X in formula represents {HEAD ~ a hydrogen atom}, a halogen atom, a hydrocarbon group having 1 to 20 carbon atoms, an alkoxy group or {TAIL ~ an amido group} 
X in formula represents { HEAD ~ a hydrogen atom } , a halogen atom , a hydrocarbon group having 1 to 20 carbon atoms , an alkoxy group or { TAIL ~ an amido group }
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL