Popping open one by one into polymers We rarely board airplanes by joining the back of a single well-ordered line. More often, we jostle around in one of several bulging crowds that merge haphazardly near the gate. Roughly speaking, these processes are analogous to the chain growth and step growth mechanisms of polymer assembly at the molecular level. Kang et al. present a strategy to link molecular building blocks through hydrogen bonding in accord with the well-controlled chain growth model. The molecules start out curled inward, as they engage in internal hydrogen bonding, until an initiator pulls one open; that molecule is then in the right conformation to pull a partner into the growing chain, poising it to pull in yet another, and so forth down the line. Science , this issue p. 646