Fructose-Based Acrylic Copolymers by Emulsion Polymerization - MDPI

polymers Article

Fructose-Based Acrylic Copolymers by Emulsion Polymerization Jessica S. Desport, Mónica Moreno and María J. Barandiaran *

ID

POLYMAT, University of the Basque Country UPV/EHU, 20018 Donostia-San Sebastián, Spain; [email protected] (J.S.D.); [email protected] (M.M.) * Correspondence: [email protected]; Tel.: +34-943-015-330 Received: 12 March 2018; Accepted: 27 April 2018; Published: 2 May 2018

Abstract: The exploration of a renewable resource for the preparation of waterborne copolymers was conducted. Low molar mass sugar resources were selected for their wide availability. A fructose-based monomer (MF) bearing a methacrylate radically polymerizable group was successfully synthesized. The latter was shown to be able to homopolymerize in emulsion. The high Tg of the resulting polymer (about 115 ◦ C) makes it of particular interest for adhesive and coating applications where hard materials are necessary to ensure valuable properties. As a result, its incorporation in waterborne acrylic containing formulations as an equivalent to petrochemical-based methyl methacrylate was investigated. It was found that the bio-based monomer exhibited similar behavior to that of common methacrylates, as shown by polymerization kinetics and particle size evolution. Furthermore, the homogeneous incorporation of the sugar units into the acrylate chains was confirmed by a unique glass transition temperature in differential scanning calorimeter (DSC). The potential of MF for the production of waterborne copolymers was greatly valued by the successful increase of formulation solids content up to 45 wt %. Interestingly, polymer insolubility in tetrahydrofurane increased with time due to further reactions occurring in storage. Most likely, the partial deprotection of sugar units was the reason for the creation of hydrogen bonding and, thus, physically insoluble entangled chains. This behavior highlights opportunities to make use of hydroxyl groups either for further functionalization or, eventually, for achieving enhanced adhesion on casted substrates. Keywords: fructose; monosaccharide; carbohydrates; renewable resources; emulsion polymerization

1. Introduction The increasing environmental regulations and strong public concern have motivated the polymer industry to move towards more environmentally friendly processes, and emulsion polymerization is, nowadays, one of the most widespread green strategies. However, these waterborne processes relied mostly on petroleum-based chemistry. This approach is non-sustainable because the feedstocks need millions of years to be formed. Therefore, the search for alternatives based on renewable resource monomers for the production of novel polymers able to substitute their petroleum-based counterparts is eagerly sought. These polymer classes may contribute to reducing the environmental impact of polymer production by reducing the carbon footprint [1]. The potential to use either natural polymers, such as casein [2], or monomers from biomass feedstocks, such as fatty acid derivatives [3,4], as raw materials to produce waterborne latexes for industrial applications has already been demonstrated. On the other hand, carbohydrates represent a rich class of natural products in terms of availability, functionality, and molecular weights. Indeed, carbohydrates represent about 75% of the annual renewable biomass (200 billon ton approximately), with only around 4% used by humans [5]. Moreover, a substantial part of the carbohydrates commercially available in the food industry are regrettably wasted as a surplus of their agricultural Polymers 2018, 10, 488; doi:10.3390/polym10050488

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production or through domestic wastage. The valorization of those food wastes appears more relevant than ever. In this context, the use of low molecular weight carbohydrates to produce bio-based monomers able to polymerize through free radical polymerization in aqueous dispersed media emerges as a promising approach towards sustainable development. However, their raw structure, which mostly consists of alcoholic functions, does not allow their direct use as monomers in conventional radical polymerization. The presence of a reactive vinyl or (meth)acrylic group in the monomer is required. Moreover, the molecule has to be hydrophobic enough to be polymerized in aqueous dispersed media. This requires the synthesis of well-defined monofunctionalized (meth)acrylate hydrophobic saccharides. Indeed, the use of hydrophobic sugar monomers in emulsion polymerization has not received much attention. Only a few examples can be cited, and all of them are based on the protected methacryloyl glucose (3-O-methacryloyl-1,1,5,6-diisopropylydene-D-glucose or 3-MDG). In the 80s, Klein et al. [6] investigated the emulsion polymerization of 3-MDG at solids content of up to 15 wt %. Using non-ionic emulsifiers, they stated conversions of up to 92%. Forcada et al. [7,8] studied the emulsion copolymerization of 3-MDG for the production of microgels for biomedical applications. Takasu et al. [9] investigated the seeded emulsion copolymerization of VAc with vinyl sugars and focused their investigations on the resulting accelerated rate of biodegradation of the material. On the other hand, Yaacoub and co-workers widely investigated the 3-MDG homopolymerization in emulsion at low solids content from 5 to 20 wt % [10]. The same group studied the batch [11] emulsion copolymerization of 3-MDG with butyl acrylate at 10 wt % solids content and subsequently succeeded in increasing the solids content up to 50 wt % with reasonable coagulum amounts by using semi-continuous processes [12–14]. They also extended their investigations to free radical [15] and RAFT mini-emulsion copolymerization [16]. In this work, and to further expand the range of application of those carbohydrates, the use of fructose as the renewable monosaccharide source for the production of bio-sourced monomers and waterborne polymers has been explored. Indeed, fructose can be obtained at an industrial scale from fruit waste. Nevertheless, very rare examples of the use of fructose as building blocks for bio-based polymers can be found in literature. Methacrylate fructose was cited as a potentially interesting monomer for the production of waterborne coatings [17]. However, to the best of our knowledge, actual polymerization results have not been reported yet. More recently, methacrylate fructose was used to synthesize nanocarriers via RAFT polymerization for biomedical applications [18]. For the purpose of this research, fructose-based monosaccharide bearing acetonide protecting groups and being commercially available was envisaged as valuable precursor. This structure was selected because it presents a primary alcohol situated in the anomeric position, which allows high selectivity in terms of functionalization, as well as high reactivity towards a common nucleophile. The incorporation of a methacrylate polymerizable group in the above-mentioned precursor was investigated first. Next, the potential use of the resulting product as a substitute of petrochemical raw materials commonly used in waterborne acrylic containing formulations—typically used as adhesives or coatings—was explored. As a first approach, low solids content emulsion polymerization with different sugar-based monomer/butyl acrylate (BA) compositions was studied. Taking advantage of the understanding gained in this first part, the study was extended to high solids content systems, which are very important for effective industrial applications. In all the cases, polymerization kinetics, polymer characteristics, thermal properties, and the quality of the resulting films are reported and discussed. 2. Materials and Methods 2.1. Materials All reactants were used without prior purification. 2,3:4,5-Di-O-isopropylidene-b-D-fructopyranose, also called diacetone-D-fructose (DAF, >98%), was obtained from Carbosynth (Compton, UK). Butyl

