A simple and efficient synthetic method for poly(ethylene terephthalate): Phenylalkyl pyrrolidinium ionic liquid as polycondensation medium

Article abstract of DOI:10.1039/c2gc35310e

A series of phenylalkyl pyrrolidinium ionic liquids (ILs) ([YBPy][X], Y = NO₂, CH₃, F, H; B = benzyl, phenethyl; X = Tf ₂N) were synthesized and found to be environmentally benign reaction media for the preparation of poly(ethylene terephthalate) (PET). ILs with specific functional groups had high thermostabilities and showed interesting properties in synthesizing PET at lower temperature (190-240 °C) and pressure (500 Pa) compared to conventional ILs. PET with M_w up to 1.9 × 10⁴ g mol^-1 was obtained. The ILs can be easily separated and reused after simple purification except for the PF ₆^- ILs. This process provides a valuable and environmentally friendly alternative to the currently available method for the preparation of PET in industry.

Full text of DOI:10.1039/c2gc35310e

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PAPER
A simple and efficient synthetic method for poly(ethylene terephthalate):
phenylalkyl pyrrolidinium ionic liquid as polycondensation medium†
Junyan Dou and Zhengping Liu*
Received 1st March 2012, Accepted 24th May 2012
DOI: 10.1039/c2gc35310e
A series of phenylalkyl pyrrolidinium ionic liquids (ILs) ([YBPy][X], Y = NO2, CH3, F, H; B = benzyl,
phenethyl; X = Tf2N) were synthesized and found to be environmentally benign reaction media for
the preparation of poly(ethylene terephthalate) (PET). ILs with specific functional groups had
high thermostabilities and showed interesting properties in synthesizing PET at lower temperature
(190–240 °C) and pressure (500 Pa) compared to conventional ILs. PET with Mw up to 1.9 × 104 g mol−1
−
was obtained. The ILs can be easily separated and reused after simple purification except for the PF6
ILs. This process provides a valuable and environmentally friendly alternative to the currently
available method for the preparation of PET in industry.
the industrial process. Because of the rigorous reaction con-
ditions, reports of PET performed at milder conditions are quite
Introduction
Since the awareness of green chemistry is progressively increas-
ing, ionic liquids (ILs) have gained considerable attention as
promising media for green processes due to their distinctive
physicochemical properties of low vapor pressure, high ionic
conductivity, excellent thermal and chemical stability and strong
solubilizing ability, which makes them attractive in several
fields, including synthetic chemistry,1–5 electrochemistry,6–8
catalysis,9–14 and polymerization processes.15–30 Moreover, the
physicochemical properties of ILs can be dramatically altered by
varying cations and anions. The facile construction of ILs with
suitable structures to optimize the reaction conditions for specific
applications continues to drive a wide range of fundamental and
applied fields in the current research.
Poly(ethylene terephthalate) (PET) has been found a particu-
larly useful material in many commercial fields. All of these
opportunities for PET applications are related to its special pro-
perties originating from structural characteristics. As an aromatic
polyester, the traditional way to synthesize PET is the melt poly-
condensation of aromatic dicarboxylic acid and diol. Although it
provides an important route to PET, the preparation of PET has
not been described as a low-energy and green process from the
view of increasing environmental awareness. The melt viscosity
increases gradually during the polycondensation process and the
removal of small molecules such as water and alcohol becomes
difficult. Consequently, preparation of PET usually requires high
temperature (>250 °C), high vacuum (10–50 Pa) as well as a
long reaction time (several hours) to remove the byproducts in
limited. Given both the problems of preparation of PET and
underlying approaches to ionic liquid design, it seems to be an
opportunity for ILs to be used for preparation of PET under
milder conditions.
To date, the possibility of using ionic liquids in polycondensa-
tions has been reported for a number of condensation polymers.
Polyamides and polyimides with high molecular weights are pre-
pared in different 1,3-dialkylimidazolium ILs reaction media.31
High molecular weight poly(12-hydroxydodecanoic acid) can be
obtained in Brønsted acid ILs (BAILs) by direct polycondensa-
tion at low to moderate temperature (90–130 °C), atmospheric
pressure and without adding any catalyst.16,17 The utilization of
ILs as new solvents for the synthesis of optically active poly-
amides has triggered unprecedented possibilities for the
advanced polymer materials.32–34 However, the molecular
weights of polyesters obtained from the polycondensation of
hydroxy acids in ILs are low due to the precipitation of the pro-
ducts with increasing of the molecular weights.35 Investigations
into lipase-catalyzed transesterification in ILs have also shown
that low molecular weight polymers are afforded due to precipi-
tation of the polyesters in ILs.36–38 High molecular weight
aliphatic polyesters are also produced in conventional ILs (diaky-
limidazolium salts). The miscibility of polyesters/ILs is the main
influencing factor in obtaining high molecular weight polymers
in the preparation of aliphatic polyesters in ILs.39
It is known that PET can only be dissolved in few traditional
organic solvents with high boiling points, high thermostabilities,
and some functional groups on benzene rings at 80–200 °C,
such as –NO2, –O–, –Cl, –OH, –CH3. According to the principle
that a substance is more likely to be dissolved in solvents with a
similar molecular structure, phenylalkyl pyrrolidinium ILs with
specific functional groups were synthesized and used as polycon-
densation media for the preparation of PET.
