A method for producing a fluoropolymer, which includes: polymerizing a monomer (I) represented by the general formula (I) below in an aqueous medium substantially in the absence of a fluorine-containing surfactant (except for the monomer (I) represented by the general formula (I)) to obtain a crude composition containing a polymer of the monomer (I); removing a dimer and a trimer of the monomer (I) contained in the crude composition from the crude composition to obtain a polymer (I) in which the content of the dimer and trimer of the monomer (I) is 1.0% by mass or less based on the polymer (I); and polymerizing a fluoromonomer in an aqueous medium in the presence of the polymer (I) to obtain a fluoropolymer:

CX.sup.1X.sup.3═CX.sup.2R(—CZ.sup.1Z.sup.2-A.sup.0).sub.m  (I)

wherein X.sup.1 and X.sup.3, X.sup.2, A.sup.0, R, Z.sup.1 and Z.sup.2 are as defined herein.

A modified polymer including (a) a main chain containing a fluorine-containing polymer and (b) a side chain represented by the following formula (b1′) and/or a side chain represented by the following formula (b2′). The formula (b1′) is —CH.sub.2—CR.sup.b1(—X)—C(═O)—Y—Z—Rf, 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 R.sup.b1 is a hydrogen atom or a monovalent organic group. The formula (b2′) is —CH.sub.2—CR.sup.b2(—R.sup.1)—C(═O)—O—Si(—CH.sub.3).sub.2—[O—Si(—CH.sub.3).sub.2].sub.n—O—Si(—CH.sub.3).sub.2—R.sup.2, wherein R.sup.1 is a hydrogen atom or a methyl group; R.sup.2 is a hydrogen atom or a methyl group; R.sup.b2 is a hydrogen atom or a monovalent organic group; and n is 3 to 200.

A modified polymer including (a) a main chain containing a fluorine-containing polymer and (b) a side chain represented by the following formula (b1′) and/or a side chain represented by the following formula (b2′). The formula (b1′) is —CH.sub.2—CR.sup.b1(—X)—C(═O)—Y—Z—Rf, 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 R.sup.b1 is a hydrogen atom or a monovalent organic group. The formula (b2′) is —CH.sub.2—CR.sup.b2(—R.sup.1)—C(═O)—O—Si(—CH.sub.3).sub.2—[O—Si(—CH.sub.3).sub.2].sub.n—O—Si(—CH.sub.3).sub.2—R.sup.2, wherein R.sup.1 is a hydrogen atom or a methyl group; R.sup.2 is a hydrogen atom or a methyl group; R.sup.b2 is a hydrogen atom or a monovalent organic group; and n is 3 to 200.

A modified polymer including (a) a main chain containing a fluorine-containing polymer and (b) a side chain represented by the following formula (b1′) and/or a side chain represented by the following formula (b2′). The formula (b1′) is —CH.sub.2—CR.sup.b1(—X)—C(═O)—Y—Z—Rf, 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 R.sup.b1 is a hydrogen atom or a monovalent organic group. The formula (b2′) is —CH.sub.2—CR.sup.b2(—R.sup.1)—C(═O)—O—Si(—CH.sub.3).sub.2—[O—Si(—CH.sub.3).sub.2].sub.n—O—Si(—CH.sub.3).sub.2—R.sup.2, wherein R.sup.1 is a hydrogen atom or a methyl group; R.sup.2 is a hydrogen atom or a methyl group; R.sup.b2 is a hydrogen atom or a monovalent organic group; and n is 3 to 200.

Provided is a method for producing polytetrafluoroethylene, which includes polymerizing tetrafluoroethylene in an aqueous medium in the presence of a nucleating agent and a polymer (I) containing a polymerization unit (I) based on a monomer represented by the general formula (I) to obtain polytetrafluoroethylene:

CX.sup.1X.sup.3═CX.sup.2R(—CZ.sup.1Z.sup.2-A.sup.0).sub.m  (I)

wherein X.sup.1 and X.sup.3 are each independently F, Cl, H, or CF.sub.3; X.sup.2 is H, F, an alkyl group, or a fluorine-containing alkyl group; A.sup.0 is an anionic group; R is a linking group; Z.sup.1 and Z.sup.2 are each independently H, F, an alkyl group, or a fluorine-containing alkyl group; and m is an integer of 1 or more.

