Impact of aryloxy initiators on the living and immortal polymerization of lactide

Article abstract of DOI:10.1039/c7dt00990a

This report describes two different methodologies for the synthesis of aryl end-functionalized poly(lactide)s (PLAs) catalyzed by indium complexes. In the first method, a series of para-functionalized phenoxy-bridged dinuclear indium complexes [(NNO)InCl]₂(μ-Cl)(μ-OPh_R) (R = OMe (1), Me (2), H (3), Br (4), NO₂ (5)) were synthesized and fully characterized. The solution and solid state structures of these complexes reflect the electronic differences between these initiators. The polymerization rates correlate with the electron donating ability of the phenoxy initiators: the para-nitro substituted complex 5 is essentially inactive. However, the para-methoxy variant, while less active than the ethoxy-bridged complex [(NNO)InCl]₂(μ-Cl)(μ-OEt) (A), shows sufficient activity. Alternatively, aryl-capped PLAs were synthesized via immortal polymerization of PLA with A in the presence of a range of arylated chain transfer agents. Certain aromatic diols shut down polymerization by chelating one indium centre to form a stable metal complex. Immortal ROP was successful when using phenol, and 1,5-naphthalenediol. These polymers were analysed and chain end fidelity was confirmed using ¹H NMR spectroscopy, MALDI-TOF mass spectrometry, and UV-Vis spectroscopy. This study shed light on possible speciation when attempting to generate PLA-lignin copolymers.

Full text of DOI:10.1039/c7dt00990a

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Impact of aryloxy initiators on the living and
immortal polymerization of lactide†
Cite this: DOI: 10.1039/c7dt00990a
L.-E. Chile,a,b T. Ebrahimi,a,b A. Wong,a D. C. Aluthge,a S. G. Hatzikiriakosb and
a
P. Mehrkhodavandi
*
This report describes two different methodologies for the synthesis of aryl end-functionalized poly
(lactide)s (PLAs) catalyzed by indium complexes. In the first method, a series of para-functionalized
phenoxy-bridged dinuclear indium complexes [(NNO)InCl]2(μ-Cl)(μ-OPhR) (R = OMe (1), Me (2), H (3),
Br (4), NO2 (5)) were synthesized and fully characterized. The solution and solid state structures of these
complexes reflect the electronic differences between these initiators. The polymerization rates correlate
with the electron donating ability of the phenoxy initiators: the para-nitro substituted complex 5 is essen-
tially inactive. However, the para-methoxy variant, while less active than the ethoxy-bridged complex
[(NNO)InCl]2(μ-Cl)(μ-OEt) (A), shows sufficient activity. Alternatively, aryl-capped PLAs were synthesized
via immortal polymerization of PLA with A in the presence of a range of arylated chain transfer agents.
Certain aromatic diols shut down polymerization by chelating one indium centre to form a stable metal
complex. Immortal ROP was successful when using phenol, and 1,5-naphthalenediol. These polymers
were analysed and chain end fidelity was confirmed using 1H NMR spectroscopy, MALDI-TOF mass
spectrometry, and UV-Vis spectroscopy. This study shed light on possible speciation when attempting to
generate PLA-lignin copolymers.
Received 18th March 2017,
Accepted 1st May 2017
DOI: 10.1039/c7dt00990a
rsc.li/dalton
architectures. One way to generate architectural density in
immortal ROP systems is to include lignocellulosic alcohols.
Introduction
Living ring opening polymerization with a chain transfer agent
The lignocellulosic biorefinery industry has been expand-
(also known as immortal ring opening polymerization)1 can be ing8 and now provides researchers access to a large number of
an efficient method of generating aryl-functionalized PLA. The bio-based composite materials9 through blending10 and co-
chain transfer step is reversible and faster than the propagat- polymerization.10c One interesting case is the use of lignin
ing step, it revives the propagating species, and the chain trans- functionalized PLA11 as an alternative to the widely-used
fer agent (CTA) becomes the polymer chain-end.2 Immortal phenol formaldehyde resins and adhesives.12 Understanding
ring opening polymerization (ROP) has been reported for a and controlling the reactivity of phenolic13 bioproducts as
range of monomers3 and has been used to generate a variety CTAs in immortal polymerization can expand their application
of end-functionalized polymers2b,4 with diverse structures5 and scope.
properties.6 Along with highly tunable catalysts,3f,7 immortal
The CTAs used in immortal polymerization are almost
polymerization has been a powerful methodology for generat- exclusively aliphatic alcohols. The use of aromatic alcohols as
ing multi-functionalized biodegradable polymers with varied CTAs6b,7a,11e,14 or phenoxy derivatives as initiators,15 for the
ROP of lactones has been underexplored due to their lower
nucleophilicity.15c Byers et al.14c have reported the use both of
internal and external phenoxy derivatives with their bis(imido)
pyridine iron complexes and found a decreasing trend in
aDepartment of Chemistry, University of British Columbia, Vancouver,
British Columbia, Canada. E-mail: mehr@chem.ubc.ca
bDepartment of Chemical and Biological Engineering, University of British Columbia,
Vancouver, BC, Canada. E-mail: savvas.hatzi@ubc.ca
activity with the electron with-drawing character of the
phenols (Chart 1B). There are few examples of main group
complexes with aryl-initiator ligands that are active for the
ROP of lactones.16
We have been studying the polymerization behaviour of
dinuclear indium complexes17 capable of stereoselective18
living19 and immortal polymerization20 of cyclic esters. In par-
†Electronic supplementary information (ESI) available: Solution and solid state
characterization of complexes, living ring-opening polymerization data of rac-
lactide, kinetic investigations, immortal ring-opening polymerization data of rac-
lactide with catalyst A and various chain transfer agents, and chain-end fidelity
data. CCDC 1526395–1526399. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c7dt00990a
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Chart 1 Previously reported [(NNO)InCl]2(μ-Cl)(μ-OEt) (A)17a,c,19b and
bis(imido)pyridine iron complex (B).14c
ticular, using dinuclear indium catalyst A supported by chiral
diaminophenolate ligands (Chart 1A), we reported the first
example21 of the formation of multiblock of PLAs with
different microstructures.22 Following these successes we have
been attempting to establish structure–property relationships
for these materials by studying the effect of tacticity23 and
architecture20b on the mechanical and rheological properties
of PLA.
Scheme 1 Synthesis of dinuclear indium complexes [(NNO)-
InCl]2(μ-Cl)(μ-OPhR) (R = OMe, Me, H, Br, NO2).
