Cellulose modification for sustainable polymers: overcoming problems of solubility and processing

Two new water-soluble cellulose derivatives were prepared by a two-step transformation with 1,3-propane sultone, followed by either maleic or succinic anhydride, thereby converting cellulose into a more easily processable form. It was found that the solubility was dependent on both the degree of substitution and the chemical properties of the substituents. The water-soluble cellulose has a molecular weight greater than 100 000 g mol−1 and both the morphology and molecular weight can be tuned by varying the reaction conditions. Furthermore, the flexible, two-step nature of the process allows for expansion of this methodology in order to prepare cellulose analogues for different applications.

Electronic Supplementary Material (ESI) for RSC Sustainability.This journal is © The Royal Society of Chemistry 2023
Infrared (IR) spectra were recorded on a FT-IR Spectrum Two with a KBr window and a LiTaO3 detector from PerkinElmer.Assignment was based on literature values. 1 Proton nuclear magnetic resonance ( 1 H NMR) spectra and carbon nuclear magnetic resonance ( 13 C NMR) spectra were recorded at 400 MHz using 400/54 Premium Shield from Agilent Technologies.The chemical shifts were reported in ppm and referenced to the solvent signal of partly deuterated D2O at 4.79 ppm. 2 Signal multiplicities are reported as singlet (s), doublet (d), triplet (t), quartet (q), quintet (quin), sextet (s) and multiplet (m).Gel permeation chromatography (GPC) was performed on a Vanquish from Thermo Scientific equipped with a 2 PSS Suprema column: (100; 1000 Å, 300 x 8 mm 10 μm) using aqueous 0.05 M NaNO3 as eluent.The column was operated at 40 °C with an injection volume of 10 μL and a flow rate of 1 mL/min.Detection was accomplished using a RID detector.The molecular weight was calculated with ethylene glycol as an internal standard using a Pullulan calibration.X-ray diffraction (XRD) patterns were recorded on a D8 Advance from Bruker using Cu K α radiation (0.154 nm), measuring between 2θ of 10 ° and 50 ° at a step rate of 0.020 °/s.Field Emission Scanning Electron Microscope (FE-SEM) images were recorded on a Nova NanoSEM, without sputtering, using a Low Vacuum Detector.Thermogravimetric analysis (TGA) was performed on a TGA5500 from TA instruments, using a platinum pan.Measurements were carried out using N2 as the carrier gas, the temperature range was between 25 and 700 °C with a ramp rate of 10 °C/min.

S2 Transformations of cellulose
For water-soluble cellulose, NMR, IR, XRD and GPC analyses were performed.For cellulose analogues that were insoluble in water, characterisation is limited to XRD and IR.

Adapted from Natus and Goethals 3
In a 100 mL round bottomed flask, cellulose (2.0 g, 12 mmol, RMM assumed to be 162.14gmol -1 based on the anhydroglucose unit, AGU) was suspended in i-PrOH (15 mL) and water (1.8 mL).Then 30 w/w NaOH (2.41 mL, 6.1 mmol, wt.% NaOH of total solution = 4.5) was added and stirred at 45 ˚C for 7 h and then overnight at RT.The suspension was poured into methanol (60 mL).The suspension was filtered with suction and washed with MeOH before drying in a desiccator under vacuum at RT overnight.A white solid, 2.142 g was recovered.
To test for water-soluble fractions, the filtrate was dried via rotary evaporation to leave a white solid.This material was analysed by 1 H NMR and IR.No cellulose was present.

S2.2 Reaction of cellulose with 1,3-propane sultone
Adapted from Natus and Goethals. 3 a 100 mL round bottomed flask, cellulose (2.0 g, 12 mmol, RMM assumed to be 162.14gmol -1 based on AGU) was suspended in i-PrOH (15 mL) and water (1.8 mL).Then 30 w/w NaOH (2.41 mL, 24 mmol, wt.% NaOH of total solution = 4.5) was added and the mixture was stirred at 45 ˚C for 1 h.1,3-Propane sultone (2.9 g, 24 mmol) was dissolved in acetone (2.3 mL) and added to the mixture.This was stirred for 6 h at 45 ˚C and then at RT overnight (16 h).The suspension was poured into methanol (60 mL).The suspension was filtered with suction and washed with MeOH before drying in a desiccator under vacuum at RT overnight.A white solid, 3.161 g, was recovered.
To test for water-soluble fractions, the filtrate was dried via rotary evaporation to leave a white solid.This material was analysed by 1 H NMR and IR.No cellulose was present.

