wherein PG is a protecting group that maybe present or absent, Y is a target molecule of interest and may be present or absent, and when Y is absent PG is an amino protecting group that may be present or absent; R1 and each R individually is hydrogen or an organic side chain; R3 is a water-soluble polymer; X is sulfur or selenium; n1 and n2 each are from 0 to 2; n3 is from 0 to 100; each L1, L2 an