Analysis of poly(bisphenol A carbonate) by size exclusion chromatography/matrix-assisted laser desorption/ionization. 2. Self-association due to phenol end groups

We report here a case of apparent failure of the size exclusion chromatogra phy/matrix-assisted laser desorption/ionization (SEC/MALDI) method to provi de polymer fractions with narrow molar mass distribution, showing that inte rmolecular chain association is responsible for this phenomenon, Poly(bisph enol A carbonate) (PC) chains terminated with hydroxyl groups undergo self- association by hydrogen bonding, providing macromolecular aggregates with h igher hydrodynamic volume. These aggregates are eluted through SEC columns at the same volume as higher molar mass chains, which remain non-associated . Thus, self-association affects negatively the SEC fractionation experimen ts, and even the sharpest SEC fractions contain a heterogeneous mixture of PC chains of different size. When the off-line SEC/MALDI procedure is appli ed, the SEC fraction is diluted in the matrix which, being a dissociating m edium (carboxylic acid) for hydrogen-bonded aggregates, suppresses the chai ns' self-association. Therefore, the MALDI spectra of these PC fractions in dicate a polydisperse character, with irregular bimodal distributions of pe aks, As a consequence, in the presence of chain association, the SEC/MALDI method for the calculation of molar masses of polymers cannot be directly a pplied. In the present case we have found that, under opportune experimenta l conditions, self-association in polycarbonates can be avoided, so that ne arly monodisperse SEC fractions can be obtained and the SEC/MALDI method ca n be applied, Our results also show that MALDI is a very sensitive techniqu e for the detection of association of polymers in dilute solutions. Copyrig ht (C) 1999 John Whey & Sons, Ltd.