Living cationic ring-opening polymerization of 2-oxazolines initiated by rare-earth metal triflates

Article abstract of DOI:10.1039/c4ra11404c

The cationic ring-opening polymerization (CROP) of substituted 2-oxazolines using rare-earth metal triflates (RE(OTf)₃) as initiator was investigated for the first time. In this work, we examined the polymerization characteristics of 2-ethyl-2-oxazoline (EtOx) initiated by Sc(OTf)₃ under conventional thermal heating and microwave irradiation, and compared the respective outcomes with those obtained with the most frequently used initiator methyl tosylate (MeOTs). The results indicated that Sc(OTf)₃ exhibits a higher catalytic efficiency to the EtOx polymerization than MeOTs under identical conditions. The controlled/living nature of the Sc(OTf)₃-catalyzed CROP was confirmed by its linear first-order kinetics and the narrow molecular weight distribution of the resultant polymers as well as the block copolymerization of EtOx and 2-phenyl-2-oxazoline (PhOx). Based on in situ NMR spectroscopic studies and SEC analysis of PEtOx samples obtained from the control termination experiments, a possible initiating/propagating mechanism has been proposed for the living cationic ring-opening polymerization. Morever, this rare-earth catalytic system can also be applied to the ring-opening polymerization of some sterically hindered or aryl-substituted 2-oxazolines.

Full text of DOI:10.1039/c4ra11404c

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Page 1 of 31
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Living cationic ring-opening polymerization of 2-oxazolines
initiated by rare-earth metal triflates
Fangyu Hu, Shoulei Xie, Liming Jiang,* and Zhiquan Shen
MOE Key Laboratory of Macromolecular Synthesis and Functionalization,
Department of Polymer Science and Engineering, Zhejiang University, Hangzhou
310027, China
(Eꢀmail: cejlm@zju.edu.cn)
ABSTRACT
The cationic ringꢀopening polymerization (CROP) of substituted 2ꢀoxazolines using
rareꢀearth metal triflates (RE(OTf)3) as initiator was investigated for the first time. In
this effort, we examined the polymerization characteristics of 2ꢀethylꢀ2ꢀoxazoline
(EtOx) initiated by Sc(OTf)3 under the conventionally thermal heating and microwave
irradiation, and compared the respective outcomes with those obtained with the most
frequently used initiator methyl tosylate (MeOTs). The results indicated that Sc(OTf)3
exhibits a higher catalytic efficiency to the EtOx polymerization than MeOTs under
identical conditions. The controlled/living nature for the Sc(OTf)3ꢀcatalyzed CROP
was confirmed by the linear firstꢀorder kinetics and narrow molecular weight
distribution of the resultant polymers as well as the block copolymerization of EtOx
and 2ꢀphenylꢀ2ꢀoxazoline (PhOx). Based on in situ NMR spectroscopic studies and
SEC analysis of PEtOx samples obtained from the control termination experiments, a
possible initiating/propagating mechanism has been proposed for the living cationic
ringꢀopening polymerization. Morever, this rareꢀearth catalytic system also can be
1

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applied to the ringꢀopening polymerization of some sterically hindered or
arylꢀsubstituted 2ꢀoxazolines.
Introduction
Poly(2ꢀoxazoline)s are a class of synthetic polyamides that are receiving considerable
attention in the past decade because of their synthetic versatility and potential
applications in many fields, especially, in the biomaterials.1−4 The polymerization of
2ꢀoxazolines usually exhibits controlled and ‘‘living’’ character, i.e. it proceeds in the
absence of chain transfer and termination reactions. This feature provides easy access
to wellꢀdefined polymers with low polydispersity index (PDI) values and adjustable
molar masses. Furthermore, a vast variety of substituted 2ꢀoxazoline monomers, as
well as functional initiators and terminating agents is available and allows the tuning
of the polymer properties.
To date, various initiators including Lewis acids, such as boron trifluoride, and
alkyl esters such as tosylates, triflates and halides have been reported for the cationic
ringꢀopening polymerization of 2ꢀoxazolines.5−7 Among them, tosylates and triflates
could be particularly effective in combination with microwave irradiation, which
enables the polymerization in minutes to a few hours.8−9 Although such a
microwaveꢀassisted polymerization was achieved in a laboratory equipment, the
usefulness of this synthesis procedure and its manipulation still remain problematic
for a scale up due to the intrinsic limitations of microwave techniques. As for tosylate
and triflate, the yet most extensively used initiators, their high toxicity is a relevant
key issue that must be taken in consideration in practical applications.10−13 Therefore,
2

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it is desirable to explore new catalytic systems to alleviate aforementioned problems
and permit the CROP of 2ꢀoxazolines to be performed on a normal apparatus. Also,
we feel that more fundamental studies on the design and synthesis of
polyoxazolineꢀbased materials with new functional properties could be beneficial to
the field.14−17
On the other hand, rareꢀearth metal triflates (RE(OTf)3) as a hard Lewis acid have
been applied in the polymer synthesis. For example, Okamoto and his coꢀworkers18−21
established a general method applicable to some polar monomers such as
(meth)acrylamides and methacrylates wherein the added RE(OTf)3 catalytically affect
the polymerization stereochemistry to isotacticꢀselective manner, producing
corresponding highly stereoregular polymers. With the same protocol, we recently
synthesized isotacticꢀrich polyacrylamide derivatives bearing a chiral oxazoline
moiety in the side chain, which can serve as a chemosensor for the enantioselective
recognition of 1,1′ꢀbiꢀ2ꢀnaphthol.22−23 More recently, RE(OTf)3 has proven to be an
efficient catalyst for the ringꢀopening polymerization of lactones,24,25
tetrahydrofuran,26 and amino acid Nꢀcarboxyanhydrides.27 In view of this background,
it was thought that this kind of rareꢀearth salts should have potential uses as initiator
for the CROP of 2ꢀoxazolines.
