Increasing interest in innovative supramolecular materials has spurred efforts to develop head-to-tail monomers possessing a host moiety as a head and a guest moiety as a tail, making them capable of forming supramolecular polymers through intermolecular associations while avoiding intramolecular cyclization. This competition between the intramolecular cyclization and the intermolecular association is influenced by conformational entropy, relying on the flexibility of the linker chain that connects the host moiety to the guest moiety in a head-to-tail monomer. However, there are limited reports presenting a quantitative thermodynamic picture describing the conformational entropy in ring-chain equilibrium processes. Here, we report the quantitative evaluation of the role of conformational entropy in the ring-chain equilibrium mechanism of the supramolecular polymerization of a headto-tail monomer possessing a bisporphyrin host and a trinitrofluorenone guest connected with various alkyl chains as linkers. The supramolecular polymerization of the head-to-tail monomers was studied using UV/vis, NMR, and diffusion-ordered spectroscopy spectroscopic techniques and viscometry. The thermodynamic parameters of the ring-chain equilibrium were determined in the initial stage of the supramolecular polymerization. The conformational entropy, which relies on the flexibility of the linker, had a significant influence on the critical polymerization concentration. This quantitative discussion of ring-chain competition is expected to provide a foundation for the proper design of artificial head-to-tail monomers that form supramolecular polymers.