Introduction
Application of cheap and easily available carbon dioxide, directly or indirectly, as a monomer for polymer synthesis due to its inertness is a big challenge. In the 70-ties of the last century Inoue [
1] as well as Kuran [
2,
3] used CO
2 directly for copolymerization with oxiranes in the presence of organometallic catalysts based on Zn and Al leading to aliphatic polycarbonates. However, it has been difficult to find wide commercial application for polycarbonates obtained according to this process due to their low glass transition temperature and relatively poor mechanical properties. Recently, new catalysts such as cobalt complexes [
4,
5], zinc adipate [
6,
7] or a double metal cyanide complex [
8] were investigated to improve the efficiency of copolymerization CO
2 with oxiranes. The main advantage of aliphatic polycarbonates similarly to aliphatic polyesters is theirs biodegradability. In contrast to polyesters, biodegradation of aliphatic polycarbonates does not lead to the local increase of pH due to formation of hydroxyl groups and CO
2 instead of OH and COOH groups. Additionally, the hydrolysis proceeds with a lower rate.
It was revealed that it is more convenient to use for synthesis of polycarbonates containing more than 3 carbon atom between carbonate linkages, monomers based on CO
2 such as alkylene carbonates (ethylene carbonate (EC) or propylene carbonate (PC)) or dimethyl carbonate (DMC) [
9‐
11].
Another attractive polymers which contain built-in CO
2 are poly(ester-carbonate)s. These copolymers can be obtained according to ring opening copolymerization or by polycondensation methods. Usually polymers obtained by polycondensation exhibit poor mechanical properties due to relatively small molar mass. To obtain high molar mass poly(ester-carbonate)s copolymerization of cyclic esters (L-lactide, ε-caprolactone) with cyclic carbonate monomer-trimethylene carbonate (TMC) is usually carried out [
12‐
14]. Besides TMC for copolymerization with cyclic esters other six-membered cyclic carbonates with different functional groups are also used [
15‐
18].
Yang et al. prepared a series of new poly(ester-carbonate)s by a simple combination of polycondensation and ring-opening-polymerization (ROP) of hydroxyl terminated poly(butylene succinate) macromonomers and various cyclic carbonate monomers [
19].
In the synthesis of aliphatic poly(ester-carbonate)s according to step-growth polymerization mode phosgene derivatives usually were used as a source of carbonate linkages. Thus, Vincenzi et al. synthesized poly(ester-carbonate)s containing poly(oxyethylene) block applying PEG-bischloroformate [
20]. Instead of phosgene or its derivatives for synthesis of aliphatic copoly(ester-carbonate)s by polycondensation diethyl carbonate or diphenyl carbonate [
21] can be used. Also Chandure et al. used for synthesis of aliphatic poly(ester-carbonate)s diethyl carbonate, adipic acid and 1,3-propanediol. Poly(butylene carbonate-
co-succinate), with low carbonate unit content (<20 mol.%), which can be used for poly(L-lactide) modification have been developed by Mitsubishi Gas Chemical Company [
22].
More and coworkers obtained poly(ester-carbonate)s from ricinoleic acid, diol and diethyl carbonate [
23]. Resultant dicarbonate monomer, 4-[(ethoxycarbonyl)oxy]butyl-12-[(ethoxycarbonyl)oxy]octadec-9-enoate was polycondensed with various biobased diols to give poly(ester-carbonate)s. In the method proposed by Paturej and El Fray poly(ester-
b-carbonate) was synthesized by coupling of oligo(trimethylene carbonate) diol with polyesterol based on dimerized fatty acid (Priplast 3192) by diisocyanate [
24]. Biodegradable poly(ester-carbonate)s based on 1,3-propylene-
co-1,4-cyclohexanedimethylene succinate were obtained from respective oligoesterols and phosgene [
25]. To introduce carbonate linkages into the polymer backbone according to a polycondensation mode bis(p-nitrophenyl) carbonate can be applied instead of toxic phosgene [
26,
27].
