Random Copolycarbonates Based on a Renewable Bicyclic Diol

Article pubs.acs.org/Macromolecules

Random Copolycarbonates Based on a Renewable Bicyclic Diol Derived from Citric Acid Xiaohan Liu, Chengcai Pang,* Jianbiao Ma, and Hui Gao* School of Material Science and Engineering, School of Chemistry and Chemical Engineering, Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, Tianjin University of Technology, Tianjin 300384, China S Supporting Information *

ABSTRACT: To address the poor thermal stability of isohexides while at the same time retain rigidity, we developed a novel bicyclic diol octahydro-2,5-pentalenediol (OPD) from naturally occurring citric acid in this study. Owing to the bicyclic skeleton composed of two fused cyclopentane rings, OPD is supposed to have perfect rigidity but higher thermal stability compared to isohexides. Herein, OPD was first converted to octahydro-2,5-pentalenediol bis(methyl carbonate) (OPBMC) by reacting with dimethyl carbonate. The absolute stereochemistry of OPBMC was investigated by 2D 1H NMR and 13 C NMR as well as single crystal X-ray diffraction. By polymerization of OPBMC with several aliphatic diols [1,8-octanediol (A8), 1,10decanediol (A10), and 1,12-dodeacnediol (A12)] and alicyclic diols [1,4cyclohexanedimethanol (CHDM), 1,2,2-trimethylcyclopentane-1,3-dimethanol (TCDM), and octahydro-2,5-pentalenediol (OPD)], a series of bio-based copolycarbonates (co-PCs) with intriguing properties were synthesized. NMR spectra revealed that the stereochemistry of OPBMC was preserved after polymerization. Both differential scanning calorimetry and wide-angle X-ray diffraction analyses revealed that co-PCs made from A8, A10, A12, and OPD are semicrystalline, while co-PCs based on CHDM and TCDM are amorphous. A relatively high T5% of 276 °C and outstanding high Tg up to 80.4 °C were detected for fully OPD-based co-PC, confirming the excellent thermal stability and rigidity of OPD. This work addresses some critical needs for high performance polymers such as improving the sustainability of raw materials and achieving both high Tg values and thermal stability.

1. INTRODUCTION Significant interests in the development of bio-based polymers have been sparked in recent years due to the diminishing reserves of petroleum.1−6 Among the wide diversity of the biobased monomers, sugar-derived cyclic structures stand out at the privileged position. In addition to their natural origin, they are able to provide polymers with excellent mechanical strength, high glass transition temperature (Tg) values, UV stability, and high transparency.7−9 The high Tgs and transparency are greatly appreciated merits in polymers that are used as beverage packing under heating or optical glasses.10 Isohexides, which are found in three major isomeric forms, namely isosorbide, isoidide, and isomannide, are a group of secondary diols composed of two fused tetrahydrofuran rings derived from starch or cellulose and have found diverse applications in polyesters, polycarbonates, polyurethanes, and coating due to their attractive rigidity and chirality.11−16 Despite the apparent merits of isohexides, two major drawbacks were detected: (i) oligomers were obtained due to the poor reactivity of the secondary hydroxyl groups at the 2and 5- positions;17 (ii) severely colored or cross-linked polymers were obtained at high temperature due to the poor thermal stability of the skeleton composed of two tetrahydrofuran rings.18,19 To overcome the first difficulty, Wu et al. © XXXX American Chemical Society

developed a one-carbon extension strategy to convert isomannide into isomannide dinitrile, a key intermediate that can be transformed into various monomers, including diol, diester, dicarboxylic acid, and diamine.20 However, the thermolability of the bicyclic skeleton remained unimproved, and severe gelation occurred at polycondensation temperatures above 170 °C.21 Moreover, in addition to isohexides, several bicyclic acetalized alditols and methyl aldarates derived from carbohydrates,22,23 particularly those consisting of fused rings,7 have been shown to be suitable to prepare polycondensates displaying high Tg values. Unfortunately, some of them are still suffering from poor thermal stability.9,24 Therefore, in order to obtain bio-based polycondensates with satisfactory general properties and enhanced Tg, the exploration of fully alicyclic diol from renewable resources, that mimic the structures of isohexides or diacetalized alditols, is an attractive strategy. However, up to now, few reports can be found on this subject. Citric acid, with a global annual production of 7 × 105 tons, is an acknowledged bio-based commodity produced by fermentation.25 Citric acid has been extensively used as Received: July 31, 2017 Revised: September 19, 2017

A

DOI: 10.1021/acs.macromol.7b01641 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Synthesis of Octahydro-2,5-pentalenediol Bis(methyl carbonate) (OPBMC) from DACa

Reaction conditions: (a) NaOH, glyoxal, 65 °C, 1 h, 65%; (b) HCl and AcOH, reflux, 3 h, 80%; (c) NaBH4, MeOH, 0 °C, 12 h, 80%; (d) dimethyl carbonate, tetrabutyl titanate, reflux, 12 h, 79%.

