Lignin-based polymeric surfactants for emulsion polymerization

Accepted Manuscript Lignin-based polymeric surfactants for emulsion polymerization Bernhard V.K.J. Schmidt, Valerio Molinari, Davide Esposito, Klaus Tauer, Markus Antonietti PII:

S0032-3861(17)30159-3

DOI:

10.1016/j.polymer.2017.02.036

Reference:

JPOL 19437

To appear in:

Polymer

Received Date: 14 December 2016 Revised Date:

7 February 2017

Accepted Date: 8 February 2017

Please cite this article as: Schmidt BVKJ, Molinari V, Esposito D, Tauer K, Antonietti M, Lignin-based polymeric surfactants for emulsion polymerization, Polymer (2017), doi: 10.1016/j.polymer.2017.02.036. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Lignin-based polymeric surfactants for emulsion polymerization

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Bernhard V.K.J. Schmidt,* Valerio Molinari, Davide Esposito, Klaus Tauer, Markus Antonietti

Mühlenberg 1, 14476 Potsdam, Germany

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Max-Planck Institute of Colloids and Interfaces; Department of Colloid Chemistry, Am

KEYWORDS Emulsion polymerization, lignin, polymeric surfactant, poly(ethylene oxide)

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ABSTRACT A non-ionic surfactant system is synthesized by standard grafting of poly(ethylene oxide) from renewable lignin fragments and used for the emulsion polymerization of styrene.

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The lignin precursors is formed by hydrogenolysis and utilized as initiator for the oxyanionic polymerization of ethylene oxide leading to amphiphilic polymers, very similar to standard

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nonionic surfactant synthesis. Subsequently, the formed amphiphilic polymers are employed as stabilizers in the heterophase polymerization of styrene with various initiators. Poly(styrene) latexes with solids contents of up to 21% depending on stabilizer concentration have been obtained. Stabilizer efficiencies and performances were nicely comparable with those of nonylphenol-based, non-ionic industrial performance surfactants.

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Introduction

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The depletion of fossil resources, but also the biomedical action of hydrophobic fragments in consumer products (e.g. nonylphenol or methylparaben) inspired the whole community of the chemical sciences to replace such building blocks by less critical components from renewable

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resources as raw materials for the future.[1-4] Polymer scientists are on the forefront of this movement and already historically have made great use of renewable materials.[5, 6] In that

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regard various raw materials like cellulose,[7] lactic acid,[8] polysaccharides,[9, 10] saccharides,[11] terpenes[12, 13] as well as plant oils[10, 14] or more modern systems as itaconic acid that can be obtained via fermentation of carbohydrates[15] are illustrative cases. Sustainable building blocks utilized in the synthesis of polyesters or polymethacrylates are

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multifunctional furans[10, 16] or isosorbide[17, 18] formed from saccharides. An interesting renewable monomer that resembles methyl methacrylate is α-methylene-γ-valerolactone, which can be obtained from levulinic acid and utilized for example in emulsion polymerizations.[19]

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For replacing nonylphenol, bisphenol-A or methylparabenes, aromatic fragments are however needed. One of the few renewable resources for aromatic molecules is lignin, which is highly

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abundant in nature, inexpensive, and obviously not too much health-interfering. It is obtained as a side-product in the paper industry and mostly burned for energy generation.[20] Most frequently utilized types of lignin are lignosulfonate, Kraft lignin and organosolv lignin.[21] Of these three commercial products lignosulfonate has a significant water solubility and average molecular weights of 10 to 30 kg mol-1.[21, 22] Kraft lignin with average molecular weights between 4 and 20 kg mol-1 is non-water soluble and shows limited solubility in organic solvents

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or water.[21, 23] On the other hand organosolv lignin comprising average molecular masses between 4 and 11 kg mol-1 is reasonably soluble in selected organic solvents.[21, 23] Nevertheless, several factors hinder valorization of lignin in the chemical sciences.[24] First of

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all, lignin has a rather undefined polymeric structure, which makes it hard to generate defined products.[24, 25] Therefore, lignin products find limited markets so far while mainly low value products are addressed.[24, 25] Secondly, pristine lignin has a limited solubility in most regular

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solvents, preventing easy processing. In order to improve the solubility and applicability of lignin, several processes have been developed, such as derivatization of the hydroxyl functional

