Thiol-ene eugenol polymer networks with chemical Degradation, thermal degradation and biodegradability
Introduction
The invention of plastics has brought great convenience to the society, however, the plastic pollution on the environments is becoming increasingly alarming. A variety of non-degradable plastic waste could remain in nature for decades, some even lasts for hundreds of years [1], [2]. By 2021, the world has produced over 9 billion tons of plastics with little recycled [3], [4], [5]. The remaining huge amount of plastic is difficult to be degraded and can only gradually enter the ecological cycle. It not only pollutes the land/water, but also imposes a serious threat to the health of living organisms [6].
The development of plastics having components from renewable natural resources is of great significance as it lessens the environmental problems associated with the dependence on petrochemicals [7], [8], [9], [10], [11], [12], [13]. It is expected that the global production capacity of bioplastics will increase from about 2.1 million tons in 2020 to 2.8 million tons in 2025 [14], [15]. However, the misconception between biobased and biodegradable does not help solving the eventual problems of plastics released into the environments, if they are not degradable.
Biodegradation of plastic wastes is critical in solving the pollution crisis [16]. Recent studies have utilized microbial cultures or specific microorganisms, such as bacteria and fungi isolated from insect intestines including plodia interpunctella [17], [18], darkling beetle larvae [19], Tenebrio molitor [20], Achatina fulica and Zophobas atratus [21], to degrade synthetic plastics into low molecular weight organics. Tenebrio molitor is a species of darkling beetle. Like all holometabolic insects, they go through four life stages: egg, larva, pupa, and adult. Larvae typically measure about 2.5 cm or more, whereas adults are generally between 1.25 and 1.8 cm in length. Tenebrio molitor is the most potential species to degrade plastics because it is easy for artificial breeding. These synthetic plastics mainly include hydrolysable (polyethylene glycol terephthalate, polyurethane, polycarbonate, etc.) and non-hydrolysable (polyethylene, polypropylene, polystyrene and polyvinyl chloride) [22]. The recent study reported that biodegradation of one expanded polystyrene with a weight-average molecular weight (Mw) 256.4 k Da and two low density polyethylene foams with respective Mw of 130.6 kDa and 288.7 kDa in T. monitor larvae. Gel permeation chromatography analysis confirmed broad depolymerization of polystyrene (i.e., a decrease in both Mw and Mn) but revealed limited extent depolymerization of low density polyethylene (i.e., increase in a number-average molecular weight (Mn) and decrease in Mw). For all materials, the size average molecular weight (Mz) was decreased [20]. There is a strong demand to develop and evaluate biodegradable biobased plastics.
Eugenol, also called clove oil, is an aromatic oil extracted from cloves. It is a yellow, pale phenylpropanoid compound with a hydroxyl group and an olefin group, which has been commonly used to prepare various polymers [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. Eugenol based thermosets with high glass transition temperatures exhibit good thermomechanical properties compared to the petroleum based epoxy cured with the same amines [25]. Eugenol based vitrimeric materials with succinic anhydride show improved shape changing, crack healing, and shape memory properties [26]. 1,3-dioxolan-4-one monomer was reported to be functionalized by naturally occurring eugenol to introduce a structural element, which could induce crosslinking reaction through cationic polymerization of the double bond [27]. Structure-adjustable thermosetting polyurethane networks with colorless transparent, high glass transition temperature were prepared from eugenol-based polyols, the tensile strength of the thermosetting polyurethane networks was up to 54.88 MPa [28]. Epoxy resins cured by imine-containing hardeners based on eugenol show high Tg (>120 °C) and tensile strength (>60 MPa) [29].Polysiloxane prepared from eugenol displayed high thermostability with an onset degradation and glass transition temperature of 400 and 201 °C, respectively. It displayed a low water uptake (<0.15 %) when immersed in distilled water for 144 h [31]. However, there is a lack of studies on thermal, hydrolytic and biological degradation of eugenol-based polymer crosslinked networks.
Thiol-ene reactions, one of the commonly used click chemistry [35], [36], [37], [38], were utilized to prepare monomers and polymer networks of eugenol. It has been also used to prepare eugenol-based polymer networks [39], [40], [41], [42].Herein we report the preparation and biodegradation evaluation of polymer networks using eugenol as a starting material. The synthesis was carried out via solvent-free thiol-ene click chemistry (Fig. 1), the schematic representation of all the polymer networks was showed in Fig. S1. Eugenol was first converted into eugenol multimer (dimer, trimer and tetramer) with thiols (Schemes S1, S2, and S3). Methacrylic monomers were then obtained through esterification between eugenol multimers and methacrylic anhydride. Finally, polymer networks were constructed via thiol–ene click reaction of methacrylic monomers and thiol crosslinkers. The chemical and biodegradation of the eugenol polymer networks were investigated in detail.
Section snippets
Materials
Eugenol (99 %, Sigma-Aldrich), methacrylic anhydride (MAA, 94 %, Sigma-Aldrich), 4-dimethylaminopyridine (DMAP, 99 %, Sigma-Aldrich), ethyl acetate (AR, Fisher Scientific), 1-hydroxycyclohexyl phenyl ketone (99 %, Sigma-Aldrich), 3,6-dioxa-1,8-octanedithiol (99 %, Sigma-Aldrich), pentaerythritol tetra(3-mercaptopropionate) (99 %, Sigma-Aldrich), and trimethylolpropane tris(3-mercaptopropionate) (99 %, Sigma-Aldrich) were purchased and used as received unless otherwise mentioned.
Results and discussion
Eugenol monomers, eugenol di-methacrylate (EDM), eugenol tri-methacrylate (ETM), and eugenol tetra-methacrylate (ETTM), were synthesized in two steps: first thiol–ene click reaction between eugenol and thiols, and then esterification with methacrylic anhydride (Schemes S1–S3). Scheme S1 shows the synthesis of eugenol dimer and EDM. As seen from FTIR spectra of eugenol (Fig. 2a), a broad peak at 3525 cm−1 and a strong sharp peak at 909 cm−1 are attributed to –OH stretching and = CH bending,
Conclusions
In summary, we developed a green and sustainable approach to the synthesis of methacrylic monomers and polymer networks using eugenol as the raw material. Variations in the chemical structures of methacrylic monomers and thiol crosslinkers produced tailorable thermal and mechanical properties in the biobased polymer networks. All polymer films showed high transmittance. Additionally, these biobased polymer networks underwent full chemical degradation into carboxylic acids and other small
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Puyou Jia reports financial support was provided by Chinese Academy of Forestry.
Acknowledgements
Puyou Jia acknowledged the support by the Fundamental Research Funds from Jiangsu Province Biomass and Materials Laboratory (JSBEM-S-202001).
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