Controlling the Stereoregularity of Polyacrylonitrile and Its Determination Using Small-Molecule Host Inclusion Compounds

Abstract

This research focuses on synthesizing highly stereoregular polyacrylonitrile (PAN) and determining its tacticity (predominantly isotactic or syndiotactic), utilizing guest monomer (acrylonitrile = AN) host inclusion polymerization. Highly stereoregular PAN, with a meso or racemic diad content ~ 80%, was prepared by γ-ray irradiation of an AN urea canal complex at a low temperature (–78° C). Several essential experimental factors for ensuring the highly stereoregular PAN production were considered. After γ-ray irradiation polymerization, the tacticity of PAN was determined from triad peak intensities of the methine (CH) and nitrile (–C≡N) carbons in the 13C-NMR spectra, assuming Bernoullian statistics. When the AN guest forms an inclusion compound (IC) with urea host, it was expected that there is a structural transformation of urea into the hexagonal crystal lattice structure with a narrow channel diameter (5.25-5.5Å). However, in our FTIR observations run at room temperature, a different type of transformation was detected. AN urea IC before and the PAN urea IC after low temperature γ-ray irradiation polymerization are both large tetragonal structures, which have a larger channel diameter (> 5.5Å) at room temperature. Because these infrared observations were not carried out below -20.8° C, known as the decomposition temperature of the hexagonal IC structure, the fact that the structures of both AN urea IC and PAN urea IC are the large tetragonal does not necessarily prove that during polymerization below –20.8°C the AN urea IC was also the large tetragonal structure. If PAN was polymerized in the hexagonal (or pseudo-hexagonal) urea canal lattice (5.5Å), which provides a more confined environment for its conformation and configuration, it would likely be syndiotactic and adopt the all trans conformer. Because of the flexible nature of urea when it forms inclusion compounds with guest molecules, if AN was polymerized in the large tetragonal (> 5.5.Å) lattice structure of urea, which gives more freedom to PAN during its inclusion polymerization or inclusion compound formation, it could have either an isotactic or a syndiotactic configuration.

Because no definitive evidence has been previously reported in the determination of γ-ray irradiated PAN by NMR spectroscopy, an effort to prepare PAN in another molecular host crystalline lattice, α-cyclodextrin (CD), was made. Synthesis of PAN in the columnar structure of AN α-CD–IC is a very promising method to reveal the original tacticity of highly stereoregular (~80%) PAN due to the fact that α-CD has a rigid small diameter (4.9Å) channel cavity, and only syndiotactic PAN in the all trans conformation is likely to be produced. However, γ-ray irradiation of a channel structure AN α-CD IC did not produce any PAN, implying that the AN urea IC that produced stereoregular PAN upon γ-irradiation was likely in a large tetragonal structure. Alternatively, because of the disparity in AN:α-CD and PAN:α-CD stoichiometries [1:1 (experimental) versus 3:1 (expected)], after γ-irradiation initiation of AN α-CD IC, a shortage of AN would result in the α-CD IC channels, possibly interrupting polymerization.

By analogy to polypropylene (PP) polymerized in host perhydrotriphenylene (PHTP) IC (d ~ 5Å) and polyvinylchloride (PVC) polymerized in urea canals, which are both found to be syndiotactic, we suggest that stereoregular PAN polymerized in urea canals is also predominantly syndiotactic.