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Resorcinol: A potentially bio-based building block for the preparation of sustainable polyesters
Accepted Manuscript Resorcinol: a potentially bio-based building block for the preparation of sustainable polyesters Claudio Gioia, Maria Barbara Banella, Micaela Vannini, Annamaria Celli, Martino Colonna, Daniele Caretti PII: DOI: Reference:
S0014-3057(15)30004-5 http://dx.doi.org/10.1016/j.eurpolymj.2015.09.030 EPJ 7088
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
17 July 2015 29 September 2015 30 September 2015
Please cite this article as: Gioia, C., Banella, M.B., Vannini, M., Celli, A., Colonna, M., Caretti, D., Resorcinol: a potentially bio-based building block for the preparation of sustainable polyesters, European Polymer Journal (2015), doi: http://dx.doi.org/10.1016/j.eurpolymj.2015.09.030
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RESORCINOL: A POTENTIALLY BIO-BASED BUILDING BLOCK FOR THE PREPARATION OF SUSTAINABLE POLYESTERS Claudio Gioia,a* Maria Barbara Banella,a Micaela Vannini,a Annamaria Celli, a Martino Colonna,a Daniele Carettib a
Alma Mater Studiorum – University of Bologna, DICAM, Via Terracini 28, 40131 Bologna, Italy b
Alma Mater Studiorum – University of Bologna, CHIMIND, Viale Risorgimento 4, 40136 Bologna, Italy
ABSTRACT Potentially bio-based aromatic polyesters with a kinked structure based on resorcinol have been successfully prepared. The process involves an optimized reaction between resorcinol and ethylene carbonate: the diol thus obtained was polymerized with several aliphatic and aromatic dicarboxylic acids. The chemical structure and the thermal properties of the new polyesters were analysed in order to find correlations between structure and properties. All the polymers present a high thermal stability and are mainly amorphous, with a wide range of glass transition temperatures, according to the diacid structures. On the basis of the results it is clear that resorcinol, which can be derived from renewable resources, is a potential bio-based aromatic monomer, suitable to prepare amorphous polyesters containing 1,3 substituted aromatic moieties, for coating and packaging applications.
Keywords: resorcinol; sustainable polyesters; renewable sources; aromatic polyesters; one-pot polymerization; structure-property relationships
INTRODUCTION In the last few years the increasing need to substitute, or at least lower, the impact of petrol-based compounds on our society led to the development of a variety of innovative, eco-friendly synthetic strategies for the main chemicals and materials. 1 Such approach is actually driven by the possibility to commercialize environmentally sustainable materials with high market penetration. The preparation of renewable aromatic polymers represents today a huge challenge in polymer science, both from the industrial and the academic points of views. 2 One of the main problems to be overcome is connected to the difficulty to find suitable natural sources and processes to obtain renewable aromatic monomers that are usually scarce or difficult to treat. As an example, molecules derived from ligno-cellulosic feedstocks suffer for the presence of multiple functional groups that hinder their exploitation in polyester synthesis. 3 In the field of aromatic polyesters, terephthalic acid (TPA) represents the most exploited aromatic compound; poly(ethylene terephthalate) (PET) is predominant in beverage packaging production, and is also widely used for fibres and as an engineering thermoplastic material. In order to develop sustainable alternatives to the petro-based PET, different processes have been recently reported. 4 In particular, recent patents and papers demonstrate the possibility to obtain TPA from bio-based chemicals such as muconic acid,5 isobutylene (obtained from isobutanol)6 or limonene.7 On the other hand, isophthalic acid (IPA) based polyesters are widely used in several industrial applications such as, for example, in the coating market. They are amorphous materials, given their kinked structure induced by the 1,3-substitution of the aromatic ring. In several cases, isophthalic groups are added to terephthalate polyesters in order to suppress their crystallinity. Moreover, IPA based polyesters provide high thermal stability and increased barrier properties when compared to TPA polyesters. This behaviour has been ascribed to the blocked rotation along the polymer main chain due to the 1,3-substitution.8 However, up to date only a route for the preparation of
sustainable polymers based on isophthalate monomers has been reported in the literature.9,10 In particular, the method starts from bio-isoprene and bio-acrylic acid to produce bio-terephthalic and isophthalic acids, even if the aim is the production of TPA, whereas IPA is only a secondary product. Therefore, new routes are to be rapidly found to produce sustainable aromatic monomers with non linear structures (e.g. 1,3-substitution) for the preparation of non-crystalline polyesters in view of applications in the coating and packaging sectors. With this purpose, the present paper aims at exploiting resorcinol that could be an interesting structure because it features two OH groups with a 1,3 pattern and therefore gives rise to kinked polymeric structures. Moreover, although its main production is currently petrol based, at present several different pathways to obtain bio-based resorcinol are reported in the literature.11-14 Resorcinol can be produced from biomass through fermentative and/or chemical processes. In particular it can be prepared from catechins by fermentation11 or, more commonly, from glucose following two routes: - via inositol: from glucose inositol can be obtained by fermentation12
to be subsequently
chemically converted into 1,3,5-benzenetriol (commonly named phloroglucinol).13 1,3,5benzenetriol can be reduced to resorcinol (Figure 1); OH O
HO
OH
HO
HO
OH OH
HO
OH
Fermentation HO
OH OH
OH
HO
- 3H2O
+H2 -H2O
[H+]
[H+]
OH
OH
Figure 1: Resorcinol synthesis from glucose via inositol. - via triacetic acid lactone (Figure 2):14 triacetic acid lactone can be prepared from glucose by fermentation and can be converted into the corresponding methyl ether. The methyl ether is transformed into 1,3,5-benzenetriol methyl ether that can be converted into resorcinol directly or passing by 1,3,5-benzenetriol as an intermediate.
O
H3C
OH
HO
O
(CH3O)3PO H3C or MeOH
O
Fermentation HO
O
O
OH O
OH
OH
CH3 Na HO
OH
O -CH3OH HO
CH3 [H+]
+H2O -CH3OH
[H+]
OH
HO
OH
OH -H2O HO
[H+]
OH
Figure 2: Resorcinol synthesis from glucose via triacetic acid lactone. The role of resorcinol in material science is usually related to the synthesis of polyarylated materials that involves harsh reaction conditions due to the poor reactivity of phenolic groups compared to aliphatic hydroxyl moieties. For this reason, in order to develop resorcinol based monomers with enhanced
reactivity,
a
chemical
functionalization
is
required.
Then,
1,3-
bis(hydroxyethoxy)resorcinol (HER), derived from resorcinol and ethylene carbonate (ETC) (Figure 3), can be an interesting monomer, whose synthesis is described in the literature.15,16
HO
O
OH
O +
HO
O
O OH
O
Figure 3: HER synthesis from resorcinol and ethylene carbonate. It is also notable that ETC is used in several environmental friendly processes such as the phosgene free production of bisphenol A polycarbonate17-19 and can be obtained from renewable resources.20 Surprisingly, the use of HER in the synthesis of polyesters has been poorly investigated. In particular, Vijayakumar et al21 have performed the synthesis and characterization of saturated and
unsaturated polyesters based on HER and adipic acid or maleic anhydride in order to study their thermal degradation (Figure 4). O
O O
O
O
O
O
O O
O
O
O
n
n
O O
O
O O
O n
Figure 4: polymers prepared from HER and aliphatic diacids. In a patent22 HER has been polymerized with 1,3-phenylenedioxydiacetic acid obtaining a polymer with a low O2 and CO2 permeability that could be useful in food and beverage packaging. This patent confirms the importance of 1,3 substituted aromatic compounds for the synthesis of high barrier polymeric materials. The fact that meta-linkages derived from resorcinol give high barrier properties is reported in literature. 23 Jabarin et al,24,25 Go26 and Takahashi et al27 claim that HER inserted in polyesters or co-polyesters gives them high gas barrier properties. However, for all patents it was reported that a few diacids in combination with HER were used. To the best of our knowledge there is no paper describing the preparation and characterization of a wide set of aromatic polyesters based on HER, and comparing their properties with those of the petro-based counterparts. For this reason, the present paper aims at reporting a detailed optimization of the HER synthesis with respect to that reported in the literature, along with its polymerization with different diacids and diesters mainly deriving from renewable resources, including a one-pot synthetic procedure from resorcinol, ETC and diacids. The chemical and thermal characterization of the polymers are also described.
