The concept of Chemical Confinement is introduced to describe a relaxation mode occurring in copolymers with a non-random distribution of monomer units. Poly(ethene-co-methacrylic acid) copolymer turns out to be an appropriate test system. By suitable selection of the polymerization conditions, the randomness of the distribution of methacrylic acid units along the polymer backbone may be tuned without significantly affecting the degree of polymerization and the methacrylic content of the copolymer. By interaction via hydrogen bonds, associating methacrylic units can confine the relaxation mode of chain segments positioned in between two such associating sites. Variation of the methacrylic content results in a shift of the relaxation mode. Mechanical spectroscopy and differential scanning calorimetry are used to validate the model of Chemical Confinement.