Tetradentate iminophenolate copper complexes in rac -lactide polymerization

Article abstract of DOI:10.1139/cjc-2018-0287

Copper(II) nitrate complexes of 2-(((2-((2-aminoethyl)amino)ethyl)imino)methyl)phenol, 2-(((2-((2-aminoethyl)amino)ethyl)imino)methyl)-4,6-dichlorophenol, 2-(((2-(piperazin-1-yl)ethyl)imino)methyl)phenol and (2,4-di-tert-butyl-6-(((2-(piperazin-1-yl)ethyl

Full text of DOI:10.1139/cjc-2018-0287

Canadian Journal of Chemistry
Tetradentate iminophenolate copper complexes in
rac-lactide polymerization
Pargol Daneshmand, Aurélie Randimbiarisolo, Frank Schaper*
Centre in Green Chemistry and Catalysis, Département de chimie, Université de Montréal, 2900
Boul. E.-Montpetit, Montréal, QC, H3T 1J4, Canada
* Frank.Schaper@umontreal.ca
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Abstract
Copper(II) nitrate complexes of 2-(((2-((2-aminoethyl)amino)ethyl)imino)methyl)phenol, 2-
(((2-((2-aminoethyl)amino)ethyl)imino)methyl)-4,6-dichlorophenol, 2-(((2-(piperazin-1-
yl)ethyl)imino)methyl)phenol and (2,4-di-tert-butyl-6-(((2-(piperazin-1-
yl)ethyl)imino)methyl)phenol, as well as a copper(II) acetate complex of 2-(((2-(piperidin-1-
yl)ethyl)imino)methyl)phenol have been prepared and characterized by X-ray diffraction
studies. In combination with benzyl alcohol, all complexes are active in rac-lactide
polymerization at 140 °C in molten monomer to provide moderately heterotactic PLA. Most
complexes showed complicated reaction kinetics, indicative of two interconverting active
species. Molecular weight control was poor and a strong tendency toward intramolecular
transesterification led to oligomeric products. There was no indication that the basic site of
the ligand is participating in the polymerization reaction by deprotonation of the alcohol
nucleophile.
Keywords
Copper complexes – Catalysis – Polymerization – rac-Lactide
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Introduction
Polylactic acid (PLA), produced from ring-opening polymerization of lactide, is one of the
most important biopolymers today.1-7 Controlled polymerization of lactide has become a
catalytic challenge,7-33 but combining isotactic stereocontrol, polymer molecular weight
control with high activity and catalyst stability remains difficult to achieve. Most studies –
including our own – focus on coordination-insertion polymerization of lactide, since the
tight four-membered metallacycle of the insertion transition state is likely to be highly
influenced by the ligand environment, which in turn would allow control over relative
reactivities (Scheme 1). A major drawback of lactide polymerization by a coordination-
insertion mechanism is the inherent lability of the metal alkoxide catalyst towards
protonation. Most coordination-insertion polymerization catalysts thus do not or would not
survive conditions currently employed in the industrial production of PLA, i. e. the presence
of notable amounts of water and lactic acid at temperatures of 140 °C and above. An
alternative pathway to ring-opening polymerization of lactide is Lewis-acid activation of the
monomer (Scheme 1).34-36 Since metal alkoxides are not required, the mechanism typically
tolerates protic substances, as long as the spectator ligand is not protonated easily.
Polymerizations by an activated-monomer mechanism are, however, more difficult to
control, since the nucleophilic attack occurs at some distance from the metal center and
since the activated-monomer complex is highly flexible. Lewis-acid-catalyzed lactide
polymerizations thus typically show only low to moderate stereocontrol, typically to
heterotactic PLA and mainly due to the interaction of the chiral polymeryl alcohol with the
monomer without much influence from the catalytic site. In 2013, Sarazin and Carpentier
explained an increased activity in rare-earth-based catalysts with a “ligand-assisted
activated monomer mechanism”.37 A basic site at the ligand can interact with the alcohol
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nucleophile by hydrogen bonding, thus facilitating the nucleophilic attack on the monomer
(Scheme 1). This parallels work by Bourissou and Maron on sulfonic acid-catalyzed
caprolactone polymerization, where they showed that sulfonic acid acts as Brönsted acid
and base at the same time.38 In contrast to ordinary activation of the monomer, interaction
with the catalytic site now requires a well-defined and relatively rigid geometry. In a series
of excellent publications, Wu and coworkers employed this concept in lactide
polymerizations with phenolate complexes of group 1 metals, mostly sodium and
potassium.36, 39-44 They could show that phenolate is not incorporated in the polymer chain
and acts (mostly) as spectator ligand. The bulky environment (crown-ether coordination
around the metal center, highly substituted phenolates) constricts the environment
sufficiently to allow stereocontrol, remarkably toward isotactic PLA. The catalysts are itself
achiral (although chirality at the metal is possible in the transition state) and the
stereocontrol mechanism was found to be chain-end control.
