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

The synthesis of telechelic poly(N-isopropylacrylamide) (PNIPAM) with carbo xylic acid termini was accomplished using the primary radical termination m echanism. Polymers with molecular weight (Mn) ranging from 32 to 5 kDa coul d be: prepared by changing the monomer-to-initiator ratio and the time of p olymerization. Modeling of the polymerization by a simple kinetic model bas ed on radical initiation, propagation, and primary radical termination by i nitiator was performed. Prediction of the molecular weights by this model s howed good agreement with the experimentally observed values. Characterizat ion of the polymer revealed a complex aggregation phenomena in DMF solution s at temperatures of 60-70 degreesC. For samples with M-n between 8.3 and 1 2.2 kDa the presence of nanometer size aggregates was evident in the elutio n chromatogram, and the laser light scattered at right angles. It was found that the aggregation appeared at a polymer concentration that was far belo w the overlap concentration of the polymer chains, and the aggregates could not be broken up by either dilution of the solutions or addition of lithiu m 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.