Aliphatic-aromatic poly(ester-carbonate)s obtained from simple carbonate esters, α,ω-aliphatic diols and dimethyl terephthalate

Due to low cost, environmentally friendly dimethyl carbonate (DMC) was used by us as a source of carbonate linkages in the synthesis of aliphatic-aromatic poly(ester-carbonate)s. First experiments were carried out directly with DMC when all monomers were added simultaneously. The main problem encountered in the synthesis of poly(ester-carbonate) from dimethyl carbonate was co-distillation of the starting material with the by-product (methanol) which distorts the molar ratio of the desired monomers. We tested several approaches to limit this fault. Firstly, the process was divided into two steps. At the first step oligo(tetramethylene terephthalate) was obtained in the reaction of dimethyl terephthalate and 1,4-butanediol. After removing almost whole theoretical amount of methanol, 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. 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. The highest amount of carbonate units in the product did not exceeded 10 mol.% after 8 h of polycondensation (Table 1, run 3).

In order to avoid uncontrolled distillation of dimethyl carbonate with methanol a new intermediate-alkylene bis(methylcarbonate) (BMC) monomer is hereby proposed [10]. Additionally, in case of using BMC the reaction can be carried out at higher temperature (up to 230 °C), so the polycondensation proceeds with higher rate. This solid monomer was obtained in the reaction of 1,4-butanediol (1,4-BD) with molar excess of DMC (1:6) in the presence of Ti(OBu)₄ as a catalyst [10]. It should be mentioned that despite of high molar excess of DMC, some amount of a tricarbonate was present in the product (Fig. 1).

The amount of the tricarbonate was calculated from the ¹H NMR spectrum of the product taking into account the intensity of signals corresponding to CH₃ protons (3.77 ppm) and CH₂O protons (4.16 ppm) adjacent to carbonate linkages, respectively (Fig. 1) [10].

where a – area of signals corresponding to methyl protons = 6 and b -area of signals corresponding to methylene protons.

The synthesis of poly(ester-carbonate)s from BMC, DMT and 1,4-butanediol was carried out in two steps (experimental section). At the first step methanol was distilled off, resulting in the formation of oligomers terminated with 4-hydroxybutyl groups (Scheme 1, step 1). The molar ratio of diol to the sum of BMC and DMC should be greater than 2 usually 2.1–2.2.

It was observed that the presence of water traces in starting monomers leads to the loss of carbonate groups in the resultant copolymer. Similar effect was observed when terephthalic acid was used in the reaction with BMC4 and 1,4-butanediol instead of DMT. Water can hydrolyze carbonate linkages or carboxylic acid, formed in situ, can react with carbonate groups leading to decarboxylation (Scheme 2).

Thus, all starting materials should be dried thoroughly prior to the synthesis. After removing more than 95–97 % of theoretical amount of methanol the temperature was raised up to 200–220 °C and the reaction was continued under reduced pressure (Scheme 1, step 2). Under such conditions residues of methanol together with small amount of 1,4-butanediol was distilled off and molar mass of carbonate oligomer increased up to 15 300 g/mol (Table 2). It was observed that in the case of the polycondensation carried out at 220 °C small amount of white crystals was formed in a distillation cooler. The spectral analysis and melting point measurement of these crystals (m.p. 173–174 °C) [35, 36] revealed that during polycondensation, besides 1,4-butanediol, 14-membered cyclobis(tetramethylene carbonate) was formed as a by-product (Fig. 2).

The reaction proceeds according to ‘back-biting’ manner (Scheme 3). It is characteristic that no seven-membered cyclic carbonate is formed, but much less volatile cyclobis(tetramethylene carbonate). It should be added that due to the presence of ester units in the formed copolymer, the probability that terminal dicarbonate units are present in poly(ester-carbonate) chains is lower than in the case of synthesis of poly(tetramethylene carbonate) alone and the yield of cyclobis(tetramethylene carbonate) was relatively low (1–2 %).

It is worth mentioning that in the case of using trimethylene bis(methylcarbonate) only small amount of trimethylene carbonate units can be introduced into resultant polymer due to back-biting reaction leading to volatile trimethylene carbonate.