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acrylate (BA, 99.5%) was purchased from Quimidroga (Barcelona, Spain). Methacrylic anhydride (94%), triethylamine (NEt3, >99.5%), 4-dimethylaminopyridine (DMAP, >99%), potassium Polymers 2018, 10, x FOR PEER REVIEW 3 of 11 persulfate (KPS, >99%), sodium dodecyl sulphate (SDS, >98.5%), and NaHCO3 (99.7%) were from (94%),(Steinheim, triethylamine (NEt3, >99.5%),Solvents 4-dimethylaminopyridine (DMAP, potassium persulfate Sigma-Aldrich Germany). were purchased from>99%), Acros-Organics (Geel, Belgium) (KPS, >99%), sodium dodecyl sulphate (SDS, >98.5%), and NaHCO3 (99.7%) were from Sigma-Aldrich and Sigma-Aldrich. 2.2.

(Steinheim, Germany). Solvents were purchased from Acros-Organics (Geel, Belgium) and SigmaAldrich. Synthesis of the Diacetone-D-Fructose Monomer 2.2. Synthesis of the Diacetone-D-Fructose Monomer

The transesterification reaction was done in dichloromethane (DCM) at an ambient temperature The(1transesterification reaction was done in dichloromethane (DCM) at anofambient by mixing DAF eq) and methacrylic anhydride (1 eq) in the presence NEt3 temperature and DMAP (0.2 eq), by mixing DAF (1 eq) and methacrylic anhydride (1 eq) in the presence of NEt3 and DMAP (0.2 eq,) as catalysts, under a nitrogen atmosphere (Scheme 1). as catalysts, under a nitrogen atmosphere (Scheme 1).

+

NEt3, DMAP, DCM 25 °C, 48 h

DAF

MF

1. Functionalization of protected fructose in solvent at ambient temperature. Scheme Scheme 1. Functionalization of protected fructose in solvent at ambient temperature.

Product isolation was performed through extraction in DCM, drying over Na2SO4, and solvent evaporation. This wasperformed followed by column chromatography, 85 wt %over yield.Na The resulting Product isolation was through extraction inobtaining DCM, adrying 2 SO4 , and solvent methacrylate protected fructose monomer (named MF hereafter) has a melting point in the range of evaporation. This was followed by column chromatography, obtaining a 85 wt % yield. The resulting 30–35 °C.

methacrylate protected fructose monomer (named MF hereafter) has a melting point in the range of 30–35 ◦ C. 2.3. Emulsion Polymerization Batch emulsion polymerization of MF was performed in a 250 mL double wall glass reactor,

2.3. Emulsion Polymerization equipped with a turbine stirrer, a nitrogen inlet, a reflux condenser, a temperature probe, and a

sample device. The reaction temperature was controlled by a Lauda water bath. The sugar-based

