be accessible at extracellular tumor ph because the ph sensitive portion in the polymer can be switched from hydrophobic at normal ph to hydrophilic one at extracellular tumor ph to endow ph sensitivity to the ligand bearing polymer a poly l histidine block is used l histidine is a basic amino acid with a pk of 6 5 this pk value is influenced by the polymerization of histidine comonomer and microenvironment the imidazole ring in l histidine has a lone pair of electrons on the unsaturated nitrogen which endows an amphoteric nature to poly l histidine polyhis polyhis is protonated at ph 5 0 7 0 being influenced by the molecular weight of poly l histidine polyhis with a molecular weight of gt 10 000 was reported to be soluble only below ph 6 0 by protonation poly l histidine was synthesized using an aminated ligand as an initiator the polymer was coupled to peg plla block copolymer the extended ligand system is easily made by mixing of peg plla and ligand poly l histidine peg plla block copolymers to demonstrate the invention two vitamins folate and biotin were selected as ligands and several micelle systems were prepared as described below carrier 1 comprised poly his5k peg2k block copolymer and plla3k peg2k 60 40 wt as a mixed micelle control such micelles had an average diameter of about 100 nm carrier 2 comprised poly his5k peg2k and plla3k peg2k folate 60 40 wt as a mixed micelle such micelles also had an average diameter of about 100 nm carrier 3 comprised poly his5k peg2k and plla3k peg2k poly his1k folate 60 40 wt as a mixed micelle such micelles also had.