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groups leading to higher solubility, e.g. via acetylation, methylation, or epoxidation.[26, 27] Nevertheless, decreased hydroxyl functionality is a drawback, since hydroxyls are useful for further derivatization or polymerization reactions. An alternative process that preserves (and even increases) the number of hydroxyl groups, is to split the biopolymer at the ether bonds into

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small fragments of increased solubility under formation of OH-groups,[28] for instance via catalytic hydrogenolysis.[29, 30]

Lignin and lignin-derived raw materials are not novel in polymer science.[31] Lignin-derived

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building blocks have been utilized in the synthesis of various polymer classes, e.g. polyesters,

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polyurethanes or resins. Recently, methacryl monomers were formed from lignin-derived precursors and utilized in radical addition-fragmentation chain transfer polymerizations to obtain polymers with tunable thermal and viscoelastic properties.[32] The functionalization of ligninprecursors with vinyl groups allowed for the acyclic diene metathesis polymerization of lignin derivatives.[33] Furthermore, various polymers were grafted from lignin precursors, e.g. poly(2dimethylaminoethyl methacrylate),[34] poly(N-isopropylacrylamide),[35] poly(lactic acid)[36] or poly(N-isopropylacrylamide) blocked with poly(ethylene oxide) and poly(propylene oxide)

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grafts.[37] Lignin poly(ethylene oxide) (PEO) blends have been studied thoroughly showing suppressed hydrogen bonding between lignin molecules due to interactions with PEO chains.[38, 39] Such blends could be used to improve carbon fiber formation from lignin[40] as well as for

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the synthesis of enhanced acrylonitrile butadiene styrene lignin composites.[41] Furthermore, PEO was utilized in the synthesis of lignin centered hyperbranched polymers.[42]

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A covalent attachment of PEO to lignin should lead to the formation of amphiphilic adducts that can act as colloidal stabilizers in aqueous heterophase polymerizations. Such adducts can

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possibly act as green alternatives for non-ionic nonylphenol surfactants which are known to

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decompose into endocrine disruptive units in the environment.[43]

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Scheme 1. Formation of lignin-based surfactants and utilization in emulsion polymerization.

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Herein, utilization of lignin-based precursors (Lig) in the oxyanionic polymerization of ethylene oxide (EO) is described (Scheme 1). Subsequently, the block copolymers were employed as surfactant in the emulsion polymerization of styrene (Scheme 1). The Lig-PEO products act as surfactant in aqueous solution due to their hydrophobic lignin-derived block and the hydrophilic PEO block. Depending on the surfactant content in the emulsion polymerization, latexes with high solids content or low solid content were obtained and were analyzed via scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). In the case of the less

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stable compositions and low solids content latexes, coagulum particles with extraordinary

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ellipsoidal morphology in the millimeter range were formed.

Experimental Part

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Materials

Acetic acid (Sigma Aldrich), chloroform (Sigma Aldrich), N,N-dimethylformamide (DMF,

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Sigma Aldrich), DOWEX 50WX4-100 (Sigma Aldrich), ethanol (Sigma Aldrich), softwood Kraft lignin (UPM BioPiva), potassium peroxodisulfate (KPS; Sigma-Aldrich), Raney Nickel (Sigma

Aldrich),

1-tert-butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)-

phosphoranylidenamino]-2λ5,4λ5-catenadi(phosphazene) solution (P4-t-Bu; 0.8

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in hexane,

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Sigma Aldrich) and toluene (99.8%, Acros Extra dry) were used as received. Ethylene oxide (EO; 99.9%, Sigma Aldrich) was dried over CaH2 and cryo-distilled before usage (Note: EO has a high vapor pressure and easily forms explosive mixtures with air). Tetrahydrofuran (THF;

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99.9%, Acros Extra dry) for polymerizations was dried over diphenylhexyl lithium and cryo-

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distilled prior to usage. Styrene (Sigma Aldrich) was distilled prior to usage.

Synthesis of Lignin precursors m-Lig and l-Lig An autoclave was loaded with lignin (10.0 g), ethanol (400 mL), Raney Nickel (2.0 g) and hydrogen gas (8 bar). The reactor was heated up to 180 °C and the mixture stirred for 16 h. The reaction mixture was filtered to isolate the products from the catalyst powders and the coked

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products. The ethanol was removed by evaporation under reduced pressure, and a brown solid containing lignin and lignin fragments were obtained. The product was suspended in chloroform to yield a chloroform-soluble fraction (m-Lig, 3.9 g, Mn,SEC = 700 g mol-1, Ɖ=2.19) and a fraction

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Acetylation of Lig-precursors and estimation of hydroxyl content

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that was non-soluble in chloroform (l-Lig, 2.4 g, Mn,SEC = 800 g mol-1, Ɖ=2.89).

