A novel approach to RE-OR bond from in situ reaction of rare earth triflates and sodium alkoxides: A versatile catalyst for living ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1016/j.polymer.2014.03.032

A series of rare earth triflates (RE(OTf)₃, RE = Sc, Y and Lu) were used for the first time as moisture-stable precursors to generate rare earth alkoxide complexes through an in situ reaction with sodium alkoxides (NaOR) in tetrahydrofuran. ¹H NMR and ¹³C NMR results confirmed the fast ligands exchange process and the formation of rare earth-oxygen (RE-OR) bond. The in situ formed catalysts displayed high reactivity toward living ring-opening polymerization (ROP) of ε-caprolactone (CL). For instance, Lu(OTf)₃/sodium isopropoxide (NaOⁱPr)-catalyzed ROP of CL with the [CL]₀/[NaO ⁱPr]₀/[Lu(OTf)₃]₀ feeding ratio of 300/3/1 produced poly(ε-caprolactone) (PCL) with controlled molecular weight (M_n,exp = 11.9 kDa vs M_n,theo = 11.8 kDa) and narrow polydispersity (PDI) of 1.08 within 3 min at 25°C. The kinetic studies and chain extension confirmed the controlled/living nature for the Lu(OTf)₃/NaOⁱPr-catalyzed ROP of CL. In addition, end-functionalized PCLs bearing vinyl or alkynyl group with narrow PDIs were obtained by using functional sodium alkoxides in the presence of Lu(OTf) ₃. ¹H NMR and MALDI-ToF MS analyses of the obtained PCLs clearly indicated the presence of the residue of OR groups at the chain ends. A coordination-insertion polymerization mechanism was proposed including a fast ligand exchange between Lu(OTf)₃ and NaOR giving the respective lutetium alkoxide complexes, and a CL insertion into RE-OR bond via acyl-oxygen cleavage.

Full text of DOI:10.1016/j.polymer.2014.03.032

Polymer 55 (2014) 2404e2410
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
A novel approach to REeOR bond from in situ reaction of rare earth
triflates and sodium alkoxides: A versatile catalyst for living
ring-opening polymerization of ε-caprolactone
a
a
b
a,
*
Lixin You , Zhiquan Shen , Jie Kong , Jun Ling
a
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University,
Hangzhou 310027, China
Shaanxi Key Laboratory of Macromolecular Science and Technology, School of Science, Northwestern Polytechnical University, Xi’an 710072, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 December 2013
Received in revised form
A series of rare earth triflates (RE(OTf) , RE ¼ Sc, Y and Lu) were used for the first time as moisture-stable
3
precursors to generate rare earth alkoxide complexes through an in situ reaction with sodium alkoxides
1
13
(
NaOR) in tetrahydrofuran. H NMR and C NMR results confirmed the fast ligands exchange process and
the formation of rare eartheoxygen (REeOR) bond. The in situ formed catalysts displayed high reactivity
toward living ring-opening polymerization (ROP) of ε-caprolactone (CL). For instance, Lu(OTf) /sodium
feeding ratio of 300/3/1
17 February 2014
Accepted 19 March 2014
Available online 27 March 2014
3
i
i
0 0 3 0
isopropoxide (NaO Pr)-catalyzed ROP of CL with the [CL] /[NaO Pr] /[Lu(OTf) ]
produced poly(ε-caprolactone) (PCL) with controlled molecular weight (Mn,exp
¼
11.9 kDa vs
Keywords:
Living ring-opening polymerization
Catalysis
ꢀ
M
n,theo ¼ 11.8 kDa) and narrow polydispersity (PDI) of 1.08 within 3 min at 25 C. The kinetic studies and
i
3
chain extension confirmed the controlled/living nature for the Lu(OTf) /NaO Pr-catalyzed ROP of CL. In
Rare earth triflates
addition, end-functionalized PCLs bearing vinyl or alkynyl group with narrow PDIs were obtained by
1
3
using functional sodium alkoxides in the presence of Lu(OTf) . H NMR and MALDI-ToF MS analyses of
the obtained PCLs clearly indicated the presence of the residue of OR groups at the chain ends. A co-
ordinationeinsertion polymerization mechanism was proposed including a fast ligand exchange be-
tween Lu(OTf)
3
and NaOR giving the respective lutetium alkoxide complexes, and a CL insertion into RE
eOR bond via acyl-oxygen cleavage.
