ON THE KINETICS OF MONOMER INCORPORATION INTO POLYACRYLAMIDE GELS, AS INVESTIGATED BY CAPILLARY ZONE ELECTROPHORESIS

The kinetics of monomer incorporation into a polyacrylamide gel have been studied in a photopolymerization system comprising 100 muM methylene blue in presence of a red-ox system, 1 mM sodium toluelesulfinate (reducer) and 50 muM diphenyliodonium chloride (oxidizer). A precise assessment of gel point (p(c)) was obtained in a droplet chamber, in which argon was gently bubbled with a fused silica capillary into the reaction mixture. At p(c), 50 % (+/-3) acrylamide was incorporated into the matrix, vs. 80 % (+/-4) N,N'-methylenebisacrylamide. This incorporation level remained the same when polymerized in the 2-36-degrees-C temperature range. Incorporation continued almost linearly for acrylamide up to 80% conversion. The reaction was continued up to 55 min (at 2-degrees-C), at which point bisacrylamide had been essentially consumed (>99.5% incorporation) and acrylamide had reacted (95 %). At 2-degrees-C, after gelation, the gel became progressively turbid (the Tyndall effect plateauing at 50 min), but it remained fully transparent if, at the gel point, reaction was continued at 50-degrees-C. The consumption of the pendant double bonds of Bis followed the progression of turbidity. It is concluded that, by gelation at 2-degrees-C, the nascent chains form clusters held together by hydrogen bonds (melting point at 28-degrees-C); such clusters are subsequently ''frozen'' in the three-dimensional space as the pendant double bonds in the chains react progressively. Such turbid matrices are more porous and less elastic than when the gel is polymerized at 50-degrees-C. This process is similar to the ''lateral aggregation'' occurring when gels are formed in presence of a polymer in solution (e.g. 10 KDa polyethylene glycol; Righetti et al., Electrophoresis 1992, 13, 587-594).