Molecular and Structural Characterization of Polydisperse Polymers and Copolymers by Combining MALDI-TOF Mass Spectrometry with GPC Fractionation

Abstract

Matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) allows detection of large molecules such as those present in synthetic and natural macromolecules. Until recently, it was reported that MALDI-TOF measurements can provide correct molecular weight (MW) averages only for samples with a narrow MW distribution (M w /M n ≤ 1.20). We have now developed a methodology for polydisperse samples. We recorded the GPC trace of two polydisperse polymeric samples, namely, poly(butylene adipate)(PBA) and poly(butylene adipate-co-butylene succinate)(PBAS), collecting about 40-50 fractions per run. Selected fractions were analyzed by MALDI-TOF, and the average MW of each fraction was determined, allowing calibration of the GPC curves against absolute MW. The two calibrated GPC traces were then used to compute average MW and molecular weight distributions (MWD) of the unfractionated samples. End group analysis from MALDI-TOF spectra revealed that the PBA sample is composed of seven different types of chains. For the copolymer sample, PBAS, analysis of the MALDI spectra established a random sequence distribution of comonomer units. The succinate/adipate molar ratio calculated from the MALDI spectra is in good agreement with the molar ratio found by NMR.