Designed for Molecular Recycling: A Lignin-Derived Semi-aromatic Biobased Polymer

Summary

An example of a closed-loop approach towards a circular materials economy is described here. A whole sustainable cycle is presented where biobased semi-aromatic polyesters are designed by polymerization, depolymerization, and then re-polymerized with only slight changes in their yields or final properties.

Abstract

The development of chemically recyclable biopolymers offers opportunities within the pursuit of a circular economy. Chemically recyclable biopolymers make a positive effort to solve the issue of polymer materials in the disposal phase after the use phase. In this paper, the production of biobased semi-aromatic polyesters, which can be extracted entirely from biomass such as lignin, is described and visualized. The polymer poly-S described in this paper has thermal properties similar to certain commonly used plastics, such as PET. We developed a Green Knoevenagel reaction, which can efficiently produce monomers from aromatic aldehydes and malonic acid. This reaction has been proven to be scalable and has a remarkably low calculated E-factor. These polyesters with ligno-phytochemicals as a starting point show an efficient molecular recycling with minimal losses. The polyester poly(dihydrosinapinic acid) (poly-S) is presented as an example of these semi-aromatic polyesters, and the polymerization, depolymerization, and re-polymerization are described.

Introduction

In contrast to the incineration of polymeric waste, chemical recycling offers the possibility to recover the monomers. Chemical recycling is a logical choice at the end of the technical life of polymeric materials since these polymeric materials are produced chemically¹. There are two ways to recycle the polymeric material chemically, pyrolysis and molecular recycling². With pyrolysis, the polymeric material is transformed into products of higher value by using extreme conditions^3,4. Molecular recycling is an efficient method for recovering the starting materials using depolymerization. After depolymerization, the monomeric units can be repolymerized into virgin polymeric materials⁵. The availability of suitable monomers to apply molecular recycling on a larger scale is wanting. The current plastic problem dictates that society demands sturdy and robust polymeric materials. At the same time, it is also preferred that the same polymeric materials are easily recyclable and do not endure in the environment. Current polymeric materials with good thermal and mechanical properties do not depolymerize easily⁶.

Lignin, commonly found in vascular plants, is responsible for 30% of the world's natural carbon content and is the second most abundant biopolymer after cellulose. Lignin has a complex amorphous structure and appears to be a suitable alternative to replace aromatics extracted from fossil materials. The three-dimensional structure of lignin provides strength and stiffness to wood, as well as resistance to degradation. Chemically speaking, lignin is a very complex polyphenolic thermoset. It consists of varying composition of three different methoxylated phenylpropane structures. Syringyl, guaiacyl, and p-hydroxyphenyl (often abbreviated as S, G, and H, respectively) are derived from the monolignols sinapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol⁷. The distribution of these units differs per biomass type, with softwood, for instance, consisting of mostly guaiacyl units and hardwood of guaiacyl and syringyl units^8,9. Renewable natural sources, such as trees and plants, are desirable for the production of redesigned monomers for innovative polymeric materials¹⁰. These monomers, isolated and synthesized from natural sources, are polymerized to so-called biobased polymers¹¹.

Aromatic carboxylic acids are several orders of magnitude less electrophilic than the equivalent aliphatic carboxylic acids for electronic reasons¹². Various commercial polyesters use aromatic carboxylic acids instead of aliphatic carboxylic acids. As a result, the fibers in polyester textiles made from poly(ethylene terephthalate) (PET) fibers are almost insensitive to hydrolysis during washing or, for example, rain¹³. When the molecular recycling of polyesters is wanted, it is advisable to use aliphatic esters in the build-up of the polymer.

For the reasons mentioned, we have investigated the possibilities of making polyesters from 4-hydroxy-3,5-dimethoxy-dihydrocinnamic acids¹⁴. Previous studies by Kricheldorf¹⁵, Meier¹⁶, and Miller^17,18 show that it is challenging to build polymers using 4-hydroxy-3,5-dimethoxy-dihydrocinnamic acid. Decarboxylation and crosslinking hindered the polymerizations, and so limiting the success of these syntheses. Also, the mechanism of the polycondensation remained unclear. The presented paper describes the conditions in which the polyester poly(dihydrosinapinic acid) can be synthesized regularly and in high yield, thus paving the way for using semi-aromatic polyesters that are molecularly recyclable.