Institute of Polymer Chemistry and Physics, College of Chemistry,
BNU Key Lab of Environmentally Friendly and Functional Polymer
Materials, Beijing Normal University, Beijing 100875, China.
E-mail: lzp@bnu.edu.cn; Fax: +86 10 58802075
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2gc35310e
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Fig. 1 The structures of phenylalkyl pyrrolidinium ILs.
Fig. 2 TGA thermograms of the ILs with different specific functional
groups at a heating rate of 10 °C min−1 and 50 mL min−1 N2 flow.
Scheme 1 Synthesis of phenylalkyl pyrrolidinium ILs. (1) phenylalkyl
bromide, N2, ethyl acetate, stirred, R.T., 2–8 h; (2) LiTf2N, H2O, stirred,
R.T., 24 h; (3) KPF6, H2O, stirred, R.T., 24 h.
phenylalkyl pyrrolidinium ILs are solid with melting points in
the range of 35–220 °C (ESI, Table S1†). Only two of their
Tf2N− compounds (3b ([4MBPy][Tf2N]) and 6b ([4FBPy]-
[Tf2N])) can be considered as room temperature ionic liquids.
Moreover, 3b and 6b display no melting points but glass transi-
tion temperatures, reflecting a stable supercooling state. It is
clear that both cation and anion significantly influence the
melting point of the phenylalkyl pyrrolidinium IL.
It is known that the size, charge, and charge distribution are
the main factors that influence the melting point for the cation.
The size effect is easily understood because the dominant forces
in ionic liquids are the Coulombic and the Van der Waals dis-
persion interactions. Charge delocalization reduces the charge
density, and then diminishes the overall lattice energy. Thus, an
important influence of the melting point in phenylalkyl pyrrolidi-
nium ILs is the confined extent of charge delocalization.
ILs possess low vapor pressure and, in contrast to volatile
organic solvents, can be used as a reaction medium under vacuum.
Due to the latter fact ILs can dilute highly viscous polymer solu-
tions and facilitate the elimination of the byproduct, thus shifting
the equilibrium. This work demonstrates a convenient approach to
afford ILs with different specific functional groups at the backbone
of phenylalkyl pyrrolidinium ([YBPy][X], Y = NO2, CH3, F, H;
B = benzyl or phenethyl; X = Tf2N). These ILs reveal excellent
thermostabilities and high yields. The polycondensations of bis-
β-hydroxyethyl terephthalate (BHET) in these ILs are investi-
gated. High to moderate molecular weights of PET were obtained
in shorter time and under relatively mild conditions.
Results and discussion
Synthesis of phenylalkyl pyrrolidinium ionic liquids
The anion plays a crucial role in determining the melting
point. The melting point increases in the following order in each
The phenylalkyl pyrrolidinium ionic liquids were prepared by
the quaternization reaction of N-methylpyrrolidine with phenyl-
alkyl bromide at room temperature. Anions (Tf2N−, PF6−) were
introduced by anion exchange reaction from the corresponding
phenylalkyl bromide precursors and, subsequently, salts
(LiTf2N, KPF6) in water. The ILs were dried under vacuum at
60 °C for 24 h, and stored at 25 °C before use . The structures of
−
case of phenylalkyl pyrrolidinium ILs: Tf2N− < PF6−. The PF6
anion is a large, highly symmetrical (Oh), pseudospherical anion.
However, the broad extent of charge delocalization and high
degree of conformational freedom in the Tf2N− anion results in
the lower melting point.40,41
1
the ILs were confirmed by FTIR, H NMR and elemental analy-
Thermal stabilities of phenylalkyl pyrrolidinium ionic liquids
sis (see ESI†). A schematic representation of the general struc-
tures and synthesis of the pyrrolidinium ILs are shown in Fig. 1
and Scheme 1, respectively.
The size, charge and charge distribution of pyrrolidinium
cation are different to the imidazolium cation. The structural
variation of the cations in this work influences the ionic inter-
actions and melting points, and thus the consequences of poly-
condensation in the ILs will be changed.
The thermal stabilities of phenylalkyl pyrrolidinium ILs were
measured by thermogravimetric analysis. Fig. 2 shows the TGA
thermograms of ILs with different functional groups. There are
four main factors, that is, the nature of the anion, the substituents
and their positions on the cation, and the catalyst, that affect the
thermal stabilities of the various ionic liquids. The effect of each
factor will be discussed in the following.
Anionic constituents of ILs have a great influence on their
thermostabilities. The phenylalkyl pyrrolidinium bromides
([YBPy][Br]) are thermally less stable than the corresponding
Effect of molecular structure of IL on melting point
−
The thermal behaviors of the salts were characterized by differ-
ential scanning calorimetry (DSC). The majority of the
PF6 and Tf2N− salts. The decomposition temperature (Td) of
phenylalkyl pyrrolidinium IL ([2FBPy][X]) apparently decreases
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with the following order: [2FBPy][Tf2N] > [2FBPy][PF6] >
[2FBPy][Br] (ESI, Table S1†). The Td of the ILs ranges from
290–400 °C except for those with Br− as the anion. The salts
−
with Tf2N− have better thermal stabilities than the PF6 salts
because PF6− is thought to be more basic than Tf2N−.
