aryl, and a mixture of alkyl and oxyalkyl, or a mixture of alkyl and oxyaryl, and R5 and {HEAD ~ R6} are {TAIL ~ monovalent electron} withdrawing groups selected from nitrile and a mixture of nitro and C.sub 
are
aryl , and a mixture of alkyl and oxyalkyl , or a mixture of alkyl and oxyaryl , and R5 and { HEAD ~ R6 } are { TAIL ~ monovalent electron } withdrawing groups selected from nitrile and a mixture of nitro and C.sub
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH