Synthesis, characterization and thermal, hydrolytic and oxidative degradation study of biobased (BisFuranic-Pyridinic) copolyesters

Accepted Manuscript Synthesis, characterization and thermal, hydrolytic and oxidative degradation study of biobased (BisFuranic-Pyridinic) copolyesters A. Bougarech, M. Abid, S. Abid, E. Fleury PII:

S0141-3910(16)30266-X

DOI:

10.1016/j.polymdegradstab.2016.09.010

Reference:

PDST 8054

To appear in:

Polymer Degradation and Stability

Received Date: 4 July 2016 Revised Date:

27 August 2016

Accepted Date: 6 September 2016

Please cite this article as: Bougarech A, Abid M, Abid S, Fleury E, Synthesis, characterization and thermal, hydrolytic and oxidative degradation study of biobased (BisFuranic-Pyridinic) copolyesters, Polymer Degradation and Stability (2016), doi: 10.1016/j.polymdegradstab.2016.09.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synthesis, Characterization and Thermal, Hydrolytic and Oxidative degradation study of biobased (BisFuranic-Pyridinic) Copolyesters A. Bougarech a,b, M. Abid a*, S. Abid a, E. Fleury b*

b

Laboratoire de Chimie Appliquée, Faculté des Sciences de Sfax, Université de Sfax, Tunisie

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a

Université de Lyon, CNRS, UMR 5223, INSA-Lyon, IMP@INSA, F-69621, Villeurbanne, France

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ABSTRACT

Polymers from renewable resources are of high interest since it could be possible to valorize

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the biomass and in parallel to tune their macromolecular structures in relation with their properties. Here novel bisfuranic-pyridinic copolyesters, namely poly (ethylene furoate-coethylene pyridinate) (PEBF-co-PEPy) were prepared by applying a two-stage melt polycondensation method at high temperature, in a presence of titanium derivatives as

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catalyst. Molecular weights of resulting copolyesters are in the range 24000-45200 g.mol-1 and their chemical microstructures statistical as shown by NMR. These furano-pyridinic copolyesters are amorphous with Tg ranging from 62 to 67°C and their thermal and

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hydrolysis stabilities are influenced by presence of the pyridinic dyads Py-EG-Py. Finally the

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oxidative degradation in H2O2/CoCl2 cannot be effective because of the chelating properties of pyridinic moieties.

Keywords: Polycondensation, Bio-based Polymer, Furanic, Pyridinic, Hydrolytic Degradation

1

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1. Introduction In recent years much has been written about renewable resources and their use in polymer science [1,2]. The molecular framework of furan has been a source of inspiration to chemists as supported by the rich literature dealing with a large variety of structures and properties [3].

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Furan-based compounds have also been explored as monomeric units for synthesis of several polymers by various groups [4–6]. A number of studies have described the synthesis of various difuranic monomers from ethyl-2-furoate or furfurylamine through a simple coupling

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procedure. They were used, in conjunction with other monomers such as diols, diamines, dianhydrides and diisocyanates, to prepare several polyesters [7–11], polyamides [12–14],

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polyhydrazides [15,16], polyimides [17] and polyureas [18]. All these polymers exhibited a regular structure, high molecular weight and physical properties which made them attractive for applications as engineering polymers. More recently our team has described furan-based poly(esteramide)s [19] and copolyesters with terephthalic [20] and/or sodium sulfoisophthalic

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[21,22] moieties allowing to cover a wide range of physicochemical properties and proving the interest of adding new functionality. To continue our investigation in such direction, we herein explore the interest of the pyridinic functions which can impart chelating properties

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due to their ability to form extremely stable complexes with metal ions [23]. Moreover the

