Renewable alternating aliphatic-aromatic poly(ester-urethane)sprepared from ferulic acid and bio-based diols

Mouandhoime Zahahe Oulame a,b,1, Florian Pion a,b,1, Shaik Allauddin c, Kothapalli V.S.N. Raju c,Paul-Henri Ducrot a,b,⇑, Florent Allais a,b,d,⇑a INRA UMR 1318 Institut Jean-Pierre Bourgin, F-78026 Versailles, Franceb AgroParisTech UMR 1318 Institut Jean-Pierre Bourgin, F-78026 Versailles, Francec Polymers & Functional Materials Division, Indian Institute of Chemical Technology, Hyderabad 500 607, Indiad AgroParisTech, Chaire Agro-Biotechnologies Industrielles (ABI), F-51100 Reims, France

a r t i c l e i n f o

Article history:Received 19 August 2014Received in revised form 20 November 2014Accepted 22 November 2014Available online 19 December 2014

Keywords:Aromatic poly(ester-urethane)BisphenolBiomassLignocelluloseFerulic acidBiobased polyols

a b s t r a c t

A new class of eight partially renewable alternating aliphatic-aromatic poly(ester-ure-thane)s (PEUs) is obtained through the polyaddition of four renewable bisphenols derivedfrom ferulic acid and bio-based diols on two commercially available diisocyanates. Thestructures and molecular weights of these PEUs are characterized by 1H and 13sC NMR, Fou-rier transform infrared spectroscopy (FT-IR), and size exclusion chromatography (SEC). ThePEUS are further studied by thermogravimetric analysis (TGA) and differential scanningcalorimetry (DSC). All these thermoplastic PEUs are transparent and rigid at room temper-ature, with Mn ranging from 4 to 10 kDa, and relatively narrow dispersities (1.7–3.5), proveto be stable up to more than 180 !C (Td 5%), with Tg varying from 28 to 128 !C depending ontheir structure.

" 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Polyurethanes (PUs), having an history of more than 70years, became one of the most dynamic groups of polymers,exhibiting versatile properties suitable for use in practicallyall fields of polymer applications such as foams, elastomers,thermoplastics, thermosets, adhesives, coatings, sealants,fibers) [1] and additionally, they are used in some specialtyapplications such as biomedical surgery. Presently, in theindustry, PUs are generally obtained by the reaction of a diolor an oligomeric polyol with isocyanates.

In recent years, increased interest appears in bio-basedmonomers and polymers. However, to be competitive,

bio-based products must exhibit functionalities at leastequivalent to those of the petroleum-based products. Thus,research should not be limited to pure and simple substitu-tion of existing products, but they must also propose inno-vative molecules and materials with new value-addedfeatures.

Many research programs on new bio-based monomers/additives and polymers are already ongoing in large publicand private research institutions [2]. The synthesis ofmonomers as well as polymers from plant fats, oils andsugars has already found some industrial applications,and recent developments in this field offer promisingnew opportunities [3]. More recently, scientists got inter-ested in the use of a new bio-based compounds issuedfrom [4]. However, all these resources mainly lead to ali-phatic compounds, whereas many key commercial chemi-cals involve aromatic structures [5]. Therefore, mostrecently, much focus has been placed on bio-based

http://dx.doi.org/10.1016/j.eurpolymj.2014.11.0310014-3057/" 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors at: INRA UMR 1318 Institut Jean-PierreBourgin, Route de Saint-Cyr, F-78026 Versailles, France.

E-mail addresses: ducrot@versailles.inra.fr (P.-H. Ducrot), florent.allais@agroparistech.fr (F. Allais).

