Photoinduced Acrylate Polymerization: Unexpected Reduction in Chain Branching

The branching stemming from midchain radical formation in n-butyl acrylate polymerization is investigated via melt-state ¹³C NMR measurements. The dependence of the degree of branching (DB) on the monomer conversion of the system is examined for photoinduced polymerizations, revealing a steady increase in branching with conversion. For polymerization at moderate light intensities, an increase in branching from 0.03% to 0.37% is observed for polymerizations at 60 C, which is fivefold below the level of branching observed in thermally initiated polymerizations under otherwise identical reaction conditions. The reason for this overall reduction in branching remains momentarily unclear; yet, a strong dependence of branching on light intensity is observed. While polymerization under a 1 W LED lamp results at almost full monomer conversion in branching degrees of 0.22%, polymerization under a 400 W lamp yields 1.81% of chain branches. The degree of branching in poly(butyl acrylate) stemming from midchain radical formation is discussed for photo-induced polymerization. At moderate light intensities, an increase in branching from 0.03% to 0.37% is observed at 60 C, which is fivefold below the level typically expected for a thermal polymerization under identical conditions.