a

chemical modifier, plasticizer of starch, and a functionalityenhancing monomer in high-tech coatings.26,27 In addition, dimethyl 1,3-acetonedicarboxylate (DAC), which is derived from citric acid, is a promising bio-based feedstock suitable for the construction of various novel structures.28 Previously, we prepared a series of self-curing furan-based polyesters starting from DAC and chloroacetaldehyde, which can be potentially used as one-component coating, elastomers, and adhesives.29 In this work, we developed a bicyclic diol octahydro-2,5pentalenediol (abbreviated as OPD, Scheme 1) via Weiss− Cook condensation and reduction reactions from DAC and glyoxal.30 Noteworthy, in addition to the renewability of DAC, glyoxal can be obtained by oxidation of ethylene glycol,31 which is a typical bio-based compound.32 So OPD can be regarded as a (potentially) fully biorenewable monomer. It is expected that incorporation of OPD in polycondensates will result in similar Tg but higher thermal stability compared to the polymers based on isohexides. In the past decade, the traditional bisphenol A (BPA) polycarbonate, which is produced from petroleum-based BPA (a synthetic estrogen mimic) and high toxic phosgene, has been frequently questioned due to the health risks related to it.2 From a sustainability and health point of view, preparation of fully bio-based polycarbonates without the use of phosgene and BPA is highly attractive.33 For instance, several isohexide-based polyesters and polycarbonates have been developed, which contributed to the introduction of stiffness to the resulting polymers in a way similar to BPA.34−36 Given the intrinsic rigidity and thermostability of OPD, we speculated that it is undoubtedly an excellent candidate to replace the ubiquitous BPA and thermolabile isohexides for the fabrication of polycarbonates with high Tg and thermal stability. In this paper, in order to obtain copolycarbonates with proper molecular weights, OPD was first converted to carbonated monomer octahydro-2,5-pentalenediol bis(methyl carbonate) (OPBMC) by reacting with dimethyl carbonate. Subsequently, OPBMC was melt polymerized with several flexible linear or rigid alicyclic diols, affording a series of bio-based co-PCs with intriguing properties. As a novel bio-based building block, the absolute stereochemistry of OPBMC was confirmed by 1D and 2D 1H NMR and 13C NMR as well as single crystal X-ray diffraction. The synthesis, chemical structure characterization, thermal properties, and crystal structure analysis of the resulting co-PCs will be described. In addition, in order to verify the aforementioned hypothesis concerning the rigidity and thermal stability of OPD, the structure−thermal property relations were investigated by comparing the thermal properties of a range of relevant polymers.

(Scheme 1). 1H NMR and 13C NMR spectra of OPD are shown in the Supporting Information (Figures S1 and S2) and detailed in the experimental section. The 1H NMR spectrum shows that OPD is a mixture of two diastereoisomers (endoendo:endo-exo = 4:1), which is comparable to the experimental results reported by Thomas et al.37 Isolation of the pure isomers is tedious and expensive, so they were used directly in the transesterification reactions with excess dimethyl carbonate, affording the corresponding bis(methyl carbonate) of OPD (abbreviated as OPBMC). Dimethyl carbonate is known as a green reactant suitable for the preparation of aliphatic or aromatic polycarbonates via phosgene-free methods.38 Peak assignments of OPBMC were established using 1H NMR and 13C NMR spectra. Signals in the 1H NMR spectrum (Figure S3) at δ 5.00, δ 3.78, δ 2.51, δ 2.21, and δ 1.76 ppm can be attributed to the protons 3 (7), 10 (12), 1 (5), 2 (4, 6, 8) (exocyclic), and 2′ (4′, 6′, 8′) (endocyclic), respectively. Furthermore, the number of 13C NMR signals (Figure 1b) revealed a symmetrical structure for OPBMC. On the basis of these results, we got the conclusion that only the endo-endo isomer of OPBMC was retained, while the endo-exo conformation was completely removed after recrystallization from methanol.

2. RESULTS AND DISCUSSION Synthesis and Characterization of OPD and OPBMC. Our synthetic approach to the octahydro-2,5-pentalenediol (OPD) began with the commercially available DAC and glyoxal

Figure 1. (a) 2D COSY spectrum of OPBMC recorded in CDCl3: for the cross-peaks, H1 = H5, H2 = H4, H6 and H8, H2′ = H4′, H6′ and H8′, H3 = H7, H10 = H12. (b) 13C NMR spectrum of OPBMC recorded in CDCl3. C1 = C5, C2 = C4, C6 and C8, C3 = C7, C9 = C11, C10 = C12. B


Article

Macromolecules Given the unique characteristics of OPBMC (rigidity and chirality), 2D COSY spectroscopy (Figure 1a) and singlecrystal X-ray (Figure 2 and Table S1) were used to elucidate its

Scheme 2. Polymerization Reactions Leading to Random coPCs

Figure 2. Molecular structure of OPBMC in the crystal.