The substituents and their positions on the benzene ring also
affect the thermostabilities of these compounds. The highest
thermal stability (Td = 400 °C) is observed for [PhEPy][Tf2N],
which has no functional group on the benzene ring (ESI,
Table S1†). In general, introducing specific functional groups
into ILs will afford new properties, but will inevitably lower
their thermostabilities. The phenylalkyl pyrrolidinium ILs with
fluoro- and nitro- substituents possess the highest and lowest
values of Td in the ILs with three types of specific functional
groups (Fig. 2; ESI, Table S1†). The positions of the fluorine
atom has little influence on the stabilities of the ILs, due to its
small volume, electrophilicity and lone-pair electrons’ conju-
gation with the benzene, and thus has a slight effect on the steric
hindrance and density of the electron cloud around the benzene
ring. Nevertheless, the ILs with bulky groups (methyl and nitro)
at the ortho-position, which possess a larger steric hindrance,
have lower Td than those with meta- and para-substituents.
There are some turning points about 17% weight loss on the
TGA curves of nitrobenzyl pyrrolidinium ILs, which correspond
to the mass fraction of methylpyrrolidine in the ILs.
The ILs show degradation in the TGA thermograms above
300 °C, but these temperature ramp experiments are not suitable
to describe the degradation during polycondensation. In fact, cat-
alysts may also lead to lower stabilities of ionic liquids, as
observed from the isothermal TGA experiments (Fig. 3). A wide
variety of ionic liquids may readily retain high thermal stabilities
in the processes of heating. However, when the catalyst (EGSb,
Sb(OAc)3, or Sb2O3) is incorporated into the ILs, the thermal be-
havior varies sharply according to the categories of anion and
Fig. 3 Isothermal TGA thermograms of ILs with different catalysts
under N2 atmosphere.
−
catalyst. The ILs with PF6 as the anion are not as stable in the
presence of catalyst as the ILs with Tf2N−, where the mass
losses of [3MBPy][PF6]/catalyst (10–15 wt%) are higher than
those of [3MBPy][Tf2N]/catalyst (<5 wt%). Nevertheless, the
thermal stabilities for IL/catalyst apparently follow the order of
IL/Sb2O3 > IL/EGSb > IL/Sb(OAc)3. The increase of stability of
IL/Sb2O3 is consistent with the enhanced Mw values in the poly-
condensation of BHET catalysed by Sb2O3 (Table 1, entries 13,
19, 22).
Although phenylalkyl pyrrolidinium ILs have a relatively
narrow operating liquid range, the properties of high thermal
stability and negligible volatility guarantee applications in poly-
condensation at high temperature.
molecular weight PET at the polycondensation step. The results
are compared with polycondensation in bulk and the common
IL, [BMIM][Tf2N]. The polycondensation is highly reversible;
hence, removal of the small molecules (diol) is essential to
afford high molecular weight products. In industry, the polycon-
densation is carried out at the temperature 280–300 °C and
under a high vacuum (10–50 Pa). In this work, the reactions are
performed at 190–240 °C and under a vacuum of 500 Pa with a
range of catalysts, including EGSb, Sb(OAc)3 and Sb2O3. The
weight fraction of IL varied from 0 to 90 wt%.
High molecular weight PET (Mw ≈ 1.9 × 104 g mol−1) is
obtained from the polycondensation of BHET in Tf2N− ILs at
relatively low temperature (230–240 °C) (Fig. 4; Table 1, entry
19). This result is superior to those of the polycondensation of
BHET in [PhEPy][Tf2N], [BMIM][Tf2N] and bulk (Table 1,
entries 41–49), which can be attributed to the influence
of aspecific functional groups on the benzene ring. In fact, the
Mw value of PET (Mw ≈ 1.9 × 104 g mol−1, Mn ≈ 1.2 × 104 g mol−1,
PDI = 1.53) is not a very high molecular weight. Presum-
ably, the low molecular weights come from the dilution of reac-
tive groups in the solution polymerization. In order to investigate
this speculation, polycondensation of BHET in [2FBPy][Tf2N]
with Sb2O3 as a catalyst was carried for a long period (Table 1,
Polycondensation of BHET in phenylalkyl pyrrolidinium ionic
