Method of producing ethylene-carboxylic acid compolymer

A method of producing an ethylene-carboxylic acid copolymer, comprising:

• supplying a monomer solution containing a carboxylic acid monomer from a discharging unit;
• and
• copolymerizing the carboxylic acid monomer with an ethylene-based monomer,
• wherein a coefficient of friction between the discharging unit and the monomer solution is maintained as 0.3 or less,
• wherein the coefficient of friction between the discharging unit and the monomer solution is defined as a value measured by using a Mini-Traction Machine 2 while rotating a disk and a ball therein,
• wherein a slide roll ratio between the disk and the ball is in a range of 0% to 70%,
• wherein the disk has the same surface roughness as that of the discharging unit,
• wherein a temperature in the Mini-Traction Machine 2 is the same as that in the discharging unit, and
• wherein a pressure in the Mini-Traction Machine 2 is the same as a contact pressure in the discharging unit.