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100 mg of specimen (lignin, m-Lig or l-Lig) were added to a 1:1 mixture of acetic anhydride and pyridine (10 mL of total volume) and stirred overnight. The mixture was cooled down in an icebath and the reaction was quenched with cold water. Chloroform (50 mL) was added to the solution, and the solution was washed three times with 1 N HCl (50 mL) to remove the pyridine. The organic phase was collected and the solvent was removed under reduced pressure, yielding

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the acetylated lignin fragments as brown powders. 1H-NMR in deuterated chloroform allowed the estimation of the acetyl functional groups and therefore of the hydroxyl content, DMF was

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added to the solution and used as internal standard for the calculation.

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Exemplary synthesis of m-Lig-PEO (Note: EO has a high vapor pressure and easily forms explosive mixtures with air)

According to the literature,[44, 45] m-Lig (250 mg, 1.20 mmol OH, 1.0 eq.) was placed in a flame dried ampoule under Argon flow. Dry toluene (15 mL) was added under Argon flow. After dissolution of the solids, the solvent was cryo-distilled off under reduced pressure. The dissolving/distillation procedure was repeated three times and the dried solid precursor was

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evacuated overnight. Next, THF (8 mL) was cryo-distilled into the ampoule and after thawing the THF, P4-t-Bu solution (0.51 mL, 0.41 mmol, 0.3 eq.) was added under Argon flow via syringe. Subsequently, EO (2.5 mL, 64.35 mmol, 41.9 eq.) was cryo-distilled into the ampoule

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and the mixture was heated to 50 °C under stirring. The mixture was stirred at 50 °C for 2 days and acetic acid (0.5 mL) was added under Argon flow. The mixture was stirred over night at ambient temperature. Afterwards, the solvent was evaporated under reduced pressure and the

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residue dissolved in water (50 mL). DOWEX 50WX4-100 was added, the mixture stirred for 2 h and filtered. The solution was concentrated under reduced pressure and the residue dialyzed

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against water for 3 days (SpectraPor, MWCO 1000) to afford the polymeric product after evaporation of the solvent as a brown high viscous oil (1.27 g, 0.51 mmol, 41%). Mn,SEC =

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2500 g mol-1, Ɖ = 1.26.

Exemplary emulsion polymerization of styrene employing l-Lig-PEO6.7k Batchwise emulsion polymerizations were carried out with styrene as the monomer in a 100 ml

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all-glass reactor. The reactor was equipped with stirrer, reflux condenser, nitrogen inlet and

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outlet, heating jacket to control the temperature, and a valve on the bottom to remove the latex. The standard procedure was as follows: 10 g of styrene, water, and 0.1 g of the stabilizer (or 0.5 g in some cases) were premixed in the reactor during the heat-up to reaction temperature (80 °C). The polymerization was started by injecting 0.32 g of KPS (or a corresponding molar amount of other initiator such as 0.673 g of PEGA200) dissolved in water. PEGA200 is a homemade symmetrical poly(ethylene glycol)-azo-initiators with an average molecular weight of the poly(ethylene glycol) of 200 g mol–1.[46, 47] The total amount of water was 40 g, which was

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split up into two portions where one was in the initial charge of the reactor and the second part was used to dissolve the initiator. Depending on the stabilizer efficiency, the duration of the polymerization to completion is quite different and hence, the polymerizations were allowed to

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run until no smell of styrene could be detected any longer but a fainted smell of benzaldehyde and / or phenylacetaldehyde. After polymerization and before any characterization the white

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latex was filtered through pore 0 glass frit to remove coagulum.

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Characterization Methods

H-NMR spectra were recorded at ambient temperature at 400 MHz with a Bruker Ascend400

NMR spectrometer. Size exclusion chromatography (SEC) in N-methyl-2-pyrrolidone (NMP; Fluka, GC grade) was conducted with 0.05 mol L-1 LiBr and BSME as internal standard at 70 °C

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using a column system by PSS GRAM 100/1000 column (8 x 300 mm, 7 µm particle size) with a PSS GRAM precolumn (8 x 50 mm) and a Shodex RI-71 detector and a calibration with PEO standards from PSS. SEC in THF (VWR, ACS grade, predistilled) was conducted with toluene

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as internal standard at 25 °C using a column system by PSS SDV 100/1000/100000 column (8 x

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300 mm, 5 µm particle size) with a PSS SDV precolumn (8 x 50 mm), a SECcurity RI detector and a SECcurity UV/VIS detector and a calibration with PS standards or PEO standards from PSS.