EXPERIMENTAL
Materials Resorcinol (RES), ethylene carbonate (ETC), potassium carbonate, succinic acid (SA), 1,4cyclohexanedicarboxylic acid dimethyl ester (100% trans isomer), 1,4-cyclohexanedicarboxylic acid (65% trans isomer), dimethyl isophthalate (DMI), dimethyl terephthalate (DMT), 2,5furandicarboxylic acid (FDCA), monobutyltin oxide (MBTO) (all from Aldrich, with purities declared from the manufacturer of 99% or more) were not purified before use.
Syntheses HER Synthesis RES (16.5 g, 150 mmol), ETC (29.0 g, 330 mmol) and potassium carbonate (500 mg, 3.6 mmol) were introduced in a 250 mL three-neck round-bottom flask equipped with a magnetic stirrer and a bubble condenser. The mixture was heated under nitrogen atmosphere at 180 °C for 1 hour. Afterwards, 60 mL of distilled water (or the same quantity of a NaOH 1M solution) were introduced and the mixture was cooled to room temperature. Crystallization germs (few milligrams of silica gel) were then introduced to promote the precipitation of the product. Crystals were filtered on a Buchner filter using paper. Water crystallization yield: 55%; NaOH crystallization yield: 94%. Melting temperature: 89 °C.
HER polymerization procedure The procedure reported here is a typical polymerization method. Modifications are reported in the results and discussion section. 3.00 g (15.0 mmol) of HER, the diacid or diester (16.5 mmol) and MBTO (2.9 mg, 0.014 mmol) were introduced in a 100 mL Schlenk tube under nitrogen atmosphere. The reaction temperature was increased from 200 °C to 240 °C in 4 hours and then kept at this temperature for 2 hours. The pressure was then slowly reduced (1 hour at 60 mBar then progressively decreased to 0.5 mbar, finally 1 hour at 0.5 mBar) and the polymer collected after one hour.
The polymers obtained have been named with the abbreviation P(HER-X) where P stands for polymer, HER for 1,3-bis(2-hydroxyethoxy)benzene and X identifies the group derived from diacids or diesters (S for succinate, CHD for cyclohexanedicarboxylate, I for isophthalate, T for terephthalate and F for furandicarboxylate groups). In the case of polymers containing the cycloxeanedicarboxylate group abbreviation is followed by a number indicating the percentage of trans isomer that is present in the initial monomer.
One-pot polymerization RES (33.0 g, 300 mmol), ETC (58.1 g, 660 mmol) and potassium carbonate (500 mg, 3.6 mmol) were introduced in a three-neck reactor equipped with a mechanical stirrer and a bubble condenser under nitrogen atmosphere. The mixture was heated at 180 °C and kept at this temperature for 1 hour. DMI (64.1 g, 330 mmol) and MBTO (57.0 mg, 0.27 mmol) were then introduced in the reactor. The same temperature and pressure profile previously reported were applied in this procedure except for a longer step (9 hours) at 0.5 mBar.
Instrumental NMR spectroscopy 1
H-NMR analysis of HER was carried out on samples dissolved in a mixture of 1,1,1,3,3,3-
hexafluoro-2-propanol (HFIP)/CDCl3 (1/1, v/v) while CDCl3 was used as solvent for oligomers and polymers. All the analyses were performed at room temperature with TMS as the internal reference using a Varian Mercury 400 spectrometer operating at 400 MHz. Gel permeation chromatography. Molecular weight data were obtained by gel permeation chromatography at 30 °C using a 1100 Agilent Series system with an UV spectrophotometer (at 254 nm wavelength) as a detector, equipped with Agilent PLgel 5µm MiniMIX-C column. A mixture of chloroform/HFIP (95/5 v/v)
was used as eluent with a 0.3 mL/min flow and a sample concentration of about 2 mg/mL. A molecular weight calibration curve was obtained using monodisperse polystyrene standards. Thermal analyses The thermo-gravimetric analysis (TGA) was performed using a Perkin–Elmer TGA4000 thermobalance under nitrogen atmosphere (gas flow 40 mL/min) at 10 °C/min heating rate from 40 to 800 °C. The temperature of thermal degradation beginning (T 0) was measured using a bi-tangent method and the temperature of the maximum degradation rate (TD), corresponding to the maximum of the differential thermo gravimetric curve was determined. Calorimetric measurements were carried out using a Perkin Elmer DSC6 instrument equipped with a liquid sub ambient accessory and calibrated with high purity standards. Weighted samples of approximately 10 mg were encapsulated in aluminium pans and heated from 40 °C to 280 °C at a rate of 20 °C/min (1st scan), held at this temperature for 2 minutes, cooled to -10 °C at a rate of 20 °C/min, held at this temperature for 10 minutes and then heated to 280 °C at a rate of 10 °C/min (2nd scan). 1st scan was done in order to erase the sample thermal history. During the 2nd scan, the glass transition temperature (Tg), the specific heat associated with it (ΔCP), and, if present, the exothermic peak of crystallization temperature (T cc) with the relative enthalpy (ΔHcc) and the melting temperature (Tm) with the relative enthalpy (ΔH m) were measured. Thermal cycles with a cooling rate of 1 °C/min were done in order to facilitate the crystallization process.
RESULTS AND DISCUSSION Optimization of HER synthesis The synthetic pathway to obtain HER from resorcinol and ethylene carbonate using K2CO3 as a catalyst has been previously reported.14 Nevertheless, a comprehensive study of the parameters that affect the reaction (e.g. type of catalyst, catalyst loading, temperature and reaction time) has never been described. On the contrary, the functionalization of bisphenol A with ethylene carbonate has
been recently deeply investigated as a model for the chemical modification of bisphenol A polycarbonate producing useful starting information.28 A set of tests has been performed in order to determine the formation of by-products and the effect of the molar ratio between the reagents. Indeed, the ratio of the reagents represents a key factor that deeply affects the selectivity of the reaction toward the desired product. Since ethylene carbonate presents inherent volatility at temperatures above 150 °C, part of it can be lost due to its evaporation. A 1:2.1 RES/ETC ratio leads to the formation of mono-functionalized resorcinol (Figure 5). a HO
d
O
a
O
OH
b
b c
b
HER
a
c e
l
HO
f
O
g
OH
h
i m
c
e
OH
d
HFIP
CHCl3 g m
7.4
7.2
7.0
f
h,i,l 6.8
6.4 6.4 6.6
6.2
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6 3.4 3.4 ppm
Figure 5: 1H-NMR of the products of the RES/ETC 1:2.1 reaction and chemical structure of HER and mono-functionalized resorcinol by-product (solvent: CDCl3/HFIP, 1/1, v/v). The 1H-NMR analysis of the products of the reactions conducted at 180 °C with different molar ratios of the monomers showed that a 1:2.2 RES/ETC molar ratio is necessary in order to obtain the full conversion of resorcinol hydroxyl groups.