In the following we explore the possibility to apply this stereocontrol mechanism to
copper(II) complexes. Iminophenolate Schiff-base ligands provide highly stable six-
membered metallacycles and are one of the most common motifs in coordination chemistry.
Iminophenolate complexes were among the first complexes employed in lactide
polymerization.45-48 In copper based lactide polymerization, typically homoleptic diphenolate
complexes or salen ligands, which can be considered their cyclic analogs, have been
employed (Scheme 2).49-53 Here we investigate heteroleptic complexes of tetradentate ligand
L1 with a weakly coordinating anion (A, Scheme 2). Formation of a cationic complex should
increase activity in an activated-monomer mechanism and the dissociation of the terminal
amino ligand upon coordination of the monomer (claimed, for example, in similar tridentate
zinc complexes)54-55 will liberate a basic site available for interaction with the alcohol
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nucleophile. It should be noted that zinc complexes with similar tetradentate linear
triaminophenolate ligands have been recently employed by Kol for isotactic lactide
polymerization.56 These complexes followed a coordination-insertion mechanism, however,
different from the mechanism targeted here.
Results and discussions
Copper complexes of type A have been reported for a variety of anions. Structurally
characterized (L1)CuX complexes have been reported for X– = PF6 , ClO4 , Br–, and Cl–.57-62
We were most interested in the respective nitrate complex, (L1)Cu(NO3), 1, which was
prepared by reaction of copper nitrate with L1H in methanol (Scheme 3). Complex 1
crystallizes as a monomeric, cationic complex with square-pyramidal geometry. A water
molecule replaced the anion and occupies the apical position (Fig. 1, Table 1). The
respective triflate complex 2 was prepared analogously and is practically isostructural
(Scheme 3, Fig. 1, Table 1). Reaction with chloro-substituted ligand L2H provided 3. In 3,
the nitrate anion instead of water is coordinated to Cu and bridges two copper centers to
form a 1D coordination polymer. Crystal quality was bad for 3 and the structure should not
be considered more than proof of connectivity.
–
–
Complexes 1 and 2 were tested for the polymerization of rac-lactide at 140 °C in molten
monomer with benzyl alcohol as a co-initiator. 1 showed only low to moderate activity,
requiring 24 h to complete conversion. Conversion-time plots of an experiment in which the
pressure tube was opened and aliquots taken (Fig. 2, squares; Tables 2 and S1, entry 7) and
the results of separate experiments quenched at a given time (Fig. 2, diamonds; Table S1,
entries 1-6) show very similar conversions. Introduction of ambient atmosphere during
sampling thus does not seem to influence the reaction. To test this, polymerizations were
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conducted in the presence of 5 equiv acetic acid or 5 equiv water (Tables 2 and S1, entries
11 and 12). Activity was unaffected by acetic acid and even increased upon addition of
water (Fig. S1). Polymer molecular weight analysis would normally provide evidence
whether water was just tolerated or act as a chain-transfer reagent. However, polymer
molecular weights for 1 were much lower than expected (Tables 2 and S1). MALDI-MS
spectra confirmed the presence of cyclic oligomers (Fig. S12), indicative of intramolecular
transesterification, and polymer molecular weights could thus not be used for mechanistic
interpretations. The same tendency for intramolecular transesterification was observed for
all other catalysts in this study, which generally produced oligomers instead of polymers
(Tables 2 and S1, Fig. S12-S15). Polymerizations in the presence of ligand L1H instead of
the respective copper complex resulted in only 11% conversion over 24 h with BnOH as
initiator (Table S1).
Activities in polymerizations following an activated monomer-mechanism typically increase
with increasing concentration of the alcohol nucleophile. Addition of 2, 4, or 8 equiv of
benzyl alcohol indeed led to the expected increase in reactivity, but the increase was not
linear and showed saturation behaviour (Fig. 3, Table S1, entries 8-10). While we did not
investigate this in detail, it is possible that the alcohol pre-coordinates either to the copper
center or via hydrogen-bonding with basic sites at the ligand. Saturation of the pre-
coordination equilibrium would account for the observed Michaelis-Menten kinetics. While
all kinetic traces could be fitted to a first-order rate law for conversions <70%, conversions
at later stages of the reaction were higher than expected, which will be discussed later.63
Triflate complex 2 likewise polymerized lactide with essentially identical activity, but
showed a pronounced induction period (Fig. 4, Tables 2 and S1, entry 17). To further test
the influence of the anion, polymerizations with 1 were conducted in the presence of 1 or 4
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equiv of [NEt4]Cl or [NEt3H][TsO] ([TsO]– = tosylate, Fig. 5, Tables 2 and S1, entries 13-16).