liquids
PET was prepared via polycondensation which was carried out
in two steps with dimethyl terephthalate (DMT) and ethylene
glycol (EG): (1) prepolymerization, and (2) polycondensation
(Scheme 2). During the prepolymerization step, DMT and EG
undergo esterification reactions to form oligomers (BHET) with
a degree of polymerization ranging from 4–10. The oligomers
react further in phenylalkyl pyrrolidinium IL to obtain high
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Table 1 Polycondensation of BHET in phenylalkyl pyrrolidinium ILsa
Entry
IL
Catalyst
Mw b/104 (g mol−1
)
PDIb
1.63
1.63
1.66
1.32
1.34
1.23
1.43
Yieldc (%)
1
2
3
4
5
6
7
[2MBPy][PF6]
[2MBPy][PF6]
[2MBPy][PF6]
[3MBPy][PF6]
[3MBPy][PF6]
[3MBPy][PF6]
[PhEPy][PF6]
EGSb
Sb(OAc)3
Sb2O3
EGSb
Sb(OAc)3
Sb2O3
0.42
0.46
0.37
0.31
0.34
0.25
0.34
ND
ND
ND
ND
ND
ND
80.8
Sb2O3
8
9
[2MBPy][Tf2N]
[2MBPy][Tf2N]
[2MBPy][Tf2N]
[3MBPy][Tf2N]
[3MBPy][Tf2N]
[3MBPy][Tf2N]
[4MBPy][Tf2N]
[4MBPy][Tf2N]
[4MBPy][Tf2N]
[2FBPy][Tf2N]
[2FBPy][Tf2N]
[2FBPy][Tf2N]
[3FBPy][Tf2N]
[3FBPy][Tf2N]
[3FBPy][Tf2N]
[4FBPy][Tf2N]
[4FBPy][Tf2N]
[4FBPy][Tf2N]
EGSb
Sb(OAc)3
Sb2O3
EGSb
Sb(OAc)3
Sb2O3
EGSb
Sb(OAc)3
Sb2O3
EGSb
Sb(OAc)3
Sb2O3
EGSb
Sb(OAc)3
Sb2O3
EGSb
Sb(OAc)3
Sb2O3
1.00
1.17
0.75
1.48
1.58
1.89
1.03
1.06
0.89
0.77
1.20
1.91
1.63
1.07
1.76
1.09
1.14
0.85
1.82
1.71
1.64
1.47
1.57
1.54
1.53
1.44
1.49
1.31
1.68
1.59
1.49
1.49
2.02
1.70
1.82
1.89
ND
ND
ND
ND
ND
ND
84.7
78.0
77.3
75.2
76.0
72.7
73.0
72.0
74.5
69.0
77.3
89.0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
f
f
f
26
27
28
29
30
31
[2NBPy][Tf2N]
[2NBPy][Tf2N]
[3NBPy][Tf2N]
[3NBPy][Tf2N]
[4NBPy][Tf2N]
[4NBPy][Tf2N]
EGSb
Sb2O3
EGSb
Sb2O3
EGSb
Sb2O3
—
—
—
—
—
—
f
f
f
1.06
1.04
1.60
74.0
78.9
1.64
f
f
f
—
—
—
0.87
1.93
81.3
32d
33d
34d
35d
36d
37d
38d
39d
40d
[2NBPy][Tf2N]
[2NBPy][Tf2N]
[2NBPy][Tf2N]
[3NBPy][Tf2N]
[3NBPy][Tf2N]
[3NBPy][Tf2N]
[4NBPy][Tf2N]
[4NBPy][Tf2N]
[4NBPy][Tf2N]
EGSb
Sb(OAc)3
Sb2O3
EGSb
Sb(OAc)3
Sb2O3
EGSb
Sb(OAc)3
Sb2O3
0.82
0.86
0.37
0.84
0.68
0.65
0.98
1.20
0.87
1.61
1.21
1.44
1.13
1.21
1.18
1.22
1.18
1.18
54.0
70.0
50.0
64.0
67.0
60.0
65.0
ND
75.0
41
42
43
[PhEPy][Tf2N]
[PhEPy][Tf2N]
[PhEPy][Tf2N]
EGSb
Sb(OAc)3
Sb2O3
1.30
1.00
0.68
1.80
1.84
1.57
77.3
88.7
72.8
44
45
[BMIM][Tf2N]
[BMIM][Tf2N]
[BMIM][Tf2N]
—
—
—
[2FBPy][Tf2N]
[2FBPy][Tf2N]
[3MBPy][Tf2N]
EGSb
Sb(OAc)3
Sb2O3
EGSb
Sb(OAc)3
Sb2O3
Sb2O3
—
0.54
1.07
1.06
0.82
1.05
1.04
0.72
0.49
0.87
2.12
2.28
2.14
1.62
1.76
1.41
2.38
2.26
2.05
70.0
75.0
74.0
67.0
74.7
80.5
86.0
65.0
85.1
46
47e
48e
49e
50g
51h
52h
—
a General polymerization conditions: BHET (50 wt%), IL (50 wt%) and catalyst (0.1 wt%) reacted at 200 °C for 0.5 h, after the mixture was heated to
230 °C, the pressure was reduced to 1.0 × 103 Pa for 1.5 h and 0.5 × 103 Pa for 1.5 h, finally the temperature was increased to 240 °C at 0.5 × 103 Pa
for 0.5 h. b Weight-average molecular weight (Mw) and polydispersity index (PDI) measured by GPC calibrated with polystyrene standards. c Yield
was calculated as the ratio between the purified product’s weight and the initial oligoester’s weight. ND = Not determined. d The reaction temperature
was 200 °C. e Bulk polycondensation at the same conditions as a. f Black products and reddish brown substances were generated through the
polycondensation, which were not determined. g The reaction time was 6 h, i.e., the mixture reacted at 200 °C for 0.5 h, then at 230 °C and 1.0 ×
103 Pa for 2.5 h and 0.5 × 103 Pa for 2.5 h, finally the temperature was increased to 240 °C at 0.5 × 103 Pa for 0.5 h. h Polycondensation of BHET
was performed in ILs without any catalyst. Reaction conditions, see footnote a.
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that there are some constraints on the use of nitrobenzyl pyrroli-
dinium ILs due to the poor thermostabilities of them. However,
it is impossible to rule out the possibility that some fraction of
condensation polymers can be obtained in nitrobenzyl pyrrolidi-
nium ILs. In fact, PETs with low molecular weight and broad
molecular weight distribution are obtained at 200 °C (Table 1,
entries 32–40).