Dynamic light scattering (DLS) of lignin surfactants was performed using an ALV-7004 Multiple Tau Digital Correlator in combination with a CGS-3 Compact Goniometer and a HeNe laser (Polytec, 34 mW, λ = 633 nm in a θ = 90° setup to the photo-detector (ALV/SO-SIPD)). Sample temperatures were adjusted to 25 °C with a toluene bath surrounding the cuvette.

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Apparent radii (Rapp) were determined by DLS and the data were fitted using the REPES algorithm. DLS of latexes was performed on a 380 DLS spectrometer (Particle Sizing Systems, Santa

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Barbara, CA) with a 90 mW laser diode operating at 658 nm equipped with an avalanche photodiode detector at 25 °C. The data was evaluated with Gaussian analysis. The scattered light was recorded at an angle of 90° to the incident beam.

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Scanning electron microscopy (SEM) was performed with a Jeol JSM 7500 F. Transmission electron microscopy (TEM) was conducted with a Zeiss EM 912 Omega microscope working at

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a voltage of 120 kV.

Solids content (SC) of latexes was characterized via with a HR 73 Halogen Moisture Analyzer (Mettler Toledo, Gießen, Germany). The surface tension liquid – vapor (γlv) of the final latex was determined with a Krüss tensiometer (Krüss, Hamburg) utilizing the du Noüy ring method.

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Optical light microscopy was carried out with a Keyence VH-X digital microscope (Keyence, Osaka, Japan) with an objective VH-Z100 allowing magnifications up to 1,000–fold.

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DSC measurements were performed on a Mettler-Toledo DSC1 System. The samples were preheated at 130 °C for 3 h to remove the residual water, then cooled to -80 °C and heated to

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130 °C at a rate of 10 K min-1. Glass transition temperatures were determined as inflection points using Mettler Toledo STARe software V9.30. Gas chromatography-flame ionization detection (GC-FID) measurements were performed using an Agilent Technologies 5975 gas chromatograph equipped with a FID detector in combination with a second HP-5MS capillary column on the back inlet. The temperature program started with an isothermal step at 50 °C for 2 min, in a second step the temperature was increased to 300 °C

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(rate of 10 °C min-1) and then kept for 20 min. The injector temperature is kept at 250 °C and the

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detector at 280 °C.

Results and Discussion

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Hydrogenolysis of lignin

To allow the utilization of lignin in oxyanionic polymerization of EO, a reductive fragmentation

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was performed from softwood Kraft lignin first (Scheme 2).[48] In such a way the lignin molar weight is decreased, while non-soluble residues can be separated. The catalytic hydrogenolysis of lignin was performed in an autoclave with Raney Nickel as catalyst for the reaction at 180 °C. The catalyst was recovered by filtration, together with a relevant amount of insoluble lignin

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fractions and lignin that was converted to coke during the reaction (~40 wt.%). Certainly, formation of 40 wt.% coke and insoluble lignin fractions is a significant amount and has to be considered with respect to sustainability of the process. Nevertheless, optimizations of lignin

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preparation or the utilization of other lignin precursors, e.g. organosolv lignin, are possible and can be investigated in future research. The hydrogenated lignin product was recovered as brown

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powder after evaporation of the solvent. The product was added to chloroform to yield a particle dispersion instead of a solution. Thus, two fractions were obtained after filtration: A chloroform non-soluble fraction (l-Lig, Table S1) and a chloroform soluble fraction (m-Lig, Table S1) that show slightly different compositions and average molecular weight. While Mn changes only marginally after hydrogenolysis, Mw decreased significantly compared to pristine lignin. Both samples exhibit minor enrichment of hydroxyl (OH) functional groups content around 4.8 to 4.9 mmol g-1 compared to the pristine lignin (Table S1, Figure S1-S4). Moreover, the content of

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accessible phenolic hydroxyl groups and the ratio of phenolic to aliphatic hydroxyl groups increase significantly. The hydroxyl groups are derived from the cleavage of the ether bonds that link the aromatic rings in lignin structure (Scheme 2). Thus, lignin precursors were formed that

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can be utilized as initiating moieties for oxyanionic polymerizations. Being a derivative of a natural product the formed m-Lig and l-Lig are a mixture of possible structures, which has to be taken into account in the practical use. Nevertheless, the lignin precursors with present hydroxyl

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groups were utilized as initiators for the polymerization of EO to generate amphiphilic polymers.

