End-capping procedure

Method (3).In determining [P ] during the THF poljnnerizatlon Initiated by superacid esters the phenoxyl end-capping procedure is shown In Scheme 1. The reaction of sodium phenoxide with the... 

The surface of C18 phases always contains unreacted silanol groups, which may form secondary polar interactions with the analyte. This is generally disadvantageous in RP-HPLC as it often causes peak broadening [33,40]. An important improvement is the introduction of the so-called end-capping procedure The residual silanol groups in the C18 phase are reacted with monofunctional chlorosi-lane, which decreases surface polarity. This very popular stationary phase is called C18ec, where the notation ec stands for end-capped. 

Current efforts are focused on improved control of molecular weight and on developing alternative end-capping procedures for use with our several classes of initiators. We are also investigating the properties of these unique fluoropolymers. 

The nanoparticles obtained in this surfactant-aided synthesis in the aqueous phase are stabilized by the surfactant. Usually, the removal of the surfactant leads to the formation of an insoluble precipitate due to the interparticle condensation. To avoid the occurrence of this process, the Si-OH groups at the particle surface are reacted with an end-cap-ping agent such as trimethylmethoxysilane (MCjSiOMe) or hexamethyldisilazane ((MCjSiljNH). After the end-capping procedure the aqueous dispersion is destabilized by the addition of methanol. The precipitate is filtered off and washed several times with methanol to remove the traces... 

The nanoparticles with a core-shell architecture are mainly prepared using the solidification in emulsion approach, as shown in Figure 4.5. The first step of the synthesis is the preparation of a surfactant-stabilized dispersion of uniform solid particles, as described in Section 4.2.1. These nanoparticles are treated as cores. Next, new precursors are introduced to the dispersion and solidification process is carried out. Finally, the nanopartides are isolated from the surfactant concomitantly with the end-capping procedure to deactivate silanol groups on the surface. As a result, the particles exhibiting heterogeneous structure are obtained. They consist of a solid core surrounded by a silicone shell. 

The procedure described here is not limited to the preparation of polymers such as 2. Starting from the difunctional silane 3 we have synthesized a copolymer, poly(dimethyl-co-isocyanopropylmethyl-siloxane) > as well as a linear homopolymer, poly(isocyanopropyl-methylsiloxane) 8 (Scheme 2). Indeed, preparation of a monofunctional analogue of 2. and h creates the potential for end-capping with an isocyanide function any polymer containing other functional groups, thereby in principle permitting mixed ligand complexes of polymers to be accessed. 

The major distribution, centred at approximately 2000 Da as expected, can be assigned as from di-hydroxyl end-capped oligomers of PPG (18). Addition of lithium salts in the MALDI sample preparation procedure leads to the generation of [18 + Li]+ ions as the major species. A series of low-intensity peaks below... 

The oxocarbenium perchlorate C(CH20CH2CH2C0+C104 )4 was employed as a tetrafunctional initiator for the synthesis of PTHF 4-arm stars [146]. The living ends were subsequently reacted either with sodium bromoacetate or bromoisobutyryl chloride. The end-capping reaction was not efficient in the first case (lower than 45%). Therefore, the second procedure was the method of choice for the synthesis of the bromoisobutyryl star-shaped macroinitiators. In the presence of CuCl/bpy the ATRP of styrene was initiated in bulk, leading to the formation of (PTHF-fc-PS)4 star-block copolymers. Further addition of MMA provided the (PTHF-fr-PS-fc-PMMA)4 star-block terpolymers. Relatively narrow molecular weight distributions were obtained with this synthetic procedure. 

Although the biocompatibility and biodegradability of these materials were rapidly determined, the bioactivity of Si02-PCL hybrid materials was not studied until recently [99]. In order to provide bioactivity to Si02-PCL hybrid materials, Rhee prepared triethoxysilane end-capped poly(s-caprolactone) which was then cocondensed with tetraethyl orthosilicate and calcium nitrate via the sol-gel method. The Ca-containing PCL/silica hybrid so obtained showed in vitro bioactivity and biodegradability. The hybridization procedure between the a,co-hydroxyl PCL and silica phases was proposed to be as follows ... 

Terpyridine moieties have been introduced as a terminal unit of macromolecules. In a subsequent procedure the two-step self-assembly process based on Rum/Run chemistry was used for polymers end-capped with the 2,2/ 6/,2 -terpyridine ligand. More precisely, the terpyridine-functionalized polymers were complexed with RUCI3 to selectively form a mono-complex. In a further step, this mono-complex was reacted under reducing conditions with other uncomplexed 2,2/ 6/,2/-terpyridine-terminated polymer blocks in order to form an asymmetrical AB ruthenium(II) frzs-complex. 

In previously employed end-capping schemes a palladium catalyst was used to directly connect a protected ethynyl group (2-methyl-3-butyn-2-ol) to a bulky intermediate. This procedure results in the entrapment of catalyst metals which must be laboriously removed from the final AT-product. If these metals are not removed, premature curing of the system occurs which narrows... 

The utility of a palladium catalyst in the synthesis of substituted aryl acetylenes is well established.(7,8,9,10) The end-capping agent I was produced by using a standard catalyst system, dichlorobls(triphenylphosphlne)palladlum (II)/copper (I) iodide/triphenylphosphlne mixture, which has been employed in previously developed ethynylation procedures.(10) The copper (I) iodide is believed to act as a cocatalyst, reducing the palladium (II) complex to the active palladium (0) catalyst. The scheme is shown in Figure 3 (diethylamine is the solvent).(11)... 

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