S2.3 Reaction of cellulose with cyclic anhydrides
Adapted from de Melo et al. 4 Cellulose (0.10 g) and maleic anhydride (2 g) were added to a 20 mL reaction vial with a stir bar (cellulose 5 wt.% compared to maleic anhydride).A vacuum adapter was added to the vial which was vacuum filled with nitrogen (x3) and nitrogen flow was maintained throughout the reaction.The vial was placed in an aluminium block pre-heated to 120 ο C. The maleic anhydride melted immediately to form a suspension.The reaction was continued for 6 h.After 6 h the vial was cooled under N2.Water (ca.20 mL) was added to dissolve the unreacted maleic anhydride and precipitate the cellulose.The mixture wad filtered and washed with water and acetone before drying overnight under vacuum in a desiccator.A white solid, 0.078 g, was obtained.To test for water-soluble fractions, the filtrate was dried via rotary evaporation to leave a white solid.This material was analysed by 1 H NMR and IR.No cellulose was present.

IR νmax/cm
An identical procedure was used to modify cellulose using:

S2.4 Preparation of water-soluble cellulose
Cellulose previously modified with propane sultone (Section S2.2, 0.10 g) maleic anhydride (2g) were placed in a 20 mL reaction vial with a stir bar (cellulose 5 wt.% compared to maleic anhydride).A vacuum adapter was added to the vial which was vacuum filled with nitrogen (x3) and nitrogen flow was maintained throughout the reaction.The vial was placed in an aluminium block pre-heated to 120 ο C. The maleic anhydride melted immediately to form a suspension.The reaction was continued for 6 h.After 6 h the vial was cooled under N2.An off-white solid formed.A 1:2 mixture of acetone and water (ca.50 mL) was added to dissolve the solid.Using rotary evaporation, acetone was first removed and the aqueous solution concentrated until just before solid began to precipitate.Acetone was added until the water-soluble cellulose precipitated.The cellulose was collected via centrifugation for 10 min at 10000 RPM and washed with acetone (3 x 10 mL), centrifuging as before.The obtained white solid was dried overnight under vacuum in a desiccator.A white solid, 0.57 g, was obtained.

S2.5 NaOH test reactions
In a 20 mL reaction vial, cellulose (0.67g, 4.13 mmol, RMM assumed to be 162.14gmol -1 based on AGU) was suspended in i-PrOH (5 mL) and water (0.6 mL).Then 30 w/w NaOH (0.81 mL, 24 mmol, wt.% NaOH of total solution = 4.5) was added and stirred at 45 ˚C for 1 h.Maleic anhydride (0.82 g, 8.36 mmol) was dissolved in acetone (0.77 mL) and added to the mixture.This was stirred for 6 h at 45 ˚C and then at RT overnight (16 h).The suspension was poured into methanol (15 mL).The suspension was filtered with suction and washed with MeOH before drying in a desiccator under vacuum at RT overnight.A white solid, 0.994 g, was recovered.
To test for water-soluble fractions, the filtrate was dried via rotary evaporation to leave a white solid.This material was analysed by 1 H NMR and IR.No cellulose was present.
This modified cellulose was reacted with maleic anhydride using the procedure below: Modified cellulose (0.10 g) and maleic anhydride (2 g) were added to a 20 mL reaction vial with a stir bar (cellulose 5 wt.% compared to maleic anhydride).A vacuum adapter was added to the vial which was vacuum filled with nitrogen (x3) and nitrogen flow was maintained throughout the reaction.The vial was placed in an aluminium block pre-heated to 120 ο C. The maleic anhydride melted immediately to form a suspension.The reaction was continued for 6 h.After 6 h the vial was cooled under N2.Water (ca.20 mL) was added to dissolve the unreacted maleic anhydride and precipitate the cellulose.The mixture wad filtered and washed with water and acetone before drying overnight under vacuum in a desiccator.0.098 g solid was obtained.
To test for water-soluble fractions, the filtrate was dried via rotary evaporation to leave a white solid.This material was analysed by 1 H NMR and IR.No cellulose was present.This procedure was identical for the modification of cellulose using:

