÀO1’=83.42(10); for 6: C1ÀZn1ÀCl1=117.19(5), O2ÀZn1ÀO2’=82.60(6).
tion). The corresponding values for
6
were À39.2 and
À62.3 kcalmolÀ1 in the gas phase and in THF (better basis set),
respectively. The dimerization energy was also calculated for
complexes 3 and 4 in the gas phase (À11.8 and À12.3 kcal
molÀ1, respectively) and in THF (À11.0 and À7.4 kcalmolÀ1, re-
spectively). These large monomer versus dimer energy differ-
ences thus support the proposal that species 5 and 6 are also
dimeric in solution, even in a coordinating solvent such as THF.
ROP of rac-LA
The catalytic performances of the synthesized Zn alkoxide
complexes (5 and 6) were evaluated in the ROP of rac-LA for
the production of PLA. Both complexes readily polymerize rac-
LA under mild conditions (100 equiv. of rac-LA per Zn, room
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Figure 2. DFT-calculated structures of the mononuclear complexes a) 5m and b) 6m and c) representation of the envisioned three- or four-coordinated possi-
ble structures.
which suggests residual transesterification reactions
Table 1. Results of ROP of rac-LA initiated by the Zn–NHC alkoxide complexes 5 and
6 and immortal ROP of rac-LA with complex 5 in the presence of BnOH.[a]
as the ROP proceeds (as commonly observed in
ligand-supported ZnÀOR catalysts).[27] The linear cor-
relation between the Mn(corrected) of the formed
PLA and monomer conversion during the polymeri-
zation reaction and a nearly constant PDI (Figure 3)
are in line with a controlled ROP process. Kinetic
studies of the ROP of rac-LA initiated by complex 5
agree with a pseudo-first-order reaction rate with re-
spect to monomer: v=kobserved[LA] (Figure 4).
Run Catalyst Zn/LA/
BnOH
t
Conversion[b] Mn(corrected)[c] Mn(theoretical)[d] PDI[e]
[min] [%]
[gmolÀ1
]
[gmolÀ1
]
1
2
3
4
5[f]
6
5
5
5
5
5
6
1/100/0
1/500/0
1/300/3
1/1000/10 120 92
1/100/0
1/100/0
90 90
180 80
90 89
11486
10307
8969
12474
10530
9672
12972
57656
12828
13260
13116
12107
1.11
1.39
1.11
1.09
1.09
1.15
90 91
90 84
The ROP of lactide mediated by dimer 5 was inves-
tigated through DFT calculations (see the Computa-
tional methods section). Several authors have report-
ed DFT calculations on the ROP of lactones to ad-
dress specific features that could affect it. However,
in most studies, the calculation methods have been
[a] Reaction conditions: CH2Cl2, [LA]0 =1m, room temperature; [b] Determined from
1H NMR analysis; [c] Determined from GPC analysis by using polystyrene standards
and applying a correction factor of 0.58;[28] [d] Calculated according to the conversion
(MLA =144.13 gmolÀ1); [e] Determined from GPC analysis; [f] ROP run in THF.
temperature, 90 min), which afforded narrowly dispersed PLA
(1.11<polydispersity index (PDI)<1.15) with a good chain
length control (see Mn(corrected) versus Mn(theoretical) in
Table 1, runs 1 and 6). The gel permeation chromatography
(GPC) analyses confirmed the monomodal mass distribution in
agreement with a single-site catalyst; one polymer chain
grows per metal center (Figures S1 and S2). All polymerizations
mostly gave atactic PLA samples, with racemic linkage proba-
bility (Pr) ranging from 0.52 to 0.58. The dimer 5, with a thioeth-
er functionality on the NHC side arm, was found to perform
best in terms of both control and activity. Compared with earli-
er reported Zn–NHC alkoxide species,[18b] initiator 5 demon-
strates a comparable ROP activity but enables the production
of PLA with narrower polydispersity. However, higher monomer
loading led to broader polydispersity and lower ROP control
(Table 1, run 2). In the presence of 3 or 10 equiv. of BnOH,
complex 5 polymerizes up to 1000 equiv. of the monomer, re-
spectively, in a controlled and immortal manner, which leads
to well-defined and narrowly dispersed PLA (PDI=1.11 and
1.09; Table 1, runs 3 and 4, respectively, and Figure S3). The
NMR and matrix-assisted laser desorption time-of-flight
(MALDI-TOF) MS data are consistent with the sole presence of
the ÀOBn end group PLA and thus with the noninvolvement
of the NHC moiety in the ROP of lactide (Figure S4). This find-
ing suggests that despite an excess of protic source (BnOH),
the ZnÀNHC bond in these Zn complexes retains its integrity
under ROP conditions. The MALDI-TOF spectra also demon-
strate low-intensity signals separated by increments of 72 Da,
described briefly, which limits the reliability of the conclusions.
