Modified polymer, composition, coating film and multilayer body

A modified polymer comprising:

• (a) a main chain containing a fluorine-containing polymer, containing a unit represented by —CHR—, wherein R is a hydrogen atom or a monovalent organic group, and containing a polymerized unit based on tetrafluoroethylene or chlorotrifluoroethylene;
• and
• (b) a side chain represented by the following formula (b2′), or a side chain represented by the following formula (b1′) and a side chain represented by the following formula (b2′),
• the formula (b1′) being:
• —CH2—CRb1(—X)—C(═O)—Y—Z—Rf (b1′)
• wherein
• X is a hydrogen atom, a methyl group, or a halogen atom;
• Y is —O— or —NH—;
• Z is a direct bond or a divalent organic group;
• Rf is a C1-C20 fluoroalkyl group;
• and
• Rb1 is a hydrogen atom or a monovalent organic group,
• the formula (b2′) being:
• —CH2—CRb2(—R1)—C(═O)—O—Si(—CH3)2—[O—Si(—CH3)2]n—O—Si(—CH3)2—R2 (b2′)
• wherein
• R1 is a hydrogen atom or a methyl group;
• R2 is a hydrogen atom or a methyl group;
• Rb2 is a hydrogen atom or a monovalent organic group;
• and
• n is 3 to 200.