An oligomer having the formula ##STR67## wherein each R3 is independently H, OH, OCH3, SCH3, OC3 H5 (O-allyl), OC3 H7 (O-propyl), SC3 H7, or F, andwherein each B is independently a purine or pyrimidine residue or an analogous residue; R is H or FMOC; and n is an integer between 1 and 200.