Effectiveness initialization

Bromide ion acts as an inliibitor through step (9) which competes for HBr02 with the rate detennining step for the autocatalytic process described previously, step (4) and step (5). Step (8) and Step (9) constitute a pseudo-first-order removal of Br with HBr02 maintained in a low steady-state concentration. Only once [Br ] < [Br ] = /fo[Br07]//r2 does step (3) become effective, initiating the autocatalytic growth and oxidation. 

Wheieas the BPO—DMA ledox system works well for curing of unsaturated polyester blends, it is not a very effective system for initiating vinyl monomer polymerizations, and therefore it generally is not used in such appHcations (34). However, combinations of amines (eg, DMA) and acyl sulfonyl peroxides (eg, ACSP) are very effective initiator systems at 0°C for high conversion suspension polymerizations of vinyl chloride (35). BPO has also been used in combination with ferrous ammonium sulfate to initiate emulsion polymerizations of vinyl monomers via a redox reaction (36). 

The methodology for preparation of hydrocarbon-soluble, dilithium initiators is generally based on the reaction of an aromatic divinyl precursor with two moles of butyUithium. Unfortunately, because of the tendency of organ olithium chain ends in hydrocarbon solution to associate and form electron-deficient dimeric, tetrameric, or hexameric aggregates (see Table 2) (33,38,44,67), attempts to prepare dilithium initiators in hydrocarbon media have generally resulted in the formation of insoluble, three-dimensionally associated species (34,66,68—72). These precipitates are not effective initiators because of their heterogeneous initiation reactions with monomers which tend to result in broader molecular weight distributions > 1.1)... 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

For achieving improved effective initiation for each step of block copolymerization, type I MAI having dual... 

In contrast, aqueous ethanolic ammonia effects initial nucleophilic displacement of bromide, which is followed by ring closure of the benzylamine so-formed at the aldehyde group to give 5//-dibenz[f, f]azepines 48.85 The (1-bromoethyl) aldehyde 47 undergoes ring closure with benzylamine to give 6-benzyl-5-methyl-5//-dibenz[c t>]azepinium bromide (49, R = Me), whereas the (bromomethyl) aldehyde 45 yields a mixture of the quaternary salt 50 and the aldimine 51, which cyclizes in situ to the aziridinophenanthrene 52 in low yield (9%). 

The first reported initiators were polyhalogeno-compounds (e.g. CC14, CHC13, CCl3CH2OH, CCl3Br). Trichloromcthanc derivatives and tctrachloromethanc appear effective initiators. Mono- and dichloromethane derivatives are inefficient initiators. Tetrachloromethane may act as a difunctional initiator. 

Dehydration reactions. In early studies of dehydration reactions (e.g. of CuS04 5 H20 [400]), the surfaces of large crystals of reactant were activated through the incorporation of product into surfaces by abrasion with dehydrated material. An advantage of this pretreatment was the elimination of the problems of kinetic analysis of the then little understood relationship between a and time during the acceleratory process. Such surface modification resulted in the effective initiation of reaction at all boundary surfaces and rate studies were exclusively directed towards measurement of the rate of interface advance into the bulk. 

The simple two-reactor series shown in Figure 1 will be analyzed to demonstrate the effect of inhibitor on the performance of continuous systems. Since inhibitor will be present in the continuously added feed stream, it will serve to reduce the effective initiation rate in the first reactor. Since inhibitor is very reactive with free radicals, all inhibitor fed must be destroyed before significant reaction can take place. Thus the effective rate of initiation in the first reactor is given by Equation 1. 

This is consistent with the flow-aided diffusion condition postulated for this region interpreted as an increase in the effective initiator concentration as (NRg)g increases. The increased initiator concentration causes the reduction in molecular weight (22). Values of (NRg)e greater than 4000 show an increasing weight as (NRe)e increases. This cannot be explained by the decrease in effective soap concentration because this would cause a further... 

This, in turn, reduced the number of polymer particles (the loci of reaction) and hence the reaction rate fell. However, this explanation is at variance with the results reported in Figure 12 where the molecular weight (weight-average) clearly increases with increasing Reynolds number. It seems more likely that the turbulent flow results could be explained by a decrease in the effective initiator concentration. This low concentration would also explain why there is no further reaction after a period of about one hour as contrasted with the batch reactions where the reaction is still proceeding after two to three hours. The current absence of corroborating evidence makes this explanation very tentative. 

Indeed, cumyl carbocations are known to be effective initiators of IB polymerization, while the p-substituted benzyl cation is expected to react effectively with IB (p-methylstyrene and IB form a nearly ideal copolymerization system ). Severe disparity between the reactivities of the vinyl and cumyl ether groups of the inimer would result in either linear polymers or branched polymers with much lower MW than predicted for an in/mcr-mediated living polymerization. Styrene was subsequently blocked from the tert-chloride chain ends of high-MW DIB, activated by excess TiCU (Scheme 7.2). 

If coordination were not important, the polymerization process would presumably resemble those initiated by alkali metal alkyls. The latter are very effective initiators for the polymerization of such monomers as styrene, butadiene, etc., and it is generally considered that propagation proceeds through intermediates of the type (43)... 

The second approach consists of synthesizing first the complex MLra 1(L X) with the desired ratio (L )/(M) this complex bears the reactive fragment X which then reacts with the surface of the silica. This method is of limited interest, because the synthesis and isolation of these functionalized complexes is not straightforward. One of the successful examples concerns the synthesis of nickel carbonyl complexes anchored to the surface via two bonds in an attempt to increase the stability through a sort of chelate effect. Initial attempts to achieve this by the methods described in Equation(5) (initial functionalization of silica) and Equation(6) (initial functionalization of complex) failed, as demonstrated by 29Si and 31P CP MAS NMR spectroscopies.51... 

Peroxidases are effective initiators of lipid peroxidation and LDL oxidation. These processes are considered in detail in Chapter 25. 

Another general procedure for effecting initial subdivision of the crude mixture of polysaccharides obtained on alkaline extraction is that of Schulze68 and O Dwyer62 which takes advantage of the fact that neutralization of the solution causes precipitation of the high molecular weight... 

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