A polymer and a resin composition thereof are provided. The polymer includes a first repeat unit represented by Formula (I) and a second repeat unit represented by Formula (II) wherein A1 is C24-48 alkylene, C24-48 alkenylene, C24-48 alkynylene, C24-48 alicyclic alkylene, C24-48 alicyclic alkenylene, or C24-48 alicyclic alkynylene. A2 and A4, independently having at least one reactive group, are independently C6-25 arylene, C4-8 cycloalkylene, C5-25 heteroarylene, divalent C7-25 alkylaryl, divalent C7-25 acylaryl, divalent C6-25 aryl ether, divalent C7-25 acyloxyaryl, or divalent C6-25 sulfonylaryl; and, A3 is substituted or unsubstituted C6-25 arylene, C4-8 cycloalkylene, C5-25 heteroarylene, divalent C7-25 alkylaryl, divalent C7-25 acylaryl, divalent C6-25 aryl ether, divalent C7-25 acyloxyaryl, or divalent C6-25 sulfonylaryl.