a radiation - polymerizable mixture comprising a compound having at least two terminal ethylenically unsaturated groups which can form a crosslinked polymer by means of free - radical initiated addition polymerization , a polymeric binder , a radiation activatable polymerization initiator , and a compound having two epoxy groups in the molecule and a molecular weight of not more than 1 , 500 ; and a photopolymerizable copying material having a flexible transparent temporary support and a transferrable thermoplastic photopolymerizable layer comprising the above - described radiation - polymerizable mixture .