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