Determination of Ethylenic Groups

At present there is no single procedure available for the determination of ethylenic groups in lignin which can be applied reliably to any type of sample without regard for the particular structural environment in which these units exist. Hence, most of the more widely accepted methods essentially consist of procedures used for the determination of larger structural elements comprising lignin, for example, cinnamaldehyde, cinnamyl alcohol, and stilbene units, in which the carbon-carbon double bond is incorporated. 

CW Dence. Determination of ethylenic groups. In SY Lin, CW Dence, eds. Methods in Lignin Chemistry. Berlin Springer-Verlag, 1992, pp. 435-445. 

Chemical methods used for the determination of hydroxyl groups or alcoholic constituents in polymers are based on acetylation [16-18], phthalation [18], and reaction with phenyl isocyanate [18,19] or, when two adjacent hydroxy groups are present in the polymers, by reaction with potassium periodate [9,17]. Alcoholic hydroxyl groups may be found in the following polymers (1) poly(ethylene terephthalate) (PET) [20], (2) poly(methyl acrylate), [21], (3) poly(methyl methacrylate) [21], and (4) polyhydric alcohols in hydrolysates of poly(ester) resins [22]. 

Ethylenic groups are present in units 47,48, and 51 and also in cyclohexadienone and quinoid units (Figure 7.6). It is difficult to determine the total number of ethylenic groups. Reported estimates suggest that there are 0.05-0.1 ethylenic groups/phenyl-propane units in softwood lignin [117]. 

Saponification procedures can be applied to the determination of ester groups in polymers. A copolymer of ethylene and vinyl acetate has the following structure which, upon hydrolysis in the presence of potassium hydroxide/p-toluene sulfonic acid catalyst [32, 33] reacts as follows ... 

Ester groups occur in a wide range of polymers (e.g., polyethylene terephthalate) and in copolymers such as, for example, ethylene vinyl acetate. The classical chemical method for the determination of ester groups, namely, saponification, can be applied to some types of polymer. For example, copolymers of vinyl esters and esters of vinyl esters and esters of acrylic acid, can be saponified in a sealed tube with 2 M sodium hydroxide. The free acids from the vinyl esters were determined by potentiometric titration or gas chromatography. The alcohols formed by the hydrolysis of the acrylate esters were determined by gas chromatography. Vinyl acetate ethylene copolymers can be determined by saponification with 1 N ethanolic potassium hydroxide at 80 C for 3 hours and back titration with standard acid or by saponification with p-toluene sulfonic acid and back titration with standard acetic acid [49, 50]. 

Haslam " has discussed infrared methods for the determination of ester groups in acrylic copolymers. NMR spectroscopy has been used for the determination of isophthalate in poly(ethylene terephthalate isophthalate) dissolved in 5% trichloroacetic acid. The NMR spectra of these polymers were measured on a high resolution NMR spectrometer at 80 C. A singlet at 7.74 ppm is due to the four equivalent protons attached to the nucleus of the terephthalate unit. The complicated signals which appear to 8.21, 7.90, 7.80, 7.35, 7.22 and 7.10 ppm are due to die four protons attached to the nucleus of die isophthalate unit. The content of the isophthalate unit can be calculated from the integrated intensities of these peaks. 

Various other workers have studied the application of infrared spectroscopy to the determination of propylene groups in ethylene-propylene copolymers. This work is summarized in Table 4.1. 

METHOD 46 - DETERMINATION OF HYDROXYL GROUPS IN POLY(ETHYLENE GLYCOL). SILATION - SPECTROPHOTOMETRY. ... 

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