molecular structure in detail. The first distinctive feature of OPBMC is the V-shaped skeleton composed of two cis-fused cyclopentane rings similar to the tetrahydrofuran rings of isohexide derivatives. The carbonate groups, situated at C3 and C7 and positioned inside the molecular skeleton, are designated as endo-endo. The dihedral angle between the envelop-like cyclopentane rings cannot be given precisely because they are not planar. Alternatively, it may be described in terms of torsion angles. Single-crystal X-ray diffraction showed that the torsion angle of C4−C5−C1−C8 is 119.7°, and the torsion angle of C2−C1−C5−C6 is 125.9°. Several articles reported that for the exo-exo type isohexides the bridge protons have no dihedral coupling with the endo-oriented protons adjacent to it.17,21 Given the similarity of the V-shaped bicyclic skeleton of OPBMC with the former, we speculated that there would be no 1 H−1H dihedral coupling between the bridge protons H1 (or H5) with neighboring endocyclic proton H2′ and H8′ (or H4′ and H6′). However, signals for H1 (or H5) appeared as a broad multipeak and cross-peak of the correlation between H1 (or H5) with H2′ and H8′ (or H4′ and H6′) were observed in the 2D COSY spectrum (see Figure 1a), indicating that the bridge protons correlate simultaneously with neighboring endocyclic protons. To elucidate this result, the estimation of the spin− spin coupling constants between the bridge protons and neighboring endocyclic protons was undertaken using the Karplus equation.39−41 The calculated coupling constants for H1−H2′, H1−H8′, H5−H4′, and H5−H6′ were 6.8, 4.3, 7.1, and 7.1 Hz, respectively. From this preliminary result, it is concluded that the stereochemistry of the OPBMC skeleton is slightly different from the exo-exo type of isohexide. Further investigation will be focused on the full elucidation of the conformation of the parent skeleton of OPBMC. Synthesis of Polymers and Chemical Structure Analysis. The polymerizations involving equimolar OPBMC and diols were performed by a two-step melt-polymerization process using lithium acetylacetonate (LiAcac, 0.1 wt % based on diol) as catalyst: prepolymerization at 170 °C under nitrogen, followed by polycondensation at elevated temperatures ranging from 170 to 230 °C under vacuum to facilitate the removal of methanol (Scheme 2). LiAcac has been reported as an efficient catalyst with high selectivity for transesterification reactions and has been frequently used for the synthesis of aliphatic or aromatic polycarbonates by melt-polymerization.42 A series of co-PCs were obtained by copolymerization of

OPBMC with A8, A10, A12, CHDM, TCDM, and OPD, which were labeled as PC-8, PC-10, PC-12, PCC, PTC, and POC, respectively. Some of the diols used are bio-based or can be potentially produced from biomass, such as A8,43 A10,44 OPD in this article, and TCDM, a rigid diol derived from naturally occurring camphor.45 Thus, PC-8, PC-10, POC, and PTC can be regarded as fully bio-based. After polymerization reactions, polymers were dissolved in a minimum amount of CHCl3 and precipitated in cold methanol. No gelation occurred during the whole polymerization process, confirming the higher thermal stability of OPBMC than isohexides. All of the obtained polymers were “water white” optically clear in nature and exhibit good solubility in CHCl3 and THF at room temperature. The molecular weights of all obtained co-PCs were estimated by GPC, and data are given in Table 1. In general, co-PCs based on OPBMC and aliphatic diols displayed medium weight-average molecular weights (Mws) confined in the 21 000−33 800 g mol−1 interval and polydispersity between 2.0 and 2.3. While for PCC, PTC, and POC based on alicyclic diols, consistently lower Mws were obtained, which were 14 900, 17 900, and 11 100 g mol−1, respectively. Noteworthy, although co-PCs with exception of POC were prepared from primary diols, however, the isomer of OPD with endo-endo conformation, which was generated from OPBMC following the mechanism shown in Scheme 3, was also present in the polycondensation process. As secondary diol similar to isohexides, the hydroxyl groups of OPD in transesterification reactions are relatively unreactive,46 leading to the low to medium molecular weights for these polymers. With regard to co-PCs based on alicyclic diols, the high molten viscosity of the melts under polycondensation conditions is another suppressing factor. Because both OPBMC and the alicyclic diols used have bulky ring structures, as the degree of polymerization and melt viscosity increased, it became harder for the removal of byproduct methanol under vacuum, while raising the reaction temperature led to thermal degradation of the obtained polymers.47 The molecular structure of the novel co-PCs was analyzed using 1H NMR, 13C NMR, and 2D COSY spectroscopy. An exemplary result of the 2D COSY and 13C NMR spectra of PC10 is presented in Figure 3. All protons and carbon signals from the repeating unit of the polymer are found at the expected C


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Macromolecules Table 1. Molar Composition, Molecular Weight, and Microstructure of co-PCs molar composition

microstructure molecular weightc

number-average sequence lengths

feeda

co-PCb

dyadsd

co-PC

XOPD(mol %)

XOPD(mol %)

Mnc

Mwc

Dc

AA

AO

OO

nAC

nOC

Re

PC-8 PC-10 PC-12 PCC PTC POC

50.0 50.0 50.0 50.0 50.0 50.0

16.7 30.3 34.7 37.0 58.8 50.0

15000 14500 10100 8700 9200 6700

33800 29100 21000 14900 17900 11100

2.3 2.0 2.1 1.7 2.0 1.7

67.6 48.8 44.3 37.3 14.0 47.4

30.4 42.4 44.7 47.8 48.5 46.0

2.0 8.8 11.1 15.0 37.5 6.6

5.4 3.3 3.0 2.6 1.6 0.3

1.1 1.4 1.5 1.6 2.5 0.7

1.1 1.0 1.0 1.0 1.0 1.1

a Theoretical molar content of OPD in the copolycarbonate chains. bmol % content in OPD relative to the total amount of diols (OPD + the diol used), determined by integration of the 1H NMR spectra. cNumber- and weight-average molecular weights in g mol−1 and dispersities measured by GPC in tetrahydrofuran against PS standards. dExperimental values obtained by means of the equations using the 13C NMR data mentioned in the text. eRandomness index of copolycarbonates statistically calculated on the basis of the 13C NMR analysis.

