Photo-induced polymerization of diethylene glycol diacrylate in the presence of poly(methyl methacrylate): 2—Polymerization with 9-phenylacridine as the photoinitiator

With a view to the manufacture of printing plates, films consisting of diethylene glycol diacrylate and poly(methyl methacrylate) acting as a binding agent and containing 9-phenylacridine as the photoinitiator were irradiated by means of fluorescent lamps, each film being covered by two sheets of poly(ethylene terephthalate) to exclude atmospheric oxygen. After irradiation the cover sheets were removed and the films were extracted with chloroform in a soxhlet apparatus. Together with residual monomer and low molecular weight compounds, nearly all the poly(methyl methacrylate) could be separated. The residue represented the crosslinked and therefore insoluble polymer. Under standard reaction conditions its yield can serve as a measure for the reaction rate.

Generally, polymerization of the monomer takes place under the influence of light, even if the films do not contain any photoinitiator. However, 9-phenylacridine as a photoinitiator enhances polymerization considerably. Best results are obtained with an initiator concentration of 0·1 wt%, based on the monomer.

The initiation mechanism is different from that derived recently for the polymerization of methyl methacrylate. The photoinitiator being in an excited state after absorption of light strips off hydrogen from the monomer, probably from the CH₂CH₂ groups, thus producing free radicals. In contrast, methyl methacrylate is not suitable as a hydrogen donor; for the polymerization of methyl methacrylate an additional hydrogen donor like diethylene glycol diethyl ether has to be added.

There is no evidence that poly(methyl methacrylate) is involved in the initiation reaction.