Endcapping

Figure Bl.7.14. Schematic cross-sectional diagram of a quadnipole ion trap mass spectrometer. The distance between the two endcap electrodes is 2zq, while the radius of the ring electrode is (reproduced with pennission of Professor R March, Trent University, Peterborough, ON, Canada).

Acetal Resins. Acetal resins (qv) are poly (methylene oxide) or polyformaldehyde homopolymers and formaldehyde [50-00-0] copolymeri2ed with ahphatic oxides such as ethylene oxide (42). The homopolymer resin polyoxymethylene [9002-81-7] (POM) is produced by the anionic catalytic polymeri2ation of formaldehyde. For thermal stabiUty, the resin is endcapped with an acyl or alkyl function. 

Chain stoppers are unifunctional molecules of the form AX or BX, where the X moiety is nonreactive. They are used to stop the polymerization at a desired point or to stabilize the polymer chains by endcapping. 

An implication of the kinetic analysis presented in Sec. IV.A is that the rate of chain scission of polyesters can be retarded by endcapping to reduce the initial carboxylic acid end-group concentration. Alternatively, the rate may be increased by acidic additives that supplement the effect of the carboxy end groups. The first expectation was confirmed by partial ethanolysis of high molecular weight... 

Binders (TbC) 671 Bipolar pulse conductivity detector (LC) 588 Bonded phases (GC) 125 crosslinked 126 estersils 125 nonextractable 126 siloxane 125 Bonded phases (LC) 324 carbon loading 335 cleavage of ligands 336 eluotropic strength (LSC) 382 endcapping 326 hydrophobicity 364 metal impurities 369 models for surface 337 physical characteristics 333, 366... 

Flgvire 4.5 The influence of endcapping on peak shape and retention of soee PTH-anino acids using a reversed-phase separation system. Peak identification 1 PTH-histidine, 2 PTH-arginine and 3 PTH-valine. (Reproduced with permission from ref. 71. Copyright Preston Publications, Inc.)... 

Norwalk, CT) Waters pBondapak -NH2 Amine-bonded silica 10 p, 125 A 3.5% C not endcapped... 

Many, if not most, pharmacologically active compounds are amines. For this reason, issues in the RPLC of substances of pharmaceutical interest tend to be similar to those encountered in the separation of amines. Incompletely endcapped silica-based phases may exhibit tailing. The use of ion pairing or ion suppression is common in the analysis of pharmacologically active substances. Also, derivatization of the amine functionality prior to analysis may be required. The RPLC retention indices of most common pharmaceutical compounds have been compiled.97... 

Kirkland, J. J., Henderson, J. W., DeStefano, J. J., van Straten, M. A., and Claessens, H. A., Stability of silica-based, endcapped columns with pH 7 and 11 mobile phase for reversed-phase high-performance liquid chromatography, J. Chromatogr., A, 762(1 2), 97, 1997. 

Although the reaction scheme shows a complete hydrolysis before condensation begins, this is likely not correct as stated earlier. The relative rates and extents of these two reactions will particularly depend on the amount of water added and the acidity of the system (10,11). The high functionality of the triethoxysilane endcapped PTMO oligomer should enhance the incorporation of PTMO molecules into the TEOS network. It was also assumed that the reactivities would be the same between silanol groups from silicic acid and endcapped PTMO. Therefore, no preferential condensation was expected and the deciding factors for which type of condensation (self- or co-) took place would be the diffusivities and local concentrations. 

By endcapping PTMO with trifunctional silanes, the resulting PTMO molecules will have a functionality, f, of 6. This should reduce the gelation time considerably and potentially enhance the dispersion of the PTMO species. 

Unlike the silanol terminated PDMS, the endcapped PTMO will have to undergo hydrolysis before the condensation can take place. 

Table IV lists the mechanical stress-strain data for a series of hybrid TEOS-PTMO materials containing different levels of Ti-isop in the starting reaction mixture. These materials, with the exception of the first one, were all made using a modified reaction scheme (see experimental section) in order to incorporate the titanium into the network. The starting reaction mixtures in all cases contained 50% by weight of the glass precursors (TEOS and Ti-isop) and 50% by weight of PTMO(2000) (endcapped with triethoxysilane). One set of samples without titanium was made in order to compare the effects of the reaction scheme on the observed mechanical properties.

The authors wish to thank Dr. James G. Carlson for providing the endcapped PTMO oligomers. 

V) If the C-18 bonded phase contained un-reacted silanol groups, how would the retention of the solutes be affected by endcapping the stationary phase ... 

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