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presence of such functions will open the scope of applications toward biology. Indeed the pyridine dicarboxylate was reported to be a competitive inhibitor of bovine liver glutamate dehydrogenase [24,25]. Surprisingly, only few examples of polymers having pyridinic moieties inside the backbone or in a side position were already published. One concerned the development of copoly(amide-ester)s as a promising alternative for optoelectronic applications [26] and the others concerned the modification of chitosan which thus has been applied to a wide range of potential applications: metal recovery according to the enhancement of the chelating properties due to the pyridinic functions [27,28], antimicrobial

2

ACCEPTED MANUSCRIPT activity [29,30], gene delivery [31], sensor application [32] and biomedical application [33,34]. In this work we present a straight forward methodology to precisely design a new family of furanic copolyesters enclosing pyridinic moieties and obtained by the melt polycondensation 5,5’-isopropylidene-bis(ethyl

2-furoate)

(DEBF),

Dimethyl

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between

2,6-

pyridinedicarboxylate (DMPy) and Ethanediol (ED). These copolyesters have potential interesting attributes: their bio-sourced origin, their relatively high thermal stability due to the

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semi-aromatic character of the furoate units, their sensibility toward hydrolysis and compatibility with biological media. Therefore, we deeply studied their thermal and

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hydrolytic properties as well as their in vitro behavior in oxidative environment which indirectly allows estimating their stability within biological media [35].

2. Experimental

Dimethyl

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2.1. Materials 2,6-pyridinedicarboxylate

(DMPy),

2-ethyl

furoate,

1,2-ethanediol

(EG),

tetrabutoxytitanium (Ti(OBu)4) and zinc acetate (Zn(OAc)2) were all purchased from Sigma-

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Aldrich and were used without further purification.

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2.2. Synthesis of 5,5’-isopropylidene-bis(ethyl 2-furoate) (DEBF) DEBF was prepared by the condensation of ethyl furoate with acetone following the procedure

described previously [7]. Its structure and purity were confirmed by spectroscopic techniques. 2.3. Synthesis of copolyesters The poly(ethylene furoate-co-ethylene pyridinate) PEBF-co-PEPy was produced by the melt copolymerization according to three Processes: 3

ACCEPTED MANUSCRIPT Process (a): (DEBF, DMPy), (0.8 /0.2: mol/mol) were reacted with (ED, 4 mol) in the presence of zinc acetate Zn(OAc)2 (10-3 of total mass) for 3 h in a nitrogen atmosphere at 200 °C. The resulting reaction was carried out at high temperature 240°C in the presence of 0.1

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wt-% Ti(OBu)4 under 0, 01 mmHg vacuum to remove the diol as efficiently as possible. Process (b): Transesterification reactions were carried out at 200°C under a nitrogen flow in the presence of 0.1 wt-% Ti(OBu)4 , the polycondensation reactions were performed at 240°C

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under 0, 01 mmHg vacuum.

Process (c): The same procedure as in the process (b), with the corresponding changes in the

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concentration of the catalyst of the transesterification steps 0.2 wt % Ti(OBu)4. The resulting copolymer (Process (a), (b) and (c)) was used for characterization without further purification. 2.4. Films preparation

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Films referenced as FilmPEBF-co-PEPy4, were prepared by casting a sample of PEBF-coPEPy4 copolyester at room temperature from a 10% (W/V) solution in CHCl3/MeOH (8/1 V/V)

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on silanized Petri dish. The films were cut and dried in vacuum at 50°C to constant weight. The thickness of the obtained films was 200 +/- 10 µm.

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2.5 Degradation Procedures

The hydrolytic degradation trials were performed with FilmPEBF-co-PEPy4. Samples were immersed in purified water under acidic aqueous conditions (pH = 4.35), neutral conditions (pH = 7.4) and basic conditions (pH = 9.5 and 11.5) at 37°C. After immersion for fixed periods of time, each sample was rinsed thoroughly in water and dried to constant weight. Sample weighting, was used to follow the evolution of the hydrolytic degradation.