Ó 2014 Elsevier Ltd. All rights reserved.
1
. Introduction
into two main categories. The first one is so-called direct synthesis
method, proceeding with the inhomogeneous and time-consuming
salt-exchange reaction in solvent using the respective rare earth
Over the past two decades, alkoxides or aryloxides of rare earth
metals have been designed and successfully evaluated in controlled/
living ring-opening polymerization (ROP) of lactones [1e5], lactides
chlorides (RECl
3
) and sodium alkoxides (NaOR) [23]. However, only
i
few rare earth alkoxides can be prepared and they, Y (meO)(O Pr)13
5
[
6e10] and other cyclic monomers [11e14]. The synthesized poly-
for example, have complicated structures and thus result in un-
predictable initiation efficiencies [24]. The other includes ligands
exchange reaction between alcohols and rare earth aryloxides
[25,26], rare earth tris(hexamethyldisilyl)amide [27,28] and rare
earth alkyl complexes [29e31]. The in situ formed rare earth alk-
oxides initiate the controlled ROP of ε-caprolactone (CL) and pre-
vent the alkoxides from clustering. However, the rare earth
complexes precursors themselves are very much moisture-sensitive
and prepared in a tedious way similar to the first method. In addi-
tion, the exchange reactions produce proton-involved aromatic
alcohol (ArOH) or 1,1,1,3,3,3-hexamethyldisilazane (HMDS), which
may have some detrimental effects on polymerization rate and side
reaction.
ester gained wide applications in the pharmaceutical, biomedical
and industrial fields due to its excellent biocompatibility in body
and miscibility with other polymers [15e17]. Compared with other
metal alkoxide complexes, rare earth metal based initiators are
much less toxic than aluminum alkoxides [18,19], and display higher
polymerization activity in mild condition than tin(II) complexes
[20e22]. Methods used in preparation of rare earth alkoxides fall
*
Corresponding author. Tel.: þ86 571 87953739; fax: þ86 571 87951773.
E-mail address: lingjun@zju.edu.cn (J. Ling).
http://dx.doi.org/10.1016/j.polymer.2014.03.032
032-3861/Ó 2014 Elsevier Ltd. All rights reserved.
0

L. You et al. / Polymer 55 (2014) 2404e2410
2405
functionalized PCLs containing vinyl and alkynyl end-groups are
also simply synthesized using Lu(OTf)
alkoxides.
3
and corresponding sodium
2. Experimental
2.1. Materials
CL (Acros, 99%) was distilled under reduced pressure after being
stirred over CaH for 48 h. 2-Propanol (Sinopharm, AR), -methallyl
2
b
alcohol (Shanghai Jingchun, AR) and propargyl alcohol (Shanghai
ꢀ
Jingchun, AR) were dried over 4A molecular sieve for 48 h and then
Scheme 1. ROP of CL using the in situ generated catalytic system of Lu(OTf)
alkoxides where x ꢁ 3.
3
/sodium
distilled under reduced pressure. THF was refluxed over potassium/
benzophenone ketyl prior to use. NaOR were prepared from the
respective alcohols and sodium metal in THF, and dried in vacuum
to remove the residual alcohol and solvent. Sodium triflate (NaOTf)
was prepared from the reaction of NaOH and triflic acid. RE(OTf)3
(RE ¼ Sc, Y, and Lu) were synthesized from corresponding RE O
Nowadays rare earth metal triflates have received much
attention toward organic synthesis in aqueous media thanks to
their air and moisture stability compared with other conventional
Lewis acids [32]. However, fewer attempts are promoted in poly-
mer chemistry [33e35]. Okada et al. reported a cationic living ROP
2
3
and triflic acid according to the reported method [39]. Isolated
i
Lu(O Pr) (1) compound, although its precise structure is still un-
3
i
of lactones using scandium triflate (Sc(OTf)
3
) as a catalyst and
13
known, was prepared in a similar way to Y5(meO)(O Pr) according
alcohols as initiators respectively via an activated monomer
mechanism [36]. Nevertheless, long time and high temperature
are indispensable to obtain proper monomer conversion due to
the relatively low activity of this special catalytic system. For
instance, it took 4 days to obtain poly(ε-caprolactone) (PCL) with
molecular weight (MW) of 2200 and polydispersity index (PDI) of
to the literature [44,1]. THF-d (Acros, 99.5% deuterated) was dried
8
over P O and distilled into an NMR tube under reduced pressure.
