Fast Hydrolysis Polyesters with a Rigid Cyclic Diol from Camphor

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Fast hydrolysis polyesters with a rigid cyclic diol from camphor Jeong Eon Park, Da Young Hwang, Gwang Ho Choi, Kyoung Hwan Choi, and Dong Hack Suh Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b00761 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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Fast hydrolysis polyesters with a rigid cyclic diol from camphor Jeong Eon Park†, Da Young Hwang†, Gwang Ho Choi†, Kyoung Hwan Choi†, Dong Hack Suh†*

†Department of Chemical Engineering, College of Engineering, Hanyang University, S eoul 133–791, Republic of Korea *Corresponding author. E-mail address: [email protected] Tel.: +82 2 2220 0523; fax: +82 2 2220 4523

KEYWORDS Biopolymers, Degradable polymers, Camphor, Hydrolysis, High Tg

ABSTRACT 2,2:3,3-Bis(4′-hydroxymethylethylenedioxy)-1,7,7-trimethylbicyclo[2.2.1]heptane, abbreviated as CaG, is a compound obtained by transforming a ketone group to a ketal group with camphorquinone and glycerol. The CaG diol has a complex and rigid structure and two primary hydroxyl groups. A polyester series was synthesized with the CaG diol, ethylene glycol and dimethyl terephthalate. The polyesters exhibited adequate thermal stability up to nearly 330 oC and had a high Tg, which steadily increased from 78 oC to 129 oC as the content of CaG increased. A high proportion of the CaG moiety led to an amorphous region that is susceptible to hydrolysis and promoted degradation of the polyester in acidic conditions. Depending on the proportion of CaG in the polymer, the hydrolytic degradation of the polyesters was adjustable.

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INTRODUCTION As the amount of plastic released into the environment increases, many problems arise. The conventional plastics industry has grown on petroleum. The use of petroleum releases hundreds of millions of tons of CO2 and other types of emissions, which are leading to health risks for the public. Additionally, petroleum-based materials do not degrade quickly, so their waste remains in the environment for centuries, leading to ecological accumulation.1-3 Some materials from petroleum such as polycaprolactone are rapidly degraded, but it is not a way to reduce CO2 emissions because of the long-term carbon cycle of petrochemicals.4 To overcome problems caused by petrochemical plastics, many researchers have tried to replace petroleum resources with bio-based resources such as terpenes, carbohydrates or vegetable oils.5-7 Particularly, cyclic monomers from renewable feed stocks have received attention for their ability to improve the properties related to degradability and polymer chain stiffness. Ferouillot’s group reported about the use of 1,4:3,6-dianhydrohexitols(isosorbide, isomannide and isoidide) in polymers8, 9 and Koning’s group also do a lot of work on polymers from isohexides.10-12 Munoz-Guerra’s group synthesized various monomers with diacetalized structure from carbohydrates to apply to aliphatic and aromatic polyesters.13 Among many polymers, polyesters that have a cleavable ester group in the polymer chain are expected to be candidates for sustainable biodegradable plastics.14-16 Typically, polyester degradation occurs in the polymer chain.17 Factors affecting polymer degradation are (i) the contact area determined by the morphology of the polymer;, (ii) the structure of the molecules constituting the polymer, which determines its nature, such as its hydrophilicity, water accessibility and velocity of hydrolysis; and (iii) the environment, in which heat, light, other forms of energy, solvents, hydrogen ions or microorganisms can be present. In the degradation by hydrolysis of the ester bond in the polymer back bone, water molecules


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penetrate the polymer matrix, access the ester bond and cause nucleophilic attack. Hydrolysis can be accelerated with an acid, base or enzyme.18 Unlike conventional polyesters, some degradable polyesters have a structure into which water molecules penetrate well because of the large free volume of the renewable monomers, such as derivatives of isosorbide and mannitol, and the wide spacing between polymer chains.13,

19, 20

However, physically

designing the molecules to facilitate water access has limitations for accelerating polymer degradation. There are three main types of plastic. The first is petroleum-based plastic that is commonly used, the second is biodegradable plastic for management of plastic waste, and the third is bio-based plastic for sustainable society. In this study, the polyester was prepared from petrochemicals and camphor which is found in rosemary leaves, other plants in the mint family and a type of large evergreen tree found in Sumatra, Indonesia and Borneo unlike the food-based materials studied extensively in the past, such as corn, grain or oil. And the polyesters were designed to accelerate and control degradation. The structure of camphor consists of a bridged six-membered ring and three independent methyl branches. The CaG moiety has this camphor structure and two five-membered ketal rings connected to it to form a spiro compound. Water molecules can penetrate well into this structure due to its large free volume. Additionally, the acceleration of degradation was made possible by introducing ketal groups at both ends of the camphor. A ketal group has been used as protection for the ketone group in other reactions under basic conditions.21 Hydrolysis experiments have proved that in acidic conditions, ketal was more hydrolysable than ester, and in basic conditions, only ester hydrolyzed.


