Sh. Hoke et al., MICROSTRUCTURE OF ALKYL ISOCYANATE COPOLYMERS COMPRISED OF ENANTIOMERIC MONOMERS DETERMINED BY DESORPTION CHEMICAL IONIZATION-MASS SPECTROMETRY, Macromolecules, 28(8), 1995, pp. 2955-2960
The microstructure of copolymers generated from enantiomeric 2,6-dimet
hylheptyl isocyanate monomers was determined by desorption chemical io
nization mass Spectrometry (DCI-MS). (R)-2,6-Dimethylheptyl isocyanate
and (S)-2,6-dimethyl-5,6-dideuteroheptyl isocyanate were prepared wit
h the deuterium label incorporated into the (S)-monomer so that the (R
)- and (S)-enantiomers could be distinguished by mass spectrometry. Ho
mo- 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 deter
mined 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 collis
ion-induced dissociation of the entire isotopic envelope representing
the (S)-trimer. This information was used to simulate the trimer regio
n of the mass spectra of two copolymers, one containing 27% (R)-enanti
omer and the other containing 74% (R)-enantiomer. Random and block str
uctures 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 be
tween enantiomeric excess and optical activity in these copolymers and
provides the first experimental correlation between the microstructur
e and the optical properties of synthetic D,L-copolymers.