Methyl end group

Methyl end groups resulting from main-chain scission in ethylene-propylene copolymers have observed by their characteristic 13C NMR resonance and determined quantitatively to give values of G(scission). 

Note 1 The preferred representations for the constitutional units are -CH(C6H5)-CH2- and -CH(4Cl-C6H4)-CH2-[2] use of the preferred representations would result in a-pentyl and co-[l-carboxy-l-(4-ehlorophenyl) methyl] end-groups. 

Free radicals are also produced by chain scission during deformation of polyethylene and FT-IR has been used to follow this process 237). The polyethylene samples were unaxially drawn and the resultant spectra corrected for orientation. An increase in the vinyl and methyl end groups created by decay of the free radicals occured in going from draw ratios of 5 to 20 44). A similar study involving deformation was made of polystyrene 246) and a comparison demonstrated between the results of thermal and mechanical degradation 24S. ... 

For the 10-fold drawn sample the A Hn remains at a very low level, about 0.4 G, over the temperature range examined. As discussed already, the narrow component for such a highly drawn sample will be contributed mainly by protons belonging to methyl end-groups or adjacent methylene groups insensible to the macroscopic drawing. Such components will be fairly mobile even at lower temperatures. The very small value of AHn for the highly drawn sample can thus be easily understood. 

The principal saturated hydrocarbon functional groups of concern are methyl, methylene and methyne (—CH3, —CH2—, = CH—). The spectra of typical hydrocarbon mixtures (for example as in gas oil and gasoline) are dominated by two pairs of strong bands in the first overtone and combination regions (5900-5500 cm-1 and 4350-4250 cm-1). These are predominantly methylene (—CH2—). The methyl end groups typically show up as a weaker higher-frequency shoulder to these methylene doublets. 

By methylation end-group assay. 1 Number-average degree of polymerization of methylated polysaccharide by id (isothermal distillation) or op (osmotic pressure) methods. c Viscosity-average degree of polymerization. 

The value of number-average molecular weights of methylated polysaccharides, used in conjunction with methylation end-group assays in the assessment of the degree of branching, has been stressed previously (see p. 435). Etherification is, however, normally carried out under alkaline con-... 

One approach to this problem is to start with the alkyl terminated surfaces and carry out chemical transformations of the methyl end group. Chidsey and co-workers employed this approach by forming sulfonyl chloride terminal groups via a photoinitiated free radical reaction of CI2 and SO2 with the original methyl-terminated monolayer [45]. These were then converted to sulfonamides by reaction with amines. Schematically this two-step reaction scheme can be written as ... 

Fig. 2a. MALDI-MS spectrum of linear PDMSs with methyl end groups...

Fig. 3 depicts a MALDI-MS spectrum (expanded plot) of linear siloxanes with methyl and hydroxyl end groups. It shows that linear siloxanes with methyl end groups may be differentiated from diols (the molar mass difference is 4 daltons) in the range 1.000-1.500 Da. Of course, there is no baseline separation between these specimen because of silicon and carbon isotope distribution in silicon-organic compounds. Smaller amounts of cyclosiloxanes, also present in the sample, are clearly separated from methyl end-capped components. 

For molecular properties of the TAG polymorphs, local molecular structural information such as methyl-end group, olefinic conformation, and chain-chain interaction are unveiled by infrared (IR) spectroscopy, especially Fourier-transformed infrared spectroscopy (FT-IR) (23, 24). Compared with a pioneering work by Chapman (25), great progress has been achieved by using various FT-IR techniques, such as polarized transmission FT-IR, reflection absorption spectroscopy (RAS), and attenuated total reflection (ATR) (26-28). 

The unit cell structure of C16C16C14 P2 is shown in Figure 11 (42). The unit cell is stacked in the quarto-chain-length structure, which is constructed by two double layers (1 and 11 in Figure 11) in such a way that the methyl end groups of one double layer are faced with those of another double layer at the center of the unit cell in the a-b plane. The chain axes of the two double layers in the unit cell are alternately... 

It cannot be excluded that the back biting depicted in Scheme 10 could be responsible for the observed absence of methyl proton.389 Scheme 10 would leave most of the dead polymer molecules without methyl end groups. The chemical shifts of the vinylic protons in the resulting oligo-acrylamides are at 5.75 and 6.25 ppm, which is similar to the methacrylate end group of polyBA which has been terminated by back biting. In oligo-acrylates produced by CCT, the vinylic protons have resonances at 6.8 and 5.85 ppm.383 The mechanism of Scheme 11 in acrylate polymerization requires additional study. 

Fig. 13. Experimental off-MAS spectrum of an aqueous dispersion of cetyl palmitate nanospheres (wc 100 MHz, full proton decoupling). The spinning frequency has been adjusted to w/27T= 1660 Hz at an angle of 13 = 52.5°. The signals A, B, and C can be assigned to the standard methylene group, the terminal methylene group and the methyl end group, respectively.

---