Thermochromic polydiacetylenes (PDAs) are known for their conjugated bonds with reversibility when the molecules are maintained with the bond distance of the polymer backbone along with the stacking angle. With this structural requirement, the fabrication of PDAs has to rely on thin films or coatings on the substrates where the alignment and packing of the molecules can be controlled at the molecular level. The present work demonstrates for the first time, a free-standing thermochromic film based on the poly(lactic acid) (PLA) film as a model case. Here, the PDA with a star structure is considered as the approach to provide the symmetrical chains for PDA so that the PDA molecules aligned under the chain distance are controlled by the steric branches. By simply constructing an eight-armed benzoxazine dimer from the tetra-branched benzoxazine core followed by lactide ring opening polymerization as well as the conjugation with diacetylenes at each terminal, a precise eight-branched diacetylene, i.e.4BzD–8PPLA–8DA, can be easily obtained. The blend of this compound with PLA resin followed by solution casting leads to a free-standing film with reversible thermochromic properties. The comparative studies on PLA films containing other types of branching PLLA terminated with PDA clarifies to us that these films do not show reversible thermochromism at all. The microstructure analyses indicate that the symmetrical eight-armed 4BzD–8PPLA core plays an important role to provide the framework for maintaining the ene–yne conjugated bond under a certain mobility 4BzD–8PPLA–8PDA/PLA upon heating and cooling to result in the reversible thermochromic behavior.