In contrast to the present Zn systems, all modeled ROP sys-
tems have typically involved monometallic derivatives, such as
Al,[29] Yb,[30] Ti,[31] and alkaline and alkaline-earth ROP initia-
tors.[32]
A mechanism involving coordination, nucleophilic
attack, and ring-opening reactions (the so-called coordination–
insertion mechanism) was determined for the ROP of rac-LA in-
itiated by b-diketiminate MgII species by a thorough DFT com-
putational analysis.[33] Likewise, the ROP of e-caprolactone and
Figure 3. Linear dependence of Mn and nearly constant PDI [Mw/Mn] of PLA
versus monomer (LA) conversion with 5 as a catalyst. Reaction conditions:
100 equiv. of rac-LA, [LA]0 =1m, CH2Cl2, RT.
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is considered the reference for the energy profile (0 kcalmolÀ1).
To coordinate the lactide with the metal center, some distor-
tion of the ZnII coordination sphere is required. The easiest
way to create a binding position is by moving one of the benz-
oxides from the bridging to the terminal position, which leaves
one of the ZnII centers only three coordinated. The binuclear
nature of the catalyst (intermediate B) is maintained by the
second bridging benzoxide ligand. Schematic representations
of these intermediates are shown in Figure 5. In the next step,
the benzoxide O atom attacks a carbonyl C atom of the lactide.
The two Zn centers are now bridged through the carbonyl O
atom of the monomer (intermediate C). This fragment rear-
ranges from this k1-O coordination mode to a k2-O,O mode (in-
termediate D). The final step consists of the cleavage (resulting
in ring opening) of the CÀO bond of the initial lactide moiety,
which yields a benzoxide end-functionalized and ring-opened
diester chain (intermediate E).
Figure 4. Semilogarithmic plots of rac-LA conversion versus time for
[Zn]=0.005, 0.01, and 0.02m. Reaction conditions: [Zn]/[LA]0 =100, CH2Cl2,
RT.
The energy profile of the reaction is shown in Figure 6. The
step with the highest barrier is the last step (i.e., from D to E,
involving the opening of six-membered lactide ring), which
leads to an overall barrier slightly below 23 kcalmolÀ1 (similar
to the value calculated in the gas phase, ꢀ24 kcalmolÀ1).
The substrate forms a weak CÀH···Cl hydrogen bond involv-
ing a CH of the lactide ring (2.587 ꢂ), and the carbonyl O
atoms behave as hydrogen bond acceptors toward one CH of
the NHC side arm (2.563 ꢂ) and one CH of the benzoxide ring
(2.870 ꢂ), which barely affects the initial structure of 5 (inter-
mediate A). The ZnÀZn bond length remains approximately
2.99 ꢂ, as in 5. The substrate approach toward the catalyst (in-
termediate A) is slightly unfavorable in terms of free energy,
because it is an associative process but is driven by the large
concentration of substrate. A small twisting of the k2-O,O
benzoxide transforms it into k1-O and opens up a coordination
position for L, which binds through one carbonyl O atom, af-
fording the new intermediate B. The ZnÀZn bond length in-
creases to approximately 3.5 ꢂ. The energy of intermediate B is
l-lactide initiated by a tridentate phosphorus pincer ZnII spe-
cies of the type (k2-PPP)ZnÀOMe was found to proceed
through
a A similar
coordination–insertion mechanism.[34]
mechanism has been proposed for the ROP of lactide promot-
ed by both Al(OH)3 and AlMe2OH.[35] Although bridging dinu-
clear metal complexes have been used in this type of polymer-
ization,[36] only some aspects of the mechanism appear to have
been addressed computationally.[37] Herein, the dimer 5 was
used as a catalyst and l-lactide as a substrate (L).
As shown in Figure 1, each ZnII center in complex 5 adopts
a slightly distorted tetrahedral coordination with one NHC and
one ClÀ ion, which is completed by two nearly symmetrical
bridging benzoxide ligands. The pending arms of the NHC
containing the SPh group protect the metal and are held in
place by weak CÀH···p hydrogen bonds. The approach of the
substrate occurs with the formation of weak CÀH···Cl and
CÀH···O hydrogen bonds, leading to an intermediate A, which
Figure 5. Relevant intermediates in the ROP of l-lactide catalyzed by complex 5.