In this study we aim to explore the role of aryloxy initiators
on the living and immortal polymerization of lactide with the
ultimate aim of incorporating complex arylated bioproducts in
this process. Herein, we report the synthesis of a family of
indium complexes with para-substituted phenoxy initiators
and investigate the effect of these substitutions on the living
ring-opening polymerization of lactide. We then explore the
immortal ring-opening polymerization of lactide with a range
of arylated alcohols as CTAs and explore possible limitations
of the system in incorporating more complex species such as
diols.
and 4.7 ppm) and the bridging phenolic protons Ph–H (multi-
plets around 6–8 ppm) (Fig. S20 and 21†).
Single crystals of complex 5 synthesized from racemic
ligands can be obtained from slow evaporation of toluene
at room temperature and analyzed by single crystal X-ray
crystallography (Fig. 1). Single crystals of complexes 2–4 can be
obtained in a similar way; their structures are included in the
ESI (Fig. S27–31 and Table S1†). Complexes 2–5 are asym-
metric homochiral dinuclear species with a configuration
similar to all related mixed bridged species generated with this
ligand family (Fig. 1 and Table S1†).17a,c,d,19b,20b The phenoxy-
bridged complexes 2–5, with increasing electron deficiency of
the bridging phenol, show longer In–O3 bonds than the
alkoxy-bridged complex A, (2.182 Å (2) versus 2.202 Å (5)). The
Results and discussion
Synthesis and characterization of metal complexes
A series of phenoxy bridged complexes, analogous to complex
A, were synthesized to investigate the role of aryl substituents
on the ROP of lactide. The reaction of previously reported
17a,c,19b
( )-(NNO)InCl2
and KOPhR (PhR = para-C6H4R; R =
OMe, Me, H, Br, NO2) forms the asymmetrically bridged com-
plexes [(NNO)InCl]2(μ-Cl)(μ-OPhR) (R = OMe (1), Me (2), H (3),
Br (4), NO2 (5)) (Scheme 1, Route 1). Alternatively, these com-
plexes can be synthesized by reaction of [(NNO)InCl]2(μ-Cl)-
(μ-OEt) (A) with a phenol through an exchange reaction
(Scheme 1, Route 2). In Route 1, reactions involving electron
poor phenoxy substituents favor the formation of the mono
phenoxy complexes 4 and 5, which are obtained in higher
yields than more electron rich complexes 1 and 2 which form
bis-phenoxy bridged dinuclear complexes readily.
Complexes 1–5 have been characterized by 1H, 13C{1H},
1
1H–1H COSY, and H–13C HSQC NMR spectroscopy and show
similar structures to A and related compounds (see ESI†).17a,c,
The H NMR spectra of complexes 1–5 are consistent
with a mono-phenoxy bridged species with characteristic
Fig. 1 Molecular structure of [(NNO)InCl]2(μ-Cl)(μ-OPhNO2
depicted with ellipsoids at 50% probability (H atoms and all solvent
molecules omitted for clarity). Complexes 2–4 have similar structures,
)
(5),
d,19b,20b
1
signals for the backbone NH–CH2–Ar protons (doublets at 3.7 see ESI.†
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para-substituted phenoxide groups are weaker nucleophiles days (Table 1 entries 18–22). These observations are in line
compared to ethoxide, which leads to weaker bonds to the with those reported for complex B and its analogues, where a
indium center. An associated shortening of the In–Cl3 bond is similar change in activity was shown as the electron-withdraw-
also observed, (2.621 Å (2) versus 2.618 Å (5)) implying that this ing ability of the para substituent was altered.14c The phenoxy-
bond strengthens to alleviate the electron deficiency of the bridged catalysts all show slower initiation than catalyst A.
indium metal center. In addition, with increasing electron
We can confirm that the phenoxy initiators become
deficiency of the phenol, the deviation from octahedral geo- polymer chain ends through the coordination insertion mech-
metry increases: the O3–In–Cl3 angle becomes less linear anism. Chain end analysis of polymer samples generated with
(171.8° (2) to 166.6° (5)) and the O2–In–O3 bond becoming catalyst 3 (50 equiv. rac-LA) by 1H NMR spectroscopy shows
more acute (95.8° (2) to 92.5° (5)).
aryl signals which do not belong to free phenol or to 3 (Fig. 2).
Living ring-opening polymerization (ROP) of lactide using
phenoxy-bridged indium catalysts 1–5
The efficiency of the different initiators 1–5 for the ring-
opening polymerization of up to 2000 equivalents of racemic
lactide (rac-LA) was examined at room temperature (Table 1
and Fig. S32†). Phenoxy-bridged complexes display less control
over the polymerization of rac-LA compared to catalyst A, giving
polymers with higher than expected molecular weights at low
monomer loadings, which is indicative of a slow initiation step.
At higher monomer loadings, the discrepancy between the
expected and experimental molecular weights decreases
(Fig. S32†). Catalysts with electron rich bridging phenols (1–2)
show the best control over molecular weight, conversely cata-
lysts 4 and 5 with electron deficient bridging phenols give poly-
mers with molecular weights which largely deviate from the
theoretical molecular weights. In addition, as expected for the
least nucleophilic initiator, the para-nitrophenol bridged
complex 5 is significantly less active than the other four
Fig. 2 1H NMR spectra (CDCl3, 25 °C) of free phenol (red spectrum)
overlaid with 1H NMR spectra (CDCl3, 25 °C) of [(NNO)InCl]2(μ-Cl)-
(μ-OPhH) (3) (green) and 1H NMR spectra (CDCl3, 25 °C) of the polymer-
phenoxy catalysts, only reaching 10–20% conversion after 3–4 ization reaction mixture (blue, Table 1 entry 10).
Table 1 Living ring-opening polymerization data of rac-lactide with complexes 1–5
Catalyst
Time (h)
Conversiona (%)
[M] : [I]
Mn,theo b (g mol−1
)
Mn,GPC b (g mol−1
)
Đc
1
2
3
4
5
6
7
8
1
1
1
1
2
2
2
2
2
3
3
3
3
4
4
4
4
5
5
5
5
5
18
18
18
18
18
18
18
18
18
4
78
93
95
97
86
89
97
97
95
21
95
98
98
12
93
97
98
12
8.6
11
17
23
58
260
560
1100
54
270
500
990
2070
49
260
510
1020
56
270
540
1080
53
6550
35 020
76 400
152 000
6690
34 300
71 200
139 000
284 000
1480
35 200
72 200
145 000
990
36 700
75 900
153 000
930
54 600
80 500
98 740
139 100
52 840
87 150
118 400
153 100
225 000
6180d
118 500
142 300
171 500
23 740
166 700
230 600
287 800
10 100
13 500
42 800
80 870
71 560
1.02
1.01
1.01
1.03
1.04
1.06
1.11
1.08
1.01
NA
1.01
1.02
1.01
1.04
1.02
1.01
1.01
1.01
1.22
1.14
1.27
1.18
9
10
11
12
13
14
15
16
17
18
19
20
21
22
4
4
4
18
18
18
18
72
72
72
72
96
240
490
1000
1010
3020
7600
25 000
33 400
Reactions were carried out in CH2Cl2 at 25 °C. a Monomer conversions determined from 1H NMR spectra. b Calculated from Mn,theo = (144 gmol−1
× conversion × [LA]/[initiator]) + MROPh.
c Molecular weights were determined by GPC-LALLS (gel permeation chromatography-low angle laser
light scattering) to the polystyrene standard calibration curve via the Mark-Houwink equation in THF at 25 °C ([η] = KMa, while [η] = intrinsic vis-
cosity, M = molecular weight, and K and a are Mark-Houwink parameters K = 1.832 × 10−4 dL g−1, and a = 0.69 dn/dc = 0.044 mL g−1). THF 4 mg
mL−1 and flow rate = 0.5 mL min−1 d Molecular weight determined from MALDI-TOF analysis.