1 These authors contributed equally to this work.

European Polymer Journal 63 (2015) 186–193

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aromatic compounds able to replace phenol, bisphenol A,styrene, phenol resins, aromatic polyesters and their bio-composites in industrial applications [6]. A promisingapproach consists in the preparation of new bio-basedmaterials using monomeric phenolics originating from lig-nocellulosic biomass [7]. Out of them, ferulic acid has beenidentified as a promising bio-based platform molecule.Many research groups have indeed recently demonstratedthe great potential of ferulic acid through the developmentof new homopolyesters exhibiting appealing thermal prop-erties [8–10]. Homo- and copolyesters of p-hydroxycin-namic acids showing remarkable liquid–crystal [11],biodegradability [12] or biocompatibility [11b] propertieswere also reported. However, they are often hard to pro-cess because of their too high melting temperature (Tm).To lower the Tm, p-hydroxycinnamic acids were copoly-merized with aliphatic monomers to increase the struc-tural flexibility of the resulting polymers [13]. Anotherapproach based on the synthesis of bisphenols from vanil-lin [14], for the preparation of alternating copolymers withaliphatic moieties was described. Following this approach,and having developed expertise in the chemistry of ligninrelated phenolic compounds [15], we recently focused onthe synthesis of a new class of bio-based molecules derivedfrom ferulic acid. We indeed designed a new methodologyfor the production of bisphenols incorporating ferulic acidand bio-based diols through a chemo-enzymatic processinvolving Candida antarctica lipase B (aka CAL-B, Novo-zyme#435) [16]. These bisphenolic compounds were effi-ciently condensed with renewable activated diacidchlorides providing alternating aliphatic-aromatic copoly-esters whose thermal properties could be finely tuned byplaying with the chemical structure of both diacid andbisphenol, giving access to copolyesters exhibiting Tg’sranging from 0 to 80 !C [17].

Herein we want to report the preparation, characteriza-tion and thermal properties of new alternating aliphatic-aromatic PEUs obtained through the polyaddition of thesenew bio-based bisphenols with two of the most widelyused isocyanates for PUs production on an industrial scale.Indeed, PEUs with aromatic urethane bonding structureshave shown good thermal degradation and mechanicalproperties. Based on overall properties analysis, such poly-mers are expected to be good candidates as protectivecoatings [18].

2. Experimental

2.1. Materials and methods

All reagents, including C. antarctica lipase B supportedon acrylic resin were purchased from Aldrich ChemicalCo. FT-IR spectra were recorded on a Thermo Nicolet Nexus470. 1H and 13C NMR spectra were recorded in CDCl3 on aBruker Ascend 400 at 400 MHz and 100.6 MHz respec-tively, reference in CDCl3: 7.26 ppm for 1H and 77.16ppm in 13C; in (CD3)2SO: 2.50 ppm in 1H and 39.52 in13C; d are given in ppm and J in Hz. Size exclusion chroma-tography (SEC) was performed in chloroform (CHCl3) witha Gilson 305 pump, an UltiMate 3000 Autosampler fromDionex, a Mixed-B column (600 mm ! 7.5 mm, 10 mm)

from PL calibrated with polystyrene standards at 25 !C,using UV detection at 254 nm with an Ultimate 3000PDA from Dionex. Results were calculated with CirrusGPC 3.0. Samples were dried in a vacuum-oven at 50 !Cfor 18 h prior to analysis. Thermo-Gravimetric Analysis(TGA) was recorded using a Q-500 from TA. About 5–10 mg of each sample were heated from 25 to 600 !C at10 !C/min under inert atmosphere. Differential ScanningCalorimetry (DSC) was recorded under inert atmosphereon a DSC 6000 from Perkin Elmer. About 5–10 mg of eachsample were weighted in an open pan that went to heat/cool/heat cycle at 10 !C/min. Data are given from the sec-ond heating scan.

2.2. Bisphenols preparation [16]

Available in ESI.

2.3. General procedures for polyurethanes preparation

2.3.1. Bulk polymerizationsIn a round-bottom flask under argon were successively

added bisphenol (EDF, PDF, BDF or IDF) (1 eq) and diisocy-anate (HDI or TDI) (1 eq). The reaction mixture was mag-netically stirred under argon and heated (100–140 !C) forthe given time. The resulting polymer was dissolved inchloroform, precipitated in cold methanol and recoveredby filtration.

2.3.2. In-solvent polymerizationsIn a round-bottom flask under argon were successively

added bisphenol (EDF, PDF, BDF or IDF) (1 eq), solvent(CHCl3, MEK, MIBK or 1,4-dioxane) and diisocyanate (HDIor TDI) (1 eq). The reaction mixture was magnetically stir-red under argon and heated (100–140 !C) for the giventime. The resulting polymer was precipitated in cold meth-anol and recovered by filtration.

2.3.3. Poly[1,6-hexamethylene diisocyanate-alt-(bis-O-dihydroferuloyl ethylene glycol)] (PHDI-EDF)

(Yields: 88% in MEK and 99% in bulk.)NMR: dH 400 MHz; CDCl3 6.98 (2 H, m, H7,70), 6.66–6.78