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Figure 6. Energy profile for the first cycle in the ROP of lactide catalyzed by complex 5 (Gibbs free energies in kcalmolÀ1) and structures of the intermediates.
approximately 15.2 kcalmolÀ1 higher than that of intermediate
A, with an activation barrier of only approximately 18.4 kcal
molÀ1. This small value is associated with small rearrangements
(no bond cleavage). The small bond length (2.855 ꢂ) between
the benzoxide O atom (charge: À0.695) and the charge-deplet-
ed C atom (charge: +0.711) of the substrate favors the nucleo-
philic attack, which leads to the formation of the new CÀO
bond (intermediate C). As a consequence, the benzoxide
ligand is no longer bound to Zn and the carbonyl O atom of L
bridges the two metal atoms, which again move closer
(3.063 ꢂ). Intermediate C is much more stable than intermedi-
ate B, and the activation barrier for the conversion is 2.6 kcal
molÀ1. This step is relatively similar to that reported using
mononuclear ZnII complexes as catalysts,[34] and the barrier for
this specific transformation is smaller in the present binuclear
system, emphasizing the assistance provided by the second Zn
center, which plays a role in the reaction. In contrast to what
has been observed with mononuclear catalysts, the lactide
ring opening does not occur immediately, but rather after a re-
arrangement through binding of one lactide ring O atom to
the second Zn atom, which leads to the new intermediate D
with a small activation barrier (ꢀ5.6 kcalmolÀ1). This new spe-
cies has a higher energy (3.7 kcalmolÀ1) and a longer ZnÀZn
bond length (ꢀ3.4 ꢂ). The longest CÀO bond shown in the
structure of D (1.474 ꢂ; Figure 6) now cleaves, simultaneously
with the ZnÀO bond, which enables the formation of a C=O
bond and gives rise to an acyclic intermediate E. As observed
previously, the O atom bound to Zn now bridges the two
metals. This species compares well with the initial complex 5,
except for the conformations of the SPh arm, which is variable
all along the reaction. PLA chain growth is likely to start from
intermediate E, with a second lactide monomer approaching
in a manner similar to that shown in Figure 6. The conforma-
tions around Zn are different, and the bridging fragment that
undergoes a nucleophilic attack on lactide is much larger.
For this reason, we decided to optimize only the intermedi-
ates of the reaction involving the second equivalent of lactide
to define the major features of the energy profile. The confor-
mational freedom in intermediates A–E was neglected. As
mentioned above, the SPh arm of the NHC is rather flexible;
however, because the steric constraints remained limited, the
DFT approach was acceptable. If the second monomer ap-
proaches intermediate E to form intermediate A’ and start the
second cycle, this limitation becomes more relevant and, to
obtain accurate energies, one would need to perform a molec-
ular dynamics study (using either QM or a hybrid QM/MM
method), which is beyond the scope of this work. Therefore,
the calculated energies should be considered as higher limits.
The second cycle then starts from intermediate A’ (taken as
the energy reference for this cycle) and follows as described in
Figure 5, exactly as in the first cycle: the approach of the lac-
tone monomer L (A’), coordination of L (B’), nucleophilic attack
(C’), rearrangement (D’), and CÀO bond opening (E’).
The energies of the intermediates are qualitatively similar for
the first and the second lactide insertion (Figure 7): intermedi-
ates B, C, and D are high-energy species whereas both E and
A are low-energy species. As mentioned previously, we have
not taken into account the optimization of side-chain confor-
mations.
The proposed mechanism includes the coordination of l-lac-
tide; the migration of a fragment (here a benzoxide ligand) to
the lactide (i.e., a nucleophilic attack), often referred to as the
lactide insertion; and the ring-opening reaction. Because we
decided not to simplify the structure of catalyst 5 and l-lactide,
the overall system is rather large and it is not easy to clearly
show the details. The transition states are shown in Figure S5.
However, this mechanism covers one complete cycle and
cannot thus be well compared with published calculated
mechanisms, which usually analyze individual steps. Important-
ly, the proposed mechanism, which in all steps involves dinu-
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perimentally (actual computed
energies would be lower after
the optimization of the environ-
ment).