On the other hand, larger amounts of ETC can favour the formation of carbonate and/or aliphatic ether derivatives.28 Nevertheless, even with the optimized ratio of reagents, small amounts of byproducts can be formed and therefore a crystallization of the crude mixture is advised. Water crystallization affords the desired product with 55% yield, while using a NaOH 1M water solution the crystallization affords the desired product with 94% yield. This high yield is probably due to the fact that a basic solution favours HER precipitation and that NaOH is able to hydrolyse the carbonate groups, affording a larger amount of HER, in agreement with the data reported in ref. 15. On the basis of such results, a RES/ETC ratio of 1:2.2 has been used in all subsequent experiments. A second set of tests has been performed in order to optimize the conditions of the reactions (catalyst level, temperature and reaction time). According to the literature, 15,29 potassium carbonate was chosen as catalytic system due to its high activity in reaction between OH groups and ETC as
Conversion (%)
well as its low cost and environmental sustainability.
100 90 80 70 60 50 40 30 20 10 0
K2CO3 meq. K meq. 2 CO32424 0 15 30 45 60 K CO 12 meq. 2 3 0 15 30 45 60 K CO 7 meq. 2 3
0
15
30
45
60
Time (min)
Figure 6: effect of catalyst level on resorcinol conversion. The effect of the catalyst level on the reaction rate at 180 °C was analysed measuring the conversion by 1H-NMR analysis (Figure 6). Conversion of resorcinol in HER was determined dividing the value of the area under the HER peak by the sum of the areas under the peaks of HER, resorcinol and by-products when present. Reactions were performed using 24, 12 and 7 milliequivalents of catalyst with respect to resorcinol. The lower amount was chosen according to the literature.15 The results indicate that with 12 and 24
meq the reaction was significantly faster with respect to what previously reported in the literature15 (7 meq). Using 24 meq of catalyst, the complete conversion was reached after 15 minutes while with 12 meq one hour was necessary to reach 100% of conversion. On the basis of these results 24 meq of catalyst were used in all subsequent tests. The effect of the temperature on the reaction rate is reported in Figure 7. The reaction was performed with 24 meq of catalyst at 150, 165, 180 and 200 °C. The best compromise between short reaction times and low temperature is 180 °C. Higher temperature, in fact, does not significantly affect the reaction rate nor favors side-reactions, while at lower temperatures the
Conversion (%)
reaction rate is significantly slower.
100 90 80 70 60 50 40 30 20 10 0
T=200°C T=180°C T=165°C T=150°C
0
15
30
45
60
Time (min)
Figure 7: effect of temperature on resorcinol conversion.
HER polymerization Succinic acid (SA), dimethyl terephthalate (DMT), dimethyl isophthalate (DMI), 2,5furandicarboxylic acid (FDCA), dimethyl 1,4-cyclohexanedicarboxylate (DMCD, with 100% of trans isomer) and 1,4-cyclohexanedicarboxylic acid (CHDA, 66% of trans isomer) have been polymerized with HER (Figure 8). Dimethyl terephthalate and dimethyl isophthalate have been chosen in order to avoid very long reaction times and high temperatures due to the high T m of the corresponding diacids (above 300 °C). DMCD (100% trans isomer) was used instead of the corresponding diacid since carboxylic acids are well known to catalyse the isomerization from
trans to cis isomer towards the thermodynamically stable cis/trans ratio (34/66 mol%), whereas the isomerization is less probable in the presence of diesters.30 The CHDA 66% trans, instead, can be used because isomerization does not occur: the thermodynamically stable cis/trans ratio is already present. O HO
O
O
OH
+
DIACID or
DIESTER
MBTO 200-240°C N2
O
O
O
O
O R
n O O
O
DIACIDS
OH
HO
HO
HO O
SA O
O
O
OH
CHDA 66% trans
FDCA
O
O O
DIESTERS
O O
OH
O
O
O O
O O
DMCD 100% trans
O
DMT
DMI
Figure 8: HER polymerization and diacids and diesters used.
All monomers, except dimethyl isophthalate, can be obtained from renewable sources.7,9,10,31-33 The diacids and diesters have been chosen in order to cover a wide range of different macromolecular structures from aliphatic to cycloaliphatic and aromatic ones. In particular, succinic acid has been used as an example of linear aliphatic diacid while cyclohexane dicarboxylic acids (or the equivalent esters) represent interesting cycloaliphatic monomers. Indeed, thanks to the different cis/trans ratio of the C6 ring, polyesters with different thermal properties and phase behaviour can be obtained. 34 For this reason, two different isomeric mixtures have been tested (100% and 66% trans). Dimethyl terephthalate, dimethyl isophthalate and furan dicarboxylic acid have been chosen in order to introduce aromatic units also in the diacid, which generally provides high thermal and mechanical performances.
The polymers have been synthesized by melt polycondensation in inert atmosphere with monobuthyl tin oxide as catalyst, that is a well-known trans-esterification and directesterification catalyst. The reaction conditions (monomer ratio and reaction temperature) have been optimized for all the acids or esters in order to obtain polymers with higher molecular weight (Table 1). A 10% excess of the diacids was used except in the cases of succinic acid and furandicarboxylic acid. This excess is due to the higher volatility of the diacids with respect to HER. Succinic acid was introduced with a higher excess (20% of excess) with respect to HER because of its tendency to sublimate. On the contrary, a 10% excess of HER with respect to the furandicarboxylic acid was employed due to the lower volatility of FDCA with respect to HER. Polymerization reaction made using DMCD has been performed at lower temperatures (220 °C) in order to prevent the possible isomerization of the trans-isomer into the cis-isomer. The prepared polyester are characterized by the trans isomer content equal to 94%. Also using furandicarboxylic acid the maximum temperature was 220 °C due to its lower thermal stability.35
Table 1: reaction conditions and molecular weight of polymers based on HER. HER/diacid or diester ratio
Mn·10-3
Mw·10-3
Da
PD b
P(HER-S)
Maximum temperature (°C) 240
1:1.2
9.50
27.0
2.8
27.4
P(HER-CHD66)
240
1:1.1
8.00
46.0
5.7
19.9
P(HER-CHD100)
220
1:1.1
8.70
29.2
3.4
21.7
P(HER-I)
240
1:1.1
10.6
28.5
2.7
26.8
P(HER-T)
240
1:1.1
17.0
55.5
3.3
43.0
P(HER-F)
220
1.1:1
9.6
25.9
2.7
44.1
Polyester
a)
D: polydispersity; b) PD: Polymerization degree
The reaction time has been optimized by evaluating the increment of the molecular weights through the GPC measurements carried out on the samples taken during the reaction.
The NMR analysis confirms the successful reactions without side-reactions. As an example, in Figure 9 the 1H-NMR spectrum of the CHDA-derived polymer is reported.
a b O
a
*
O
d
O
b
O
b c
2
a
O
2 1
c
*
2
e
2
1 O
n
2t
2t
c,d CHCl3
7.5
7.0
6.5
6.0
1.5
1 .0
0.5
e
7.0
TMS
2c 2c
1t
0.0
1c
6.0
5.0
4.0
3.0
2.0
1.0
0.0 ppm
Figure 9: 1H-NMR spectrum of the polymer obtained from HER and CHDA 66% trans (t and c indicate trans and cis isomers). Thermal properties The results of the DSC analysis (Figure 10 and Table 2) confirm that the materials are generally amorphous, with glass transition temperatures ranging between 18 and 63 °C, according to the rigidity of the diacid units. In particular, P(HER-S) is characterized by the lowest Tg value compared to those of the polymers containing cycloaliphatic and aromatic units. This is due to the high flexibility of the sequences of methylene moieties. On the other hand, the cyclic aliphatic structure of DMCD introduces a higher stiffness of the macromolecular chain, which induces an increment of Tg. It is worth noting that the conformation of the aliphatic ring slightly affects the glass transition temperature. In particular, the prevalence of the stretched trans isomer tends to increase the Tg value due to its higher symmetry with respect to the cis isomer.34
Normalized heat flow (W/g)
P(HER-S) P(HER-SA)
Endo
P(HER-CHD66)
P(HER-CHD100)
P(HER-I) P(HER-T)
P(HER-F)
-10
10
30
50
70
90
110
130
150
170
Temperature ( C)
Figure 10: DSC curves measured at 10 °C/min (2nd heating scan).