The presence of a more coordinating anion resulted in an observable induction period
(conversions are below those of 1 without added salt for t < 60 min), but final activities were
3 times higher than without addition of salt (kobs = 0.4 h–1). Given that [NEt4]Cl and
[NEt3H][TsO], either at at 1 or 4-fold concentration relative to 1, yield virtually identical
kinetic traces, the observed increase in activity is most likely due to a rate increase of 1
under those conditions. In combination with the positive deviation of conversion from the
theoretical curves observed above and the higher activity in presence of water, this
indicates that 1 slowly converts into a more active state and that this conversion is more
efficient under more polar conditions.
Kinetics of polymerizations with 3, with a dichloro-substituted phenolate ligand, supported
this interpretation. Conversion-time plots of 3 in the presence of either 1 or 4 equivalents of
benzyl alcohol are close to linear (Fig. 6, Tables 2 and S1, entries 18+19). There is no
mechanistic explication for a zero-order dependence on lactide concentration, in particular
since the immediate and consistent colour change of the reaction mixture indicates that 3
dissolves readily in lactide monomer. The semilogarithmic plot for both reactions does not
show the gradual increase to a linear regime expected for first-order reactions with an
induction period either, but rather two linear regimes with different rate constants (Fig. 6).
1 and 3 thus slowly convert from a state of lower reactivity (active immediately or with very
short induction periods) to a state of higher reactivity throughout the reaction, which occurs
at an earlier stage for 3.
Polymerizations with 1-3 all afforded moderately heterotactic PLA (Pr = 0.7-0.85, Table 2).
Heterotacticities differed notably between different experiments under otherwise similar
conditions, but there was no clear correlation between stereocontrol and either the catalyst
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employed, addition of benzyl alcohol or ammonium salts (Table S1). To investigate in more
detail the influence of a basic group on the ligand on catalyst reactivity, the ethylene
diamine moiety was replaced by piperazine (L3H, Scheme 4). Although sterically similar,
coordination of both amino groups in piperazine would enforce an unfavourable boat-
conformation, and consequently in structurally characterized copper complexes with L3 or
ligands similar to L3 the ligand was tridentate.64-74 Complexes 4 and 5 were prepared
analogous to 1. In addition, 6 was prepared, which is sterically similar but does not contain
a basic group on the ligand. Reaction with copper(II) nitrate did not afford crystalline
material upon reaction with L4H (even in the presence of base), but 6 was readily obtained
with an acetate counteranion (Scheme 4). Complexes 4 and 5 also crystallized as monomeric
complexes with square-pyramidal coordination geometry. A derivative structure of 4 with
nitrate replaced by a water ligand has been reported previously.68 6 forms a 1D
coordination polymer by bridging coordination of the acetate anion, similar to the structure
of 3. In all three complexes, there is a weak interaction with a second oxygen on the nitrate
or acetate anion. 4 and 5 were obtained as the nitric acid adducts with an additional anion
and a protonated terminal amino group on the piperazine. This is in accordance with all
other structurally characterized copper complexes of ligands of type L5.64-74 Several
attempts to prepare the nitric-acid-free complex by introducing bases in the reaction failed
to provide crystalline material. The anion is coordinated in all three complexes in an
equatorial position. The apical position is either occupied by a second anion or methanol,
with the elongation of the bond length expected for the ligand in the apical position (Table
1).
The kinetic profile of rac-lactide polymerizations with 4 showed again an apparent linear
conversion-time plot, which deviates significantly from the sigmoidal curve expected for a
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simple induction period. Conversions from four independent kinetic experiments and from
four separate experiments quenched at a given polymerization time agree remarkably well
with each other (Fig. 8, Tables 2 and S1, entries 21-30), indicating again that sample-taking
did not influence polymerization and that the observed deviations from simple first-order
behaviour are reproducible. The semilogarithmic plot shows again two linear regimes, in
agreement with slow transformation from one active species into another (Fig. 8). Activities
somewhat increase with addition of benzyl alcohol (Fig. S2-S4, Table S1, entries 20,31-33),
but the saturation behaviour is even more pronounced than in 1. Polymerizations at lower
and increased catalyst loading yield reduced or increased rate constants for the slow regime
at the beginning of the reaction as well as for the fast regime at the end of the reaction (Fig.