(4 H, m, H8,80 ,10,100), 5.22 (bs, Ar-OH), 5.17 (bs, OCONH), 4.26(4 H, s, H16,160), 3.81 (6 H, s, H12,120), 3.23 (4 H, m, H3), 2.85–2.93 (4 H, m, H13,130), 2.59–2.65 (4 H, m, H14,140), 1.56 (4 H,m, H2), 1.38 (4 H, m, H1). dC 100.6 MHz; CDCl3 172.9(s, C150), 172.7 (s, C15), 154.7 (s, C5), 151.6 (s, C11), 146.6(s, C60), 144.2 (s, C110), 138.9 (s, C9), 138.6 (s, C6), 132.4 (s,C90), 123.4 (d, C7), 120.9 (d, C80), 120.4 (d, C8), 114.5(d, C70), 112.7 (d, C10), 111.1 (d, C100), 62.3 (t, C16), 62.2 (t,C160), 56.0 (q, C12,120), 41.3 (t, C3), 36.1 (t, C140), 35.7(t, C14), 30.8 (t, C13), 30.6 (t, C130), 29.8 (t, C2), 26.4 (t, C1);FT-IR: mmax/cm"1 3360 (NH), 1732 (C@O), 1506 (C@C);UV: kmax(THF)/nm 242 and 278.

2.3.4. Poly[1,4-toluene diisocyanate-alt-(bis-O-dihydroferuloyl ethylene glycol)] (PTDI-EDF)

(Yields: 85% in MEK and 99% in bulk.)NMR: dH (400 MHz; (CD3)2SO) 10.62 (bs, H2), 9.36 (1 H,

m, H10) 7.61–6.59 (9 H, m, H4,6,7,13,14,18,130 ,140 ,180), 5.52 (bs,Ar-OH), 5.29 (bs, OCONH), 4.25 (4 H, s, H22,220), 3.76 (6 H,

M.Z. Oulame et al. / European Polymer Journal 63 (2015) 186–193 187

bs, H18,180), 2.88 (4 H, m, H19,190), 2.60 (4 H, m, H20,200), 2.21(3 H, s, H9). dC (100.6 MHz; CDCl3) 172.9 (s, C210), 172.8 (s,C21), 151.6 (s, C1,11), 146.6 (s, C120), 144.2 (s, C170), 139.4 (s,C15), 138.1 (s, C12), 136.6 (TDI), 136.0 (TDI), 132.4 (s, C150),131.0 (TDI), 123.2 (d, C13), 123.2 (TDI), 121.0 (d, C140), 120.4(TDI), 114.5 (d, C130), 112.8 (d, C16), 111.2 (d, C160), 62.4 (t,C22), 62.3 (t, C220), 56.0 (q, C18,180), 36.1 (t, C200), 35.7 (t,C20), 30.8 (t, C190), 30.7 (t, C19), 17.0 (q, C9); FT-IR: mmax/cm"1 3320 (NH), 1733 (C@O), 1509 (C@C); UV: kmax

(THF)/nm 242 and 279.

2.3.5. Poly[1,6-hexamethylene diisocyanate-alt-bis-O-dihydroferuloyl 1,3-propanediol)] (PHDI-PDF)

(Yields: 95% in MEK and 99% in bulk.)NMR: dH (400 MHz; CDCl3) 6.99 (2 H, m, H7,70), 6.59-

6.79 (4 H, m, H8,80 ,10,100), 5.22 (bs, Ar-OH), 5.12 (bs, OCONH),4.12 (4 H, m, H16,160), 3.82 (6 H, m, H12), 3.25 (4 H, m, H3),2.90 (4 H, m, H13,130), 2.61 (4 H, m, H14,140), 1.92 (2 H, m,H17), 1.57 (4 H, m, H2), 1.39 (4 H, m, H1). dC (100.6 MHz;CDCl3) 173.0 (s, C150), 172.9 (s, C15), 154.7 (s, C5), 151.7 (s,C11), 146.6 (s, C60), 144.2 (s, C110), 139.0 (s, C9), 138.6(s, C6), 132.5 (s, C90), 123.3 (d, C7), 121.0 (d, C80), 120.4 (d,C8), 114.5 (d, C70), 112.7 (d, C10), 111.1 (d, C100), 61.2 (t,C16), 61.1 (t, C160), 56.1 (q, C12), 56.0 (q, C120), 41.3 (t, C3),36.3 (t, C140), 35.9 (t, C14), 31.0 (t, C13), 30.8 (t, C130), 29.9(t, C2), 28.1 (t, C17), 26.4 (t, C1); FT-IR: mmax/cm"1 3350(NH), 1728 (C@O), 1506 (C@C); UV: kmax(THF)/nm 242and 278.

