<p>Rotors are common in nature — from rotating membrane-proteins to superfluid-vortices. Yet, little is known about the collective dynamics of heterogeneous populations of rotors. Here, we show experimentally, numerically, and analytically that at small but finite inertia, a mixed population of oppositely spinning rotors spontaneously self-assembles into active chains, which we term gyromers. The gyromers are formed and stabilized by fluid motion and steric interactions alone. A detailed analysis of pair interaction shows that rotors with the same spin repel and orbit each other while opposite rotors spin-pair and propagate together as bound dimers. Rotor dimers interact with individual rotors, each other, and the boundaries to form chains. A minimal model predicts the formation of gyromers in numerical simulations and their possible subsequent folding into secondary structures of lattices and rings. This inherently out-of-equilibrium polymerization process holds promise for engineering self-assembled metamaterials such as artificial macroscale proteins.</p>