Effectiveness of Esterification Catalysts in the Synthesis of Poly

catalysts

Article Effectiveness of Esteriﬁcation Catalysts in the Synthesis of Poly(Ethylene Vanillate)

Eleftheria Xanthopoulou 1,2 , Alexandra Zamboulis 2 , Zoi Terzopoulou 1,2 , Margaritis Kostoglou 3, Dimitrios N. Bikiaris 2,* and George Z. Papageorgiou 1,4,*

1 Department of Chemistry, University of Ioannina, P.O. Box 1186, 45110 Ioannina, Greece; [email protected] (E.X.); [email protected] (Z.T.) 2 Laboratory of Polymer Chemistry and Technology, Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; [email protected] 3 Laboratory of Chemical and Environmental Technology, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece; [email protected] 4 Institute of Materials Science and Computing, University Research Center of Ioannina (URCI), 45110 Ioannina, Greece * Correspondence: [email protected] (D.N.B.); [email protected] (G.Z.P.)

Abstract: Over the last few decades, bio-based polymers have attracted considerable attention from both academic and industrial ﬁelds regarding the minimization of the environmental impact arising from the excessive use of petrochemically-based polymeric materials. In this context, poly(ethylene vanillate) (PEV), an alipharomatic polyester prepared from 4-(2-hydroxyethoxy)-3-methoxybenzoic acid, a monomer originating from lignin-derived vanillic acid, has shown promising thermal and mechanical properties. Herein, the effects of three different catalysts, namely titanium butoxide (TBT), titanium isopropoxide (TIS), and antimony trioxide (Sb2O3), on the synthesis of PEV via a two-stage melt polycondensation method are investigated. The progress of the reaction is assessed using Citation: Xanthopoulou, E.; Zamboulis, A.; Terzopoulou, Z.; various complementary techniques, such as intrinsic viscosity measurement (IV), end group analysis Kostoglou, M.; Bikiaris, D.N.; (AV), nuclear magnetic resonance spectroscopy (NMR), Fourier-transformed infrared spectroscopy Papageorgiou, G.Z. Effectiveness of (FTIR), and differential scanning calorimetry (DSC). The thermal stability of the produced polyesters Esteriﬁcation Catalysts in the is studied by evolved gas analysis mass spectrometry (EGA-MS). Moreover, as the discoloration in Synthesis of Poly(Ethylene Vanillate). polymers affects their applications, color measurement is performed here. Finally, theoretical kinetic Catalysts 2021, 11, 822. https:// studies are carried out to rationalize the experimental observations. doi.org/10.3390/catal11070822 Keywords: poly(ethylene vanillate); synthesis; bio-based polyesters; vanillic acid; catalysts; ther- Academic Editor: Eric M. Gaigneaux mal properties

Received: 8 June 2021 Accepted: 3 July 2021 Published: 6 July 2021 1. Introduction

Publisher’s Note: MDPI stays neutral Contemporary industrial polymerization technologies make the production of ver- with regard to jurisdictional claims in satile polymeric materials with highly tunable properties possible and subsequently a published maps and institutional affil- plethora of applications in areas including the packaging, automotive, and engineering iations. sectors. In Europe, more than 61 million tons of polymers are produced annually [1,2], with the great majority of them being petrochemically-sourced; however, the interest in bio-based materials has flourished over the last few decades, both in academic and industrial fields due to increasing concerns about the depletion of fossil resources and the footprint of synthetic polymers on the environment, as it is estimated that roughly 80% Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. of synthetic polymers are discarded in landfills or natural recipients [3–5]. In fact, the This article is an open access article European Commission supports the synthesis of high added value products based on distributed under the terms and renewable sources by offering financial support for research, innovation, and industrial conditions of the Creative Commons upscaling in this field [6]. Attribution (CC BY) license (https:// The conversion of biomass to bio-based monomers, and subsequently polymers creativecommons.org/licenses/by/ by means of biochemical and/or chemical transformations, constitutes the most impor- 4.0/). tant route toward the preparation of bio-based plastics like poly(lactic acid) (which, at

Catalysts 2021, 11, 822. https://doi.org/10.3390/catal11070822 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 822 2 of 17

the moment, is the most important bio-based polymer with many applications [7]) and other polymers, owing to the abundance and relatively low cost [8]. Through this path, bio-based forms of conventional plastics such as bio-poly(ethylene terephthalate) and bio-poly(ethylene) [9] may be produced, as well as novel building blocks based on new aromatic precursors, thus reinforcing the transition from a linear to a circular economy [10]. One of the most promising monomers derived from lignin biomass is vanillic acid (VA), which constitutes an oxidation product of vanillin, which is the most commonly produced aroma chemical substance [11,12]. Owing to its structural similarity with terephthalic acid (TPA), VA-based materials are expected to demonstrate comparable thermal and mechanical properties to poly(alkylene terephthalates), which are a series of high performance petrochemically-based thermoplastic polyesters. Furthermore, the continuous progress concerning lignin depolymerization and purification [13–15], combined with the biotechnological production of vanillin [16–19], is defining more sustainable routes towards the preparation of vanillin, thus promoting vanillic acid as an excellent building block for the preparation of fully bio-based polymers. Poly(ethylene vanillate) (PEV) is a new bio-based alipharomatic polyester produced from vanillic acid that is gaining attention as a potential substitute of the popular poly(ethylene terephthalate) (PET), which is the most common polymeric material in packaging industry. The aforementioned polyesters possess similar alipharomatic structures and subsequently exhibit comparable thermal transitions and nanomechanical properties. Despite its great potential as a packaging material, the synthesis of high molecular weight PEV with high purity and an absence of coloration is still requires investigation. Catalysts play a key role in the synthesis of polyesters as they affect the reaction and color of the resulting materials [20–23]. More specifically, organometallic compounds of tin, lead, and titanium, such as titanium butoxide (TBT), titanium isopropoxide (TIS), and dibutyltin oxide (DBTO), are known to catalyze the esterification and polycondensation steps. Metal-oxide catalysts (e.g., TiO2 and GeO2) are generally preferred for the esterification step, while metal acetates or carbonates (e.g., Zn(CH3COO)3 and Sb(CH3COO)3) have been extensively used during polycondensation processes [24–26]. Moreover, it has been proved that the valence of the metal atom and the concentration of the used catalyst alters its activity during the reaction [27]. Finally, toxicity constitutes a crucial parameter upon the selection of the proper polymerization catalyst, especially for the preparation of materials used in the food packaging industry [28–31]. Several attempts to produce PEV using different catalysts have been reported. Mialon et al. polymerized 4-(2-hydroxyethoxy)-3-methoxybenzoic acid using an antimony trioxide −1 (Sb2O3) catalyst, yielding a polyester with an average molecular weight of 5390 g mol and a melting temperature of 239 ◦C[32]. Furthermore, Gioia et al. reported the preparation −1 of PEV from vanillic acid with ethylene carbonate catalyzed by DBTO (Mn = 4700 g mol , ◦ Tm = 264 C)[33]. More recently, our research group has investigated the thermal transitions and degradation mechanisms of the abovementioned polyester in the presence of two different catalysts, namely TBT and Sb2O3, indicating that PEV is a promising alternative to its terephthalate homologues [34]; however, there is no comparative analysis concerning the effect of the catalyst on the molecular weight and the thermal properties of the final polyester. Herein, a kinetic study of PEV synthesis using three different catalysts is reported for the first time. Titanate catalysts (TBT and TIS) have been selected due to their high efficiency in polyesterification reactions [35], while Sb2O3 is the most common catalyst for PET production in an industrial context (Figure1). The evaluation of the reaction progress is monitored using various techniques, including intrinsic viscosity measurement (IV), end group analysis (AV), nuclear magnetic resonance spectroscopy (NMR), Fourier- transformed infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The thermal stability of the produced polyesters is investigated with the implementation of evolved gas analysis mass spectrometry (EGA-MS). As coloration in polymeric materials CatalystsCatalysts 20212021,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 33 ofof 1717

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thermalthermal stabilitystability ofof thethe producedproduced polyesterspolyesters isis investigatedinvestigated withwith thethe implementationimplementation ofof evolvedevolved gasgas analysisanalysis massmass spectrometryspectrometry (EGA-M(EGA-MS).S). AsAs colorationcoloration inin polymericpolymeric materialsmaterials affectsaffects their their field ﬁeldfield of of application,application, crucial crucial color color parameters parameters are are deﬁned.defined.defined. Finally, Finally, theoreticaltheoretical kinetickinetic studies studies have have been been performed performed toto supportsupport thethe experimentalexperimental observations.observations.

FigureFigure 1. 1. ChemicalChemicalChemical structuresstructures structures ofof of thethe the usedused used catalysts:catalysts: catalysts: ((aa)) (titaniumtitaniuma) titanium butoxidebutoxide butoxide (TBT),(TBT), (TBT), ((bb)) titanium (titaniumb) titanium iso-iso- propoxidepropoxide (TIS),(TIS), andand ((cc)) antimonyantimony trioxidetrioxide (Sb(Sb22OO33).). isopropoxide (TIS), and (c) antimony trioxide (Sb2O3).