Phase behavior of polycondensation
The evolution of phase behavior for the processes of polycon-
densation with different ILs ([4FBPy][PF6] and [2FBPy][Tf2N])
was recorded on a photographic camera and is displayed in
Fig. S2 (ESI†). As the molecular weight of PET gradually
increases, phase separation starts to occur. The initially homo-
geneous sample (Fig. S2 (F)†) becomes opaque and turbid with
the reaction proceeding. The mixture of polymer and IL forms a
hard solid phase at 240 °C and 500 Pa. Consequently, the effect
of stirring on homogenous dispersion is critical at the later stages
of polycondensation. A similar observation is obtained in
[BMIM][Tf2N]; however, the mixture of PET/[BMIM][Tf2N] is
hygroscopic and soft because [BMIM][Tf2N] is a liquid at room
temperature. The effects of dilution and viscosity are mainly con-
sidered. Dilution is a static effect, and the viscosity influences
transportation.
Scheme 2 Synthesis of PET by two-step polycondensation in phenyl-
alkyl pyrrolidinium ILs.
The phase-separated domain structure of the fractured surface
was measured by SEM and the observed information is pres-
ented in Fig. S3 (ESI†). The networks and bulk aggregates
are separated on the edge area adjacent to the bottle
(Fig. S3(B, C)†). The networks and aggregates correspond to the
PET and IL phases, respectively. In contrast, a homogeneous and
continuous phase is formed near to the center. Instead of large
holes observed at the edge area, small holes surrounded by
viscous mixture of polymer and IL appear at the center in
Fig. S3 (D, E†). In other words, formation of a uniform and
homogenous morphology will occur if stirred well.
Fig. 4 The dependence of Mw of PET on IL and catalyst. (Three points
for each line, for example, (1, 1, Mw), corresponding to the results of
three different reactions, those are polycondensations of BHET using
EGSb as catalyst in [2MBPy][Tf2N] (1b), [3MBPy][Tf2N] (2b) and
[4MBPy][Tf2N] (3b).).
Effect of catalyst on polycondensation
Catalysis plays a critical role in enhancing the rate of the poly-
condensation reaction. The activity of three types of antimony
catalysts is dependent on the anions of the ILs, as reflected in the
molecular weight of PET (Table 1). The reddish brown crude
product is generated through the polycondensation of BHET in
ILs with PF6− as the anion. After purifying, yellowish PETs with
a relatively low molecular weight are obtained (Mw < 4.5 ×
103 g mol−1; Table 1, entries 1–7; Fig. S2(D)†), which is lower
than the one obtained in bulk polycondensation (Mw: (0.8–1.1) ×
104 g mol−1; Table 1, entries 47–49). The same trends have been
observed in the preparation of aliphatic polyesters in ILs.35,36
They supposed that the low catalytic activity of the metal cation
in IL was caused by the preferential combination of the metal
cation with the anion of the IL.
entry 50) and then a low molecular weight of PET (Mw = 0.72 ×
104 g mol−1, PDI: 2.38, Yield: 86.0%) was obtained. This result
indicates that the reaction time does indeed have a crucial effect
on the molecular weight of PET. However, in contrast to the
dilution of reactive groups, long reaction time and water content
of the reaction also favors the degradation of the polymer. The
water will act to reduce the equilibrium molecular weight of the
polymer. One problem with IL solvents is the difficulty to
remove all water, even under the harsh conditions employed
during polymerization.
For the polycondensation of BHET in phenylalkyl pyrrolidi-
nium Tf2N− ILs, deviations from the molecular weight of PET
or the catalytic activity of the catalyst depend on the different
specific functional groups on the benzene ring in the ILs. The
ILs with methyl- and fluoro- substituents have distinct advan-
tages over those ones with nitro- substituents in the molecular
weight of available products under the same conditions (Fig. 4;
Table 1, entries 8–31). Practical experience in synthesis shows
In fact, if only small quantities of water or residues from
the preparation of ionic liquids are present as impurities, their
thermostability will collapse. Additional evidence of the water
contents in samples (ILs, catalysts, and polymer/IL) is provided
by Karl Fischer titration at 24 °C (ESI, Table S4†). Under the
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Fig. 6 GPC curves of PET synthesized in [4MBPy][Tf2N] with EGSb
as the catalyst at different temperatures.
Fig. 5 The dependence of Mw on amount of [4FBPy][Tf2N].
same drying conditions, the catalyst has the highest water
content. ILs with PF6 and Tf2N− as anions are hydrophobic;
−
Thus it requires a high temperature to ensure the product is in a
molten state. However, this generates side reactions, like the
degradation of the PET chains. It is noted that ILs already have
demonstrated their ability to decrease reaction temperature in
polycondensation processes.43
however, they can absorb water from the atmosphere. It was also
reported that the instability of PF6− ILs has been noted due to its
hydrolysis in contact with moisture forming volatiles, including
HF (CAUTION: HF IS AN EXTREMELY CAUSTIC LIQUID
AND VAPOR AND EXTREME CARE SHOULD BE TAKEN
DURING HANDLING), POF3, etc., which can dissolve glass-
ware.42 The difference in performances are more significant for
ILs with PF6− and Tf2N− as counterions. In terms of the thermal
properties, it is reasonable to suppose that there are some kind of
interactions between the IL and catalyst, and water, probably as
the important forces effective in this process.
