Syringaldehyde, malonic acid, ferulic acid, Pd/C, norbornene-endo-2,3-dicarboxylic anhydride and lithium aluminum hydride were purchased from Sigma-Aldrich. Candida antarctica Lipase B was purchased from Novozyme. Solvents were purchased from ThermoFisher Scientific and VWR. Deuterated chloroform (CDCl₃) and acetone ((CD₃)₂CO) were purchased from Euriso-top. Column chromatographies were carried out with an automated flash chromatography (PuriFlash 4100, Interchim) and pre-packed INTERCHIM PF-30SI-HP (30 μm silica gel) columns using a gradient of cyclohexane and ethyl acetate for the elution. NMR analyses were recorded on a Bruker Fourier 300 (Supplementary Data Sheet 1). ¹H NMR spectra of samples were recorded in CDCl₃ at 300 MHz, chemical shifts were reported in parts per million (CDCl₃, CHCl₃ residual signal at δ = 7.26 ppm; Acetone-d₆, acetone residual signal at δ = 2.05 ppm). ¹³C NMR spectra of samples were recorded at 75 MHz (CDCl₃ signal at δ = 77.16 ppm; Acetone residual signal at δ = 29.84 and 206.26 ppm). HRMS were recorded by the PLANET platform at URCA on a Micromass GC-TOF.

Thermo-gravimetric analyses (TGA) were recorded on a Q500, from TA. About 10 mg of each sample was heated at 10 °C.min⁻¹ from 30 to 500 °C under nitrogen flow (60 mL.min⁻¹). Differential scanning calorimetry (DSC) thermograms were obtained using a DSC Q20 and Tzero Hermetic Lid pans from TA, under inert atmosphere (N₂), with a calibration using indium, n-octadecane and n-octane standards. For each sample, about 10 mg were weighed in a pan which was then sealed and submitted to 3 heat/cool/heat cycles: heating from 30 °C to 200 °C at 10 °C.min⁻¹, cooling from 200 °C to −50 °C at 20 °C.min⁻¹. Glass transition temperatures (Tg) were determined at the inflexion value in the heat capacity jump.

Gel permeation chromatography (GPC) analyses were performed on an Agilent Infinity 1,260 equipped with four detectors (UV, λ = 280 nm, RI, LS, viscosimeter). About 3 mg of polymer of each sample was dissolved in 1 mL THF (stabilize with BHT). The column PL Gel MIXED-D (5 μm, 300 × 7.5 mm) was calibrated with polystyrene standards in THF flow (1 mL.min⁻¹).

In a typical experiment, a 25 mL flask is charged with monomer (NDF) or (NDS) (500 mg) under N₂. CH₂Cl₂ (so that the final volume of the solution is 7.15 mL) and Grubbs 1^st generation or Hoveyda-Grubbs 2^nd generation catalyst solution in dichloromethane (8 mmol/L), were added with a syringe in sequence at different monomer to catalyst molar ratio ([M]/[C]) (from 25 to 1000). The mixture was stirred at room temperature for 3 h under N₂. Ethyl vinyl ether (1 mL) was added to the mixture to quench the reaction and stirred for additional 10 min. The resulting polymer (poly(NDF) or poly(NDS)) was precipitated with an excess of diethyl ether at 0 °C. The polymer was filtered and washed with diethyl ether. A solid is obtained.

In a typical experiment, a 25 mL flask is charged with monomers (NDF) or (NDS) and norbornene (N) (500 mg, molar ratio [N]/[NDX] from 0.33 to 9) under N₂. CH₂Cl₂ (so that the final volume of the solution is 6 mL) and Grubbs 1st generation catalyst solution in dichloromethane (8 mmol/L, 2 mol%) were added with a syringe in sequence (total volume of solvent was 6 mL). Ethyl vinyl ether (1 mL) was added to the mixture to quench the reaction and stirred for additional 10 min. The resulting polymer [poly(N-co-NDF) or poly(N-co-NDS)] was precipitated with an excess of diethyl ether at 0 °C. The polymer was filtered and washed with diethyl ether. A solid is obtained.

The polymer is charged with p-toluenesulfonyl hydrazine (5 eq per double bond calculated) into a round-bottom flask. 10 mL of xylene are added with MOPS buffer (40 mg/mL). The reaction is heated at 135 °C. After 4 h, the reaction medium is allowed to cool down and then is filtered (Hahn, 1992). The polymer is precipitated into diethyl ether. Powders are obtained.

Hundred and ninety microliter of homogeneous DPPH solution (200 μM) in ethanol are added to a well containing 10 μL of potential antiradical molecule solution in ethanol at different concentrations (from 300 μM to 9.3 μM). The reaction is followed by a microplate Multiskan FC, 1 scan every 5 min for 7.5 h at 515 nm. The use of different amounts of potential antioxidant give the EC₅₀ value, which is describe as the efficient concentration needed to reduce half the initial population of DPPH. Each analysis was performed 4 times to have a mean value.

