Microstructure of Alkyl Isocyanate Copolymers Comprised of Enantiomeric Monomers Determined by Desorption Chemical Ionization Mass Spectrometry

The microstructure of copolymers generated from enantiomeric 2,6-dimethylheptyl isocyanate monomers was determined by desorption chemical ionization mass spectrometry (DCI-MS). (R)-2,6-Dimethylheptyl isocyanate and (S)-2,6-dimethyl-5,6-dideuteroheptyl isocyanate were prepared with the deuterium label incorporated into the (S)-monomer so that the (R)- and (S)-enantiomers could be distinguished by mass spectrometry. Homo- and copolymers of these enantiomers were prepared and examined by desorption chemical ionization, a method which causes depolymerization and generates protonated 1,3,5-tris(2,6-dimethylheptyl)cyanuric acids. The degree of deuterium incorporation into the (S)-monomer was determined by a tandem mass spectrometry procedure which avoids the errors associated with signals due to adventitious ions present in the single-stage mass spectra. The experiment involved mass selection and collision-induced dissociation of the entire isotopic envelope representing the (S)-trimer. This information was used to simulate the trimer region of the mass spectra of two copolymers, one containing 27% (R)-enantiomer and the other containing 74% (R)-enantiomer. Random and block structures were considered, and comparison with experiment revealed that the monomers are randomly distributed in the original copolymers. This analysis sets the stage for understanding the unusual nonlinearity between enantiomeric excess and optical activity in these copolymers and provides the first experimental correlation between the microstructure and the optical properties of synthetic D,L-copolymers.