The influence of crosslinking process on the resulting structural properties of phthalonitrile matrices is studied through theoretical and experimental investigations. Multiscale procedure for generating fully atomistic phthalonitrile networks with simulation of radical polymerization reactions and specific reactions of triazine formation at the mesoscale level is presented and applied to the case of phthalonitrile resin based on low-melting monomer bis(3-(3,4-dicyanophenoxy)phenyl)phenyl phosphate. The structural properties of the generated networks of various conversions and with various amount of triazine are analyzed using the dissipative particle dynamics and atomistic molecular dynamics. Triazine-containing networks are much sparser in comparison with triazine-free ones in terms of simple cycle size. The values of density, coefficients of linear thermal expansion and glass transition temperatures (T(g)s) agree with obtained experimental data, and are very similar for different crosslinking mechanisms. The dependence of T-g on conversion correlates well with the sol-gel transition in network structure. (c) 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018, 56, 362-374