Synthesis of a Liquid Lignin-Based Methacrylate Resin and Its Application in 3D Printing without Any Reactive Diluents

3D printing of bio-based and renewable polymers such as lignin has gained research attention during the last few decades. We report on the synthesis and characterization of a liquid lignin-based photopolymer and its application in additive manufacturing (AM). Wheat straw soda lignin is liquified in an oxyalkylation reaction with propylene oxide under alkaline conditions and modified with methacryloyl chloride to obtain a lignin-based methacrylate resin. Ninety percent of the functional hydroxyl groups are grafted during the synthesis. The photopolymerization efficiency was evaluated by real-time-NIR-photorheology experiments with two different photoinitiators, leading to double bond conversions (DBC) of ≥80%. 3D-printing experiments of the methacrylated lignin were performed with the hot lithography technology. For the first time, a light-curable lignin derivative with a lignin content of over 30% was successfully 3D printed via vat photopolymerization without any reactive diluents, which is a significant improvement over current state-of-the-art solutions. This outstanding result is a motivating proof of concept and a promising starting point for the in-depth evaluation of bio-based precursors as an alternative to nonrenewable derivatives for 3D printing.


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: Assignment of the 13 C-1 H cross-signals in the 2D HSQC spectrum of PB1000. 1 III Figure S1: Aromatic (left) and side-chain (right) regions in the 2D-HSCQ-NMR spectrum of PB1000. Figure S2: 13 C-NMR spectrum of PB1000 in DMSO-d6.   Table S3. An exemplary quantitative 31 P-NMR-spectrum of PB1000 with detailed information of the functional OH groups is shown in Figure S4.  Figure S4: Quantitative 31 P-NMR-spectrum of PB1000 with assignment of the different functional OH groups.

Calculation of lignin content via quantitative 31 P-NMR-spectroscopy
The lignin content in L-PO and L-PO-MAC samples is determined by the OH number obtained from 31 P-NMR-spectroscopy. The results are given in Table S5.

Determination of grafted chain length by 1 H-NMR-spectroscopy
For determination of the grafted chain length, 1 g L-PO is acetylated with 8.1 mL pyridine (0.1 mol, 1 eq) and 5 mL acetic anhydride (0.1 mol, 1.6 eq). The mixture is stirred for 24 h at room temperature. After 24 h the acetylated L-PO was cooled with an ice bath and 50 mL ethanol was added in order to esterify the remaining excess of acetic anhydride. The solution was stirred for 30 min and the solvent was evaporated to dryness with a membrane pump vacuum (100 mbar) afterward. The procedure was repeated 3 times. The residue was extracted with a dichloromethane/water mixture and the organic phase was dried with sodium sulfate. After filtration, the product was dried with membrane pump vacuum. 1 H-NMR spectra were measured in 0.6 µL CDCl3 with a Bruker Avance 400 or Bruker Avance 600 MHz spectrometer (16 scans).
The 1 H-NMR spectrum of L-PO 1 is shown in Figure S6. Afterward, the signal of CDCl3 was referenced to 7.26 ppm and then the integral of the CH group from the chain end at 5 ppm was set to 1. All other signals were integrated as described in Table S6.  Figure S6: 1 H-NMR-spectrum of acetylated L-PO 1 measured in CDCl3.
The statistical chain length nstat. of grafted PPG chains can be calculated by dividing integral (D) by integral (C). During oxyalkylation of PB1000 phenolic and carboxylic OH groups are grafted near to quantitative with PPG chains. In contrast, some aliphatic OH groups may remain unmodified after oxyalkylation. It is possible to distinguish between unreacted aliphatic OH groups and grafted aliphatic OH groups in the 31 P-NMR-spectrum. However, unmodified aliphatic OH groups will be acetylated as well in the acetylation process and would falsify the value of the statistical chain length. Therefore, the average chain length naverage can be calculated since this value is only related to the PPG repetition unit.  Table S7 and Table   S8.

Calculation of theoretical yield and practical outcome of L-PO-MAC (DS=100%)
The theoretical yield myield,theo. L-PO-MAC is calculated by multiplication of the used molar ratio of nL-PO with the molecular weight of the methacrylate group Mmethacryl group followed by the addition of the used L-PO mass mL-PO. This is the theoretical yield if 100 % of the available double bonds would be converted into reactive double bonds. This theoretical yield can be corrected by multiplication of the real OH conversion

Gel permeation chromatography (GPC) data
GPC measurements were conducted according to the procedure described in the main manuscript. OmniSEC Version 5.10.461 was used for the analysis of the chromatograms. First, a conventional calibration curve XIV was created from the measured polystyrene standards before result evaluation. For each chromatogram, baseline and peak limits were set manually.