The present work aims at examining (i) the catalytic performance of rareꢀearth
metal triflates for the cationic ringꢀopening polymerization of 2ꢀoxazolines, and (ii) its
monomer application scope. For this purpose, we investigated the Sc(OTf)3ꢀcatalyzed
polymerizations of EtOx under the conventionally thermal heating and microwave
3

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irradiation conditions in comparison with those by using methyl tosylate as initiator.
In addition, the influence of the temperature on polymerization kinetics was studied.
Based on the observations from in situ NMR analyses and control termination
experiments, a possible polymerization mechanism was proposed for the rareꢀearth
catalysis. Finally, we briefly screened the polymerization behavior of other substituted
2ꢀoxazolines in the presence of Sc(OTf)3. As depicted in Scheme 1, these monomers
can be divided into two categories, namely, the sterically hindered
2ꢀbutylꢀ2ꢀoxazolines bearing an alkyl group at the 4ꢀ or 5ꢀposition, and
2ꢀphenylꢀ2ꢀoxazoline derivatives where tꢀbutyloxycarbonyl (Boc)ꢀprotected Lꢀproline
moiety was bound to the benzene ring via amide or ester linkage. Despite some Lewis
acids had been utilized as an initiator for the polymerization of 2ꢀoxazolines as early
as 1960′s,28 we note that Lewis acidic rareꢀearth triflates have not yet been reported,
to our knowledge.
N
O
N
O
N
O
N
O
N
O
EtOx
4-EtBuOx
4-MeBuOx
5-MeBuOx
N
O
N
O
O
N
O
H
HN
N
N
N
Boc
N
Boc
Boc
O
O
O
PhOx
ProPhOx-1
ProPhOx-2
ProPhOx-3
Scheme 1. 2ꢀOxazoline monomers tested in this study.
Experimental section
Material
2ꢀEthylꢀ2ꢀoxazoline (EtOx) and 2ꢀphenylꢀ2ꢀoxazoline (PhOx), purchased from Acros
Organics, were dried and vacuumꢀdistilled over barium oxide before use. Acetonitrile
4

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(CH3CN), dimethyl sulfoxide (DMSO), N, Nꢀdimethylformamide (DMF), methyl
pꢀtosylate (MeOTs) and piperidine were distilled from calcium hydride. Piperazine
(Aldrich) was distilled to use. Other chemicals, such as valeronitrile,
4ꢀaminobenzonitrile, methyl salicylate, isatoic anhydride, dicyclohexylcarbodiimide
(DCC), 1ꢀethylꢀ3ꢀ(3ꢀdimethylaminopropyl) carbodiimide hydrochloride (EDC),
4ꢀdimethylaminopyridine
(DMAP),
NꢀBocꢀLꢀproline,
2ꢀaminobutanol,
2ꢀaminopropanol, 1ꢀaminoꢀ2ꢀpropanol, 2ꢀchloroethylamine hydrochloride and
ethanolamine (Aldrich) were used as received. Rareꢀearth metal triflates (RE(OTf)3,
RE = Sc, Y, La, Dy, Lu) were prepared following the reported method29 and used after
drying in vacuum.
Analytical instrumentation and characterization techniques
1H NMR spectra were recorded on a Bruker Avance DMX−500 spectrometer. The
chemical shifts are reported in ppm relative to tetramethylsilane (TMS) as an internal
standard. Melting points were taken with a micro melting point apparatus (Shanghai
Precision & Scientific Instrument Co. LTD, the accuracy = 0.1°C). Sizeꢀexclusion
chromatography (SEC) was measured on a Waters−150C apparatus equipped with two
PLgel 5 ꢀm MIXEDꢀC 300 × 7.5 mm columns and a differential refractometer
detector using DMF with LiBr (0.05 mol L−1) as the eluent (flow rate 1 mL min−1,
60°C). The numberꢀaverage molecular weight (Mn) and polydispersity index (PDI) of
the polymers were calculated on the basis of a polymethylmethacrylate (PMMA)
calibration. Matrixꢀassisted laser desorption/ionization timeꢀofꢀflight (MALDIꢀTOF)
mass spectra were acquired on a Bruker Ultraflextreme mass spectrometer in the
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positive reflector mode with an acceleration voltage of 25 kV, using sodium iodide
(10 mg mL−1 in THF) as ionization salt and dithranol (20 mg mL−1 in THF) as matrix.
Electrospray ionizationꢀmass spectrometry (ESIꢀMS) measurements were performed
with a Varian 500 mass spectrometer. The Xꢀray diffraction data for the complex
Sc(OTf)3⋅EtOx were measured using an Atlas Gemini Ultima diffractometer (MoꢀKα
radiation, ωꢀscan technique,
λ = 0.71073 Å) at 170K. The crystal structure was solved
by direct methods using SHELXSꢀ97.
H
O
N
HO
NH2
CO2CH3
OH
N
O
N
O
OH
N
1) SOCl
r.t. over 2night
OH
Boc
OH
OH
O
O
N
150oC, 2h
DCC, DMAP
Toluene, 3h
2) NaHCO3
Boc
O
1a
1b
ProPhOx-1
O
HO
N
O
N
O
N
H
ClCH2CH2NH3Cl
Boc
N
O
O
NH2
N
DCC, DMAP
Toluene, 3h
DMF, NEt3
70-80oC, 2.5h
Boc
O
N
H
O
2a
HO
ProPhOx-2
N
O
CN
NH2
N
OH
O
N
Boc
O
H
N
N
130oC, 24h
EDC, DMAP,THF, 6h
Boc
O
NH2
NH2
3a
ProPhOx-3
Scheme 2. Synthesis of 2ꢀphenylꢀ2ꢀoxazoline (PhOx) derivatives bearing
Bocꢀprotected Lꢀproline moiety.
