Monomer mixture, resin composition, resin molded article, panel for aquariums, illuminated sign, building component, method for producing resin composition, and method for manufacturing resin cast board

A resin composition comprising:

• a (meth)acrylic polymer (P);
• and
• a phosphine compound (D),
• wherein the (meth)acrylic polymer (P) comprises a repeating unit derived from an ethylenically unsaturated monomer,
• a content proportion of the ethylenically unsaturated monomer which is unreacted, in the resin composition, is 4% by mass or less,
• the repeating unit derived from the ethylenically unsaturated monomer comprises a repeating unit derived from a (meth)acrylic acid ester (M) having, in a side chain, an aromatic hydrocarbon group or an alicyclic hydrocarbon group having 3 to 20 carbon atoms, and a content proportion of the repeating unit derived from the (meth)acrylic acid ester (M) is 2% to 10% by mass with respect to 100% by mass of a total mass of the (meth)acrylic polymer (P),
• the (meth)acrylic polymer (P) comprises a repeating unit derived from methyl methacrylate, and a content proportion of the repeating unit derived from methyl methacrylate is 90% to 98% by mass with respect to 100% by mass of a total mass of the (meth)acrylic polymer (P), and
• the phosphine compound (D) is represented by Formula (I),
• in Formula (I), R1, R2, and R3 each independently represent a hydrogen atom, an alkyl group having 1 to 12 carbon atoms, a cycloalkyl group having 5 to 12 carbon atoms, a (cycloalkyl) alkyl group having 6 to 12 carbon atoms, an aryl group having 6 to 12 carbon atoms, or an aralkyl group having 7 to 12 carbon atoms.