Scheme 3. Mechanisms Leading to the Low Content of OPD in PC-8, PC-10, PC-12, and PCC

chemical shift values with matching multiplicities. Signal emerging at δ 4.08−4.13 ppm corresponding to H11 (or H11′), and the disappearance of signal at δ 3.78 ppm corresponding to methyl carbonate of OPBMC, indicate the formation of carbonate linkage. Multipeaks at δ 4.95, δ 2.44, δ 2.23, and δ 1.68 ppm are attributed to the protons H3 (or H7), bridge protons H1 (or H5), exocyclic protons H2 (or H4, H6, and H8), and endocyclic protons H2′ (or H4′, H6′, and H8′) of the molecular skeleton moiety of OPD. Peaks at δ 1.64−1.67 and δ 1.28−1.35 ppm are assigned to H12 (or H12′) and methylene protons of the A10 moiety, respectively. Noteworthy, except for the multipeak located at δ 4.95 ppm corresponding to H3 and H7, no additional peaks appeared at around δ 5.00 ppm, suggesting that the original stereochemistry of OPBMC was preserved after polymerization. This finding was further supported by the cross-peaks 3/2′ (4′), 3/2 (4), 7/6′ (8′), and 7/6 (8) at δ 4.95 ppm, which was identical to the 2D COSY spectrum of OPBMC (see Figure 1), implying that the protons H3 and H7 adopt an exo-exo configuration (and hence the two hydroxyl groups are endo-endo oriented). 1 H NMR and 13C NMR spectra for PC-8, PC-12, PCC, PTC, and POC can be found in the Supporting Information. In the case of PCC (Figure S6), δ 4.07, δ 3.96, δ 1.93, δ 1.87, δ 1.60, δ 1.56, δ 1.46, and δ 1.03 ppm were assigned hydrogen atoms 11(c), 11(t), 12(c), 13a(t), 12(t), 13a(c), 13b(c), and 13b(t), respectively. By integrating the peaks corresponding to 11(c) and 11(t), the calculated ratio of trans- to cis-isomers in CHDM is 7:3, this concurs with the starting monomers, so epimerization of CHDM did not occur in the polymerization

Figure 3. (a) 2D COSY spectrum of PC-10 recorded in CDCl3: for the cross-peaks, H1 = H5, H2 = H4, H6 and H8, H2′ = H4′, H6′ and H8′, H3 = H7. (b) 13C NMR spectrum of PC-10 recorded in CDCl3.

process. For POC, much more peak split was observed in the H NMR spectrum (Figure S8) due to the coexistence of the endo-endo and endo-exo isomers of OPD in the POC chains. Apart from the increased complexity of the endo-exo isomer itself, the stereoirregularity of the polycarbonate (i.e., endo-endo, endo-exo, exo-exo) results in peak split and hence significant peak overlapping in the 1H NMR spectrum. Chemical shifts at 5.19, 2.71, 2.61, 2.55, 2.44, 1.59, and 1.28 ppm were attributed to hydrogen atoms g, a, e, 1, 5, h′ (f′), and b′ (d′), respectively. Peaks 2 (4, 6 and 8) and 2′ (4′, 6′ and 8′) overlap exocyclic hydrogen atoms b (d) and h (f) of the endo-exo OPD moiety, respectively. In the 1H NMR spectrum of the endo-exo OPD moiety, δ 2.08−2.15 and δ 1.80−1.84 ppm were assigned to hydrogen atoms b (d) and h (f), respectively. 1

D


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Macromolecules

at the beginning of the polymerization reactions, leading to the conclusion that the two isomers have similar reactivity in the transesterification reactions. On the basis of the above analysis, we speculated that scrambling transesterification reactions occurred throughout the whole polycondensation process and hence all of the obtained co-PCs should be random. In order to verify this hypothesis, the microstructure of all co-PCs was determined by 13 C NMR spectra, and signals were reasonably assigned as shown in the Supporting Information (Figures S4−S8). In the 13 C NMR spectrum, the carbonyl carbons with a chemical shift at around 155 ppm were used to clarify the dyad sequence distribution because this shift is more sensitive to sequence effects than any other carbon atoms. Typically, in the expanded 13 C NMR spectrum of PC-10 (Figure 4), the signals of