We have developed a green and efficient way to synthesize sinapinic acid using a condensation reaction between syringaldehyde and malonic acid^19,20. After this Green Knoevenagel, hydrogenation produces dihydrosinapinic acid, which is suitable for reversible polycondensation. This publication visualizes the synthetic steps to the molecularly recyclable polymer poly(dihydrosinapinic acid), referring to the base units of lignin, called poly-S. After analyzing the polymeric material, poly-S is depolymerized to the monomer dihydrosinapinic acid under relatively favorable conditions and repolymerized over and over again.

Protocol

1. Green Knoevenagel condensation of syringaldehyde towards sinapinic acid with 5 mol% ammonium bicarbonate

1. Add malonic acid (20.81 g, 200.0 mmol) together with syringaldehyde (36.4 g, 200.0 mmol) to a 250 mL round-bottom flask. Dissolve both constituents in 20.0 mL of ethyl acetate and add ammonium bicarbonate (790 mg, 10.0 mmol) to the flask.
   NOTE: To ensure full completion of the condensation reaction, the rotary evaporator can be used to distill the ethyl acetate and concentrate the reaction mixture, resulting in a solvent-less reaction.
2. Keep the reaction mixture for 2 hours at 90 °C in the open flask without stirring for complete conversion to sinapinic acid.
   NOTE: During this reaction, several morphological changes are observed. The reaction mixture changes from a dense grey mixture to a dissolved yellow mix due to the formation of water during the condensation step of the reaction. After evaporation of the condensate, the reaction mixture solidifies again, indicating full conversion.
3. In the work-up of the sinapinic acid product, dissolve the residue in 100 mL of a saturated aqueous NaHCO₃-solution. Transfer the solution to a beaker and subsequently acidify it to a pH of 2 using 6 M HCl.
   NOTE: While adding the saturated aqueous NaHCO₃-solution, the crude product dissolves slowly and requires some time and manual scraping to dissolve fully. Once wholly dissolved and transferred to the beaker, the acidic solution of 6 M HCl is added drop-wise. Sinapinic acid will precipitate quickly, and the release of CO₂ gas will cause the product to foam.
4. Separate the resulting residue by vacuum filtration and wash it with demineralized water.
   NOTE: At this point, the purity of the product can be analyzed by HPLC. If the product appears contaminated, the next purification step (e.g., recrystallization) must be applied.
5. After recrystallization in a mixture of water-ethanol (4:1, v/v), separate the crystals by vacuum filtration. Dry the residue at 60 °C in a vacuum oven, resulting in 42.56 g of pure sinapinic acid.
   NOTE: The purity of sinapinic acid is measured by melting point analysis and HPLC.

2. Hydrogenation of sinapinic acid towards dihydrosinapinic acid with Raney^TM nickel.

1. Charge a 450 mL flask (using a hydrogenation apparatus) with sinapinic acid (33.6 g, 150 mmol). Dissolve the sinapinic acid in 300 mL of 2 M NaOH solution and add 1.5 g of nickel slurry (see Table of Materials) before fitting the flask to the apparatus.
   NOTE: The color of the reaction mixture changes due to the addition of the alkaline solution. Sinapinic acid will be yellow in its crystalline form, but when deprotonated by the base, the color of the solution will change to red. The addition of nickel will turn the color black.
2. Pressurize the reactor with three bar of hydrogen gas and mechanically shake the reaction at 80 °C for three hours.
3. Cool the reactor to room temperature and depressurized slowly. Recover most of the nickel catalyst with a magnet and subsequently filter the solution under reduced pressure.
   NOTE: The reaction mixture should change from red to green color, visible after filtration. The notable color change is because the conjugated system is interrupted by hydrogenation of the double bond.
4. Acidify the solution with a 4 M HCl solution towards a pH of 2. Subsequently, perform an extraction with ethyl acetate (4 times 50 mL). Remove the solvent under reduced pressure after drying over MgSO₄. Dry the solid product dihydrosinapinic acid at 60 °C in a vacuum oven.
   NOTE: Theoretically, the polycondensation reaction can be processed on dihydrosinapinic acid, but there are some practical drawbacks. Due to the molecular structure, dihydrosinapinic acid sublimates above the melting point. An acetylation reaction can be performed to prevent sublimation and increase reactivity.

