FREE-RADICAL CROSS-LINKING COPOLYMERIZATION - EFFECT OF CYCLIZATION ON DIFFUSION-CONTROLLED TERMINATION AT LOW CONVERSION

The pregelation period of free-radical methyl methacrylate/ethylene glycol dimethacrylate (MMA/EGDM) and acrylamide/N,N'-methylenebisacrylamide (AAm/BAAm) copolymerization systems was studied by means of the dilatometric technique. The amount of cyclization in MMA/EGDM copolymers was estimated using proton nuclear magnetic resonance and analytical titration methods. The results suggest that, in MMA/EGDM copolymerization and at a monomer concentration of 220 gl(-1), nearly half of the pendent double bonds are consumed by cyclization. It was found that the average termination rate constant of macroradicals at zero conversion, k(t0), is dependent on the crosslinker concentration. k(t0) increases first but then decreases continuously as the amount of crosslinker increases. Moreover, termination rate constants become less sensitive to conversion in the presence of small amounts of crosslinker. All of these phenomena can be explained in terms of cyclization, as it decreases the size of macroradical coils by producing intramolecular links.