S5-S7, Tables 2 and S1, entries 34+35). Given the two regimes present, the quality of the
data is insufficient to determine the actual reaction order in catalyst with confidence, but
the obtained data agrees reasonably well with a linear dependence on catalyst
concentration with a “dead” concentration of 0.3 mol% catalyst (Fig. S7). At 4:lactide = 1:50,
the reaction follows pure first-order kinetics (Fig. S5, S6), eventual for the trivial reason
that the reaction already reached 90% conversion at the time the second active species
typically starts to be noticeable in the reaction kinetics.
Polymerizations with 4 showed the same moderate heterotacticity as 1-3 with Pr = 0.7-0.8.
Polymerizations were conducted in the presence of base (1 equiv of triethylamine, pyridine,
or tBuOK per 4) to ensure that the ammonium group on piperazine is available for
interaction with the alcohol (Fig. S8+S9, Table S1, entries 36-38). An induction period was
observed for triethylamine and pyridine, to lead afterwards to faster conversion than
without added base. Polymerizations showed slightly higher stereocontrol in the presence of
pyridine and triethylamine (Pr = 0.73-0.87, Table 2, S1), but lower stereocontrol for tBuOK
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(Pr = 0.5-0.7), which would be suspected to be most effective in deprotonating the
ammonium group. The slightly increased stereocontrol can thus not be correlated with
liberation of a basic side on the ligand.
Introduction of tert-butyl substituents at the ortho- and para-position of the phenolate
ligand in 5 leads to a minor reduction in activity (approx. 2/3 compared to 4), but otherwise
identical polymerization behaviour (Fig. S10 and S11, Tables 2 and S1, entries 39-41).
Stereocontrol was only slightly increased with the more bulky phenolate ligand (Pr = 0.75-
0.85). Polymerizations with 6, carrying a tridentate ligand without additional basic site,
show the same general polymerization behaviour as 1-5 (Fig. 9, Tables 2 and S1, entries 42-
44). Coordination/dissociation of the 3rd amino group is thus not responsible for the
existence of two active species. Rate constants identical in the range of error to those of 4
and unaffected stereocontrol of Pr = 0.75-0.80 further support that the 3rd amino group in 4
does not participate in the polymerization.
Conclusions
While the structure of the active species is not known, close to identical activities and
polymerization kinetics of complexes with diethylenetriamine, piperazineethyleneamine
and piperidineethyleneamine substituents propose a tridentate coordination of the ligand in
the active species. However, there is no evidence that a basic site facilitates polymerization
via a ligand-assisted chain-end control mechanism in these complexes.
Complexes 1-6 show up to 85% heterotacticity, an impressive stereocontrol for
polymerizations at 140 °C and unprecedented for copper complexes. The high amount of
intramolecular transesterification observed for all complexes, however, and the at best
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mediocre activity argues against the suitability of this catalyst system for lactide
polymerization in general. Given the fact that copper diketiminate complexes with a
bidentate ligand, square-planar geometry and notable steric bulk oriented below and above
the complex plane did not show any evidence of transesterification reactions even under
monomer-starved conditions,75-76 it can be argued that ligands which permit square-
pyramidal coordination might not provide a constricted enough coordination environment to
avoid transesterification reactions.
Experimental section
General. 4,6-di-tert-Butylsalicyladehyde,77 1,3-dichlorosalicylaldehyde,77 L1H,60 L3H,72 L4H,78 and
L5H,79 were prepared according to literature. rac-Lactide (98%) was purchased from Sigma–Aldrich,
◦
purified by 3x recrystallization from dry ethyl acetate and kept at –30 C. All other chemicals were
1
purchased from common commercial suppliers and used without further purification. H and 13C
NMR spectra were acquired on Bruker Avance 300 and 400 spectrometers. Chemical shifts were
referenced to the residual signals of the deuterated solvents (CDCl3: H: δ 7.26 ppm, 13C: δ 77.16).
1
Proton and carbon signals of minor isomers are respectively reported in brackets. Abbreviations for
peak multiplicities are s (singlet), d (doublet), t (triplet), q (quadruplet), qu (quintuplet), m
(multiplet) and br (broad). Certain 13C NMR chemical shifts values were extracted from HSQC and
HMBC spectra. Elemental analyses were performed by the Laboratoire d’analyse élémentaire
(Université de Montréal). All UV-Vis measurements were done in MeOH at RT in a sealed quartz
cell on a Cary 500i UV-Vis-NIR Spectrophotometer.
2-(2-(2-aminoethylamino)ethylimino)methyl)-4,6-dichlorophenol, L2H. A procedure from literature
was adapted as follows:78 To a yellow solution of 1,3-dichlorosalicyladehyde (1.00 g, 5.2 mmol) in
ethanol (10 mL) was added dropwise a solution of diethylenetriamine (540 mg, 5.2 mol) in ethanol
(10 mL), followed by the addition of 5 drops of formic acid. The obtained yellow solution was refluxed
for 1 hour. The solvent was removed under vacuum to yield a yellow oil (1.26 g, 87%).
