UV-curing of acrylic formulations by means of polymeric photoinitiators with the active 2,6-dimethylbenzoylphosphine oxide moieties pendant from a tetramethylene side chain

Polymeric photoinitiators (PIs) bearing the 2,6-dimethylbenzoylphosphine oxide moiety pendant from flexible tetramethylene oxide side chains were synthesised by reacting their polymeric precursors, based on 4-(4'-acryloyloxy-1'-butoxy)-2,6-dimethylbenzoic acid (ABDBA), with diphenylmethylphosphinite. The resulting 4-(4'-acryloyloxy-1'-butoxy)-2,6-dimethylbenzoylphosphine oxide (ABDBPO) homo- and copolymers with butyl acrylate (BA), poly(ABDBPO) and poly(ABDBPO-co-BA), were employed in UV-curing experiments using model clear-coating (hexandiol diacrylate/butyl acrylate, HDDA/BA) and adhesive acrylic formulations. The photoinitiated radical polymerisations were followed by microwave dielectrometry, which allows real-time monitoring of the main kinetic parameters such as induction time, t(0), polymerisation rate, and absolute monomer conversion. The low molecular weight 2,4,6-trimethylbenzoylphospine oxide (TMBPO) and the couple ethyl 4-dimetllylaminobenzoate/isopropyl thioxantone (EPD/TTX) were also used as reference acylphosphine oxide (APO) and nonAPO PIs in parallel experiments. The kinetic data from the HDDA/BA system were compared with analogous literature data on acrylic PI with the APO moiety directly attached to the backbone. The presence of the flexible spacer does not significantly affect the initiation efficiency of the APO, whereas the initial viscosity of the medium resulting from the presence of macromer resins in the adhesive formulations markedly reduces both polymerisation rate and final conversion.