2.3.6. Poly[1,4-toluene diisocyanate-alt-bis-O-dihydroferuloyl1,3-propanediol)] (PTDI-PDF)

(Yields: 84% in MEK and 98% in bulk.)NMR: dH (400 MHz; (CD3)2SO) 10.65 (bs, H2), 6.52-7.40

(9 H, m, H13,14,16,130 ,140 ,160), 5.54 (bs, ArOH), 5.30 (bs,OCONH), 4.11 (4 H, m, H22,220), 3.77 (6 H, m, H18,180), 2.91(4 H, m, H19,190), 2.61 (4 H, m, H20,200), 2.24 (3 H, s, H9),1.90 (2 H, m, H23). dC (100.6 MHz; CDCl3) 173.0 (1s, C210),172.9 (s, C21), 151.6 (s, C1,11), 146.6 (s, C120), 144.2 (s,C170), 139.4 (s, C15), 138.2 (s, C12), 136.7 (TDI), 136.0(TDI), 132.5 (s, C150), 131.1 (TDI), 123.3 (d, C13), 123.2(TDI), 121.0 (d, C140), 120.5 (TDI), 114.5 (d, C130), 112.8 (d,C16), 111.1 (d, C160), 61.2 (t, C22,220), 56.0 (q, C18,180), 36.3(t, C200), 35.9 (t, C20), 31.0 (t, C19), 30.8 (t, C190), 17.1 (q,C9); FT-IR: mmax/cm"1 3336 (NH), 1731 (C@O), 1509(C@C); UV: kmax(THF)/nm 242 and 279.

2.3.7. Poly[1,6-hexamethylene diisocyanate-alt-bis-O-dihydroferuloyl 1,4-butanediol)] (PHDI-BDF)

(Yields: 91% in MEK and 93% in bulk.)NMR: dH (400 MHz; CDCl3) 6.98 (2 H, m, H7,70), 6.67-

6.82 (4 H, m, H8,80 ,10,100), 5.56 (bs, ArOH), 5.12 (bs, OCONH),4.07 (4 H, m, H16,160), 3.81 (6 H, m, H12,120), 3.24 (4 H, m, H3),2.90 (4 H, m, H13,130), 2.60 (4 H, m, H14,140), 1.63 (4 H, m,H17), 1.57 (4 H, m, H2), 1.39 (4 H, m, H1). dC (100.6 MHz;CDCl3) 173.1 (s, C150), 173.0 (s, C15), 154.7 (s, C5), 151.7 (s,C11), 146.6 (s, C60), 144.2 (s, C110), 139.1 (s, C9), 138.6 (s,C6), 132.5 (s, C90), 123.3 (d, C7), 121.0 (d, C80), 120.4(d, C8), 114.5 (d, C70), 112.7 (d, C10), 111.1 (d, C100), 64.1(t, C16), 64.0 (t, C160), 56.0 (q, C12,120), 41.3 (t, C3), 36.4 (t,C140), 36.0 (t, C14), 31.0 (t, C13), 30.8 (t, C130), 29.9 (t, C2),

26.4 (t, C1), 25.4 (t, C17); FT-IR: mmax/cm"1 3318 (NH),1703 (C@O), 1510 (C@C); UV: kmax(THF)/nm 242 and 279.

2.3.8. Poly[1,4-toluene diisocyanate-alt-bis-O-dihydroferuloyl1,4-butanediol)] (PTDI-BDF)

(Yields: 93% in MEK and 99% in bulk.)NMR: dH (400 MHz; CDCl3) 10.67 (m, H2), 7.54-6.57 (9

H, m, H4,6,7,13,130 ,14,140 ,16,160), 5.52 (bs, Ar-OH), 4.06 (4 H, m,H22,220), 3.72 (6 H, m, H18,180), 2.87 (4 H, m, H19,190), 2.61(4 H, m, H20,200), 2.21 (3 H, s, H9), 1.57 (4 H, m, H23). dC

(100.6 MHz; (CDCl3) 173.3 (s, C210), 173.1 (s, C21), 151.6(s, C1), 151.3 (s, C11), 146.6 (s, C120), 144.2 (s, C170), 139.5(s, C15), 138.1 (s, C12), 136.8 (TDI), 136.0 (TDI), 132.5(s, C150), 131.1 (TDI), 123.3 (d, C13), 121.0 (d, C140), 120.4(TDI), 114.5 (d, C130), 112.7 (d, C10), 111.1 (d, C160), 64.2 (t,C22), 64.0 (d, C220), 56.0 (q, C18,180), 36.4 (t, C200), 36.0(t, C20), 31.1 (t, C19), 30.9 (t, C190), 25.4 (t, C23,230), 17.1 (q,C9); FT-IR: mmax/cm"1 3350 (NH), 1730 (C@O), 1509(C@C); UV: kmax(THF)/nm 241 and 279.

