Method of stabilizing polymerizable compounds

A liquid phase comprising at least 10% by weight of:

• at least one compound selected from the group consisting of acrylic acid, methacrylic acid and a respective ester thereof;
• at least one metal selected from the group consisting of copper, manganese and cerium;
• and
• at least one ligand selected from the group consisting of quinoline compounds of formula (I):
• N-oxides of compounds of the formula (I), 2,2′-bis(2,3-dihydro-3-oxoindolylidene), and aliphatic y-dentate ligands having y=2-6 and comprising at least two nitrogen atoms joined by aliphatic or aromatic C1-C4 bridges comprising y-2 further coordinating nitrogen atoms or heteroatoms,
• wherein
• X is OH, NH2, O—C1-C4-alkyl, O—C(O)—C1-C4-alkyl, O—C(O)—C2H5 or O—C(O)-phenyl,
• R2 is H, or (C1-C4)-alkyl,
• R2 is H, C1-C4-alkyl, Cl, Br or SO3H, and
• R3 is H, Cl or Br.

A complex, comprising:

• at least one metal selected from the group consisting of copper, manganese and cerium;
• and
• at least one ligand selected from the group consisting of quinoline compounds of formula (I):
• N-oxides of compounds of the formula (I), 2,2′-bis(2,3-dihydro-3-oxoindolylidene), and aliphatic y-dentate ligands having y=2-6 and comprising at least two nitrogen atoms joined by aliphatic or aromatic C1-C4 bridges comprising y-2 further coordinating nitrogen atoms or heteroatoms,
• wherein
• X is OH, NH2, O—C1-C4-alkyl, O—C(O)—C1-C4-alkyl, O—C(O)—C2H5 or O—C(O)-phenyl,
• R1 is H, or (C1-C4)-alkyl,
• R2 is H, C1-C4-alkyl, Cl, Br or SO2H,
• R3 is H, Cl or Br, and
• the complex is suitable for stabilizing at least one selected from the group consisting of acrylic acid, methacrylic acid and a respective ester thereof, as a polymerization inhibitor.