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ACCEPTED MANUSCRIPT The oxidative degradation was also performed with FilmPEBF-co-PEPy4 in solution composed of 30 vol. % H2O2 and in solution composed of 20 vol. % H2O2 in combination with 0.1 M CoCl2 for 15 days at 37 °C [36]. Sample weighting and NMR spectroscopy were

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used to follow the rate of the oxidative degradation. 2.6. Analytical methods 1

H spectra were recorded on a BruckerAvance III 400 spectrometer at 90°C. Samples were

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dissolved in DMSO. Chemical shifts were referenced to the peak of TMS at 0.0 ppm. The

Pyridinic units (‫ܮ‬ത௡,௉௬ ) are given by:

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degree of randomness (B) and the number-average sequence length of BisFuranic (‫ܮ‬ത௡,஻ி ) and

B = PPyBF + PBFPy =

L n , BF =

PBFPy 1

=

=

PPyBF

2 FPy

2 FBF

FBF − ED − Py 2 FPy

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L n , Py =

1

FBF − ED − Py

FBF − ED − Py

+

FBF − ED − Py 2 FBF

(1)

(2)

(3)

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(ܲ௉௬஻ி ) The probability of finding a BisFuranic unit next to an Pyridinic unit (ܲ஻ி௉௬ ) The probability of finding a Pyridinic unit next to an BisFuranic unit

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Size exclusion chromatography (SEC) analysis was performed with Viscotek machine using a system of three columns in series (Styragel HR 4E DMF) in dimethyl formamide (DMF) mixture to (LiBr 0.05 M) at 70°C. Inherent viscosities were determined at 25°C from DMSO solutions with a polymer concentration of 0.15 g dL−1 using an Ubbelhode AVS-400 microviscosimeter. The differential scanning calorimetry experiments (DSC) were performed with TA-

instruments Q 23 to study the thermal properties and crystallization behavior. The samples were cut and placed in closed capsules. The samples were tested under dry nitrogen flow, a 5

ACCEPTED MANUSCRIPT heating rate at 10°C/min, covering the range of 30-230°C, a cooling rate at 10°C/min, covering the range of 230-30°C. Glass transition temperatures (Tg) were measured on the second heating curves. It was taken at the inflection point. Thermogravimetric analysis (TGA) was performed with a (TA-instruments Q500). Data were

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collected on samples of 10-20 mg during a ramp of temperature from 30 to 550°C at

10°C/min under a dry nitrogen atmosphere.

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3. Results and discussion

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3.1. Synthesis of poly (ethylene bisfuroate-co-ethylene pyridinate) PEBF-co-PEPy The synthesis of poly(ethylene bisfuroate-co-ethylene pyridinate) PEBF-co-PEPy was performed according to a process inspired from that previously described by El Gharbi et al. [7] and comprising a two-step melt polycondensation reactions (scheme1) between 5,5’-

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isopropylidene-bis(ethyl 2-furoate) (DEBF), dimethyl 2,6-pyridinedicarboxylate (DMPy) and 1,2-ethanediol (ED). The first step was carried out for 3h under a nitrogen atmosphere in the presence of zinc acetate as transesterification catalyst. The temperature was gradually raised

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from 180 to 200°C during this step in order to avoid sublimation of reagent and side reactions [21]. The second step was completed at higher temperature (240°C) under vacuum in order to

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release the excess of diol and to shift the reaction equilibrium toward the formation of high molecular weight. This latter step, i.e. the polycondensation was catalysed by the addition of Ti(OBu)4 [21].

