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

The pregelation period of free-radical methyl methacrylate/ethylene gl ycol dimethacrylate (MMA/EGDM) and acrylamide/N,N'-methylenebisacrylam ide (AAm/BAAm) copolymerization systems was studied by means of the di latometric 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 copolymerizat ion and at a monomer concentration of 220 gl(-1), nearly half of the p endent 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) increase s first but then decreases continuously as the amount of crosslinker i ncreases. Moreover, termination rate constants become less sensitive t o conversion in the presence of small amounts of crosslinker. All of t hese phenomena can be explained in terms of cyclization, as it decreas es the size of macroradical coils by producing intramolecular links.