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