6

ACCEPTED MANUSCRIPT CH3 H5C2 O C O

O

C

C O C2H5 + O

O

CH3

H3CO

DEBF

C O

N

C O

OCH3

+

4 HO (CH2)2

OH

EG

DMPy

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1st step: 180 to 200°C, 3h, N2 Zn(OAc)2 2nd step: 240°C, 6h, 0.1 mmHg Ti(OBu)4

O

C O O

(CH2)2 O n

C O

N

C O (CH2)2 O O

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C O

CH3 C O CH3

EG

m

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Scheme 1. Synthesis of Poly(ethylene furoate-co-ethylene pyridinate) or PEBF-co-PEPy [bisF/Py ; 80/20] (process (a))

The result of this first trial (Process a) was investigated by 1H NMR. Thus, the NMR spectrum given (Fig.1a) shows the presence of peaks at 1.3 ppm related to a high quantity of

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residual -O-CH2-CH3 end groups (~ 80 molar %) attesting that the yield of the transesterification reaction remained low. This was ascribed to a deactivation of the catalyst

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due to the acid-base interactions that takes place between the amine function of the pyridine

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and the metal of the zinc salt.

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Fig. 1.1H NMR spectrum of PEBF-co-PEPy [ F/Py ; 80/20] (Process (a),(b) and (c)) [400

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MHz, DMSO-d6, reference: d (TMS) δ = 0 ppm].

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To counteract this drawback a more robust catalyst, tetrabutoxytitanium (Ti(OBu)4), was tested. Indeed the mechanism of transesterification with titanium derivatives catalyst involves

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the fast ligands exchange between 4-butanol and 1,2-ethanediol and the coordination of the ester carbonyl to the metal atom which favors the nucleophilic attack of alkoxy ligand [37]. Therefore acid-base interactions with the pyridine nitrogen atom should be improbable and consequently the Ti(OBu)4 should remain active. A PEBF-co-PEPy copolyester synthesis (Process b) was thus performed in the presence of 0.1 wt-% Ti(OBu)4 introduced at the beginning of the transesterification step, the rest of the process being the same as described above. The hypothesis was confirmed since the residual signal corresponding to the protons of ethyl ester end groups observed at 1.3 ppm on the 1H-NMR spectrum Fig. 1b is fainter (~ 9 8

ACCEPTED MANUSCRIPT molar %) than the one of the copolyester achieved from process a (see Fig.1a) thus indicating a better yield of polymerization. A higher concentration of catalyst: 0.2 wt-% Ti(OBu)4 (Process c), was tested with success since in that case no residual peak was visible on the 1HNMR spectrum (see Fig. 1c). A series of copolyesters was further prepared by using the

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process (c) and by changing the molar ratio DEBF/DMPy: 80:20 (PEBF-co-PEPy1), 70:30 (PEBF-co-PEPy2), 60:40 (PEBF-co-PEPy3), 50:50 (PEBF-co-PEPy4) and 40:60 (PEBF-coPEPy5).

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3.2. Characterization

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Structural features of these copolyesters were investigated by 1H NMR. The NMR spectrum of PEBF-co-PEPy1 copolyester given in Fig. 2 shows the presence of the expected signals of the protons of the furoate moieties at 6.33, 7.12 and 1.61 ppm and those of the pyridinic units at around 8.16 ppm. Between 4.40 to 4.80 ppm a range of peaks matches to the protons of

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oxyethylene units which are the counterpart of diacid units in the copolyester backbone. As there are two different diacid units, the signal is complex and corresponds to the three possible different dyad sequences BF-EG-BF, BF-EG-Py and Py-EG-Py attesting that

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copolymerization reaction (see scheme 1) has really taken place. Another set of signals from 3.50 to 4.40 was assigned to the presence of dioxyethylene units (DEG) tethered either to a

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furoate or to a pyridinic unit. These units were already observed with sulfonated furanic copolyesters and were ascribed to etherification side reactions between ethylene glycol and/or hydroxyester end groups [21]. Finally a rather small peak at 3.60 ppm (Fig. 2) is related to the protons of the trioxyethylene units (TEG): OCOCH2CH2OCH2CH2OCH2CH2OCO. [22]

9

ACCEPTED MANUSCRIPT BisFuranic units

(a)