.
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MALDI-TOF mass spectral analysis revealed a major series of by the bridging ethoxy moiety.24 The longer initiation period
peaks separated by 144 mass units which can be attributed to of the phenoxy bridged catalysts can be attributed to the lower
PLA with phenol at the end-group (Fig. S35†).
nucleophilicity of the phenoxy compared to alkoxy moieties
The tacticity of polymers formed from the aryl initiators leading slower activation and a lower concentration of the
can be determined by inspecting the 1H{1H} NMR spectra. active catalytic species in reaction medium. After initiation,
From analysis of defect tetrad patterns, we verified that the propagation rates are identical. This phenomenon also causes
stereo-control mechanism is maintained when moving from the observed higher-than-expected molecular weights.
alkyl to aryl bridging moiety (Fig. S34†).
Immortal ring-opening polymerization (ROP) of lactide with
catalyst A and aromatic alcohols
Polymerization of rac-LA using catalysts 1–4 ([LA] : [I] =
50 : 1) was monitored using 1H NMR spectroscopy at room
temperature. The plots of ln[LA] versus time for all catalysts
One of the main goals of this study is to investigate the use of
show an initiation period followed by a linear propagation aromatic alcohols in the immortal ROP of rac-LA. Shifting
period. The initiation times correlate with the electron- from alkyl alcohols to using aryl alcohols has an effect on the
deficiency of the bridging phenoxide, thus catalyst 1 displays a
initiation step of the living ROP (as shown above). Therefore,
shorter initiation period than catalysts 2–4 (Fig. 3). The esti- we were interested in examining the influence of phenols on
mated rates of propagation for 1–4 are one order of magnitude the chain transfer reaction needed to achieve immortal ROP.
slower than that for catalyst A (Table 2).17a
To probe the kinetics of this exchange we followed the substi-
Previously reported computational studies on the polymer- tution reaction between catalyst A and phenol (HOC6H5), para-
ization of lactide isomers with catalyst A showed that the rate methoxyphenol
(HOC6H4OMe)
and
para-bromophenol
1
determining step is the insertion into the carbonyl C–O bond
(HOC6H4Br) by in situ H NMR spectroscopy. A 1 : 1 mixture of
[A] : [aryl alcohol] was dissolved in CDCl3 and the loss of the
bridging ethoxy moiety for A was monitored at room tempera-
ture for 60 minutes (Fig. 4 and S37†). Plots of ln[A] versus time
show an initial decrease in the [A] followed by a steady state.
The slope of this graph gives an indication of the chain-trans-
fer step (6.8(4) × 10−4 s−1 ([A] + phenol)) which is on the same
order as the propagation rate kobs, for catalysts 1–4.
The immortal ring-opening polymerization of rac-LA was
attempted with a variety of aromatic alcohols as chain transfer
agents (CTAs) at room temperature (Chart 2 and Table 3).
Solutions of rac-LA and CTA in dichloromethane were prepared
under an inert atmosphere. Subsequently, complex A was
Fig. 3 Plot of ln[LA] versus time for polymerization of rac-LA catalysed
with A ( ) 1 ( ) 2 ( ), 3 ( ) and 4 ( ). All reactions were carried out with
50 equiv. of LA in CD2Cl2 at 25 °C with 1,3,5-trimethoxybenzene (TMB)
as internal standard. [A] = 0.0046 M, [LA] = 0.22 M. [1] = 0.0039 M, [LA]
= 0.19 M. [2] = 0.0043 M, [LA] = 0.21 M. [3] = 0.0037 M, [LA] = 0.22
M. [4] = 0.0041 M, [LA] = 0.22 M.
Table 2 Propagation rates for the ROP of rac-LA with 1–4
Catalyst
kobs (×10−4 s−1
)
1
2
3
4
5
A
1
2
3
4
18.9(5)
8.5(8)
5.3(2)
3.3(2)
1.1(1)
Fig. 4 Plot of ln[A] versus time for exchange between catalyst A,
Carried out in an NMR tube at 25 °C with 1,3,5-trimethoxybenzene
(TMB) as internal standard with 50 equivalents of LA in CD2Cl2 and
followed by 1H NMR spectroscopy. [A] = 0.0046 M, [LA] = 0.22 M.
[1] = 0.0039 M, [LA] = 0.19 M. [2] = 0.0043 M, [LA] = 0.21 M. [3] = 0.0037 M,
[LA] = 0.22 M. [4] = 0.0041 M, [LA] = 0.22 M. The value of kobs was deter-
mined from the slope of ln[LA] versus time, averaged from at least 3 experi-
ments (Fig. S36).
HOC6H4OMe ( ) and HOC6H4Br ( ) and HOC6H5
( ). Reactions carried
out in an NMR tube (CDCl3, 25 °C). 1,3,5-Trimethoxybenzene (TMB) was
used as internal standard. [A] = 0.0046 M, [HOC6H4OMe] = 0.0050 M,
[HOC6H4Br] = 0.0059 M, [HOC6H5] = 0.00411 M. The value of kexchange
was determined from the slope of ln[A] versus time, averaged from three
experiments.
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Chart 2 Aryl alcohols used as chain transfer agents (CTAs) for the
immortal ROP of rac-LA.
added and the resulting homogenous mixtures and further
reacted at room temperature. Catalyst to CTA ratios were kept
constant at 1 : 20, while catalyst to monomer ratios were varied
from 1050 to 21 000 to generate polymer chains from 50 to
1000 units in length. The resulting polymers, generated from
ethoxy-bridged A, using phenol (Ph) and 1,5-naphthalenediol
(1,5-Nap) as CTA’s, showed excellent control over molecular
weight and dispersity, compared to the polymerization reac-
tions with aryloxy initiators (Table 2 and Fig. S38†). End-group
Fig. 5 Plot of molecular weight and dispersity with respect to conver-
sion immortal polymerization of lactide with
A
with phenol
([LA] : [A] : [phenol] = 10 500 : 1 : 20).