Batchmonomer emulsion polymerization of MF was performed in a 250 mL double wall glass reactor, was heated up and introduced as a melted substance in the reactor containing an aqueous equipped with turbine stirrer, awater, nitrogen inlet,(SDS, a reflux condenser, a temperature probe, preand a sample phase amade of deionized emulsifier 4 %wbm), and buffer (NaHCO3, 4 %wbm) heated at 60 °C. The pre-emulsion prepared under purgingbath. and strong stirring monomer device. The reaction temperature waswas controlled by a nitrogen Lauda water Theagitation sugar-based (800 rpm). the temperature increased to 70 °C and the initiator 2 %wbm), an previously was heated up andThen, introduced as a was melted substance in the reactor(KPS, containing aqueous phase dissolved in water, was introduced in the media. A low solids content of 10 wt % was targeted. made of deionized water, emulsifier (SDS, 4 %wbm), and buffer (NaHCO , 4 %wbm) pre-heated at 3 Emulsion copolymerization reactions were carried out in batches by varying the bio-based 60 ◦ C. Themonomer pre-emulsion was prepared under nitrogen purging and strong agitation stirring content and the initiator amount in some cases. A 30 wt % solids content, as well as 45 wt % (800 rpm). ◦ C and the solids latexes, were of initiator the emulsion, the organic phasepreviously composed by dissolved Then, the temperature wassynthesized. increasedFor to the 70 preparation (KPS, 2 %wbm), the monomer mixture (MF and BA) and the aqueous containing emulsifier the buffer, used in water, was introduced in the media. A low solidsphase content of 10thewt % wasand targeted. to avoid drops in pH during the reaction, were mixed under magnetic stirring (15 min at 900 rpm). The Emulsion copolymerization reactions were carried out in batches by varying the bio-based resultant mixture was poured in a double wall glass reactor equipped with a condenser, a nitrogen monomer content and the initiator amount inturbine some stirrer. cases. Reactions A 30 wtwere % solids inlet, a sample device and a stainless steel carriedcontent, out at 70 as °C well underas 45 wt % agitation (250synthesized. rpm) at a pH of For approximately 9. The initiator KPSemulsion, was injectedthe as aorganic shot whenphase the pre-composed solids latexes, were the preparation of the emulsion mixture temperature reached °C, and defining beginningphase of the reaction. The system was then and the by the monomer (MF and70BA) thethe aqueous containing the emulsifier allowed to react in batches. Samples were withdrawn at regular intervals and the inhibitor buffer, used to avoid drops in pH during the reaction, were mixed under magnetic stirring (15 min at (hydroquinone) was used to stop reaction and allow monomer conversion follow-up time, which was 900 rpm). The resultant mixtureAwas poured in a double glass reactor equipped a condenser, performed by 1H-NMR. standard formulation for wall the synthesis of 30 wt % solidswith content includeddevice 2 %wbmand SDS,a1 stainless %wbm KPS, and 1.5 %wbmstirrer. NaHCO3.Reactions were carried out at a nitrogencopolymers inlet, a sample steel turbine The copolymer composition in of terms of both the weight the molar of the as a shot 70 ◦ C under agitation (250 rpm) at a pH approximately 9. Theand initiator KPSfraction was injected investigated latexes, together with the nomenclature to be used throughout this manuscript, is when the pre-emulsion temperature reached 70 ◦ C, defining the beginning of the reaction. The system reported in Table 1. The first number of the latex name refers to the weight percentage of the sugarwas then allowed to react in the batches. were withdrawn regular intervals and the inhibitor based monomer while second Samples number corresponds to the solidsatcontent. (hydroquinone) was used to stop reaction and allow monomer conversion follow-up time, which was performed by 1 H-NMR. A standard formulation for the synthesis of 30 wt % solids content copolymers included 2 %wbm SDS, 1 %wbm KPS, and 1.5 %wbm NaHCO3. The copolymer composition in terms of both the weight and the molar fraction of the investigated latexes, together with the nomenclature to be used throughout this manuscript, is reported in Table 1. The first number of the latex name refers to the weight percentage of the sugar-based monomer while the second number corresponds to the solids content.

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Table 1. Studied methacrylate sugar-based copolymer compositions (in terms of both weight and molar fractions) and their nomenclature. Latex Name Low Solids

High Solids

MF20-BA/30

MF20-BA/45

MF30-BA/30 MF30-BA/45 Polymers 2018, 10, x FOR PEER REVIEW MF40-BA/30

MF40-BA/45

MF/BA (wt %)

MF/BA (mol %)

20/80 30/70 40/60

9/91 14/86 21/79

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Table 1. Studied methacrylate sugar-based copolymer compositions (in terms of both weight and molar fractions) and their nomenclature.

2.4. Characterization

Latex name

MF/BA

MF/BA

1 H-NMR (wt %) %) Samples were withdrawn during a High polymerization reaction and (mol spectroscopy was used Low Solids Solids MF20-BA/30NMRMF20-BA/45 20/80 was carried 9/91 out with 50 µL of latex and to monitor monomer conversion. sample preparation MF30-BA/30 MF30-BA/45 30/70 450 µL of a solution containing benzene-1,3,5-tricarboxylic acid (BTC) 14/86 as a reference at a concentration MF40-BA/30 MF40-BA/45 40/60 21/79 1 of 1 g/L. H-NMR spectra were recorded on Bruker 400 MHz equipment, using the WATERGATE method to2.4. suppress the signal of water, and theintegration of the vinyl proton signals and reference Characterization signal was performed. Samples were withdrawn during a polymerization reaction and 1H-NMR spectroscopy was used monitor monomer conversion. NMR sample preparation was using carried out with 50 µLlight of latexscattering and 450 The to Z-average particle diameter was determined dynamic (DLS, µL of a Z, solution containing benzene-1,3,5-tricarboxylic acid (BTC) as a were reference at a concentration of 1 Zetasizer Nano Malvern Instruments, Malvern, UK). Latexes diluted with deionized water g/L. 1H-NMR spectra were recorded on Bruker 400 MHz equipment, using the WATERGATE method before measurements. to suppress the signal of water, and theintegration of the vinyl proton signals and reference signal The gel was determined by Soxhlet extraction of the dried polymer with tetrahydrofuran wascontent performed. The Z-average particle was determined using dynamic the lightweight scattering Zetasizer polymer (THF) under reflux for 24 h, and itdiameter is reported as the ratio between of(DLS, the insoluble Nano Z, Malvern Instruments, Malvern, UK). Latexes were diluted with deionized water before fraction and that of the initial sample. measurements. The glassThe transition temperature (Tg ) of the polymers was measured through means of a gel content was determined by Soxhlet extraction of the dried polymer with tetrahydrofuran differential scanning calorimeter, DSC Q1000 (TA Instruments, Castle, (THF) under reflux for 24 h, and itseries is reported as the ratio between the weight of theNew insoluble polymerDE, USA). fraction and were that of carried the initialout sample. The measurements in a range from −70 to 200 ◦ C at a heating rate of 10 ◦ C/min. The glass measurements, transition temperature (Tg) of the polymersanalyzer was measured meansTA of instrument a For thermal resistance a thermogravimetric TGAthrough 2950/Q500 differential scanning calorimeter, series DSC Q1000 (TA Instruments, New Castle, DE, USA). The ◦ was used,measurements and the experiments were carried out with a heating rate of 10 C/min from room were carried out in a range from −70 to 200 °C at a heating rate of 10 °C/min. For ◦ C. temperature to 800 thermal resistance measurements, a thermogravimetric analyzer TGA 2950/Q500 TA instrument was

used, and the experiments were carried out with a heating rate of 10 °C/min from room temperature