C O

C

O

O

CH3 CH3

C O

6 C O CH2 CH2 O C O O BF-ED-BF

C

O

9 10 C O CH2 CH2 O O

O

CH3

CH3 O

C

O

CH3

CH3 CH2 CH2 O C O

BF-DEG-BF

Pyridinic units 1'

C O

N

5 C O CH2 CH2 O C O O

Py-ED-Py

C O

8 7 C O CH2 CH2 O O

N

N

C O

CH2 CH2 O C O

C

O

CH3

N

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Py-DEG-Py

O

C O

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1 1'

C O

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2 CH3 3 4

C O

BisFuranic – Pyridinic units CH3 O

CH3

11 12 C O CH2 CH2 O C O O BF-ED-Py

N

C O

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(b)

O

C

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C O

Fig. 2.1H NMR characterization of PEBF-co-PEPy1 [F/Py; 80/20] (Process (c)), (a) Atom numbering in pyridinate-furoate copolyesters, (b) 1H NMR spectrum [400 MHz, DMSO-d6, reference: d (TMS) δ = 0 ppm]. 10

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The integration of the 1H NMR peaks allows quantifying the molar ratio: furoate (FF) /pyridinic (FPy) units, for the five copolymers PEBF-co-PEPy1-5 (SI 1). The results gathered Table 1 show clearly that the values are close to the theoretical ones. The degree of

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randomness (B) was also calculated and was found ranging from 0.89 to 1 proving that the copolymers microstructure is statistic [38]. The DEG content is in the same range (5-6 molar %) whatever the polyesters: copolyesters (PEBF-co-PEPy1-5) and PEBF. This level is

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relatively low compared to the one calculated in the case of sulfonated copolyesters for which the DEG content increased with the increase of the sulfonated functions. Thus, pyridinic

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moieties have probably a limited role in the chemistry of the side product formation, the etherification reactions being more favored by acidic media [21].

Finally, the molar masses of copolyesters were found to be ranged from 13300 to 26600 g.mol-1 and from 24000 to 45200 g.mol-1 for M n and M w respectively. These values

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calculated from a calibration with polystyrene standards are relatively high in comparison with polymers obtained by polycondensation reaction. This confirms that the two reactions: transesterification and polycondensation, occurred with high yields even if it should be

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underlined that the increasing of pyridinic moieties lead to the decreasing of the molecular

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weight of the resulting copolyesters. The basic character of pyridinic monomers may probably reduce the catalytic activity of Ti(OBu)4. The dispersity (Ɖ) was close to 2, between 1.67 to 1.86 (Table 1), as expected according to the Flory law polycondensation reactions.

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ACCEPTED MANUSCRIPT Table 1 Characteristics of the PEBF and PEBF-co-PEPy1-5 copolyesters. [BF]/[Py]NMR1

%DEG2

%TEG2

ηinh3(dLg−1)

80/20

85.0 : 15.0

6.5

3.2

0.72

PEBF-co-PEPy2

70/30

76.0 : 24.0

5.3

2.4

0.65

PEBF-co-PEPy3

60/40

62.0 : 38.0

4.6

2.5

0.61

PEBF-co-PEPy4

50/50

46.0 : 54.0

5.7

5.1

0.45

PEBF-co-PEPy5

40/60

46.5 : 53.5

5.0

3.4

PEBF

100 :00

100 : 00

7.5

-

4

26600

(g /mol)

Ɖ4

B5

1.70

1

4

45200

25000

46800

1.86

0.99

22500

37600

1.67

0.97

17800

30900

1.73

0.89

0.38

13300

24000

1.80

0.98

0.34

10800

15800

1.46

-

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1

(g/mol)

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PEBF-co-PEPy1

Mw

Mn

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[BF]/[Py]1

Sample

BisFuroate to 2,6-pyridinedicarboxylate molar ratio in the initial reaction mixture and in the copolymer determined by 1H

NMR. Composition in mol% determined by 1H NMR.

