Doubling down on polymerization In biology, structural polymers such as cytoskeletal fibers assemble from covalently polymerized monomers through weaker supramolecular interactions such as hydrogen bonds. Yu et al. report the synthesis of cylindrical fibers when three monomers react, two covalently and one in a supramolecular fashion. When the reaction proceeded stepwise, lower-molecular-weight flat tapes formed instead, which suggests that supramolecular interactions helped to catalyze the covalent polymerization. Science , this issue p. 497