The effect of covalent crosslinking on material tensile toughness. The crosslinking density of the covalent network played an essential role in ductility. We altered the crosslinking densities by varying the molar ratios between EBMP and TEMPIC. We used average functionality (f) to describe the composition of monomers and crosslinking density. As the f values decreased from 2.5, 2.25, to 2.125 -the molar percentage of EBMP increased from 0, 50%, to 75% -Tgs decreased from 78 °C, 57 °C, to 42 °C and the rubbery moduli decreased from 6.3 MPa, 3.3 MPa, to 1.8 MPa, respectively (Figure and). When 100% TEMPIC was used (f=2.5), we observed a glassy but brittle polymer (yellow sample, Figure ). As the f value decreased, we again observed glassy but ductile materials (Figure , blue and red). We conclude that the covalent crosslinking network has an essential effect on material properties since lower crosslinking densities result in decreased tensile strengths and increased elongation-at-break. Ductility could only be obtained at relatively low crosslinking densities. When f value equals to 2, i.e., 100 mol% of EBMP was used as the thiol monomer, a linear thiol-ene polyamide was obtained, which showed viscous flow at elevated temperature. The linear polymer showed an impressively high elongation-at-break of 681% but a low tensile strength of 5.7 MPa. Interestingly, when we used a smaller dithiol monomer, namely EDT, in replacement of EBMP, we obtained a linear polymer with a Tg of 33 °C and a drastically enhanced ultimate tensile strength to 27 MPa, while maintaining a high elongation-at-break of 604% (Figure ).