The chemical compositions of all co-PCs were ascertained using 1H NMR spectra, and the results are listed in Table 1. Integration of the proton signals arising from OPBMC and corresponding diol led to quantification of the composition of the co-PCs in such units. For example, by integration of the signals of the methine protons H3 (H7) and methylene protons H11 (H11′) in PC-12 (Figure S5), we got the mol % content of OPD relative to the total amount of diols (OPD + the corresponding comonomeric diol) of 34.7%, which is lower than the expected value of 50%. Similar results were also observed for PC-8, PC-10, and PCC, which were 16.7%, 30.3%, and 37.0%, respectively. We tried to figure out the probable causes that led to the deviation of OPD content from the feed ratio and proposed a mechanism as shown in Scheme 3. In our strategy of base-catalyzed polycondensation process, transesterification reactions continually occur throughout the entire polycondensation process. In the beginning of the polymerization reactions, chain growth occurs predominantly by the transesterification reaction between −OH and −OC(O)OCH3 end-groups, with formation of methanol. However, transesterification reactions between primary diols and the formed carbonate linkages may partially lead to the formation of free OPD as a byproduct (Scheme 3, path a). As a secondary diol similar to isomannide, the endo-endo hydroxyl groups in OPD exhibited rather low reaction activity, and the “kicked” OPD is constantly eliminated from the system by volatilization under the polymerization conditions. Therefore, for OPD, the chances that reincorporation into the polymer chains via chain scission in the polycondensation is much lower than the primary diols, resulting in the low content of OPD. In order to confirm this hypothesis, we performed a simple kinetic study on the polymerization process of PC-10 in detail. The reaction was performed under polymerization conditions identical to the reaction course for PC-10 with a 2:1 molar ratio of A10 to OPBMC, and the content of OPD was monitored by integration of characteristic 1H NMR signals at various reaction time (Figure S9 in the Supporting Information). The recorded curve showed that the resulting content of OPD units decreased steadily with the prolonged reaction time, from 24.8% to 12.5% within 18 h (Figure S10), implying the lower reactivity of OPD compared to primary diols. In contrast, a higher content of OPD was observed for PTC because sublime of TCDM above 70 °C under full evacuation caused the imbalance of stoichiometric ratio of TCDM and OPBMC.48 For POC, the accurate molar ratio of the endo-endo and endoexo isomers of OPD in the main chains is a noteworthy issue because it is closely related to the properties of POC. Based on the literature data,47 the reactivity of the exo hydroxyl group in isosorbide is 1.5 times higher than that of endo hydroxyl group in transesterification reaction. So we speculate that the reaction between the endo-exo isomer and OPBMC will precede the reaction of the endo-endo isomer and OPBMC, and a large amount of the endo-endo isomer will be lost during polymerization, resulting in the high content of the endo-exo isomer in POC. To ascertain this, the molar ratio of the two isomers was analyzed by integration of the proton signals arising from H(3)/H(7) and H(c)/H(g), as shown in Figure S8. The obtained ratio of the integral area corresponding to protons (H(c) + H(3) + H(7)) to H(g) is 18.94:1.00, and the integral area of H(c) should be identical to H(g), so the molar ratio of the endo-endo and endo-exo isomers in POC is very close to 9:1. This result is in good accordance with the feed ratio of OPBMC (endo-endo) to OPD (endo-endo:endo-exo = 4:1) of 1:1

Figure 4. Extended 13C NMR spectra of various co-PCs in the carbonyl carbon region.

carbonyl carbons were split into three peaks corresponding to the A10A10, A10O, and OO dyads, where A10 and O represent 1,10-decanediol and OPD units, respectively. By the same reason, PC-8, PC-12, and POC exhibited similar NMR patterns in the carbonyl carbon region. For POC, three peaks were present in the carbonyl carbon region, corresponding to the occurrence of stereoirregularity in POC (exo-exo, exo-endo, and endo-endo triads). While in the case of PTC (Figure 4), six peaks are observed since three types of dyads (TT, TO, and OO) are possible, and the T unit is asymmetric, so the TO dyad will produce two peaks and three peaks of equal intensity arise from TT dyad due to the two possible orientations that T units may adopt when incorporated into the polymer chains. Similarly, for PCC, six peaks were present corresponding to the three possible dyads (OC, OO, and CC) and the two possible orientations of cis/trans isomers of CHDM in the polymer chains. By integration of corresponding peaks, the different dyad contents (N), the number-average sequence lengths (n), and the degree of randomness (R) were determined using the following equations, and the data are listed in Table 1. Results showed that the value of R is close to 1 in all cases, leading to the conclusion that the microstructure of all co-PCs was random.49−51 nAC = E

NAC/OC + 2NAC/AC NAC/OC DOI: 10.1021/acs.macromol.7b01641 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules nOC = R=

