Poly(α-methylolbenzoin methyl ether acrylate) and copolymers were prepared from a monomeric benzoin ether derivative, α-methylolbenzoin methyl ether acrylate, and photopolymerizations and photoreactions of those polymers were studied. The photosensitive polymer having benzoin methyl ether side chains showed a greater efficiency in photoinitiating capabilities than nonpoly-meric benzoin ether derivatives, such as benzoin, benzoin methyl ether, α-methylbenzoin methyl ether, and benzoin methyl ether acetate. Gelation by the polymeric photoinitiators predominated in photopolymerizations with vinyl monomers.