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Scheme 2. Representative overview over lignin hydrogenolysis.

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Lig-initiated oxyanionic polymerization of EO In order to perform the oxyanionic polymerization of EO with hydroxyl-containing lignin fragments as initiator in a controlled way (Scheme 3), the precursor was dried via azeotropic distillation of toluene first. In a reference experiment, the polymerization was tried without prior azeotropic distillation to study the effect of residual water on the polymerization. All polymerizations were performed in THF with t-Bu-P4 as non-nucleophilic base for 2 days at

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50 °C.[44] Due to the difference of the solubility of the lignin fragments and the formed PEO

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chains, amphiphiles are formed that are probed for utilization as surfactants.

Scheme 3. Surfactant preparation via oxyanionic polymerization of EO (Note: several OH-

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groups might be present in the Lignin precursor leading to star polymer structures).

Table 1. Results of oxyanionic polymerizations of EO started from lignin precursors. Lignin precursor

Lig-OH/EO/ t-Bu-P4 eq.a

Yield [%]

Mn,SEC [g mol-1]

Ɖ

68

1800b

2.10b

1.0/20.9/0.3

m-Lig

1.0/41.9/0.3

41

2500b

1.26b

m-Lig

1.0/63.8/0.3

74

4600b

1.49b

m-Ligd

1.0/63.8/0.3

74

9200b

1.71b

1.0/62.2/0.3

90

6700c

2.23c

1.0/124.4/0.3

86

13000c

1.70c

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l-Lig

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l-Lig

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m-Lig

a) Polymerization in THF at 50 °C for 2 days, b) determined via SEC in THF against PEO standards, c) determined via SEC in NMP against PEO standards, d) without azeotropic distillation prior to polymerization

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The amount of lignin initiator was varied to change the molecular weight of the obtained polymer products as shown in Table 1. In such a way the Mn of the obtained polymer product doubles if the initiator amount is bisected, as expected for a living polymerization. Molecular

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weights ranging from 1800 to 13000 g mol-1 with Ɖ from 1.26 to 2.23 were obtained as concluded from SEC measurements in NMP or THF against PEO standards. As the lignin initiators are rather undefined, multi modal molecular mass distributions are obtained (Figure

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1a). From the analysis of lignin precursors it can be concluded that a major fraction of precursors features several hydroxyl groups per molecule. Thus, the formed products can be rather

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considered as lignin centered PEO star polymers (Scheme 1), which is common for nonionic surfactants, e.g. in the case of Tween 20. In any case, a clear trend of initiator amount on the obtained molecular weights is visible. Without azeotropic distillation higher molecular weights

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were obtained, which we attribute to residual water that led to chain terminations.

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Figure 1. a) SEC traces of m-Lig-PEO surfactants measured in THF and b) SEC traces of l-Lig-

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PEO surfactants measured in NMP.

The formed polymer structures were studied via 1H NMR as well (Figure 2a and Figure S5-S9). Although the lignin precursor is rather polydisperse, several signals can be assigned. Signals from both blocks, lignin and PEO, can be observed and attributed to aromatic fragments, aliphatic parts as well as the PEO backbone and the hydroxyl end groups in the product. Notably,

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the signals of aldehyde protons in the lignin precursor vanish after reaction, which is most likely

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due to reaction of active chain ends with the reactive aldehyde moieties.

Figure 2. a) 1H NMR spectrum of m-Lig (top) and m-Lig-PEO2.5k (bottom) measured in DMSOd6 and b) particle size distributions of aqueous l-Lig-PEO6.7k solutions obtained via DLS (ambient temperature and a concentration of 5, 10 and 20 mg mL-1).