These biodegradable polymers can be applied in the production of medical materials such as sutures, implants and tissue scaffolds. Furthermore, polycarbonates have lower degradation rate than polyesters [
28,
29], which would be advantageous in the applications where relatively higher stability is desired.
To achieve better mechanical properties, polymers, especially those obtained by polycondensation, should contain aromatic rings in their backbones. Thus, typical aliphatic-aromatic polyester - poly(ethylene terephthalate) exhibits good mechanical strength even when its molar mass does not exceed 40000 g/mol. However, to achieve biodegradability aliphatic-aromatic random copolyester should contain less than 50 mol.% of aromatic dicarboxylic acid in acidic components and usually 1,4-butanediol instead of ethylene glycol (Ecoflex of BASF). Succinic or adipic acids are usually used as aliphatic dicarboxylic acids [
30,
31]. When the number-average sequence of butylene phthalate units is lower than 3 the copolyester exhibit complete biodegradation [
32]. Such copolyesters are used as packaging films as well as compost bags.
However, there is relatively little information concerning aliphatic-aromatic poly(ester-carbonate)s based on α,ω-diols, aromatic dicarboxylic acid and derivatives of carbonic acid. This paper deals with the synthesis of poly(alkylene phthalate-co-carbonate)s carried out by polycondensation method using dimethyl carbonate or propylene carbonate as precursors of carbonate linkages in the reaction with dimethyl terephthalate (DMT) and 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol and 1,10-decanediol. The macromolecular structures as well as mechanical and thermal properties of the obtained copolymers were characterized and discussed.
Experimental section
Materials
Dimethyl carbonate (DMC) (99 %), titanium(IV) butoxide (Ti(OBu)4) (≥97 %), (Sigma-Aldrich), propylene carbonate (≥99 %) (Merck) and n-Heptane (≥99 %) (Roth) were used as received. Dimethyl terephthalate (≥99 %) (Fluka Analytical) was dried for 24 h under pressure of 0.2 mbar at room temperature before use. 1,10-Decanediol (98 %), 1,6-hexanediol (97 %) (Aldrich), 1,5-pentanediol (≥97 %) (Fluka) and 1,4-butanediol (99 %) (Sigma-Aldrich) were dried for 8 h in 80 °C under pressure of 20 mbar before use .
Characterization techniques
FTIR absorption spectra were recorded on a Biorad FTS-165 BIO-RAD spectrometer as KBr pellets. The measurements were carried in the range of 400–4000 cm-1 with a resolution of 2 cm-1.
1H NMR and
13C NMR spectra were recorded at room temperature on Varian VXR 400 MHz spectrometer using tetramethylsilane as an internal reference and CDCl
3 as solvent and analyzed with MestReNovav.6.2.0–7.238 (Mestrelab Research S.L) software. The carbonate unit content in the copolymers was calculated from
1H NMR spectra according to our earlier report [
33].
The molar mass and molar mass distribution were determined by GPC on a Viscotek system comprising GPCmax and TDA 305 triple detection unit (RI, IV, LS) equipped with one guard and two DVB Jordi gel columns (102–107, linear, mix bed) in CH2Cl2 as eluent at 35 °C at a flow rate of 1.0 mL/min. Triple detection was used for determination of absolute molar mass and DI of condensation poly(ester-carbonate)s. Standard PS calibration was used in case of low molar mass products.
Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF/MS) measurements were performed on a Bruker Ultra Flex MALDI TOF/TOF spectrometer (Bremen, Germany) in a linear mode using DHB (2,5-dihydroxybenzoic acid) or HABA (2-(4′-hydroxybenzeneazo)benzoic acid matrix and Bruker Peptide Calibration Standard (1047.19–3149.57 Da) as a calibrant and analyzed with flexAnalysis v.3.3 (Bruker Daltonik GmbH) and Polymerix v. 2.0 (Sierra Analytics Inc.) software.
