The synthesis and characterisation of hyperbranched polyesters
A new route to the AB(_2) monomer dimethyl 5-(2-hydroxyethoxy) isophthalate and its subsequent polymerisation is reported. Hyperbranched polyesters were prepared by melt condensation polymerisation of the AB(_2) monomer and also by copolymerisation with suitable core molecules. Unusual molecular weight growth characteristics were demonstrated. The number average molecular weight (Mn) reached a plateau value after relatively short polymerisation time, whereas the weight average molecular weight (M(_n)) continued to increase. This limit to the attainable number average molecular weight is thought to be a consequence of intramolecular cyclisation reactions, evidence of which was provided by MALDI-TOF MS. The weight average molecular weight continues to increase after complete cyclisation and it is postulated that this is due to a redistribution of polymer chains by ester interchange reactions. The plausibility of the occurrence of ester exchange processes was demonstrated by the successful incorporation of a dimethyl isophthalate core molecule into a fully cyclised hyperbranched polyester. The amount of branching in the hyperbranched polyesters was in agreement with the theoretical value for a statistical distribution at high conversion, as determined by quantitative (^13)C NMR spectroscopy. The solution properties of the hyperbranched polyesters were studied and revealed these materials to be soluble in common organic solvents. Solutions of the hyperbranched polyesters had characteristically low intrinsic viscosities ([n]) and Mark-Houwink plots showed a linear relationship between log M(_w) and log[n]. The thermal properties of the hyperbranched polyesters were examined by differential scanning calorimetry, which showed the polymers to be amorphous materials exhibiting a glass transition temperature (T(_g)) but no melting point. The T(_g) varied with molecular weight, was proportional to 1/M(_w) and reached a limiting value of 86ºC at high conversion.