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EXPERIMENTAL SECTION Materials. DL-Camphorquinone (CQ, 98%), glycerol (99.5%), p-toluenesulfonic acid (PTSA, monohydrate, 98.5%), benzene (anhydrous, 99.8%), sodium bicarbonate (NaHCO3, 99.0%), ethylene glycol (≥ 99.8%), dimethyl terephthalate (99%), dibutyl tin oxide (DBTO, 98%) and chloroform-d (99.8 atom % D) were purchased from Sigma-Aldrich. All materials were used as received without further purification. Measurements. 2,2:3,3-Bis(4′-hydroxymethylethylenedioxy)-1,7,7-trimethylbicyclo[2.2.1]heptane

was

synthesized following the procedure reported in a previous paper.22 1H-NMR spectra were recorded on a Mercury Plus spectrometer at 25 oC operating at 299.9 MHz. Samples were dissolved in a mixture of deuterated trifluoroacetic acid and chloroform (1:9). The thermal stability of the polymers was determined by thermogravimetric analysis (TGA) with an SDT Q6000 apparatus from TA Instruments. The samples were heated from 50 °C to 600 °C at a heating rate of 10 °C/min under a nitrogen flow of 100 mL/min. Glass transition temperatures (Tg) were measured via differential scanning calorimetry (DSC) using a DSC Q100 from TA Instruments. The measurements were carried out at a heating and cooling rate of 10 °C/min from 60 °C to 200 °C. Thermal data acquisition was carried out using the Thermal Analysis software from TA Instruments. Gel permeation chromatography (GPC) was performed on a Water 1515 isocratic HPLC pump and a refractive index detector (Water 2414) at 30 oC. The samples were eluted with chloroform using Agilent PLgel 5µm Mixed-C Column 7.5 x 300 mm and PS standard was used to establish a calibration curve. Polymer synthesis. PET and PCaGT were synthesized by reacting dimethyl terephthalate with ethylene glycol and CaG, respectively. PExCaGyT copolyesters were synthesized from a mixture of ethylene


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glycol, dimethyl terephthalate and the CaG monomer at the given compositions and DBTO as a catalyst. The polymerizations were performed in a one-neck round-bottom flask equipped with a vacuum distillation outlet, a nitrogen inlet and a mechanical stirrer. Transesterification was carried out under nitrogen flow and then polycondensation progressed under a 0.03-0.06 mbar vacuum. After the polymerizations is terminated, the reactants were cooled to room temperature and the low pressure state was restored with nitrogen. The reactants were dissolved in a mixture of chloroform and trifluoroacetic acid (9:1) and then precipitated in methanol. Finally, the polymers were filtered, washed with methanol and dried in a vacuum oven. PET homopolyester. 20% molar excess of ethylene glycol was added to dimethyl terephthalate. Transesterification reactions were performed at 200 oC for 2 h and 240 oC for 0.5 h. Polycondensation reactions were performed at 260 oC under a 0.03–0.06 mbar vacuum for 2 h. 1H-NMR (299.9 MHz, CDCl3–TFA), δ (ppm): 8.13 (s, 4H, ArH), 4.79 (s, 4H, CH2). 13C-NMR (299.9 MHz, CDCl3– TFA), δ (ppm): 166 (C=O), 133.5 (ArC), 130 (ArCH), 63.5(CH2). PExCaGyT copolyesters. 5% molar excess of the diol mixture was added to dimethyl terephthalate. Transesterification reactions were performed at 160 oC for 1 h, 200 oC for 1 h and 240 oC for 0.5 h under a low nitrogen flow. Polycondensation reactions were performed at 240 oC for 3 h under a 0.03– 0.06 mbar vacuum. 1H NMR (299.9 MHz, CDCl3), δ (ppm): 8.08 (s, 4H, ArH), 4.69 (s, x·4H, CH2), 0.44 (s, y·2H, OCH2CH) 4.38 (s, y·2H, OCH2CH) 4.22-4.10 (m, y·2H, OCH), 3.953.48 (m, y·4H, OCH2) 1.89 (m, y·1H, CCH2CH2), 1.71 (m, y·1H, CCH), 1.68 (m, y·1H, CHCH2CH2), 1.55 (m, y·1H, CCH2CH2), 1.35 (m, y·1H, CHCH2CH2), 1.18 (broad s, y·3H, CCH3), 0.85 (broad s, y·3H, CCH3), 0.81 (broad s, y·3H, CCH3). 13C NMR (299.9 MHz, CDCl3), δ (ppm): 166 (C=O), 133.5 (ArCOO), 130 (CH), 114 (OCO), 74 (CHO), 65


