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