Fluidized bed ring-opening polymerization process

In the process for the ring-opening polymerization of a ring-opening polymerizable cyclic compound which comprises contacting the cyclic compound under polymerizing conditions in a reaction zone with a metal or metal compound polymerization initiator effective to initiate the polymerization of the cyclic compound to a solid polymer, the improvement which comprises contacting a vaporizable, cyclic ring-opening polymerizable compound having 6 to 8 atoms in the ring in the vapor state with a particulate solid initiator maintained in the reaction zone as a fluidized bed by a fluidizing flow of a gas stream whereby the cyclic compound is converted to a particulate solid polymeric reaction product.