3

Inherent viscosities measured in DMSO at a concentration of 0.15g/dL at 25 °C.

4

Number and weight average molecular weights determined by SEC in DMF mixture to LiBr using polystyrene standards

(eq Polystyrene). 5

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2

Degree of randomness determined by 1H NMR.

3.3 Thermal properties

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DSC thermograms of PEBF-co-PEPy1-5 copolyesters do not show melting/crystallization

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events but reveal glass transition signal between 62 and 67 °C (Fig. 3). This evidences the amorphous character of copolyesters and suggests that the crystallization is hampered by the presence of pyridinic units and probably by the ether moieties since the polyester PEBF is also amorphous. By comparing PEBF-co-PEPy1-5 copolyesters and PEBF one can also assume that the pyridinic units has a significant effect on the increase of Tg level arising from the stiffening of the polymer backbone which is already known for aromatic moieties such as isophthalic or terephthalic. However the increase of the Tg of the present copolyesters does not vary a lot with the increase of the pyridinic moieties concentration as reported table 2.

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ACCEPTED MANUSCRIPT This is probably due to the change of molecular weight of copolyesters which decreases with the increase of pyridinic concentration (v.s) and therefore gives rise to an underestimate of the effect of co-monomer units. Nevertheless the two PEBF-co-PEPy4-5 copolyesters with the highest level of pyridinic unit have clearly the highest Tg despite their lowest molecular

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weights.

Table 2 Thermal properties of PEBF and PEBF-CO-PEPY1-5 copolyesters. Td(5%)(°C)4

% Degradationbetween 300-350(°C) 5

324

13

%DEG2

Tg(°C)3

PEBF-co-PEPy1

85.0 : 15.0

6.5

63

PEBF-co-PEPy2

76.0 : 24.0

5.3

64

324

21

PEBF-co-PEPy3

62.0 : 38.0

4.0

62

314

30

PEBF-co-PEPy4

46.0 : 54.0

5.7

67

308

39

PEBF-co-PEPy5

46.5 : 53.5

5.0

66

310

48

PEBF

100 : 00

57

321

-

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1

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[BF]/[Py]NMR1

Sample

7.5

Furoate to 2,6-pyridinedicarboxylate molar ratio in the copolymer determined by 1H NMR. Composition in diethylene glycol units in mol% determined by 1H NMR.

3

Glass-transition measured by DSC

4

Temperature degradation at 5% weight loss measured by TGA

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2

5

Determined from TGA curves

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Fig. 3.DSC curves of PEBF-co-PEPy1-5 copolyesters.

The influence of pyridinic moieties in thermal stability of PEBF-co-PEPy1-5 copolyesters was studied by TGA under nitrogen. From the TGA curves, it appears that thermal degradation

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starts at temperature higher than 300°C and takes place according a two-step mechanism, whatever the copolyesters (Fig. 4). The first ranging between 300 and 350°C gives a

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degradation level proportional to the concentration of pyridinic units and thus may be ascribed to the decomposition of copolyester sequences with pyridinic units. The second at 380°C may

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arise from the decomposition of furanic units since the degradation temperature is comparable to the one of pure polyethylene furoate (PEBF). To support these assumptions the 1H NMR spectrum of initial PEBF-co-PEPy3 [BF/Py: 60/40] was compared with the one drawn after the heating under nitrogen of the sample at temperature between 25 to 320°C. Figure 5 shows the decrease of the signal of the pyridinic protons at 8 ppm and the signals, between 4.55-4.7 ppm, assigned to the ethylene units of the pyridinic dyads as well as Py-EG-Py and BF-EGPy. In fact, the calculation gives a decrease of pyridinic units of 8 % molar (from 38% in the initial sample to 30.8 % in the degraded sample) due to the initial degradation of the pyridinic 14

ACCEPTED MANUSCRIPT moieties and a decrease of number-average block length of pyridinic units (‫ܮ‬ത௡, ) from 1.66

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to 1.42 (Table 3).

