Polydispersity of Hyperbranched Polyesters Based on 2,2-Bis(hydroxymethyl)propanoic Acid: SEC/MALDI−TOF MS and 13C NMR/Kinetic-Recursive Probability Analysis

Abstract

The absolute average molar masses and polydispersity index of a series of polyesters based on 2,2-bis(hydroxymethyl)propanoic acid (BMPA, AB2-type monomer), 2,2-bis(hydroxymethyl)-1,3-propanediol (PE, B4-type monomer), and 1,1,1-tris(hydroxymethyl)propane (TMP, B3-type monomer) were determined by SEC/MALDI−TOF off-line coupling. The M̅w/M̅n values, in the 1.8−3.1 range, were surprisingly very close to those found by SEC using the conventional linear polystyrene calibration method, but dramatically lower than those calculated from conversion and composition of initial monomer mixture assuming equal reactivity of functional groups. The mol ratios of the various monomer units in final polyesters were determined by 13C NMR. The kinetic-recursive probability approach assuming first-shell substitution effects was then applied to calculate the branching factors and the average molar masses of these polyesters from their final composition. The AB- and B-branching factors were significantly lower than expected for random polymerizations, reflecting the existence of both negative A−B and B−B substitution effects, i.e. lower reaction rate of a given A or B group after reaction of the other groups present on the same monomer unit. The calculated values of M̅w and M̅w/M̅n were reasonably close to the experimental ones for all hyperbranched polyesters. Intramolecular esterifications and intermolecular etherifications side reactions were taken into account in the calculations with some simplifying assumptions and found to marginally broaden MW distributions.