PLA degradation
The dinuclear Zn–NHC com-
plexes were not only efficient in-
itiators for the ROP of lactide
but also found to mediate the
controlled depolymerization of
polylactide through extensive
transesterification reactions in
the presence of MeOH. Two
treatments, that is, evaporation
to dryness and methanolysis of
crude PLA resulting from the
ROP of rac-LA by species 5 (reac-
tion conditions: catalyst 5,
100 equiv. of rac-LA, CH2Cl2,
room temperature, t=90 min,
95% conversion), were initially
found to yield different materi-
als. Thus, although the 1H NMR
spectrum of the sample that was
evaporated to dryness contains
signals corresponding to PLA along with the traces of unreact-
Figure 7. ROP of the first and the second (values in italics) monomer of lactide initiated by complex 5 (Gibbs ener-
gies in kcalmolÀ1), with energies of the intermediates and the activation barriers (first monomer).
clear Zn species, is in line with and further complements the
experimental observations on the possibility that dimer 5 is
the actual ROP initiator. In addition, the assistance of a second
Zn center for a lower-energy ROP process (versus classical
mononuclear initiators) has been confirmed and it further
highlights that cooperative effects (through, for instance, the
use of multinuclear initiators) could be beneficial to the ROP of
lactide.
1
ed rac-LA, the H NMR data for methanolized crude PLA [with
an excess of cold MeOH (1.2 mL, ꢀ2500 equiv. per Zn), CH2Cl2,
room temperature, t=30 min] agree with the presence of PLA
along with methyl lactate and associated oligomers (methyl
lactate/oligomers ratio 1:3; Figure 8). This observation was con-
firmed by GPC analyses of both the non-MeOH and MeOH
The data on the ROP of lactide initiated by these Zn–NHC
benzyloxide dimers agree with the finding of one PLA chain
growing per Zn center. Two propagating polymer chains are
thus expected to grow from the dimeric catalyst; accordingly,
we also calculated the energy of the intermediates involved in
the formation of the second chain, starting from complex E.
We assumed that the next monomer approaches on the side
of the benzoxide ligand to form intermediate A2 (Figure S6),
and the other growing PLA chain is on the opposite side. The
energies of A2, B2, C2, D2, and E2 parallel those shown in
Figure 6; of these intermediates, intermediate B2 is a high-
energy species (corresponding to the lactide coordination to
one of the Zn centers along with a cleavage of the ZnÀOBnÀ
Zn bridge), and intermediates C2, D2, and E2 are lower-energy
species (Figure S6). This finding suggests a similar path to that
calculated for the first growing chain, with the same highest
energy barrier for the ring-opening transition state leading to
E2. In addition, the energies of 5 (dimeric Zn catalyst with two
benzoxide ligands), E (one benzoxide ligand and one growing
PLA chain), and E2 (two growing PLA chains) are 0, 10.0, and
22.6 kcalmolÀ1 (Gibbs energies), respectively, which indicates
the possibility of a second PLA chain growth as observed ex-
Figure 8. 1H NMR spectra for the ROP of rac-LA initiated by complex 5 with-
out quenching with MeOH (bottom) and after treatment with MeOH (top).
Reactions conditions for the lower spectrum: 100 equiv. of rac-LA,
[LA]0 =1m, 1.17 mL of CH2Cl2, RT, t=90 min, polymer isolated by evapora-
tion of the solvent, 95% conversion. Reactions conditions for the upper
spectrum: Same conditions, but after 90 min of the reaction time, 1.2 mL of
MeOH was added, and after 30 min the mixture was evaporated to dryness.
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quenched samples. One peak corresponding to a Mn(GPC)
value of 18787 gmolÀ1 was observed for the former, and two
peaks corresponding to 5567 and 148 gmolÀ1 for the latter,
which was consistent with the formation of smaller oligomers
and methyl lactate from the initial PLA (Figures S7 and S8, re-
spectively). Likewise, the treatment of crude PLA (derived from
the ROP of rac-LA by 5; Mn(GPC)=7955 gmolÀ1) with MeOH
(0.5 mL, ꢀ1000 equiv. per Zn center, room temperature, t=
1 h) led to the nearly quantitative formation of methyl lactate
along with residual oligomers (methyl lactate/oligomers ratio
10:1), as deduced from GPC (two peaks: Mn values of 140 and
691 gmolÀ1) and NMR data. On the basis of these preliminary
results, the possibility of a Zn-catalyzed mild PLA degradation
process proceeding through transesterification with MeOH
emerged quickly.
