PURPOSE: To obtain a product or coating which is polymerized at high photopolymerization rate and does not cause yellowing under ultraviolet rays or sunlight by adding a specific photopolymerization initiator to an ethylenic unsaturated monomer or prepolymer.

CONSTITUTION: 0.1-10% of a polyaromatic, aliphatic ketone {e.g. poly [2- hydroxy-2-methyl-1-(4-vinylpropiophenone)], etc.}, of the formula [(n) is 2-50; Ar is phenylene; X is OH or Cl; R and R1 are 1-4C alkyl; R2 is H or 1-4C alkyl; R3, R4, A, and B are H] is added as a photopolymerization initiator to an ethylenic unsaturated monomer and/or prepolymer. The above-mentioned photopolymerization initiator has an excellent photopolymerization initiating property and provides a higher photopolymerization rate than that of conventional non- polymers or non-polymer type photopolymerization initiators. Moreover, the photopolymerization initiator produces a product or coating which does not cause yellowing under the ordinary aging condition in the presence of ultraviolet rays and sunlight. The by-product of the photochemical reaction hardly has an odor, and is compatible with the polymerized system.