2.3.9. Poly[1,6-hexamethylene diisocyanate-alt-bis-O-dihydroferuloyl isosorbide)] (PHDI-IDF)

(Yields: 93% in MEK and 99% in bulk.)Because of the asymmetry of isosorbide, multiple peaks

are observed for both 1H and 13C NMR spectra with strongoverlapping that did not allow unambiguous assignments.

NMR: dH (400 MHz; CDCl3) 6.98 (2 H, m, H7,70), 6.65-6.82 (4 H, m, H8,80 ,10,100), 5.58 (bs, OCONH), 5.17 (1 H, s,H160), 5.14 (1 H, m, H16), 4.75 (1 H, m, H180), 4.31 (1 H, m,H18), 3.73–3.90 (4 H, m, H17,170), 3.81 (6 H, s, H12,120), 3.23(4 H, m, H3), 2.92 (4 H, m, H13,130), 2.66 (4 H, m, H14,140),1.56 (4 H, m, H2) 1.39 (4 H, m, H1). dC (100.6 MHz; CDCl3)172.3 (s, C150), 172.0 (s, C15), 154.6 (s, C5), 151.7 (s, C11),146.6 (s, C60), 144.2 (s, C110), 138.7 (s, C9), 138.7 (s, C6),123.3 (d, C7), 121.0 (d, C80), 120.4 (d, C8), 114.5 (d, C70),112.7 (d, C10), 111.1 (d, C100), 86.0 (d, C180), 80.9 (d, C18),78.1 (d, C160), 74.1 (d, C16), 73.3 (t, C170), 70.5 (t, C17), 56.1(q, C12,120), 41.3 (t, C3), 36.2 (t, C14), 35.6 (d, C140), 30.9 (t,C13,130), 29.8 (t, C2), 26.4 (t, C1); FT-IR: mmax/cm"1 3360(NH), 1734 (C@O), 1509 (C@C); UV: kmax(THF)/nm 242and 278.

2.3.10. Poly[1,4-toluene diisocyanate-alt-bis-O-dihydroferuloyl isosorbide)] (PTDI-IDF)

(Yields: 95% in MEK and 99% in bulk.)Because of the asymmetry of isosorbide, multiple peaks

are observed for both 1H and 13C NMR spectra with strongoverlapping that didn0t allow unambiguous assignments.

NMR: dH (400 MHz; (CD3)2SO) 10.47 (m, H2), 8.00–6.50(9 H, m, H4,6,7,13,130 ,14,140 ,16,160), 5.65 (bs, Ar-OH), 5.45-5.13 (2H, m, H22,220), 4.76 (0.7 H, m, H240), 4.62 (0.3 H, m, H240), 4.37(0.3 H, m, H24), 4.28 (0.7 H, m, H24), 4.02-3.68 (10 H, m,H18,180 ,23,230), 2.94 (4 H, m, H19,190), 2.68 (4 H, m, H20,200),2.27 (3 H, m, H9). dC (100.6 MHz; CDCl3) 171.7 (s, C210),171.6 (s, C21), 151.7 (s, C1,11), 146.6 (s, C120), 144.3 (s,C170), 139.5 (s, C15), 138.1 (s, C12), 136.8 (TDI), 136.0(TDI), 132.5 (s, C150), 132.3 (s, C150), 131.1 (TDI), 123.2(d, C13), 121.0 (d, C140), 120.5 (TDI), 114.4 (d, C130), 112.8(d, C10), 111.1 (d, C160), 86.0 (d, C240), 80.9 (d, C24), 78.0 (d,C220), 74.2 (d, C22), 73.2 (t, C230), 70.4 (t, C23), 56.0(q, C18,180), 36.0 (t, C200), 35.2 (t, C20), 31.0 (t, C19,190), 17.1

188 M.Z. Oulame et al. / European Polymer Journal 63 (2015) 186–193

(q, C9); FT-IR: mmax/cm"1 3331 (NH), 1734 (C@O), 1507(C@C); UV: kmax(THF)/nm 242 and 279.

3. Results and discussion

Four renewable bisphenols, namely EDF, PDF, BDF andIDF, were prepared from ferulic acid and respectively eth-ylene glycol, 1,3-propanediol, 1,4-butanediol or isosorbide,as previously described [16].