linkage is more susceptible to heating than the alicyclic ones,52 the first step can be attributed to the decomposition of AC short sequence, and the second step should be mainly caused by the degradation of the OC ones. Furthermore, the TGA observations described above differ remarkably from their analogous copolycarbonates reported by Li et al.,53 in that case the thermal degradation mechanism involved only one main step and the values of T5% and Td were found in the range of 320−330 and 360−380 °C, respectively. The lower OPD content in the polymer chains, which is in the range of 15%− 35% (Table 1), is the reason for their lower thermal stability. Similarly, a two-step pyrolysis profile was observed for PCC with maximal decomposition rates at the proximities of 305 and 368 °C, respectively, indicative of the different thermal stability of CC and OC short sequences. In the case of PTC and POC, similar thermal decomposition behaviors involving only one narrow degradation step with the same maximum degradation rate taking place at approximately 310 °C were observed for both cases. PTC initially decomposed at a lower temperature than POC, and above 270 °C it decomposed much more quickly at a temperature similar to that of POC. In contrast, no detectable weight loss took place up to 250 °C, and a T5% value as high as 276 °C was observed for POC. Given that the molecular weight of POC is the lowest in this study, it still shows a high T5% value compared to PCC and PTC, confirming the outstanding thermal stability of OPD. The melting-crystallization behavior of all co-PCs, during heating and cooling from −50 to 125 °C, are determined by DSC as shown in Figure 6, and the thermal transition data are given in Table 2. In general, fully aliphatic polycarbonates without substituents are highly crystalline polymers displaying low Tg, high melting enthalpy (ΔHm), and crystallization enthalpy (ΔHc) values.42 Bigot et al. prepared two aliphatic homopolycarbonates by polycondensation of A10 and A12 with dimethyl carbonate with Mns around 3000 g mol−1.54 The two polymers exhibited Tms at 45 and 60 °C, respectively, with melting enthalpies higher than 100 J g−1. In our present study, the main factors that governing the thermal transition behaviors of the co-PCs are the molar content of OPD and the type of diols used. As observed from DSC analysis, co-PCs made from aliphatic diols were found to be semicrystalline with low to moderate enthalpies, as evidenced by one or more melting and crystallization peaks from the first and second heating DSC curves. For instance, PC-8 exhibited broad melting transitions on both first and second heating, with Tms at 26.2 °C (ΔHm = 14.3 J g−1) and 26.9 °C (ΔHm = 17.8 J g−1), respectively. PC-10 exhibited melting peaks on both first and second heating traces,

NAC/OC + 2NOC/OC NAC/OC

1 1 + nAC nOC

The chemical structures of all co-PCs were additionally analyzed using FT-IR spectroscopy. The entire series of polymers were of similar FT-IR absorption patterns as shown in Figure S11. All the FT-IR spectra show characteristic strong absorption band corresponding to the CO stretching vibration at 1740 cm−1, the different types of diols, aliphatic or alicyclic, likely have negligible effect on the absorption pattern of carbonyl groups. More details about FT-IR spectra of OPD-based co-PCs can be found in the Supporting Information. Thermal Properties. The thermal stability of the co-PCs was comparatively studied by TGA in a nitrogen atmosphere. TGA traces measured from 40 to 600 °C are shown in Figure 5,

Figure 5. TGA traces of co-PCs recorded from 40 to 600 °C at 10 °C min−1 under a N2 atmosphere.

and the temperatures at 5% weight loss and the maximal decomposition rates of the co-PCs are presented in Table 2. The overview of the collected TGA data revealed an anomalously result that the co-PCs made from aliphatic diols exhibited higher thermal stability than the ones based on alicyclic diols, which was caused by their high Mns. Generally, polymers with higher molecular weight exhibited better thermal stability.51 A closer inspection of the DTG curves of PC-8, -10, and -12 revealed two major degradation steps for all cases, where the first one appears as a shoulder of the second peak, with maximal decomposition rates at 310−320 and 330−370 °C, respectively (see Figure S12). Since the aliphatic carbonate

Table 2. Thermal Properties of co-PCs Measured by TGAa and DSCb TGA

DSC first heating

cooling

co-PC

T5% (°C)

Td (°C)

Tg (°C)

Tm (°C)

ΔHm (J/g)

PC-8 PC-10 PC-12 PCC PTC POC

281 289 271 273 257 276

311/346 318/366 313/333 305/368 309 312

−41.4

26.2 30.6 61.3

14.3 20.2 30.7

32.8 39.7 82.4

113.7

2.9

Tc (°C)

101.2

second heating

ΔHc (J/g)

5.0

Tg (°C)

Tcc (°C)

ΔHcc (J/g)

Tm (°C)

ΔHm (J/g)

−40.5 −32.9

−4.6 −12.4

20.1 16.0

26.9 27.9 39.6/52.4c

17.8 24.0 18.9

34.4 40.1 80.4

109.1

3.4

a T5% = temperature of 5% mass loss, Td = temperature for maximal decomposition rate. bTg = glass transition temperature, Tm = melting temperature, Tc = crystallization temperature, Tcc = cold crystallization temperature upon heating, ΔH = enthalpy of transition. cMultiple melting temperatures.