3. Acetylation of dihydrosinapinic acid towards acetylated monomers and oligomers (prepolymer).

1. Add dihydrosinapinic acid (22.6 g, 100 mmol) to a 250 mL round-bottom flask, followed by acetic anhydride (14.2 mL, 150 mmol) and sodium acetate (0.82 g, 10 mmol).
   NOTE: The dihydrosinapinic acid does not fully dissolve in the acetic anhydride at room temperature.
2. Heat the solution to 90 °C while stirring for one hour for complete conversion of dihydrosinapinic acid towards 4-acetoxy-dihydrosinapinic acid and its acetylated oligomers.
   NOTE: All the products remain dissolved in acetic anhydride. To increase the total precipitation of 4-acetoxy-dihydrosinapinic acid, add a small amount of water-soluble solvent in the next step.
3. Dissolve the solid in 25 mL of acetone, precipitate (solvent-non solvent) into 250 mL of 0.1 M HCl while stirring vigorously and filter under vacuum.
   NOTE: The best result shows an entirely precipitated white, sticky solid. If the product solution is added to the HCl solution too quickly, the product remains as a brown liquid at the bottom of the acidic solution. In this situation, extract the entire product solution with ethyl acetate (4 times 50 mL). After drying over MgSO₄, remove the ethyl acetate under reduced pressure. The product is 4-acetoxy-dihydrosinapinic acid and prepolymer.

4. Polymerization of acetylated monomers and oligomers.

1. Add the monomers, 4-acetoxy-dihydrosinapinic acid (20.8 g, 100 mmol), and prepolymer to a 100 mL round-bottom flask and add finely powdered NaOH (400 mg, 10.0 mmol). Heat the reaction mixture for three hours at a set temperature of 140 °C in the open flask while stirring at 100 rpm.
   NOTE: During this reaction, water and acetic acid condense from the open flask. The pH of the vapor can be measured to confirm the condensation of acetic acid. The morphology of the reaction product changes from a molten state to a compact, light-brown product. The setup is subjected to a nitrogen flow to promote the evaporation of the condensate and prevent oxidation. Such reactions are usually stirred with a mechanical stirrer due to the high viscosity of the mixture. However, the use of a magnetical stirrer is sufficient, and the difference is negligible on this scale.
2. Set up a solvent assisted polymerization by adding zinc(II)acetate (180 mg, 1 mmol) and 25.0 mL of 1,2-xylene to the flask. Raise the set temperature to 160 °C.
   NOTE: Raise the set temperature to 160 °C to ensure the reflux of 1,2-xylene and full reactivity of the polymerization. The temperature of the reaction itself remains at 144 °C, due to the boiling point of the 1,2-xylene.
3. Reflux the mixture at 144 °C for three hours with constant water and acetic acid removal using a Dean-Stark head.
   NOTE: After adding the 1,2-xylene and heating the reaction flask to the required temperature, the reagents form a brown slurry with a lower viscosity. This lower viscosity is presumed to benefit the mobility of the end-groups of the polymer, promoting reactivity.
4. Cool the reaction mixture down and remove the 1,2-xylene by applying a vacuum (<10 mbar).
   NOTE: The reaction mixture solidifies while evaporating the solvent and turns into an off-white solid.
5. Raise the temperature to 240 °C during the final stage of the polymerization and apply a high vacuum <1 mbar for thirty minutes.
   NOTE: Only a slight amount of condensate evaporates during this step because only a small amount of condensation is needed to boost the chain length substantially at this stage of polymerization.
6. Cool the polymer poly-S to room temperature and wash with methanol to eliminate 4-acetoxy-dihydrosinapinic acid and prepolymers. The obtained product is a light-brown solid.
   NOTE: After the process, the chain length and thermal properties of the polyester are investigated by GPC and DSC.