It was found that the reaction temperature should not be higher than 220–225 °C. Above this temperature decarboxylation takes place leading to drastically reduced concentration of tetramethylene carbonate units in the resultant polymer (Table 2, run 10). It should be added that in contrast to the synthesis of polyester, synthesis of poly(ester-carbonate) proceeds with lower rate. The attack of hydroxyl groups at carbonyl group in terminal carbonate linkages can lead to volatile 1,4-butanediol and oligomer (1a) or two oligomers (1b), whereas in case of the reaction with terminal ester group only volatile product and oligomer are formed (2) (Scheme 4). Thus, the reaction (1b) is unproductive and does not lead to the increase in the polymer molar mass.

It was not possible to obtain high molar mass poly(tetramethylene terephthalate)s containing more than 60 % of terephthalate when planned molar ratio BMC to DMT was used. In this case due to high melting point of the copolymer the stirring of the reaction mixture at temperature lower than 225 °C was difficult. Copolymers with higher than 60 % of terephthalate units contribution were obtained when excess of tetramethylene bis(methylcarbonate) was used and the process was proceeded at 240 °C at which temperature partial decarboxylation took place.

Poly(pentamethylene terephthalate-co-carbonate) based on 1,5-pentanediol was obtained according to a similar manner (Table 2, run 13).

In case of longer diols such as 1,6-hexanediol and 1,10-decanediol it was impossible to obtain copolymers of high molar mass because of high boiling points of diols (250 and 297 °C, respectively) (Table 2, run 14). To solve this problem we have performed the experiments in which the molar excess of 1,6-hexanediol or 1,10-decanediol was replaced by that of 1,4-butanediol, which distills more easily under used reaction conditions (Table 2, runs 15–16 and Fig. 3). The small intensity signals which can be assigned to protons C(O)OCH₂CH₂CH₂CH₂OC(O) which are present in the ¹H NMR spectrum of poly(decamethylene terephthalate-co-carbonate) (Fig. 3d) correspond to small amount of tetramethylene units (TC-4) which are incorporated in the structure of the polymers TC10/ ₄ -50.

Structure of the obtained poly(alkylene terephthalate-co-carbonate)s was also confirmed by ¹³C NMR spectra analysis (exemplary spectra is shown for TC4-46 in Fig. 4). In the range of 64–67 ppm there are signals which can be assigned to carbon atoms of CH₂O groups characteristic for heterodiads and homodiads. There are two signals from carbonyl ester and carbonate carbon atoms (166 and 155 ppm). The ¹³C NMR spectrum additionally confirmed random structure of the poly(tetramethylene terephthalate-co-carbonate).

The chemical structure of the obtained poly(alkylene terephthalate-co-carbonate)s was confirmed by MALDI-TOF mass spectrum (Figs. 5 and 6). In the spectrum the series differ in the number of terephthalate units (220.23 Da). The peaks of these series are characterized by mass increment 130.14 Da from one peak to the next, and is equal to mass of the tetramethylene carbonate unit.

The amount of macromolecules of exact combination of terephthalate and carbonate units is visualized in Fig. 6. The dark areas show the combinations of the greatest intensity. It is seen that copolymer contains mainly macromolecules with 0 to 12 carbonate and 5 to 14 terephthalate units per molecule. The second area of macromolecules of high intensity is observed for macromolecules containing 0–3 terephthalate and 20–35 carbonate units. It should be underlined that it is rather difficult for interpretation results obtained by MALDI-TOF mass spectrometry. The results refer only to the fractions of the polymer which were ionized during the MALDI-TOF experiments. The ionization ability depends on polymer chemical structure.

Propylene carbonate (PC) is cheap and non-toxic monomer which can be used as a carbonate linkages precursor. It is easily obtained from propylene oxide and CO₂. The synthesis of poly(ester-carbonates) was carried out in two steps. Firstly, PC was reacted with 1,4-butanediol (Scheme 5). The amount of 1,4-butanediol used in the first step was calculated for molar ratio of 1,4-BD to the sum of DMT and PC equals 1.5:1.

The reaction was carried out in an azeotropic solvent (n-heptane) [9, 37], so the by-product (1,2-propylene glycol) was removed from the reaction mixture as an hetero-azeotrope. The attempts of removing of 1,2-propylene glycol by distillation failed due to its co-distillation together with 1,4-butanediol.

As far as catalyst is concerned, it was found that the same catalyst (Ti(OBu)₄) used in the first step can be applied in the polycondensation step. It should be mentioned that the use of basic catalyst such as K₂CO₃, which is suitable in the synthesis of alkylene bis(methylcarbonate), cannot be applied in the reaction of diol with propylene carbonate. Basic catalysts promote the formation of ether linkages in a high extent (Scheme 6) [9].

