[Problem] To provide a resin composition containing a novel polymer, and a method for forming a pattern using the resin composition, as well as a method for synthesizing the polymer. [Solution] A resin composition for forming an insulating film, the resin composition containing an organic solvent and a polymer that has structural units represented by formula (1a) and structural units represented by formula (1b). (In the formulas, T0 represents a divalent organic group including at least one arylene group in which at least one hydrogen atom has been substituted by an amino group, T1 represents a divalent organic group including at least one arylene group that has at least one substituent, and the substituent is represented by formula (2): (in the formula, Z represents an optionally substituted divalent aliphatic group, aromatic group, or alicyclic group).)