3. Resultsto 800 °C. 3. Results Monomers 3.1. Fructose-Based 3.1. Monomers1 H-NMR spectrum of the pure resulting monomer. The methacrylate Figure 1 Fructose-Based reports the proton 1H-NMR functional group is1recognizable: protons are giving signalsmonomer. at 5.8 and ppm and methyl Figure reports the protonvinyl spectrum of the pure resulting The6.2 methacrylate functional group is recognizable: vinylThe protons are giving signals at 5.8 andprotons 6.2 ppm and methyl protons appear as one singlet at 1.9 ppm. presence of the sugar ring is confirmed by the protons appear as one singlet at 1.9 ppm. The presence of the sugar ring protons is confirmed by the signals between 3.5 and 4.8 ppm. signals between 3.5 and 4.8 ppm.

Figure 1. 1H-NMR spectrum of the methacrylate protected fructose (MF) monomer in DMSO-d6.

Figure 1. 1 H-NMR spectrum of the methacrylate protected fructose (MF) monomer in DMSO-d6 .

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13 C-NMR

The spectrum given in Figure 2 corroborates these results by highlighting a5 structure Polymers 2018, 13 x FOR PEER REVIEW of 11 The 10, C-NMR spectrum given in Figure 2 corroborates these results by highlighting a structure bearingbearing 16 well16identified carbons. well identified carbons. The 13C-NMR spectrum given in Figure 2 corroborates these results by highlighting a structure bearing 16 well identified carbons.

13 Figure 13 2. C-NMR spectrum of the methacrylate protected fructose (MF) monomer in DMSO-d6.

C-NMR spectrum of the methacrylate protected fructose (MF) monomer in DMSO-d6 .

Figure 2.

3.2. Homopolymer and Its Thermal Characterization

Figure 2. C-NMR spectrum of the methacrylate protected fructose (MF) monomer in DMSO-d6. 3.2. Homopolymer and Its Thermal Characterization Full conversion was achieved in the emulsion homopolymerization of MF, as the complete 13

the proton was observed after 1 h of the reaction withasH-NMR. 3.2.disappearance Homopolymer and Its vinyl Thermal Characterization Full conversionof was achieved insignals the emulsion homopolymerization of MF, the complete Moreover, small particle sizes (67 nm) were obtained, and the molecular weight (1 × 106 Da) was 1in disappearance of the vinyl proton signals was observed after 1 h of the reaction with H-NMR. conversion was achieved inhomopolymer the emulsionexhibited homopolymerization of MF, temperature as the complete theFull range of emulsion polymers. The a high glass transition of 6 1 Moreover, small particle nm)signals were and the molecular ×rigidity 10H-NMR. Da) disappearance of the sizes vinyl proton was observed 1 h of the weight reaction with about 115 °C, which can be(67 explained by theobtained, cyclic nature ofafter the sugar combined with (1 the of was in the range emulsion polymers. Thenm) homopolymer exhibited a highofglass transition of Moreover, small particle sizes (67 were obtained, and the molecular weight (1 Because × 106 temperature Da)ofwas theof diacetonide protecting groups, which limits the mobility amplitude the chains. the in ◦ C, range emulsion polymers. The homopolymer exhibited a petrochemical high glass transition temperature of high Tg,of this monomer appears as aby potential equivalent toof thethe methyl with methacrylate aboutthe 115 which can be explained the cyclic nature sugar combined the rigidity of 1H-NMR, GPC about 115 °C, which can be explained by the cyclic nature of the sugar combined with the rigidity of raw material. For detailed chart, and DSC see the Supporting Information section. the diacetonide protecting groups, which limits the mobility amplitude of the chains. Because of the protecting groups, limits the mobility of the chains. Because of the high the Tg ,diacetonide this monomer appears as a which potential equivalent toamplitude the petrochemical methyl methacrylate 3.3.TFructose-Based Emulsion Copolymerization high g, this monomer appears as a potential equivalent to the petrochemical methyl methacrylate raw material. For detailed 1 H-NMR, GPC chart, and DSC see the Supporting Information section. 1 1

raw material. detailed GPC chart, andsize DSC see the Supporting Information section. Figure For 3 presents theH-NMR, kinetics and the particle evolution of the different copolymerizations

carried out with the methacrylate fructose and BA at 30% solids content and 1 wbm% of KPS. All 3.3. Fructose-Based Emulsion Copolymerization

3.3.reactions Fructose-Based Emulsion Copolymerization occurred very fast and no differences among the different compositions were observed,

Conversion (wt%)

Figure 3 for presents thekinetics kinetics and the particle size evolution of the different copolymerizations except faster of the MF40-BA/30. Figure 3the presents the kinetics and the particle size evolution of the different copolymerizations carried out out with thethemethacrylate fructoseand andBABA at 30% solids content 1 wbm% of KPS. carried with methacrylate fructose at 30% solids content and 1 and wbm% of KPS. All 100 100 All reactions very fast fast and andno nodifferences differencesamong among the different compositions observed, reactions occurred occurred very the different compositions were observed, (b) were (a) except for the kinetics of of the MF40-BA/30. except for faster the80faster kinetics the MF40-BA/30. 80 100 60

(a)

Conversion (wt%)