2 5
2.2. Polymerization
CL polymerizations were carried out in THF in previously flamed
and argon-purged 30 mL ampoules using Schlenk techniques. Solid
NaOR was added and followed by predetermined amount of
RE(OTf)3 in THF solution. They were allowed to stir for 3 min at
1
.30 when Lu(OTf)
pure end-functionalized polyesters were not accessible using
Sc(OTf) /alcohol catalytic system since H O initiated contami-
3
and benzyl alcohol were used [37]. Moreover,
3
2
ꢀ
nated polyesters could not be excluded [35,38]. Another example
is a living/controlled cationic ROP of tetrahydrofuran (THF) using
rare earth triflate catalyst in the presence of epoxide to produce
well-defined polyethers [39].
room temperature (20 C) before CL was added. After pre-
determined time, a portion of the polymerization mixtures was
taken out and added by a drop of trifluoroacetic acid to determine
1
the monomer conversion in H NMR measurements. Polymeriza-
Despite the many rare earth compounds reported for ROP of
different cyclic esters, rare eartheoxygen (REeOR) bond is actually
the growing active site for monomer insertion [40e43]. However,
only one lutetium alkoxide complex from alkyl lutetium and
alcohol was reported as initiator for ROP of lactones [31]. Here we
report novel lutetium alkoxide complexes in situ generated by a fast
ligand exchange reaction of moisture-stable lutetium triflate
tion reaction was quenched by addition of an excess of 1 mol/L HCl
solution. The polymer was isolated by reprecipitation from CHCl in
3
cold methanol and dried under vacuum to constant weight.
2.3. Kinetic study
i
NaO Pr (37.4 mg, 0.456 mmol), THF (9.8 mL) and Lu(OTf)
3
(
Lu(OTf)
excluding the formation of any other deleterious byproducts
Scheme 1). It exhibits high catalytic activity and excellent
3
) with sodium alkoxides (NaOR) in THF within only 3 min
(141.8 mg, 0.228 mmol) were mixed under argon in a 30 mL
Schlenk tube. After stirring for 3 min at room temperature, the
polymerization started upon addition of CL (5.2 g, 45.6 mmol).
Samples were taken during the polymerization, and subjected to H
NMR and SEC analyses.
(
1
controllability towards ROP of CL in mild conditions, producing
PCLs with predictable MWs and narrow PDIs below 1.10. Pure end-
Table 1
i
a
Ring-opening polymerization of ε-caprolactone (CL) catalyzed by rare earth triflates (RE(OTf)
3
) and sodium isopropoxide (NaO Pr) in THF.
Convb (%)
M
n(exp) (kDa)
c
PDIc
M
n(theo.) (kDa)
d
Run
RE(OTf)
3
0
[Lu] :[Na]
0
:[CL]
0
molar ratio
Time (min)
1
2
3
4
5
6
7
8
9
Lu(OTf)
Lu(OTf)
Lu(OTf)
Lu(OTf)
Lu(OTf)
Lu(OTf)
Lu(OTf)
Lu(OTf)
3
3
3
3
3
3
3
3
1:1:100
1:2:200
1:3:300
1:1:200
1:2:400
1:3:600
1:4:800
1:6:1200
1:3:600
1:3:600
0:1:100
1:0:300
50
35
3
240
110
8
3
2
0.5
90
2
90.0
91.0
99.0
70.0
50.6
98.0
97.0
95.3
90.2
50.2
92.2
0
9.7
10.1
11.9
13.6
11.0
21.8
24.1
21.7
23.3
13.0
9.8
1.09
1.06
1.08
1.23
1.05
1.07
1.28
1.53
1.27
1.11
2.01
e
10.2
10.4
11.8
15.9
11.5
22.3
22.1
21.7
20.5
11.4
10.5
e
Y(OTf)
3
1
1
1
0
1
2
Sc(OTf)
e
3
e
Lu(OTf)
3
4 day
e
a
b
c
ꢂ1
ꢀ
[CL]
0
¼ 3.0 mol L , 20 C.
1
Determined by H NMR.
Determined by SEC.
Calculated from ([CL]
d
e
i
0
/[NaO Pr]
0
) ꢃ Conv ꢃ (MW of CL) þ (MW of isopropanol).
No RE(OTf) was used.