Table 2: DSC and TGA data of HER containing polymers Tg
ΔCp
Tcc
ΔHcc
Tm
ΔHm
T0 a
Td b
P(HER-S)
(°C) 18
(J/g °C) 0.67
(°C) --
(J/g) --
(°C) --
(J/g) --
(°C) 404
(°C) 431
P(HER-CHD66)
32
0.42
--
--
--
--
446
468
P(HER-CHD100)
33
0.37
111
21
147
20
445
470
P(HER-I)
51
0.47
--
--
--
--
435
459
P(HER-T)
58
0.37
--
--
--
--
434
460
P(HER-F)
63
0.46
--
--
--
--
387
414
Polyester
a) b)
T0: temperature of degradation beginning Td: temperature of the maximum rate of degradation
The combination between HER and the aromatic units of isophthalic acid, terephthalic acid and furandicarboxylic acid produces materials with high T g values (respectively 51, 58, and 63 °C). The most symmetric structure of terephthalic acid justifies the higher Tg value of P(HER-T) with respect to P(HER-I). Furanic rings tend to confer higher Tg values to P(HER-F) with respect to phthalic rings due to its high rigidity and polarity.36 The effect of the structure of the diacids on the final thermal properties of the polyesters is confirmed by comparing data reported in the literature: some examples are collected in Table 3, for aliphatic, cycloaliphatic and aromatic polyesters derived from ethylene glycol or 1,4-
butanediol, combined with the same diacids used here. Along a column of Table 3, Tg increases from an aliphatic to an aromatic diacid, i.e. as a function of the structure of the diacids.
Table 3: Tg data of some polyesters from literature Polyester
Tg (°C)
Mw·10-3
Polyester
Tg (°C)
Mw ·10-3
PESa
-11
43.8
PBSe
-27
-
PECHD57b
14
55.3
PBCHD72f
-2
77.6
PECHD100
-
-
PBCHD100f
10
73.4
PEIc
63
48.7
PBIg
26
57.4
PETd
75
15.2
PBTd
43
29.8
13.4
d
36
21.2
d
PEF
77
PBF
a)
PES: poly(ethylene succinate), heating rate: 10 °C/min, ref.37; b) PECHD57: poly(ethylene 1,4cyclohexane dicarboxylate) with 57% of trans isomer, heating rate: 10 °C/min, ref. 38; c) PEI: poly(ethylene isophthalate), heating rate: 20 °C/min, ref.39; d) PEF: poly(ethylene 2,5furandicarboxylate), PBT: poly(butylene terephthalate), PBF: poly(butylene 2,5furandicarboxylate), heating rate: 10 °C/min, ref.35; e) PBS: poly(butylene succinate), heating rate: 10 °C/min, ref.40; f) PBCHD72: poly(butylene 1,4-cyclohexane dicarboxylate) with 72% of trans isomer, PBCHD100: poly(butylene 1,4-cyclohexane dicarboxylate) with 100% of trans isomer, heating rate: 10 °C/min, ref.41; g) PBI: poly(butylene isophthalate), heating rate: 20 °C/min ref.42
On the other hand, by considering the effect of the structure of the diol, as Tables 2 and 3 show, it is also worth noting that the presence of the aromatic HER units along the macromolecules has a huge effect on the rigidity of the chains and induces significantly higher Tg values only in the presence of an aliphatic diacid. For example, P(HER-S) and PBS have Tg equal to 18 and -27 °C, respectively. Instead, when the diacid is aromatic, this effect is remarkably reduced and the difference in Tg between an aromatic diol (HER) and an aliphatic diol (ethylene glycol or 1,4butanediol) are no longer evident. For example, P(HER-T), PET and PBT have Tg equal to 58, 75 and 43, respectively. The amorphous character of the HER-derived polyesters has been confirmed also by analysing the materials in DSC at a very slow cooling rate (1 °C/min). With the only exception of the P(HER-CHD100), the polymers are unable to rearrange towards an ordered state. The reason for such behaviour can be ascribed to the non-linear shape of HER that prevents the formation of
crystalline domains. As a confirmation, polymers derived from meta-substituted isophthalic acid present a remarkably lower crystallization rate than the analogous para-substituted (for example, PEI vs PET). Only in the case of P(HER-CHD100) the stretched trans configuration improves the symmetry of the chain and the packing towards stable crystals. All the polyesters obtained present good thermal stability since the degradation processes occur at temperatures higher than 380 °C (Table 2). It must be underlined that the samples containing the 1,4-cyclohexylene rings have an outstanding thermal resistance with onset temperature of degradation up to 446 °C. P(HER-F), instead, has the lowest degradation temperature whereas the polyesters based on aromatic diacids achieve high stabilities. Such result is in agreement with some data reported in the literature36 on PEF and PET (thermal degradation onset= 389 and 413 °C, respectively), that indicate a lower thermal stability for the furanic polymer.
One-pot synthesis from ETC, RES and isophthalic acid The synthesis of HER has also been performed in the same reactor used for the subsequent polycondensation (Figure 11). This procedure was investigated in order to determine a simple and even more sustainable procedure starting directly from resorcinol, thus avoiding the purification step.
Figure 11: one-pot synthetic procedure.
Using this process a polyester with Mn equal to 9200, Mw of 37400 and Tg of 47 °C has been obtained. The results are consistent with those obtained by the polymerization procedure of HER with dimethyl isophthalate, with a slightly lower T g (4 °C) for the one-pot procedure. Figure 12-a and 12-b show the comparison between the 1H-NMR spectra of the polymers obtained using the one-pot polymerization and using the purified HER with the procedure previously described.
O
a
*
O
d
O
O
b
O
b c
O
1
a
*
2
c
2
e
CHCl3
n
3
TMS
a b
c,d
2 1 3
e
4.80
4.60
4.40
4.20
4.00
3.80
a a b
CHCl3
TMS c,d
2 1
3 e 4.80
4.60
4.40
4.20
4.00
3.80
b 9.0 9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0 0.0 ppm
Figure 12: 1H-NMR spectra of P(HER-I) from one-pot polymerization (a) and P(HER-I) from twosteps polymerization using purified HER (b). 1
H-NMR spectra show that the polymers obtained with the two methods are similar. A small
difference can be noted in the zone between 3.9 and 4.05 ppm due to the development of aliphatic ether linkages. Indeed, the formation of ether bonds is due to a well-known dehydration side-reaction of alcoholic end groups, by following the mechanism depicted in Figure 13. 43 This by-product increaseses the amout of CH2 groups in the polymer chain wich can be reponsible for the lower Tg of the polymer obtained by the one-pot process.
Figure 13: dehydration reaction of alcoholic end groups with formation of aliphatic ether units
CONCLUSIONS A new successful and green strategy for the synthesis of polyesters with a kinked structure based on resorcinol has been developed. The process involves the reaction of resorcinol with ethylene carbonate (ETC); the resulting aromatic diol (HER) can be polymerized with several aliphatic and aromatic dicarboxylic acids or esters by melt polycondensation using a tin based catalyst. Moreover, a novel one-pot procedure has also been developed: the product obtained from ETC and resorcinol reaction is directly polymerized with a diacid without any intermediate purification step. The final material features properties similar to those obtained in polyesters prepared by two-step synthesis. The optimized synthetic polymerization starting from HER provided a wide range of polyesters, which are generally amorphous and characterized by glass transition temperatures varying over a
wide range, according to the diacid chemical structure and rigidity. Moreover, all the polymers present a high thermal stability. On the basis of the results obtained it is clear that resorcinol, that can be derived from renewable sources, can be used in the synthesis of amorphous aromatic polyesters for coating and packaging applications.