Figure 9. PLLA molecular number (Mn) and formation of methyl lactate (rela-
tive to PLLA) versus time in the presence of the 3–MeOH component mix-
These results prompted us to study the possible use of Zn–
NHC complexes described herein for the (controlled) degrada-
tion (through transesterification) of poly(l-lactide) (PLLA), the
commercial form of PLA. Therefore, a sample of PLLA (200 mg;
Mn =18410 gmolÀ1) was first mixed with the NHC–zinc ethyl
complex 3 (reaction conditions: CH2Cl2, room temperature, t=
ture. Reaction conditions: 200 mg of PLLA with Mn(GPC)=18410 gmolÀ1
,
5 mg of catalyst 3 (0.014 mmol), 1.43 mL of CH2Cl2, t=5 min, RT, followed by
the addition of 45 mL of MeOH (1.4 mmol, 100 equiv. per Zn). t0 =Time of
MeOH addition.
5 min) for the in situ generation of the corresponding NHCÀ signed to methyl lactate with Mn(GPC)=145 gmolÀ1. The
1
ZnÀPLLA species (as deduced from H NMR data) through pro-
tonolysis of the ZnÀEt bond (Scheme 2). MeOH (100 equiv. per
MALDI-TOF spectrum of a depolymerization sample (isolated
after t=22.5 h) demonstrated a mass distribution with peaks
separated by increments of 72 Da and consistent with the sole
presence of MeO end-functionalized lactyl oligomers, which
confirmed that the extensive degradation–transesterification of
the initial PLA sample had occurred (Figure S12). In an inde-
pendent NMR control experiment, to probe the stability of the
ZnÀNHC bond (in complex 5) in the presence of excess MeOH,
species 5 was reacted with 40 equiv. of MeOH (reaction condi-
tions: CH2Cl2, room temperature, t=2 h), which led to a com-
plete substitution of the ZnÀOBn groups by ZnÀOMe moieties
and, interestingly, only marginal Zn-NHC protonolysis, as de-
1
duced from H NMR data. Altogether, the above kinetic and
characterization data on the degradation of PLA with a robust
Zn–NHC/MeOH component mixture suggest a fairly well con-
trolled process.
Conclusions
These studies confirm that relatively simple dinuclear Zn–N-
heterocyclic carbene alkyl/alkoxide complexes can mediate
either the ring-opening polymerization of lactide (in an effec-
tive and controlled manner) for the production of chain
length-controlled polylactide (PLA) or, in the presence of an al-
cohol source such as MeOH, the controlled depolymerization
of PLA through extensive transesterification reactions. Al-
though the polymerization of lactide mediated by Zn-based
catalysts has been widely studied, herein we first showed the
controlled degradation of PLA under mild conditions to methyl
lactate and higher oligomers by well-defined Zn systems,
which is a rare instance of a mild catalytic process for the deg-
radation of PLA. We then performed a thorough DFT computa-
tional analysis of the polymerization mechanism (of lactide)
with these dinuclear Zn-based initiators, which revealed the
beneficial effect of a second Zn center for a lower-energy ring-
Scheme 2. PLA degradation starting from an isolated sample of PLLA and
mediated by the NHCÀzinc ethyl complex 3.
Zn) was then added, and the degradation of PLA with the con-
comitant formation of methyl lactate was monitored by using
1H NMR spectroscopy and GPC. A kinetic study of the depoly-
merization process could be performed over a period of 24 h.
The evolution of the PLLA Mn(GPC) together with the forma-
tion of methyl lactate (determined from 1H NMR data) as
a function of time are depicted in Figure 9. A linear decrease in
the PLLA Mn along with a linear increase in methyl lactate/
smaller oligomers are observed (Figure S9). The GPC analysis of
the final product (after a reaction time of 24 h; Figures S10
and S11) revealed two peaks: one corresponding to an oligo-
meric material with Mn(GPC)=2005 gmolÀ1, and the other as-
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For 4 (D=O): The same method was followed but with ZnEt2
(3.74 mL, 461 mg, 3.74 mmol, 2 equiv.) in CH2Cl2 (4 mL) and 2·HCl
(330 mg, 1.87 mmol, 1 equiv.) in CH2Cl2 (10 mL). A light brown oil
was obtained. Yield: 90%; 1H NMR (300 MHz, CD2Cl2): d=0.16 (q,
opening polymerization process; this result may be of interest
for the future design of well-defined and high-performance
metal-based catalysts.
3
3J=8.1 Hz, 2H; ZnCH2CH3), 1.13 (t, J=8.1 Hz, 3H; ZnCH2CH3), 3.38
(s br, 3H; OCH3), 3.72 (t, 3J=4.8 Hz, 2H; NCH2CH2O), 3.90 (s, 3H;
Experimental Section
3
NCH3), 4.43 (t, J=4.8 Hz, 2H; NCH2CH2O), 6.94 and 7.07 ppm (1H
and 1H, AB spin system, 3J=1.5 Hz; CH=CH); 13C{1H} NMR
(75.5 MHz, CD2Cl2): d=4.41 (br, ZnCH2CH3), 11.53 (br, ZnCH2CH3),
37.89 (NCH3), 50.45 (NCH2CH2O), 59.50 (OCH3), 72.55 (NCH2CH2O),
122.02 and 122.58 (CH=CH), 175.40 ppm (NCN); elemental analysis
calcd (%) for C9H17ClN2OZn (270.08): C 40.02, H 6.34, N 10.37;
found: C 39.76, H 6.71, N 10.22.