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(CH2OC=O, CH2O), 63.5 (CH2), 52 (CCH2CHCH2), 44.6 (CCH3), 28.8 (CH2C), 20.9 (CH3, CH3, CH2CH), 9.8 (CH3) CGB synthesis 1,7,7-trimethyl-spiro[bicyclo[2.2.1]heptane-2,2'-[1,3]dioxolane]-4'-methanol

(CGB)

was

synthesized according to reference.23 Hydrolysis procedures Films with a thickness of about 200µm were prepared by casting from solution (100 g/L) in a mixture of chloroform and hexafluoroisopropanol (4:1). The films were dried under atmospheric pressure and room temperature for 72h and finally remained between 15 to 20 mg. The polymers were immersed in 10 mL of citric acid buffer (pH 2.0) at 37 oC. After the scheduled periods, the films were rinsed with distilled water, dried. The molecular weight of hydrolyzed polymers was analyzed by GPC chromatography and the average value repeated three times was used. 15 mg CGB was dissolved in deuterium buffer solution and CD3CN(volume ratio 7:3). After 18 h at 80 oC, the solution is analyzed by 1H NMR.


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RESULTS AND DISCUSSION Synthesis and chemical structure Scheme 1. Polymerization from CaG diol, ethylene glycol and dimethyl terephthalate (DMT)

with DBTO catalyst. PExCaGyT copolyesters were prepared by a melt polymerization process from the CaG diol, ethylene glycol and dimethyl terephthalate with a DBTO catalyst as depicted in Scheme 1. The melt polymerization process was used as an industrial method and was a two-step process. The first step was transesterification, which proceeded at a relatively low temperature and atmospheric pressure to reduce volatilization of the diol and to prevent crystallization of oligomers that were produced early in the process. The second step was polycondensation, which was performed at low pressure to efficiently remove by-products, methanol and excess diols. After purification by precipitating the polymer in chloroform or a mixture of chloroform and trifluoroacetic acid with methanol, the polyesters were obtained.


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The chemical structure and composition of the polyesters were analyzed by 1H-NMR, and their molecular weights were measured by GPC. The results are given in Table 1.

Figure 1. 1H NMR spectra of PExCaGyT copolyesters.


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The molar ratios of the resulting polyesters were calculated by 1H-NMR spectra (Figure 1). In the 1H-NMR analysis of the copolyesters, all signals were assigned to different protons. The peaks of the CaG moiety were assigned at 4.6-3.4 ppm and 2.1-0.8 ppm, and the peaks of the ethylene glycol moiety was assigned at 4.79 ppm. Integration of the proton signal indicated the content of each moiety. The CaG diol content in the copolyesters was slightly higher than their corresponding feedstock because the reactivity of CaG and ethylene glycol was similar while the ethylene glycol was more volatile. Similar results were obtained with the alditol-based polyester synthesis.13 Table 1. Molar composition and molecular weight of PExCaGyT copolyesters.

a

Molar composition determined by integration of the 1H-NMR spectra. bNumber- and weight-average molecular

weights in g mol-1 and dispersities measured by GPC in HFIP against PMMA standards.

Thermal properties.


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Table 2. Thermal properties of PExCaGyT copolyesters. a

Temperature at which 5% weight loss was observed.

b

Temperature for maximum degradation rate.

c

Remaining weight at 600 oC. d Glass-transition temperature measured by DSC trace at a second heating.

The thermal behavior of the PExCaGyT copolyesters was measured by TGA and DSC, and results are shown in Table 2. The TGA traces (Figure 2) show that PCaGT and the PExCaGyT copolyesters were considerably stable at high temperature. The weight loss of all polyesters started above 320 oC, and these copolyesters decomposed in one main step, leaving less than 10% residue after being heated. The consequence of such behavior is that the resistance to heat of these polyesters is enough to allow for their conventional processing.24 Figure 2. TGA traces of PExCaGyT copolyesters measured from 30 to 600 oC at 10/min under nitrogen atmosphere.


