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