1
fidelity was confirmed by H NMR spectroscopy, UV-Vis spec-
naphthalenediol completely hindered the polymerization
activity. Previously reported computational studies showed
that both metal centres are active during polymerization with
complex A.24 Chelation of the diols to one or both metals
would be expected to hinder polymerization.
We were able to isolate one such species. Reaction of A with
2 equivalents of dichlorophene in toluene at room temperature
for 16 h forms the dinuclear complex [(NNO)InCl]2(μ-Cl)-
(κ2,μ-diClPh) (6) (Scheme 2). The 1H NMR spectrum of
complex 6 shows separate signals for each ligand on the two
metal centres, indicating a break-down in symmetry compared
to the previous complexes in the series (Fig. S22†).
troscopy, and MALDI-TOF mass spectroscopy (Fig. S40–46†).
The 1H NMR spectra show peaks in the aromatic region corres-
ponding to the aryl-chains ends (Fig. S40–43†). The inte-
grations of the aromatic peaks and the methylene proton
signals (5.10–5.25 ppm) are in agreement with the calculated
molar ratios of [LA] : ([A] + [CTA]) (Table 3).
To probe the immortal ROP reaction further, a polymeriz-
ation with a ratio of 10 500 : 1 : 20 ([LA] : [A] : [phenol]) in
CH2Cl2 at room temperature was performed and aliquots
taken at regular intervals over 8 hours. The extracted aliquots
were quenched and precipitated with wet methanol and ana-
lysed using GPC analysis (Fig. 5 and S39†). We observed a
linear growth in molecular weight with respect to conversion
which is expected of a living process.
Single crystals, suitable for X-ray crystallography, were
obtained from the slow diffusion of toluene. The solid state
structure of 6 shows a deprotonated dichlorophene molecule
chelated to one indium centre with one of the oxygen atoms
Of the diols, only 1,5-naphthalenediol was incorporated
into the polymer chain; the biphenol, dichlorophene and 1,8-
Table 3 Molecular weight data for immortal ring-opening polymerization of rac-LA with various aryl chain transfer agents
b
b
Mn,theo
Mn,GPC
Entry
CTA
Time (h)
Conversiona (%)
[LA] : ([A] + [CTA])
(g mol−1
)
(g mol−1
)
Đc
1
Dichlorophene (diClPh)
Dichlorophene (diClPh)
Biphenol (biPh)
1,8-Naphthalenediol (1,8-Nap)
1,8-Naphthalenediol (1,8-Nap)
1,5-Naphthalenediol (1,5-Nap)
1,5-Naphthalenediol (1,5-Nap)
1,5-Naphthalenediol (1,5-Nap)
Phenol (Ph)
168
65
168
168
65
18
18
18
18
18
18
0
44
0
71
79
53
45
26
—
—
—
—
—
—
2d
3
5310
3155e
—
—
—
4
0
—
5d
6
33
93
99
98
99
92
98
97
1400
8754e
14 380
109 100
197 800
8750
—
91
12 198
124 853
244 640
7084
33 776
139 972
70 322
1.04
1.04
1.01
1.01
1.00
1.01
1.00
7
8
9
10
11
12
875
1732
49.8
255
993
503
Phenol (Ph)
Phenol (Ph)
Phenol (Ph)
30 390
116 500
54 410
18
Reactions carried out in CH2Cl2 under a nitrogen atmosphere at room temperature. a Monomer conversions determined from 1H NMR spectra.
b Calculated from Mn,theo = (144 gmol−1 × conversion × [LA]/([A] + [CTA])) + MCTA c Molecular weights were determined by GPC-LALLS (gel per-
meation chromatography-low angle laser light scattering) to the polystyrene standard calibration curve via the Mark-Houwink equation in THF at
.
25 °C ([η] = KMa, while [η] = intrinsic viscosity, M = molecular weight, and K and a are Mark-Houwink parameters K = 1.832 × 10−4 dL g−1
,
and a = 0.69 dn/dc = 0.044 mL g−1). THF 4 mg mL−1 and flow rate = 0.5 mL min−1 d Reactions carried out in THF. e Molecular weight obtained
.
from MALDI-TOF mass spectra.
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correlation between electron deficiency of the bridging phenol
for the aryloxide-bridged complexes 1–5, and catalytic activity
towards the living ring-opening polymerization of rac-lactide,
was investigated. In situ studies showed that increasing elec-
tron deficiency of the bridging phenol leads to an associated
increase in the initiation period and decrease in the rate of
propagation. This effect is attributed to the lower nucleophili-
city of the phenolic oxygen, which hinders the initial insertion
into the monomer ester C–O bond and reduces the concen-
tration of active catalyst in solution, leading to higher than
expected molecular weights. Although this effect is not surpris-
ing, the electron rich aryloxy species showed reaction rates
only half that of the ethoxy catalysts, showing that these moi-
eties have the potential to be active in catalysis.
Scheme 2 Synthesis of complex 6.
Phenols were also used in tandem with ethoxy-bridged
complex A to investigate the use of aromatic alcohols as chain
transfer agents in the immortal ring-opening polymerization
of lactide. Aromatic diols inhibit polymerization by chelating
indium centres to form a stable metal complex, however in
coordinating solvents these chelating metal complexes are dis-
favoured and polymerization can proceed. Immortal ROP was
successful when using phenol, and 1,5-naphthlaenediol.
Polymers with well-controlled molecular weights and mole-
cular weight distributions were generated and the aryl chain
1
ends were confirmed using H NMR, MALDI-TOF and UV-Vis
analyses.
Fig. 6 Molecular structure of [(NNO)InCl]2(μ-Cl)(κ2,μ-diClPh) (6)
depicted with ellipsoids at 50% probability (H atoms and all solvent
molecules omitted for clarity).
These results show that both alkyl and electron rich aryl
alcohols can be active chain transfer agents with this family of
dinuclear indium complex. Ultimately, we are interested in uti-
lizing phenol rich bioproducts as chain transfer agents in
immortal ring opening polymerization reactions. Our future
efforts will be focused on expanding this fundamental work to
complex arylated molecules such as lignin.
bridging both indium centres (Fig. 6). The related complex
[(NNO)InCl]2(μ-Cl)(κ2,μ-biPh) can be generated from an ana-
logous reaction with biphenol. Although [(NNO)InCl]2(μ-Cl)
(κ2,μ-biPh) was not further characterized, we assume a similar
structure to that of 6 based on the 1H NMR spectrum (Fig. S26†).