3

2406
L. You et al. / Polymer 55 (2014) 2404e2410
n
Fig. 3. Dependence of number-average molecular weight (M ) and polydispersity
i
(
2
PDI) on monomer conversion for the CL ROP catalyzed by Lu(OTf)
3
/NaO Pr in THF at
ꢀ
ꢂ1
ꢂ1
i
ꢂ1
0
C, [CL]
0
¼ 3.0 mol L , [Lu(OTf)
3
]
0
¼ 0.015 mol L , and [NaO Pr]
0
¼ 0.03 mol L
.
i
Dotted line is the Mn,theo values calculated as ([CL]
0
/[NaO Pr]
0
) ꢃ Conv ꢃ (MW of CL).
i
Fig. 1. SEC traces of the obtained PCLs catalyzed by Lu(OTf)
run 3) and by NaO Pr alone (dashed line, Table 1, run 11).
3
/NaO Pr (solid line, Table 1,
i
3. Results and discussion
2
.4. Measurements
3
.1. ROP of CL using the in situ generated catalyst derived from rare
i
Nuclear magnetic resonance (NMR) spectra were recorded on a
3
earth triflates (RE(OTf) ) and sodium isopropoxide (NaO Pr)
1
13
Bruker Avance DMX 500 MHz ( H: 500 MHz and C: 125 MHz)
spectrometer in CDCl
reference or in THF-d
3
with tetramethylsilane (TMS) as an internal
. Size exclusion chromatography (SEC) was
Compared with the rare earth complex catalysts previously re-
ported [27,46e48], the unique advantages of the present catalytic
8
performed on a Waters-150C apparatus in THF with a flow rate of
3
system include moisture-stable RE(OTf) , commercially available
NaO Pr, and simple in situ ligand exchange reaction without time-
consuming isolation. Several polymerizations of CL were carried
i
ꢀ
1
.0 mL/min at 40 C using narrow PDI polystyrene (PS) standards
for calibration. Number-average MW of PCL (Mn,PCL) was calculated
according to the relationship Mn,CL ¼ 0.54 ꢃ Mn,PS [45].
out in THF, using the in situ generated catalyst consisting of
i
Matrix-assisted laser desorption/ionization time-of-flight
RE(OTf)
3
(RE ¼ Sc, Y, Lu) and NaO Pr as summarized in Table 1.
i
(
MALDI-ToF) mass spectra were measured on an Applied Bio-
When 1/3 molar ratio of Lu(OTf)
3
with respect to NaO Pr was added,
systems Voyager System 4350 equipped with a 337 nm nitrogen
laser (3 ns pulse width). All polymer (3e5 mg/mL PCL) mass spectra
were recorded in the reflection mode with an acceleration voltage
of 20 kV. 2,5-Dihydroxybenzoic acid (20 mg/mL in THF) was used as
a matrix and NaI (10 mg/mL in THF) was used as the cationic agent.
the conversion of CL was as high as 99% within 3 min (Table 1, run
3), maintaining a high propagation rate close to that observed in
i
NaO Pr and producing PCL with narrow PDI of 1.08. As illustrated in
i
Fig. 1, SEC trace of PCL obtained by Lu(OTf)
3
/NaO Pr (Table 1, run 3)
ꢂ1
i
Fig. 2. First-ordered kinetic plot for the polymerization of CL catalyzed by Lu(OTf)
3
/
Fig. 4. SEC traces of PCL precursor (solid line, [CL]
[Lu(OTf)
¼ 120/2/1, polymerization time ¼ 3 min) and final product after the
addition of a second batch of CL monomer (dotted line, polymerization time ¼ 5 min).
0
¼ 3.0 mol L , [CL]
0 0
/[NaO Pr] /
i
ꢀ
ꢂ1
ꢂ1
NaO Pr in THF at 20 C, [CL]
0
¼ 3.0 mol L , [Lu(OTf)
3
]
0
¼ 0.015 mol L , and
3 0
]
i
ꢂ1
[
NaO Pr]
0
¼ 0.03 mol L
.

L. You et al. / Polymer 55 (2014) 2404e2410
2407
Fig. 7. 1H NMR spectrum of the alkynyl-ended PCL (Table 2, run 1) in CDCl
of CL catalyzed by Lu(OTf) /sodium propargyloxide.
from ROP
Fig. 5. 1H NMR spectrum of the obtained PCL (Table 1, run 3) in CDCl
3
.
