Molecular Weight Characterization of Poly(N-isopropylacrylamide) Prepared by Living Free-Radical Polymerization

A series of relatively monodisperse samples of poly(N-isopropylacrylamide) were synthesized by reversible addition−fragmentation chain transfer (RAFT) over the molecular weight range 2 × 103−3 × 105. For molecular weights below 4 × 104, conditions were found so that polydispersity remained below 1.4 up to high conversion (72%). Molecular weight distributions of polymer obtained using GPC in THF and MALDI were in accord for the low molecular weight range (typically 105). Particular care is required in GPC sample preparation: it is necessary to ensure that trace amounts of water are initially present when drying a polymer sample prior to dissolution in THF, to avoid irreversible chain aggregation. The log/log plot of intrinsic viscosity against molecular weight for polyNIPAM was found to be linear for molecular weights <105, after which the hydrodynamic volume seems to be independent of molecular weight. The Mark−Houwink parameters obtained from the lower molecular weight data are K = 10-(4.24±0.42) dL g-1 and a = 0.78 ± 0.09.