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