Monomer synthesis (Scheme 2)
2-[2-(N-Boc-L-prolyloxy)phenyl]-2-oxazoline (ProPhOx-1): Methyl salicylate (7.6
mL, 59 mmol) was mixed with ethanolamine (3.6 mL, 60 mmol) and stirred for 2h at
150°C. To the reaction mixture was added toluene (3 mL) under stirring. The
precipitated product was recrystallized from ethyl acetate to give a white solid (1a,
7.25 g, 63%). M.p. = 38.8−39.2°C (lit.30 36−37°C). 1a (3.38 g, 20 mmol) was
dissolved in 50 mL of dichloromethane, then to the solution added dropwise 4.5 mL
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of SOCl2 (62 mmol) at 0°C. After stirring at room temperature overnight, the reaction
mixture was filtered, and the filtrate was concentrated under reduced pressure. The
residue was dissolved in water and then saturated with aqueous sodium bicarbonate to
precipitate 1b as a pink solid (2.40 g, 73.7%). To a solution of NꢀBocꢀLꢀproline (1.72
g, 8 mmol) and 1b (0.815 g, 5 mmol) in toluene (25 mL), DCC (1.65 g, 8 mmol) was
added followed by DMAP (0.98 g, 8 mmol), and the reaction mixture was stirred at r.t.
for 3h. The volatiles were removed under vacuum, and the residue was subjected to
silica gel column chromatography (petroleum ether/ethyl acetate, 3:1), giving
1
ProPhOx-1 (1.55g, 86%) as a white crystal. M.p. = 123.2−124.2°C. H NMR (400
MHz; CDCl3):
δ = 1.47 (s, OC(CH3)3, 9H), 1.91−2.11 (m, N(CH2)2CH2, 2H),
2.23−2.59 (m, NCH2CH2CH2, 2H), 3.41−3.65 (m, NCH2(CH2)2, 2H), 4.02 (t,
NCH2CH2O, 2H), 4.35 (t, NCH2CH2O, 2H), 4.51−4.60 (m, OCCHN, 1H) 7.08−7.19
(m, Ph, 1H), 7.27−7.32 (m, Ph, 1H), 7.45−7.52 (m, Ph, 1H), 7.91−7.95 (m, Ph, 1H);
13C NMR (125 MHz; DMSOꢀd6):
δ = 22.9, 23.9, 28.0, 29.3, 46.3, 54.5, 58.8, 66.7,
79.0, 120.9, 123.3, 126.2, 130.3, 132.5, 148.9, 153.7, 160.2, 170.4. MS (ESI+): m/z
(%) = 383.0 (39.6) [M+Na]+, 743.1 (100) [2M+Na]+.
2-[2-(N-Boc-L-prolinamido)phenyl]-2-oxazoline (ProPhOx-2): To a solution of
isatoic anhydride (3.26 g, 20 mmol) and 2ꢀchloroethylamine hydrochloride (2.32 g, 20
mmol) in DMF (50 mL) was added 7 mL of NEt3. After stirring for 2.5h at 80°C, the
reaction mixture was poured into 200 mL of water, and then extracted with
dichloromethane (3 × 40 mL). The organic layer was washed successively with water,
followed by drying with anhydrous MgSO4. Further purification was performed with
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a silica gel column using petroleum ether/ethyl acetate mixture (3:1) as an eluant, and
the intermediate 2a was obtained as a white crystal (1.84 g, 56.7%). M.p. =
53.4−54.3°C (lit.31 54.5−55.5°C). The amidation of 2a was conducted by the same
procedure as the esterification of 1b, and the desired product ProPhOx-2 was
1
obtained as a white crystal (89.5%). M.p. = 130.3−131.5°C. H NMR (400 MHz;
CDCl3),
δ = 1.32 (s, OC(CH3)3, 9H), 1.91−1.95 (m, N(CH2)2CH2, 2H), 2.13−2.40 (m,
NCH2CH2CH2, 2H), 3.40−3.72 (m, NCH2(CH2)2, 2H), 4.02−4.49 (m, NCH2CH2O,
NCH2CH2O and OCCHN, 5H), 7.09−7.13 (m, Ph, 1H), 7.46−7.48 (m, Ph, 1H),
1
7.84−7.87 (m, Ph, 1H), 8.80−8.83 (m, Ph, 1H), 12.79 (s, NHCO, 1H); C NMR (125
MHz; DMSOꢀd6):
δ = 23.2, 23.9, 27.7, 28.0, 30.2, 31.0, 46.4, 54.3, 61.9, 66.3, 78.8,
112.8, 118.5, 122.4, 128.9, 132.5, 139.0, 152.9, 153.9, 163.4, 172.1. MS (ESI+): m/z
(%) = 382.1 (93.8) [M+Na]+, 741.1 (100) [2M+Na]+.
