Degradable Thermosets from Cellulose Acetate Allyl Carbonate via Thiol-ene Click Chemistry

Abstract

The development of degradable thermosets from renewable resources addresses the pollution associated with fossil fuel-based plastics. Here, degradable thermosets with controllable hydrophobicity and mechanical properties were prepared from renewable cellulose. Microcrystalline cellulose was dissolved in the polarity-switchable green solvent system, CO2/DBU/DMSO, followed by a dual modification of allyl carbonation and acetylation using allyl bromide and vinyl acetate, respectively, to produce cellulose acetate allyl carbonate. Reaction with allyl bromide exclusively made allyl carbonation without any allyl etherification confirmed by a 1H, 13C, and 13C-H HSQC NMR. The degree of substitution (DS) of polymers prepared by allyl carbonation or acetylation were systematically controlled over the range 0.07 − 0.45 and 0.29 − 2.92, respectively, by varying the allyl bromide and vinyl acetate contents in the reaction mixture. Cellulose acetate allyl carbonate was cross-linked with trimethylolpropane tris(3-mercaptopropionate) via a UV-induced thiol-ene click reaction. The ratio of the thiol-to-alkene ratio impacted the mechanical properties where more thiol resulted in softer thermosets. The tensile strength was controlled from 2.9 to 24.0 MPa, where elongation at break varied from 8.6 to 56.4%. Hydrophobicity of the thermosets was tuned by varying the concentration of unsubstituted –OH groups in the cellulose acetate allyl carbonate for thermoset formulations. The most hydrophilic thermoset swelled significantly in water with 136% mass gain, while the most hydrophobic formulation swelled in acetone with 205% mass gain. A viable pathway for the development of more sustainable thermosets with tunable properties could reduce the environmental footprint of plastic waste and contribute to a circular economy.