The importance of the molecular weight distribution for the direct determination of the kinetic constants of free radical polymerization

Complete text of publication follows. In principle, each molecular weight distribution bears the imprint of the genesis of the polymer. Several problems are associated with the interpretation of measured distribution curves (by size exclusion chromatography): 1. How can the information about the kinetic constants be extracted? 2. How strong is the influence of axial dispersion? Stationary polymerization processes (at low conversions) will lead to monomodale distributions characterized either by their moments or by the extrema. Without any additional information only the type of termination can theoretically be determined, individual rate constants are not accessible. Pulsed laser polymerization techniques (= pseudostationary techniques) lead to the first direct determination of the rate constant of propagation, kp. The periodic modulation of the total concentration between a minimum and maximum value gives rise to molecular weight distributions, which are characterized by the appearance of so called additional peaks. For the first peak the location of the point of inflection on the low molecular weight side (and/or maximum), L0, are directly related to the monomer concentration [M], the duration of the dark period separating two laser pulses t0 and kp by L0 = kp[M]t0. A similar procedure turned out successful for quenched instationary polymerization systems for which it is essential that the contribution of the quenched radical chain length distribution dominates the total distribution. Pseudostationary and instationary techniques are suitable for the investigation of copolymerization systems, polymerization in solution as well as in heterogeneous systems. A simple method for the direct determination of axial dispersion was developed