2.2. Results Results and and Discussion Discussion 2.1.2.1. Effect Effect of of Catalysts Catalysts on on the the Pr ProgressProgressogress of of the the Esterification EsterificationEsterification ReactionReaction Poly(ethylenePoly(ethylene vanillate) vanillate)vanillate) was waswas produced producedproduced using usinusin ag two-stepg aa two-steptwo-step polycondensation polycondensationpolycondensation method methodmethod using usingthreeusing different threethree differentdifferent metal-based metal-basedmetal-based catalysts, catalysts,catalysts, starting startingstarting from 4-(2-hydroxyethoxy)-3-methoxybenzoic fromfrom 4-(2-hydroxyethoxy)-3-methox-4-(2-hydroxyethoxy)-3-methox- ybenzoicacid, which acid, is which a VA-based is a VA-based monomer monomer (Scheme (Scheme1). The esterification 1). The esterifi stepcation was step performed was per- ybenzoic acid, ◦which is a VA-based monomer (Scheme 1). The esterification step was per- formedfor 3 h atfor 200 3 h atC, 200 while °C, while the polycondensation the polycondensation stage stage took took place place at gradually at gradually increasing increas- formed for 3 h at 200 °C,◦ while the polycondensation stage took place at gradually increas- ingtemperaturesing temperaturestemperatures up to upup 265 toto 265265C in °C°C a in vacuumin aa vacuumvacuum for the forfor preparation thethe preparationpreparation of the ofof final thethe polymericfinalfinal polymericpolymeric materials. ma-ma- The studied catalysts, i.e., TBT, TIS, and Sb2O3, were introduced/added in the beginning of terials.terials. TheThe studiedstudied catalysts,catalysts, i.e.,i.e., TBT,TBT, TIS,TIS, andand SbSb22OO33,, werewere introduced/addedintroduced/added inin thethe be-be- the polymerization. Samples were taken from the reaction mixture every 30 min to observe ginningginning ofof thethe polymerization.polymerization. SamplesSamples werewere takentaken fromfrom thethe reactionreaction mixturemixture everyevery 3030 the progress of the polymerization. minmin toto observeobserve thethe progressprogress ofof thethe polymerization.polymerization.

SchemeScheme 1.1. SynthesisSynthesis of of poly(ethylene poly(ethylene vanillate)vanillate) polypolyesterpolyesterester withwith threethree differentdifferent catalysts.catalysts.

TheThe conversionconversion conversion ofof of thethe the carboxyliccarboxylic carboxylic groupgroup group (-C(-COOH) (-COOH)OOH) ofof thethe of monomermonomer the monomer intointo thethe into esterester the groupgroup ester (COOR)group(COOR) (COOR) ofof PEVPEV ofwaswas PEV evaluatedevaluated was evaluated usingusing titrationtitration using titrationandand nuclearnuclear and magneticmagnetic nuclear magnetic resonanceresonance resonance spectros-spectros- copyspectroscopycopy (NMR).(NMR). (NMR).NMRNMR spectraspectra NMR spectra werewere recordedrecorded were recorded inin deuterateddeuterated in deuterated chloroform/tchloroform/t chloroform/triﬂuoroaceticrifluoroaceticrifluoroacetic acidacid 1 1 13 13 (CDClacid(CDCl (CDCl33)/(TFA-d)/(TFA-d3)/(TFA-d11)) (4/1(4/11 v)v/ (4/1/vv).). IndicativeIndicativev/v). Indicative examplesexamples examples ofof 1HH NMRNMR of H andand NMR 13CC and NMRNMRC spectraspectra NMR spectra ofof thethe crudeofcrude the reactionreaction crude reaction mixturemixture mixture areare illustratedillustrated are illustrated inin FigureFigure in S1S1 Figure andand FigureFigure S1 and 2,2, Figure respectively.respectively.2, respectively. InIn 11HH NMRNMR In 1 spectra,spectra,H NMR thethe spectra, methylenemethylene the methylene protonsprotons ofof protons thethe O-CHO-CH of22b-CH theb-CH O-CH22a-OHa-OH2b-CH partpart 2 werewerea-OH selectedselected part were toto monitormonitor selected thethe to reactionmonitorreaction progress. theprogress. reaction InIn detail, progress.detail, asas aa In resultresult detail, ofof asththee a esterificationesterification result of the of esteriﬁcationof thethe hydroxylhydroxyl of group, thegroup, hydroxyl aa shiftshift group, a shift of the aforementioned protons from 4.42 (CH a) and 3.91 ppm (CH b) in ofof thethe aforementionedaforementioned protonsprotons fromfrom 4.424.42 (CH(CH22a)a) andand 3.913.91 ppmppm2 (CH(CH22b)b) inin thethe monomermonomer2 toto 4.48the4.48 monomer andand 3.993.99 ppmppm to 4.48 inin PEV,PEV, and respectively, 3.99respectively, ppm in PEV,isis visible.visible. respectively, AsAs thethe reactionreaction is visible. timetime As increased,increased, the reaction thethe mon-mon- time omerincreased, peaks the gradually monomer diminished, peaks gradually and the diminished,corresponding and polymer the corresponding peaks dominated polymer the omer peaks gradually diminished, and the corresponding13 polymer peaks dominated the spectrum.peaksspectrum. dominated RegardingRegarding the spectrum.thethe 1313CC NMRNMR Regarding spectra,spectra, theaa singlesingleC NMR peak,peak, spectra, attributedattributed a single toto thethe peak, carboxyliccarboxylic attributed acidacid to the carboxylic acid carbon of the monomer, appeared at 172.0 ppm (carbon a), while carboncarbon ofof thethe monomer,monomer, appearedappeared atat 172.0172.0 ppmppm (carbon(carbon a),a), whilewhile thethe correspondingcorresponding peakpeak the corresponding peak of the ester carbon in PEV appeared at 168.4 ppm (carbon a’). As ofof thethe esterester carboncarbon inin PEVPEV appearedappeared atat 168.4168.4 ppmppm (carbon(carbon a’).a’). AsAs thethe reactionreaction proceeded,proceeded, the reaction proceeded, the intensity of the peak at 172.0 ppm reduced, along with the increase of the peak a’. The conversion after every 30 min interval was calculated from the

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Catalysts 2021, 11, 822 the intensity of the peak at 172.0 ppm reduced, along with the increase of the peak a’.4 The of 17 the intensity ofconversion the peak at after 172.0 every ppm 30 reduced, min interval along waswith calculated the increase from of the the peak 13C NMR a’. The spectra by inte- conversion aftergrating every the 30 peaksmin interval corresponding was calculated to the acid from and the the 13C ester NMR carbon spectra following by inte- Equation (1): grating the peaks corresponding to the acid and the ester carbon following Equation (1): 13C NMR spectra by integrating the peaks corresponding𝑎′ to the acid and the ester carbon Conversion (%) = (1) following Equation (1): 𝑎′ 𝑎+𝑎′ Conversion (%) = a0 (1) where 𝑎 is the integral of the esterConversion carbon𝑎+𝑎′ and(% )𝑎′= is the integral of the acid carbon. (1) a + a0 where 𝑎 is the integral of the ester carbon and 𝑎′ is the integral of the acid carbon. where a is the integral of the ester carbon and a0 is the integral of the acid carbon.

a a' a a'

3 h 3 h 2.5 h 2.5 h 2 h 2 h 1.5 h 1.5 h 1 h 1 h 0.5 h 0.5 h 180 170 160 150 140 130 120 110 100 90 80 70 60 50 δ 180 170 160 150 140 130 120 110 100 (ppm) 90 80 70 60 50 δ (ppm) Figure 2. 13C NMR spectra of the crude reaction mixture of the esterification performed at 200 °C Figure 2. 13FigureC NMR 2. spectra13C NMRin of the thespectra presence crude of reactionthe of crudeTIS. mixture reaction of mixture the esterification of the esterification performed performed at 200 ◦C in at the 200 presence °C of TIS. in the presence of TIS. TheThe assessment assessment of of the the reaction reaction progress progress during during the the esterification esterification step, in the the presence presence The assessmentofof each each of catalyst, catalyst, the reaction is is presented presented progress in in duringFigure Figure 33the and esterification Table S1.S1. As step, shown, in the the presence calculated conversion of each catalyst,ofof is-COOH -COOH presented groups in Figure usingusing 3 titanium-basedtitani and Tableum-based S1. As catalysts catalysts shown, ranged rangedthe calculated from from 80% 80% conversion to to 85%, 85%, while while the the use use of of -COOH groupsofSb Sb2O using23Oresulted3 resulted titani inum-based in a a 73% 73% conversion. conversion. catalysts ranged These These resultsfrom results 80% indicateindicate to 85%, thatthat while TBTTBT the and use TIS demonstrate of Sb2O3 resultedgreatergreater in a 73% catalytic catalytic conversion. activity activity These during during results the the esterification indicate esterification that stage TBT stage and of PEV, of TIS PEV, demonstrate in accordance in accordance with with its ter- its greater catalyticephthalateterephthalate activity duringand and furan-2,5-dicarboxylate the furan-2,5-dicarboxylate esterification stage homologues of homologues PEV, in accordance [22,23,27,35]. [22,23,27 with,35 ]. its terephthalate and furan-2,5-dicarboxylate homologues [22,23,27,35]. 100 100

80 80

60 60

40 PEV/TBT, titration 40 PEV/TBT, NMR Conversion (%) PEV/TBT, titration PEV/TBT, NMR PEV/TIS, titration Conversion (%) 20 PEV/TIS, titration PEV/TIS, NMR PEV/Sb O , titration 20 PEV/TIS, NMR 2 3 PEV/Sb O , NMR PEV/Sb2O3, titration 2 3 0 PEV/Sb O , NMR 0 20 40 60 80 1002 3 120 140 160 180 0 0 20 40 60 80 100 120Time 140 (min) 160 180

Time (min) Figure 3. Conversion measured by titration and NMR during the esteriﬁcation step in the presence of each catalyst.

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100

40 PEV/TBT, titration PEV/TBT, NMR

Conversion (%) Conversion PEV/TIS, titration 20 PEV/TIS, NMR

PEV/Sb2O3, titration

PEV/Sb2O3, NMR 0 0 20 40 60 80 100 120 140 160 180 Time (min)

Figure 3. Conversion measured by titration and NMR during the esterification step in the presence Catalysts 2021, 11, 822 5 of 17 of each catalyst.

