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Abstract
Antioxidant norbornene-based monomers bearing biobased sterically hindered phenols (SHP)—NDF (norbornene dihydroferulate) and NDS (norbornene dihydrosinapate)—have been successfully prepared through biocatalysis from naturally occurring ferulic and sinapic acids, respectively, in presence of Candida antarctica Lipase B (Cal-B). The ring opening metathesis polymerization (ROMP) of these monomers was investigated according to ruthenium catalyst type (GI) vs. (HGII) and monomer to catalyst molar ratio ([M]/[C]). The co-polymerization of antioxidant functionalized monomer (NDF or NDS) and non-active norbornene (N) has also been performed in order to adjust the number of SHP groups present per weight unit and tune the antioxidant activity of the copolymers. The polydispersity of the resulting copolymers was readily improved by a simple acetone wash to provide antioxidant polymers with well-defined structures. After hydrogenation with p-toluenesulfonylhydrazine (p-TSH), the radical scavenging ability of the resulting saturated polymers was evaluated using α,α-diphenyl-β-picrylhydrazyl (DPPH) analysis. Results demonstrated that polymers bearing sinapic acid SHP exhibited higher antiradical activity than the polymer bearing ferulic acid SHP. In addition it was also shown that only a small SHP content was needed in the copolymers to exhibit a potent antioxidant activity.
Highlights
The fundamental principle for the stabilization of polymeric materials consists in preserving the initial aesthetic and mechanical properties of a polymer during processing, storage, and application
We report a new and greener pathway to biobased antioxidant phenolic polymers through the chemoenzymatic synthesis of new hindered phenol-functionalized norbornene derivatives from naturally occurring ferulic and sinapic acids using Candida antarctica Lipase B (Cal-B) and their homo/copolymerization using ring opening metathesis polymerization (ROMP)
The first step consisted in synthesizing ethyl dihydroferulate (1, 98%) and ethyl dihydrosinapate (2, 92%) from ferulic acid and sinapic acid, respectively, through a two-step one-pot route involving Fisher esterification and palladium-mediated hydrogenation. 5-Norbornene-2endo,3endo-dimethanol (3) was synthesized through the reduction of 5-norbornene2,3-dicarboxylic anhydride in presence of LiAlH4 (94% yield)
Summary
The fundamental principle for the stabilization of polymeric materials consists in preserving the initial aesthetic and mechanical properties of a polymer during processing, storage, and application. The first approach involves the post-functionalization of reactive groups of natural (Arefjev et al, 1999; Sousa et al, 2009) or synthetic (Kim and Oh, 2004) polymer chains with sterically hindered phenol groups (SHP) with the aim to improve their antioxidant activity This method is quite efficient and accessible, but may trigger undesirable side reactions, especially regarding the difficulty to obtain full distribution of stabilizers at the reactive sites of the matrix. We report a new and greener pathway to biobased antioxidant phenolic polymers through the chemoenzymatic synthesis of new hindered phenol-functionalized norbornene derivatives from naturally occurring ferulic and sinapic acids using Candida antarctica Lipase B (Cal-B) and their homo/copolymerization using ROMP.
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