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The solubility and aggregation behavior of the formed products in water was probed via DLS

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(Figure 2b for l-Lig-PEO6.7k). In the observed concentration range (5-20 mg mL-1) aggregates with intensity averaged hydrodynamic diameters (Di) between 251 nm and 398 nm were observed for aqueous solutions of l-Lig-PEO6.7k. Furthermore, the critical micelle concentration (CMC) of m-Lig-PEO9.2k and l-Lig-PEO6.7k in water was determined via tensiometry. Values of

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2.02 mg L-1 and of 1.91 mg L-1, respectively, were obtained, which is reasonable for non-ionic

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surfactants (Figure S10-S11). Therefore, it can be concluded that amphiphilic surfactant molecules are formed in the oxyanionic polymerization of EO from the synthesized lignin precursors.

Lig-PEO stabilizer in emulsion polymerizations of styrene

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The lignin-based PEO surfactants were exemplarily utilized in emulsion polymerizations of styrene. Mainly potassium peroxodisulfate (KPS) was used as initiator leading to sulfate endgroups, which contributes to the stabilization of the latex particles (Table 2). For purposes of

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comparison, few polymerizations were initiated with non-ionic PEGA200 leading to EO end

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groups (Table S2). Initially, 1.0 wt.% of surfactant relative to the amount of monomer was employed, which is for the target solids content of 20 wt.% a comparably low amount of nonionic surfactant. For, classical ionic stabilizers such as sodium dodecyl sulfate, 1 wt.% is sufficient to stabilize particles with diameter of about 100 nm at 20 wt.% solids content. Contrary, the solids content of the latex obtained with Lig-PEO stabilizers stays at values between 2.5 and 7.9 wt.% (Table 2 and Table S2) at 1.0 wt.% of surfactant in the polymerization reaction. Nevertheless, white latexes were formed in all cases. The monomer conversion was

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complete and hence, most of the polymer was therefore obtained in the form of coagulum and removed from the latex during filtration (refer to the discussion at the end of the section). The average size of the latex particles Di varies between 322 to 405 nm. Electron microscopy reveals

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in addition to spherical latex particles the presence of larger non-spherical particles with various shape and size in the range of 1 µm (Figure 3 and S12-19). Figure 3b shows also the presence of elongated structures arranged in a fan-like fashion occurring after drying. These structures stem

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from parts of the Lig-PEO surfactant which are not bound at the latex particles, as suggested by

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reference TEM micrographs of the pure surfactant (Figure S20).

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Figure 3. SEM and TEM images of the formed latex particles via utilization of 1.0 wt.% l-Lig-

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PEO6.7k a) and b) and 5.0 wt.% l-Lig-PEO6.7k c) and d).

Typically, increasing concentration leads to higher stabilizing power of the surfactant, which applies to the Lig-PEO stabilizers as well. Using 5.0 wt.% of stabilizer relative to the mass of

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monomer significantly reduces the amount of coagulum and increases the solids content of the latex. Interestingly, the effect on the average particle size is much less pronounced (Table 2 and

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Figure 3/S18), while no fan-like structures that originate from surfactant aggregates are detectable on TEM micrographs anymore. The latter observation is in accordance with the significant increased latex stability. The missing presence of fiber-like structures can be explained with the higher surface area of the latex particles compared to coagulum, binding the

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surfactants. Again, latexes appearing white were obtained as required for many applications. Table 2. Results of styrene emulsion polymerizations employing Lig-PEO surfactants and KPS

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initiator.

E (106cm2 g-1)d

γlv (mN m-1)e

4.98

62.6

329

2.37

54.8

5.9

380

3.76

-f

1

7.9

405

4.84

55.8

l-Lig-PEO13.0k

1

2.5

322

1.82

53.1

l-Lig-PEO6.7k

5.0

18.8

383

13.82

56.1

l-Lig-PEO13.0k

5.0

20.9

374

16.15

51.2

Lig-PEO [%]a

SC [%]b

Di [nm]c

m-Lig-PEO2.5k

1

6.9

340

m-Lig-PEO4.6k

1

3.3

m-Lig-PEO9.2k

1

l-Lig-PEO6.7k

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Surfactant

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a) in relation to the mass of monomer, b) obtained via gravimetry, c) intensity weighted particle diameter determined via DLS at ambient temperature, d) stabilizer efficiency calculated with w SC 6 ⋅107 , e) surface tension liquid – vapor of the ⋅ ⋅ s 100 − SC ρ p Di

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final latex, f) non-sufficient amount of latex for the measurement

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experimental data according to E =

In order to compare the action of various stabilizers during emulsion polymerization the

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stabilizer efficiency, E, has turned out to be a valuable quantitative measure (Table 2).[49] The value of E describes the overall latex particles surface area that can be stabilized per mass of stabilizer employed for given polymerization conditions and recipe. Evaluating these values one should bear in mind the quite profound influence of the initiator, specifically its hydrophilicity.

