End Group Effect on the Thermal Response of Narrow-Disperse Poly(N-isopropylacrylamide) Prepared by Atom Transfer Radical Polymerization

Four series of narrow-disperse poly(N-isopropylacrylamide) (PNIPAM) with well-controlled molecular weights and with end groups of varying hydrophobicity were synthesized by room temperature atom transfer radical polymerization in 2-propanol using the corresponding chloropropionate and chloropropionamide initiators. The thermal phase transitions of aqueous solutions of these PNIPAMs were studied by turbidimetry and high-sensitivity differential scanning calorimetry (HS-DSC) and showed an inverse molecular weight (MW) dependence of their cloud points. The magnitude of the MW dependence decreases when using more hydrophobic end groups. The choice of end group further affected the shape of the cloud point curves and the enthalpy of the phase transition. Above the cloud point, narrow-disperse PNIPAM sedimented more rapidly than polydisperse PNIPAM produced by conventional free radical polymerization, especially at concentrations above 1%. Thus, multiple HS-DSC scans of PNIPAM prepared by ATRP typically gave repeatable results only at lower concentrations.