Glass transition temperature (Tg) was calculated from the inflection point in the break in the DSC heat flow curves. DSC studies were carried out using a TA Instruments DSC Q200 apparatus. Samples were heated in a temperature range from -50 to 200 °C with the heating rate 5 Kmin-1, then cooled backward to -50 °C with the heating rate of 10 Kmin-1 and heated again to 200 °C with 20 Kmin-1.
Thermal stability was studied by thermogravimetric analysis (TGA) performed using MOM Deriwatograph Q1500D. Heating rate was 10 Kmin-1, scans were carried out starting at 20 °C up to 600 °C. The reference substance was aluminum oxide.
Measurements of tensile strength were performed on Instron 5566 machine with a constant stretching rate of 5 % of the length/min. For the data processing Bluehill software was used. Samples were prepared by compression in a hydraulic press at 170 °C and then cut into the form of dumbbell shape with the measuring section dimensions of 28 mm length /4 mm width / 2 mm thickness. Tests were performed at room temperature. The values of Young′s modulus (E), the relative elongation at break (εbreak), tensile strength at break (εbreak) and the stress at yield (σyield) (if any) were calculated.
Synthesis of poly(tetramethylene terephthalate-co-carbonate) directly from dimethyl carbonate, 1,4-butanediol and dimethyl terephthalate
In a 250 cm3 three-neck round-bottomed flask equipped with a magnetic stirrer, thermometer, distilling condenser and nitrogen supply system, 30.63 g (0.3399 mol) of 1,4-butanediol, 30.01 g (0.1545 mol) of dimethyl terephthalate (DMT) and 0.17 g (0.5 mmol) of Ti(OBu)4 were placed. The reaction was carried out at 150–220 °C under atmospheric pressure with a nitrogen flow, until no methanol was observed in the distillate (the refractive index was measured). After removing almost whole theoretical amount of methanol (2 h) the reaction temperature was reduced to 95–115 °C and 83.50 g (0.9270 mol) DMC was added and the polycondensation was continued under reflux without removing methanol from the reaction mixture for 2 h. Then, methanol/DMC was gradually distilled off using a long Vigreoux column. When the content of methanol in the distillate was lower than 3 % (refractive index) the reaction again was carried out under reflux for 2 h to generate the next portion of methanol. This procedure was repeated 3 times.
Alternatively, DMC was introduced into reaction mixture in 3 portions and the process was carried out according to the above mentioned manner.
TC4-9: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 8.03 (s, 4H), 4.38 (t, 4H, J = 5.9 Hz), 4.35 (t, 4H, J = 5.9Hz), 4.13 (t, 4H, J = 5.9 Hz), 4.10 (t, 4H, J = 5.8 Hz), 3.71-3.66 (two overlapping triplets, 8H, J = 6.4, 6.3 Hz), 1.81 (bs, 4H), 1.71 (bs, 8H), 1.60 (bs, 4H).
FTIR (KBr): 2960, 2859, 1743, 1442, 1320, 1258, 1022, 935, 792 cm-1.
Synthesis of alkylene bis(methylcarbonate)s
Alkylene bis(methylcarbonate)s (BMC) were obtained in reaction of 6-fold molar excess of DMC with α,ω-diol. The procedure was described by us earlier [
10]. Ti(OBu)
4 (0.1 mol.%) was used as a catalyst. BMC means alkylene bis(methyl carbonate) and the number after signifies the length of hydrocarbon chain in the α,ω-diol, for example
BMC4 means tetramethylene bis(methylcarbonate).
BMC4: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 4.15 (t, 4H, J = 6.2 Hz), 3.77 (s, 6H), 1.74 (bs, 4H).
BMC5: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 4.06 (t, 4H, J = 6.7 Hz), 3.69 (s, 6H), 1.63 (bs, 4H), 1.39 (bs, 2H).
BMC6: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 4.09 (t, 4H, J = 6.6 Hz), 3.75 (s, 6H), 1.70 (bs, 4H), 1.34 (bs, 4H).