General
All experiments were performed under N2 by using standard
Schlenk techniques or in a MBRAUN UNIlab glovebox. Toluene and
pentane were collected after passing through drying columns
(MBRAUN Solvent Purification Systems) and stored over activated
molecular sieves (4 ꢂ) for 24 h in a glovebox before use. THF was
distilled over the Na–benzophenone complex and stored over acti-
vated molecular sieves (4 ꢂ) for 24 h in a glovebox before use.
CH2Cl2, CD2Cl2, and C6D6 were distilled from CaH2, degassed under
a N2 flow, and stored over activated molecular sieves (4 ꢂ) in a glo-
vebox before use. Anhydrous BnOH (99.8%) was purchased from
Aldrich and stored over activated molecular sieves (4 ꢂ) for 24 h in
a glovebox before use. All deuterated solvents were obtained from
Eurisotop (Groupe CEA, Saclay, France). rac-LA (98% purity) was
purchased from Aldrich and was sublimed once before use. All
other chemicals were purchased from Aldrich and were used as re-
ceived. The NMR spectra were recorded on Bruker AC 300 or
400 MHz NMR spectrometers with Teflon-valved J. Young NMR
Synthesis of the [(S,CNHC)2ZnCl2] complex 3’
The same method as for the synthesis of 3 was followed but with
ZnEt2 (785 mL, 97 mg, 0.785 mmol, 1 equiv.) in CH2Cl2 (4 mL) and
1·HCl (200 mg, 0.785 mmol, 1 equiv.) in CH2Cl2 (8 mL). No pure
sample could be extracted (THF) from the mixture of 3’ and 3
1
(<5% remaining). Yield: ꢀ60%; H NMR (300 MHz, CD2Cl2): d=3.35
3
3
(t, J=7.8 Hz, 2H; NCH2CH2S), 3.83 (s, 3H; NCH3), 4.51 (t, J=7.6 Hz,
2H; NCH2CH2S), 6.87 and 6.99 (1H and 1H, AB spin system, 3J=
1.8 Hz; CH=CH), 7.17–7.34 ppm (m, 5H; HAr); 13C{1H} NMR (CD2Cl2,
75.5 MHz): d=35.17 (NCH2CH2S), 37.50 (NCH3), 49.36 (NCH2CH2S),
122.20 and 126.43 (CH=CH), 129.45 (C phenyl), 129.61 (C phenyl),
129.83 (C phenyl), 135.28 (C ipso-phenyl), 175.27 ppm (NCN).
tubes at ambient temperature. H and 13C chemical shifts were de-
1
termined by reference to the residual 1H and 13C solvent peaks.
The diffusion-ordered NMR spectroscopy experiments were per-
formed with a Bruker 600 MHz NMR spectrometer. Elemental analy-
ses for all compounds were performed at the Service de Microana-
lyse of the Universitꢀ de Strasbourg (Strasbourg, France). GPC anal-
yses were performed on a system equipped with a Shimadzu
RID10A refractive index detector with HPLC grade THF as an eluant
(with molecular masses and PDIs calculated by using polystyrene
standards). These were adjusted with appropriate correction fac-
tors for the Mn values. MALDI-TOF-MS analyses were performed at
the Service de Spectromꢀtrie de Masse de l’Institut de Chimie de
Strasbourg and run in a positive mode: samples were prepared by
mixing a solution of the polymers in CH2Cl2 (0.5 mg100mLÀ1); 2,5-
dihydroxybenzoic acid was used as the matrix in a volume ratio of
5:1. Imidazolium salts 1·HCl and 2·HCl were prepared according to
literature methods.[19e,20]
Synthesis of [(D,CNHC)ZnCl(OBn)]2 complexes 5 and 6
For 5 (D=S): A solution of BnOH (51 mL, 53 mg, 0.49 mmol) in