The FTIR spectrum of the monomer is displayed in Figure 4, as well as the spectra of theThe crude FTIR reacting spectrum mixtu of there at monomer different istime displayed intervals in during Figure4 the, as first well stage as the of spectra the reac- of tion.the crude Briefly, reacting the monomer mixture atexhibit differented a timebroad intervals absorption during band the at first 335 stage0–3500 of thecm−1 reaction., which canBriefly, be the attributed monomer to exhibited the stretching a broad absorption vibrations band of the at 3350–3500 hydroxyl cm group−1, which of can the 2 2 −1 Obe-CH attributed-CH -OH to thesegment, stretching two sep vibrationsarate bands of the at hydroxyl2600–3000 group cm , oflinked the O-CHto the2 stretching-CH2-OH vibrationssegment, twoof the separate bound, bands through at 2600–3000hydrogen bonds, cm−1, linkedcarboxylic to the O- stretchingH groups, vibrationsand a sharp, of characteristicthe bound, through band at hydrogen 1677 cm− bonds,1, owing carboxylic to the C=O O-H stretching groups, vibration and a sharp, of the characteristic carboxylic acidband [36] at. 1677 Concerning cm−1, owing the first to the2 h C=Oof the stretching esterification vibration step, two of the absorption carboxylic peaks acid were [36]. apparentConcerning in the firstcarbonyl 2 h of area the of esterification the spectrum, step, representing two absorption the acid peaks and were the apparent ester bond in (atthe 1684 carbonyl and 1708 area cm of the−1) stretching spectrum, vibrations, representing respectively. the acid and As thethe esterreaction bond proceed (at 1684ed, and the first1708 peak cm− 1diminishe) stretchingd, and vibrations, the formation respectively. of a unique As the peak reaction at 1720 proceeded, cm−1 was the visible. first peakFur- thermore,diminished, the and decrease the formation of the absorption of a unique bands peak at at 17203350 cm–3500−1 wascm−1 visible. and 260 Furthermore,0–3000 cm−1 provedthe decrease the successful of the absorption esterification bands of at 3350–3500the hydroxyl cm − group1 and 2600–3000 and carboxylic cm−1 acid.proved These the observationssuccessful esterification are in agreement of the hydroxyl with the group reaction and progresscarboxylic determined acid. These by observations NMR and are titrimetry.in agreement with the reaction progress determined by NMR and titrimetry.

(a) (b)

3h 3 h

2.5h 2.5 h

2h 2 h

1.5h 1.5 h

Absorbance

Absorbance 1h 1 h

0.5h 0.5 h

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4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500

  Wavenumber (cm ) Wavenumber (cm ) (c)

3 h

2.5 h

2 h

1.5 h

Absorbance 1 h

0.5 h monomer

4000 3500 3000 2500 2000 1500 1000 500  Wavenumber (cm ) Figure 4. Figure 4. FTIR spectraspectra ofof thethe crudecrude reaction reaction mixture mixture during during the the esteriﬁcation esterification step step at at different different time time intervals intervals in thein the presence pres- of the (a) TBT, (b) TIS, and (c) Sb O catalysts. ence of the (a) TBT, (b) TIS, and (2c) 3Sb2O3 catalysts.

2.2.2.2. Catalyst Catalyst Effect Effect on on PEV PEV Molecular Molecular Weight Weight Inc Increaserease ExceptExcept for for the the effect effect of of the the catalyst catalyst type type on onthe theesterifi esterificationcation step, step, emphasis emphasis was wasalso givenalso given to the to evaluation the evaluation of their of their activity activity on the on the polycondensation polycondensation stage. stage. The TheIV of IV the of samplesthe samples was wasmeasured measured in a phenol/1,1,2,2 in a phenol/1,1,2,2-tetrachloroethane-tetrachloroethane mixture mixture at 25 °C. at As 25 show◦C. Asn inshown Figure ins Figure5 and 5S2, and the Figure samples S2, theexhibit samplesed different exhibited intrinsic different viscosity intrinsic values viscosity and valuessubse- quentlyand subsequently various number various average number molecular average weights, molecular owing weights, to the owing different to thereactivity different of eachreactivity catalyst. of each More catalyst. specifically, More PEV/TBT specifically, with PEV/TBT [η] = 0.27 with dL/g, [η ]PEV/TIS = 0.27 dL/g, with [ PEV/TISη] = 0.29 2 3 dL/g,with [andη] = PEV/S 0.29 dL/gb O, andwith PEV/Sb[η] = 0.252O dL/g3 with were [η] =prepared. 0.25 dL/g Similarly, were prepared. the presence Similarly, of a TIS the catalystpresence result of a TISed in catalyst a final resulted polymer in where a final Mn polymer = 5014 where g/mol, Mn while = 5014 TBT g/mol, and Sb while2O3 led TBT to polyesters where Mn = 4375 g/mol and Mn = 3886 g/mol, respectively. It is evident that the use of titanium-based based catalysts provided higher IV values, and consequently final products with a higher molecular weight. Furthermore, as the reaction progressed, regardless of the catalyst type used, there was an obvious increase of both the IV and Mn values. Overall, the two-step melt polycondensation reaction was interrupted at preselected times to retrieve samples from the reacting mixture. Despite the brevity of each sampling, the interruptions may affect the final IV and Mn values of the produced polyesters.

0.30

0.25

0.20

0.15

0.10

Intrinsic vicosity (dL/g) vicosity Intrinsic PEV/TBT 0.05 PEV/TIS

PEV/Sb2O3 0.00 0 60 120 180 240 Polycondensation time (min) Figure 5. Intrinsic viscosity values at different time intervals during the polycondensation step.

IV measurements were further supported by FTIR analysis. As depicted in Figure S3, in all samples, as the polycondensation proceeded, the band at 3500–3600 cm−1 gradually decreased. This phenomenon can be linked to the reaction between the hydroxyl

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Catalysts 2021, 11, 822 6 of 17 each catalyst. More specifically, PEV/TBT with [η] = 0.27 dL/g, PEV/TIS with [η] = 0.29 dL/g, and PEV/Sb2O3 with [η] = 0.25 dL/g were prepared. Similarly, the presence of a TIS catalyst resulted in a final polymer where Mn = 5014 g/mol, while TBT and Sb2O3 led to

polyestersand Sb2O3 whereled to polyestersMn = 4375 whereg/mol Mnand = Mn 4375 = g/mol3886 g/mol, and Mnrespectively. = 3886 g/mol, It is respectively.evident that theIt is use evident of titanium-based that the use ofbased titanium-based catalysts provided based catalysts higher IV provided values, and higher consequently IV values, finaland consequentlyproducts with ﬁnal a higher products molecular with aweight. higher Furthermore, molecular weight. as the Furthermore,reaction progressed, as the regardlessreaction progressed, of the catalyst regardless type used, of the there catalyst was an type obvious used, increase there was of anboth obvious the IV increaseand Mn values.of both theOverall, IV and the Mn two-step values. melt Overall, polycond the two-stepensation melt reaction polycondensation was interrupted reaction at prese- was lectedinterrupted times at to preselected retrieve samples times to from retrieve the samplesreacting frommixture. the reactingDespite mixture.the brevity Despite of each the sampling,brevity of eachthe interruptions sampling, the may interruptions affect the fina mayl IV affect and theMn ﬁnalvalues IV of and the Mn produced values ofpoly- the esters.produced polyesters.

0.30

0.25

0.20

0.15

0.10

Intrinsic vicosity (dL/g) vicosity Intrinsic PEV/TBT 0.05 PEV/TIS

PEV/Sb2O3 0.00 0 60 120 180 240 Polycondensation time (min) Figure 5. IntrinsicIntrinsic viscosity values at different time intervals during the polycondensationpolycondensation step.step.

IV measurements were further supported by FTIR analysis. As depicted depicted in in Figure Figure S3, S3, −1 in all samples, asas thethe polycondensationpolycondensation proceeded,proceeded, thethe bandband at at 3500–3600 3500–3600 cm cm−1 gradually decreased. This This phenomenon phenomenon can can be be linked linked to tothe the reaction reaction between between the thehydroxyl hydroxyl and andcar- boxylcarboxyl end end groups groups of ofi-mers i-mers (i (i= =2,3,4,…,n) 2,3,4, ... ,n) towards towards the the formation formation of of polymers; polymers; however, however, −1 the C=OC=O stretchingstretching bandband at at 1720 1720 cm cm−1appeared appeared to to not not be be strongly strongly influenced influenced during during the 4-hthe 4-hduration duration of this of this step, step, supporting supporting that thethat conversion the conversion of the of -COOH the -COOH groups groups was completed was com- pletedat a high at degreea high degree during during the esterification the esterification step. step. As thermal thermal transitions transitions and and phase phase changes changes do dominateminate the theprocessing processing of polymeric of polymeric ma- materials and determine the temperature window of this procedure, melting (T ), cold terials and determine the temperature window of this procedure, melting (Tm), coldm crys- crystallization (Tcc), and glass transition (Tg) temperatures of the synthesized polyesters tallization (Tcc), and glass transition (Tg) temperatures of the synthesized polyesters dur- ingduring the thepolycondensation polycondensation step step were were measured measured by by DSC. DSC. The The curves, curves, shown shown in in Figure Figure 66,, clearly depict that Tg, Tcc, and Tm gradually shift to higher values, following the increase of clearly depict that Tg, Tcc, and Tm gradually shift to higher values, following the increase the molecular weight with increasing polycondensation time and temperature, as analyzed of the molecular weight with increasing polycondensation time and temperature, as ana- above. In detail, the sample that was prepared in the presence of TIS catalyst after the lyzed above. In detail, the sample that was ◦prepared in the presence of TIS catalyst after esterification step exhibits a Tg value of 73.2 C, and due to its highly amorphous nature, it the esterification step exhibits a Tg value of◦ 73.2 °C, and due to its highly amorphous na-◦ exhibits cold crystallization (Tcc) at 106.7 C, while the formed crystals melt at 253.1 C. ture, it exhibits cold crystallization (Tcc) at 106.7 °C, while the formed crystals◦ melt at 253.1 Not surprisingly, by increasing the reaction time and temperature (4 h, 265 C), Tg,Tcc, and °C. Not surprisingly, by increasing the reaction time and temperature (4 h, 265 °C), Tg, Tcc, Tm values were observed at higher temperatures. A similar pattern was noticed when the and Tm values were observed at higher temperatures. A similar pattern was noticed when reaction was catalyzed by the other two catalysts (TBT and Sb2O3). Thermal transitions the reaction was catalyzed by the other two catalysts (TBT and Sb2O3). Thermal transitions occur at higher temperatures for the polymer prepared in the presence of TIS, where it occur at higher temperatures for the polymer prepared in the presence of TIS, where it can thus be concluded that TIS exhibited a higher catalytic activity. The characteristic can thus be concluded that TIS exhibited a higher catalytic activity. The characteristic tem- temperatures of each polymer and at each time interval during the second heating scan are peraturespresented of in each detail polymer in Table 1and. at each time interval during the second heating scan are presented in detail in Table 1.