1H NMR (CDCl3, 300 MHz): δ 8.22 (br s, 1H, H3), 7.37 (dd, J = 3, 3 Hz, 1H, H1), 7.12 (d, J = 3 Hz,
0.6H, H2), [7.08 (d, J = 3 Hz, 0.4H, H2)], 3.67 (br s, 2H, H4), 2.97 (br s, 2H, H5), 2.80 (t, J = 6 Hz, 2H,
H7), 2.68 (t, J = 6 Hz, 2H, H6); 13C{1H} NMR (CDCl3, 75 MHz): δ 164.5 (HSQC, CH3), [158.3 (HMBC,
C-OH)], 157.8 (HMBC, C-OH) [132.6 (CH1)], 132.2 (CH1), [129.1, CH2], 129.0 (CH2), 123.3 (CCl1),
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122.4 (2C, CCl2 + C(C=N)), 58.1 (CH4), 52.4 (CH6), 49.8 (CH5), 41.9 (CH7), [41.6 (CH7)]. ESI-HRMS
(m/z): [M+H]+ (C11H16Cl2N3O) calcd 276.0670; found 276.0675.
[(L1)Cu][NO3]·H2O, 1. Cu(NO3)2·2.5 H2O (112 mg, 0.48 mmol) was added to a freshly prepared yellow
solution of 2-(2-(2-aminoethylamino)ethylimino)methyl)phenol (L1H) (100 mg, 0.48 mmol) in
methanol (10 mL). After stirring the suspension for 30 min, the obtained dark blue solution was left
to slowly evaporate to yield blue crystals. The crystals were recrystallized from a minimum amount
of boiling methanol (38 mg, 23%).
UV-vis (MeOH, 5·10-6 M) [λmax, nm (ε, mol-1 cm2)]: 266 (33310), 362 (104300), 577 (3300). Anal. Calcd
for C11H18CuN4O5·H2O: C, 35.92; H, 5.48; N, 15.23; Found: C, 35.77; H, 5.09; N, 15.88.
[(L1)Cu][CF3SO3], 2. Analogous to 1, from Cu(CF3SO3)2 (174 mg, 0.48 mmol), 2-(2-(2-
aminoethylamino)ethyl)iminomethyl)phenol (L1H) (100 mg, 0.48 mmol) in methanol (10 mL) stirred
30 min. The obtained blue solution was left to slowly evaporate to yield very few purple crystals,
which were used without characterization other than X-ray studies.
[(L2)Cu][NO3], 3. Analogous to 1, from Cu(NO3)2·2.5 H2O (85 mg, 0.36 mmol), 2-(2-(2-
aminoethylamino)ethyl)iminomethyl)-4,6-dichlorophenol (L2H) (100 mg, 0.36 mmol) in ethanol (10
mL) stirred 30 min to yield dark blue oil. The oil was crystallized from boiling ethanol (3 mL), to
yield dark blue crystals which were then re-crystallized from a minimum amount of boiling ethanol
(13 mg, 9%).
UV-vis (MeOH, 5·10-6 M) [λmax, nm (ε, mol-1 cm2)]: 267 (90000), 359 (25000), 570 (1000). Anal. Calcd
for C11H14Cl2CuN4O4·EtOH: C, 34.95; H, 4.51; N, 12.54; Found: C, 34.84; H, 3.95; N, 12.75.
[(L3)Cu][NO3]2·MeOH, 4. Analogous to 1, from Cu(NO3)2·2.5 H2O (100 mg, 0.43 mmol), 2-((2-
(piperazin-1-yl)ethyl)iminomethyl)phenol (L3H) (100 mg, 0.43 mmol) in methanol (10 mL) stirred 30
min. The desired complex directly crystallized out of the dark green solution (65 mg, 33%).
UV-vis (MeOH, 7.5·10-6 M) [λmax, nm (ε, mol-1 cm2)]: 278 (76900), 300 (sh), 374 (22100), 632 (200).
Anal. Calcd for C13H19CuN5O7·CH3OH: C, 37.13; H, 5.12; N, 15.46; Found: C, 36.74; H, 5.01; N,
15.43.
