High molecular weight cyclic poly(ε-caprolactone)s (cPCLs) with variable ring size are synthesized via light-induced ring closure of α,ω-anthracene-terminated PCL (An-PCL-An). The ring size of cPCL is tunable simply by adjusting the polymer concentration from 10 to 100 mg mL^?1 in THF. The cyclo­addition via the bimolecular cyclization of An-PC-An is well characterized by a variety of analyses such as ¹H NMR and UV–vis spectroscopies, gel-permeation chromatography, and differential scanning calorimetry. The reversible dimerization of An induced by heating enables the cyclic PCL to have a switchable “on–off” capability. This novel light-induced ring-closure technique can be one of the most powerful candidates for producing various well-defined cyclic polymers in highly concentrated polymer solution.