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In general, charges on the particles surface whether from the stabilizer and / or the initiator increase the efficiency. Consequently, the use of an initiator leading to charged polymer endgroups is advantageous in combination with non-ionic stabilizers with respect to higher E

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values. Comparative experiments (Figure 4, Table 2 and S2), confirm this statement.

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109

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PEGA200 - CO880 PEGA200 - SDS KPS - SDS KPS - CO880 KPS - m-Lig-PEO2.5k KPS - m-Lig-PEO4.6k

108

KPS - m-Lig-PEO9.2k

KPS - 1% l-Lig-PEO6.7k KPS - 5% l-Lig-PEO6.7k

7

10

KPS - 1% l-Lig-PEO13.0k

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E [cm2 g-1]

PEGA200 - m-Lig-PEO9.2k

KPS - 5% l-Lig-PEO13.0k

105 0

100

200

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106

300

400

500

600

700

800

Di [nm]

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Figure 4. Correlation between the stabilizer efficiency E and the intensity weighted average particle size Di for styrene emulsion polymerization with various initiator - stabilizer systems illustrating both the influence of the initiator and the competitiveness of the Lig-PEO stabilizers

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with commonly used ones (anionic sodium dodecyl sulfate, SDS, and non-ionic ethoxylated

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nonylphenolether CO880);[50] if not otherwise stated the amount of surfactant is 1 wt.% in relation to the mass of monomer.

The data compiled in Figure 4 are quite instructive and corroborate the common behavior of lignin – PEO adducts as stabilizers in emulsion polymerization. Firstly, they prove the strong influence of the nature of the initiator on the outcome of emulsion polymerization for Lig-PEO

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stabilizers as known for common non-ionic emulsifiers.[50] Secondly, aqueous heterophase polymerization in the absence of any charges (here with the non-ionic PEGA200 initiator) leads to the lowest efficiency, in the particular case about one order of magnitude lower than the

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values made with KPS co-stabilization. Low efficiency in completely non-ionic heterophase polymerization systems is absolutely common.[51] The significant different stabilizer efficiency observed for charged and non-charged emulsifiers together with KPS and PEGA200 might have

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two reasons. The ionic strength is drastically reduced if ionic initiators are replaced with neutral initiators, which has significant influence on all processes and reactions in aqueous environment,

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e.g. particle nucleation, interactions between particles and reaction kinetics. Moreover, there are specific interactions between PEO chains and surfactants that have been the subject of intense research.[50] Consequently, stabilizer efficiency of the investigated Lig-PEO systems depends significantly on the utilized initiators. Interestingly, the polymerizations with 5% l-Lig-PEO6.7k

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or l-Lig-PEO13.0k stabilizer in relation to the mass of monomer leads to an efficiency absolutely comparable with the low molecular weight commercial stabilizer CO880, which is a non-ionic ethoxylated nonylphenolether. The amount of stabilizer (5 wt.% in relation to the monomer

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mass) is also technically in an absolutely reasonable range; it can be therefore stated that the formed lig-PEO surfactants can indeed be utilized as a substitute for nonylphenolether

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surfactants.

The surface tension liquid – vapor (γlv) of the final latexes is a measure of the free stabilizer concentration in the aqueous phase. The closer to 72.8 mN m-1, the value of pure water at room temperature, the lower the free surfactant concentration in water, or in other words, the more stabilizer is adsorbed and used for stabilization. The γlv value is of importance for the application of latexes because the free surfactant concentration determines the hydrophilicity of the obtained

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polymeric product, and thereby its water sensitivity. The higher the calculated values of γlv and E simultaneously are, the better is the stabilizer suited for emulsion polymerization. The γlv – values given in Table 2 show quite a narrow range between 51 and 56 mN m-1 for PEO

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molecular weights between 4.6 and 13 g mol-1. Values in this range of γlv are quite common also for low molecular weight stabilizers. However, for m-Lig-PEO2.5k the value is with 62.6

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considerably higher.