CH2Cl2 (5 mL) was added slowly to a solution of complex 3
(170 mg, 0.49 mmol) in CH2Cl2 (10 mL) at À408C. The reaction mix-
ture was then allowed to warm to RT and stirred for 2 h. The vola-
tiles were then removed under reduced pressure, and the residue
was washed twice with pentane (10 mL). Complex 5 was isolated
as a white solid after recrystallization from a CH2Cl2/pentane solu-
1
3
tion. Yield: 78%; H NMR (300 MHz, CD2Cl2): d=3.29 (t, J=8.0 Hz,
2H; NCH2CH2S), 3.79 (s, 3H; NCH3), 4.38 (t, 3J=8.0 Hz, 2H;
2
NCH2CH2S), 4.67 and 4.85 (1H and 1H, AB spin system, J=12.3 Hz;
OCH2Ph), 6.73 and 6.88 (1H and 1H, AB spin system, 3J=1.8 Hz;
CH=CH), 7.04–7.34 ppm (m, 10H; HAr); 13C{1H} NMR (75.5 MHz,
CD2Cl2): d=34.51 (NCH2CH2S), 37.79 (NCH3), 50.14 (NCH2CH2S),
69.04 (OCH2Ph), 114.78 (CAr), 121.38 (CAr), 122.24 (CAr), 123.05 (CAr),
126.73 (CAr), 127.70 (CAr), 128.23 (CAr), 129.82 (CAr), 145.48 (CAr),
158.64 (CAr), 171.82 ppm (NCN); elemental analysis calcd (%) for
C38H42Cl2N4O2S2Zn2 (852.56): C 53.53, H 4.97, N 6.57; found: C 53.37,
H 4.79, N 6.54.
Synthesis of [(D,CNHC)ZnClEt] complexes 3 and 4
For 3 (D=S): A solution (1m in hexane) of ZnEt2 (4.75 mL, 587 mg,
4.75 mmol, 2 equiv.) diluted in CH2Cl2 (5 mL) was slowly added to
a solution of the imidazolium chloride 1·HCl (605 mg, 2.37 mmol,
1 equiv.) in CH2Cl2 (12 mL) at À408C. The reaction mixture was
then allowed to warm to RT and stirred overnight. The volatiles
were then removed under reduced pressure, and the residue was
washed twice with pentane (2ꢃ10 mL), which yielded a light
For 6 (D=O): The same method was followed but with BnOH
(58 mL, 60.0 mg, 0.56 mmol) in CH2Cl2 (5 mL) and 4 (150 mg,
0.56 mmol) in CH2Cl2 (10 mL). Complex 6 was isolated as a white
solid after recrystallization from a CH2Cl2/pentane solution. Yield:
1
3
brown oil. Yield: 93%; H NMR (400 MHz, CD2Cl2): d=0.18 (q, J=
3
8.0 Hz, 2H; ZnCH2CH3), 1.11 (t, J=8.0 Hz, 3H; ZnCH2CH3), 3.37 (t,
1
3J=6.4 Hz, 2H; NCH2CH2S), 3.88 (s, NCH3, 3H), 4.53 (t, 3J=6.4 Hz,
2H; NCH2CH2S), 6.84 and 6.96 (1H and 1H, AB spin system, 3J=
1.6 Hz; CH=CH), 7.19–7.33 ppm (m, 5H; HAr); 13C{1H} NMR
(100.7 MHz, CD2Cl2): d=4.62 (br, ZnCH2CH3), 11.74 (br, ZnCH2CH3),
35.83 (NCH2CH2S), 37.95 (NCH3), 49.78 (NCH2CH2S), 122.16 and
126.85 (CH=CH), 129.43 (C phenyl), 129.65 (C phenyl), 129.87 (C
phenyl), 135.18 (C ipso-phenyl), 175.65 ppm (NCN); elemental anal-
ysis calcd (%) for C14H19ClN2SZn (348.21): C 48.29, H 5.50, N 8.04;
found: C 48.41, H 5.34, N 7.90.
86%; H NMR (300 MHz, CD2Cl2): d=3.38 (s br, 3H; OCH3), 3.72 (t,
3
3J=4.8 Hz, 2H; NCH2CH2O), 3.90 (s, 3H; NCH3), 4.43 (t, J=4.8 Hz,
2H; NCH2CH2O), 6.94 and 7.07 ppm (1H and 1H, AB spin system,
3J=1.5 Hz; CH=CH); 13C{1H} NMR (75.5 MHz, CD2Cl2): d=37.89
(NCH3), 50.09 (NCH2CH2O), 60.10 (OCH3), 68.55 (OCH2Ph), 72.74
(NCH2CH2O), 114.8 (C phenyl), 121.55 (C phenyl), 122.22 and 122.38
(CH=CH), 129.86 (C phenyl), 154.57 (C phenyl), 171.86 ppm (NCN);
elemental analysis calcd (%) for C28H38Cl2N4O4Zn2 (696.26): C 48.30,
H 5.50, N 8.05; found: C 47.91; H 5.32; N 8.20.
