Solution Properties of Star and Linear Poly(N-isopropylacrylamide)

Abstract

The LCST transitions of novel N-isopropylacrylamide (NIPAM) star polymers, prepared using the four-armed RAFT agent pentaerythritoltetrakis(3-(S-benzyltrithiocarbonyl)propionate) (PTBTP) and their hydrolyzed linear arms were studied using 1H NMR, PFG-NMR, and DLS. The aim was to determine the effect of polymer architecture and the presence of end groups derived from RAFT agents on the LCST. The LCST transitions of star PNIPAM were significantly depressed by the presence of the hydrophobic star core and possibly the benzyl end groups. The effect was molecular weight dependent and diminished once the number of repeating units per arm ≥70. The linear PNIPAM exhibited an LCST of 35 °C, regardless of molecular weight; the presence of both hydrophilic and hydrophobic end groups after hydrolysis from the star core was suggested to cancel effects on the LCST. A significant decrease in RH was observed below the LCST for star and linear PNIPAM and was attributed to the formation of n-clusters. Application of a scaling law to the linear PNIPAM data indicated the cluster size n = 6. Tethering to the hydrophobic star core appeared to inhibit n-cluster formation in the lowest molecular weight stars; this may be due to enhanced stretching of the polymer chains, or the presence of larger numbers of n-clusters at temperatures below those measured.