0.004 mmol of polymer are solubilized in 0.5 mL of chloroform. 0.1 mL of the polymer solution are dropped off in a quartz cell. Then the cell is placed under N₂ for 1 h. We obtained a polymer film on the bottom of the cell. Two milliliters of a DPPH solution (0.1 mM) are added in the cell and the absorbance at 515 nm is measured at t = 0 and t = 3 h.

The two norbornene-derived ester monomers bearing SHP groups, NDF and NDS, were prepared as described in Scheme 1. 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 then synthesized through the reduction of 5-norbornene-2,3-dicarboxylic anhydride in presence of LiAlH₄ (94% yield). Finally, the solvent-free CAL-B-mediated transesterification (Pion et al., 2013) of 1 and 2 by 3 was performed to provide NDS (60%) and NDF (80%) monomers, respectively. One of the main advantages of using CAL-B in this synthesis is its inactivity toward phenols. Indeed, this particular selectivity renders unnecessary atom- and solvent-consuming protection/deprotection sequence. In addition, this bio-catalysis can be conducted under mild conditions (70 °C, solvent-free, under reduced pressure) and CAL-B can be readily recovered by filtration at the end of reaction. Finally, compared to the chemical synthesis of the similar monomer reported by Xue et al. (2007), this biocatalytic process is not only greener but also provides higher yields (60–80 vs. 37% (Xue et al., 2007)).

The monitoring of ethyl dihydroferulate (1) transesterification with 3 using ¹H NMR spectroscopy showed that it takes about 3 days to reach high yields, providing NDF in 87% yield after flash chromatography. In the case of NDS, for the same reaction time, a very low reactivity of ethyl dihydrosinapate (2) toward the enzyme was observed as NDS was obtained in 10% yield while up to 60% of norbornene mono-sinapate (NMS) was produced (Figure 2). It was shown that the reaction had to be run for more than 10 days to reach a NDS yield of 60%.

Both compounds were tested for their antiradical activity by using the DPPH method that determines the H-donor capacity of the antioxidant to quench the stable DPPH free radical, as previously reported (Reano et al., 2015). In this study, EC₅₀ value corresponds to the amount of antioxidant (nmol) needed to reduce half of the initial population of DPPH radicals. The lower the EC₅₀ value, the higher the antiradical activity. The total stoichiometry n_tot (number of DPPH molecules reduced by one molecule of antioxidant) of the monomers were also expressed in Table 1, according to: n_tot = n_DPPH/2^*EC₅₀ [n_DPPH: initial DPPH mole number (nmol)].

As expected, data in Table 1 confirm the significant positive impact of methoxy group on both radical stability (+M, mesomeric effect) and antiradical activity since the higher the degree of methoxylation (NDF (1) vs. (NDS (2)), the lower the EC₅₀ (NDF (EC₅₀ = 3.43 nmol) vs. NDS (EC₅₀ = 2.29 nmol)). NDS also exhibits a slightly higher antiradical activity than Iganox1010®. It is also noteworthy to mention that antiradical activity reported by the phenol numbers for NDF and NDS (i.e., 2 phenols for NDF and NDS and 4 phenols for Irganox1010®) are significantly higher than that of commercial Irganox1010®.

In DPPH analysis, radical scavenging ability (%RSA) is linked to the ability of SHP molecules to scavenge α,α-diphenyl-β-picrylhydrazyl (DPPH) free radical (Blois, 1958; Brand-Williams et al., 1995). In this study, %RSA was defined as the rate of absorption loss for DPPH solution measured at 515 nm after shaking for 3 h in presence of equimolar amount of poly-H-(NDF), poly-H-(NDS), poly-H-(N-co-NDF) or poly-H-(N-co-NDS) copolymers films. Only copolymers with [N]/[NDX] ratio of 9 were submitted to the RSA test as the others copolymers were poorly soluble.

Table 6 shown RSA < 2% for polynorbornene. Bearing no SHP group, this very low activity was due only to auto-reduction of DPPH in solution during analysis time (3 h). Homopolymers poly-H-(NDF) and poly-H-(NDS) exhibited interesting activities as both reduced more than 80% of the initial population of free radical. In addition, as already observed at the monomer level, poly-H-(NDS) is more antioxidant than poly-H-(NDF). Logically, due to a lower SHP content, the RSA of copolymers poly-H-(N-co-NDF) and poly-H-(N-co-NDS) is lower than that of poly-H-(NDF) and poly-H-(NDS), respectively. However, it is noteworthy to mention that a 9-fold decrease of SHP content (poly-H-(N-co-NDF) vs. poly-H-(NDF), poly-H-(N-co-NDS) vs. poly-H-(NDS)) only resulted in a ca. 2-fold decrease in RSA, demonstrating that not only the radical scavenging activity is not proportional to the SHP content, but also that only a relatively small SHP content is needed to provide a potent antioxidant activity to the polymers.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.