3
3
PDIs above 2.0 [49]. On the other hand, no PCL was obtained using
Lu(OTf) alone even after 4 days (Table 1, run 12).
With fixed [CL]
or runs 4e6), propagation rate increased as [NaO Pr]
3
i
0
0
/[NaO Pr] ratio of 100 or 200 (Table 1, runs 1e3
i
0
3 0
/[Lu(OTf) ]
molar ratios raised from 1 to 3. In addition, experimental MWs of
the obtained PCLs were in agreement with those calculated from
i
initial ratios of [CL]
0
/[NaO Pr]
0
and monomer conversions, regard-
i
less of the various feeding ratios of [NaO Pr]
0
/[Lu(OTf)
3 0
] . Further
i
increasing ratios of [NaO Pr] /[Lu(OTf) ] to 4 and 6 resulted in
0 3 0
continuous rise of propagation rate, however, the controllability of
the polymerization became poor as the PDIs of the obtained PCLs
broadened to 1.5 (Table 1, runs 7 and 8). As a crucial component of
the novel catalytic system, different RE(OTf)
3
exhibited distinctive
activity towards ROP of CL (Table 1, runs 3, 9 and 10). For instance,
i
[
NaO Pr]
of 90.2% in 30 s (Table 1, run 9), even faster than NaO Pr alone
Table 1, run 11). However, the broadened PDIs of PCLs up to 1.27
indicated the existence of adverse side reactions (Table 1, run 9).
0 3 0
/[Y(OTf) ] with ratio of 3 produced PCL with conversion
i
(
Sc(OTf)
RE(OTf)
3
exhibited the lowest catalytic activity among the three
3
, giving monomer conversion of 50.2% in 90 min under the
Fig. 6. 13C NMR spectrum of the obtained PCL (Table 1, run 3) in CDCl
3
.
same condition (Table 1, run 10), in contrast to its relatively high
reactivity in the cationic cases [37]. The relatively low polymeri-
zation rate of scandium triflate may be contributed to the low ac-
tivity of the corresponding scandium alkoxide as described in the
scandium tris(2,6-di-tert-butyl-4-methylphenolate)-mediated ROP
of CL [50].
was symmetrical and narrow, in sharp contrast with that using
i
i
NaO Pr alone (Table 1, run 11). As a typical anionic initiator, NaO Pr
gave very fast polymerization of CL, but severe inter and intra-
molecular transesterification reactions occurred, leading to broad
Table 2
Synthesis of end-functionalized PCLs using different sodium alkoxides in the presence of Lu(OTf)
3
in THF.a
c
b
d
PDId
1.08
1.09
Run
1
NaOR
[Lu]
0
:[NaOR]
0
:[CL]
0
molar ratio
Convb (%)
M
n(theo.) (kDa)
M
n(NMR) (kDa)
Mn(SEC) (kDa)
1:2:100
1:3:300
1:2:100
99.2
5.7
11.4
5.7
5.6
11.8
5.8
5.5
11.6
5.9
2
99.0
3
99.3
1.04
a
ꢂ1
ꢀ
[CL]
0
¼ 3.0 mol L , 20 C, 3 min.
b
c
1
Determined by H NMR in CDCl
Calculated from ([CL]
Determined by SEC.
3
.
i
0
/[NaO Pr]
0
) ꢃ Conv ꢃ (MW of CL) þ (MW of corresponding alcohol).
d

2408
L. You et al. / Polymer 55 (2014) 2404e2410
Fig. 10. 1H NMR spectra of the in situ formed catalytic system by Lu(OTf)
and 3
3
i
i
i
3 8
equivalence of NaO Pr (A), Lu(O Pr) (1) (B), NaO Pr (C) in THF-d .
Fig. 8. 1H NMR spectrum of the vinyl-ended PCL (Table 2, run 3) in CDCl
CL catalyzed by Lu(OTf) /sodium -methallyloxide.
from ROP of
3
3
b
with the theoretical ones calculated by the feeding molar ratios of
i
i
0 0
[CL] /[NaO Pr] and the monomer conversions. From the SEC results,
3.2. Living ROP of CL catalyzed by Lu(OTf)
3
/NaO Pr
the PDIs of the obtained PCLs show rather narrow values ranging
from 1.05 to 1.09 throughout the polymerization. All of these evi-
dences support the livingness of ROP.