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X-ray structures of complexes 5 and 6 were used in the calcula-
Typical polymerization method
tions of the dimer and to build models of monomers.
In a glovebox, the initiator was charged in a vial equipped with
a Teflon tight screw cap and a monomer solution ([M]0 =1m; THF
or CH2Cl2) was added with a syringe all at once. The solution was
stirred vigorously for an appropriate time and at RT. Upon reaching
the desired time, aliquots were taken and analyzed by using
1H NMR spectroscopy to estimate the conversion. The reaction mix-
ture was exposed to air, and volatiles were removed under
vacuum; the resulting solid was then quickly washed several times
with MeOH, dried in vacuo until constant weight was achieved,
and then analyzed by using 1H NMR spectroscopy and GPC. In
some cases, a MALDI-TOF-MS analysis was performed.
All the reported energies (Gsolution) were calculated by using the fol-
lowing relationship: Gsolution =Esolution +(GgasÀEgas). Ggas and Egas were
obtained from the gas phase calculation, and Esolution, which is the
electronic energy in solution, was obtained by using the integral
equation formalism of the polarizable continuum solvation model
(IEFPCM)[44] on the electronic density (SMD).[45] Three-dimensional
representations of calculated structures were obtained with Chem-
craft.[46]
Acknowledgements
We thank Dr. Lydia Brelot and Corinne Bailly for their assistance
in the resolution of X-ray structures. We are also grateful to the
Fundażo para a CiÞncia e Tecnologia, Portugal, for funding proj-
ects (PTDC/QUI-QUI/099873/2008 and PEst-OE/QUI/UI0612/2013)
and fellowships (SFRH/BPD/73253/2010 to C.F. and SFRH/BD/
81017/2011 to D.V.V.). We also acknowledge the CNRS and the
University of Strasbourg for financial support.
“Immortal” polymerization conditions
A method similar to that described above was used but with the
addition of a monomer solution ([M]0 =1m; CH2Cl2) containing the
desired amount of BnOH onto complex 5.
Initial experiments on Zn-mediated PLA degradation
A typical polymerization method was used with the Zn initiator 5,
and the ROP process was performed till complete monomer con-
sumption. The resulting mixture was divided into two equal sam-
ples: one was treated as described above for the typical ROP pro-
cess, and MeOH was added to the other sample and the resulting
mixture was stirred at RT until the desired time was reached. In
both cases, the mixtures were evaporated in vacuo and subse-
Keywords: carbenes
degradation · ring-opening polymerization · zinc
·
methyl lactate
·
polylactide
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2009, 38, 1139; b) C. Jꢀrꢄme, P. Lecomte, Adv. Drug Delivery Rev. 2008,
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Kinetic studies of the Zn-mediated degradation of PLLA
In a glovebox, the NHCÀzinc ethyl complex 3 was charged in a vial
equipped with a Teflon tight screw cap and a PLLA solution
(CH2Cl2) was added with a syringe all at once. The resulting solu-
tion was stirred for 5 min at RT (the disappearance of the ZnÀEt
1
signals was monitored by using H NMR spectroscopy). MeOH was
then added, and the solution was vigorously stirred for an appro-
priate time and at RT. Aliquots were taken at regular intervals of
time and analyzed to monitor the formation of methyl lactate (by
1H NMR spectroscopy) and the loss of molecular mass of PLLA (by
GPC).
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[5] For selected reviews, see: a) S. Dagorne, M. Normand, E. Kirillov, J.-F.
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Received: November 29, 2013
Published online on && &&, 0000
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11
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C. Fliedel, D. Vila-ViÅosa, M. J. Calhorda,*
S. Dagorne,* T. Avilꢀs*
Mild thing: Simple dinuclear zinc–N-
heterocyclic carbene (NHC) alkyl/alkox-
ide complexes mediate, under mild con-
ditions, either the ring-opening poly-
merization of lactide (in an effective and
controlled manner) for the production
of chain length-controlled polylactide
or, in the presence of an alcohol source
such as MeOH, the controlled depoly-
merization of polylactide through exten-
sive transesterification reactions.
&& – &&
Dinuclear Zinc–N-Heterocyclic
Carbene Complexes for Either the
Controlled Ring-Opening
Polymerization of Lactide or the
Controlled Degradation of Polylactide
Under Mild Conditions
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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