2-[4-(N-Boc-L-prolinamido)phenyl]-2-oxazoline (ProPhOx-3): To the melted
4ꢀcyanoaniline (5.91 g, 50 mmol) was added Zn(OAc)2⋅2H2O (0.35 g, 1.6 mmol) and
ethanolamine (4 mL, 66 mmol), and then the mixture was allowed to stir for 24h at
130°C. The reaction mixture was poured into cold deionized water (200 mL), and the
resulted precipitate was purified by chromatography on a silica gel column eluted
with THF/petroleum (v/v, 3: 1) to give 3a as yellowish solid (3.67 g, 45.4%). M.p. =
159.5−160.5°C (lit.32 160−161°C). ProPhOx-3 was prepared by a similar way to that
of ProPhOx-2 except for the use of EDC as the dehydrant and THF as solvent (53.7%,
1
white crystal). H NMR (400 MHz; CDCl3):
δ = 1.44 (s, OC(CH3)3, 9H), 1.84−1.98
(m, N(CH2)2CH2 and NCH2CH2CH2, 4H), 3.22−3.61 (m, NCH2(CH2)2, 2H), 4.04 (t,
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NCH2CH2O, 2H), 4.40−4.44 (m, NCH2CH2O and OCCHN, 3H), 7.56−7.59 (m, Ph,
2H), 7.88−7.90 (m, Ph, 2H), 9.77 (br, NHCO, 1H); 1C NMR (125 MHz; DMSOꢀd6):
δ
= 171.6, 162.6, 153.2, 141.7, 128.5, 122.1, 118.5, 78.5, 67.2, 60.2, 54.3, 46.6, 33.3,
30.9, 30.1, 27.9, 23.6. MS (ESI+): m/z (%) = 360.1 (100) [M+H]+, 719.0 (62.6)
[2M+H]+, 741.1 (23.8) [2M+Na]+.
2ꢀButylꢀ2ꢀoxazoline derivatives, including 4ꢀethylꢀ2ꢀbutylꢀ2ꢀoxazoline (4ꢀEtBuOx),
2ꢀbutylꢀ4ꢀmethylꢀ2ꢀoxazoline (4ꢀMeBuOx), and 2ꢀbutylꢀ5ꢀmethylꢀ2ꢀoxazoline
(5ꢀMeBuOx) were synthesized according to a general procedure described in the
literature.33
1
4-EtBuOx: H NMR (400 MHz; CDCl3):
δ = 0.88−0.96 (br, OC(CH2)3CH3 and
NCHCH2CH3, 6H), 1.36−1.61 (br, NCHCH2CH3 and OCCH2(CH2)2CH3, 6H), 2.15 (t,
OCCH2(CH2)2CH3, 2H), 3.75 (t, CH2O, 1H), 3.95 (q, CHN, 1H), 4.25 (t, CH2O, 1H).
1
4-MeBuOx: H NMR (400 MHz; CDCl3):
δ = 0.86 (t, OC(CH2)3CH3, 3H), 1.18 (d,
NCHCH3, 3H), 1.32−1.55 (br, OCCH2(CH2)2CH3, 4H), 2.20 (t, OCCH2(CH2)2CH3,
2H), 3.67 (t, CH2O, 1H), 4.08 (q, CHN, 1H), 4.24 (t, CH2O, 1H).
5-MeBuOx: 1H NMR (400 MHz; CDCl3):
δ = 0.92 (t, OC(CH2)3CH3, 3H), 1.29−1.40
(m, OC(CH2)2CH2CH3 and OCHCH3, 5H), 1.62 (q, OCCH2CH2CH2CH3, 2H), 2.25 (t,
OCCH2(CH2)2CH3, 2H), 3.37 (t, CH2O, 1H), 3.89 (t, CHN, 1H), 4.61 (t, CH2O, 1H).
Polymerization procedure
All the polymerizations under thermal heating conditions were carried out on the
Schlenk line with a predetermined initial monomer concentration. Taking the solution
polymerization of EtOx as an example, a typical experimental procedure is as follows.
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To a flame dried flask EtOx (1 mL, 9.9 mmol) and 1.2 mL of an acetonitrile solution
of Sc(OTf)3 (8.25 × 10−2 M) was added via a syringe under nitrogen atmosphere. The
flask was then immersed in a constantꢀtemperature oil bath of 80°C. After being
heated for a definite period of time with magnetic stirring, the reaction was quenched
by the addition of deionized water. The resulting solution was left to stir at ambient
temperature for 1 h and the crude product was precipitated in diethyl ether. To
completely remove the catalyst residue and unreacted monomer, the collected powder
matter was redissolved in CH3CN and then precipitated from Et2O. This process was
repeated three times and the desired polymer PEtOx was dried under vacuum. The
polymerization of other 2ꢀoxazoline monomers and the purification are similar to that
described above.
For comparison, the polymerization of EtOx with microwave assistance was carried
out in a CEM Discover SP microwave synthesizer. In this case, a stock solution of the
monomer and Sc(OTf)3 or MeOTs as initiator with various monomerꢀtoꢀinitiator ratios
was prepared in acetonitrile. The microwave vial was charged with 1−2 mL of this
stock solution and heated in the microwave to a certain temperature. After a given
time, the reaction mixture was cooled with compressed air and quenched by adding
deionized water. The subsequent treatment of the polymerization product is the same
as those described above.
Kinetic study of EtOx polymerization initiated with Sc(OTf)3
The kinetics of Sc(OTf)3ꢀcatalyzed cationic ringꢀopening polymerization of EtOx was
studied under the conventionally thermal heating conditions in acetronotrile. In order
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to obtain the monomer conversion, aliquots taken periodically from the reaction
1
mixture were directly subjected to H NMR analysis in CDCl3. The conversion was
determined by comparing the signal intensity from the released CH2 of polymer at
3.6−3.3 ppm with that from the remaining CH2 of EtOx at 4.3 and 3.8 ppm (see:
Fig.S1 in Supporting Inf). Meanwhile, the collected samples were dissolved in DMF
containing LiBr (50 mM) for SEC measurement to determine the molecular weight
and polydispersity.
Results and discussion
Monomer synthesis and polymerization features
Except for EtOx and PhOx, other 2ꢀoxazoline derivatives shown in Scheme 1 were
prepared from the commercially available starting materials (nitriles, carboxylates, or
isatoic anhydride) by relatively facile functionalꢀgroup transformations in moderate
yields. Details of the synthetic preparations and structural characterization data of
these monomers can be found in the experimental part and the Supporting
Information.
