Dependence of CC stretching wavenumber on polyene length in degraded polyvinyl chloride: a comparative empirical, classical mechanics, and DFT study.

Mathematically describing the length-dependence of vibrational fingerprints of polyenes is challenging, yet crucial in understanding and predicting polyene-associated molecular properties of industrially-important and vital substances. To this end, we develop an analytical relationship between the wavenumbers ν∼C=C of the Raman-active CC stretching mode in polyene sequences (CHCH)n and the polyene length (n) using classical mechanics laws. Noteworthy, this relationship is derived from Newton's equations instead of regression approximations and validated against experimental data for degraded polyvinyl chloride (PVC), t-butyl end-capped all-trans polyenes, ß-carotenes, and carotenoids. Furthermore, given this fundamental tool, we carefully re-examined or validated the up-to-now applied empirical tools; we find that: (i) A phenomenological exponential regression function ν~C=C=1461+151.2×exp-0.07808n proves fairly suitable for describing polyenes with lengths below 24 in degraded PVC. (ii) The derived analytical relationship agrees more closely with a long-established reciprocal-length regression function ν~C=C=1459+720/n+1 for describing carotenoids. Moreover, extensive DFT calculation results on all-trans polyenes H(CHCH)nH (n = 3-30) and polyenes end-capped with terminal vinyl chloride oligomers agree with experiment for shorter polyenes and are similar, showing that complicated calculations of ν∼C=C for infinite degraded PVC chains reduce to the calculations on finite polyene sequences. Noteworthy, unlike other polyene length-determination tools, the proposed analytical polyene length-determination based on intrinsic physical properties could well prove to be an even more versatile tool, as it comes with the added potential for determining or correcting the elasticity constants of carbon bonds in polyene chains.

Raman spectroscopic detection of polyene-length distribution for high-sensitivity monitoring of photo- and thermal degradation of polyvinylchloride.

The degradation of the ubiquitous polyvinylchloride (PVC) material under the influence of various factors is known to result commonly in polyene formation. Such polyene defects occur in the form of conjugated aliphatic chains with different lengths and contents, and their sensitive and length-specific monitoring is important for the assessment of PVC degradation. Here, we report on the resonance-enhanced Raman signatures of polyene sequences of varying lengths in photo- and thermally degraded PVC films. The lengths of polyene segments have been estimated based on their selectively enhanced and spectrally resolved contributions to the Raman bands assigned to the stretching vibrations of conjugated double carbon bonds. Using deconvolution analysis of a characteristic Raman band of polyenes, we especially demonstrate that the spectral signatures of polyene segments corresponding to other various electronic resonances contribute to the Raman spectral envelope observed at a given resonant excitation. In most cases, we observe an asymmetric band profile, indicating an asymmetric length distribution of polyene defects formed in PVC films upon a mild degradation extent less than 1% mass loss. We also demonstrate that the wavenumber (ν1) of the stretching vibrations of single carbon bonds in the polyene sequences of degraded PVC is inversely related to the number (n) of double carbon bonds by an empirical equation n=476·cm-1/ν1-1082·cm-1. To the best of our knowledge, while considering different laser excitations spanning the range of possible electronic resonances from blue to near-infrared for Raman investigations, the present work includes (i) the first Raman spectral deconvolution study for the 532.0 nm excitation wavelength used in most portable Raman probes nowadays and (ii) the screening of polyene defects also beyond the red edge of the visible spectrum and the evidence of a resonance-enhanced polyene signal at 647.1 nm. Important new information has been obtained about polyene lengths and spectral distribution for PVC, whose critical physical properties ranging from flexibility to electrical resistance are severely affected by polyene formation.

Cyclic Methacrylate Tetrahydropyrimidinones: Synthesis, Properties, (Co)Polymerization.

During radical polymerization of novel biocidal methacrylate guanidine monomers, a cyclic byproduct was discovered and identified as 2-imino-5-methyltetrahydropyrimidin-4(1H)-one (THP). Its methacrylate salt (MTHP) was synthesized and characterized via 1H and 13C NMR and pyrolysis chromatography. Synthesis conditions of both THP and MTHP were optimized to high yields, and both MTHP homopolymerization (in aqua) and copolymerization with diallyldimethylammonium chloride (in aqua in salt form) were successfully carried out with middle to high yields, providing a promising platform for potential tailored biocide polymers.