80 40 MF20-BA/30 MF30-BA/30 MF40-BA/30

60 20 40

0

0

20

40

60

80

100

dp (nm)

dp (nm)

100 60

(b)

40 80 MF20-BA/30 MF30-BA/30 MF40-BA/30

20 60 0 40 0

20

40

60

80

100

Polymerization time (min)


MF20-BA/30 MF20-BA/30 20 20 MF30-BA/30 MF30-BA/30 Figure 3. Methacrylate fructose-based copolymers. (a) Kinetics and (b) particle MF40-BA/30 size evolution for MF40-BA/30

copolymerizations carried out with 1.0 wbm% of KPS. 0

0

20

40

60

80

100

0

0

20

40

60

80

100

time (min) Polymerization time (min) In order to clarify the behavior of MF40-BA/30, another set ofPolymerization reactions using a lower amount of initiator (0.5 wbm%) were performed (Figure 4). As expected, slower kinetics were observed but, Figure 3. Methacrylate fructose-basedcopolymers. copolymers. (a) and (b)(b) particle sizesize evolution for Figure 3.the Methacrylate fructose-based (a)Kinetics Kinetics and particle evolution again, reaction containing 40 wt % of MF was the fastest. It can be speculated that due to the much for copolymerizations carried out with 1.0 wbm% of KPS.

copolymerizations carried out with 1.0 wbm% of KPS.

In order to clarify the behavior of MF40-BA/30, another set of reactions using a lower amount of

In order(0.5 to clarify behavior of MF40-BA/30, another set of reactions usingobserved a lowerbut, amount initiator wbm%)the were performed (Figure 4). As expected, slower kinetics were again, the containing 40 wt % of(Figure MF was 4). the As fastest. It can beslower speculated that due to observed the much but, of initiator (0.5reaction wbm%) were performed expected, kinetics were again, the reaction containing 40 wt % of MF was the fastest. It can be speculated that due to the much

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higher reactivity of the methacrylates to BA, BA,the theeffect effect composition onoverall the overall higher reactivity of the methacrylateswith withrespect respect to ofof thethe composition on the Polymers 2018, 10, x FOR PEER REVIEW 6 of 11 conversion would be more pronounced when dealing with a larger methacrylate composition [19]. conversion would be more pronounced when dealing with a larger methacrylate composition [19]. higher reactivity of the methacrylates with respect to 100 BA, the effect of the composition on the overall 100 (b) conversion would be more pronounced when dealing with a larger methacrylate composition [19]. (a)

80

60 80

(a)

40 60

MF20-BA/30 MF30-BA/30 MF40-BA/30

20 40 0 0 20

dp (nm)

100 (b)

60 80 40

dp (nm)

ConversionConversion (wt%) (wt%)

80 100

60

MF20-BA/30 MF30-BA/30 MF40-BA/30

20 40 0

MF20-BA/30 20 40 60 80 100 MF30-BA/30 Polymerization timeMF40-BA/30 (min)

20

0

0

40 60 MF20-BA/30 80 100 MF30-BA/30 Polymerization timeMF40-BA/30 (min)

20

0

Figure 4. Methacrylate fructose-based copolymers. copolymers. (a) and (b)(b) particle size size evolution for for Figure 4. Methacrylate fructose-based (a)Kinetics particle evolution 0 20 40 60 80 100 0 Kinetics 20 and 40 60 80 100 copolymerizationsPolymerization carried out with 0.5 wbm% of KPS. time0.5 (min) Polymerization time (min) copolymerizations carried out with wbm% of KPS. 4. Methacrylate fructose-based copolymers. (a) Kinetics and (b) particle size evolution for 3.3.1.Figure Polymer Characterization

3.3.1. Polymer Characterization copolymerizations carried out with 0.5 wbm% of KPS.

The gel content (an insoluble polymer in THF) was measured using Soxhlet extraction. Large

