Synthetic polymers constitute a cornerstone of modern functional materials, underpinning a wide range of applications from advanced coatings and membranes to biomedical devices and electronic components. The development of efficient polymerization strategies—particularly those featuring “clickable” or modular reactivity—is highly desirable, as it enables rapid diversification of polymer architectures and facilitates systematic exploration of structure–property relationships. We have recently developed a versatile and efficient synthetic platform for constructing isoindoline-1-one-based alternating copolymers via a novel mode of step-growth polycondensation. The process employs functionalized bis-ortho-phthalaldehyde and diamine monomers as building blocks, at room temperature under the cooperative catalytic action. This mild and operationally simple protocol affords high-molecular-weight linear copolymers with well-defined alternating sequences. A key advantage of this strategy lies in its modular design: by varying the linker segments in the monomer structures, a wide range of functional moieties can be seamlessly incorporated into the polymer backbone without altering the overall synthetic procedure. This tunability not only broadens the accessible chemical space but also streamlines the preparation of tailor-made materials for targeted applications. Beyond linear architectures, the methodology was readily extended to the construction of branched and crosslinked polymer networks by introducing multifunctional monomers—specifically, a triamine brancher and a dithiol crosslinker—into the polymerization system. These modifications produced materials with higher dimensional connectivity, imparting enhanced mechanical and thermal characteristics. Notably, several of the isoindoline-1-one-based copolymers exhibited thermoplastic elastomer behavior, combining elastic recovery with melt processability.