Summary:In this work an investigation of the chain end groups produced in the free radical copolymerization of vinilydene fluoride (VDF) and hexafluoropropylene (HFP) is performed. Type and amount of chain end groups are evaluated by a meticulous analytical characterization of VDF/HFP copolymer. At first pulsed gradient spin-echo nuclear magnetic resonance (spin-echo NMR) is used to identify all the chain end groups also at very low concentration (equal to 0.1 mmol Á Kg
À1). The instrument sensitivity is increased of an order of magnitude in comparison with the traditional NMR. Moreover potentiometric titration and ion chromatography (IC) are also used to study the chain end groups and, as a consequence, the nature and the amount of the acidity showed by the polymer chains. In details two intensity of acidity are detected by potentiometric titration, namely strong and weak. The strong acidity is associated to the presence of residual surfactant and can be removed washing the polymer, while the weak acidity is due to free molecules of fluoride acid (HF). The standard ion chromatography facility is properly modified to quantify the fluoride in the polymer matrix without any pre extraction in water. Thanks to this the HF concentration in the polymer is evaluated with high accuracy. A detailed kinetic scheme for the VDF/HFP polymerization is also proposed taking into account all the findings obtained studying the chain end groups.
A novel silane standard, 1,4-bis(trimethylsilyl)benzene (BTMSB), is introduced for the generic quantitation of small organic molecules in DMSO-d(6) solution by (1)H NMR. This standard is an easily weighable solid and is stable for at least 1 month in DMSO solution, and its (1)H NMR spectrum contains a strong singlet in a region usually free of signals. With a set of certified standards, concentration determination with about 2% precision and accuracy is verified after solution preparation with fully automated procedures, thus making very effective the characterization of small combinatorial chemistry libraries for identity and purity when combined with other physicochemical or biochemical tests. As an example, for a set of about 400 compounds, results of (1)H NMR characterization are compared to the more customary LC-UV-MS method. NMR and MS data agree for identity on the vast majority of cases (84% positive and 5% negative), whereas the remaining cases (11%) are marked as highly impure only after NMR spectra analysis. Most importantly, determination of concentration rather than that of relative purity appears the right choice for a correct evaluation of biochemical potency.
Experimental evidence is reported to illustrate the role of
quantitative 1H NMR in the analytical characterization of new
synthetic molecules. This evidence includes comparison with
results obtained for reference materials and for extensively
characterized substances, precision data on a statistically
significant set of different molecules, elemental analysis data.
These data prove that 1H NMR estimates quantitative purity
with a precision of about 1% and with a comparable or better
accuracy. Moreover, many of the limitations that generally
afflict other methods (variability of response factors, incomplete
accounting for inorganic or volatiles) do not apply to the
quantitative 1H NMR method, whose only requirement is the
presence of at least one integrable signal in the spectrum.
Because of the widespread usage of 1H NMR in modern
synthetic chemistry, it is suggested that spectra are routinely
run under quantitative conditions, with a corresponding reduction of other quantitative characterization tools (elemental
analysis, HPLC, loss on drying and residue on ignition determination) and speed-up of the whole analytical process. For a
budget-conscious laboratory either connected to a discovery
environment or in the field of process chemistry, this could be
a considerable economic advantage, without a compromise on
quality.
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