Effect of Cross-Linker Reactivity on Experimental Gel Points during ATRcP of Monomer and Cross-Linker

A systematic study of the effect of cross-linker reactivity on experimental gel points was conducted using the atom transfer radical copolymerization (ATRcP) of methyl acrylate (MA) with different types and amounts of divinyl cross-tinkers, including ethylene glycol diacrylate (EGDA), ethylene glycol dimethacrylate (EGDMA), and an asymmetric cross-linker EGAMA containing one acrylate group and one methacrylate group. The variations of experimental gel points based on the monomer conversions were determined and compared with each other. Compared to the equivalent reactivity of the acrylate groups in MA and EGDA, the methacrylate groups from EGDMA had higher reactivity and led to faster incorporation of cross-linker unit into polymers and quicker generation of branching points (acceleration effect). However, the branched polymers containing the gradient distribution of pendant vinyl groups and branching points resembled a starlike structure, in which the pendant vinyl groups, located in the more densely cross-linked core, were isolated by the surrounded shell and reduced probability of the intermolecular reaction, which retarded the gelation (deceleration effect). These two effects intercorrelated with each other and significantly affected the experimental gel points of the poly(MA-co-EGDMA) system, depending on the amount of added EGDMA. With more EGDMA added, the acceleration effect became dominant, which led to a quicker gelation at lower MA conversion compared to the ATRcP by using the same molar amount of EGDA. When the asymmetric cross-linker EGAMA was used, these two effects became much weaker because the pendant vinyl groups were mainly acrylate groups, which had a similar reactivity to free MA monomer and slower branching compared to methacrylate pendant groups.