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The glass transition temperature values of the copolyesters were studied by DSC (Figure 3). The DSC curve revealed that the PCaGT homopolyester had a Tg that was 51 oC higher than the PET homopolyester, and the Tg steadily increased as the amount of the CaG moiety in the polyester increased. The increase in the Tg of these copolyesters could extend the available temperature range of polymers. Because the CaG diol had a structure that consisted of a rigid bridged six-membered ring, three methyl branches and two five-membered spiro-rings, it induced stiffness in polymer chain, leading to a high glass transition temperature. The sixmembered ring was tightly bound in the bridged form, which prevented ring-flipping, and the methyl branches interrupted the movement of the ring. This bulky and rigid structure of CaG enhanced the Tg by limiting the oscillation. PET had a distinct melting point which is one of the two major first-order phase transitions of crystalline polymers, but it gradually disappeared as the content of CaG increased. PExCaGyT with CaG below 20% and PET showed the heating traces with melting exotherms implying that they are semicrystalline. The polymer with a small amount of CaG had a lower melting temperature than that of PET, and the shape of the graph was also broad. It was reported that the presence of a bicyclic unit in


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the polymer chain produced a decrease of crystallinity.13 Because the cyclic structure has a large free volume as compared with linear hydrocarbon chains, CaG spread the spacing of the polymer chains. The chemical structure of CaG is disadvantageous for crystallization, but it has a relatively low segmental mobility and thermosstability.

Figure 3. DSC curves of PExCaGyT copolyesters at a second heating with a rate of 10 oC/min.

Degradability


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Figure 4. Molecular weight change of PE90CaG10T, PE70CaG30T and PE50CaG50T at pH 10.0

(a) and pH 2.0 (b). It has been reported that common polyesters and also the polyesters containing sugar-drived monomers are reported to be hydrolyzed at higer rated when incubated under acidic or basic pH than at neutral pH.8,

21, 25, 26

PExCaGyT copolyesters showed a significantly faster

degaradation rate in acidic buffer solution. The hydrolytic degradation of the copolyesters was observed by measuring the molecular weight with time. In the basic solution, the molecular weight of all the polymers was reduced by about 10%, but the decreasing rate was different in the acidic solution. The higher the proportion of the CaG diols in the copolyester, the faster the molecular weight was reduced. This is because the PExCaGyT copolyesters contained both ester and ketal groups in the main polymer chain, and the hydrolysis of ketal bonds is faster than that of ester bonds in acidic conditions.23 The substance eluted from the PE50CaG50T by hydrolysis and eluted with the acidic aqueous solution was detected by NMR,


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but no substance was detected. If the ester bond was degraded, signals of ethylene glycol dissolved in water would have appeared. Hydrolysis of the ketal bond produced a waterinsoluble camphorquinone, so no signal was seen in NMR spectra. Scheme 2. Synthesis of CGB.

Figure 5. NMR peaks of CGB corresponding to the camphor part before (a) and after hydrolysis in pH 2.0 and 80oC for 18h (b). NMR peaks of CGB corresponding to the benzene ring before (c) and after hydrolysis in pH 10.0 and 80oC for 18h (d).

To demonstrate that the hydrolysis of the ketal bond occurs more rapidly in acidic condition, CGB which had both ketal and ester groups and was the same structure as a part of PCaGT was synthesized(Scheme 2). In consideration of the stability of the ketal group, CGB was formed using benzoic anhydride under basic conditions. The hydrolysis of CGB was carried out in acidic and basic conditions. The results were observed with NMR and represented in


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Figure 5. In acidic conditions, CGB was hydrolyzed to camphor and 2,3-dihydroxypropyl benzoate. The NMR spectra of CGB (Figure 5a) before hydrolysis showed proton peaks of three CH3 groups in the camphor part between 0.76 and 1.0 ppm and no peaks between 2.3 and 2.4 ppm, corresponding to exo-hydrogen located at the 2C of camphor. After 18 h at 80 o

