This application describes methods of forming an object. The methods described include flowing a polyhemiaminal (PHA), polyhexhydrotriazine (PHT), or polyoctatriazacane (POTA) precursor mixture to a nozzle of a 3D printer, heating the PHA, PHT, or POTA precursor to a temperature of at least 50° C., dispensing the PHA, PHT, or POTA precursor in a pattern; and, hardening the PHA, PHT, or POTA precursor into a polymer. The PHA and PHT polymers are formed by reacting a primary diamine with a formaldehyde-type reagent. The POTA polymer is formed by reacting a primary diamine with a formaldehyde-type reagent and formic acid. The objects formed using the methods described herein may be made of a single polymer, a single polymer type using multiple diamine monomers, or a mixture of PHA, PHT, and/or POTA polymers with different desired physical properties.