Isosorbide, was primarily chosen for its robust bicyclicstructure [19], providing high thermal and mechanicalproperties to the resulting polymers [20]. The influenceof its bicyclic structure was demonstrated by comparisonof the final properties of the resulting polymers with thoseobtained with aliphatic segments of different length suchas ethylene glycol, 1,3-propanediol and 1,4-butanediol.Bisphenols EDF, PDF, BDF and IDF were reacted with twocommercially available diisocyanates – aliphatic 1,6-hex-amethylene diisocyanate (HDI), and aromatic 1,4-toluenediisocyanate (TDI) – in various conditions to form eightthermoplastic PEUs through a thermal process thus avoid-ing the use of metal catalysts (Scheme 1).

Having chosen a thermal polymerization process for thesynthesis of these PEUs, all polyadditions were conductedat relatively high temperatures under inert atmosphere(argon), in bulk as well as in solvent (ca. 100–140 !C and60–100 !C, respectively), with a ratio NCO:OH fixed at1:1. After cooling the reaction mixtures to room tempera-ture, the resulting polymers were directly recovered byprecipitation in cold methanol (for solvent polyadditions)or after dissolution in chloroform prior precipitation (forbulk polyadditions). Formation of alternating aliphatic-aromatic PEUs was supported by NMR, FT-IR and SEC anal-yses. FT-IR and SEC were used to follow the kinetics of

polyadditions. Average molecular weights and polydisper-sities were determined by SEC using calibration with poly-styrene standards. The thermal properties were evaluatedby TGA and DSC under inert atmosphere.

3.1. Optimization of polyaddition parameters – Polymercharacterization

In order to optimize the preparation of PEUs, two mainparameters were studied: the polymerization temperatureand the reaction media (in bulk or in solvent). Polyadditionkinetics were studied with a fixed system PDF/HDI.

We first considered the temperature and ran experi-ments in bulk at 100, 120 and 140 !C. As expected, dueto the low reactivity of phenols in polyurethane systems,raising the temperature to 140 !C was required to achievehigher Mw in reasonable reaction times. Firstly polyaddi-tions were performed without solvent, but this led tohighly viscous systems almost gel in nature at 140 !C. Thus,low boiling point or high evaporation rate solvents, thatare easier to remove from PEUs after precipitation, wereselected as viscosity reducer. Chloroform, methyl ethylketone (MEK), 1,4-dioxane and methyl isobutyl ketone(MIBK) were therefore considered and tested in polyaddi-tion reactions at their respective boiling temperature witha concentration of 1000 g/L (total weight of monomers).MIBK was not selected to run the polymerization reactionin homogenous media as it only dissolved monomers andshort oligomers but not the polymers.

Boiling point of chloroform (62.1 !C) proved too low toallow polymers formation in reasonable time. MEK and1,4-dioxane both solubilized this system and possessedhigh boiling points (79.6 and 101.1 !C respectively) ableto lead to interesting kinetics. Faster kinetic being

Scheme 1. Synthesis of eight PEUs from two commercially available isocyanates (HDI, TDI) and four bio-based bisphenols (EDF, PDF, BDF and IDF).

M.Z. Oulame et al. / European Polymer Journal 63 (2015) 186–193 189

observed at lower temperature with MEK, the latter waspreferred to conduct polymerizations.

From these kinetic studies, the two polyadditions pro-cedures optimized for the PDF/HDI system were appliedto the eight A2/B2 monomeric systems (four bisphenolsand two diisocyanates): polyaddition in bulk at 140 !C dur-ing 72 h, and polyaddition in MEK at 1000 g/L under refluxduring 72 h (Fig. 1). Comparing bulk and in-solvent poly-merizations (Table 1, entries 1, 3, 5, 7, 9, 11, 13, 15 vs. 2,4, 6, 8, 10, 12, 14, 16) demonstrated the interest of usingMEK as viscosity reducer in polymerization media(1000 g/L). Indeed, for the same reaction time (72 h) andsystem (A2/B2), higher Mn and DPw were obtained in MEKcompared to the bulk procedure (Mn: 5–10 vs. 1.7–4 kg mol"1, DPw: 16–38 vs. 6–24) as well as higher conver-sion rates, despite the lower temperature (80 vs. 140 !C).Strangely, except for EDF- and PDF-based PEUs, resultswere quite similar when either HDI or TDI was used,despite their well-known difference of reactivity. It isimportant to note that all resulting polyurethanes aretransparent or can be obtained as white powders whenprecipitated in cold methanol.

3.2. Spectral characterization

3.2.1. FT-IR analysesMonitoring the reaction by FT-IR spectroscopy showed

the progressive disappearance of the absorption band

corresponding to the isocyanate moiety (ca. 2270 cm"1)and the presence of aromatic carbon-carbon bounds, esterlinkages coming from the bisphenols and urethane bounds(ca. 1720 cm"1, ca. 1730 and 1500 cm"1, respectively). Theevolution of the isocyanate signal can be therefore used toaccurately follow the consumption of isocyanate duringthe polyaddition process (Fig. 1).