The chelating initiator explains the lack of reactivity in this
system. In contrast, 1,5-naphthalenediol with hydroxyl groups
on opposite sides of the naphthalene ring, is active as a chain
transfer agent (Table 2 and Fig. S38c, S45†). To further
confirm that these stable chelate species were inhibiting reac-
tivity, immortal ROP using A and CTAs dichlorophene and 1,8-
naphthalenediol were conducted in THF. Under these con-
ditions polymers were formed (Table 2, entries 2 and 5). We
attribute this effect to the blocking of a coordination site by
solvent molecules which inhibits the formation of the chelat-
ing complex. Additionally, reaction of the stable chelate
complex 6 with rac-LA in THF also generated polymer, albeit
with only 30% conversion after 5 days, showing that coordinat-
ing solvents can disrupt the chelate leading to polymer growth
(Fig. S47–48†).
Experimental
General methods
All the air and moisture sensitive manipulations were carried
out in an MBraun glovebox or using standard Schlenk line
techniques. A Bruker Avance 300 or 400 MHz spectrometer was
used to record H NMR spectra. H NMR chemical shifts are
given in ppm versus residual protons in deuterated solvents as
follows: δ 7.27 CDCl3, δ 5.31 CD2Cl2.
Molecular weights, hydrodynamic radii and intrinsic visco-
sities were determined by GPC-LALLS using an Agilent liquid
chromatograph equipped with an Agilent 1200 series pump
and autosampler, three Phenogel 5 μm Narrow Bore columns
(4.6 × 300 mm with 500 Å, 103 Å and 104 Å pore size), a Wyatt
Optilab differential refractometer, Wyatt tristar miniDAWN
(laser light scattering detector) and a Wyatt ViscoStar visco-
meter. The column temperature was set at 40 °C. A flow rate of
0.5 mL min−1 was used and samples were dissolved in THF
1
1
Conclusions
In this work we explored the role of phenols as initiators and (ca. 4 mg mL−1). The measurements were carried out at laser
as chain transfer agents in the polymerization of lactide cata- wavelength of 690 nm, at 25 °C. The data were processed using
lysed by diaminophenolate-supported indium complexes. The the Astra software provided by Wyatt Technology Corp.
Dalton Trans.
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Diffraction measurements for X-ray crystallography were crystallized twice from a 2 : 3 mixture of CHCl3 : pentane to
1
made on a Bruker APEX DUO diffractometer with graphite give a white solid (45.0 mg, 28%). H NMR (400 MHz, CDCl3):
3
4
monochromated Mo-Kα radiation and a Bruker X8 diffracto- δ 7.45 (d, JH–H = 8.8 Hz, 2H, Ph–H), 7.08 (d, JH–H = 2.4 Hz,
4
3
meter with Mo-Kα microsource radiation. The structure was 2H, Ar–H), 6.60 (d, JH–H = 2.4 Hz, 2H, Ar–H), 6.47 (d, JH–H
=
2
solved by direct methods and refined by full-matrix least- 8.8 Hz, 2H, Ph–H), 4.68 (d, JH–H = 11.7 Hz, 2H, Ar–CH2–N),
squares using SHELXL-2016.25 Compounds (2) and (6) 3.54 (s, 3H, Ph–OCH3), 3.54(dd, 2H, JH–H = 1.76, Ar–CH2–N),
3
4
3
3
included disordered lattice solvent that could not be reason- 3.17 (d, JH–H = 7.6 Hz, 2H, NH), 2.98 (td, JH–H = 10.0, JH–H
=
ably modelled; as a result SQUEEZE26 was employed to gene- 1.8 Hz, 2H, DACH), 2.69 (s, 6H, N(CH3)2), 2.59–2.38 (m, 4H,
rate ‘solvent-free’ data sets for these two structures. Unless DACH), 1.93–1.85 (br m, 2H, DACH), 1.813(s, 6H, N(CH3)2),
specified, all non-hydrogen atoms were refined with an- 1.34 (s, 9H, C(CH3)3), 1.22 (s, 9H, C(CH3)3), 1.17–0.86 (m, 2H,
isotropic displacement parameters, and all hydrogen atoms DACH). 13C NMR (151 MHz, CDCl3): δ 162.4, 153.9, 152.8,
were constrained to geometrically calculated positions but 138.2, 135.9, 129.0, 128.2, 125.9, 125.2, 123.9, 118.8, 113.3,
were not refined.
64.4, 55.1, 53.0, 44.2, 37.8, 35.1, 35.1, 33.7, 30.7, 29.9,
Materials. THF, toluene and diethyl ether was taken from an 29.7, 24.9, 24.8, 21.8. Elemental analysis calculated for
IT Inc. solvent purification system with activated alumina C53H85Cl3In2N4O4: C, 54.03; H, 7.27; Cl, 9.03; In, 19.49;
columns and degassed before use. HPLC grade dichloro- N, 4.76; O, 5.43; found: C, 53.95, H, 7.09, N, 4.79.
methane was purchased from Fischer Chemicals and was
Synthesis of [(NNO)InCl]2(μ-Cl)(μ-OPhMe
) (2) via salt
dried over CaH2, transferred under vacuum and degassed methathesis. To
a
suspension of (NNO)InCl2 (153 mg,
before use. CDCl3 was purchased from Cambridge Isotope 0.280 mmol) in toluene (5 mL) was added a suspension of
Laboratories Inc. and dried over CaH2, transferred under potassium para-methoxyphenoxide (61.0 mg, 0.417 mmol) in
vacuum and degassed through three freeze–pump–thaw cycles toluene (2 mL) under N2. The solution was made up to 8 mL
before use. Pentane, acetonitrile and, CD2Cl2 were purchased and stirred at ambient temperature for 18 h. After this time
from Sigma-Aldrich and dried over CaH2, transferred under the solution was a cloudy yellow colour and was concentrated
vacuum and degassed before use. Indium(III) trichloride, to give a pale yellow solid. The solid was the dissolved in 2 mL
1,5-naphthalenediol, 1,8-naphthalenediol, p-methoxyphenol, of CHCl3 and filtered through Celite. The solution was con-
p-bromophenol, p-nitrophenol, and p-methylphenol were pur- centrated to give an off-white solid which was then stirred
chased from Sigma-Aldrich and were used as received. Phenol in acetonitrile. The crude product was then isolated by
was purchased from Fisher Chemicals and used as received. filtration and finally recrystallized from a 2 : 3 mixture of
2,2-methylenebis(4-chlorophenol) (95%) (dichlorophene) was CHCl3 : pentane to give a white solid (34.1 mg, 21%). X-ray
purchased from Alfa Aesar and recrystallized from hot toluene quality crystals were grown from slow evaporation of toluene.
