Synthesis and characterization of poly(N-isopropylacrylamides)s initiated with siloxane oligomer-(NH₄)₂Ce(NO₃)₆ redox pair

Poly(N-isopropylacrylamide)s (PNIPAAM)s were synthesized via free-radical polymerization using a ceric ammonium nitrate, Ce(IV)-α-ω-dihydroxy(polydimethylsiloxane) (Tegomer H-Si 2111, PDMS) redox pair in hexane at 30°C in a nitrogen atmosphere. The dependence of the initiation and termination steps on the [NIPAAM]/[Ce(IV)] and [NIPAAM]/[PDMS] ratios were studied using gravimetry and FTIR, ¹H-NMR, UV-vis, and GFAA spectroscopy techniques. Gravimetric results indicated that, in the case of high concentrations of PDMS, the percentage of the solid portions of the products decreased while the amount of the oligomeric NIPAAM chains increased, that is, as the amount of PDMS in hexane was increased, the number of the short NIPAAM chains having PDMS segments at the two ends, also increased. UV-vis results showed that the LCST of PNIPAAM initiated with Ce(IV) alone was higher than those of the ones that were synthesized using common initiator systems such as an ammonium persulfate-N,N,N′,N′-tetramethylethylenediamine redox pair and azobis(isobutyronitrile). Further, it was observed that both siloxane blocks and -NH- groups forming coordination bonds with free Ce(IV) ions and/or metal-ligand complexes had an important effect on the aggregation process of the chains.