The gel content (anobtained insoluble polymer THF)inwas measured using extraction. amounts of gelCharacterization were in all cases, asinshown Figure 5. On one hand,Soxhlet a large amount of gelLarge 3.3.1. Polymer is expected whenobtained dealing with BA This of intermolecular chain amounts of gel were in alllarge cases, ascontent shown latexes. in Figure 5. is Onbecause one hand, a large amount of gel is The to gelpolymer content followed (an insoluble polymer in THF) wascombination, measured using Soxhlet in extraction. Large transfer by termination through as reported literature [20]. expected when dealing with large BA content latexes. This is because of intermolecular chain transfer amounts ofitgel werementioned obtained in all the cases, as shown inofFigure 5. On one hand, (MMA) a large amount gel However, is also that incorporation methyl methacrylate unitsHowever, in of butyl to polymer followed by termination through combination, as reported in literature [20]. it is is expected when dealingthe with large BA content latexes. This is MMA because of intermolecular chain acrylate chains promotes decrease of gel content [21]. Indeed, terminated chains are less also mentioned that thefollowed incorporation of methyl methacrylate (MMA)reported units in in butyl acrylate chains transfer to polymer by termination combination, literature [20]. reactive towards hydrogen abstraction and the through absence of abstractableas hydrogens in the MMA units, promotes the decrease of gel content [21]. Indeed, MMA terminated chains are less reactive towards However, it is the alsofact mentioned thatradicals the incorporation of methyl methacrylate (MMA) units in butyl together with that MMA terminate predominantly by disproportionation, explain hydrogen abstraction and the absence of abstractable hydrogens in the MMA units, together acrylate chains promotes the decrease of gel content [21]. Indeed, MMA terminated chains are less with the significant reduction of gel content. Thus, since the bio-based monomer is carrying a methacrylate the fact that towards MMA radicals predominantly by disproportionation, explain the significant reactive hydrogen abstraction and the of abstractable hydrogens in the MMA units, function, a decrease in gelterminate could be expected asabsence more methacrylate sugar is incorporated. This effect together with the fact that MMA radicals terminate predominantly by disproportionation, explain reduction of gel content. Thus, since the bio-based monomer is carrying a methacrylate was observed for the two sets of reactions involving MF/BA copolymerization, i.e., using bothfunction, 0.5 the significant reduction gel content. the bio-based monomer is a methacrylate a decrease in geland could beofexpected asThus, more methacrylate sugar is incorporated. Thisofeffect wbm% KPS 1% KPS. Additionally, less since gel content was found when a carrying smaller amount KPS was function, a decrease in gel could be40% expected as MF/BA more methacrylate sugar is This effect was employed. Latexes containing of methacrylate fructose exhibited a incorporated. geli.e., content 28% 0.5 when observed for the two sets of reactions involving copolymerization, usingofboth wbm% was observed for the two sets of reactions involving MF/BA copolymerization, i.e., using both 0.5 wbm% KPS and a gel of 72% prepared 1 wbm%amount KPS (Figure 5a). KPS prepared and 1% with KPS.0.5Additionally, less gelcontent content waswhen found whenwith a smaller of KPS was wbm% KPS and 1% KPS. Additionally, less that gel content was foundconcentration when a smaller amount of KPS This unexpected result goes against theory a larger initiator induces a superior employed. Latexes containing 40% of methacrylate fructose exhibited a gel content of 28% when was employed. Latexes containing 40% of methacrylate fructose exhibited a gel content 28% when number of 0.5 radicals per KPS particle, promoting termination reactions and, hence, lower gel of contents. prepared with wbm% of72% 72%when when prepared with 1 wbm% (Figure prepared with 0.5 wbm% KPSand andaagel gelcontent content of prepared with 1 wbm% KPS KPS (Figure 5a). 5a). This This unexpected result goes against theory that a larger initiator concentration induces a superior unexpected result goes against theory that a larger 100 initiator concentration induces a superior 100 number of radicals perper particle, promoting reactionsand, and, hence, lower gel(b)contents. (a) number of radicals particle, promotingtermination termination reactions hence, lower gel contents. Gel content Gel(wt%) content (wt%)

(a)

60 80 40 60 20 40 0 20

40/60

30/70

20/80

Gel content Gel(wt%) content (wt%)

80 100

80 100

(b)

60 80 40 60 20 40 0 20

40/60

30/70

20/80

Figure 5. The gel content of sugar-based latexes prepared with different MF/BA ratios at 1 wbm% KPS 0 0 (▲●■) and 0.5 wbm% KPS 30/70 (▲●■). (a) Initial20/80 measurements; (b)40/60 Measurements 30/70 repeated after 1520/80 days. 40/60 Figure 5. The content sugar-basedlatexes latexes prepared prepared with MF/BA ratios at 1 wbm% KPS KPS Figure 5. The gel gel content ofof sugar-based withdifferent different MF/BA ratios at 1 wbm% (▲●■) and 0.5 wbm% KPS (▲●■). (a) Initial measurements; (b) Measurements repeated after 15 days. (N•) and 0.5 wbm% KPS (N•). (a) Initial measurements; (b) Measurements repeated after 15 days.

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However, it is important to note that the measurement of the gel was not performed systematically but after few days the latex was synthesized. Indeed, the gel presented in Figure 5a corresponding to 1 wbm% KPS was measured after 26 days of its synthesis. Meanwhile, the gel with 0.5 wbm% KPS was measured just after 3 days of storage. Thus, the unexpected lower gel amount for 0.5 wbm% KPS latexes could be attributed to some evolution of the latex during storage. In fact, recent studies in our lab have shown that the gel content of MMA/BA (50/50) latex increases during storage when the latex does not contain hydroquinone (HQ), likely because of the presence of tertiary radicals which are very stable and can survive long time. In our case, the latexes were stored without HQ. Therefore, this hypothesis could help explain these results. Another aspect to take into consideration is the probability of sugar side chains to suffer partial deprotection, yielding –OH groups that could aggregate due to hydrogen bonding. It is well known that the isopropylidene protecting groups can be cleaved under acidic conditions [22]. However, it was reported that protecting groups within non-water soluble polymer chains are rather hard to cleave. Bird et al. [23] detailed the particular resistance to deprotection of methacrylate galactose homopolymers, showing that heating at 100 ◦ C in HCl for several hours failed in deprotecting the carbohydrate units. In our work, reactions were performed at a basic pH by using NaHCO3 as buffer, so that hydrolysis of the protecting groups was not favored. In any case, it is worth noting that if the polymer chains have a significant molecular weight, just a few –OH in the chain would be enough to lead to association by hydrogen bonding and to form polymer chains that are not soluble in THF. To verify if the latexes evolved during storage, gel measurement of the latexes were repeated 15 days later. The copolymer were 18 days and 41 days old at this point (latexes synthesized with 0.5 wbm% KPS and 1 wbm% KPS respectively). Figure 5b summarizes the results. A clear increase of gel was observed after longer storage for all the latexes of the 0.5 wbm% KPS series. On the other hand, the comparison of the 1 wbm% KPS series also shows that the latex is evolving during storage. From the observed evolution, it can be concluded that the gel fraction of the three series are comparable and that the insoluble fraction in THF most likely increases with time because of a little deprotection of the sugar chains and hydrogen bond formation. 3.3.2. Thermal Characterization and Film Properties The glass transition temperatures of the resulting MF/BA copolymers were measured using DSC. It was observed that all copolymers exhibited one single Tg (curves are given in Supporting Information) and, as revealed in Table 2, the values were in good agreement with those predicted by the Fox equation (Equation (1)), 1 w w = 1 + 2 (1) Tg Tg1 Tg2 where w1 and w2 are the weight fractions of MF and BA, considering the Tg of the sugar-based homopolymer at 115 ◦ C and that of the BA at −50 ◦ C. Table 2. Measured and predicted Tg of the different methacrylate sugar-based copolymers. Monomer Composition MF/BA (◦ C)

Tg measured Tg (◦ C) Fox-equation prediction

20/80

30/70

40/60

−30 −29

−25 −17

−2 −4

The thermal stability of the different sugar-based copolymers was evaluated by thermogravimetric analysis. It can be seen in Figure 6 that thermal stability was independent from the carbohydrate-monomer content. The copolymers exhibited very good thermal stability, since no significant degradation was observed below 350 ◦ C. Moreover, copolymer degradation was achieved in a single step, suggesting homogeneity in chain composition.


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Figure 6. Thermogravimetric analysis of copolymers MF/BA.

Figure 6. Thermogravimetric analysis of copolymers MF/BA. visual appearance of films casted on analysis silicon moulds at ambient temperature Figure 6. Thermogravimetric of copolymers MF/BA.

The and 55% humidity is given in Figure 7. In all cases, continuous transparent films were obtained. This result was in accordance the casted thermal properties of the copolymer shown above, indicating a good The visual appearance ofwith films on silicon moulds at ambient temperature and 55% humidity The visualincorporation appearance of films on chains. silicon moulds at ambient temperature and 55% of the sugar units incasted butyl acrylate

is given inisFigure 7. Figure In all cases, continuous transparent films were obtained. This result in humidity given in 7. In all cases, continuous transparent films were obtained. Thiswas result accordance with the with thermal of the copolymer above, indicating a good incorporation was in accordance theproperties thermal properties of the shown copolymer shown above, indicating a good of the sugar units in butyl acrylate chains. incorporation of the sugar units in butyl acrylate chains.

Figure 7. Pictures of polymer films containing (from left to right) 20, 30, or 40 wbm% methacrylate fructose.

3.4. 45 wt % Solids Content Sugar-Based Copolymer Latexes 30 wt % solids content latexes prepared from sugar-based methacrylate monomers presented excellent colloidal and thermal stability, as well as a homogeneous incorporation of the sugar units in butyl acrylate chains. However, it was not possible to properly cast thin films because of wettability issues. Latexes tend to shrink when applied on various substrates such as glass, metal, or plastic. Moreover,ofsince high solids content latexes are soughtleft for to industrial formulation Figure 7. Pictures polymer films containing (from right)applications, 20, 30, or the 40 wbm% methacrylate Figure 7. Pictures of polymer films (fromlatexes. left to right) 20, 30, or 40 wbm% methacrylate fructose. was modified to produce 45 containing wt % solids content To enhance electrostatic stabilization of the fructose. particles, a disulfonate carrying emulsifier (Dowfax 2A1, 2 wbm%) was used. Acrylamide is a watersoluble monomer which can generate oligomers or polymers in the aqueous phase, causing more 3.4. 45 wt % Solids Content Sugar-Based Copolymer Latexes interactions and Sugar-Based resulting in an increase in viscosity. Raising viscosity helps have better wetting when 3.4. 45 wt % Solids Content Copolymer Latexes casting films; also, amide functions are well-known to promote adhesion [24]. Thus, 2 wbm% of 30 wt % solids content latexes preparedand from sugar-based monomers presented acrylamide was added to the formulation, 0.75 wbm% of KPS plusmethacrylate 1 wbm% NaHCO3 was used. 30 wt % solids content latexes prepared from sugar-based methacrylate monomers presented The latexes made were very stable, with final average particle sizes of approximately 110 nm and excellent colloidal and thermal stability, as well as a homogeneous incorporation of the sugar units in

excellent colloidal and thermal stability, as well as a homogeneous incorporation of the sugar units butyl acrylate chains. However, it was not possible to properly cast thin films because of wettability in butyl acrylate chains. However, it was not possible to properly cast thin films because of wettability issues. Latexes tend to shrink when applied on various substrates such as glass, metal, or plastic. issues. Latexes tend to shrink when applied on various substrates such as glass, metal, or plastic. Moreover, since high solids content latexes are sought for industrial applications, the formulation Moreover, since high solids content latexes are sought for industrial applications, the formulation was modified to produce 45 wt % solids content latexes. To enhance electrostatic stabilization of was modified to produce 45 wt % solids content latexes. To enhance electrostatic stabilization of the the particles, a disulfonate carrying emulsifier (Dowfax 2A1, 2 wbm%) was used. Acrylamide is a particles, a disulfonate carrying emulsifier (Dowfax 2A1, 2 wbm%) was used. Acrylamide is a waterwater-soluble monomer which can generate oligomers or polymers in the aqueous phase, causing soluble monomer which can generate oligomers or polymers in the aqueous phase, causing more more interactions and resulting in an increase in viscosity. Raising viscosity helps have better wetting interactions and resulting in an increase in viscosity. Raising viscosity helps have better wetting when when casting films; also, amide functions are well-known to promote adhesion [24]. Thus, 2 wbm% casting films; also, amide functions are well-known to promote adhesion [24]. Thus, 2 wbm% of of acrylamide was added to the formulation, and 0.75 wbm% of KPS plus 1 wbm% NaHCO3 was acrylamide was added to the formulation, and 0.75 wbm% of KPS plus 1 wbm% NaHCO3 was used. used. The latexes made were very stable, with final average particle sizes of approximately 110 nm The latexes made were very stable, with final average particle sizes of approximately 110 nm and and having a narrow distribution (Ð < 0.04; Ð = (width/mean)2 in DLS Malvern equipment). Particle size distributions can be found in Supporting Information. The evolution of the reaction was also very fast, as exemplified in Figure 8.