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(a) TBT (b) TIS

4h 4h

3h 3h

2h 2h

1h 1h

3h N2 3h N2 Normalized Heat Endo Up Flow (W/g) 1 W/g 1 W/g Normalized Heat Flow (W/g) Endo Up Normalized Heat Flow (W/g) Endo

50 100 150 200 250 300 50 100 150 200 250 300 o o Temperature ( C) Temperature ( C)

3h N2

1 W/g Normalized Heat Flow (W/g) Endo Up

50 100 150 200 250 300 o Temperature ( C) Figure 6. DSCDSC thermograms thermograms of the the condensation condensation step at diffe differentrent reaction times in the presence of the ( a) TBT, TBT, ( b) TIS, and (c) Sb2OO33 catalysts.catalysts. 2.3. Effect of Catalysts on the Thermal Stability of PEV Table 1. Thermal characteristics, including the glass transition (Tg), cold crystallization (Tcc), melting pointThe (Tm), thermal and stabilitycrystallization of the temperature produced (T polyestersc) of melt-quenched was evaluated PEV samples using when EGA/MS. using As differentcan be observed catalysts. in Figure7, the catalyst used appeared to have a small impact on the decomposition temperature peak of the final sample; however, not surprisingly, PEV PEV/TBT synthesized with a titanium-based catalyst showed higher thermal stability [29], with a Melt-Quenched peak (correspondingTime (min) to the temperature where the largest quantity of degradation products Tg (°C) Tcc (°C) Tm (°C) are produced) at 407 ◦C and 412 ◦C in the presence of TBT and TIS, respectively, while the 0 66.7 101.9 248.9 Sb O -catalyzed material degraded at 405 ◦C. These observations support the hypothesis 2 3 60 72.4 105.9 258 of the superior performance of these catalysts in the synthesis of PEV polyesters. 120 72.6 106.1 259.7 2.4. Effect of Used180 Catalysts on PEV Coloration73 106.6 260.2 240 73.5 107.7 260.9 Besides their effect on the molecularPEV/TIS weight of the final polyesters, the three catalysts also resulted in differences in the coloration of the materials. As shown in Figure8, the Time (min) Tg (°C) Tcc (°C) Tm (°C) titanium-based0 catalysts led to reddish-yellowish73.2 products,106.7 while the use of Sb253.12O3 gave a greyish color to60 the synthesized polymer.75.2 106.7 264.6 Concerning120 CIEL*a*b* color system75.9 values, in all three106.8 polymers, there was264.8 a relatively large decrease180 in lightness (∆L* = 10) after76.7 the first hour of108.8 thepolycondensation 266.5 reaction, followed by a240 gradual one as time increased.77.3 Furthermore,109.1 the a* and b* values267.5 (linked

to the green–red and blue–yellow axes,PEV/Sb respectively)2O3 of each sample were also calculated (Figure9b,c),Time demonstrating (min) that TBT Tg and (°C) TIS offered stronger Tcc (°C) red–yellowhues Tm (°C) to the corresponding0 materials in comparison61.1 with Sb2O3. Finally,98.7 the K/S fraction,238.6 which is connected to the60 concentration of the color69.1 on the polyester, was104.4 determined (Figure2539 d ). As 120 70.2 104.5 256.4

Catalysts 2021, 11, 822 8 of 17

can be seen, samples that were synthesized with the presence of Sb2O3 catalyst possessed higher concentrations of color on their surface.

Table 1. Thermal characteristics, including the glass transition (Tg), cold crystallization (Tcc), melting point (Tm), and crystallization temperature (Tc) of melt-quenched PEV samples when using different catalysts.

PEV/TBT Melt-Quenched Time (min) ◦ ◦ ◦ Tg ( C) Tcc ( C) Tm ( C) 0 66.7 101.9 248.9 60 72.4 105.9 258 120 72.6 106.1 259.7 180 73 106.6 260.2 240 73.5 107.7 260.9 Catalysts 2021, 11, x FOR PEER REVIEW 8 of 17 PEV/TIS ◦ ◦ ◦ Time (min) Tg ( C) Tcc ( C) Tm ( C) 0 73.2 106.7 253.1 60180 75.272.3 106.7106 264.6257.8 120240 75.972.4 106.8106.2 264.8258.9 180 76.7 108.8 266.5 2.3. Effect of240 Catalysts on the Thermal 77.3 Stability of PEV 109.1 267.5 The thermal stability of the producedPEV/Sb poly2estersO3 was evaluated using EGA/MS. As ◦ ◦ ◦ can be Timeobserved (min) in Figure 7, the Tg catalyst( C) used appeared Tcc ( toC) have a small impact Tm ( C) on the decomposition0 temperature peak 61.1of the final sample; however, 98.7 not surprisingly, 238.6 PEV synthesized with60 a titanium-based catalyst 69.1 showed higher 104.4thermal stability [29], with 253 a peak (corresponding120 to the temperature 70.2 where the largest quantity 104.5 of degradation products 256.4 are produced) 180at 407 °C and 412 °C 72.3in the presence of TBT 106 and TIS, respectively, 257.8 while the Sb2O3-catalyzed240 material degraded 72.4 at 405 °C. These observatio 106.2ns support the 258.9 hypothesis of the superior performance of these catalysts in the synthesis of PEV polyesters.

1.0

PEV/ Sb2O3 PEV/TIS PEV/TBT 0.8

0.6

0.4 Relative intensity intensity Relative

0.2

0.0 100 200 300 400 500 600 Temperature (oC)

FigureFigure 7. 7.EGAEGA pyrograms pyrograms of of PEV PEV in in the the presence presence of of each each catalyst. catalyst.

2.4. Effect of Used Catalysts on PEV Coloration Besides their effect on the molecular weight of the final polyesters, the three catalysts also resulted in differences in the coloration of the materials. As shown in Figure 8, the titanium-based catalysts led to reddish-yellowish products, while the use of Sb2O3 gave a greyish color to the synthesized polymer.

Figure 8. Appearances of the final (a) PEV/TBT, (b) PEV/TIS, and (c) PEV/Sb2O3 samples after 4 h of melt polycondensation.

Concerning CIEL*a*b* color system values, in all three polymers, there was a relatively large decrease in lightness (ΔL* = 10) after the first hour of the polycondensation reaction, followed by a gradual one as time increased. Furthermore, the a* and b* values (linked to the green–red and blue–yellow axes, respectively) of each sample were also

Catalysts 2021, 11, x FOR PEER REVIEW 8 of 17

180 72.3 106 257.8 240 72.4 106.2 258.9

2.3. Effect of Catalysts on the Thermal Stability of PEV The thermal stability of the produced polyesters was evaluated using EGA/MS. As can be observed in Figure 7, the catalyst used appeared to have a small impact on the decomposition temperature peak of the final sample; however, not surprisingly, PEV synthesized with a titanium-based catalyst showed higher thermal stability [29], with a peak (corresponding to the temperature where the largest quantity of degradation products are produced) at 407 °C and 412 °C in the presence of TBT and TIS, respectively, while the Sb2O3-catalyzed material degraded at 405 °C. These observations support the hypothesis of the superior performance of these catalysts in the synthesis of PEV polyesters.

1.0

PEV/ Sb2O3 PEV/TIS PEV/TBT 0.8

0.6

0.4 Relative intensity intensity Relative

0.2

0.0 100 200 300 400 500 600 Temperature (oC)

Figure 7. EGA pyrograms of PEV in the presence of each catalyst.

2.4. Effect of Used Catalysts on PEV Coloration Besides their effect on the molecular weight of the final polyesters, the three catalysts Catalysts 2021, 11, 822 also resulted in differences in the coloration of the materials. As shown in Figure 98, of the 17 titanium-based catalysts led to reddish-yellowish products, while the use of Sb2O3 gave a greyish color to the synthesized polymer.

