alkyl radicals, R2 and {HEAD ~ R3} are {TAIL ~ ##STR18}## or a mixture of ##STR19## where R7 and R8 are selected from C.sub 
are
alkyl radicals , R2 and { HEAD ~ R3 } are { TAIL ~ # # STR18 } # # or a mixture of # # STR19 # # where R7 and R8 are selected from C.sub
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL 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