PURPOSE: To provide a method for degrading a polymer by subjecting the polymer having a terminal repeating unit comprising a nucleophile and a backbone carbonyl carbon to the attack of the nucleophile to the backbone carbonyl carbon by raising the energy level to provide a ring and to release the ring, and to sequentially degrade the polymer.

CONSTITUTION: This method for degrading a polymer comprises allowing a polymer having a terminal repeating unit comprising a backbone carbonyl carbon of formula I CV is OH, SH, NH2 or NHR (R is a 1-5C alkyl or the like); W is H, R, OR, SR, NHR, a halogen or the like; Y1 to Yq-1 is O, S, NH or NR; Z is H, OH, OR, NH2, NHR, SH, a natural amino acid or the like; A1 to Aq are each a single bond or a group of formula II (S is 1-5); L1 to Lq are each W, a nucleic acid base or the like; (n) and (m) are each 1-2; (p) is 0-1,000,000] or the like distant from a nucleophile, and the nucleophile to be subjected to the attack of the nucleophile to the backbone carbonyl carbon by raising the energy level to form a ring comprising the terminal repeating unit, and to simultaneously release the ring and provide a shortened polymer having the terminal repeating unit comprising the backbone carbonyl carbon, and sequentially degrading the polymer.