## 2 Materials and Methods
### Synthesis  
The prepolymers were obtained through a two-step bulk polycondensation reaction (described below), following the formulations listed in Table 1 and with the chemical structure depicted in Figure 1, which illustrates how lignin is incorporated in the bio-based prepolymer. The lignin structure is linked by covalent bonds in the structure of the linear vinyl ester prepolymer. Depending on the lignin content, each vinyl ester backbone can have one or more lignin derived segments but not more than three to avoid premature reticulation. Both BVE and BVEL resins were formulated with a mixture of unsaturated (PA) and saturated (FA) acids, at a mass ratio of 1:2.5. BVE and BVEL resins were synthesized using a glycol excess of 15% and 12% in mol, respectively (in the latter case, the glycol excess aimed to compensate for uncertainties arising from the polycondensation reaction of the lignin). In the BVEL resin, 0.1 mol % of isosorbide (regarding the total glycol amount) was replaced by 35.74 g of lignin, corresponding to a total incorporation ratio of lignin of ca. 4% \(w/w\), (slightly) increasing the prepolymer bio-based mass content from 82.1% to 82.8%. The reactants were placed inside a 2 1 reactor equipped with a bottom drain valve, a glass lid with 5 ground joints, an anchor blade mechanical stirrer, a nitrogen gas inlet, and an addition funnel (to add the FA in the second stage of the synthesis, as described below). The reactional medium was heated at 206 \({}^{\circ}\)C \(\pm\) 4 \({}^{\circ}\)C using a heating mantle, and the temperature inside the reactor was monitored with a thermocouple. The residual water formed during synthesis and the water derived from the solubilization of the raw materials was collected using a condenser coupled to a distilling receiver (Dean-Stark type apparatus). In the first stage of the synthesis, the reactants were added to the reactor at ambient temperature, and then stirred and heated to the set-point temperature to promote the bonding between the OH groups of glycols and the COOH groups of PA. During this stage, the reaction progress was monitored through acid-base titration, and the reaction proceeded until the acid number (AN) stabilized below 30 mgKOH/g. In a second stage, the reactor was cooled to 100 \({}^{\circ}\)C and the stirring motion was slowed to incorporate the FA in the reaction medium. This two-step procedure ensured that the unsaturated double bonds of FA were formed at the extremities of the polymeric chains, making the resin less hydrophilic than an unsaturated polyester (UP) resin (as there are fewer unsaturation points in the polymeric chain), and thus enabling a vinyl ester molecular structure (with less saturation points over the polymeric chain) to be obtained. The second stage of the polycondensation reaction proceeded until the AN stabilized (same value in two consecutives measurements, one hour spaced) or until the maximum viscosity reached 16,000 cp (the viscosity was measured at 125 \({}^{\circ}\)C in a high-temperature ICI cone and plate viscosimeter); once one of these criteria was satisfied, the prepolymers were cooled to a temperature of 160 \({}^{\circ}\)C. Afterwards, they were diluted (without prior purification) in a balanced mix of STY and HEMA at an incorporation temperature between 60 \({}^{\circ}\)C and 70 \({}^{\circ}\)C. The use of reactive diluents in the processing stage allowed the adjustment of the viscosity of the final product, whereas in the hardening stage they promoted the connection between the linear chains of VE (through the crosslinking points of the unsaturated double bonds of FA), thus forming a thermosetting crosslinked structure. Of the various formulations tested (in which the ratio between PA and FA was varied between 1.5 and 3.5), the resin compositions shown in Table 1 were the ones that presented a better balance between mechanical and thermomechanical properties and processing parameters (for different manufacturing techniques of FRP composites), and they aimed at maximizing the overall bio-based content.