The relationship between the absolute molecular weight and the degree of quaternisation of N-trimethyl chitosan chloride

N-Trimethyl chitosan chloride (TMC) is a partially quaternised derivative of chitosan with enhanced water solubility and superior drug absorption enhancing properties, especially in neutral and basic environments where chitosan is insoluble. The degree of quaternisation of TMC plays an important role on the solubility, the absorption enhancing ability and mucoadhesive properties of this polymer. A range of TMC polymers with different degrees of quaternisation were synthesised by varying the number and duration of reaction steps. Size exclusion chromatography (SEC) and multi-angle laser light scattering (MALLS) were used to determine the absolute molecular weight, radius and polydispersity of these TMC polymers. The viscosities of TMC polymer solutions were measured at different concentrations and the calculated intrinsic viscosity values were used as a further indication of the molecular weight of each polymer. The decrease in absolute molecular weight correlated well with the intrinsic viscosities of the TMC polymers and was related to the increase in their degrees of quaternisation. This polymer degradation was explained by increased exposure time of the polymer molecules to reaction conditions, such as the strong alkaline environment and elevated temperatures, that is necessary to produce higher degrees of quaternisation during the synthesis of TMC polymers.