1. IN A PROCESS FOR POLYMERIZING A FLUID MONOMER ADAPTABLE TO IONIC POLYMERIZATION IN THE PRESENCE OF BF3, SAID MONOMER BEING SELECTED FROM THE GROUP CONSISTING OF TETRAHYDROFURAN AND CYCLIC ACETALS HAVING 6 TO 8-MEMBERED RINGS, THE IMPROVEMENT WHICH COMPRISES DISPERSING AN ARYLDIAZONIUM FLUOROBORATE IN SAID MONOMER AND THEN DECOMPOSING SAID FLUOROBORATE TO FORM BF3 IN SAID MONOMER BY IRRADIATION THEREOF WITH RADIANT ENERGY COMPRISING WAVE LENGTHS BETWEEN THE WAVE LENGTHS OF INFRARED AND ULTRAVIOLET LIGHT.