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Highlights -
A sustainable synthesis of 1,3-bis(hydroxyethoxy)resorcinol (HER) was performed. Polyesterification of HER with different diacids was carried out. The obtained polyesters showed amorphous character and high thermal stability. The relationship between structure and properties of these materials was studied. A one-pot polyesterification avoiding purification steps was developed.
S0014-3057(15)30004-5 http://dx.doi.org/10.1016/j.eurpolymj.2015.09.030 EPJ 7088
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
17 July 2015 29 September 2015 30 September 2015
Please cite this article as: Gioia, C., Banella, M.B., Vannini, M., Celli, A., Colonna, M., Caretti, D., Resorcinol: a potentially bio-based building block for the preparation of sustainable polyesters, European Polymer Journal (2015), doi: http://dx.doi.org/10.1016/j.eurpolymj.2015.09.030
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RESORCINOL: A POTENTIALLY BIO-BASED BUILDING BLOCK FOR THE PREPARATION OF SUSTAINABLE POLYESTERS Claudio Gioia,a* Maria Barbara Banella,a Micaela Vannini,a Annamaria Celli, a Martino Colonna,a Daniele Carettib a
Alma Mater Studiorum – University of Bologna, DICAM, Via Terracini 28, 40131 Bologna, Italy b
Alma Mater Studiorum – University of Bologna, CHIMIND, Viale Risorgimento 4, 40136 Bologna, Italy
ABSTRACT Potentially bio-based aromatic polyesters with a kinked structure based on resorcinol have been successfully prepared. The process involves an optimized reaction between resorcinol and ethylene carbonate: the diol thus obtained was polymerized with several aliphatic and aromatic dicarboxylic acids. The chemical structure and the thermal properties of the new polyesters were analysed in order to find correlations between structure and properties. All the polymers present a high thermal stability and are mainly amorphous, with a wide range of glass transition temperatures, according to the diacid structures. On the basis of the results it is clear that resorcinol, which can be derived from renewable resources, is a potential bio-based aromatic monomer, suitable to prepare amorphous polyesters containing 1,3 substituted aromatic moieties, for coating and packaging applications.
Keywords: resorcinol; sustainable polyesters; renewable sources; aromatic polyesters; one-pot polymerization; structure-property relationships
INTRODUCTION In the last few years the increasing need to substitute, or at least lower, the impact of petrol-based compounds on our society led to the development of a variety of innovative, eco-friendly synthetic strategies for the main chemicals and materials. 1 Such approach is actually driven by the possibility to commercialize environmentally sustainable materials with high market penetration. The preparation of renewable aromatic polymers represents today a huge challenge in polymer science, both from the industrial and the academic points of views. 2 One of the main problems to be overcome is connected to the difficulty to find suitable natural sources and processes to obtain renewable aromatic monomers that are usually scarce or difficult to treat. As an example, molecules derived from ligno-cellulosic feedstocks suffer for the presence of multiple functional groups that hinder their exploitation in polyester synthesis. 3 In the field of aromatic polyesters, terephthalic acid (TPA) represents the most exploited aromatic compound; poly(ethylene terephthalate) (PET) is predominant in beverage packaging production, and is also widely used for fibres and as an engineering thermoplastic material. In order to develop sustainable alternatives to the petro-based PET, different processes have been recently reported. 4 In particular, recent patents and papers demonstrate the possibility to obtain TPA from bio-based chemicals such as muconic acid,5 isobutylene (obtained from isobutanol)6 or limonene.7 On the other hand, isophthalic acid (IPA) based polyesters are widely used in several industrial applications such as, for example, in the coating market. They are amorphous materials, given their kinked structure induced by the 1,3-substitution of the aromatic ring. In several cases, isophthalic groups are added to terephthalate polyesters in order to suppress their crystallinity. Moreover, IPA based polyesters provide high thermal stability and increased barrier properties when compared to TPA polyesters. This behaviour has been ascribed to the blocked rotation along the polymer main chain due to the 1,3-substitution.8 However, up to date only a route for the preparation of
sustainable polymers based on isophthalate monomers has been reported in the literature.9,10 In particular, the method starts from bio-isoprene and bio-acrylic acid to produce bio-terephthalic and isophthalic acids, even if the aim is the production of TPA, whereas IPA is only a secondary product. Therefore, new routes are to be rapidly found to produce sustainable aromatic monomers with non linear structures (e.g. 1,3-substitution) for the preparation of non-crystalline polyesters in view of applications in the coating and packaging sectors. With this purpose, the present paper aims at exploiting resorcinol that could be an interesting structure because it features two OH groups with a 1,3 pattern and therefore gives rise to kinked polymeric structures. Moreover, although its main production is currently petrol based, at present several different pathways to obtain bio-based resorcinol are reported in the literature.11-14 Resorcinol can be produced from biomass through fermentative and/or chemical processes. In particular it can be prepared from catechins by fermentation11 or, more commonly, from glucose following two routes: - via inositol: from glucose inositol can be obtained by fermentation12
to be subsequently
chemically converted into 1,3,5-benzenetriol (commonly named phloroglucinol).13 1,3,5benzenetriol can be reduced to resorcinol (Figure 1); OH O
HO
OH
HO
HO
OH OH
HO
OH
Fermentation HO
OH OH
OH
HO
- 3H2O
+H2 -H2O
[H+]
[H+]
OH
OH
Figure 1: Resorcinol synthesis from glucose via inositol. - via triacetic acid lactone (Figure 2):14 triacetic acid lactone can be prepared from glucose by fermentation and can be converted into the corresponding methyl ether. The methyl ether is transformed into 1,3,5-benzenetriol methyl ether that can be converted into resorcinol directly or passing by 1,3,5-benzenetriol as an intermediate.
O
H3C
OH
HO
O
(CH3O)3PO H3C or MeOH
O
Fermentation HO
O
O
OH O
OH
OH
CH3 Na HO
OH
O -CH3OH HO
CH3 [H+]
+H2O -CH3OH
[H+]
OH
HO
OH
OH -H2O HO
[H+]
OH
Figure 2: Resorcinol synthesis from glucose via triacetic acid lactone. The role of resorcinol in material science is usually related to the synthesis of polyarylated materials that involves harsh reaction conditions due to the poor reactivity of phenolic groups compared to aliphatic hydroxyl moieties. For this reason, in order to develop resorcinol based monomers with enhanced
reactivity,
a
chemical
functionalization
is
required.
Then,
1,3-
bis(hydroxyethoxy)resorcinol (HER), derived from resorcinol and ethylene carbonate (ETC) (Figure 3), can be an interesting monomer, whose synthesis is described in the literature.15,16
HO
O
OH
O +
HO
O
O OH
O
Figure 3: HER synthesis from resorcinol and ethylene carbonate. It is also notable that ETC is used in several environmental friendly processes such as the phosgene free production of bisphenol A polycarbonate17-19 and can be obtained from renewable resources.20 Surprisingly, the use of HER in the synthesis of polyesters has been poorly investigated. In particular, Vijayakumar et al21 have performed the synthesis and characterization of saturated and
unsaturated polyesters based on HER and adipic acid or maleic anhydride in order to study their thermal degradation (Figure 4). O
O O
O
O
O
O
O O
O
O
O
n
n
O O
O
O O
O n
Figure 4: polymers prepared from HER and aliphatic diacids. In a patent22 HER has been polymerized with 1,3-phenylenedioxydiacetic acid obtaining a polymer with a low O2 and CO2 permeability that could be useful in food and beverage packaging. This patent confirms the importance of 1,3 substituted aromatic compounds for the synthesis of high barrier polymeric materials. The fact that meta-linkages derived from resorcinol give high barrier properties is reported in literature. 23 Jabarin et al,24,25 Go26 and Takahashi et al27 claim that HER inserted in polyesters or co-polyesters gives them high gas barrier properties. However, for all patents it was reported that a few diacids in combination with HER were used. To the best of our knowledge there is no paper describing the preparation and characterization of a wide set of aromatic polyesters based on HER, and comparing their properties with those of the petro-based counterparts. For this reason, the present paper aims at reporting a detailed optimization of the HER synthesis with respect to that reported in the literature, along with its polymerization with different diacids and diesters mainly deriving from renewable resources, including a one-pot synthetic procedure from resorcinol, ETC and diacids. The chemical and thermal characterization of the polymers are also described.