[(L4)Cu(MeOH)][NO3]2·½MeOH, 5. Analogous to 1, from Cu(NO3)2·2.5 H2O (67 mg, 0.29 mmol), 2,4-
di-tert-butyl-6-((2-(piperazin-1-yl)ethyl)iminomethyl)phenol (L4H) (100 mg, 0.29 mmol) in methanol
(10 mL) refluxed 1 h to yield dark green crystals. The crystals were re-crystallized from a minimum
amount of boiling methanol and washed with hexane (3 x 5 mL). (18 mg, 10%)
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UV-vis (MeOH, 2.5·10-6 M) [λmax, nm (ε, mol-1 cm2)]: 280 (173600), 316 (41400), 390 (45200), 632
(600). Anal. Calcd for C22H39CuN5O8·½ C6H14: C, 49.39; H, 7.50; N, 11.47; Found: C, 49.66; H, 7.12;
N, 11.86. (Replacement of methanol by hexane assumed during drying and washing for EA.)
[(L5)Cu(OH2)][AcO]·H2O, 6. Analogous to 1, from Cu(OAc)2 (78 mg, 0.43 mmol), 2-((2-(piperidin-1-
yl)ethyl)iminomethyl)phenol (L5H) (100 mg, 0.43 mmol) in methanol (10 mL) stirred 30 min. The
obtained green solution was left to slowly evaporate to yield green crystals. The crystals were
recrystallized from a minimum amount of boiling methanol. (36 mg, 23%)
UV-vis (MeOH, 7.5·10-6 M) [λmax, nm (ε, mol-1 cm2)]: 272 (81400), 306 (sh), 378 (22000). Anal. Calcd
for C16H24CuN2O4.·H2O: C, 47.11; H, 6.92; N, 6.87; Found: C, 47.59; H, 6.71; N, 6.68.
rac-Lactide polymerization. In a glove box, the desired amount of rac-lactide was placed into a
pressure tube together with the catalyst. If required, a highly concentrated stock solution of an
additive (BnOH, etc.) in toluene was added. The pressure tubes were then placed in a preheated oil
bath at 140 ºC. In kinetic experiments, samples were taken at specific time intervals, dissolved in
CDCl3, filtered through a short silica plug to remove copper catalyst, which was rinsed with
1
additional CDCl3, and studied by H NMR. After drying, polymers were stored at –80 °C for further
analysis.
1
Conversion was determined from H NMR by comparison to remaining lactide. Pr values were
determined from homodecoupled 1H NMR spectra and calculated from Pr = 2·I1/(I1+I2), with I1 = 5.20
– 5.25 ppm (rmr, mmr/rmm), I2 = 5.13 – 5.20 ppm (mmr/rmm, mmm, mrm). The integration of the
left multiplet and right multiplet (I1 and I2) required only one, very reproducible dividing point of the
integration, which was always taken as the minimum between the two multiplets. Nevertheless, Pr
values showed a much higher variability than typically observed in these polymerizations.
Investigations indicated incomplete removal of Cu(II) as the source of the high variations. Molecular
weight analyses were performed on a Waters 1525 gel permeation chromatograph equipped with
three Phenomenex columns and a refractive index detector at 35 ◦C. THF was used as the eluent at a
flow rate of 1.0 mL·min-1 and polystyrene standards (Sigma–Aldrich, 1.5 mg·mL-1, prepared and
filtered (0.2 mm) directly prior to injection) were used for calibration. Obtained molecular weights
were corrected by a Mark-Houwink factor of 0.58.80
X-ray diffraction studies. Crystal for X-ray diffraction were obtained from synthesis as described
above. Diffraction data were collected on a Bruker Venture METALJET diffractometer (Ga
K radiation) or a Bruker APEX II microsource (Cu K radiation).81 Data reduction was performed
with SAINT,82 absorption corrections with SADABS.83 Structures were solved by dual-space
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refinement (SHELXT).84 All non-hydrogen atoms were refined anisotropic using full-matrix least-
squares on F2 and hydrogen atoms refined with fixed isotropic U using a riding model
(SHELXL97).85 Further experimental details can be found in Table 3 and the supporting information
(CIF). All crystals but 1 and 4 were weakly diffracting and yielded poor structural data. Structures
should be considered as proof of connectivity mainly.
Acknowledgements
Funding was supplied by the NSERC discovery program (RGPIN-2016-04953) and the
Centre for Green Chemistry and Catalysis (FQRNT). We thank Francine Bélanger for
support with X-ray diffraction, Elena Nadezhina for elemental analysis, Marie-Christine
Tang and Dr. Alexandra Furtos for MALDI analyses, and Dr. Antoine Douchez for support
with 2D NMR. J. L. Jimenez Santiago contributed during his MITACS internship to ligand
synthesis.
Supporting Information
Additional graphics and details of all polymerizations. Details of the crystal structure
determinations (CIF). CCDC 1852773-1852778 contains the supplementary crystallographic
data for this paper. The data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.
