Molecular weight distribution in anionic polycaprolactam

Abstract We examined the molecular weight distribution in polycaprolactam obtained at 180 with sodium hydride and acetylcaprolactam. The polymer was fractionated in a continuous procedure with phenol/water system at 70. Titration of acetyl end groups in the various polymer fractions led to values of M n . At all stages of the polymerization, the molecular weight distribution seems very narrow in agreement with the step-addition process of the lactam molecules. Fractionations of samples, made with the same catalyst system but above the polymer melting point (220°), were also carried out. In this case, the method just mentioned could not be used to find M n because of the presence of free amino end groups. According to recent theories, this could indicate the presence of branched products together with smaller linear molecules. Indirect measurements suggested a progressive broadening of the distribution. This effect suggests the occurrence of transamidation reactions in the molten state.