Influence of preparation conditions of poly (ethylene glycol)/poly (N-isopropylacrylamide) block copolymers on their properties

Thermosensitive block copolymers of poly (N-isopropylacrylamide) (PNIPAM) and poly (ethylene glycol) (PEG) were synthesized by the redox reaction of the hydroxyl end group of PEG with a ceric ion. The effects of polymerization temperature and NIPAM concentration on the molecular weight of block copolymers were investigated. The molecular weight of the copolymer was found to increase in an exponential manner with an increase in NIPAM concentration when the polymerization was performed below the lower critical solution temperature (LCST) of PNIPAM. At a temperature above the LCST, on the other hand, few NIPAM monomers were polymerized regardless of NIPAM concentration, and the molecular weight of the block copolymers obtained was almost the same as that of the original PEG. These results indicate that above the LCST polymeric micelles are formed during polymerization and the polymerization hardly proceeds within the hydrophobic core of the polymeric micelles.