3.2.2. NMR analysesIn 13C NMR spectroscopy, PEUs formation is supported

by the appearance of a new carbonyl peak correspondingto the urethane moiety (ca. 155 ppm for HDI-based struc-tures and 151 ppm for TDI-based structures). Moreover,when a phenol was reacted with an isocyanate moiety, ashift of the aromatic carbon signals was observed, thusallowing an easy monitoring of the consumption of the phe-nol moieties (Fig. 2). In the case of TDI-based PEUs, TDIbeing commercially available as a mixture of two isomers,the aromatic carbons zone is poorly defined, thus prevent-ing an easy monitoring of the polyaddition (see correspond-ing 13C NMR spectra in Supporting Information). However,the incorporation of TDI units during the polyaddition issupported by the appearance of a well-defined methyl sig-nal (around 17 ppm in 13C and 2.2 ppm in 1H). In 1H NMRspectra, conversion of phenols into urethanes was also sup-ported by the disappearance of the phenoxyl signal (5.5–6.0 ppm) concomitant with the appearance of the urethaneone (5.0–5.5 ppm, AC(O)ANHA). Moreover, the regular

Fig. 1. Monitoring of isocyanate consumption by FT-IR.

190 M.Z. Oulame et al. / European Polymer Journal 63 (2015) 186–193

alternating nature of the PEUs was supported by theintegration of the proton signals from the 1H NMR spectraconfirming the 1:1 bisphenol:diisocyanate stoichiometry.

Unfortunately, because of overlapping and also differ-ent end-units for the polymers (phenol–phenol, phenol–isocyanate or isocyanate–isocyanate) due to the use of a1:1 monomer ratio, the determination of average molecu-lar weights was not possible through 1H NMR spectros-copy. Heteronuclear 2D correlations were also performedbut did not afford supplementary informations.

3.3. Thermal characterization

3.3.1. TGA analysesThermal stability of the eight PEUs was characterized by

TGA under nitrogen at 10 !C/min (Fig. 3).By changing the diisocyanate structure in polymer

backbone, thermal stability of coating films is also chan-ged. The early stage degradation temperature of PEU filmsbased on TDI is lower than PEU films based on HDI.Moreover, the number of methylene units in the polymer

Table 1Results of the polymerizations of the 8 poly(ester-urethane)s conducted during 72 h either in bulk at 140 !C or in MEK (1000 g/L) at reflux.

Entry Polymer Media Mna (!103 g mol"1) Mw

a (!103 g mol"1) Conversion (%)b ÐM DPwa Td (5%) (!C)c Td (50%) (!C)c Tg (!C)d

1 PHDI-EDF Bulk 1.7 4.2 88 2.5 7 221 333 342 MEK 7.1 12.8 >99 1.8 22 220 340 403 PTDI-EDF Bulk 1.8 3.7 85 2.0 6 200 319 804 MEK 8.6 19.0 >99 2.2 32 201 325 965 PHDI-PDF Bulk 3.8 14.6 95 3.8 24 213 311 376 MEK 10.3 22.6 >99 2.2 37 225 338 387 PTDI-PDF Bulk 2.2 7.1 84 3.1 12 197 321 728 MEK 4.8 9.7 98 2.0 16 203 332 749 PHDI-BDF Bulk 2.8 5.9 91 2.0 10 225 336 28

10 MEK 5.4 10.6 93 2.0 18 224 326 2811 PTDI-BDF Bulk 2.9 7.3 93 2.5 11 184 330 6212 MEK 6.6 12.3 99 1.8 20 214 320 7813 PHDI-IDF Bulk 2.6 6.4 93 2.4 10 223 360 6714 MEK 11.7 25.5 >99 2.2 38 228 357 6815 PTDI-IDF Bulk 2.6 8.0 95 3.0 12 211 362 10616 MEK 7.9 24.2 >99 3.1 36 204 348 127

a Values determined by SEC analysis with polystyrene calibration.b Calculated with the residual monomer amounts determined from the corresponding peak area on SEC traces of crude reaction mixtures prior

precipitation.c Values determined by TGA under nitrogen at 10 !C/min.d Values determined by DSC under nitrogen at 10 !C/min.

Fig. 2. 13C NMR signals for reacted and unreacted phenol moieties (PHDI-BDF).

