{HEAD ~ each} of X is independently chosen from the moieties —N—, —C—, —O—, {TAIL ~ —OC}—, —C—, —NHC—, and —NH—C— 
is independently chosen from
{ HEAD ~ each } of X is independently chosen from the moieties — N — , — C — , — O — , { TAIL ~ — OC } — , — C — , — NHC — , and — NH — C —
HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL REL REL OTH OTH OTH OTH OTH OTH OTH OTH 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 OTH OTH