Catalysts 2021, 11, x FOR PEER REVIEW 9 of 17

calculated (Figure 9b,c), demonstrating that TBT and TIS offered stronger red–yellow hues to the corresponding materials in comparison with Sb2O3. Finally, the K/S fraction, which

isFigure connected 8. Appearances to the concentration of the final (a )of PEV/TBT, the colo (br )on PEV/TIS, the polyester, and (c) PEV/Sb was determined2O3 samples after (Figure 4 h Figure 8. Appearances of the ﬁnal (a) PEV/TBT, (b) PEV/TIS, and (c) PEV/Sb2O3 samples after 4 h 9d). As can be seen, samples that were synthesized with the presence of Sb2O3 catalyst ofof meltmelt polycondensation.polycondensation. possessed higher concentrations of color on their surface. Concerning CIEL*a*b* color system values, in all three polymers, there was a rela- 65 tively large decrease in lightness 4.5(ΔL* = 10) after the first hour of the polycondensation (a) PEV/TBT (b) PEV/TBT reaction, followed by a gradual one4.0 as time increased. Furthermore, the a* and b* values PEV/TIS PEV/TIS (linked to the green–red and blue–yellow axes, respectively) of each PEV/Sb sampleO were also 60 PEV/Sb2O3 3.5 2 3

3.0

55 2.5

2.0 a* L* 50 1.5

1.0 45 0.5

0.0 40 0 60 120 180 240 0 60 120 180 240 Polycondensation time (min) Polycondensation time (min)

8 5.0 (c) PEV/TBT (d) PEV/TBT PEV/TIS 4.5 PEV/TIS

PEV/Sb O PEV/Sb2O3 6 2 3 4.0

3.5

4 3.0 K/S b* 2.5 2 2.0

1.5 0 1.0

0 60 120 180 240 0 60 120 180 240 Polycondensation time (min) Polycondensation time (min) Figure 9. 9. VariationVariation of of (a ()a L*,) L*, (b ()b a*,) a*, (c) ( cb*,) b*, and and (d) (K/Sd) K/S values values of PEV of PEVsamples samples prepared prepared with different with different catalysts catalysts and poly- and condensationpolycondensation times. times. Overall, the formation of the colored polycondensation products can be attributed to prolongedOverall, the heating formation at a temperatureof the colored above polyco 240ndensation◦C. At these products temperatures, can be attributed a high de-to prolongedgree of yellowing heating canat a betemperature caused either above due 240 to the°C. possibleAt these decarboxylationtemperatures, a high of the degree starting of yellowingmonomer can [34] be or caused due to theeither potential due to formationthe possible of organicdecarboxylation contaminants of the during starting the mono- poly- mer [34] or due to the potential formation of organic contaminants during the polymeri- merization. Moreover, the grey discoloration of PEV in the case of Sb2O3 catalysts may zation.be attributed Moreover, to the the reduction grey discolor of Sbation+3 to elementaryof PEV in the Sb 0casein the of Sb presence2O3 catalysts of carbon may oxides. be at- tributedThese observations to the reduction are in of agreement Sb+3 to elementary with theeffect Sb0 in of the the presence aforementioned of carbon catalysts oxides. onThese the observationsdiscoloration are of PETin agreement and poly(ethylene with the furan-2,5-dicarboxylate)effect of the aforementioned (PEF). catalysts [37–39]. on the discoloration of PET and poly(ethylene furan-2,5-dicarboxylate) (PEF). [37–39].

2.5. Reaction Kinetic Analysis In general, there are three approaches to model the kinetics of esterification/polycondensation reactions. In order of increasing complexity, they are (i) the simple empirical laws for the rate of decrease of reactant concentration (e.g., in [40]); (ii) the functional group models (e.g., in [41]) which are restricted to the description of the evolution of the active molecular groups (e.g., hydroxyl and carboxyl in the present case); and (iii) detailed

Catalysts 2021, 11, 822 10 of 17

2.5. Reaction Kinetic Analysis In general, there are three approaches to model the kinetics of esterification/polycondensation Catalysts 2021, 11, x FOR PEER REVIEW 10 of 17 reactions. In order of increasing complexity, they are (i) the simple empirical laws for the rate of decrease of reactant concentration (e.g., in [40]); (ii) the functional group models (e.g., in [41]) which are restricted to the description of the evolution of the active molecular moleculargroups (e.g., species hydroxyl models, and carboxylwhich describe in the present in detail case); all andthe reaction (iii) detailed occurring molecular in the species process.models, The which number describe of unknown in detail kinetic all the parameters reaction occurring increases in from the process.(i) to (iii). The The number physical of meaningsunknown kineticof parameters parameters also increases increase fromas we (i) move to (iii). from The empirical physical meanings (i.e., in (i)) of parametersto detailed (i.e.,also increasein (iii)) models. as we move Here, from a combination empirical (i.e., of approaches in (i)) to detailed (ii) and (i.e., (iii) in will (iii)) be models. followed. Here, a combinationThe particular of approaches problem (ii) at andhand (iii) is willlinear be chain followed. polymerization with a single monomer. Actually,The particular the same problem reaction at hand occurs is linear in both chain esterification polymerization and transesterification with a single monomer. steps. TheActually, detailed the kinetic same reaction model for occurs the evolution in both esteriﬁcation of the concentration and transesteriﬁcation of the i-mers (molecules steps. The consisteddetailed kineticof i-primary model units) for the is ( evolutioni = 1,2,…Nmax of) the concentration of the i-mers (molecules

consisted of i-primary units) is (i = 1, 2, . . . , Nmax) 𝑑𝐶 1 = 𝐾 𝐶 𝐶 −𝐶 𝐾 𝐶 i , Nmax , (2) dCi𝑑𝑡 1 2 = ∑Ki−j,jCi−jCj − Ci ∑Ki,jCj (2) dt 2 j=1 j=1 where Nmax corresponds to the larger i-mer in the system and Ki,j is the reaction constant for the reaction between an i-mer and a j-mer. The above equation is used in an approxi- where Nmax corresponds to the larger i-mer in the system and Ki,j is the reaction constant for matethe reaction way to betweenmodel the an esterification i-mer and a j-mer.step. The The conversion above equation of a functional is used in group an approximate is experi- mentallyway to model measured the esterification in this step. step.This Theconversi conversionon corresponds of a functional to the grouploss of is monomers experimentally from themeasured system in(i.e., this ev step.olution This of variable conversion C1). correspondsIt can be assumed to the that loss during of monomers esterification from the mainsystem mechanism (i.e., evolution for monomer of variable lossC 1is). the It canreac betion assumed between that monomers. during esterification In reality, some the largermain mechanismmolecules are for formed, monomer but loss the isconcentrations the reaction betweenare too low monomers. and the reaction In reality, rates some of monomerslarger molecules with them are formed, are negligible. but the In concentrations such a situation, are Equation too low and(2) is the transformed reaction rates to the of followingmonomers form: with them are negligible. In such a situation, Equation (2) is transformed to the following form: 𝑑𝐶 dC =−𝑘𝐶 (3) 𝑑𝑡1 = −kC2 (3) dt 1 This is a simple second-order reaction (with k = K1,1). This equation is integrated and This is a simple second-order reaction (with k = K1,1). This equation is integrated and transformed to give the percentage conversion R as 𝑅 = 100(1 − 1 ), where Co is the transformed to give the percentage conversion R as R = 100(1 − ), where C is the 1+kCot o initialinitial concentration concentration of of the the reactant. reactant. To To expl exploitoit the the experimental experimental conversions conversions measured measured by bothby both techniques techniques employed employed in the in present the present work worktheir average their average values values are considered. are considered. Then, theThen, above the aboveexpression expression for R foris fitted R is fittedto the to av theerage average conversion conversion data. data. The Theresults results of the of thefit- tingsfittings are are presented presented in inFigure Figure 10. 10 .

100

40 PEV/TBT PEV/TIS

Conversion (%) Conversion PEV/Sb2O3 20 PEV/TBT PEV/TIS

PEV/Sb2O3 0 0 60 120 180 Time (minutes)

FigureFigure 10. 10. ComparisonComparison of of experimental experimental (symbols) (symbols) and and mo modeldel (continuous (continuous lines) lines) conversions conversions of of the the esterificationesteriﬁcation reaction. The The experimental experimental data are averaged from both techniques.

The values found for the single fitting parameter kCo are 0.018, 0.02, 0.0137 min−1 for the catalysts TBT, TIS, and Sb2O3, respectively. Considering that Co is the same for all catalysts, the esterification reaction constant appears to be largest for TIS, followed by TBT, leaving Sb2O3 as the least effective catalyst. The validity of the approximation of the system by Equation (3) is confirmed by assuming the reaction order exponent as a second

Catalysts 2021, 11, 822 11 of 17

−1 The values found for the single fitting parameter kCo are 0.018, 0.02, 0.0137 min for the catalysts TBT, TIS, and Sb2O3, respectively. Considering that Co is the same for Catalysts 2021, 11, x FOR PEER REVIEW 11 of 17 all catalysts, the esterification reaction constant appears to be largest for TIS, followed by TBT, leaving Sb2O3 as the least effective catalyst. The validity of the approximation of the system by Equation (3) is confirmed by assuming the reaction order exponent as a second fittingfitting parameter. The fitting fitting with two paramete parametersrs led to values of the exponent very close toto two. two. TheThe next step was the examination of the polycondensationpolycondensation reaction. The reactions inin the the presence presence of of the the three three catalysts catalysts started started from from different initial conditions (i.e., average molecularmolecular weight) weight) as as a aconsequence consequence of of the the diff differenterent extents extents of preceding of preceding esterification esterification reactions.reactions. To To eliminate eliminate the the effect effect of of initial initial conditions, conditions, the the normalized normalized molecular weight M/MM/Moo evolutionevolution is is presented presented in in Figure Figure 1111,, wherewhere M isis the the average average molecular molecular weight in the reactionreaction mixture mixture and Mo denotesdenotes its its value value at at the the beginning beginning of polycondensation step.