EXPERIMENTAL
Materials Resorcinol (RES), ethylene carbonate (ETC), potassium carbonate, succinic acid (SA), 1,4cyclohexanedicarboxylic acid dimethyl ester (100% trans isomer), 1,4-cyclohexanedicarboxylic acid (65% trans isomer), dimethyl isophthalate (DMI), dimethyl terephthalate (DMT), 2,5furandicarboxylic acid (FDCA), monobutyltin oxide (MBTO) (all from Aldrich, with purities declared from the manufacturer of 99% or more) were not purified before use.
Syntheses HER Synthesis RES (16.5 g, 150 mmol), ETC (29.0 g, 330 mmol) and potassium carbonate (500 mg, 3.6 mmol) were introduced in a 250 mL three-neck round-bottom flask equipped with a magnetic stirrer and a bubble condenser. The mixture was heated under nitrogen atmosphere at 180 °C for 1 hour. Afterwards, 60 mL of distilled water (or the same quantity of a NaOH 1M solution) were introduced and the mixture was cooled to room temperature. Crystallization germs (few milligrams of silica gel) were then introduced to promote the precipitation of the product. Crystals were filtered on a Buchner filter using paper. Water crystallization yield: 55%; NaOH crystallization yield: 94%. Melting temperature: 89 °C.
HER polymerization procedure The procedure reported here is a typical polymerization method. Modifications are reported in the results and discussion section. 3.00 g (15.0 mmol) of HER, the diacid or diester (16.5 mmol) and MBTO (2.9 mg, 0.014 mmol) were introduced in a 100 mL Schlenk tube under nitrogen atmosphere. The reaction temperature was increased from 200 °C to 240 °C in 4 hours and then kept at this temperature for 2 hours. The pressure was then slowly reduced (1 hour at 60 mBar then progressively decreased to 0.5 mbar, finally 1 hour at 0.5 mBar) and the polymer collected after one hour.
The polymers obtained have been named with the abbreviation P(HER-X) where P stands for polymer, HER for 1,3-bis(2-hydroxyethoxy)benzene and X identifies the group derived from diacids or diesters (S for succinate, CHD for cyclohexanedicarboxylate, I for isophthalate, T for terephthalate and F for furandicarboxylate groups). In the case of polymers containing the cycloxeanedicarboxylate group abbreviation is followed by a number indicating the percentage of trans isomer that is present in the initial monomer.
One-pot polymerization RES (33.0 g, 300 mmol), ETC (58.1 g, 660 mmol) and potassium carbonate (500 mg, 3.6 mmol) were introduced in a three-neck reactor equipped with a mechanical stirrer and a bubble condenser under nitrogen atmosphere. The mixture was heated at 180 °C and kept at this temperature for 1 hour. DMI (64.1 g, 330 mmol) and MBTO (57.0 mg, 0.27 mmol) were then introduced in the reactor. The same temperature and pressure profile previously reported were applied in this procedure except for a longer step (9 hours) at 0.5 mBar.
Instrumental NMR spectroscopy 1
H-NMR analysis of HER was carried out on samples dissolved in a mixture of 1,1,1,3,3,3-
hexafluoro-2-propanol (HFIP)/CDCl3 (1/1, v/v) while CDCl3 was used as solvent for oligomers and polymers. All the analyses were performed at room temperature with TMS as the internal reference using a Varian Mercury 400 spectrometer operating at 400 MHz. Gel permeation chromatography. Molecular weight data were obtained by gel permeation chromatography at 30 °C using a 1100 Agilent Series system with an UV spectrophotometer (at 254 nm wavelength) as a detector, equipped with Agilent PLgel 5µm MiniMIX-C column. A mixture of chloroform/HFIP (95/5 v/v)
was used as eluent with a 0.3 mL/min flow and a sample concentration of about 2 mg/mL. A molecular weight calibration curve was obtained using monodisperse polystyrene standards. Thermal analyses The thermo-gravimetric analysis (TGA) was performed using a Perkin–Elmer TGA4000 thermobalance under nitrogen atmosphere (gas flow 40 mL/min) at 10 °C/min heating rate from 40 to 800 °C. The temperature of thermal degradation beginning (T 0) was measured using a bi-tangent method and the temperature of the maximum degradation rate (TD), corresponding to the maximum of the differential thermo gravimetric curve was determined. Calorimetric measurements were carried out using a Perkin Elmer DSC6 instrument equipped with a liquid sub ambient accessory and calibrated with high purity standards. Weighted samples of approximately 10 mg were encapsulated in aluminium pans and heated from 40 °C to 280 °C at a rate of 20 °C/min (1st scan), held at this temperature for 2 minutes, cooled to -10 °C at a rate of 20 °C/min, held at this temperature for 10 minutes and then heated to 280 °C at a rate of 10 °C/min (2nd scan). 1st scan was done in order to erase the sample thermal history. During the 2nd scan, the glass transition temperature (Tg), the specific heat associated with it (ΔCP), and, if present, the exothermic peak of crystallization temperature (T cc) with the relative enthalpy (ΔHcc) and the melting temperature (Tm) with the relative enthalpy (ΔH m) were measured. Thermal cycles with a cooling rate of 1 °C/min were done in order to facilitate the crystallization process.
RESULTS AND DISCUSSION Optimization of HER synthesis The synthetic pathway to obtain HER from resorcinol and ethylene carbonate using K2CO3 as a catalyst has been previously reported.14 Nevertheless, a comprehensive study of the parameters that affect the reaction (e.g. type of catalyst, catalyst loading, temperature and reaction time) has never been described. On the contrary, the functionalization of bisphenol A with ethylene carbonate has
been recently deeply investigated as a model for the chemical modification of bisphenol A polycarbonate producing useful starting information.28 A set of tests has been performed in order to determine the formation of by-products and the effect of the molar ratio between the reagents. Indeed, the ratio of the reagents represents a key factor that deeply affects the selectivity of the reaction toward the desired product. Since ethylene carbonate presents inherent volatility at temperatures above 150 °C, part of it can be lost due to its evaporation. A 1:2.1 RES/ETC ratio leads to the formation of mono-functionalized resorcinol (Figure 5). a HO
d
O
a
O
OH
b
b c
b
HER
a
c e
l
HO
f
O
g
OH
h
i m
c
e
OH
d
HFIP
CHCl3 g m
7.4
7.2
7.0
f
h,i,l 6.8
6.4 6.4 6.6
6.2
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
3.8
3.6 3.4 3.4 ppm
Figure 5: 1H-NMR of the products of the RES/ETC 1:2.1 reaction and chemical structure of HER and mono-functionalized resorcinol by-product (solvent: CDCl3/HFIP, 1/1, v/v). The 1H-NMR analysis of the products of the reactions conducted at 180 °C with different molar ratios of the monomers showed that a 1:2.2 RES/ETC molar ratio is necessary in order to obtain the full conversion of resorcinol hydroxyl groups.
On the other hand, larger amounts of ETC can favour the formation of carbonate and/or aliphatic ether derivatives.28 Nevertheless, even with the optimized ratio of reagents, small amounts of byproducts can be formed and therefore a crystallization of the crude mixture is advised. Water crystallization affords the desired product with 55% yield, while using a NaOH 1M water solution the crystallization affords the desired product with 94% yield. This high yield is probably due to the fact that a basic solution favours HER precipitation and that NaOH is able to hydrolyse the carbonate groups, affording a larger amount of HER, in agreement with the data reported in ref. 15. On the basis of such results, a RES/ETC ratio of 1:2.2 has been used in all subsequent experiments. A second set of tests has been performed in order to optimize the conditions of the reactions (catalyst level, temperature and reaction time). According to the literature, 15,29 potassium carbonate was chosen as catalytic system due to its high activity in reaction between OH groups and ETC as
Conversion (%)
well as its low cost and environmental sustainability.