The reaction progress was monitored by IR spectroscopy. When in the spectrum of the reaction mixture an absorption band at 1795 cm^-1 (characteristic for carbonyl group of five-membered cyclic carbonate) disappeared, the solvent was distillated off and DMT was added. Next steps were very similar to the process in which tetramethylene bis(methylcarbonate) was used.

In contrast to the method in which dimethyl carbonate was used, the synthesis with propylene carbonate can be carried out in one pot, and there is no need to separate semi-product to obtain poly(ester-carbonate) with intended carbonate units participation.

The ¹H NMR spectrum of poly(tetramethylene terephthalate-co-carbonate) (TC4-46), together with the corresponding chemical shift assignments, is presented in Fig. 7.

Poly(ester-carbonate)s’ composition was calculated using the relative areas of the signals of the OCH₂ protons of the terephthalate units located at 4.35–4.45 ppm (TT + TC) and the OCH₂ protons of the carbonate units at 4.13–4.23 ppm (CC + CT). To estimate microheterogeneity the degree of randomness (R) was calculated. This was accomplished by taking into account the intensity of resonance peaks of the OCH₂ protons. The degree of randomness (R) according to Yamadera and Murano is defined [38] as:

where P _TC and P _CT are the probability of finding a T unit next to a C unit and the probability of finding a C unit next to a T unit, respectively, while f _TT , f _TC , f _CT , and f _CC represent the triads fraction, calculated from the integral intensities of the resonance signals TT, TC, CT, and CC, respectively. L _nC and L _nT stand for the number-average sequence length, the so-called block length, of the C and T units, respectively. In the case of random copolymers R takes a value equal to 1, while for alternate copolymers equal to 2 and for block copolymers close to zero. For all polymers, as seen in Table 2, the degree of randomness was very close to 1, indicating the random nature of the prepared poly(ester-carbonate)s. Similar results were obtained for copolymers based on both dimethyl carbonate and propylene carbonate as carbonate linkage sources. According to Yamadera and Murano, the number-average sequence length for terephthalate (L _nT) and carbonate (L _nS) units, also known as the corresponding block length, can be calculated using the same equations. The observation of signals corresponding to terminal groups HOCH₂ also indicated that molar ratio of 4-hydroxybutyl carbonate to 4-hydroxybutyl terephthalate end groups is similar to that of terephthalate to carbonate units in the whole polymer.

In Table 2 calculated values for the degree of randomness and the number average block length for terephthalate and carbonate are collected. It is seen that for 50/50 molar ratio, the block length is ca. 1, what indicates that polymer has a random structure (Fig. 7, Table 2). The length of terephthalate block has decisive influence on copolymer biodegradation. When the number average block length is 3 or higher degradation rate is small and it is difficult to achieve complete biodegradation of aliphatic-aromatic polyester [29].

For poly(ester-carbonate)s obtained from other diols it was difficult to estimate the copolymer randomness by ¹H NMR method, due to overlapping of the signals corresponding to OCH₂ protons (CC with CT and TT with TC) in the region 4.0–4.5 ppm (Fig. 3).

The highest molar mass (M_w = ca. 50 000 g/mol) was obtained when the mixture of 1,4-butanediol and 1,5-butanediol as well as BMC4 and BMC5 were used in the reaction with DMT (Table 2, run 12).

Thermal analysis of the poly(ester-carbonate)s was carried out using DSC. The first heating scan of the samples showed that the poly(ester-carbonate)s and polycarbonate are semicrystalline. The melting points and heat of fusion decreased with increasing tetramethylene carbonate (BC) unit content. The respective values are summarized in Table 3.

The glass transition temperature decrease with the increasing of carbonate units content in the polymer chain, from 17.5 °C for TC4-27 to 39 °C for CD4 (100 % carbonate units).

In the DSC thermodiagrams two melting points can be observed (Fig. 8). The melting point of first type of crystallites is decreasing with the increase of the distortion of the polymer chains regular structure-in case of CD4, where is no aromatic units in the structure, the melting point is the highest (59 °C) (Table 3, run 1) and decrease down to 41.2 °C for TC4-39 (Table 3 run 5). The melting enthalpy (ΔH_m1) of first type of crystallites increases with the increasing content of aliphatic carbonate units. The second type of the crystallites origins from aromatic ester units and melting point increases with the increasing number of terephthalic units - from 124 °C (TC4-57) to 188 °C for TC4-27. Figure 9 shows the influence of the aliphatic carbonate units content on the thermal characteristics of the obtained products.