3
before use. 2,2′-Biphenol was purchased from ACOS and used 1H NMR (400 MHz, CDCl3): δ 7.40 (d, JH–H = 8.2 Hz, 2H,
4
3
as received. Racemic lactide was purchased from PURAC Ph–H), 7.10 (d, JH–H = 2.4 Hz, 2H, Ar–H), 6.69 (d, JH–H
=
4
America Inc. and was recrystallized thrice from hot dried 8.2 Hz, 2H, Ph–H), 6.60 (d, JH–H = 2.4 Hz, 2H, Ph–H), 4.66
toluene prior to use. The racemic proligand 2,4-t-butyl-6- (d, JH–H = 12.3 Hz, 2H, Ar–CH2–N), 3.54 (d, 3H, JH–H
(((2-(dimethylamino)cyclohexyl)amino)methyl)phenol ( )-H(NNO), 12.3 Hz, 2H, Ar–CH2–N), 3.20 (d, JH–H = 10.6 Hz, 2H, NH),
2
2
=
4
3
3
dichloro complex (NNO)InCl2 and [(NNO)InCl]2(μ-Cl)(μ-OEt) 3.00 (td, JH–H = 11.4, JH–H = 3.5 Hz, 2H, DACH), 2.68 (s, 6H,
(A) can synthesized according to previously reported N(CH3)2), 2.58–2.40 (m, 4H, DACH), 1.92–1.80 (br m, 2H,
methods.17a,19b Complexes 1–5 were synthesized using similar DACH), 1.80 (s, 6H N(CH3)2), 1.34 (s, 9H, C(CH3)3), 1.22 (s, 9H,
methodology, see ESI.†
C(CH3)3), 1.17–0.86 (m, 2H, DACH). 13C NMR (151 MHz,
Synthesis of phenolic potassium salts. Inside a glovebox, a CDCl3): δ 162.5, 157.6, 138.3, 135.8, 128.8, 125.9, 123.8, 122.4,
suspension of potassium tert-butoxide (0.008 mg) in 3 mL of 118.9, 64.4, 53.2., 50.7, 44.3, 37.9, 35.1, 33.7, 31.7, 30.8,
Et2O was added to a solution of phenol in Et2O (0.098 g, 29.9, 24.9, 24.8, 21.9, 20.6. Elemental analysis calculated
0.234 M). This was stirred at room temperature for one hour. for C53H85Cl3In2N4O3: C, 54.77; H, 7.37; Cl, 9.15; In, 19.76;
After this time the solid was filtered off, washed with aceto- N, 4.82; O, 4.13; found: C, 55.05; H, 7.34; N, 5.20.
nitrile and dried under vacuum.
Synthesis of [(NNO)InCl]2(μ-Cl)(μ-OPhOMe) (1) via salt meta- methathesis. To
Synthesis of [(NNO)InCl]2(μ-Cl)(μ-OPhH) (3) via salt
suspension of (NNO)InCl2 (111 mg,
a
thesis. To a suspension of (NNO)InCl2 (15 mg, 0.278 mmol) in 0.204 mmol) in toluene (5 mL) was added a suspension of
toluene (5 mL) was added a suspension of potassium para- potassium phenoxide (39.6 mg, 0.300 mmol) in toluene (2 mL)
methoxyphenoxide (57.3 mg, 0.353 mmol) in toluene (2 mL) under N2. The solution was made up to 8 mL and stirred at
under N2. The solution was made up to 8 mL and stirred at ambient temperature for 18 h. After this time the solution was
ambient temperature for 18 h. After this time the solution was a cloudy yellow colour and was concentrated to give a pale
a cloudy yellow colour and was concentrated to give a pale yellow solid. The solid was the dissolved in 2 mL of CHCl3 and
yellow solid. The solid was the dissolved in 2 mL of CHCl3 and filtered through Celite. The solution was concentrated to give
filtered through Celite. The solution was concentrated to give an off-white solid which was then stirred in acetonitrile. The
an off-white solid which was then stirred in acetonitrile. The crude product was then isolated by filtration and finally re-
crude product was then isolated by filtration and finally re- crystallized from a 2 : 3 mixture of CHCl3 : pentane to give a
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white solid (66.6 mg, 57%). X-ray quality crystals were grown 0.173 mmol) under N2. The solution was stirred for 18 h.
1
from slow evaporation of toluene. H NMR (400 MHz, CDCl3): Yellow precipitate appeared in the solution as the reaction
3
4
δ 7.54 (d, JH–H = 7.6 Hz, 2H, Ph–H), 7.08 (d, JH–H = 2.4 Hz, progressed. The solution was concentrated to yield a dark
3
4
2H, Ar–H), 6.93 (t, JH–H = 8.2, Ph–H) 6.59 (d, JH–H = 2.4 Hz, yellow solid, which was washed with ACN (3 × 2 mL) and con-
2H, Ar–H), 6.54 (t, JH–H = 7.6 Hz, 2H, Ph–H), 4.66 (d, JH–H
11.7 Hz, 2H, Ar–CH2–N), 3.55 (d, 3H, JH–H = 10.0 Hz, 2H, crystallized from 3 : 2 pentane/CH3Cl at −30 °C to yield
Ar–CH2–N), 3.20 (d, JH–H = 10.6 Hz, 2H, NH), 3.00 (td, JH–H
3
2
=
sequently dried under vacuum. The crude product was re-
2
4
3
=
product as short yellow crystalline needles (23.2 mg, 43%).
3
11.1, JH–H = 2.4 Hz, 2H, DACH), 2.70 (s, 6H, N(CH3)2), X-ray quality crystals were grown from slow diffusion of
2.59–2.39 (m, 4H, DACH), 1.93–1.84 (br m, 2H, DACH), 1.83 pentane into CH3Cl. 1H NMR (400 MHz, CDCl3) δ 7.85 (d,
(s, 6H N(CH3)2), 1.34 (s, 9H, C(CH3)3), 1.20 (s, 9H, C(CH3)3), 3J
= 8.9 Hz, 2H, Ph–H), 7.60 (d, JH–H = 9.0 Hz, 2H, Ph–H),
3
H–H
1.16–0.86 (m, 2H, DACH). 13C NMR (100 MHz, CDCl3): δ 162.4, 7.09 (s, 2H, Ar–H), 6.59 (s, 2H, Ar–H), 4.53 (d, JH–H = 12.6 Hz,
2
2
160.0, 138.2, 137.9, 129.0, 128.2, 125.9, 125.2, 123.9, 122.9, 2H, Ar–CH2–N), 3.56 (d, JH–H = 12.4 Hz, 2H, Ar–CH2–N), 3.17
120.2, 118.8, 64.4, 53.1, 50.6, 44.2, 37.8, 35.1, 33.7, 31.7, 30.7, (d, JH–H = 9.4 Hz, 2H, NH), 2.96 (t, JH–H = 10.6 Hz, 2H,
29.9, 24.9, 24.8, 12.8, 21.4. DACH), 2.69 (s, 6H, N(CH3)2), 2.58–2.38 (m, 6H, N(CH3)2),
4
3
Synthesis of [(NNO)InCl]2(μ-Cl)(μ-OPhBr) (4) via alkoxide- 1.35–1.27 (m, 22H), 1.20 (s, 18H, C(CH3)3). 13C NMR (151 MHz,
phenoxide exchange. To a solution of A (115 mg, 0.112 mmol) CDCl3): δ 166.9, 161.9, 138.3, 136.7, 129.0, 128.2, 126.0, 124.7,
in toluene (5 mL) was added p-bromophenol (59.2 mg, 124.2, 123.5, 118.5, 64.6, 53.2, 50.7, 44.3, 37.9, 35.1, 34.6, 33.8,
0.342 mmol) under N2. The solution was stirred for 18 h. 31.7, 30.8, 30.0, 29.9, 24.7, 21.8. Elemental analysis calculated
White precipitate slowly appeared as the reaction progressed. for C52H82Cl3In2N5O5: C, 52.34; H, 6.93; Cl, 8.91; In, 19.24;
After stirring, the solution was concentrated to yield a white N, 5.87; O, 6.70; found: C, 52.66, H, 6.97, N, 5.53.