4.0

3.5

3.0

2.5

o 2.0 M/M 1.5

1.0 PEV/TBT PEV/TIS

0.5 PEV/Sb2O3 model 0.0 0 60 120 180 240 Polycondensation time (minutes)

FigureFigure 11. 11. ExperimentalExperimental normalized normalized average average molecular molecular weight weight (symbols) (symbols) and and the the corresponding corresponding modelmodel (continuous (continuous line).

ItIt is is very interesting that the evolution curves of M/Mo practically coincided for the threethree catalysts, catalysts, implying implying that that the the performances performances of of the the three three catalysts catalysts for for polycondensation polycondensation were similar to each other.other. ToTo modelmodel the the (unique (unique for for all all catalysts) catalysts) polycondensation polycondensation kinetics, kinetics,the the system system (2) of(2)N ofmax Nmaxordinary ordinary differential differential equations equations must must be be numerically numerically integrated. integrated. A Achoice choice of of the the kinetic kinetic function functionK Ki,ji,jis is needed. needed. It It isis expectedexpected thatthat thethe motionmotion of the polymer −λ −λ chainschains is is inhibited inhibited as as their their molecular molecular weight weight increases increases so the so choice the choice Ki,j ∝K i,j(i−λ∝ +( ji−λ) (where+ j ) λ(where is a positiveλ is a parameter) positive parameter) is a reasonable is a reasonable one. There one.are no There experimental are no experimental data for the com- data pletefor the molecular complete weight molecular distribution, weight but distribution, only for its but average only value for its so average the system value is solved so the approximatelysystem is solved using approximately the monodisperse using the method monodisperse of moments method to give of the moments following to give simple the equationfollowing for simple the evolution equation of for the the ratio evolution M/Mo (details of the ratio of theM/M derivationo (details can of the be found derivation in [22]): can be found in [22]): 𝑀 M 1//()(1+λ) ==(1+(1+𝜆)𝐾 (1 + (1 + λ)Kot)𝑡) (4) M𝑀o

DataData fitting ﬁtting (shown (shown in inFigure Figure 11) was11) wasachieved achieved using usingthe values the λ values = 2.5 andλ = Ko 2.5 = 0.037 and −1 minKo =−1 0.037(for all min the catalysts).(for all the The catalysts). value λ = The2.5 implies value λ a =large 2.5 impliesreduction a largeof reaction reduction rate as of thereaction chain ratelength as theof the chain reactant length increases. of the reactant This is due increases. to a combination This is due of to reduced a combination molecule of mobilityreduced molecule(i.e., diffusivity) mobility and (i.e., increased diffusivity) mixture and increasedviscosity, mixtureas the size viscosity, of the molecules as the size in- of creases.the molecules increases.

3. Materials and Methods 3.1. Materials Vanillic acid (VA, purum 99%) was supplied by J & K Scientific (GmbH, Pforzheim, Germany). Sodium iodide (>99%), used in the synthesis of the monomer, was purchased by Acros Organics N.V. (Geel, Belgium). Furthermore, the 2-chloroethanol (>99%), titanium butoxide (Ti(OBu)4), titanium isopropoxide (Ti(iΟPr)4), and antimony trioxide (Sb2O3) (analytic grade) catalysts were provided by Sigma-Aldrich Co (Chemie GmbH,

Catalysts 2021, 11, 822 12 of 17

3. Materials and Methods 3.1. Materials Vanillic acid (VA, purum 99%) was supplied by J & K Scientiﬁc (GmbH, Pforzheim, Germany). Sodium iodide (>99%), used in the synthesis of the monomer, was purchased by Acros Organics N.V. (Geel, Belgium). Furthermore, the 2-chloroethanol (>99%), titanium butoxide (Ti(OBu)4), titanium isopropoxide (Ti(iOPr)4), and antimony trioxide (Sb2O3) (analytic grade) catalysts were provided by Sigma-Aldrich Co (Chemie GmbH, Steinheim, Germany). Phenol (99+%), and 1,1,2,2-tetrachloroethane (98+%), utilized as a mixture for intrinsic viscosity measurements, were purchased from Alfa Aesar (Kandel, Germany). All other solvents and materials used were of an analytical grade.

3.2. Synthesis of 4(2-Hydroxyethoxy)-3-Methoxybenzoic Acid In short, 33.6 g of vanillic acid (0.20 mol) and 12 g of sodium iodide (0.08 mol) were dissolved in an aqueous potassium hydroxide solution (33.75 g (0.6 mol) in 260 mL of water). Then, 17.7 g (0.22 mol) of 2-chloroethanol, dissolved in 520 mL of ethanol, was ◦ added dropwise over 4 h at 100 C, after having been degassed by N2 bubbling. The reacting mixture was refluxed and 6.0 g of 2-chloroethanol was added approximately every 12 h. After 4 days, the obtained mixture was concentrated using a rotary evaporator, and the residue was diluted in water. The aqueous solution was washed with ether and acidified with aqueous hydrochloric acid solution. The precipitated solid was separated by filtration and purified by two subsequent recrystallizations in ethanol to obtain the product in 64% yield [34].

3.3. Synthesis of Poly(Ethylene Vanillate) Polyesters The synthesis of PEV polyesters was carried out via a two-stage melt polycondensation procedure in a glass batch reactor according to previous studies by our research group [42]. First, 8 g of 4-(2-hydroxyethoxy)-3-methoxybenzoic acid was charged into the reaction tube of the polycondensation apparatus. Then, 500 ppm (0.5 mL) of TBT, TIS, or Sb2O3 was subsequently added in the polycondensation reactor and the apparatus was subsequently evacuated and ﬁlled with N2 three times to remove oxygen. The reacting mixture was ◦ heated at 200 C under N2 ﬂow (50 mL/min) for 3 h. At 30 min time periods, samples were collected from the reaction mixture for analysis. During the second stage, a vacuum (5.0 Pa) was progressively applied over 20 min and the temperature was gradually increased to 240 ◦C while the stirring speed was increased from 360 to 750 rpm over an hour. The reacting mixture was heated for 2 h at 240 ◦C and the temperature was further increased to 255 ◦C and 265 ◦C for 1 h each time. Finally, the resulting materials were milled and washed with methanol. Samples after 1, 2, 3, and 4 h of the second step were gathered and characterized using various techniques.

3.4. Polyesters’ Characterization 3.4.1. Acid Value Measurements The carboxyl end group content was deﬁned by titration with a potassium hydroxide methanolic solution (0.1 M) and phenol red as an indicator. For each sample, the titration was repeated in triplicate and the mean value was estimated. The acid value (AV) represents the unreacted acid groups, and it is deﬁned as the milliliters of potassium hydroxide solution required to neutralize one gram of sample. The AV of the resulted polyesters was determined based on Equation (5):

C × V × 56.11 AV = (5) m where V is the volume of KOH solution consumed (in mL); C is the concentration of the KOH solution (in M); m stands for the mass of the sample (in grams); and 56.11 is potassium hydroxide molar mass (in g/mol). Catalysts 2021, 11, 822 13 of 17

Furthermore, the conversion of the monomer to polyester was subsequently calculated with the implementation of Equation (6):

AV − AV Conversion (%) = 0 t (6) AV0

where AV0 is the initial AV and AVt is the AV value at each sampling interval.

3.4.2. Nuclear Magnetic Resonance (NMR)

NMR spectra were recorded in deuterated chloroform/trifluoroacetic acid (CDCl3)/(TFA- d1) (4/1 v/v), using an Agilent 500 spectrometer (Agilent Technologies, Santa Clara, CA, USA) at room temperature. Spectra were calibrated utilizing the residual solvent peaks.

3.4.3. Intrinsic Viscosity Measurements (IV) The intrinsic viscosity was measured with an Ubbelohde viscometer (Schott Gerate GMBH, Hofheim, Germany) at 25 ◦C in a phenol and 1,1,2,2-tetrachloroethane (60/40, w/w) solution. To reach complete dissolution, the sample was heated in the solvent mixture at 70 ◦C for 20 min. After cooling, the solution was filtered through a disposable Teflon filter, to eliminate possible solid residues. The calculation of the intrinsic viscosity values of the produced polyesters performed applying the Solomon–Cuita equation (Equation (7)) for a single point measurement:

1 t t 2 [2{ t − ln t − 1}] [η] = 0 0 (7) c

where c is the solution concentration, t is the ﬂow time of the solution and t0 is the ﬂow time of the solvent. The experiment was performed three times and the average value was estimated.

3.4.4. Molecular Weight Intrinsic viscosity values [η] were used to calculate the number average molecular weight (Mn) of the PEV samples, using the Berkowitz equation (Equation (8)), as it was modiﬁed in our previous work [43]:

Mn = 3.29 × 104 [η]1.54 (8)

3.4.5. Fourier-Transformed Infrared Spectroscopy (FTIR) FTIR spectra of the produced polyesters were obtained by FTIR-2000 (Perkin Elmer, Waltham, MA, USA). A small amount of each sample was triturated with a proper amount of potassium bromide (KBr) and the disks were formed under pressure. All spectra were collected in the range from 4000 to 500 cm−1 using a resolution of 4 cm−1 and 32 co-added scans. The presented spectra were further baseline corrected, normalized, and converted into an absorbance mode.