Interest in phenylalkyl pyrrolidinium ionic liquids will
increase if the polycondensation of BHET was performed in
ILs without any catalyst for comparison. The results of these
cases reveal condensation polymers with molecular weights
(0.5–0.9) × 104 g mol−1 (Table 1, entries 51–52), leading to the
conclusion that the polycondensation can be truly considered as
green.
In this work, the polycondensations of BHET in [4MBPy]-
[Tf2N] catalyzed by EGSb at different temperatures were investi-
gated (Fig. 6; ESI, Table S3†). The reaction temperature was
controlled between the glass transition temperature and the
melting point of PET, which is much lower than the temperature
of melt polycondensation and limits thermal degradation. As can
be seen in Fig. 6, the Mw of PET reaches the maximum value at
230 °C. The molecular weight distributions (MWDs) of PET
become increasingly bimodal and the shoulder peaks at the low
molecular weight part increase with decreasing reaction tempera-
ture. Lower Mw values are attributed to the decrease of monomer
concentration, which is caused by the conversion of BHET
forming polymer in the IL.
Isolation and reuse of ILs
Effect of amount of IL on polycondensation
It is imperative that ILs can be recovered when they are serving
as reaction media in green chemistry processes. Based on the
difference of solubility between PET and phenylalkyl pyrrolidi-
nium ILs, PET can be isolated by simple ethanol extraction,
which is considered as a convenient and environmentally
friendly process. The IL can be efficiently recovered from
ethanol.
Polycondensation of BHET in [4FBPy][Tf2N] catalyzed by
Sb(OAc)3 was chosen as a model system to quantitatively
demonstrate the effect of amount of IL (see ESI, Table S2†). The
variation of Mw of PET versus the concentration of IL is plotted
in Fig. 5. The Mw of PET increases with the increase of IL,
because the mobility of PET chains and the probability of col-
lision of the terminal groups on BHET are enhanced with the
increased amount of IL. It also favors the polycondensation pro-
cesses, which require eliminating the small molecules at high
temperature, due to the diluting abilities of ILs.
Most of the recovered ILs are deep color solids or viscous
liquids. As can be seen from the FTIR spectra (see ESI,
Fig. S1†), the absorption peak observed at 830 cm−1, which is
−
assigned to the P–F bond of PF6 in recovered ILs, disappeared.
It indicates that the P–F bond decomposed during the polycon-
−
densation or isolation processes. The PF6 anion is subject to
hydrolysis in the presence of water traces even at 70 °C.42
During the polycondensation process ethylene glycol is evolved,
while during the prepolymerization the traces of methanol could
still remain in BHET even after its drying. Additionally ethanol
is used as the solvent for the separation of the polymer from the
Effect of temperature on polycondensation
The temperature is an important influence parameter in the
process of polycondensation. The reaction is diffusion-controlled
at the later stage of polycondensation due to high melt viscosity.
2310 | Green Chem., 2012, 14, 2305–2313
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Table 2 The polycondensation of BHET in recycled ILsa
Entry
IL
Catalyst
Temperature (°C)
Mw/104 (g mol−1
)
PDI
Yield (%)
1
2
3
[3NBPy][Tf2N]
[4FBPy][Tf2N]
[4MBPy][Tf2N]
EGSb
EGSb
EGSb
200
200
200
0.62b
0.81b
0.50b
1.13
1.19
1.65
NDc
76.3
77.5
a Polymerization conditions, see footnotes of Table 1. Only the reaction temperature was changed as follows: BHET, IL and catalyst reacted at 180 °C
for 0.5 h, after the system was heated to 190 °C, the pressure was reduced to 1.0 × 103 Pa for 1.5 h and 0.5 × 103 Pa for 1.5 h, finally the temperature
was increased to 200 °C under 0.5 × 103 Pa for 0.5 h. b Bimodal distribution. c ND = Not determined.
ionic liquid. In general, any of the mentioned factors can cause
the hydrolysis of PF6− ionic liquids.
procedures in the literatures.44–46 All catalysts used were dried at
60 °C for 6 h. 1-Butyl-3-methylimidazolium bis(trifluoromethyl-
sulfonyl)imide ([BMIM][Tf2N]) was prepared according to lit-
erature procedures.47
However, the Tf2N− ILs have less changes before and after
use. For this study, the content of catalyst is determined by
inductively coupled plasma spectrometry. It is demonstrated that
the product is formed with 0.12 μg mL−1 Sb catalyst after
separation from the IL. A decrease of catalyst content is observed
for the recycled IL ([3MBPy][Tf2N]: 0.05 μg mL−1). Conse-
quently, a new portion of Sb catalyst was added in the polycon-
densation performed in recycled IL. PETs with relative low
molecular weight and bimodal distribution are obtained at low
reaction temperature (Table 2).
Methods
1
All compounds were characterized by H NMR. Nuclear mag-
netic resonance (NMR) spectra were taken on an Avance
Brucker-400 MHz, and chemical shifts of NMR were reported in
parts per million (ppm, δ). Splitting patterns were designated as
s (singlet), d (doublet), t (triplet), m (multiplet), and br (broad).