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having a narrow distribution (Ð < 0.04; Ð = (width/mean)² in DLS Malvern equipment). Particle size distributions Polymers 2018, 10,can 488 be found in Supporting Information. The evolution of the reaction was also 9very of 11 fast, as exemplified in Figure 8. 100

Conversion (wt%)

80 60 40 MF20-BA/45 MF40-BA/45

20 0

0

20

40

60

80

100


Figure 8. The Figure 8. The evolution evolution of of overall overall conversion conversion for for the the copolymerization copolymerization of of MF MF with with BA BA at at different different ratios and 45% solids content.

Excellent wettability features were observed on different substrates, such as plastic, glass, and Excellent wettability features were observed on different substrates, such as plastic, glass, metal, and the resulting films were continuous and transparent. A summary of the final copolymer and metal, and the resulting films were continuous and transparent. A summary of the final copolymer characteristics is given in Table 3. In this case, rather large amounts of gel fraction were also obtained. characteristics is given in Table 3. In this case, rather large amounts of gel fraction were also obtained. Values around 80% were measured independent of monomer type and composition. It is important Values around 80% were measured independent of monomer type and composition. It is important to to note that Soxhlet extractions were carried out after several days of storage. note that Soxhlet extractions were carried out after several days of storage. Table 3. Final copolymer characteristics. Table 3. Final copolymer characteristics.

Run Run MF20-BA/45 MF20-BA/45 MF40-BA/45 MF40-BA/45

dp (nm)

Tg (°C)

dp 107 (nm)

Tg (◦−24 C)

107 106 106

−243 3

Gel (wt %) Gel (wt %) 76 76 75 75

4. Conclusions 4. Conclusions A methacrylate fructose monomer bearing acetonide protecting groups was synthesized from a A methacrylate fructose monomer bearing acetonide protecting groups was synthesized from commercially available protected fructose-based monosaccharide. The resulting homopolymer a commercially protected The resulting homopolymer presented a Tg ofavailable approximately 115fructose-based °C, appearing monosaccharide. as an interesting alternative to petrochemical◦ C, appearing as an interesting alternative to petrochemical-based presented a T of approximately 115 based methylg methacrylate, which is commonly used in waterborne acrylic formulations for methyl methacrylate, adhesives and coatings.which is commonly used in waterborne acrylic formulations for adhesives and coatings. The copolymerization of fructose-based monomers with butyl acrylate in batch emulsion The copolymerization of fructose-based monomers butylin acrylate batch polymerization was investigated. High conversions were with achieved all cases,inand the emulsion polymer polymerization investigated. conversions were achieved all cases, and that the polymer particle size waswas independent fromHigh the bio-based monomer content. in It was observed gel was particle size wasstorage, independent from theofbio-based monomerof content. It was observed that gelbond was evolving during likely because some deprotection the sugar chains and hydrogen evolving during storage, likely because of some deprotection of the sugar chains and hydrogen bond formation. Furthermore, latexes led to continuous transparent films with a unique glass transition formation. Furthermore, latexes led to transparent films with a unique transition temperature, which was in the range ofcontinuous the theoretical value calculated from the Foxglass equation. The temperature, which was in the range stability. of the theoretical value calculated from the Fox equation. films also had excellent thermal The potential of the sugar-based monomerThe forfilms the also had excellent thermal copolymers stability. Thewas potential the sugar-based monomer for the production of production of waterborne greatlyofevident through the successful increase of solids waterborne copolymers was evident through the successful increase of solids content in the content in the formulation to greatly up to 45%, which is important for effective industrial applications. formulation to up to 45%, which is important for effective industrial applications. Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1. Figure S1: 1H-NMR Supplementary Materials: The following areWATERGATE available online at http://www.mdpi.com/2073-4360/10/5/488/s1. method. Figure S2: Molecular weight distribution of spectrum of the MF homopolymer in D2O. 1 H-NMR spectrum of the MF homopolymer in D O. WATERGATE method. Figure S2: Molecular weight Figure 2 the MFS1: homopolymer. Figure S3: DSC of the MF homopolymer., Figure S4: Particle size distribution of different distribution of the MF homopolymer. Figure S3: DSC of the MF homopolymer., Figure S4: Particle size distribution copolymers: (a) MF40-BA/30, with 1% KPS; (b) MF40-BA/30, with 0.5% KPS; and (c) MF40-BA/45. Figure S5: of different copolymers: (a) MF40-BA/30, with 1% KPS; (b) MF40-BA/30, with 0.5% KPS; and (c) MF40-BA/45. DSCs MF/BA of differentof compositions: (a) MF20-BA; MF30-BA; (c) MF40-BA. FigureofS5: DSCs copolymers of MF/BA copolymers different compositions: (a) (b) MF20-BA; (b) and MF30-BA; and (c) MF40-BA. Author Contributions: Mónica Moreno and María J. Barandiaran conceived and designed the experiments. Jessica S. Desport performed the experiments. All the authors analyzed the data and wrote the paper.

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Funding: This research was funded by the Industrial Liaison Program of POLYMAT, Eusko Jaurlaritza (IT999-16), MINECO (CTQ2014-59016-P), and UPV/EHU (UFI11/56). Conflicts of Interest: The authors declare no conflict of interest.

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