Quantitative Measurement of Isomer Composition in Polypentenamer Using Carbon-13 Nuclear Magnetic Resonance

Polypentenamer ((−CH2CH2CH=CHCH2−)n, is produced by the ring-opening polymerization of cyclopentene. In the past, the determination of the relative concentrations of cis and trans structure was based upon an infrared method developed for analyzing polybutadiene. However, inexact absorption coefficients and problems with band overlap left the resultant analyses open to question. Carbon-13 nuclear magnetic resonance spectroscopy (13C NMR) seemed ideally suited for determining the isomer composition in polypentenamers. We have used 13C NMR as the primary analytical method to precisely determine the isomer composition of a series of eight polypentenamers. These results and samples will provide standards to determine infrared absorption coefficients, and thus provide a new infrared analysis. Two recently published books clearly show that the flurry of research over the past few years has firmly established 13C NMR as a valuable spectroscopic tool for the organic chemist. Its potential for establishing polymer molecular structure has equally been exciting and encouraging. The advantage of carbon-13 over proton NMR has been the dispersion of chemical shifts over a much wider range. This has meant that unique, separate NMR peaks have been obtained which describe molecules with subtle differences in molecular structure. The analytical use of 13C NMR has been suggested and used to a limited extent to quantitatively measure stereoconfiguration and monomer sequence distribution. However, no detailed investigation has been reported on precision or accuracy if 13C NMR is used as the primary analytical method for measuring polymer microstructure. As with proton NMR, measurements of peak areas are necessary to obtain quantitative analyses. Allerhand has predicted that 13C integrated intensity should be valid as a carbon count in spectra of complex molecules. Schaefer subsequently has shown that within a polymer system the carbons undergo equal nuclear Overhauser enhancement (NOE), even though the total NOE of different polymers may not be equal or maximum. Hence one can compare relative areas within a 13C NMR spectrum without fear of inadequately accounting for all of the area of a given structural feature.