C (Figure 5b), the multiplet peaks of the three CH3 groups were simplified to three singlet

peaks and shifted to down field. The hydrogen peaks of 2C in camphor appeared between 2.3 and 2.4 ppm. Their shape and position were exactly the same as the hydrogen peaks of camphor. The signal of hydrogen on the benzene ring was unchanged. This provides evidence that the ester bonds were retained and the ketal bonds were completely broken. In contrast, CGB was hydrolyzed into CG and benzoic acid under basic conditions. The signals corresponding to the hydrogens on the camphor body did not change, but the signals split in two. Existing peaks that are relatively up-field corresponded to the hydrogens on the benzene ring of CGB, and the newly formed peaks were identical to the peaks of hydrogens on benzoic acid. At 80 oC for 18 h, the esters were 50% degraded while all of the ketals did not degrade. These results show that the ketal bond is stronger in basic conditions than the ester bond and the ester bond under basic state is decomposed more slowly than the ketal bond under acidic state. Conclusion In conclusion, degradable polyesters with high Tg and satisfactory molecular weight were obtained from the camphor-based CaG diol and dimethyl terephthalate. The incorporation of the complex ring structure from the camphor enhanced the glass transition temperature of the resulting polyesters. The difference between the glass transition temperatures for PET and PCaGT is 51 oC, and the Tg of the polyester can be controlled by adjusting the content of the CaG moiety because the increase in Tg is proportional to the increase in the content of the CaG moiety. When these polyesters were incubated under pH 2.0, hydrolytic degradation


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proceeded rapidly as the content of the CaG moiety in the polyesters increased. The ketal group was introduced into the polymer main chain, which led to the rapid degradation of the polymers under acidic conditions. In addition, since only the ester bond was hydrolyzed under basic conditions, the degradation degree of the polymer according to external conditions was adjustable by varying the polymer constituents. The polyesters in this paper have the potential for use as degradable polymers in many fields, such as agrochemistry, medicine or the packaging industry. AUTHOR INFORMATION Corresponding Author *Dong Hack Suh. * Hanyang University, Advanced Materials & Chemical Engineering Building 311, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea Author Contributions Jeong Eon Park conducted the synthesis and polymerization. Dong Hack Suh planned the synthesis and polymerization. Da Young Hwang, Gwang Ho Choi and Kyoung Hwan Choi supported the research and confirmed the results of the research.


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polyesters: Effect of pH and time. Polymer Testing 2016, 52, 192-199. 20. Rajput, B. S.; Chander, U.; Arole, K.; Stempfle, F.; Menon, S.; Mecking, S.; Chikkali, S. H., Synthesis of Renewable Copolyacetals with Tunable Degradation. Macromolecular Chemistry and Physics 2016, 217, (12), 1396-1410. 21. Sureshan, K. M.; Shashidhar, M. S.; Praveen, T.; Das, T., Regioselective Protection and Deprotection of Inositol Hydroxyl Groups. Chemical Reviews 2003, 103, (11), 44774504. 22. Choi, G.-H.; Hwang, D. Y.; Suh, D. H., High Thermal Stability of Bio-Based Polycarbonates Containing Cyclic Ketal Moieties. Macromolecules 2015, 48, (19), 68396845. 23. Liu, J.-H.; Wang, H.-Y., Synthesis and characterization of novel copolymers with acid-labile ketal moieties derived from camphor. Journal of Applied Polymer Science 2003, 90, (11), 2969-2978. 24. Lavilla, C.; de Ilarduya, A. M.; Alla, A.; García-Martín, M. G.; Galbis, J. A.; MuñozGuerra, S., Bio-Based Aromatic Polyesters from a Novel Bicyclic Diol Derived from dMannitol. Macromolecules 2012, 45, (20), 8257-8266. 25. Munoz-Guerra, S.; Lavilla, C.; Japu, C.; Martinez de Ilarduya, A., Renewable terephthalate polyesters from carbohydrate-based bicyclic monomers. Green Chemistry 2014, 16, (4), 1716-1739. 26. Lavilla, C.; Alla, A.; Martínez de Ilarduya, A.; Benito, E.; García-Martín, M. G.; Galbis, J. A.; Muñoz-Guerra, S., Carbohydrate-Based Polyesters Made from Bicyclic Acetalized Galactaric Acid. Biomacromolecules 2011, 12, (7), 2642-2652.


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Biomacromolecules

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Title Fast hydrolysis polyesters with a rigid cyclic diol from camphor Author list Jeong Eon Park, Da Young Hwang, Gwang Ho Choi, Kyoung Hwan Choi, and Dong Hack Suh


Fast Hydrolysis Polyesters with a Rigid Cyclic Diol from Camphor

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