solid, which was washed with ACN (3 × 2 mL) and con-
Synthesis of [(NNO)InCl]2(μ-Cl)(μ-OPhNO2) (5) via salt meta-
sequently dried under vacuum. The crude white product was thesis. To a suspension of (NNO)InCl2 (148 mg, 0.272 mmol)
recrystallized from 3 : 2 pentane/CH3Cl at −30 °C to afford in toluene (5 mL) was added a suspension of potassium para-
product as long white crystalline needles (60.2 mg, 47%). methoxyphenoxide (73.8 mg, 0.417 mmol) in toluene (2 mL)
Suitable crystals for X-ray crystallography were grown from under N2. The solution was made up to 8 mL and stirred at
slow evaporation of toluene. 1H NMR (600 MHz, CDCl3): δ 7.43 ambient temperature for 18 h. After this time the solution was
3
4
(d, JH–H = 8.7 Hz, 2H, Ph–H), 7.11 (d, JH–H = 2.6 Hz, 2H, a cloudy yellow colour and was concentrated to give a pale
3
4
Ar–H), 7.01 (t, JH–H = 8.7, Ph–H) 6.61 (d, JH–H = 2.6 Hz, 2H, yellow solid. The solid was the dissolved in 2 mL of CHCl3 and
2
Ar–H), 4.60 (d, JH–H = 12.8 Hz, 2H, Ar–CH2–N), 3.56 (d, 3H, filtered through Celite. The solution was concentrated to give a
2JH–H = 12.8 Hz, 2H, Ar–CH2–N), 3.17 (d, JH–H = 10.2 Hz, 2H, yellow solid which was then stirred in acetonitrile. The crude
4
3
3
NH), 2.98 (td, JH–H = 11.8, JH–H = 3.1 Hz, 2H, DACH), 2.84 product was then isolated by filtration and finally recrystallized
(s, 6H, N(CH3)2), 2.57–2.43 (m, 4H, DACH), 1.92–1.83 (br m, from a 2 : 3 mixture of CHCl3 : pentane to give a yellow solid
2H, DACH), 1.82 (s, 6H N(CH3)2), 1.34 (s, 9H, C(CH3)3), 1.23 (90.9 mg, 56%).
(s, 9H, C(CH3)3), 1.16–0.92 (m, 2H, DACH). 13C NMR
Synthesis of [(NNO)InCl]2(μ-Cl)(κ2,μ-diClPh) (6). To a solu-
(151 MHz, CDCl3): δ 162.2, 159.3, 138.2, 136.2, 131.0, 126.0, tion of A (83.3 mg, 0.076 mmol) in toluene (4 mL) was added
124.8, 124.0, 118.7, 112.6, 64.4, 53.1, 50.7, 44.2, 37.8, 35.1, dichlorophene (40.8 mg, 0.151 mmol) under N2. The solution
33.7, 31.7, 30.8, 29.9, 24.9, 24.8, 21.8. Elemental analysis calcu- was stirred for 18 h. White precipitate appeared in the solution
lated for C52H82BrCl3In2N4O3: C, 50.90; H, 6.74; Br, 6.51; as the reaction progressed. The solution was concentrated to
Cl, 8.67; In, 18.71; N, 4.57; O, 3.91; found: C, 51.07, H, 6.70, yield an off-white solid, which dissolved in CHCl3 and filtered
N, 4.37.
Synthesis of [(NNO)InCl]2(μ-Cl)(μ-OPhBr
methathesis. To
through Celite. The solvent was again removed and the white
(4) via salt solid washed with ACN (3 × 2 mL) and consequently dried
suspension of (NNO)InCl2 (148 mg, under vacuum. The crude product was recrystallized from 3 : 2
)
a
0.272 mmol) in toluene (5 mL) was added a suspension of pot- pentane/CH3Cl at −30 °C to yield product as a white powder
assium para-methoxyphenoxide (107 mg, 0.6200 mmol) in needles. X-ray quality crystals were grown from slow evapo-
1
toluene (2 mL) under N2. The solution was made up to 8 mL ration of toluene (50.8 mg, 52%). H NMR (400 MHz, CDCl3)
3
4
and stirred at ambient temperature for 18 h. After this time δ 7.58 (d, JH–H = 8.8 Hz, 1H, diClPh–H), 7.21 (d, JH–H
=
4
the solution was a cloudy yellow colour and was concentrated 2.3 Hz, 1H, Ar–H), 7.12 (d, JH–H = 2.3 Hz, 1H, Ar–H), 7.02
to give a pale yellow solid. The solid was the dissolved in 2 mL (d, JH–H = 2.9 Hz, 1H, diClPh–H), 7.01 (d, JH–H = 2.9 Hz, 1H,
of CHCl3 and filtered through Celite. The solution was diClPh–H), 6.97 (d, JH–H = 8.5 Hz, 1H, diClPh–H), 6.83
concentrated to give an off-white solid which was then stirred (d, JH–H = 2.7 Hz, 1H, diClPh–H), 6.76 (d, JH–H = 2.6 Hz, 1H,
in acetonitrile. The crude product was then isolated by diClPh–H), 6.74 (d, JH–H = 2.6 Hz, 1H, diClPh–H), 6.70
filtration and finally recrystallized from a 2 : 3 mixture of (d, JH–H = 2.3 Hz, 1H, Ar–H), 6.54 (d, JH–H = 2.3 Hz, 1H,
3
3
3
3
3
3
4
4
2
CHCl3 : pentane to give a white solid.