Provided is a method for producing polytetrafluoroethylene, which includes polymerizing tetrafluoroethylene in an aqueous medium in the presence of a nucleating agent and a polymer (I) containing a polymerization unit (I) based on a monomer represented by the general formula (I) to obtain polytetrafluoroethylene:

CX.sup.1X.sup.3═CX.sup.2R(—CZ.sup.1Z.sup.2-A.sup.0).sub.m  (I)

wherein X.sup.1 and X.sup.3 are each independently F, Cl, H, or CF.sub.3; X.sup.2 is H, F, an alkyl group, or a fluorine-containing alkyl group; A.sup.0 is an anionic group; R is a linking group; Z.sup.1 and Z.sup.2 are each independently H, F, an alkyl group, or a fluorine-containing alkyl group; and m is an integer of 1 or more.

A sealing member includes a thermoplastic fluororesin composition including a thermoplastic fluororesin (A) having a Shore D hardness of less than or equal to 50, the Shore D hardness being measured at 23° C. in accordance with ASTM D2240, and a cross-linked structure forming agent (B) that is selected from the group consisting of a polyfunctional unsaturated compound (b-1), a polyamine compound (b-2), and a polyhydroxy compound (b-3) and that is capable of forming a cross-linked structure through a reaction with the thermoplastic fluororesin (A), the thermoplastic fluororesin composition not substantially containing a peroxide compound when containing the (b-1), not substantially containing an acid acceptor when containing the (b-2), and not substantially containing at least one of the acid acceptor and an onium compound when containing the (b-3); and a method for producing a cross-linked body including a step of cross-linking the thermoplastic fluororesin composition by an ionizing radiation.

A sealing member includes a thermoplastic fluororesin composition including a thermoplastic fluororesin (A) having a Shore D hardness of less than or equal to 50, the Shore D hardness being measured at 23° C. in accordance with ASTM D2240, and a cross-linked structure forming agent (B) that is selected from the group consisting of a polyfunctional unsaturated compound (b-1), a polyamine compound (b-2), and a polyhydroxy compound (b-3) and that is capable of forming a cross-linked structure through a reaction with the thermoplastic fluororesin (A), the thermoplastic fluororesin composition not substantially containing a peroxide compound when containing the (b-1), not substantially containing an acid acceptor when containing the (b-2), and not substantially containing at least one of the acid acceptor and an onium compound when containing the (b-3); and a method for producing a cross-linked body including a step of cross-linking the thermoplastic fluororesin composition by an ionizing radiation.

A sealing member includes a thermoplastic fluororesin composition including a thermoplastic fluororesin (A) having a Shore D hardness of less than or equal to 50, the Shore D hardness being measured at 23° C. in accordance with ASTM D2240, and a cross-linked structure forming agent (B) that is selected from the group consisting of a polyfunctional unsaturated compound (b-1), a polyamine compound (b-2), and a polyhydroxy compound (b-3) and that is capable of forming a cross-linked structure through a reaction with the thermoplastic fluororesin (A), the thermoplastic fluororesin composition not substantially containing a peroxide compound when containing the (b-1), not substantially containing an acid acceptor when containing the (b-2), and not substantially containing at least one of the acid acceptor and an onium compound when containing the (b-3); and a method for producing a cross-linked body including a step of cross-linking the thermoplastic fluororesin composition by an ionizing radiation.

A fluorine-containing curable composition which contains, as essential components, component (A) that is composed of a compound which is a linear polymer having a fluoropolyether in the main chain, while having a trifluoromethyl group at one end of the molecular chain and two or more (meth)acryl groups at the other end, component (B) that is composed of a compound which is a linear polymer having a fluoropolyether in the main chain, while having two or more (meth)acryl groups at each end of the molecular chain, and which has four to ten (meth)acryl groups in each molecule on average, and component (C) that is composed of a non-fluorinated acrylic compound which contains no fluoropolyether structure, while having two or more (meth)acryl groups in each molecule on average; the total blending amount of component (A) and component (B) relative to 100 parts by mass of component (C) is from 0.05 to 50 parts by mass; and the blending amount of component (A) relative to 100 parts by mass of component (B) is 1 to 100 parts by mass. This fluorine-containing curable composition is capable of forming a cured film that exhibits excellent antifouling properties and excellent water repellency, while having a low coefficient of kinetic friction.