To evaluate the catalytic performance of rareꢀearth triflates for CROP of
2ꢀoxazolines, we first investigated the solution polymerization of EtOx under
different conditions. The polymerizations with RE(OTf)3 (RE = Sc, Y, La, Dy or Lu)
were found to proceed smoothly at 80°C in acetonitrile. As can be seen from Table 1
(runs 1−5), after a reaction period of 2 hours the monomer conversion of 47−84% was
observed by 1H NMR analysis, yielding corresponding polymers with
numberꢀaverage molecular weight of 5270−8440 and low polydispersity indices (PDI
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= 1.09−1.12). Of five triflates tested, Sc(OTf)3 seems furnish best results in terms of
the monomer conversion and PDIs of the resultant PEtOxs. In contrast, methyl
tosylate (MeOTs) exhibited a poor initiating efficiency, which gave a monomer
conversion of ~16% and a lowꢀmolecularꢀweight polymer under the same conditions
(run 6). Noteworthy is also the strong influence of the solvent property on the catalytic
activity of Sc(OTf)3, with a decrease even deactivation observed when the
polymerization was carried out in other solvents such as nꢀBuCl, DMSO or DMF
instead of CH3CN (runs 7−9).
Table 1 ROP of 2ꢀethylꢀ2ꢀoxazoline (EtOx) under various conditions a
Temp. Time
PEtOx
[M]:[I]
Run Catalyst Solvent
(min) Conv./%c
Mn
PDId
1.09
1.12
1.12
1.10
1.11
1.18
1.21
−
d
(°C)
80
80
80
80
80
80
80
80
1
2
3
4
5
6
7
8
9
Sc(OTf)3 CH3CN 100
Y(OTf)3 CH3CN 100
La(OTf)3 CH3CN 100
Dy(OTf)3 CH3CN 100
Lu(OTf)3 CH3CN 100
MeOTs CH3CN 100
Sc(OTf)3 nꢀBuCl 100
Sc(OTf)3 DMSO 100
120
120
120
120
120
120
120
120
120
84
65
47
73
71
16
48
0
5270
6220
8440
6270
6290
3440
4230
−
Sc(OTf)3
DMF
100 120
0
−
−
53
4080
1.09
10b
Sc(OTf)3 CH3CN 100
Sc(OTf)3 CH3CN 100
80
90
60
(93)
(7880)
(1.10)
66
4490
1.10
11b
40
5
(91)
(8910)
(1.11)
12 Sc(OTf)3 CH3CN 100 140e
>99
5560
1.14
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13
14 Sc(OTf)3
15 MeOTs
16 Sc(OTf)3
17 MeOTs
18 Sc(OTf)3
19 MeOTs
MeOTs
93
>99
69
5320
12210
15320
19060
14850
27240
14100
1.27
1.31
1.52
1.56
1.97
1.67
2.08
CH3CN 400 140e
20
20
20
>99
67
CH3CN 800 140e
CH3CN 1600 140e
89
44
a The monomer concentration was 3 M for runs 1−9 and 4.5 M for runs 10−19.
b The data given in parentheses are values obtained under microwave (ꢀW) radiation.
c Monomer conversion was determined by 1H NMR.
d Determined by SEC, PMMA calibration, DMF containing 50 mM LiBr as the eluent.
e Under ꢀW conditions.
In a set of comparable experiments, we found that the microwave (ꢀW) irradiation
makes Sc(OTf)3 more effective for the CROP of EtOx compared to the conventional
heating, i.e., the rareꢀearth salt can catalyze the polymerization affording high yields
(>90%) of higherꢀmolecularꢀweight polymers with narrow PDIs under microwave
conditions in acetonitrile at 80 or 90°C (runs 10 and 11). Such a significant
improvement in catalytic efficiency pointed to the existence of a nonꢀheat microwave
effect in the Sc(OTf)3ꢀcatalyzed polymerization. Nevertheless, for the reported
ꢀWꢀassisted CROPs of 2ꢀoxazolines there seemed to be no distinct correlation
between the accelerated polymerization rate and the microwave effect, as
demonstrated by Hoogenboom and Schubert et al.34
It is also interesting to note that Sc(OTf)3 always exhibited higher catalytic
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efficiency than MeOTs in the ꢀWꢀassisted polymerization process (runs 12−19). For
example, the Sc(OTf)3ꢀcatalyzed reaction came to completion within 5−20 minutes at
140°C in the cases of monomerꢀtoꢀinitiator ratio of 100, 400 and 800, even at a [M]/[I]
of 1600 the monomer conversion reached ~90% yielding Mn as high as 27000.
However, with decreasing of the catalyst loading a relatively broad molecularꢀweight
distribution was observed for the resultant PEtOxs (PDI value changing from 1.14 to
1.67), which may be attributed to chain transfers during the CROP.35
Table 2 summarizes the bulk polymerization results of EtOx using Sc(OTf)3 as the
initiator under conventionally thermal heating. It can be seen that this bulk
polymerization process allowed the synthesis of polymers with molar masses in the
range 5 400 g⋅mol−1 < Mn < 18 800 g⋅mol−1 (PMMA standard) through regulation of
reaction parameters. In experimenting we found that with the proceeding of
polymerization the reaction mixture became highly viscous and later solidified. It
usually occurred at the point that the monomer conversion was 70−80%. Most likely,
the presence of such a viscous or heterogeneous system at the late stage of
polymerization is responsible for the broader molecular weight distributions (PDI =
1.27−1.37). This speculation is supported by the data from Table 2. More specifically,
in the polymerization with a monomerꢀtoꢀinitiator ratio of 100 (runs 1−3), elongating
the reaction time from 30 min to 60 min resulted in a distinct increase in both Mn and
PDI; however, both the values almost remained constant when further extending
periods of time. From the results shown in runs 6−8 of Table 2, it appears that the
combination of a lower temperature and longer reaction time is beneficial to the
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improvement of molar masses while keeping narrower polydispersity for the case of
high [M[/[I] ratio.