Ar–H), 5.41 (d, JH–H = 15.2 Hz, 1H, diClPh–CH2–PhCl), 4.63
2
3
Synthesis of [(NNO)InCl]2(μ-Cl)(μ-OPhNO2) (5) via alkoxide- (d, JH–H = 11.6 Hz, 2H, Ar–CH2–N), 3.65 (dd, JH–H = 13.2 Hz,
phenoxide exchange. To a solution of A (50.0 mg, 0.049 mmol) 3JH–H = 2.9 Hz, 2H, CH), 3.49 (d, JH–H = 12.6 Hz, 2H, Ar–CH2–
2
2
in toluene (4 mL) was added p-nitrophenol (24.1 mg, N), 3.37 (d, JH–H = 15.8 Hz, 1H, diClPh–CH2–PhCl), 3.13
Dalton Trans.
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3
3
(dd, JH–H = 13.2 Hz, JH–H = 2.6 Hz, 2H, CH), 2.91 (s, 3H, dissolved in THF (∼2 mL). Catalyst 6 (2.5 mg, 0.002 mmol) in
N(CH3)), 2.89 (m, 1H,), 2.62 (s, 3H, N(CH3)), 2.61(m, 1H), 2.59 1 mL of THF was then added. The reaction mixtures were
(m, 3H), 2.29 (m, 3H), 1.93 (s, 3H, N(CH3)), 1.88 (m, 5H), 1.72 stirred at room temperature. Every few hours a 0.5 mL aliquot
(s, 3H, N(CH3)), 1.41 (s, 9H, C(CH3)3), 1.32 (s, 9H, C(CH3)3), was taken to check conversion.
1.25 (s, 9H, C(CH3)3), 1.22 (s, 9H, C(CH3)3), 1.17–0.85 (m, 5H),
0.7 (d, J = 9.7 Hz, 1H). 13C NMR (151 MHz, CDCl3): δ 162.63,
157.8, 137.9, 135.4, 133.2, 132.7, 131.2, 130.6, 127.2, 126.8,
Acknowledgements
126.0, 124.7, 124.1, 121.7, 120.5, 99.9, 65.0, 64.2, 53.3, 44.8,
PM and SGH would like to acknowledge support from NSERC
44.2, 37.7, 35.9, 33.6, 31.7, 30.1, 29.7, 25.1, 24.9.
in the form of a joint Strategic Grant. TE would like to thank
Representative living ring-opening polymerization. Inside a
Brian Patrick for guidance with the crystallographic aspects of
glovebox under nitrogen, appropriate volumes of a standard
this work.
solution of catalyst in dichloromethane (20 mg into 1 mL) was
added to
a solution of rac-lactide in dichloromethane.
The solutions were stirred at room temperature for at least
18 hours before the solvent was evaporated. A small sample of
crude polymer was taken for H NMR analysis, the remaining
polymer was dissolved in minimal dichloromethane (∼2 mL)
and precipitated with cold methanol. The methanol was de-
canted off and the precipitation repeated twice. The isolated
polymer was then dried under vacuum for at least 24 hours
before further analysis.
Notes and references
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1 S. Asano, T. Aida and S. Inoue, J. Chem. Soc., Chem.
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(b) S. M. Guillaume and J. F. Carpentier, Catal. Sci.
Technol., 2012, 2, 898–906.
3 (a) M. J. Stanford and A. P. Dove, Macromolecules, 2009, 42,
141–147; (b) L. Li, B. Liu, D. Liu, C. Wu, S. Li and D. Cui,
Organometallics, 2014, 33, 6474–6480; (c) L. Clark,
M. G. Cushion, H. E. Dyer, A. D. Schwarz, R. Duchateau
and P. Mountford, Chem. Commun., 2010, 46, 273–275;
(d) W. Zhao, D. Cui, X. Liu and X. Chen, Macromolecules,
2010, 43, 6678–6684; (e) Y.-C. Liu, B.-T. Ko and C.-C. Lin,
Macromolecules, 2001, 34, 6196–6201; (f) L. F. Wang,
V. Poirier, F. Ghiotto, M. Bochmann, R. D. Cannon,
J. F. Carpentier and Y. Sarazin, Macromolecules, 2014, 47,
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X. Liu and D. Cui, Polym. Chem., 2014, 5, 4580; (h) Y. Wang,
W. Zhao, X. Liu, D. Cui and E. Y. X. Chen, Macromolecules,
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Sci. Technol., 2013, 3, 3268–3277; (o) V. Poirier, T. Roisnel,
S. Sinbandhit, M. Bochmann, J. F. Carpentier and
Representative immortal ring-opening polymerization.
Inside a glovebox under nitrogen, appropriate amounts of
rac-lactide were dissolved in dichloromethane. Appropriate
volumes of a standard solution of chain transfer agent in
dichloromethane was added and the solution stirred until
homogenous. A standard solution of catalyst A in dichloro-
methane (10 mg into 1 mL) was the added the reaction mix-
tures were stirred at room temperature for at least 18 hours
before the solvent was evaporated. A small sample of crude
polymer was taken for 1H NMR analysis, the remaining
polymer was dissolved in minimal dichloromethane (∼2 mL)
and precipitated with cold methanol. The methanol is
decanted off and the precipitation repeated twice. The isolated
polymer was then dried under vacuum for at least 24 hours
before further analysis.
Representative in situ living ring-opening polymerization.
Inside a glovebox under nitrogen, 0.25 mL of a standard solu-
tion of catalyst in d2-dichloromethane (20 mg, 0.004 M) was
added to a J-Young tube. This was frozen in liquid nitrogen
before 0.25 mL of d2-dichloromethane was added then frozen.
Finally, 0.5 mL of a standard solution of rac-lactide and tri-
methoxybenzene (internal standard) in d2-dichloromethane
(123 mg, 0.21 M) was added and frozen. The J-Young tube was
evacuated and thawed before insertion into the NMR probe.
Representative in situ phenoxy-ethoxy exchange. Inside a
glovebox under nitrogen, 0.50 mL of a standard solution of
catalyst in d2-dichloromethane (20 mg, 0.005 M) was added to
a J-Young tube. This was frozen in liquid nitrogen before
0.25 mL of d2-dichloromethane was added then frozen.
Finally, 0.25 mL of a standard solution of phenol and tri-
methoxybenzene (internal standard) in d2-dichloromethane
(2.5 mg, 0.005 M) was added and frozen. The J-Young tube was
evacuated and thawed before insertion into the NMR probe.
Living ring-opening polymerization with complex 6. Inside a
glovebox under nitrogen, rac-lactide (31 mg, 0.215 mmol) was
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