FTIR (cm−1) spectra were recorded on an Omnic AVATAR 360,
Nicolet, USA. The intensity of the infrared absorptions and
reflections were designated as vs (very strong), s (strong), m
(medium), w (weak), br (broad). Elemental analyses were
obtained on a Vario El elemental analyser (Elementar). Thermo-
gravimetric analysis (TGA, ZRY-2P, Balance Instrument Factory,
Shanghai Precision Scientific Instrument Co., Ltd, China) was
carried out in an Al2O3 pan from 25 to 600 °C at a heating rate
of 10 °C min−1 under 50 mL min−1 N2 flow. Decomposition
temperature (Td) was obtained via TGA and defined as the temp-
erature at which a 10% weight loss was observed. Isothermal
TGA measurements were conducted from 25–200 °C at a
heating rate of 10 °C min−1 and held at 200 °C for 30 min, then
heated to 230 °C at 10 °C min−1 and isothermally for another
30 min, finally brought to 300 °C at 10 °C min−1. Unless other-
wise noted, this was done under 50 mL min−1 N2 flow with
open Al2O3 pans. A Mettler DSC1 (STARe System) differential
scanning calorimeter, operated under 40 mL min−1 nitrogen flow
and equipped with a liquid nitrogen cooling accessory, was used
to detect the evolution of heating or cooling during the phase
transition. Temperature and heat flow were calibrated by indium
standards. A dry and constant flow of nitrogen was maintained
in order to eliminate thermal gradients and ensure the validity of
the calibration standard from sample to sample. Each sample
was heated from 25 to 250 °C at 10 °C min−1 and maintained at
this temperature for 5 min to allow the complete melting of the
crystallites, followed by cooling to −100 °C, then heating again
up to 250 °C at 10 °C min−1. The glass transition temperature
(the midpoint temperature of the heat capacity change, Tg) was
determined from the DSC thermograms. The values of enthalpy
and entropy were normalized with respect to the amount of IL.
The samples of PET (3.0 mg) were dissolved in o-chlorophenol
(0.3 mL) at 95–105 °C for 15 min and diluted with chloroform
to give a final concentration of 1.2 mg mL−1. The solution was
filtered through a syringe filter before injection. Molecular
Conclusion
In summary, a series of novel phenylalkyl pyrrolidinium ILs
were synthesized and have been demonstrated to be reaction
media for the preparation of PET with relatively high molecular
weight (Mw ≈ 1.9 × 104 g mol−1). The ILs with specific func-
tional groups showed high thermostabilities. The effects of the
activity of antimony catalysts, amount of IL and reaction temp-
erature were investigated. All ILs could be easily recycled and
reused after simple purification except for the PF6− ILs.
The high and moderate molecular weights, green reaction
media, as well as manipulative simplicity in relative mild con-
ditions make this process a valuable and environmentally
friendly alternative to the currently available methods for the
preparation of PET in industry.
Experimental section
Materials
N-Methylpyrrolidine (99%, Dalian Rich Fortune Chemicals Co.,
Ltd, China), lithium bis(trifluoromethylsulfonyl)imide (Li-
(CF3SO2)2N, ≥99%, TCI, Tokyo Chemical Industry Co., Ltd,
Tokyo, Japan), potassium hexafluorophosphate (KPF6, 99%,
Tianjin Guangfu Fine Chemical Research Institute, Tianjin,
China), 2-phenethyl bromide (98%, Alfa Aesar China) were
used as received. Benzyl bromides (97%) were purchased from
Tianjin Shentuo Jingyi Chemical R & D Center (Tianjin, China)
and used without further purification. Ethylene glycol (AR,
Beijing Chemical Plant, Beijing, China) was distilled under
reduced pressure. Dimethyl terephthalate (DMT, CP, Beijing
Chemical Plant, Beijing, China) was recrystallized in methanol
before used. Antimony glycolate (Sb2(OCH2CH2O)3, EGSb)
and antimony acetate (Sb(OAc)3) were prepared according to
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weight and polydispersity of the polymer were determined by
gel permeation chromatography (GPC) analysis relative to poly-
styrene calibration (PLGPC 50, Polymer Laboratories Corpor-
ation) using chloroform as eluant at a flow rate of 1.0 mL min−1
at 40 °C. Water content was analyzed by Karl Fischer titration
technique (Mettler Toledo DL38 KF titrators) at 24 °C. The
sample was dissolved in methanol and titrated. Inductively
coupled plasma (ICP) data were collected on a JY Ultima ICP
spectrometer (France). The microscopic image was recorded on
a HITACHI S-4800 scanning electron microscope.
times, and dried under vacuum at 60 °C for 24 h to provide the
title product as a white solid.
Yield: 2.68 g (80.0%); Td: 320 °C; Found: C, 46.55%, H,
5.93%, N, 4.12%; Calc. for C13H20NPF6 (335.12): C, 46.57%,
H, 6.01%, N, 4.18%; 1H NMR (400 MHz, d-DMSO):
δ = 7.48–7.28 (br, 4H, C6H4), 4.59 (s, 2H, NCH2C6H4),
3.50–3.42 (br, 4H, NCH2CH2CH2CH2), 2.86 (s, 3H, NCH3),
2.42 (s, 3H, C6H4CH3), 2.10–2.08 (br, 4H, NCH2CH2CH2CH2)
ppm; IR (KBr pellet): 3079 (m), 3024 (m, νAr–H), 2987 (m),
2936 (m, νC–H), 1608 (w), 1470 (s), 1385 (m), 1295 (w), 1218
(w), 834 (s, νP–F), 557 (s) cm−1
.
