Photoreactive cyclohexanone-formaldehyde and acetophenone-formaldehyde resin-bound benzoin and benzoin isobutyl ether resins were successfully prepared by the method of in situ modification of ketonic resins. These photoinitiators were used to polymerize styrene using UV lamp with wavelength of 350 nm. Initiating efficiencies of ketonic resin-bound benzoin and benzoin ether were much higher than benzoin and benzoin ether. The products were ketonic-resin-polystyrene block copolymers.