F


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Macromolecules

higher than PC-8, which is opposite to general observations that Tg value decreases with increasing the carbon chain of the α,ω-diols.55 This is caused by the rather low content of OPD units in PC-8, which is only about half of the value in PC-10 (Table 1). With regard to PCC, PTC, and POC, different thermal behaviors were observed, depending mainly on the diols used. PCC displayed an amorphous feature in both the first and second DSC heating, which was caused by the existence of trans and cis isomers of CHDM in the polymer chains. Actually, the trans isomer of the CHDM shows a more stretched and symmetrical form with better packing ability, while the cis isomer introduces a kink in the polymer chains, hindering the formation of stable crystals.47 Compared to the higher Tg value of PCIC (88 °C) reported by Li et al.,53 the lower Tg of PCC (34.4 °C) finds its origin in the combined effects of the low content of OPD (37%) and the low Mn (8700 g mol−1). The latter is more remarkable because a low Mn implies a high concentration of end-groups (and hence free volume) as well as insufficient possibilities for chain entanglement of the polymer chains.56 In the case of PTC, DSC analysis shows completely amorphous characteristic that resembles the thermal behavior of PCC, which was caused by the occurrence of stereoirregular structures in the polymer chains due to the asymmetry of TCDM. Noteworthy, PTC displayed a higher Tg up to 40.1 °C in the second DSC heating compared to PCC, which can be attributed to two possible reasons. The chemical composition analysis has shown that PTC possesses the highest OPD content (58.8%) among all the polymers, so a relatively high Tg value can be expected. In addition, the three additional methyls attached to the cyclopentane ring in TCDM will increase the motion resistance of the chain segments and hinder the flips of cyclopentane rings. We thus speculate that TCDM is capable of raising the Tg of polymers in a manner similar to 2,2,4,4tetramethyl-1,3-cyclobutanediol due to its rigidity.57 In a patent by Wu et al.,58 they reported that PET copolyesters containing 60% and 80% CHDM units displayed Tg values of 82.1 and 87.7 °C, respectively. In contrast, PET copolyesters containing merely 10% and 25% TCDM units showed Tg values of 84.0 and 87.1 °C, respectively. Accordingly, TCDM has a more remarkable ability in inducing rigidity in the polymer chains than that exerted by the incorporation of CHDM. It is not clear yet which is the main factor determining the glass transition temperature of PTC because the differences of molecular weights and OPD contents between PCC and PTC preclude a quantitative comparison. Compared with PCC and PTC, the fully OPD-based POC is semicrystalline, despite the fact that the OPD units in the POC chains is actually a mixture of two diastereoisomers (endo-endo:endo-exo = 9:1). A sharp and intense melting peak can be seen in the first heating at 113.7 °C with ΔHm of 2.9 J g−1, which became broad in the second DSC heating at around 100.0−115.0 °C with ΔHm of 3.4 J g−1. Moreover, a crystallization peak was present during the cooling run at 101.2 °C with a ΔHc of 5.0 J g−1. On the basis of the above data, it is clear that POC is able to crystallize. This phenomenon can be elucidated by the perfect regularity of the POC backbones because it is exclusively composed of the OPD units and carbonate linkages, despite the fact that OPD is a conformationally restricted diol that hard to crystallize and the coexistence of the small amount of endo-exo isomers will disturb crystallization. More importantly, a highest Tg value up to 80.4 °C was found for POC among all the co-PCs. Based on the literature data, PC made from BPA with Mn of 12 000 g mol−1

Figure 6. First heating (a) and second heating (b) DSC curves of coPCs measured between −50 and 125 °C at 10 °C min−1.

with Tms at 30.6 °C (ΔHm = 20.2 J g−1) and 27.9 °C (ΔHm = 24.0 J g−1), respectively. No exothermal crystallization peaks were detected at the cooling DSC traces for both of them. In the case of PC-12, a broad melting peak appeared at 61.3 °C on the first heating (ΔHm = 30.7 J g−1), while bimodal melting peaks between 26 and 61 °C with moderate melting enthalpies (ΔHm = 18.9 J g−1) were observed on the second heating, indicative of imperfect polymer crystals or existence of polymorphism.18 Furthermore, PC-8 and PC-10 show cold crystallization behavior in the second DSC heating runs at −4.6 °C (ΔHcc = 20.1 J g−1) and −12.4 °C (ΔHcc = 16.0 J g−1), respectively, which is attributed to the known crystallization behavior of long chain aliphatic diols.42 Collectively, these results imply that both the degree of crystallinity and the crystallization rate from the melt for each sample were much lower than the fully aliphatic polycarbonates. This is understandable considering the chemical constitution and structures of the diols present in the polymer chains. On one hand, the high flexible long chains of aliphatic diols have high crystallize ability, leading to a better organization of the polycarbonate chains and permitting crystallization. On the other hand, OPD can be regarded as a conformationally restricted counterpart of isohexides due to its V-shaped skeleton composed of two cisfused cyclopentane rings. Therefore, incorporation of OPD units in aliphatic polycarbonates will induce a considerable amount of free volume due to the ring puckering, which prevents close packing of polymer chains. In addition, stiffness should be expected in view of the bicyclic skeleton of OPD, and we speculate that PC-8 and PC-10 will show much higher Tg values compared to those fully aliphatic polycarbonates. However, only slightly elevated Tg values were observed for both of them, which were −40.5 and −32.9 °C, respectively. The unexpected result can be explained by the low contents of OPD units in respective polymer chains, which were 16.7% and 30.3%, respectively. In addition, the Tg value of PC-10 is 7.6 °C G