Preparation of phenylalkyl pyrrolidinium ionic liquids
Preparation of bis-β-hydroxyethyl terephthalate (BHET)
1-Methyl-1-(2-methylbenzyl)pyrrolidinium bromide [2MBPy]-
[Br] (1a). To a stirred solution of N-methylpyrrolidine (4.26 g,
50 mmol) in ethyl acetate (50 mL) was slowly added 2-methyl-
benzyl bromide (9.25 g, 50 mmol) under nitrogen atmosphere.
The mixture was stirred for 8 h at room temperature, and then
filtered to remove the solvent. The crude product was washed
with ethyl acetate several times, and dried under vacuum at
60 °C for 24 h to provide 1-methyl-1-(2-methylbenzyl)pyrrolidi-
nium bromide as a white solid.
Into a 250 mL three-necked flask, ethylene glycol (19.22 g,
0.31 mol) was added into stirred dimethyl terephthalate (29.14 g,
0.15 mol) at 110 °C under nitrogen atmosphere. The catalyst
Mn(OAc)2 and Mg(OAc)2 were added to the mixture at 190 °C.
Methanol was distilled off after 30 min at the same temperature.
The reaction was kept at 190 °C for 6 h and quenched by
cooling to room temperature. The product was smashed into bits
and dried at 60 °C for 12 h before being used in subsequent
polycondensation.
Yield: 12.98 g (96.1%); Td: 210 °C; Found: C, 57.73%,
H, 7.34%, N, 5.15%; Calc. for C13H20NBr (269.08): C, 57.78%,
1
H, 7.46%, N, 5.18%; H NMR (400 MHz, D2O): δ = 7.34–7.18
Typical synthesis of PET
(br, 4H, C6H4), 4.45 (s, 2H, NCH2C6H4), 3.47–3.32 (br, 4H,
NCH2CH2CH2CH2), 2.82 (s, 3H, NCH3), 2.30 (s, 3H,
C6H4CH3), 2.07–2.06 (br, 4H, NCH2CH2CH2CH2) ppm;
IR (KBr pellet): 3021 (m, νAr–H), 2998 (s), 2963 (s), 2888 (s),
2832 (m, νC–H), 1603 (m), 1467 (s), 1455 (s), 1389 (m), 1158
(m, νC–N), 898 (s), 886 (s), 876 (s), 829 (m), 783 (s), 755 (s),
BHET (1.5 g, 50 wt%), IL (1.5 g, 50 wt%) and catalyst
(0.0015 g, 0.1 wt%) were charged into a dry vial. The mixture
was degassed by three consecutive vacuum–nitrogen cycles. The
reaction was performed in an oil bath thermostated at a pre-
scribed temperature and pressure. The polycondensation was
conducted at 200 °C for 0.5 h, after that the mixture was heated
to 230 °C, the pressure was reduced to 1.0 × 103 Pa for 1.5 h
and 0.5 × 103 Pa for another 1.5 h, finally the temperature was
increased to 240 °C at 0.5 × 103 Pa for 0.5 h. The polymerization
was terminated by adding nitrogen and cooling to room tempera-
ture. The purified PET was obtained by Soxhlet extraction with
ethanol for 12 h to afford white solid, and the ILs were recycled
by rotary evaporation to remove the solvent.
732 (m), 702 (m) cm−1
.
1-Methyl-1-(2-methylbenzyl)pyrrolidinium bis(trifluoromethyl-
sulfonyl)imide [2MBPy][Tf2N] (1b). To a stirred solution of
[2MBPy][Br] (2.69 g, 10 mmol) in deionized water (20 mL) was
slowly added an aqueous solution of LiTf2N (2.87 g, 10 mmol)
at room temperature for 24 h under a nitrogen atmosphere.
[2MBPy][Tf2N] was separated from the aqueous solution by
filtration, washed with water several times, and dried under
vacuum at 60 °C for 24 h to provide the title product as a white
solid.
Acknowledgements
Yield: 4.14 g (88.1%); Td: 340 °C; Found: C, 38.26%,
H, 4.44%, N, 5.85%; Calc. for C15H20N2S2O4F6 (470.08): C,
38.30%, H, 4.28%, N, 5.95%; H NMR (400 MHz, d-DMSO):
The authors thank the National Science Foundation of China
(Grant no. 50873016), Beijing Municipal Commission of
Education, the Fundamental Research Funds for the Central
Universities and Measuring Fund of Large Apparatus of Beijing
Normal University for financial support.
1
δ = 7.46–7.30 (br, 4H, C6H4), 4.59 (s, 2H, NCH2C6H4), 3.45
(br, 4H, NCH2CH2CH2CH2), 2.86 (s, 3H, NCH3), 2.42 (s, 3H,
C6H4CH3), 2.08 (br, 4H, NCH2CH2CH2CH2) ppm. IR (KBr
pellet): 3073 (w), 3030 (w), 2994 (m), 1480 (m), 1465 (m),
1352 (s, νSvO), 1217 (s), 1139 (s), 1055 (s, νC–F), 928 (m), 911
(m), 790 (m), 779 (m), 762 (w), 752 (m), 740 (m), 616 (s),
Notes and references
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