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