Determination of gel point during atom transfer radical copolymerization with cross-linker

Atom transfer radical polymerization (ATRP) of methyl acrylate (MA) and ethylene glycol diacrylate (EGDA) provided a series of poly(MA-co-EGDA) branched and cross-linked copolymers using various initial molar ratios of cross-linker to initiator. In agreement with the Flory -Stockmayer theory, the gelation occurred when the concentration of reacted pendant vinyl groups was larger than that of primary chains. This critical point depends on the initiation efficiency and the reactivity of vinyl groups in both cross-linker and monomer. The experimental gelation point occurred when both the molecular weight and the weight fraction of branched polymers among the total sols (f(branch)) reached the maximum, as determined by GPC measurement of the sols at different conversions. The kinetic study based on experimental results and Predici simulation demonstrated that all acrylate groups had a similar reactivity. In all reactions, the gel points theoretically predicted by Predici simulation, based on the conversions of MA and EGDA (conV(MA),(gel), and conV(EGDA),(gel)), were only slightly lower than the experimental results. These results indicate that the gels synthesized by ATRP have a more homogeneous structure and contain a smaller amount of cyclization products in contrast to gels synthesized by conventional free radical polymerization.