As my experiences: the functional parts on Polymer and on drugs - or - the conjungated parts (hydrophobic interaction to make micell). For example: PEG and folic acid and DOX in this publication
Indeed important, with regard to formation of polymer-drug conjugates and impurities. Also relevant and often under-estimated is the reaction between polymer impurities (e.g residual monomers, oligomers, plasticizers...) and drugs. This speaks in favour of well characterized polymers and pure polymers.
for example, new drug candidate (GEMSP) is made up of fatty acids, vitamins and amino acids or their derivatives; all these compounds were linked to Poly-L-Lysine