{HEAD ~ Y} is hydrogen or the organic residue of a saturated or unsaturated organic compound containing at least one isocyanate reactive active hydrogen atom, {TAIL ~ which residue} remains after reaction of at least one such active hydrogen atom with an isocyanate moiety 
{ HEAD ~ Y } is hydrogen or the organic residue of a saturated or unsaturated organic compound containing at least one isocyanate reactive active hydrogen atom , { TAIL ~ which residue } remains after reaction of at least one such active hydrogen atom with an isocyanate moiety
HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH 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 OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH