New mass spectral methods to measure the absolute molecular
mass distribution and molecular architecture of saturated hydrocarbon
polymers, particularly polypropylene, are under development.
Polyolefins dominate the synthetic polymer market. Their dominance
has been spurred by new developments in metallocene catalysts
that provide an unprecedented level of synthetic control. A
polymer’s molecular mass and molecular-mass distribution
are critical in determining its performance properties. Mass
spectrometry is currently the most promising method for obtaining
accurate molecular mass and absolute molecular mass distributions.
Molecular mass determination by mass spectrometry requires the
formation of intact macromolecular ions in the gas phase. This
is typically accomplished by the association of metal cations
(e.g. Na+, Ag+) with polar or polarizable functional groups
on the polymer. Polymers that lack such functional groups (like
polypropylene) cannot be analyzed by mass spectrometry using
existing techniques of metal cationization.
Experimental Approach
A new method was introduced by us for the mass spectrometry
of saturated polyolefins whereby an organic cation is covalently
bonded to the vinyl end-group of the polymer to produce the
necessary ionization for the creation of intact gas-phase macromolecules
by matrix-assisted laser desorption/ionization (MALDI). This
eliminates the dependence of ionization efficiencies on gas-phase
ion-molecule reactions yielding high sensitivity and resolution.
Results
Shown below are the first MALDI time-of-flight (TOF) mass
spectra of polypropylene (PP), which can be used in pioneering
chemical structure and quantitation studies.
Contributors
Polymers Division: Mark A. Arnould, David L. Vanderhart,
William E. Wallace University of Massachusetts: Prof.Bryan Coughlin (polymer
synthesis)
Characterization and Measurement Group
Polymers Division
Materials Science and Engineering Laboratory
