Expandable thermoplastic polymer beads with method of producing same

Expandable thermoplastic polymer beads comprising a core and a skin substantially surrounding and adhered to said core, and a foaming agent contained in at least said core, said core consisting of a polymer made from one or more kind of vinyl monomers, said skin comprising a cross-linked olefin polymer produced by polymerizing one or more vinyl monomers in the presence of an olefin polymer and a cross-linking agent, and said foaming agent being gaseous or liquid at ordinary temperature and pressure and having a boiling point lower than the softening point of the core, wherein the weight ratio of said core to said skin is in the range of from 95:5 to 30:70.