3.4.6. Differential Scanning Calorimetry (DSC) Differential scanning calorimetry measurements were performed using a PerkinElmer Pyris Diamond DSC (PerkinElmer Corporation, Waltham MA, USA), updated to the DSC 8500 level, combined with an Intracooler 2P cooling accessory, calibrated with pure indium and zinc standards. Samples of 5 ± 0.1 mg sealed in aluminum pans were applied for assessing the thermal behavior of the produced polymers. All samples were initially heated at 20 ◦C/min up to 300 ◦C and held at this temperature for three minutes to erase any previous thermal history. The glass transition (Tg) and cold crystallization (Tcc) temperatures were measured by performing a heating scan of the melt-quenched samples at 20 ◦C/min. Catalysts 2021, 11, 822 14 of 17

3.4.7. Color Measurements

Catalysts 2021, 11, x FOR PEER REVIEW Color measurements were performed using a Datacolor Spectraflash SF60014 of 17 plus CT UV reflectance colorimeter (Datacolor, Marl, Germany) with the implementation of the D65 illuminant, a 10◦ standard observer with the UV component excluded and the specular component included. In each case, five measurements were performed using a special holder (Datacolor) and the mean values were calculated. The color values were calculated holder (Datacolor) and the mean values were calculated. The color values were calculated via the CIE L*a*b* color space system and the modified CIELCH system [44]. In this sys- via the CIE L*a*b* color space system and the modified CIELCH system [44]. In this system, tem, L* represents the lightness (L* = 0, black, L* = 100, white). As depicted in Figure 12, L* represents the lightness (L* = 0, black, L* = 100, white). As depicted in Figure 12, the the a* value corresponds to the green–red axis, where negative a* values indicate green a* value corresponds to the green–red axis, where negative a* values indicate green and and positivepositive a*values a*values indicate indicate red red hues. hues. Th Thee b* b* value represents thethe blue–yellowblue–yellow axis,axis, where where negativenegative b*b* valuesvalues indicateindicate blueblue andand positivepositive b*b* valuesvalues indicateindicate yellowyellow hues.hues.

Figure 12. Illustration of CIE L*a*b* color space system. Figure 12. IllustrationFurthermore, of CIE the L*a*b* fraction color R/S, space linked system. to the concentration of the color on the polyester, was determined following Equation (5): Furthermore, the fraction R/S, linked to the concentration of the color on the polyester, was determined following Equation (5):K (1 − R)2 = (9) 𝐾 S (1 − 𝑅)2R = (9) where K is the absorbance coefficient,𝑆 S stands2𝑅 for the scattering coefficient, and R is the where percentK is thereflectance absorbance of coefficient, each sample. S stands for the scattering coefficient, and R is the percent reflectance of each sample. 3.4.8. Evolved Gas Analysis Mass Spectroscopy (EGA/MS) 3.4.8. EvolvedFor Gas the Analysis performance Mass of Spectroscopy the Py-GC/MS (EGA/MS) analysis of the resulting polyesters, a very Forsmall the quantityperformance of the of materialthe Py-GC/MS has been analysis set primarily of the resulting into the “Double-Shot”polyesters, a very EGA/PY- small quantity3030D Pyrolyzer of the material (Frontier has Laboratories been set primarily Ltd., Fukushima into the Japan) “Double-Shot” with a CGS-1050Ex EGA/PY- (Kyoto, 3030D Japan)Pyrolyzer carrier (Frontier gas selector. Laboratories For evolved Ltd., gas Fukushima analysis (EGA), Japan) the with furnace a CGS-1050Ex temperature was programmed from 100 ◦C to 600 ◦C with a heating rate of 20 ◦C/min, using He as the (Kyoto, Japan) carrier gas selector. For evolved gas analysis (EGA), the furnace tempera-◦ ture waspurge programmed gas and air from as the 100 cooling °C to gas. 600 The°C with GC oven a heating temperature rate of was20 °C/min, set at 300 usingC for He25 min. as the Thepurge pyrolyzates gas and air were as the separated cooling using gas. The the temperatureGC oven temperature programmed was capillaryset at 300 column °C of for 25 min.a Shimadzu The pyrolyzates QP-2010 were Ultra separat Plus (Shimadzu,ed using the Kioto, temperature Japan) gas programmed chromatograph capillary and were columnanalyzed of a Shimadzu by the mass QP-2010 spectrometer Ultra Plus MS-QP2010SE (Shimadzu, ofKioto, Shimadzu Japan) (Shimadzu, gas chromatograph Kioto, Japan) at 70 eV. For the mass spectrometer, the following conditions were used: ion source heater at and were analyzed by the mass spectrometer MS-QP2010SE of Shimadzu (Shimadzu, 200 ◦C, interface temperature at 320 ◦C, vacuum of 10−4–100 Pa, m/z range of 45–500 amu, Kioto, Japan) at 70 eV. For the mass spectrometer, the following conditions were used: ion and scan speed of 10,000 u/s. source heater at 200 °C, interface temperature at 320 °C, vacuum of 10−4–100 Pa, m/z range of 45–5004. Conclusions amu, and scan speed of 10,000 u/s. Within this study, the efficiency of three different metal-based catalysts (TBT, TIS, and 4. Conclusions Sb2O3) during two-step melt polycondensation of PEV was examined for the first time. The Withinevolution this ofstudy, the esterification the efficiency step of was three investigated different metal-based using various catalysts techniques, (TBT, such TIS, as NMR, and Sb2O3) during two-step melt polycondensation of PEV was examined for the first time. The evolution of the esterification step was investigated using various techniques, such as NMR, end group analysis, and FTIR. Based on the acquired results, titanium- based catalysts seem to exhibit higher catalytic activity. In the second stage of the reaction, the efficiency of the used catalysts followed the same pattern, resulting in polyesters with

Catalysts 2021, 11, 822 15 of 17

end group analysis, and FTIR. Based on the acquired results, titanium-based catalysts seem to exhibit higher catalytic activity. In the second stage of the reaction, the efficiency of the used catalysts followed the same pattern, resulting in polyesters with higher molecular weights and subsequently superior thermal transitions (higher Tg,Tcc, and Tm values), as well as elevated thermal stability. Since coloration in materials plays an important role toward their future applications, color measurements were also performed, proving that the reaction time and temperature strongly affect the discoloration of the final materials, resulting in reddish-yellowish products in the case of titanium-based catalysts and a greyish polymer when in the presence of Sb2O3. Finally, theoretical kinetic studies were carried out and rate constants for the reactions that took place in each step were calculated, indicating that TIS possesses superior catalytic activity during the esterification stage, while all three catalysts exhibited similar behavior during the polycondensation process.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/catal11070822/s1. Figure S1: 1H NMR spectra of the crude reaction mixture of the esterification performed at 200 ◦C, in the presence of TIS. Table S1: Conversions obtained/observed in the presence of TBT, TIS and Sb2O3 catalysts. Figure S2: Evaluation of number average molecular weight at different time intervals during the polycondensation step. Figure S3: FTIR spectra of the crude reaction mixture during the polycondensation step at different time intervals in the presence of (a) TBT, (b) TIS and (c) Sb2O3 catalyst. Author Contributions: Investigation, methodology, formal analysis, writing—original draft E.X.; investigation, methodology, formal analysis A.Z. and Z.T.; methodology, formal analysis M.K.; supervision, writing—review and editing D.N.B. and G.Z.P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are provided within the article. Acknowledgments: This publication is based upon work from COST Action FUR4Sustain, CA18220, supported by COST (European Cooperation in Science and Technology). Conflicts of Interest: The authors declare no conflict of interest.

References 1. PlasticsEurope (2019) Plastics–the Facts 2019. An Analysis of European Plastics Production, Demand and Waste Data. Available online: https://www.plasticseurope.org/en/resources/publications/1804-plastics-facts-2019 (accessed on 15 May 2021). 2. Filho, W.L.; Salvia, A.L.; Bonoli, A.; Saari, U.A.; Voronova, V.; Klõga, M.; Kumbhar, S.S.; Olszewski, K.; De Quevedo, D.M.; Barbir, J. An assessment of attitudes towards plastics and bioplastics in Europe. Sci. Total Environ. 2021, 755, 142732. [CrossRef] 3. Chen, Y.; Li, T.; Hu, H.; Ao, H.; Xiong, X.; Shi, H.; Wu, C. Transport and fate of microplastics in constructed wetlands: A microcosm study. J. Hazard. Mater. 2021, 415, 125615. [CrossRef] 4. Geyer, R.; Jambeck, J.R.; Law, K.L. Production, use, and fate of all plastics ever made. Sci. Adv. 2017, 3, 25–29. [CrossRef] 5. Yang, L.; Zhang, Y.; Kang, S.; Wang, Z.; Wu, C. Science of the Total Environment Microplastics in soil: A review on methods, occurrence, sources, and potential risk. Sci. Total Environ. 2021, 780, 146546. [CrossRef] 6. European Commission. A Sustainable Bioeconomy for Europe: Strengthening the Connection between Economy, Society and the Environment. COM(2018) 673 Final; European Union: Brussels, Belgium, 2018. 7. Balla, E.; Daniilidis, V.; Karlioti, G.; Kalamas, T.; Stefanidou, M.; Bikiaris, N.D.; Vlachopoulos, A.; Koumentakou, I.; Bikiaris, D.N. Poly(lactic Acid): A Versatile Biobased Polymer for the Future with Multifunctional Properties—From Monomer Synthesis, Polymerization Techniques and Molecular Weight Increase to PLA Applications. Polymers 2021, 13, 1822. [CrossRef][PubMed] 8. Storz, H.; Vorlop, K.D. Bio-based plastics: Status, challenges and trends. Appl. Agric. For. Res. 2013, 63, 321–332. 9. Nea¸tu,F.; Culică, G.; Florea, M.; Parvulescu, V.I.; Cavani, F. Synthesis of Terephthalic Acid by p-Cymene Oxidation using Oxygen: Toward a More Sustainable Production of Bio-Polyethylene Terephthalate. ChemSusChem 2016, 9, 3102–3112. [CrossRef][PubMed] 10. Poveda-Giraldo, J.A.; Solarte-Toro, J.C.; Cardona Alzate, C.A. The potential use of lignin as a platform product in bioreﬁneries: A review. Renew. Sustain. Energy Rev. 2021, 138, 110688. [CrossRef] 11. Fache, M.; Boutevin, B.; Caillol, S. Vanillin, a key-intermediate of biobased polymers. Eur. Polym. J. 2015, 68, 488–502. [CrossRef] Catalysts 2021, 11, 822 16 of 17