As a further evidence of the controlled/living nature, a second
batch of CL monomer with the same amount of the first one was
The kinetic and postpolymerization experiments were carried
out to confirm the controlled/living nature of the Lu(OTf) /NaO Pr-
3
i
mediated ROP. Fig. 2 shows a distinct first-order relationship be-
tween reaction time and monomer consumption indicating a con-
stant concentration of active species during the polymerization [51].
No induction period is observed according to the fact that the fitting
line passes the origin, indicating that no substantial rearrangement
of the catalytic species was required before the polymerization
started [52,25]. MWs of the obtained PCLs linearly increase with the
monomer conversions as high as 99% (Fig. 3). More importantly, the
experimental Mn,(exp) values of the obtained PCLs agree very well
ꢂ1
added to a living PCL precursor with Mn,(exp) ¼ 6760 g mol and
1
PDI ¼ 1.06 after monomer conversion of 99.2% determined by
H
NMR. The living chains kept growing and the MW of final PCL
doubled at a total CL conversion of 98.9%. The narrow poly-
dispersity (PDI ¼ 1.07) in the SEC curves (Fig. 4) indicated the
absence of chain termination and transfer reactions.
3
.3. Synthesis of end-functionalized PCLs using different sodium
alkoxides in the presence of Lu(OTf)
3
The results based on 1H NMR and 13C NMR analyses conclude
that the obtained PCL possesses an isopropyl ester group at one
Fig. 9. MALDI-ToF MS spectrum of the vinyl-ended PCL (Table 2, run 3) from ROP of CL
Fig. 11. 13C NMR spectra of in situ formed catalytic system by Lu(OTf)
3
and 3 equiva-
(C) and NaOTf (D) in THF-d
i
i
catalyzed by Lu(OTf)
3
/sodium
b-methallyloxide.
lence of NaO Pr (A), Lu(O Pr)
3
(1) (B), Lu(OTf)
3
8
.

L. You et al. / Polymer 55 (2014) 2404e2410
2409
chain end and a hydroxyl group at the other, as illustrated in Figs. 5
and 6 [53]. The characteristic signals of the isopropyl ester end
good agreement with three doublets at 1.63, 1.41 and 1.18 ppm
13
found in 1. Additional evidence is found in C NMR (Fig. 11). The
peak at far left of the quadruple carbon signal of OSO CF in the
/NaO Pr is found at 125.36 ppm which is the
same as that in NaOTf rather than that in Lu(OTf) at 124.20 ppm
(Fig. 11). All the above suggests a fast ligand exchange reaction
a
b
group are observed at 1.21 ppm (H ) and 4.98 ppm (H ) [53,45], and
2
3
g
i
the methylene protons of the other chain end (H ) is found at
mixture of Lu(OTf)
3
1
3
3
.62 ppm (Fig. 5). In addition, the corresponding C signals are
3
a
b
found at 21.8 ppm (C ) and 67.4 ppm (C ).
Because PCLs produced by the Lu(OTf)
i
i
3
/NaO Pr are capped by
3
between Lu(OTf) and NaO Pr generating lutetium complex 2 con-
i
an isopropyl ester end group from the residue of the sodium
alkoxide used, it is a straightforward way to change the alcohol so
as to tailor the PCL end group and to contribute to the macro-
molecular engineering of PCL. Table 2 summarizes the PCLs end
capped by vinyl and alkynyl groups prepared by the corre-
taining LueO Pr bond similar to that in metal alkoxide. According to
þ
hardesoft acidebase (HSAB) theory [54,55], Na is considered as
3
þ
ꢂ
i
ꢂ
þ
3þ
harder acid than Lu and OTf is a harder base than O Pr . Na
ꢂ
prefers to ionically bind with harder Lewis bases of OTf and Lu
with O Pr. By comparison, ROP of CL using Lu(O Pr)
i
i
3
(1) and NaOTf
sponding sodium alkoxides in the presence of Lu(OTf)
3
. All the
as initiators were carried out in THF and toluene (Table S1). All the
polymerizations initiated by 1 show good reactivity but low initi-
ation efficiency. The Mn(exp)s are higher than the theoretical values,
Lu(OTf) /NaOR catalytic systems initiate ROP of CL proceeded in a
3
well-controlled manner to yield PCLs with predictable MWs and
narrow PDIs below 1.10 (Fig. S1 in Supporting Information). These
indicating a slow equilibrium between aggregated and monomeric
1
i
functional end groups are confirmed by H NMR analysis (Figs. 7
Lu complex similar to the reported Y
5
(m
eO)(O Pr)13 catalyst [19,45].