5. Representative procedure for the depolymerization of poly-S in 1 M NaOH:

1. Finely grind and sieve poly-S into particles smaller than 180 μm to measure hydrolytic degradation.
   NOTE: To grind the particles to the required size, use a mortar and pestle with liquid nitrogen-cooled poly-S, followed by a mechanical sieving step.
2. Load several test tubes with 20 mg of poly-S and add 1.0 mL of 1 M NaOH solution. Incubate the tubes at three different temperatures (RT, 50, and 80 °C), with an agitation of 500 rpm using a controlled environment incubator shaker.
   NOTE: Use a heating-block/shaker with multiple entries for this reaction to ensure all the reactions take place in precisely the same (thermal) conditions.
3. Neutralize a test tube at regular time intervals with 1.0 mL of 0.5 M H₂SO₄ and add 2.0 mL of methanol after cooling.
   NOTE: Small amounts of poly-S and only 1.0 mL of 1 M NaOH are used because these concentrations can, after the addition of methanol, be directly injected in HPLC and do not require any further dilution.
4. Filter all samples using 0.45 μm PTFE syringe filters and inject (20 μL) on HPLC using an autosampler. Monitor the absorbance at λ = 254 nm, and calculate the concentrations from a calibration curve of known dihydrosinapinic acid standard solutions.
   NOTE: The standard solutions of the calibration curve should be made in the same solvent mixture as the work-up after the depolymerization.

Results

Sinapinic acid was synthesized in high purity and high yield (> 95%) from syringaldehyde using the Green Knoevenagel condensation. (Supporting Information: Figure S1) The E-factor is an indication of waste production where a higher number indicates more waste. The E-factor is calculated by taking the total material input, subtracting the amount of the desired end product, and dividing the whole by the amount of the end product. This Green Knoevenagel condensation has an E-factor of 1.0, which can be ...

Discussion

When dihydrosinapinic acid was heated in a reaction vessel, sublimation of the starting material occurred, and this effect was enhanced when a vacuum was applied. Acetylation has been performed on dihydrosinapinic acid to avoid sublimation. Kricheldorf et al.^12,27 recognized that not only acetylation but similarly di- and oligomerization occurred. However, these esterified monomers and oligomers no longer sublimate and are suitable as monomers for the melt polyco...

Materials

Name	Company	Catalog Number	Comments
Reaction 1: Green Knoevenagel condensation
Ammonium bicarbonate	Sigma Aldrich	>99%
Ethanol	Boom	Technical grade
Ethyl acetate	Macron	99.8%
Hydrochloric acid	Boom	37%
Malonic acid	Sigma Aldrich	99%	used as received
Sodium bicarbonate	Sigma Aldrich	>99.7%
Syringaldehyde	Sigma Aldrich	98%	used as received
Reaction 2: Hydrogenation
Magnesium sulfate	Macron	99%	dried
Raney™ nickel	Sigma Aldrich	>89%
Sodium hydroxide	Boom	Technical grade	dissolved
Reaction 3: Acetylation
Acetic anhydride	Macron	>98%
Acetone	Macron	>99.5%
Sodium acetate	Sigma Aldrich	>99%
Reaction 4A: Polymerisation
1,2-xylene	Macron	>98%
Sodium hydroxide	Boom	Technical grade	finely powdered
Zinc(II)acetate	Sigma Aldrich	99.99%
Reaction 4B: Depolymerisation
Sodium hydroxide	Boom	Technical grade	dissolved
Sulfuric acid	Macron	100%
Analysis
CDCl3	Cambride Isotope Laboratories, Inc.	99.5%
CF3COOD	Cambride Isotope Laboratories, Inc.	98%
Dimethylformamide	Macron	>99.9%
Hexafluoro-2-propanol	TCI Chemicals	>99%
Methanol	Macron	>99.8%
Tetrahydrofuran	Macron	>99.9%