Table 2 Bulk polymerization of 2ꢀethylꢀ2ꢀoxazoline with Sc(OTf)3
PEtOx
Run
[M]:[I]
Time/min
Temp./°C
Yielda /%
34.2
Mn
PDIb
1.09
1.29
1.29
1.28
1.37
1.26
1.64
1.32
b
1
2
3
4
5
6
7
8
100
100
100
100
200
500
500
500
80
80
30
60
5450
12040
12920
8860
89.0
80
90
89.4
100
100
100
100
80
30
86.4
30
86.1
9030
30
70.1
13800
12410
18730
180
180
97.8
85.0
a Isolated yield determined by gravimetry.
b
Determined by GPC, PMMA calibration, DMF containing 50 mM LiBr as the
eluent.
Kinetic studies of EtOx polymerization with Sc(OTf)3
The polymerization kinetics of EtOx were determined in acetonitrile with Sc(OTf)3 as
the initiator under conventional conditions, using an initial monomer concentration of
4.5 M and a monomerꢀtoꢀinitiator ratio of 100. The resulting kinetic plots at five
different polymerization temperatures are depicted in Figure 1a. It can be seen that at
100°C or 90°C the monomer revealed linear firstꢀorder kinetics up to ~90%
conversion (ln([M]0/[M]t ≈ 2.3) in less than 2 h. For the polymerization conducted at
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80°C and 70°C, this linear relationship was observable in the range of 86% and 68%
conversion within 3 h, respectively. In all experiments, SEC analyses of the obtained
polymers gave unimodal traces. As shown in Figure 1b, the numberꢀaverage
molecular weight (Mn) increased linearly in proportion to the conversion, while the
PDIs remained around 1.10. As such, it can be concluded that the polymerizations
proceeded in a controlled/living manner.
Fig. 1 (a) Kinetic plots for 2ꢀethylꢀ2ꢀoxazoline polymerizations initiated with
Sc(OTf)3 ([M]0/[I]0 = 100) in acetonitrile ([M]0 = 4.5 M) at 60, 70, 80, 90, and 100°C
under thermal heating; the apparent rate constants (kp) are expressed in L mol−1 min−1.
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(b) Evolution of the molar mass (Mn) and the PDI value with monomer conversion
(determined by SEC, RI detection, PMMA calibration, DMF containing 50 mM LiBr
as the eluent. See: Fig. S2 in Supporting Information).
The fact that ln([M]0/[M]t increased linearly with reaction time (Fig.1a) indicates
that the concentration of propagating species remained constant throughout the
polymerization. Assuming that the concentration of the active species [P*] was equal
to the initial initiator concentration [I]0, the formula (eq 1) for determing the apparent
rate constant (kp) was integrated in eq 2. Then, kp values can be deduced by the solpe
of the ln([M]0/[M]t versus time, as shown in Fig.1a. From the graphical resolution of
the Arrhenius equation (eq 3 and Fig. 2), the activation energy (Ea) and the frequency
factor (A) are calculated as 69.54 kJ mol−1 and 9.77 × 107 L mol−1 s−1, respectively.
The Ea value is comparable to that obtained in the MeOTs intiating system.8
ꢀꢁꢂꢃ
∗
ꢀꢄ
ꢁ ꢃ
−
ꢅ ꢆꢇ P ꢁMꢃ
(1)
(2)
(3)
ꢁꢂꢃ
ꢈ
ln
ꢅ ꢆꢇꢁIꢃꢊꢋ
ꢁꢂꢃ
ꢉ
ꢆꢇ ꢅ ꢌeꢍꢎ /ꢐꢑ
ꢏ
Fig. 2 Arrhenius plot for the polymerizations of 2ꢀethylꢀ2ꢀoxazoline with Sc(OTf)3 in
acetoniltrile (see Fig. 1a for kp values).
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As evidenced by the kinetic results, Lewis acidic Sc(OTf)3 is more reactive towards
EtOx polymerization than other existing initiators such as methyl tosylate8 and alkyl
iodides.36 With the rareꢀearth catalyst the high yield synthesis of PEtOx was achieved
in a rapid manner (within several hours) under mild conditions. Furthermore, this
living system can be expected for the preparation of block copolymers with a
wellꢀdefined structure. For instance, we have successfully synthesized a diblock
polymer comprised of PEtOx and PPhOx segments via a sequential polymerization
route, wherein the formation of copolymer was confirmed by the observed SEC
analyses of a peak shift from 20.3 to 18.4 min elution time from the first (PEtOx) to
second block growth profiles and a narrow molecular weight distribution (PDI = 1.18)
(Fig.3).
Fig. 3 SEC profiles of PEtOx and the in situ prepared poly(EtOxꢀbꢀPhOx) by using
the sequential addition method (SEC: RI detection, PMMA calibration, THF as eluent,
with a flow rate of 1 mL min−1, 40°C). Copolymerization: Sc(OTf)3/EtOx/PhOx =
1:28:60 (molar ratio), [M]total = 4.5 M, acetonitrile, 90°C; the second monomer was
added when EtOx conversion reached >99% (traced by 1H NMR). The Mn’s of PEtOx
and poly(EtOxꢀbꢀPhOx) were 1190 and 3340, respectively.