and 8). A characteristic triplet peak of 2.50 ppm ascribed to
In contrast, NaOTf alone cannot initiate ROP of CL, indicating that
the NaOTf is not an active site. Together with the living manner in
the previous kinetic study, it supports that the real active site of 2 is
LueOR bond analogous to 1.
a
alkynyl proton (H ) and a doublet of 4.71 ppm to methylene
b
protons (H ) confirm the successful introduction of the alkynyl
group at PCL chain end. With respect to the
b-methallyl ester
h
a
group, methyl protons (H ), vinyl protons (H ) and methylene
A coordinationeinsertion mechanism of CL ROP is illustrated in
b
protons adjacent to the ester linkage (H ) appear at 1.76, 4.95 and
Scheme 2. A fast ligand exchange reaction of Lu(OTf)
3
and NaOR
þ
4
.50 ppm, respectively. In addition, the MALDI-ToF mass spec-
trum of PCL catalyzed by Lu(OTf) /sodium -methallyloxide re-
veals only one population of polymers possessing a -methallyl
residue and a hydroxyl chain end (Fig. 9). Thus we reach the
conclusion that Lu(OTf) /NaOR is an efficient catalyst for the ROP
leads to the in situ formed lutetium alkoxide complex 2, where Na
ꢂ
3
b
and OTf combine into tight ion pairs surrounding and stabilizing
b
the lutetium metal centers to prevent their aggregation. The reason
for the ligand exchanged reaction is faster than that of RECl
3
and
3
NaOR is the good solubility of Lu(OTf) in THF leading to a ho-
3
þ
ꢂ
of CL to generate well-defined PCLs applicable for future modi-
fication by Click reactions.
mogenous mixture and the higher affinity between Na and OTf
ꢂ
than Cl [39]. Because one Lu(OTf)
3
can only exchange with at most
i
i
three Na OPr, excessive Na OPr initiates anionic ROP of CL leading to
broad PDIs (Table 1, runs 6e8). The coordination of the lutetium
complex onto the carbonyl group of CL activates the selective acyl-
oxygen cleavage of the CL followed by its insertion into the lute-
tiumeoxygen bond in a way that maintains the growing chain
bound to the lutetium through an active alkoxide bond [42,43].
Therefore, an ester group with R residue is generated at the PCL
chain end during the initiation. When quenched by acid, a hydroxyl
group caps the other end. This structural feature agrees with the
“coordinationeinsertion” mechanism reported in the case of
aluminum alkoxide initiators and rare-earth alkoxides as well
[19,52].
3
.4. Mechanisms
Based on chain end analysis of PCL, the alkyloxy residue group
supports that the real active site is REeOR bond derived from ligand
exchange reaction of Ln(OTf) and NaOR. A reaction of Lu(OTf)
with 3 equivalent NaO Pr in THF-d was carried out for the analyses
of H NMR and C NMR spectroscopy. Fig. 10 compares the signals
of the in situ mixture of Lu(OTf) and NaO Pr, Lu(O Pr) (1) and
3 3
NaO Pr. The broad methine signal covering 3.7e4.7 ppm and mul-
tiple peaks of methyl group at 1.1e1.6 ppm move downfield from
3
3
i
8
1
13
i
i
i
i
those of NaO Pr at 4.12 and 0.99 ppm, respectively. The peaks are in
3
Scheme 2. Mechanism of ring-opening polymerization of CL catalyzed by the in situ generated catalytic system of Lu(OTf) and NaOR.

2
410
L. You et al. / Polymer 55 (2014) 2404e2410
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bond was formed by simply mixing Lu(OTf)
3
with NaOR in THF for
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min at room temperature without any tedious intermediate pu-
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quite stable and easy to handle compared with those used in other
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1
thesize pure end-functionalized PCL with vinyl and alkynyl end-
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These novel rare earth alkoxide complexes are also expected as
effective initiators for ROP of lactide and other cyclic esters.
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Financial supports from the National Natural Science Founda-
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Projects (G2011CB606001), and the Zhejiang Provincial Natural
Science Foundation of China (Y4110115) are gratefully
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