Copolymer and stainproofing agent containing the same

A copolymer comprising:

• (I) 5 to 80% by weight, based on the copolymer, of a structure unit derived from a monomer which has a fluoroalkyl group and optionally has a urethane or urea linkage,
• (II) about 30 to 80% by weight, based on the copolymer, of a structure unit derived from a monomer which contains no fluorine and has at least one urethane or urea linkage and one carbon-carbon double bond, and
• (III) 1 to 50% by weight, based on the copolymer, of a structure unit derived from a monomer which has a carbon-carbon double bond and a functional group selected from the group consisting of a carboxyl group, a carboxylic anhyride group, an epoxy group, a phosphoric group, an alkoxysilane group, an imine group, a sulfonic group, an imino group, an isocyanate group and a blocked adduct thereof and a chlorine atom,
• wherein the monomer constituting the structure unit (III) is other than (meth)acrylic acid, glycidyl (meth)acrylate, vinyl chloride and vinylidene chloride.