A method comprising contacting at least one metal salt of an organic acid with at least one aromatic compound in a reaction zone under conditions suitable for the formation of a polymer, wherein the metal salt of an organic acid comprises a metal and at least one unsaturated organic acid moiety. A composition comprising polystyrene and a metal salt of cinnamic acid. An article made from a composition comprising polystyrene and a metal salt of cinnamic acid. A composition comprising polystyrene and a salt of a fatty acid. An article made from a composition comprising polystyrene and a salt of a fatty acid.

A petroleum resin that is a polymer of a raw material component including a C9 fraction containing an aromatic olefin and dicyclopentadiene, in which a ratio C.sub.2/C.sub.1 (mass ratio) of a content C.sub.2 of the aromatic olefin to a content C.sub.1 of the dicyclopentadiene in the raw material component is 0.35 or more and 0.5 or less, and a softening point is 150? C. or higher.

The purpose of the present invention is to provide: a cross-copolymer in which a residual catalyst component remains in a reduced amount and which has improved transparency, applicability to medical materials and yellowish discoloration resistance; and a method for producing the cross-copolymer. According to the present invention, a cross-copolymer is provided, wherein the cross-copolymer is produced through a coordination polymerization step of carrying out copolymerization of an olefin monomer, an aromatic vinyl compound monomer and an aromatic polyene using a single-site coordination polymerization catalyst to synthesize an olefin-(aromatic vinyl compound)-(aromatic polyene) copolymer and a subsequent anionic polymerization step of carrying out polymerization in the co-presence of the olefin-(aromatic vinyl compound)-(aromatic polyene) copolymer and an aromatic vinyl compound monomer using an anionic polymerization initiator, the cross-copolymer being characterized in that the total mass of aluminum and lithium, which are residual catalyst components, contained in the cross-copolymer is 200 ppm or less.