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Mechanistic aspects
The cationic ringꢀopening polymerization of 2ꢀoxazolines proved to proceed via two
different types of active species, i.e. ionic and covalent types, depending on the nature
of the initiator and on the monomer.5 For the present system, the ionic polymerization
−
mechanism seems to be most possible because the counteranion OTf is of poor
nucleophilicity. Morever, judging from the low polydispersity (PDIs ≤ 1.10) of the
purified kinetic samples, the cationic species of an oxazolinium is likely to be the only
propagating end existed during the polymerization. Further support to this speculation
was given by analysis of the MALDIꢀTOF mass spectrum of PEtOx with piperidine
terminals. These polymer samples were obtained by endꢀtermination of the living
oxazolinium species resulting from the CROP of EtOx with excess piperidine. As can
be seen from Fig. 4, the mass difference in the series of signals (m/z 99.1) agrees well
with the mass of one EtOx repeating unit, confirming that the sodium adducts form a
homologue series originating from endꢀfunctionalized polymers. Similar results
observations were made for the higher molecular weight polymers, shown in Fig. S3
in the Supporting Information.
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Fig. 4 A typical MALDIꢀTOF mass spectrum of PEtOx having a piperidine group at
the −terminal end (Mn,SEC = 3560, Mw,SEC/Mn,SEC = 1.11; PMMA standard, eluent:
ω
DMF with 50 mM LiBr). The inset shows the corresponding full spectrum.
Polymerization conditions: [M]0 = 4.5 M, [M]0/[I]0 = 100, CH3CN, Sc(OTf)3 initiator,
90oC, 1h, piperidine terminater.
Singleꢀcrystal Xꢀray diffraction analysis of Sc(OTf)3/EtOx complex revealed
extensive coordinative interactions between the imine units of the monomer and the
scandium cation (Fig. S4 in the Supporting Information). It is conceivable that the
coordination would promote the nucleophilic attack of free monomers at the Cꢀ5
position of the bound oxazoline unit to generate an oxazolinium propagating species,
which has been confirmed by in situ NMR spectroscopic studies. As shown in Fig. 5,
the EtOx polymerization system before being subjected to the termination (i.e. a
living polymer) displays the weak proton signals at 4.26 and 4.90 ppm (denoted as a
and b). The two peaks should come from the active oxazolinium units, because they
are different from those due to methylene protons of monomer (δ: 3.71, 4.17 ppm)
and those of the polymer main chain (δ: 3.6−3.3 ppm) as well; furthermore, they
vanished totally upon addition of terminating agents such as piperidine (see: Fig. S1
in the Supporting Information).
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Fig. 5 1H NMR spectrum of a living polymer sample formed in the EtOx
polymerization in CD3CN at 80°C for 30 min ([EtOx]/[Sc] = 30).
Simultaneously, 13C NMR spectroscopy provides us with more information about
the propagating species. In the 13C NMR spectra of Sc(OTf)3, the anionic ligand
displays a characteristic quartet at 114.4−123.8 ppm, and the position of these signals
does not change distinctly in the presence of EtOx at ambient temperature (Fig. 6a,b).
However, a ca. 1.2 ppm downfield shift was observed upon heating the mixture at
80°C for 30 min (Fig. 6c), indicating that the ligand moved to the propagating ends as
a bound counterion with the polymerization went on. This situation is similar to that
seen in the ligand exchangement of rareꢀearth triflates reported by Ling et al.37
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Fig. 6 13C NMR spectra in CD3CN of (a) Sc(OTf)3, (b) Sc(OTf)3 + EtOx (30 eq.),
25°C, and (c) the mixture composed of Sc(OTf)3 and EtOx (30 eq.) after heating for
30 min at 80°C.
−
It is noteworthy that the 13C NMR signal of the ligand OTf still remained its
intrinsic quartet feature in the living polymerization system (Fig.6), which means they
are in the identical chemical environments as counterions. Hence we presume that
every metallic active species in situꢀgenerated from the Sc(OTf)3−EtOx complexation
may possess three identical chainꢀgrowing sites, in other words, one Sc(OTf)3 initiated
three equivalent PEtOx chains. To clarify this point, we devised a set of control
experiments, in which the parallel EtOx polymerizations initiated with Sc(OTf)3 under
the same conditions ([M] = 4.5 M, [M]/[I] = 100, 90°C, 60 min) were terminated by
two kinds of terminating agents, the monofunctional piperidine and the bifunctional
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piperazine. Rather, the termination reaction was conducted at a predetermined
terminatorꢀtoꢀinitiator molar ratio ([T]/[Sc]) to prepare the desired polymers (P1−P5)
for SEC analysis. Among them, P1, as a reference, was obtained by using
intentionally excess piperidine ([T]/[Sc] = 6) in the termination step, while P2−P5
resulted from the piperazine termination with [T]/[Sc] of 0.5, 1.0, 1.5, and 2.0,
respectively.
Fig. 7 Normalized SEC traces (THF, RI detection) of PEtOx samples obtained by
terminating the EtOx polymerization with piperidine (P1) and piperazine (P2−P5);
the added amounts of terminators ([T]/[Sc]) are noted in the inset (PD = piperidine,
PZ = piperazine).
As shown in Fig.7, P1 gives a unimodal molecular weight distribution with Mn =
3310 (PDI = 1.09) in THF. In contrast, the SEC traces of P2−P5 exhibit a bimodal
shape and the peak of low molecular weight components (LMW, arbitrarily defined as
Mn ≈ 3 kD) was found to decrease in going from P2 to P4 in concert with an increase
in the terminator amount used in termination reactions. The Mn values of high
molecular weight (HMW) polymers are roughly twice Mn’s of LMW parts. These
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observations may be explained by the idea proposed above that one Sc(OTf)3 leads to
three living polymer chains combination with Scheme 3 illustrating the
initiating/propagating mechanism. In the piperazineꢀtermination with [T]/[Sc] = 0.5,
the nucleophilic sites (secondary amino groups) were not in sufficient quantities to
stop all the living chains and thereby the resultant polymer P2 is composed of the
piperazineꢀcoupled HMW PEtOx and nonꢀcoupled LMW PEtOx, as evidenced by its
SEC curve (Fig.7). It was also the case for the termination with [T]/[Sc] = 1.0, but the
relative content of LMW components in P3 decreased remarkably compared with P2.