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Figure 1. X-ray structures of 1 (left), 2 (middle) and 3 (right). Thermal ellipsoids are drawn at 50%
probability. Hydrogen atoms other than those of water, non-coordinated anions, the second
independent molecule for 3, and the minor fraction of the disordered nitrate in 3 omitted for clarity.
Figure 2. Conversion-time plot for the polymerization of rac-lactide with 1/benzyl alcohol at 140 °C.
lactide:1:BnOH = 100:1:1. Black squares: single experiment with aliquots taken at desired times.
Reaction was exposed to air during sampling. Blue diamonds: Series of 5 independent polymerization
experiments quenched after 2, 4, 7 and 24 h. Reactions were not exposed to air. The inset shows the
semi-logarithmic plot. Solid lines are theoretical conversions using the apparent first-order rate
constant determined from the linear region of the semi-logarithmic plot
Figure 3. rac-Lactide polymerization with 1/BnOH with different ratios of benzyl alcohol. Conditions:
140 °C, neat monomer, lactide:1 = 100:1, BnOH:1 = 1 (blue diamonds, 5 separate experiments), 1
(brown squares), 2 (black triangles), 4 (blue hollow diamonds), and 8 (red circles). The solid lines are
theoretical conversions based on the pseudo-first-order rate constant determined from the linear
region of the semilogarithmic plot (conversion < 70%). Left: Conversion-time plots, Upper Right:
Semilogarithmic plots, Lower Right: Dependence of the observed pseudo-first-order rate constant on
benzyl alcohol concentration.
Figure 4. Conversion-time plot for the polymerization of rac-lactide with 2 (blue diamonds). Data for
1 under identical conditions is provided for comparison (black squares). Conditions: 140 °C,
lactide:[Cu]:BnOH = 100:1:1. The inset shows the semilogarithmic plot. Solid lines represent
theoretical conversions based on the pseudo-first-order rate constant determined by linear regression
of the semilogarithmic plots.
Figure 5. Conversion-time plot for the polymerization of rac-lactide with 1 in the presence of
ammonium salts: 1 equiv [NEt4]Cl (black triangle), 4 equiv [NEt4]Cl (hollow triangle), 1 equiv
[NEt3H][TsO] (red circle),
4
equiv [NEt3H][TsO] (hollow circle). Conditions: 140 °C,
lactide:[Cu]:BnOH = 100:1:1. The inset shows the semilogarithmic plot. Solid lines represent
theoretical conversions based on the pseudo-first-order rate constant determined by linear regression
of the semilogarithmic plots for t>120 min.
Figure 6. Conversion-time plot and the semilogarithmic plot for rac-lactide polymerizations with 3.
Conditions: lactide:3:BnOH = 100:1:1 (blue diamonds), 100:1:4 (red circles).
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Figure 7. X-ray structures of 4-6. Thermal ellipsoids drawn at 50% probability level. Hydrogen
atoms, the second independent molecule in 5 and the anion disorder in 4 omitted for clarity.
Figure 8. Conversion-time plot and the semilogarithmic plot for rac-lactide polymerizations with 4.
Conditions: lactide:4:BnOH = 100:1:1. Hollow diamonds are independent reactions quenched after
0.5, 2, 4, and 7 h without exposing the reaction to air. In the remaining four experiments samples
were taken in the desired intervals, exposing the reaction to air. Two reactions were conducted with
twice (diamonds) or half (triangles) the total amount of reactants to verify the influence of external
impurities.
Figure 9. Conversion-time plot and the semilogarithmic plot for rac-lactide polymerizations with 6.
Conditions: lactide:6:BnOH = 100:1:1 (squares, triangles), 100:1:4 (circles). Lines represent
theoretical conversions calculated from the pseudo-first-order rate constants obtained from the linear
regions of the semilogarithmic plot.