100 90 80 70 60 50 40 30 20 10 0
K2CO3 meq. K meq. 2 CO32424 0 15 30 45 60 K CO 12 meq. 2 3 0 15 30 45 60 K CO 7 meq. 2 3
0
15
30
45
60
Time (min)
Figure 6: effect of catalyst level on resorcinol conversion. The effect of the catalyst level on the reaction rate at 180 °C was analysed measuring the conversion by 1H-NMR analysis (Figure 6). Conversion of resorcinol in HER was determined dividing the value of the area under the HER peak by the sum of the areas under the peaks of HER, resorcinol and by-products when present. Reactions were performed using 24, 12 and 7 milliequivalents of catalyst with respect to resorcinol. The lower amount was chosen according to the literature.15 The results indicate that with 12 and 24
meq the reaction was significantly faster with respect to what previously reported in the literature15 (7 meq). Using 24 meq of catalyst, the complete conversion was reached after 15 minutes while with 12 meq one hour was necessary to reach 100% of conversion. On the basis of these results 24 meq of catalyst were used in all subsequent tests. The effect of the temperature on the reaction rate is reported in Figure 7. The reaction was performed with 24 meq of catalyst at 150, 165, 180 and 200 °C. The best compromise between short reaction times and low temperature is 180 °C. Higher temperature, in fact, does not significantly affect the reaction rate nor favors side-reactions, while at lower temperatures the
Conversion (%)
reaction rate is significantly slower.
100 90 80 70 60 50 40 30 20 10 0
T=200°C T=180°C T=165°C T=150°C
0
15
30
45
60
Time (min)
Figure 7: effect of temperature on resorcinol conversion.
HER polymerization Succinic acid (SA), dimethyl terephthalate (DMT), dimethyl isophthalate (DMI), 2,5furandicarboxylic acid (FDCA), dimethyl 1,4-cyclohexanedicarboxylate (DMCD, with 100% of trans isomer) and 1,4-cyclohexanedicarboxylic acid (CHDA, 66% of trans isomer) have been polymerized with HER (Figure 8). Dimethyl terephthalate and dimethyl isophthalate have been chosen in order to avoid very long reaction times and high temperatures due to the high T m of the corresponding diacids (above 300 °C). DMCD (100% trans isomer) was used instead of the corresponding diacid since carboxylic acids are well known to catalyse the isomerization from
trans to cis isomer towards the thermodynamically stable cis/trans ratio (34/66 mol%), whereas the isomerization is less probable in the presence of diesters.30 The CHDA 66% trans, instead, can be used because isomerization does not occur: the thermodynamically stable cis/trans ratio is already present. O HO
O
O
OH
+
DIACID or
DIESTER
MBTO 200-240°C N2
O
O
O
O
O R
n O O
O
DIACIDS
OH
HO
HO
HO O
SA O
O
O
OH
CHDA 66% trans
FDCA
O
O O
DIESTERS
O O
OH
O
O
O O
O O
DMCD 100% trans
O
DMT
DMI
Figure 8: HER polymerization and diacids and diesters used.
All monomers, except dimethyl isophthalate, can be obtained from renewable sources.7,9,10,31-33 The diacids and diesters have been chosen in order to cover a wide range of different macromolecular structures from aliphatic to cycloaliphatic and aromatic ones. In particular, succinic acid has been used as an example of linear aliphatic diacid while cyclohexane dicarboxylic acids (or the equivalent esters) represent interesting cycloaliphatic monomers. Indeed, thanks to the different cis/trans ratio of the C6 ring, polyesters with different thermal properties and phase behaviour can be obtained. 34 For this reason, two different isomeric mixtures have been tested (100% and 66% trans). Dimethyl terephthalate, dimethyl isophthalate and furan dicarboxylic acid have been chosen in order to introduce aromatic units also in the diacid, which generally provides high thermal and mechanical performances.
The polymers have been synthesized by melt polycondensation in inert atmosphere with monobuthyl tin oxide as catalyst, that is a well-known trans-esterification and directesterification catalyst. The reaction conditions (monomer ratio and reaction temperature) have been optimized for all the acids or esters in order to obtain polymers with higher molecular weight (Table 1). A 10% excess of the diacids was used except in the cases of succinic acid and furandicarboxylic acid. This excess is due to the higher volatility of the diacids with respect to HER. Succinic acid was introduced with a higher excess (20% of excess) with respect to HER because of its tendency to sublimate. On the contrary, a 10% excess of HER with respect to the furandicarboxylic acid was employed due to the lower volatility of FDCA with respect to HER. Polymerization reaction made using DMCD has been performed at lower temperatures (220 °C) in order to prevent the possible isomerization of the trans-isomer into the cis-isomer. The prepared polyester are characterized by the trans isomer content equal to 94%. Also using furandicarboxylic acid the maximum temperature was 220 °C due to its lower thermal stability.35
Table 1: reaction conditions and molecular weight of polymers based on HER. HER/diacid or diester ratio
Mn·10-3
Mw·10-3
Da
PD b
P(HER-S)
Maximum temperature (°C) 240
1:1.2
9.50
27.0
2.8
27.4
P(HER-CHD66)
240
1:1.1
8.00
46.0
5.7
19.9
P(HER-CHD100)
220
1:1.1
8.70
29.2
3.4
21.7
P(HER-I)
240
1:1.1
10.6
28.5
2.7
26.8
P(HER-T)
240
1:1.1
17.0
55.5
3.3
43.0
P(HER-F)
220
1.1:1
9.6
25.9
2.7
44.1
Polyester
a)
D: polydispersity; b) PD: Polymerization degree
The reaction time has been optimized by evaluating the increment of the molecular weights through the GPC measurements carried out on the samples taken during the reaction.
The NMR analysis confirms the successful reactions without side-reactions. As an example, in Figure 9 the 1H-NMR spectrum of the CHDA-derived polymer is reported.
a b O
a
*
O
d
O
b
O
b c
2
a
O
2 1
c
*
2
e
2
1 O
n
2t
2t
c,d CHCl3
7.5
7.0
6.5
6.0
1.5
1 .0
0.5
e
7.0
TMS
2c 2c
1t
0.0
1c
6.0
5.0
4.0
3.0
2.0
1.0
0.0 ppm
Figure 9: 1H-NMR spectrum of the polymer obtained from HER and CHDA 66% trans (t and c indicate trans and cis isomers). Thermal properties The results of the DSC analysis (Figure 10 and Table 2) confirm that the materials are generally amorphous, with glass transition temperatures ranging between 18 and 63 °C, according to the rigidity of the diacid units. In particular, P(HER-S) is characterized by the lowest Tg value compared to those of the polymers containing cycloaliphatic and aromatic units. This is due to the high flexibility of the sequences of methylene moieties. On the other hand, the cyclic aliphatic structure of DMCD introduces a higher stiffness of the macromolecular chain, which induces an increment of Tg. It is worth noting that the conformation of the aliphatic ring slightly affects the glass transition temperature. In particular, the prevalence of the stretched trans isomer tends to increase the Tg value due to its higher symmetry with respect to the cis isomer.34
Normalized heat flow (W/g)
P(HER-S) P(HER-SA)
Endo
P(HER-CHD66)
P(HER-CHD100)
P(HER-I) P(HER-T)
P(HER-F)
-10
10
30
50
70
90
110
130
150
170
Temperature ( C)
Figure 10: DSC curves measured at 10 °C/min (2nd heating scan).