Interestingly, the SEC elution trace of P4 is close to a unimodel shape, indicative of
this sample consisted of the coupled polymers predominantly. This result seems to be
understandable, because in the case of [T]/[Sc] = 1.5 the bifunctional terminator just
provides equivalent nucleophilic cites relative to the cationic propagating ends,
whereby every piperazine molecule can catch two active chains in average during the
termination process yielding the couped product. The very minor distribution
occurred in the SEC profile reveals the presence of a few PEtOx polymers resulting
from chain transfer to the moisture or/and nucleophilic impurity that enter reaction
vessel. On the other hand, an excess of piperazine would retard such coupling
reactions owing to partial terminators is bound to serve as the monofuctional capping
agent in this case, which increasing the content of LMW polymers, as shown in SEC
curve of P5 (Fig.7).
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Scheme 3 Schematic representation of the cationic ringꢀopening polymerization of
2ꢀethylꢀ2ꢀoxazoline (EtOx) with Sc(OTf)3 as the initiator.
Extension of Sc(OTf)3-catalysis to other substituted 2-oxazolines
Finally, we tried to extend the rareꢀearth catalysis to CROPs of some sterically
hindered and functional 2ꢀoxazoline derivatives. The preliminary results suggest that
4/5ꢀalkylated 2ꢀbutylꢀ2ꢀoxazolines (i.e., 4ꢀEtBuOx, 4ꢀMeBuOx and 5ꢀMeBuOx) as
well as 2ꢀphenylꢀ2ꢀoxazoline (PhOx) can be polymerized in a thermal heating process
using Sc(OTf)3 as the initiator, leading to corresponding polymers with low molecular
weights and a relatively narrower polydispersity (runs 1−4, Table 3). At a high
catalyst loading ([M]/[I] = 20), the polymerization of ProPhOxꢀ1 containing ester
linkage was achieved at 90°C, while its amide analogs (ProPhOxꢀ2 and ProPhOxꢀ3)
did not give the desired polymers (runs 5−7). Different from ProPhOxꢀ2, however,
ProPhOxꢀ3 can be polymerized at 140°C under microwave irradiation resulting in the
polymer with Mn of ~6.0 kD and a narrower molecular weight distribution (PDI~1.4).
The poor polymerizability of ProPhOxꢀ2 is probably attributed to the fact that the
intramolecular hydrogen bonding interaction between the adjacent oxazoline unit and
amide group decreases the cyclicꢀimine basicity to such an extent that the
oxazolinium initiating species failed to form. However, a relevant case was reported
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by Jordan and coworkers,13 who showed that methyl triflate did not efficiently initiate
the ringꢀopening polymerization of a kind of 2ꢀoxazoline monomer with a Boc
protected amino function (2ꢀ[NꢀBocꢀ5ꢀaminopentyl]ꢀ2ꢀoxazoline). They presumed the
initiator does not only attack the 2ꢀoxazoline ring, but mainly attacks the secondary
amide group in the monomer side chain.
Table 3 Polymerization of various substituted 2ꢀoxazoline monomers with Sc(OTf)3
Polymer a
[M]
Run Monomer
[M]:[I] Solvent
t/min
T/°C
(mol·L−1)
c
Yield b/% Mn PDIc
1
2
3
4
5
6
7
8
9
4ꢀEtBuOx
4ꢀMeBuOx
5ꢀMeBuOx
PhOx
neat
neat
neat
4.5
60
60
60
bulk
bulk
bulk
120 180
120 180
120 180
48
38
26
31
85
−
4650 1.13
4530 1.15
4230 1.20
5510 1.21
2460 1.14
100 CH3CN 90 180
ProPhOxꢀ1
ProPhOxꢀ2
ProPhOxꢀ3
ProPhOxꢀ2
ProPhOxꢀ3
1.0
20
20
20
20
20
CH3CN 90 180
CH3CN 90 180
CH3CN 90 180
CH3CN 140c 20
CH3CN 140c 20
1.0
−
−
−
−
−
−
1.0
−
1.0
−
1.0
46
5840 1.42
a The structural characterization of polymers are given in Supporting Information.
b Isolated yield.
c Determined by SEC, PMMA calibration, DMF containing 0.05 M LiBr as the eluent.
d Under ꢀW conditions.
Conclusions
In summary, we have demonstrated that rareꢀearth metal triflates could be used as an
efficient catalyst for the cationic ringꢀopening polymerization of 2ꢀoxazolines under
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mild conditions. As evidenced by linear firstꢀorder kinetics, the polymerization of
2ꢀethylꢀ2ꢀoxazoline with Sc(OTf)3 in acetonitrile proceeded in a living/controlled
manner, which was also supported by unimodel molar mass distribution for the
resultant polymers with PDI values below 1.15. In situ NMR spectroscopic studies in
combination with SEC analysis of PEtOx samples obtained from the control
termination experiments showed strong evidence that during the Sc(OTf)3ꢀcatalyzed
polymerization every in situꢀformed metallic active species consists of three identical
onium sites and therefrom initiates simultaneously three equivalent propagating
chains. Such a multiꢀsite propagating mechanism seems to be consistent with the
higher polymerization reaction rate observed in the Sc(OTf)3ꢀcatalysis. In comparison
with other initiating systems, the rareꢀearth catalysis has the advantage of no
requirement for microwave assistance, which would allow the whole polymerization
process to be easily scaled up. We expect that the polyoxazolines bearing a chiral
Lꢀproline moiety in the sideꢀchain synthesized herein will find potential applications
in asymmetric catalysis, the related research is currently underway.
Acknowledgements
The authors are indebted the financial support by the National Natural Science
Foundation of China (Grant No. 21274122).
Notes and references
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