Polyhexahydrotriazine (PHT) and polyhemiaminal (PHA) materials chemically modified to include thermoplastic polymer bridging groups, and methods of making such materials, are disclosed. The materials are formed by a process that includes heating a mixture comprising i) a solvent, ii) paraformaldehyde, iii) a diamine monomer comprising two primary aromatic amine groups, and iv) a polymer diamine at a temperature of about 20 C. to less than 150 C. This heating step forms a stable PHA in solution, which can be isolated. The PHA includes covalently bonded thermoplastic polymer groups. The PHA is then heated at a temperature of 150 C. to about 280 C., thereby converting the PHA material to a PHT material that includes covalently bonded thermoplastic polymer groups.