12. Fache, M.; Darroman, E.; Besse, V.; Auvergne, R.; Caillol, S.; Boutevin, B. Vanillin, a promising biobased building-block for monomer synthesis. Green Chem. 2014, 16, 1987–1998. [CrossRef] 13. Gomes, E.D.; Rodrigues, A.E. Lignin biorefinery: Separation of vanillin, vanillic acid and acetovanillone by adsorption. Sep. Purif. Technol. 2019, 216, 92–101. [CrossRef] 14. Mota, M.I.F.; Pinto, P.C.R.; Loureiro, J.M.; Rodrigues, A.E. Recovery of Vanillin and Syringaldehyde from Lignin Oxidation: A Review of Separation and Purification Processes. Sep. Purif. Rev. 2016, 45, 227–259. [CrossRef] 15. Pandey, M.P.; Kim, C.S. Lignin Depolymerization and Conversion: A Review of Thermochemical Methods. Chem. Eng. Technol. 2011, 34, 29–41. [CrossRef] 16. Priefert, H.; Rabenhorst, J.; Steinbüchel, A. Biotechnological production of vanillin. Appl. Microbiol. Biotechnol. 2001, 56, 296–314. [CrossRef][PubMed] 17. Gallage, N.J.; Møller, B.L. Vanillin-bioconversion and bioengineering of the most popular plant flavor and its de novo biosynthesis in the vanilla orchid. Mol. Plant 2015, 8, 40–57. [CrossRef] 18. Martău, G.A.; Călinoiu, L.F.; Vodnar, D.C. Bio-vanillin: Towards a sustainable industrial production. Trends Food Sci. Technol. 2021, 109, 579–592. [CrossRef] 19. Banerjee, G.; Chattopadhyay, P. Vanillin biotechnology: The perspectives and future. J. Sci. Food Agric. 2019, 99, 499–506. [CrossRef] 20. Thiele, U.K. The Current Status of Catalysis and Catalyst Development for the Industrial Process of Poly (ethylene terephthalate) Polycondensation. Int. J. Polym. Mater. Polym. Biomater. 2001, 50, 387–394. [CrossRef] 21. Banella, M.B.; Bonucci, J.; Vannini, M.; Marchese, P.; Lorenzetti, C.; Celli, A. Insights into the Synthesis of Poly(ethylene 2,5- Furandicarboxylate) from 2,5-Furandicarboxylic Acid: Steps toward Environmental and Food Safety Excellence in Packaging Applications. Ind. Eng. Chem. Res. 2019, 58, 8955–8962. [CrossRef] 22. Terzopoulou, Z.; Karakatsianopoulou, E.; Kasmi, N.; Tsanaktsis, V.; Nikolaidis, N.; Kostoglou, M.; Papageorgiou, G.Z.; Lam- bropoulou, D.A.; Bikiaris, D.N. Effect of catalyst type on molecular weight increase and coloration of poly(ethylene furanoate) biobased polyester during melt polycondensation. Polym. Chem. 2017, 8, 6895–6908. [CrossRef] 23. Papadopoulos, L.; Zamboulis, A.; Kasmi, N.; Wahbi, M.; Nannou, C.; Lambropoulou, D.A.; Kostoglou, M.; Papageorgiou, G.Z.; Bikiaris, D.N. Investigation of the catalytic activity and reaction kinetic modeling of two antimony catalysts in the synthesis of poly(ethylene furanoate). Green Chem. 2021, 23, 2507–2524. [CrossRef] 24. Ahmadnian, F.; Velasquez, F.; Reichert, K.H. Screening of different titanium(IV) catalysts in the synthesis of poly(ethylene terephthalate). Macromol. React. Eng. 2008, 2, 513–521. [CrossRef] 25. MacDonald, W.A. New advances in poly(ethylene terephthalate) polymerization and degradation. Polym. Int. 2002, 51, 923–930. [CrossRef] 26. Shigemoto, I.; Kawakami, T.; Taiko, H.; Okumura, M. A quantum chemical study on the polycondensation reaction of polyesters: The mechanism of catalysis in the polycondensation reaction. Polymer 2011, 52, 3443–3450. [CrossRef] 27. Gruter, G.-J.; Sipos, L.; Adrianus Dam, M. Accelerating Research into Bio-Based FDCA-Polyesters by Using Small Scale Parallel Film Reactors. Comb. Chem. High Throughput Screen. 2011, 15, 180–188. [CrossRef] 28. Filella, M.; Hennebert, P.; Okkenhaug, G.; Turner, A. Occurrence and fate of antimony in plastics. J. Hazard. Mater. 2020, 390, 121764. [CrossRef] 29. Terzopoulou, Z.; Karakatsianopoulou, E.; Kasmi, N.; Majdoub, M.; Papageorgiou, G.Z.; Bikiaris, D.N. Effect of catalyst type on recyclability and decomposition mechanism of poly(ethylene furanoate) biobased polyester. J. Anal. Appl. Pyrolysis 2017, 126, 357–370. [CrossRef] 30. Sánchez-Martínez, M.; Pérez-Corona, T.; Cámara, C.; Madrid, Y. Migration of antimony from PET containers into regulated EU food simulants. Food Chem. 2013, 141, 816–822. [CrossRef] 31. Payán, L.; Poyatos, M.T.; Muñoz, L.; La Rubia, M.D.; Pacheco, R.; Ramos, N. Study of the influence of storage conditions on the quality and migration levels of antimony in polyethylene terephthalate-bottled water. Food Sci. Technol. Int. 2017, 23, 318–327. [CrossRef] 32. Mialon, L.; Vanderhenst, R.; Pemba, A.G.; Miller, S.A. Polyalkylenehydroxybenzoates (PAHBs): Biorenewable aromatic/aliphatic polyesters from lignin. Macromol. Rapid Commun. 2011, 32, 1386–1392. [CrossRef] 33. Gioia, C.; Banella, M.B.; Marchese, P.; Vannini, M.; Colonna, M.; Celli, A. Advances in the synthesis of bio-based aromatic polyesters: Novel copolymers derived from vanillic acid and -caprolactone. Polym. Chem. 2016, 7, 5396–5406. [CrossRef] 34. Zamboulis, A.; Papadopoulos, L.; Terzopoulou, Z.; Bikiaris, D.N.; Patsiaoura, D.; Chrissafis, K.; Gazzano, M.; Lotti, N.; Papageorgiou, G.Z. Synthesis, Thermal Properties and Decomposition Mechanism of Poly (Ethylene Vanillate) Polyester. Polymers 2019, 11, 1672. [CrossRef] 35. Karayannidis, G.P.; Roupakias, C.P.; Bikiaris, D.N.; Achilias, D.S. Study of various catalysts in the synthesis of poly(propylene terephthalate) and mathematical modeling of the esterification reaction. Polymer 2003, 44, 931–942. [CrossRef] 36. Belkov, M.V.; Brinkevich, S.D.; Samovich, S.N.; Skornyakov, I.V.; Tolstorozhev, G.B.; Shadyro, O.I. Infrared spectra and structure of molecular complexes of aromatic acids. J. Appl. Spectrosc. 2012, 78, 794–801. [CrossRef] 37. Finelli, L.; Lorenzetti, C.; Messori, M.; Sisti, L.; Vannini, M. Comparison between titanium tetrabutoxide and a new commercial titanium dioxide based catalyst used for the synthesis of poly(ethylene terephthalate). J. Appl. Polym. Sci. 2004, 92, 1887–1892. [CrossRef] Catalysts 2021, 11, 822 17 of 17

38. Wang, S.Q.; Cheng, S.; Lin, P.; Li, X. A phenomenological molecular model for yielding and brittle-ductile transition of polymer glasses. J. Chem. Phys. 2014, 141, 094905. [CrossRef] 39. Aharoni, S.M. The cause of the grey discoloration of PET prepared by the use of antimony-catalysts. Polym. Eng. Sci. 1998, 38, 1039–1047. [CrossRef] 40. Kostoglou, M.; Bikiaris, D. Kinetic Analysis of Nanocomposites Prepared in situ Consisting of an Aliphatic Biodegradable Polyester and Fumed Silica Nanoparticles. Macromol. React. Eng. 2011, 5, 178–189. [CrossRef] 41. Bikiaris, D.N.; Achilias, D.S. Synthesis of poly(alkylene succinate) biodegradable polyesters I. Mathematical modelling of the esteriﬁcation reaction. Polymer 2006, 47, 4851–4860. [CrossRef] 42. Xanthopoulou, E.; Terzopoulou, Z.; Zamboulis, A.; Papadopoulos, L.; Tsongas, K.; Tzetzis, D.; Papageorgiou, G.Z.; Bikiaris, D.N. Poly(propylene vanillate): A Sustainable Lignin-Based Semicrystalline Engineering Polyester. ACS Sustain. Chem. Eng. 2021, 9, 1383–1397. [CrossRef] 43. Kasmi, N.; Papageorgiou, G.Z.; Achilias, D.S.; Bikiaris, D.N. Solid-State polymerization of poly(Ethylene Furanoate) biobased Polyester, II: An efﬁcient and facile method to synthesize high molecular weight polyester appropriate for food packaging applications. Polymers 2018, 10, 471. [CrossRef][PubMed] 44. Savvidis, G.; Karanikas, V.; Zarkogianni, M.; Eleftheriadis, I.; Nikolaidis, N.; Tsatsaroni, E. Screen-Printing of Cotton with Natural Pigments: Evaluation of Color and Fastness Properties of the Prints. J. Nat. Fibers 2017, 14, 326–334. [CrossRef]