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Fig. 4. TGA curves of PEBF and PEBF-co-PEPy1-5 copolyesters.

Fig. 5. 1H NMR spectrum of PEBF-co-PEPy3, (a) initial sample; (b) degraded sample at 320°C [400 MHz, DMSO-d6]. 15

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Table 3 Chain microstructure of initial sample PEBF-co-PEPy3 and degraded sample PEBF-co-PEPy3 at 320°C copolyesters: degree of randomness B and number-average sequence length of Furanic (Lത୬,୊ ) and Pyridinic units (Lത୬,୔୷ ).

PEBF-co-PEPy3

ത ࢔,ࡼ࢟ ࡸ

FPy-EG-Py

FPy-EG-F

FF-EG-F

62.0 : 38.0

0.14

0.46

0.39

2.70

1.66

0.97

69.2 : 30.8

0.11

0.40

0.48

3.43

1.42

0.99

(Initial sample)

PEBF-co-PEPy3

ത ࢔,ࡲ ࡸ

[F]/[Py]RMN

B

SC

(Degraded sample at 320°C)

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Copolyesters

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3.4 Degradation in aqueous solution

It is obvious that copolyesters are susceptible to chemical hydrolysis under acidic or basic media. In order to evaluate the hydrolytic degradation ability of the furano-pyridinic copolyesters, films of copolyesters PEBF-co-PEPy4 (BF/Py: 50/50), chosen to clearly

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evidenced the role of the pyridinic moieties, were incubated in water at pH respectively equal to 4.35, 7.4, 9.5 and 11.5 over a period of four weeks. The temperature was fixed at 37°C in order to mimic the one of human body. As performed in our previous study [21, 22], the

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remaining weight of FilmPEBF-co-PEPy4 showed an overall decreasing trend over the entire period of degradation at all pHs. Acidic and alkaline hydrolysis degradation of FilmPEBF-

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co-PEPy4 led to a higher mass loss in both cases than the neutral conditions (Fig. 6). A similar decreasing trend for residual mass was observed for degradation at pH 9.5 and 11.5, with the residual mass at about 40 and 37% respectively at the end of week 4. At pH 4.35, this hydrolytic degradation was slightly lower in relation to the basic conditions. A possible reason for this is that, under acidic aqueous conditions, the partial neutralization of the acid by the pyridinic functions may protect the ester linkage toward the hydrolytic degradation. In contrast degradation at higher pH is expected to be accelerated because the presence of higher concentration of hydroxyl ion (OH-) which can catalyze the cleavage of the ester bonds in the 16

ACCEPTED MANUSCRIPT PEBF-co-PEPy4 chains. Same degradation’s trials with FilmPEBF clearly shown the highest stability of this polyester whatever the pH (see SI 2 and SI 3) confirming the role of the pyridinic units on the degradation susceptibility of PEBF-co-PEPy copolyesters, probably

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molecules accessibility towards the ester functions.

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because the pyridinic moieties are more hydrophilic and thus allow to enhance the water

Fig. 6. Remaining weight (%) of PEBF-co-PEPy4 copolyester films after degradation at (pH

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4.35, 7.4, 9.5 and 11.5) at 37°C.

PEBF-co-PEPy4 and PEBF films were also submitted to the oxidative degradation within

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two different ways: i/ in aqueous solution containing 30 vol. % H2O2 and ii/ in aqueous solution composed of 20 vol. % H2O2 and 0.1 M CoCl2, These media were supposed to reproduce the in vivo degradation due to reactive oxygen species released by adherent macrophages [36].