BMC10: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 4.11 (t, 4H, J = 6.8 Hz), 3.77 (s, 6H), 1.66 (bs, 4H), 1.34 (bs, 4H,), 1.28 (bs, 8H).
FTIR (KBr): 2962, 2860, 1745, 1444, 1323, 1260, 1027, 936, 793 cm-1.
Synthesis of poly(tetramethylene terephthalate-co-carbonate) from butylene bis(methylcarbonate), DMT and 1,4-butanediol
In a 250 cm3 three-neck round-bottomed flask equipped with a magnetic stirrer, thermometer, distilling condenser and nitrogen supply system, 75.53 g (0.8381 mol) of 1,4-butanediol, 43.63 g (0.1983 mol) of BMC4, 69.96 g (0.3605 mol) of dimethyl terephthalate (DMT) and 0.19 g (0.6 mmol) of Ti(OBu)4 were placed. The reaction was carried out at 180–190 °C for 8 h under atmospheric pressure with a nitrogen flow, until no methanol was observed in the distillate (the refractive index was measured). Then, the reaction was continued under reduced pressure: 20 mbar (190 °C, 2 h), 0.2 mbar (190 °C, 2.5 h) and 0.1 mbar (215 °C, 8 h). 123.70 g of the poly(tetramethylene terephthalate-co-carbonate) (TC4-50) was obtained as an semicrystalline colorless solid with Mn = 8260 containing 50 mol.% of butylene carbonate units.
Poly(alkylene terephthalate-co-carbonate)s based on 1,6-hexanediol, 1,5-pentanediol and 1,4-butanediol/1,5-pentanediol mixture (1,4-BDO:1,5-PDO = 1:1 and BMC4:BMC5 = 1:1) were obtained in the same manner. Products were white semicrystalline solids.
TC means aliphatic-aromatic poly(alkylene terephthalate-co-carbonate) and the number after signifies the length of hydrocarbon chain in the α,ω-diol and content of carbonate units, respectively. For example TC4-50 means poly(tetramethylene terephthalate-co-carbonate) with 50 mol.% of carbonate units.
TC4-50: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 8.09 (s, 4H), 4.43 (t, 4H, J = 6.2 Hz), 4.37 (t, 4H, J = 6.2 Hz), 4.21 (t, 4H, J = 6.2 Hz), 4.17 (t, 4H, J = 6.2 Hz), 3.73 (t, 4H, J = 6.0 Hz), 3.68 (t, 4H, J = 6.0 Hz), 1.87 (bs, 4H), 1.76 (bs, 8H), 1.66 (bs, 4H).
TC4/5-67: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 8.07 (bs, 4H), 4.42 (t, 4H, J = 6.4 Hz), 4.39-4.32 (m, 8H), 4.20 (t, 4H, J = 5.9 Hz), 4.16-4.12 (m, 8H), 3.74- 3.63 (m, 8H), 1.96 – 1.85 (m, 8H), 1.79 – 1.72 (m, 8H), 1.72 – 1.64 (m, 8H), 1.55 (bs, 4H), 1.46 (bs, 4H).
TC5-49: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 8.08 (bs, 4H), 4.36 (two overlapping triplets, 8H, J = 7.8, 6.6 Hz), 4.14 (two overlapping triplets, 8H, J = 8.1, 6.5 Hz), 3.67 (bs, 8H), 1.77 (bs, 4H), 1.71-1.57 (m, 16H), 1.57-1.44 (m, 8H), 1.44-1.30 (m, 8H).
TC6-47: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 8.07 (bs, 4H), 4.34 (two overlapping triplets, 8H, J
1
= 7.9 Hz, J
2
= 6.6 Hz), 4.12 (two overlapping triplets, 8H, J
1
= 8.5 Hz, J
2
= 6.9 Hz), 3.64 (two overlapping triplets, 8H, J
1
= 7.7 Hz, J
2
= 6.5 Hz), 1.79 (bs, 8H), 1.67 (bs, 8H), 1.53 (bs, 4H), 1.47 (bs, 4H), 1.40 (bs, 8H).
