End groups, structures

The variations in the end group structure and conformation are determined by the carotenoid biosynthesis enzymes. These structural features are likely to determine localization as well as functions of these xanthophylls in vivo (Hashimoto et al., 2001 Young et al., 2002). 

Several modem analytical instruments are powerful tools for the characterisation of end groups. Molecular spectroscopic techniques are commonly employed for this purpose. Nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy and mass spectrometry (MS), often in combination, can be used to elucidate the end group structures for many polymer systems more traditional chemical methods, such as titration, are still in wide use, but employed more for specific applications, for example, determining acid end group levels. Nowadays, NMR spectroscopy is usually the first technique employed, providing the polymer system is soluble in organic solvents, as quantification of the levels of... 

The combination of a number of these techniques often provides complementary information on end group structure and hence the mechanism of polymerisation. The synergy between MALDI-TOF MS and NMR spectroscopy is particularly powerful, with MS/MS providing additional information where necessary. MALDI-TOF MS provides information on individual oligomers,... 

This section focuses on describing on how end group structures can be determined from one particular polymer that was generated by free radical polymerisation (see Figure 1), namely poly(methyl methacrylate) (PMMA, 1). 

Several techniques for determination of end group structure in polyesters have been described in the literature [25-31]. Many analytical procedures have... 

Figure 11 Reaction schemes for formation of PET oligomers with different end group structures.

Di-hydroxyl end-capped PPG (18) is an intermediate in the formation of a common polyurethane prepolymer (20). End group functionality of this intermediate is important, as this hydroxyl functionality is modified to form the prepolymer. Any different end group structures could lead to the presence of prepolymer that will not form polyurethane of the desired structure. The desired reaction of the intermediate (18) to form the prepolymer (20) is described in Figure 21. Reaction of one unit of the intermediate (18) and two units of methylene diphenyl 4,4 -diisocyanate (MDI) results in the formation (nominally) of the prepolymer (20). 

Fig. 10. Complex dynamic viscosity as function of temperature for three different aliphatic hyperbranched polyesters based on bismethylol propionic acid and having different end-group structure - (O) propionate end-groups, ( ) benzoate end-groups, ( ) hydroxyl end-groups [118]...

The use of hyperbranched polymers as the base for various coating resins has been described in the literature. Different resin types are obtained depending on the reactive end-group structure which is attached to the hyperbranched polymer. 

Mahapatro, A., Kalra, B Kumar, A. and Gross, R. A., Lipase-catalyzed polycondensations effect of substrates and solvent on chain formation, dispersity, and end-group structure. Biomacromolecules, 2003, 4, 544. 

A similar procedure was also used for the synthesis of methacrylate functional poly(a-MeS) [80]. Thus, 31 was used in conjunction with SnBr4 in CH2C12 at -78 °C, to obtain the macromonomer with Mns substantially (-50%) higher than the theoretical value. This was probably due to the formation of terminated low MW oligomers with indanyl end group structure. The eliminated proton was as-... 

The research team then followed up another end group which had been designed by the chemists at SK F, who had continued to explore other polar end group structures in place of cyanoguanidine. One moiety which had not been used previously in a drug was that of l,l-diamino-2-nitroethene. This was de-... 

Ashby, R. D., Shi, F.-Y., and Gross, R. A. 1997. Use of polyfethylene glycol) to control the end group structure and molecular weight of poly(3-hydroxybutyrate) formed by Alcaligenes latus DSM 1122. Tetrahedron Lett., 53,15209-15223. 

Because of the results to be presented to illustrate composition drift, we prefer to define the average weight fraction of styrene Ws(V) in equation 5. Determinations of Ks, Ka, and Kb were reported previously (J7). It was demonstrated that for the range of molar masses studied there was no dependence of refractometer proportionality constants on chain length or end group structure. 

Introduction of postpolymerization steps to obtain the copolymer with end group structure, thus significantly altering the properties of the resulting SBR grades. 

Having elucidated fhe end group structure for norbornene polymerized in the presence of ethylene, a catalytic cycle showing initiation, propagation, and chain transfer can be constructed (Fig. 4.14). 

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