u201d or tag is thus used herein to refer to the component or components of the medium that are responsible for the thermal induced formation of acid the second acid generator generally comprises only a single compound that produces acid directly the acid produced from the second acid generator may be either a bronsted acid or lewis acid provided of course that the acid is of a type and strength which will induce cationic polymerization of the monomer when the acid produces a bronsted acid this acid preferably has a pk a less than about 0 the second acid generator can be the first acid generator such as when it is present in the medium in amounts in excess of the concentration required for radiation induced formation of acid in such cases the sensitizer which renders the first acid generator sensitive to a particular wavelength must first undergo a process that provides for complete photobleaching known superacid precursors such as diazonium sulfonium phosphonium and iodonium salts may be used in the present medium but sulfonium salts are generally preferred specifically those which are only thermal acid generators benzyldialkyl sulfonium salts have been found to perform well in the present media with specific preferred benzyldialkylsulfonium salts being 4 vinylbenzyl tetramethylene sulfonium tetrakis 3 5 bis trifluoromethyl phenyl borate 4 methylbenzyl tetra methylenesulfonium tetrakis 3 5 bis trifluoromethyl phenyl borate and 4 methylbenzyl tetramethylene sulfonium tetrakis pentafluorophenyl borate other examples of sulfonium salts include for example dialkylphenacyl sulfonium salts see polymer journal vol 17 no.