FTIR (KBr): 2963, 2902, 1744, 1716, 1247, 1119, 928, 728 cm-1.
Synthesis of poly(tetramethylene terephthalate-co-carbonate) on larger scale
The synthesis of poly(tetramethylene terephthalate-co-carbonate) was carried out on larger scale in a stainless steel reactor of 2 dm3. The reactor was equipped with a mechanical stirrer, a distillation system, a heating-cooling mantle, nitrogen inlet and outlet and oil vacuum pump. The reactor contained 4 electrically powered heating zones controlled automatically (trigger, bottom, top, cover). Based on the power consumption of the mechanical stirrer the information about increase of the reaction mixture viscosity during the synthesis was possible to monitor. 389.91 g (2.0080 mol) of dimethyl terephthalate, 394.82 g (4.3811 mol) of 1,4-butanediol, 200.80 g (0.9127 mol) of BMC4 were placed. Reaction was carried out at 150–220 °C first under atmospheric pressure with nitrogen flow until no methanol was observed in distillate. The temperature was increased to 220 °C slowly. Than the process was continued under reduced pressure of 0.3 mbar until the viscosity of the product indicated a significant increase in molar mass, and when no further evolution of 1,4-butanediol was observed. 695.91 g of poly(tetramethylene terephthalate-co-carbonate) as a white solid with Mn = 24 790 and 46 mol.% of carbonate units was obtained.
TC4-46: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 8.08 (4H, bs), 4.43 (t, 4H, J = 5.8 Hz), 4.38 (t, 4H, J = 5.9 Hz), 4.21 (t, 4H, J = 6.1 Hz), 4.17 (t, 4H, J = 5.8 Hz), 3.71 (t, 4H, J = 6.4 Hz), 3.68 (t, 4H, J = 6.3 Hz), 1.97 (4H, bs), 1.88 (8H, bs), 1.77 (4H, bs).
Synthesis of poly(decamethylene terephthalate-co-carbonate)
In a 100 cm3 three-neck round-bottomed flask equipped with a magnetic stirrer, thermometer, distilling condenser and nitrogen supply system, 38.48 g (0.2208 mol) of 1,10-decanediol, 22.23 g (0.0783 mol) of BMC10, 9.95 g (0.1104 mol) of 1,4-butanediol, 27.66 g (0.1425 mol) of dimethyl terephthalate (DMT) and 0.06 g (0.2 mmol) of Ti(OBu)4 were placed. The reaction was carried out at 180–190 °C for 8 h under atmospheric pressure with a nitrogen flow, until no methanol was observed in the distillate (the refractive index was measured). Then, the reaction was continued under reduced pressure: 20 mbar (190 °C, 2 h), 0.2 mbar (190 °C, 2.5 h) and 0.1 mbar (215 °C, 8 h). 45.6 g of the poly(decamethylene-co-tetramethylene terephthalate-co-carbonate) (TC10/
4
-50) was obtained as a white solid with Mn = 17 320 containing ca. 50 mol.% of carbonate units.
TC10/
4
-50: 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 8.09 (bs, 4H), 4.43 (t, 4H, J = 5.8 Hz), 4.33 (t, 8H, J = 6.7 Hz), 4.20 (t, 4H, J = 5.9 Hz), 4.11 (two overlapping triplets, 8H, J = 8.4, 6.7 Hz), 3.63 (two overlapping triplets, 8H, J = 7.8, 6.6 Hz), 1.87 (bs, 4H), 1.77 (bs, 4H), 1.65 (bs, 4H), 1.42 (bs, 4H), 1.42-1.25 (m, 16H).
Poly(alkylene terephthalate-co-carbonate) based on 1,6-hexanediol was obtained in the same manner. 56.9 g of the poly(hexamethylene-co-tetramethylene terephthalate-co-carbonate) (TC6/
4
-49) was obtained as an white semicrystalline solid with Mn = 18400 containing 49 mol.% of carbonate units.
