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