Influence of monomer structure on the propagation kinetics of acrylate and methacrylate homopolymerizations studied via PLP-SEC in fluid CO2

Propagation kinetics of free-radical homopolymerizations of methyl acrylate, dodecyl acrylate, butyl methacrylate, dodecyl methacrylate, glycidyl methacrylate, cyclohexyl methacrylate, and isobornyl methacrylate in solutions containing 40 wt.-% CO2 were studied applying the PLP-SEC technique. The obtained apparent propagation rate coefficients, k(p,app), are by up to 40% below the associated bulk k(p) values. This reduction is assigned to a lowering of local monomer concentration, c(M,loc), at the site of the free-radical chain end rather than to a decrease of the actual propagation rate coefficient. With the alkyl (meth)acrylates, intersegmental interactions between polar groups of the same polymer molecule are responsible for deviations of c(M,loc) from the analytical overall monomer concentration, c(M,a). Increasing size of the flexible alkyl ester group reduces the differences between c(M,loc) and C-M,C-a due to shielding effects. Methacrylates with cyclic ester groups do not follow this trend. In case of isobornyl methacrylate, which polymerizes to a rigid material with large side groups, relative size of monomer and CO2 matters and reduces c(M,loc) significantly below C-M,C-a.