Telechelic poly(N-isopropylacrylamide):: Polymerization and chain aggregation in solution

The synthesis of telechelic poly(N-isopropylacrylamide) (PNIPAM) with carboxylic acid termini was accomplished using the primary radical termination mechanism. Polymers with molecular weight (Mn) ranging from 32 to 5 kDa could be: prepared by changing the monomer-to-initiator ratio and the time of polymerization. Modeling of the polymerization by a simple kinetic model based on radical initiation, propagation, and primary radical termination by initiator was performed. Prediction of the molecular weights by this model showed good agreement with the experimentally observed values. Characterization of the polymer revealed a complex aggregation phenomena in DMF solutions at temperatures of 60-70 degreesC. For samples with M-n between 8.3 and 12.2 kDa the presence of nanometer size aggregates was evident in the elution chromatogram, and the laser light scattered at right angles. It was found that the aggregation appeared at a polymer concentration that was far below the overlap concentration of the polymer chains, and the aggregates could not be broken up by either dilution of the solutions or addition of lithium bromide to DMF. The solvent was found to play a role, and aggregation was observed in another polar solvent such as N-methylpyrollidone but not in a nonpolar solvent such as tetrahydrofuran.