Computational prediction of optimal metal ions to induce coordinated polymerization of muscle-like [c2]daisy chains

Recently, a muscle-like organometallic polymer has been successfully synthesized using Fe²⁺ as a linker atom. The polymer exhibits acid-base controllable muscle-like expansion and contraction on the micrometer scale. Further development could be facilitated by revealing the polymerization mechanism and by searching for optimal linker atoms. In this work, we have examined possible equilibrium and intermediate polymer structures, which consist of [c2]daisy chains linked by divalent transition metal ions (Sc²⁺, Ti²⁺, Fe²⁺, Co²⁺, Ni²⁺ or Zn²⁺) with various hexa-coordination arrangements, based on calculations using density functional theory. We find that the metal linkers in polymers are weaker in acid than in base due to excess positive charges on the polymer, leading to their thermodynamical instability or even decomposition. This can explain the experimental difficulty in improving the degree of polymerization for metal-linked polymers. We also find that the polymers with either Fe²⁺ or Co²⁺ are the most favorable, with the latter extending 1.4% longer than with the former. Since Fe²⁺ has been confirmed experimentally to be a successful linker, Co²⁺ would function equally well and thus could be used as an alternative choice for polymerization.