Abstract: A composite injector is provided for handling the introduction to a thick-walled, high pressure polymerization autoclave of both monomer and catalyst streams. Said injector comprises an inner, separately enclosed, high pressure catalyst conduit of relatively small cross-section terminating in a fine orifice at its downstream end which substantially coincides with the inner wall boundary of said autoclave, and, surrounding the upstream portion of said catalyst conduit at least, a pressure-tight outer housing of much larger inner cross-section having an upstream monomer supply connection and a downstream fitting for sealing engagement with a matched opening through said thick-walled autoclave. By introducing the catalyst and the monomer through their respective separate passageways of this composite injector, the resultant catalyst stream is released into the monomer stream just as said streams are both emerging into the open space beyond the inner wall boundary of said autoclave.