1.2 Polydispersity

The polymers in this context are a polymeric linear structures with n repeating units of monomers. Depending whether the polymer is consisting of one single molecular weight (only one n existing) or of a range of compounds with an average mass and a distribution of n around a mean value, polymers are referred to as “monodisperse” or “polydisperse”. If the polymer is polydisperse it shows a mass spectrum as shown in the figure. In order to quantify the distribution of the molecular weight, the Polydispersity D is defined as the ratio between the weight average molecular weight Mw ° and the number average molecular weight Mn °. The weight average molecular weight does not “count” species just by their number, but takes into account the total weight of each species and is therefore a much more realistic indicator of the gross mechanical property. For a homogeneous sample, where the polymer chains have all the same length, Mw ° is equal to Mn °, the polydispersity D is then equal to 1 and the sample is referred to be monodisperse. Whenever there is a distribution of molecular weights, the weight average Mw ° is always greater than the number average Mn ° and the polydispersity is greater than 1. The polydispersity D of PEGs, PGAs and PSRs typically used in polymer therapeutics is between 1.05 and 1.20. Though, whenever a PEGylated new drug compound needs to be approved by EMEA, FDA and other authorities, it is easier and faster if this compound shows only one signal in the mass spectrum and not a distribution pattern. Therefore the need for high molecular weight but monodisperse compounds is increasing. PAS polymers are the choice in this case. As they are produced through recombinant methods, only one specific molecular weight exists although it is a large molecule.

Reference:

• Fundamentals of Polymer Science: An Introductory Text; P. C. Painter and M. M. Coleman; CRC Press 1997: 22. ISBN 1-56676-559-5