S2.6 Preparation of a fluorescent cellulose derivative
Cellulose previously modified with propane sultone (Section S2.2, 0.20 g), maleic anhydride (1g), and 1,8-naphthalic anhydride (1 g) were placed in a 20 mL reaction vial with a stir bar (cellulose 10 wt.% compared to total anhydride).A vacuum adapter was added to the vial which was vacuum filled with nitrogen (x3) and nitrogen flow was maintained throughout the reaction.The vial was placed in an aluminium block pre-heated to 120 ο C. The maleic anhydride melted immediately to form a suspension of cellulose and 1,8-naphthalic anhydride.The reaction was continued for 16 h.After 16 h the vial was cooled under N2.
Purification was carried out in one of two ways: A 1:2 mixture of acetone and water (ca.50 mL) was added to dissolve the solid.Using rotary evaporation, acetone was first removed and the aqueous solution concentrated until just before solid began to precipitate.Acetone was added until the water-soluble cellulose precipitated.The cellulose was collected via centrifugation for 10 min at 10000 RPM and washed with acetone (3 x 10 mL), centrifuging as before.The mixture wad filtered and washed with acetone before drying under vacuum.
Acetone (ca.30 mL) was added to dissolve unreacted anhydride.The mixture wad filtered and washed with acetone before drying under vacuum.

S3.1 Homo-Substituted Cellulose
For homo-substituted cellulose the degree of substitution was calculated from elemental analysis.Unmodified cellulose was analysed and found to have a carbon content of 42.03.Graphs of theoretical carbon content at degree of substitution 1, 2, and 3 were plotted for maleic anhydride (Figure S1) and succinic anhydride (Figure S2).With these graphs, the measured carbon content of the modified cellulose was used to determine the degree of substitution.For 1,3-propane sultone modified cellulose, a similar procedure was used with sulfur content (Figure S3).S3: The graph used to determine the degree of substitution for 1,3-propane sultone modified cellulose.

S3.2 Hetero-substituted cellulose
Elemental analysis could not be used to calculate the degree of substitution of hetero-substituted cellulose as the theoretical carbon content could not be accurately estimated.Therefore, a modified back-titration method was used to determine the DS of the carboxylate groups.This was added to the DS of the sulfate groups previously determined by elemental analysis, by assuming that the sulfate group existed as the sodium salt. 5,6A representative procedure and calculation is given below: 0.035 g modified cellulose was suspended in 22 mL 0.009 M NaOH and stirred for 1 h.The pH of the NaOH was measured before adding modified cellulose and after the 1 h reaction.The pH before = 11.45,pH after = 11.08 The moles of carboxylate substituent groups was calculated using equations ia-iiia.Spectrum S18: 13 C NMR (D2O, 100 MHz) spectrum of water-soluble cellulose prepared using a 6 h reaction with maleic anhydride (Section S2.4).

S6 Gel Permeation Chromatography (GPC) Data
Figure S4: A plot of the GPC traces for water-soluble cellulose.

Figure S10 :
Figure S10: Low vacuum FE-SEM image of cellulose at 2000 x magnification.

Figure S13 :
Figure S13: Low vacuum FE-SEM image of sulfonated cellulose at 1000 x magnification.

Figure S17 :
Figure S17: Low vacuum FE-SEM image of water-soluble cellulose from 6 h maleic anhydride reaction at 2000 x magnification.

Figure S18 :Figure S19 :
Figure S18: Low vacuum FE-SEM image of water-soluble cellulose from 6 maleic anhydride reaction at 8000 x magnification.

Figure S20 :
Figure S20: Low vacuum FE-SEM image of water-soluble cellulose from 6 h succinic anhydride reaction at 2000 x magnification.

Figure S21 :
Figure S21: Low vacuum FE-SEM image of water-soluble cellulose from 6 h succinic anhydride reaction at 4000 x magnification.

Figure S22 :Figure S23 :
Figure S22: Low vacuum FE-SEM image of water-soluble cellulose from 6 h succinic anhydride reaction at 8000 x magnification.
The graph used to determine the degree of substitution for maleic anhydride modified cellulose.The graph used to determine the degree of substitution for succinic anhydride modified cellulose.

Table S1 :
Mw and Mn values of water-soluble cellulose obtained from GPC using a Pullulan calibration.XRD pattern obtained for cellulose, sulfonated cellulose, maleic anhydride modified cellulose and succinic anhydride modified cellulose (Main Paper, Scheme 1).