Article

Macromolecules has a Tg value of 146 °C,59 and homopolycarbonate made from isosorbide with Mn of 15 000 g mol−1 has a Tg value of 162 °C.53 Here, given the rather low Mn value of POC, which is only half of the values of PCs made from BPA or isosorbide, it is concluded that the fused bicyclic OPD is distinctively effective in providing the polymer chains with a degree of stiffness similar to isohexides. One can expect that the Tg of POC can be dramatically improved if it has Mn comparable to the aforementioned PCs made from BPA or isosorbide. In a word, OPD is a promising bio-based diol that combining excellent thermal stability, rigidity, and chirality and can be an excellent candidate to replace BPA for the synthesis of high performance polycarbonates. Wide-Angle X-ray Diffraction. Wide-angle X-ray diffraction (WAXD) measurements were carried out to gain some more insight into the crystallinity of all the synthesized co-PCs. The diffraction profiles are presented in Figure 7. The degrees

for PC-8, PC-10, and PC-12 are determined using peak separation, which were 13.6%, 37.5%, and 48.6%, respectively. Such degrees of crystallinity were indicative of the low crystallize ability of the polymer chains from the melt, which further resulted in the absence of crystallization exotherms during the cooling DSC runs. In contrast to the weak reflection peaks observed for PC-8 to PC-12, the fully OPD-based POC displayed two sharp reflection peaks at 2θ of 16° and 18° with moderate degrees of crystallinity of 32.7%, which confirm the previous DSC observations that the incorporation of symmetric OPD improves the ability of POC to crystallize.

3. CONCLUSIONS For the first time, co-PCs from the citric acid-based OPBMC monomer, obtained by four step chemical transitions from dimethyl 1,3-acetonedicarboxylate and glyoxal, have been successfully synthesized via non-phosgene methods. Singlecrystal X-ray revealed the distinctive V-shaped skeleton of OPBMC composed of two cis-fused cyclopentane rings similar to isohexides derivatives. By melt polymerization reactions of OPBMC and aliphatic or alicyclic diols, “water white” optically clear co-PCs with moderate to high molecular weights were obtained in good yields. No gelation occurred throughout the whole polymerization process, confirming the higher thermal stability compared to isohexides. NMR analyses indicated that the original stereochemistry of OPBMC was preserved after melt polymerizations. The 1H NMR spectrum showed that the OPD content in PC-8, PC-10, PC-12, and PCC are consistently lower than the theoretical value of 50%, a result caused by the lower reactivity of OPD compared to the primary diols. By analyzing the dyad sequence distributions using the expanded 13 C NMR spectra, it was found that the microstructures for all co-PCs were random. Both DSC and WAXD analyses showed the semicrystalline characteristic with low to moderate degrees of crystallinity for PC-8, PC-10, PC-12, and POC and amorphous natures for PCC and PTC due to the asymmetry of the CHDM and TCDM. A systematic investigation on the structure−thermal properties relationship showed that the thermal stability of the three alicyclic diol-based co-PCs are basically in the order of PTC < PCC < POC, and Tg values are in the order of PCC < PTC < POC. Although the molecular weights and OPD contents of the three polymers differ from one another, a qualitative trend can be clearly observed that the fully OPD-based POC has the highest thermal stability and Tg value. Overall, on the basis of the polymerization process, molecular weights, color formation, and thermal properties of the resulting polymers, we have shown that the citric acid-based OPD is a highly intriguing bio-based building block: combining adequate rigidity and thermal stability suitable for the construction of performance polymers. We are currently further exploring the potential application of this new building block.

Figure 7. X-ray powder diffraction profiles of co-PCs.

of crystallinity were calculated according to the ratio between the surfaces of peaks, corresponding to crystalline and amorphous component. As an example, the result of the peak separation applied to the PC-12 is shown in Figure S13. As can be clearly seen from Figure 7, the crystallographic structures of these co-PCs show strong dependence on the type of diols used and chemical compositions, which corroborated the DSC results: PC-8, PC-10, PC-12, and POC are semicrystalline materials with weak to moderate reflection peaks at different diffraction angles, while PCC and PTC displayed amorphous characteristics by showing rather broad diffraction signals centered at 2θ angles around 18°. For PC-8, two characteristic reflections located at 2θ of 20° and 23° were observed, while in the case of PC-10 and PC-12, the diffraction patterns undergo substantial changes, which is evidenced by the different diffraction peaks located at 2θ of 21° (110) and 24° (200) close to those of linear polyethylene.60 Thus, we speculate that the crystallization of these three co-PCs are mainly caused by the methylene segments of the alkanediols. In order to verify this hypothesis, three homopolycarbonates, which were abbreviated as h-PC-8, h-PC-10, h-PC-12, have been synthesized from diphenyl carbonate and A8, A10, and A12, respectively. Their X-ray diffraction profiles are present in Figure S14. Obviously, these three homopolycarbonates exhibited diffraction patterns identical to co-PCs having the same alkanediols in this text, only differing in the degree of sharpness. So we got the conclusion that the crystallographic structures of co-PCs were mainly determined by the long methylene chains of the alkanediols. The degrees of crystallinity

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01641. Table S1, Figure S13, assigned 1H/13C NMR spectra, FT-IR spectra, and DTG plots for all synthesized monomers and polymers (PDF) H


Article

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AUTHOR INFORMATION

Corresponding Authors

*Tel +86 22 60214251; e-mail [email protected] (C.P.). *Tel +86 22 60214251; e-mail [email protected] (H.G.). ORCID

Hui Gao: 0000-0002-5009-9999 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (No. 21674080 and No. 51503150), Tianjin Municipal Natural Science Foundation (No. 17JCQNJC03400), 131 talents program of Tianjin, and the leading talents program of Tianjin Educational Committee.

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Random Copolycarbonates Based on a Renewable Bicyclic Diol

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