Salil K. Jha

The road to chiral amplification in polymers originated in Italy

The fact that stereoregular polymers produced by Ziegler-Natta polymerization methods could be crystallized and studied by X-ray diffraction methods lead to the discovery that these polymers adopted helical conformations in the crystalline solid. In addition to the nearly perfect asymmetric synthesis demonstrated by the highly isotactic stereoregularity produced by the catalyst, the conformation adopted in the crystalline state also involves chirality. The large number of units polymerized produced though gives rise to a cryptochiral material in which the mirror helical conformations are necessarily equally populated and therefore the measurement of optical activity can never be a consideration in these experiments. At that time though Nattas students began to prepare stereoregular vinyl polymers with chiral nonracemic pendant groups and discovered that the optical activities of such materials in solution showed nonlinear relationships between enantiomeric content and optical activity. This phenomenon arising from the effect of the chiral pendants on the population of the helical senses of the polymer backbone, acted to demonstrate that these stereoregular polymers adopted helical conformations in solution comparable to those investigated in the solid state. In recent times this effect has reached an extreme in the polyisocyanates which show a much larger sensitivity to chiral effects than found for the vinyl polymers although the underlying forces at work are identical to those studied earlier by the Italian School. Chirality 9:424–427, 1997.

Block microstructural characterization of copolymers formed from fluorinated and non-fluorinated alkyl polyisocyanates using desorption chemical ionization mass spectrometry

Abstract Homopolymers and copolymers of 1-isocyanato-4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononane (monomer F ) and n -hexylisocyanate (monomer H) were examined by desorption chemical ionization mass spectrometry (DCI-MS) to obtain information on the monomer distribution in the copolymers. Tandem mass spectrometry (MS/MS) was used to characterize ions generated by DCI in the mass spectrometer ion source; ammonia and isobutane were selected as chemical ionization (CI) reagent gases. The major peaks in the ammonia DCI mass spectrum of poly(4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononyl)isocyanate (poly ( F )) are protonated and ammoniated trimers. This result suggests that on pyrolytic degradation poly ( F ) forms cyclic trimers as do alkyl isocyanate polymers. The isobutane DCI mass spectra display the characteristic alkene elimination sequence characteristic of poly( n -hexyl)isocyanate and poly(2,6-dimethylheptyl)isocyanate but with additional extensive fragmentation. The major fragment ion is the protonated monomer. The monomer distributions in copolymers comprised of monomer F and monomer H were deduced from the abundances of various protonated and ammoniated trimers in the ammonia DCI mass spectra using Markovian statistics. Both soluble and insoluble copolymer samples were isolated and found to have non-random monomer distributions. The soluble fraction is dominated by monomer H blocks while the insoluble fraction also contains a majority of monomer H blocks but relatively more monomer F blocks. This forms an example in the polyisocyanates, which hitherto exhibited only random copolymerization, of a non-living method of polymerization yielding a block microstructure for a mixture of two monomers with virtually identical polymerizable functions. Mass spectrometry offers information on chain microstructure which would be unavailable by other means.

Microstructure of alkoxy and alkyl substituted isocyanate copolymers determined by desorption chemical ionization mass spectrometry

Abstract Copolymers comprised of 3-(2-(2-methoxy)ethoxy)propyl isocyanate (monomer G ) with 1-isocyanato-4,4,5,5,6,6,7,7,8,8,9,9,9-tridecafluorononane (monomer F ) as well as copolymers of monomer G and n -hexylisocyanate (monomer H ) were examined by desorption chemical ionization mass spectrometry (DCI-MS) to obtain information on the monomer distributions in the copolymers. The monomer distributions were deduced from the abundances of various protonated and ammoniated trimers in the ammonia DCI mass spectra using zero-order Markovian statistics. Both copolymers comprised of monomers G and F as well as monomers G and H were found to have random monomer distributions. This behavior is in contrast to copolymers comprised of monomers F and H whose dissimilar side chains resulted in non-random distributions.