Expectedly, because the average particle size only is in a narrow range and in any case greater

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than 100 nm, the molecular weight of the samples considered should not vary much (Table S3). Indeed, Mn and Mw of the polymer in the latex particles vary between 23,100 – 43,900 and 117,000 – 244,000 g mol-1, respectively. The Mw – values of the polymer in the coagulum are in a similar range but the Mn – values are considerably lower. These values for the molecular

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weight are again very typical for styrene emulsion polymerization under conditions leading to average particle sizes between 300 and 400 nm.

As already stated above, significant coagulation takes place at low concentrations of Lig-PEO of

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1 wt.% in relation to the mass of monomer. Typically, the coagulum is considered as waste and not studied in more detail but discarded. However, recently published results prove that valuable

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information on the polymerization and aggregation process are missed that way.[52] Interestingly, the coagulum obtained with the Lig-PEO stabilizers is not an ill-defined bulky polymer lump or layer around the reactor wall but well dispersed in the latex. The shape of the coagulum particles is not spherical, well-defined elongated and quite flat with a size in the mm range (Figure 5, Figure S15 and S19).

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Figure 5. Coagulum formed in a weakly stabilized emulsion polymerization after filtration: a) Snapshot of coagulum obtained from l-Lig-PEO6.7k with KPS as initiator, b) optical micrograph of coagulum obtained from m-Lig-PEO9.2k with KPS as initiator and c-d) optical micrograph of coagulum obtained from m-Lig-PEO9.2k with PEGA200 as initiator.

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The optical micrographs (Figure 5) reveal that many particles of the coagulum particles retain bended and/or drawn-out shapes apparently generated under the influence of the stirrer. Some

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particles are broken and possess a hollow interior. The SEM micrographs depicted in Figure 6 prove that the surface of the coagulum particles is locally quite diverse. It contains smooth and bended regions as shown in Figure 6a and lower left

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side of Figure 6b. Furthermore, there are also regions where a huge number of sub-micrometer sized particles are stuck together and pile up (Figure 6b). Some regions show a kind of porous

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structure where the pores are partly filled with sub-micrometer sized particles (Figure 6c and d). The diversity of formed coagulum structures, particularly the formation of hollow and porous morphologies is in contrast to simple random aggregate formation of solid particles and points to anisometric stabilization with the chosen surfactant. This special phase behavior is typical for

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some amphiphilic polymers[53, 54] but however needs further investigations.

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Figure 6. SEM micrographs showing details of the morphology of the coagulum particles obtained via emulsion polymerization with Lig-PEO surfactants: a) l-Lig-PEO6.7k, b) l-LigPEO6.7k, c) and d) m-Lig-PEO2.5k.

Conclusion

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In conclusion a renewable-materials based, non-ionic surfactant system for the emulsion polymerization of styrene has been presented. Renewable lignin fragments were utilized as initiator for the oxyanionic polymerization of ethylene oxide. The thus formed amphiphiles were

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utilized as stabilizers to form emulsions of styrene in water. Heterophase polymerizations were performed as a well-documented model reaction that led to the formation of poly(styrene) latexes with solids contents of up to 21% depending on stabilizer concentration. All stabilizer

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efficiencies and performances were nicely comparable with those of nonylphenol-based, nonionic industrial performance surfactants. Moreover, unexpected coagulum morphologies were

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observed that show particulate, pill-like structures pointing to complex phase hehavior.

Supporting Information. Additional synthetic procedures, NMR, and electron microscopy data

at http://pubs.acs.org.

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are collated in Supporting Information. This material is available free of charge via the Internet

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AUTHOR INFORMATION Corresponding Author Tel

(+49)

331

567

9509;

Fax

(+49)

331

567

9502;

Email

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*(BVKJS)

[email protected] Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources

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Max-Planck society ACKNOWLEDGMENT

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This work was supported by the Max-Planck society. The authors would like to acknowledge Marlies Gräwert for SEC measurements, Chungxiang Wei for SEM and TEM measurements, Irina Shekova for tensiometry measurements, Ursula Lubahn for conducting emulsion

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polymerizations and Jochen Willersinn for DLS measurements. REFERENCES

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Table of Contents Graphic

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Renewable Lignin precursors are utilized as initiators for oxanionic polymerization of ethylene oxide. The formed amphiphilic Lignin-PEO adducts are utilized as surfactants in the emulsion polymerization of styrene. Latexes with weight contents up to 20% are obtained. Performances are comparable to nonylphenol-based, nonionic industrial performance surfactants. Low surfactant concentration leads to the formation of millimeter sized anisotropic coagulum particles.

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