M.Z. Oulame et al. / European Polymer Journal 63 (2015) 186–193 191

backbone has a significant effect on the thermal stability ofthe prepared PEU samples since increasing the carbonchain length also increases the thermal stability of coatingfilms. All polymers were stable up to more than 180 !C (Td

5%), an interesting feature for polyurethanes systems [21].As expected, compared to their HDI-based counterparts,

TDI-based polymers showed lower thermostability due tothe lower stability of aromatic–aromatic urethane linkagescompared to aromatic–aliphatic ones [22].

Similarly to what was observed with the previouslyreported copolyesters derived from these ferulic acid-basedbisphenols [17], varying the bisphenol structure had a lowimpact on the thermal stability of the resulting PEUs.

3.3.2. DSC analysesAs shown in Table 1 and Fig. 4, PEUs obtained from the

polyaddition of aliphatic HDI and EDF, PDF and BDFbisphenols prepared from aliphatic diol in MEK showedthe lowest Tg (28–41 !C) demonstrating that the shorterthe internal diol, the higher the Tg (EDF > PDF > BDF,derived respectively from ethylene glycol, 1,3-propanedioland 1,4-butanediol). The use of the bisphenol preparedfrom isosorbide (IDF), because of its rigid bicyclic struc-ture, considerably increased the Tg of the resulting poly-mers (up to 128 !C). The increase of chain flexibilitywhile introducing methylene units does play a major partin the crystallization rate. By increasing the methylenenumber, from EDF to BDF, the chain becomes more flexibleand the crystallization process faster. Accordingly, thehigher flexibility of the chains in BDF causes a lower Tg

value with respect to EDF. From the isocyanate point ofview, for a given bisphenol, replacing aliphatic HDI by aro-matic TDI dramatically increased the Tg due to the higheraromatic content.

The changes in Tg (Table 1, 28–128 !C) associated withthe nature of the internal diol and isocyanate structureindicate that the aromatic based PEUs have a wider glasstransition region over which their mechanical propertieswill vary as compared to the aliphatic based PEUs. Theeffect of hard segment length impacts PU films propertiesfrom dilute solution to bulk. McKay et al. [23] found thatmethyl ethyl ketone was the most effective among numer-ous common solvents tested for the viscosity reduction ofhydroxyl bearing high solids oligomers. Segments firstinteract intramolecularly and intermolecularly, whichfavors more compact conformation, finally resulting in acluster formation. [24] These results confirm that the PEUsin MEK solution exhibit very different structures comparedto that bulk PUs [25].

4. Conclusion

We present herein the synthesis, structural character-ization and thermal properties of a new class of renewablealiphatic-aromatic poly(ester-urethane)s. Their prepara-tions involve bio-based bisphenols and commerciallyavailable isocyanates through a catalyst-free thermal pro-cess that can be conducted in bulk as well as in solvent.All these PEUs are transparent and rigid at room tempera-ture, or can be obtained as white powders when precipi-tated in cold methanol. These new aromatic basedpoly(ester-urethane)s exhibit a high thermal stability anda wide range of Tg according to the aliphatic segmentsassociated to ferulic acid and the nature of the isocyanate.

Though these PEUs were not obtained with relativelyhigh DPIw and Mw values, these preliminary results never-theless constitute a proof of concept and demonstrate thepotentialities of such alternating aliphatic–aromatic PEUs.Indeed, these low molecular weight PEUs may be used asaliphatic–aromatic prepolymers in applications such ascoatings. The biodegradability and mechanical propertiesof these new polymers will be studied soon and reportedin due course. Further work will also address improvingthe efficiency of the polymerization in terms of Mw valuesby using other polyaddition methods (e.g., metal-catalyzedpolyaddition).

Acknowledgements

The authors are grateful to the Region Ile-de-France forproviding Florian Pion with a PhD fellowship. Thanks alsogo to Armando F. Reano for his help in TGA analysis, andto Laurent Cézard for his priceless technical help with theSEC system.

Appendix A. Supplementary material

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.eurpolymj.2014.11.031.

180

190

200

210

220

230

EDF PDF BDF IDF

T d5%

(°C)

TDI HDI

Fig. 3. 5% weight loss temperature (Td 5%, determined by TGA undernitrogen) of the eight polyurethanes (MEK, 1000 g/L).

0

50

100

150

T g (°

C)

HDI TDIEDF PDF BDF IDF

Fig. 4. Glass transition temperatures (Tg, determined by DSC) of the eightpolyurethanes (MEK, 1000 g/L).

192 M.Z. Oulame et al. / European Polymer Journal 63 (2015) 186–193

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