In case of samples with approximately 50 mol.% of carbonate units content and relatively high molar mass (Table 3 run 8–9) one broad melting area in range of higher temperatures is observed what indicates that aliphatic and aromatic units create new type of crystallites. It can be also seen that in case of the polymers of higher molar mass T_g increases significantly (Table 3, run 4 and 8).

Thermal characteristics of the obtained aliphatic-aromatic poly(alkylene terephthalate-co-carbonate)s based on different diols are collected in Table 4. With the increasing length of aliphatic hydrocarbon chain due to the chains flexibility T_g decrease from 18.9 °C in case of TC4-50 to -24.1 °C in case of TC10/ ₄ -50. The melting point of the polymers is the highest for tetramethylene copolyester (149 °C). In case of usage of other diols (and mixture with 1,4-butanediol) the melting point was shifted to much lower temperature and increases with the increasing number of methylene groups in the hydrocarbon chain from 57.6 °C for TC5-49 up to 84.2 °C for TC10/ ₄ -50 (Tab. 4, run 3–5).

It should be mentioned that polymers based on 1,5-pentanediol (odd number of carbon atoms in a molecule) and mixture of 1,5-pentanediol and 1,4-butanediol crystallize very slowly and during cooling with 10 Kmin^-1 no crystallization was observed (Table 4, run 2–3). The same phenomenon was observed in case TC6/ ₄ -49, most probably due to the even less regular polymer structure (Table 4, run 4).

Taking into account that thermal stability is important parameter for synthesis as well as polymer processing, thermogravimetric analysis of poly(tetramethylene terephthalate-co-carbonate)s was performed. Two samples with different content of carbonate units were investigated – TC4-39 and TC4-77. In both cases two-step degradation process was observed (Fig. 10). Based on DTG curve the decomposition temperature at 5 % weight loss (T_5%), the temperature of the highest degradation rate of each step of degradation (T_max1, T_max2), the temperatures after the first (T₁) and the second (T₂) degradation step as well the weight loss of the sample at T₁ and T₂ were estimated. In Table 5 weight loss of the sample at 600 °C (T₆₀₀) was also shown.

The T_5% is higher in case of TC4-39, what indicated that thermal stability is increasing with the increasing amount of aromatic units. As it can be observed from the values of weight loss at T₁ and T₂ (Table 5) the first step of the degradation is connected to the carbonate units of the polymer chains, and the second step to the terephthalate units. The temperature at which degradation is completed is influenced by the aromatic ester content in the molecule. The T_max1 and T_max2 values were almost the same in case of both investigated polymers.

Preliminary results of mechanical properties of poly(tetramethylene terephthalate-co-carbonate) of different content of carbonate units in molecules are presented in Table 6. The values of Young′s modulus (E), the relative elongation at break (ε_break), tensile strength (σ_break) and the stress at yield (σ_yield) (if any) were measured. As can be seen aliphatic-aromatic poly(tetramethylene phthalate-co-carbonate)s exhibit good mechanical strength. The tensile strength of 37 and 27 MPa and elongation at break of 690 and 610 %, respectively were recorded for copolymers containing 43 and 39 mol.% of carbonate units (Table 6, runs 3 and 4). During the stretching of these polymer samples orientation of polymer chains was observed – samples changed from white to transparent. Higher amount of carbonate units in the polymer leads to lower tensile strength. It should be mentioned that these samples are characterized by lower molar mass.

Mechanical properties of poly(tetramethylene terephthalate-co-carbonate) TC4-43 are very similar to those of commercially available (BASF) aliphatic-aromatic copolyester Ecoflex^® which contains 50 % of adipate and 50 % of terephthalate units (Table 6, runs 3 and 7). It is worth mentioning that the poly(ester-carbonate) containing 40–45 % of butylene carbonate units are characterized also by similar thermal properties to Ecoflex^® (T_m = 115 °C) [39].

In case of products based on longer diols (1,5-pentanediol, TC5-49 and mixture of 1,6-hexanediol and 1,4-butanediol, TC6/ ₄ -49) mechanical properties were also measured (Table 6, runs 5 and 6). In both cases the tensile strength was lower than that when 1,4-butanediol was used. The lower strength was observed because of the polymer poor crystallization capacity caused by usage of diol with odd number of methylene groups as well as a mixture of two different diols.