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Table 1. Bond lengths in the X-ray structures of 1-6
1
2
3
4
5
6
Cu-OPhenol
Cu-N(=C)
Cu-N’ a
Cu-N’’ a
Cu-X
1.924(2)
1.944(3)
2.014(3)
2.011(3)
1.906(6)
1.939(7)
1.995(8)
2.009(7)
1.91(2)
1.97(2)
1.97(2)
2.03(2)
1.921(3)
1.931(4)
2.079(4)
1.909(13)
1.942(15)
2.067(16)
1.923(11)
1.920(13)
2.094(14)
2.50(1),
2.72(2)
1.998(3),
2.440(4),
(2.618(4))
1.987(13), 1.942(10),
(2.769(14)) 2.275(11),
(2.721(12))
Cu-OH2/O(H)Me
2.320(2)
0.1
2.425(6)
0.1
2.352(13)
0.1
0.1
0.2

a N’ : first amino group. N’’ : second amino group
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Table 2. Summary of rac-lactide polymerizations with 1-6/1 equiv benzyl alcohol a
e
g
Catalyst
LA:Cu:ROH b
100:1:1
Additive
Conversion c
94-96%
96%
kobs [h–1] d
0.15(1)
Mw/Mn
2.9
#chains f
17
Pr
1
1
1
2
3
4
4
4
5
6
0.65-0.85
0.7-0.85
0.6-0.85
0.8-0.85
0.8-0.85
0.6-0.75
0.85-0.9
0.75-0.8
0.8
100:1:1
5 H2O
0.37(1)
1.7
17
100:1:1
1-4 [NEt3R]X
93-97%
95%
0.37(1)-0.43(4)
0.13(2)
1.6-1.8
1.5
9-18
9
100:1:1
100:1:1
95%
0.081(3), 0.26(4)
0.16(1), 0.52(2)
0.57(3)
2.3
28
100:1:2 h
50:1:2 h
200:1:2 h
100:1:2 h
100:1:1
94-99%
97%
1.3-1.4
1.7
12
12
94%
0.072(4), 0.15
0.10(1), 0.28(2)
0.14(1), 0.34(4)
1.1
>100
15-27
12-17
94-96%
95-96%
1.5-2.0
2.6-2.9
0.75-0.8
a
b
See Table S1 for all polymerization data. LA = rac-lactide, ROH = total amount of alcohol present, i. e. co-crystallized alcohol + benzyl
alcohol added. c Final conversion after overnight reaction. This value does not represent reactivity. d Determined by linear regression of the
semi-logarithmic plot. Two values are provided if two linear regions were identified. In this case, the lower value always describes the rate at
the beginning of the reaction. Determined by GPC, see experimental section. Number of polymer chains per Cu calculated from
e
f
g
1
h
(conversion*[lactide]/[catalyst]*Mlactide)/Mn(GPC). Determined from decoupled H NMR, see experimental section. Crystal structure
contains co-crystallized methanol.
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Table 3. Details of X-ray Diffraction Studies
1
2
3
4
5
6
Formula
Mw (g/mol).
T (K); F(000)
Crystal System
Space Group
Unit Cell:
b (Å)
C11H18CuN4O5 C12H18CuF3N3O5S C11H14Cl2CuN4O4
C14H23CuN5O8 C45H82Cu2N10O17 C16H24CuN2O4
349.83
150; 724
Monoclinic
P21/c
436.89
150; 1784
Monoclinic
P21/n
400.70
150; 812
Triclinic
P(-1)
452.91
150; 940
Monoclinic
P21/n
1162.28
150; 2464
Orthorhombic
Pna21
371.91
150; 1560
Monoclinic
P21/c
a (Å)
9.1046(4)
14.6096(6)
10.8435(4)
90
20.6661(8)
7.2817(3)
24.2161(9)
90
7.1434(7)
10.8066(13)
19.909(2)
96.025(8)
90.517(8)
98.325(9)
1511.8(3); 4
10.067
9.5497(5)
14.8857(7)
13.4561(6)
90
20.8207(13)
8.3864(6)
33.203(2)
90
9.7008(10)
18.6698(17)
23.501(3)
90
c (Å)
 (°)
102.117(2)
90
109.637(2)
90
98.530(3)
90
90
101.775(7)
90
 (°)
90
 (°)
V (Å3); Z
 (mm–1).
1410.21(10); 4
8.542
3432.2(2); 8
3.511
1891.68(16); 4
6.543
5797.5(7); 4
4.353
4166.7(7); 8
5.751
Absorption correction
 range (°)
multiscan
4.3-60.9
1.0
multiscan
2.4-72
multiscan
3.6-42.4
multiscan
2.6-60.7
1.0
multiscan
3.9-59.9
0.98
multiscan
2.7-42.0
0.99
Completeness
1.0
0.96
17672; 0.0380
3198; 0.0567
0.0530
122587; 0.1259
72266; 0.1724
0.1071
7438; 0.1500
3106; 0.1205
0.1648
25277; 0.0670
4351; 0.0935
0.0812
45433; 0.1881
12938; 0.1750
0.1461
23048; 0.1223
4372; 0.1797
0.1289
Collected refl.; R
Unique refl.; Rint
R1(F) (I > 2(I))
wR(F2) (all data)
GoF(F2); Flack-x
Res. electron density
0.1139
0.3005
0.4410
0.2140
0.4089
0.3823
1.15; -
1.04; -
1.04; -
1.04; -
1.13; 0.43(2)
2.97; – 0.76
1.20; -
0.35; –0.45
2.81; –1.91
1.21; –0.67
0.72; –0.88
0.92; –0.47
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Scheme 1
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Scheme 4
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Figure 1
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Figure 2
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Figure 4
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