Seeding techniques

Soluble analogues of these difunctional initiators have been prepared either by addition of small amounts of weakly basic additives such as triethylamine (73) or anisole (74) which have relatively minor effects on diene microstmcture (37). Another method to solubilize these initiators is to use a seeding technique, whereby small amounts of diene monomer are added to form a hydrocarbon-soluble, oligomeric dilithium-initiating species (69,75). 

Another approach, the so-called seeding technique, provides preferential sites for the nucleation of scale, which permits the heat-transfer surfaces to remain clean of scale. Extensive studies of this technique have been conducted, and field use was reported ia the former USSR as early as the mid-1960s (42). The use of ion-exchange methods is another possible approach. Eor calcium, the exchange can be represented as... 

The seeding procedure is described in Fig. 20 by the line abed. Its inspection shows that the concentration of the monomer left at the time when all the initiator is consumed remains the same whether the monomer is added at once or in two portions. Moreover, the total concentration of the added monomer must exceed a critical value to allow for quantitative consumption of the initiator. Thus, the seeding technique does not eliminate the broadening of molecular weight distribution caused by slow initiation of a virtually irreversible polymerization. This conclusion is confirmed experimentally 133). 

Oligomers, oligomeric, oligomerization —, formation by seeding technique 130... 

Seeding technique, procedure 130, 131 Sequential addition of monomers 164, 167 Silicon-carbide fibers 8 Silicon-nitride fibers 8 Silicone rubber, crosslinked 4, 7-9, 31, 67 Siloxane, definition of 5 Siloxane-acrylate copolymers 27, 29, 56, 57, 64, 70, 71, 73, 74... 

Seeding technique. Al-free Ti-beta obtained by use of dealuminated zeolite-beta seeds Fluoride method. Al-free Ti-beta synthesis from a reaction mixture containing TEAOH and fluoride ions (HF) at near-neutral pH. Gel composition Ti02 60SiO2 32.9NEt4OH 32.9HF 20H2O 457.5 H20. Crystallization at 413 K with rotation of the autoclave (60 rpm)... 

An alternative means of conducting trials with a similar outcome to seeding, but without the need to handle crystals, is by dilution. An additional benefit of dilution methods is that they are more amenable to automation compared with seeding techniques (Chayen, 2005). The aim of dilution is to start the trial at nucleation conditions and after a given time... 

The seeding technique can also be used for controlling the size of the monodis-persed hematite particles. If we combine seeding technique with a shape controller such as sulfate or phosphate ions, the size and shape with different aspect ratios of... 

Fig. 1.3.12 TEM images of ellipsoidal hematite particles of different sizes prepared by the seeding technique at 100°C with different contents of seed suspension (S.S.) in the presence of a fixed concentration of sulfate ions at 3.0 X 10-2 mol dm-3. The aging times were fixed at 2 days for (a), (b), and (c) and 3 days for (d), (e), and (f). (From Ref. 15.)...

Analogously, the use of m-divinylbenzene [108-57-6] has been reported (76). However, oligomerization occurs on treatment of divinylbenzene with butyllithium resulting in initiators with functionalities greater than two (68). From a commercial perspective this oligomerization and lack of precise functionality control is not necessarily a problem and useful multifunctional initiators have been prepared from the reaction of butylithium with varying amounts of divinylbenzene (commercial divinylbenzene contains 22% meta, 11%para, and 66% o-, / -, and%ethylvinylbenzene) (77) often in the presence of styrene or diene monomer to provide solubility (seeding technique). 

Diene substrate latices were prepared by seeding techniques following the scheme shown in Figure 1. Persulfate anion was the initiator, and an alkyl aryl sulfonate was the emulsifier. Reaction time varied between 13 and 33 hours. Since all the substrates were prepared by similar techniques, the volumetric swell index and gel gravimetric data (toluene) varied little with particle size. Small differences in swell and gel characteristics should not be a variable in this study because of the polarity of the grafting monomer mixture and rapidity of the graft reactions. 

A similar seeding technique can be used to prepare monodispersed polymer latex dispersions by emulsion polymerisation (see page 17). 

FIGURE 10.10 A scanning electron microscopy (SEM) analysis of l>-oriented SIL-1 layers over Si wafers (prepared by secondary growth using dip-coating as seeding technique) as a function of synthesis time. 

Kumakiri I. Preparation and permeation mechanism of zeolite membranes. PhD Dissertation, The University of Tokyo, Japan, 2000. Pera-Titus M, Llorens J, Cunill F, MaUada R, and Santamaria J. Preparation of zeolite NaA membranes on the inner side of tubular supports by means of a controlled seeding technique. Catal Today, 2005, 104 281-287. 

Once chain initiation is complete, the monomer consumption rate is determined only by the chain propagation step. With the less efficient lithium alkyl initiators in hexane or cyclohexane, rather large amounts of monomer are needed to complete chain initiation. The appearance of a first order decay in monomer concentration, invariably obtained in these experiments, is not a very sensitive indication of the complete absence of initiator. Analysis of trial samples for hydrolysis products of lithium alkyls or spectroscopic determination that the polymer anion concentration has reached a plateau are preferable. A seeding technique is often used [32, 59] where the real initiator is a pre-formed active polymer... 

By using this technique, operations free of calcium sulfate scaling have been demonstrated in laboratory and field installations. This seeding technique is the key to closed loop operation in which liquid leaves the system only by evaporation or in combination with the solid by-product of the scrubbing system. Disposal of this solid is discussed later. 

Thaller, C., Weaver, L. H., Eichele, G., Wilson, E., Karlsson, R., and Jansonius, J. N. 1981. Repeated seeding technique for growing large single crystals of proteins. J. Mol. Biol. 147 465-469. 

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