(TC6/
4
-49): 1H NMR (400 MHz, d-CHCl3): δ (ppm) = 8.07 (4H, bs), 4.42 (t, 4H, J = 5.7 Hz), 4.34 (two overlapping triplets, 8H, J = 8.2 Hz, 6.6 Hz), 4.20 (t, 4H, J = 5.8 Hz), 4.13 (two overlapping triplets, 8H, J = 8.1, 6.4 Hz), 3.65 (bs, 4H), 1.79 (4H, bs), 1.69 (4H, bs), 1.53 (2H, bs), 1.47 (4H, bs), 1.40 (2H, bs).
Synthesis of poly(tetramethylene terephthalate-co-carbonate) based on propylene carbonate
Poly(tetramethylene terephthalate-
co-carbonate) was alternatively obtained from propylene carbonate as a source of carbonate linkages according to the procedure described by us earlier [
37].
In a 250 cm3 three-neck round-bottomed flask equipped with a magnetic stirrer, thermometer, Dean-Stark distilling trap, reflux condenser and nitrogen supply system, 78.75 g (0.8750 mol) of 1,4-butanediol, 38.28 g (0.3750 mol) of propylene carbonate and 0.09 g (0.25 mmol) of Ti(OBu)4 were placed. The reaction was carried out for 6 h at 165–170 °C under atmospheric pressure with continuously removal of 1,2-propylene glycol by co-distillation with n-heptane, until no 1,2-propylene glycol was observed in the distillate. Then, the solvent was distilled off and 48.55 g (0.2500 mol) of dimethyl terephthalate was added. The reaction with a DMT was carried out for 5 h at 150 °C under inert gas flow until no distillation of methanol was observed. The final step - removal of an excess of propylene carbonate and the polycondensation was proceeded under reduced pressure (0.2 mbar) at 150–200 °C till required molar mass of oligo(tetramethylene terephthalate-co-carbonate) was attained. About 93.4 g of the product (TC4-39) with molar mass of 19 500 g/mol containing 39 mol.% of carbonate units was obtained.
TC4-39: 1H NMR (CDCl3, 400 MHz): δ (ppm) = 8.08 (s, 4H), 4.43 (t, 4H, J = 6.0 Hz), 4.32 (t, 4H, J = 6.0 Hz), 4.20 (t, 4H, J = 6.2 Hz), 4.15 (t, 4H, J = 6.2 Hz), 3.72 (t, 4H, J = 6.1 Hz), 3.66 (t, 4H, J = 6.1 Hz), 1.95 (bs, 4H), 1.89 (bs, 4H), 1.77-1.54 (m, 8H).
FTIR (KBr): 3550, 2964, 2903, 1745, 1715, 1247, 1120, 929, 728 cm-1.
Synthesis of poly(tetramethylene carbonate)
In a 1000 cm3 three-neck round-bottomed flask equipped with a magnetic stirrer, thermometer, distilling condenser and nitrogen supply system, 22.96 g (0.2546 mol) of 1,4-butanediol, 37.33 g (0.1697 mol) of BMC4 and 0.09 g (0.2 mmol) of Ti(OBu)4 were placed. The reaction was carried out at 165–180 °C for 6 h under atmospheric pressure with a nitrogen flow, until no methanol was observed in the distillate (the refractive index was measured). Then, the reaction was continued under reduced pressure - 0.5 mbar (160 °C, 1 h). 30.6 g of the poly(tetramethylene carbonate) (CD4) was obtained as a white solid with Mn = 11 300 g/mol.
CD4: 1H NMR (CDCl3, 400 MHz): δ (ppm) = 4.14 (t, 4H, J = 6.2 Hz), 3.67 (t, 4H, J = 6.0 Hz), 3.42 (t, 4H, J = 6.1), 1.74 (bs, 8H).
FTIR (KBr): 2964, 1742, 1247, 928, 772 cm-1.