Table 2: DSC and TGA data of HER containing polymers Tg
ΔCp
Tcc
ΔHcc
Tm
ΔHm
T0 a
Td b
P(HER-S)
(°C) 18
(J/g °C) 0.67
(°C) --
(J/g) --
(°C) --
(J/g) --
(°C) 404
(°C) 431
P(HER-CHD66)
32
0.42
--
--
--
--
446
468
P(HER-CHD100)
33
0.37
111
21
147
20
445
470
P(HER-I)
51
0.47
--
--
--
--
435
459
P(HER-T)
58
0.37
--
--
--
--
434
460
P(HER-F)
63
0.46
--
--
--
--
387
414
Polyester
a) b)
T0: temperature of degradation beginning Td: temperature of the maximum rate of degradation
The combination between HER and the aromatic units of isophthalic acid, terephthalic acid and furandicarboxylic acid produces materials with high T g values (respectively 51, 58, and 63 °C). The most symmetric structure of terephthalic acid justifies the higher Tg value of P(HER-T) with respect to P(HER-I). Furanic rings tend to confer higher Tg values to P(HER-F) with respect to phthalic rings due to its high rigidity and polarity.36 The effect of the structure of the diacids on the final thermal properties of the polyesters is confirmed by comparing data reported in the literature: some examples are collected in Table 3, for aliphatic, cycloaliphatic and aromatic polyesters derived from ethylene glycol or 1,4-
butanediol, combined with the same diacids used here. Along a column of Table 3, Tg increases from an aliphatic to an aromatic diacid, i.e. as a function of the structure of the diacids.
Table 3: Tg data of some polyesters from literature Polyester
Tg (°C)
Mw·10-3
Polyester
Tg (°C)
Mw ·10-3
PESa
-11
43.8
PBSe
-27
-
PECHD57b
14
55.3
PBCHD72f
-2
77.6
PECHD100
-
-
PBCHD100f
10
73.4
PEIc
63
48.7
PBIg
26
57.4
PETd
75
15.2
PBTd
43
29.8
13.4
d
36
21.2
d
PEF
77
PBF
a)
PES: poly(ethylene succinate), heating rate: 10 °C/min, ref.37; b) PECHD57: poly(ethylene 1,4cyclohexane dicarboxylate) with 57% of trans isomer, heating rate: 10 °C/min, ref. 38; c) PEI: poly(ethylene isophthalate), heating rate: 20 °C/min, ref.39; d) PEF: poly(ethylene 2,5furandicarboxylate), PBT: poly(butylene terephthalate), PBF: poly(butylene 2,5furandicarboxylate), heating rate: 10 °C/min, ref.35; e) PBS: poly(butylene succinate), heating rate: 10 °C/min, ref.40; f) PBCHD72: poly(butylene 1,4-cyclohexane dicarboxylate) with 72% of trans isomer, PBCHD100: poly(butylene 1,4-cyclohexane dicarboxylate) with 100% of trans isomer, heating rate: 10 °C/min, ref.41; g) PBI: poly(butylene isophthalate), heating rate: 20 °C/min ref.42
On the other hand, by considering the effect of the structure of the diol, as Tables 2 and 3 show, it is also worth noting that the presence of the aromatic HER units along the macromolecules has a huge effect on the rigidity of the chains and induces significantly higher Tg values only in the presence of an aliphatic diacid. For example, P(HER-S) and PBS have Tg equal to 18 and -27 °C, respectively. Instead, when the diacid is aromatic, this effect is remarkably reduced and the difference in Tg between an aromatic diol (HER) and an aliphatic diol (ethylene glycol or 1,4butanediol) are no longer evident. For example, P(HER-T), PET and PBT have Tg equal to 58, 75 and 43, respectively. The amorphous character of the HER-derived polyesters has been confirmed also by analysing the materials in DSC at a very slow cooling rate (1 °C/min). With the only exception of the P(HER-CHD100), the polymers are unable to rearrange towards an ordered state. The reason for such behaviour can be ascribed to the non-linear shape of HER that prevents the formation of
crystalline domains. As a confirmation, polymers derived from meta-substituted isophthalic acid present a remarkably lower crystallization rate than the analogous para-substituted (for example, PEI vs PET). Only in the case of P(HER-CHD100) the stretched trans configuration improves the symmetry of the chain and the packing towards stable crystals. All the polyesters obtained present good thermal stability since the degradation processes occur at temperatures higher than 380 °C (Table 2). It must be underlined that the samples containing the 1,4-cyclohexylene rings have an outstanding thermal resistance with onset temperature of degradation up to 446 °C. P(HER-F), instead, has the lowest degradation temperature whereas the polyesters based on aromatic diacids achieve high stabilities. Such result is in agreement with some data reported in the literature36 on PEF and PET (thermal degradation onset= 389 and 413 °C, respectively), that indicate a lower thermal stability for the furanic polymer.
One-pot synthesis from ETC, RES and isophthalic acid The synthesis of HER has also been performed in the same reactor used for the subsequent polycondensation (Figure 11). This procedure was investigated in order to determine a simple and even more sustainable procedure starting directly from resorcinol, thus avoiding the purification step.
Figure 11: one-pot synthetic procedure.
Using this process a polyester with Mn equal to 9200, Mw of 37400 and Tg of 47 °C has been obtained. The results are consistent with those obtained by the polymerization procedure of HER with dimethyl isophthalate, with a slightly lower T g (4 °C) for the one-pot procedure. Figure 12-a and 12-b show the comparison between the 1H-NMR spectra of the polymers obtained using the one-pot polymerization and using the purified HER with the procedure previously described.
O
a
*
O
d
O
O
b
O
b c
O
1
a
*
2
c
2
e
CHCl3
n
3
TMS
a b
c,d
2 1 3
e
4.80
4.60
4.40
4.20
4.00
3.80
a a b
CHCl3
TMS c,d
2 1
3 e 4.80
4.60
4.40
4.20
4.00
3.80
b 9.0 9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0 0.0 ppm
Figure 12: 1H-NMR spectra of P(HER-I) from one-pot polymerization (a) and P(HER-I) from twosteps polymerization using purified HER (b). 1
H-NMR spectra show that the polymers obtained with the two methods are similar. A small
difference can be noted in the zone between 3.9 and 4.05 ppm due to the development of aliphatic ether linkages. Indeed, the formation of ether bonds is due to a well-known dehydration side-reaction of alcoholic end groups, by following the mechanism depicted in Figure 13. 43 This by-product increaseses the amout of CH2 groups in the polymer chain wich can be reponsible for the lower Tg of the polymer obtained by the one-pot process.
Figure 13: dehydration reaction of alcoholic end groups with formation of aliphatic ether units
CONCLUSIONS A new successful and green strategy for the synthesis of polyesters with a kinked structure based on resorcinol has been developed. The process involves the reaction of resorcinol with ethylene carbonate (ETC); the resulting aromatic diol (HER) can be polymerized with several aliphatic and aromatic dicarboxylic acids or esters by melt polycondensation using a tin based catalyst. Moreover, a novel one-pot procedure has also been developed: the product obtained from ETC and resorcinol reaction is directly polymerized with a diacid without any intermediate purification step. The final material features properties similar to those obtained in polyesters prepared by two-step synthesis. The optimized synthetic polymerization starting from HER provided a wide range of polyesters, which are generally amorphous and characterized by glass transition temperatures varying over a
wide range, according to the diacid chemical structure and rigidity. Moreover, all the polymers present a high thermal stability. On the basis of the results obtained it is clear that resorcinol, that can be derived from renewable sources, can be used in the synthesis of amorphous aromatic polyesters for coating and packaging applications.
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Highlights -
A sustainable synthesis of 1,3-bis(hydroxyethoxy)resorcinol (HER) was performed. Polyesterification of HER with different diacids was carried out. The obtained polyesters showed amorphous character and high thermal stability. The relationship between structure and properties of these materials was studied. A one-pot polyesterification avoiding purification steps was developed.