Practically, whereas PEBF films are highly stable, the remaining weight of collected PEBFco-PEPy4 films after 15 days at 37 °C highlighted a difference between the two oxidative media (Table 4). Thus in H2O2/CoCl2 aqueous solution the remaining weight reached 75% whereas the one in H2O2 aqueous solution was fallen to 23.8%. If we suppose that the 17

ACCEPTED MANUSCRIPT hydrogen peroxide/cobalt (II) ion system should have reacted according the Haber-Weiss reaction to produce reactive hydroxyl radicals a series of oxidative reaction leading to chain’s scission and/or crosslinking should have occured [36]. In reality, solubility tests revealed that all samples remained soluble proving that none radical coupling occurred whatever the

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oxidative medium. In addition, the monitoring of the degradation by means of the 1H NMR showed that no double bond aroused during the degradation and that the only variation between the different NMR traces was the decrease of the signal at 4.7 ppm, assigned to the

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ethylene units of the pyridinic dyads Py-EG-Py (Fig. 7) previously described. The curve in Fig. 8 confirms this trend since the value of molar fraction of pyridinic dyads (FPy-EG-Py ) was

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half reduced after 48h of oxidative degradation, ranging from 21 to 10.2 %. The pH measurement of the aqueous solution: 3.8 for H2O2 and pH 6.2 for H2O2/CoCl2 finally allowed to confirms that the degradation level is similar than the one observed with hydrolysis at various pH (Table 4). All these data suggest that the degradation in H2O2/CoCl2 or in H2O2

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media can be interpreted as being the result of a hydrolysis reaction rather than an oxidative process as expected. The possible interpretation of this result is that the pyridinic moieties

radicals.

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may chelate the Co atom which hampering the possibility to have the formation of hydroxyl

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Table 4 Remaining weight (%) of PEBF-co-PEPy4 after 15 days of degradation Sample

Degradation conditions

Hydrolysis

pH (4.35) PEBF-co-PEPy4

60.68

Oxidation H 2O 2 pH (7.4)

86.93

pH (9.5)

59.92

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(30 Vol %)

H2O2 (20 Vol %) CoCl2 (0.1M)

23.80

75.00

pH (11.5)

58.58

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Fig. 7. 1H NMR characterization of intial sample FilmPEBF-co-PEPy4 (a) [F/Py; 50/50] and degraded sample FilmPEBF-co-PEPy4 in oxidative conditions (H2O2 (30 Vol %)) (b.c.d.e.f and g) respectively after (1.3.6.12.24 and 48) hours [400 MHz, DMSO-d6].

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time of oxidative degradation

4. Conclusion

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Fig. 8. Evolution of the molar fraction of pyridinic dyads Py-EG-Py of PEBF-co-PEPy4 with

We investigated the feasibility and properties of novel bio-based bis-furanic-pyridinic

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copolyesters (PEBF-co-PEPy1-5) performed thanks to a melt polycondensation reaction between ethanediol and a mixture of bifuranic and pyridinic diesters. It was established that the choice of tetrabutoxytitanium (Ti(OBu)4) as catalyst is helpful to efficiently control the

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transesterification reactions and thus obtaining high molecular weights. The microstructure of

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the resulting copolyesters was studied by 1H NMR and their statistical character was evidenced. This study also clearly underlines the role of the pyridinic moieties onto the thermal and degradations properties. Indeed the presence of these aromatic moieties favors the amorphous character of copolyesters and the high value of their Tg. In parallel the pyridinic dyads F-EG-Py and Py-EG-Py are sensitive to the thermal degradation and also to the hydrolysis at acid or basic pH. Finally, the inefficiency of the in vitro degradation in oxidative condition in H2O2/CoCl2 solutions may be attributed to the chelation of the cobalt. Studies on

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ACCEPTED MANUSCRIPT the mechanical properties of these furano-pyridinic copolyesters are now in progress in relation to their potential applications including biological applications and metal recovery.

ACKNOWLEDGMENTS

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The authors acknowledge the financial support of the Ministry of Higher Education and Scientific Research in Tunisia.

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