The molecular weight and molecular weight distribution of hydroxypropyl methylcellulose used in the film coating of tablets

The molecular weight and molecular weight distributions of nine grades of hydroxypropyl methylcellulose used in the film coating of tablets have been measured using gel permeation chromatography. With the exception of Pharmacoat 603, there is a high molecular (greater than 5 x 10(5)) component present in all grades ranging from relatively small amounts in grades with low nominal viscosity to relatively large amounts in the 50 mPas grade. The peak molecular weight taken from the distribution curve--an indication of the molecular weight of the main component--could be calculated from the nominal viscosity using the equation. Peak molecular weight = 23.54 x 10(3) (viscosity)0.45. The relationship could also be expressed in the standard form [eta]=KM alpha where M is the peak molecular weight [eta] is the intrinsic viscosity and K and alpha are constants (in this case 9.94 x 10-4 and 1.096 respectively). The wide molecular weight distribution of these samples and the presence of quite high proportions of very low molecular weight (less than 5 x 10(3)) components especially in samples with nominal viscosity designations of less than 15 mPas appears to affect their mechanical properties.