Proton traps

Unless working with superdried systems or in the presence of proton traps, adventitious water is always present as a proton source. Polymeriza tion rates, monomer conversions, and to some extent polymer molecular weights are dependent on the amount of protic impurities therefore, weU-estabHshed drying methods should be followed to obtain reproducible results. The importance is not the elimination of the last trace of adventitious water, a heroic task, but to estabhsh a more or less constant level of dryness. 

A series of graft polymers on polychloroprene were made with isobutjiene, /-butyl vinyl ether, and a-methylstyrene by cationic polymerization in solution. The efficiency of the grafting reaction was improved by use of a proton trap, eg, 2,6-di-/-butylpyridine (68). 

Illustrated in Scheme 7.8 are the mechanisms that give rise to the products shown in Scheme 7.7. These mechanisms involve either electrophilic attack or an internal redox reaction. The internal redox reaction shown in Scheme 7.8 involves proton trapping from the solvent or from the hydroquinone hydroxyl group as shown. This process has been documented for the mitomycin system50 and also occurs in many quinone methide systems.25,30,31... 

Figure 5.1.7 shows the propagator of the motion measured for a clean and a biofilm impacted capillary [14,15] and the residence time distributions calculated for each from these velocity distributions. The clean capillary gives an experimental propagator equal to the theoretical velocity distribution convolved with a Gaussian diffusion curve [14], as shown in Figure 5.1.2. For the flow around the biofilm structure note the appearance of a high velocity tail indicating higher probability of large displacements relative to the clean capillary. The slow flow peak near zero displacement results from the protons trapped within the EPS gel matrix where the...

Hexaepoxy squalene, HES (Scheme 70) was used as a multifunctional initiator in the presence of TiCU as a coinitiator, di-f-butylpyridine as a proton trap, and N,N-dimethylacetamide as an electron pair donor in methylcy-clohexane/methyl chloride solvent mixtures at - 80 °C for the synthesis of (PIB-fc-PS)n star-block copolymers [145]. IB was polymerized first followed by the addition of styrene. The efficiency and the functionality of the initiator were greatly influenced by both the HES/IB ratio and the concentration ofTiCL, thus indicating that all epoxy initiation sites were not equivalent for polymerization. Depending on the reaction conditions stars with 3 to 10 arms were synthesized. The molecular weight distribution of the initial PIB stars was fairly narrow (Mw/Mn < 1.2), but it was sufficiently increased after the polymerization of styrene (1.32 < Mw/Mn < 1.88). 

In this section, it should be mentioned that star poly(tetrahydrofuran) has been prepared by coupling cationically polymerized THF with multifunctional diethylenetriamine [92] in the presence of 2,2 6,6 tetramethylpiperidine as a proton trap. When the MW of poly(THF) is 1600 seven chains are added to the triamine, when the MW is 8000 a five-arm star has been obtained. 

Majerska K, Duda A, Penczek S (2000) Kinetics and mechanism of cyclic esters polymerization initiated with tin(II) octoate, 4. Influence of proton trapping agents on the kinetics of E-caprolactone and L,L-dilatce polymerization. Macromol Rapid Commun 21 1327-1332... 

Introducing the Tau residue into a peptide according to the first approach demands protection of the amino group, usually in the form of a Z-derivative and turning the sulfonic acid into sulfonyl chloride. Synthesis of (j-su Ifonamidopeptides via an iterative process, both in solution and in the solid phase, has been described.11201 Chiral methylene sulfinamide peptides can be synthesized both in solution and in the solid phase using the sulfonyl chlorides derived from enantiomerically pure 2-substituted taurines under mild coupling conditions (DMAP catalysis and excess methyl trimethylsilyl dimethylketene acetal as a proton trap).11261... 

This direct oxidation generates itself a proton and is probably only a minor pathway under superacidic condition where, in the absence of the proton trap, protolysis of the C—H and C—C bond occurs very rapidly. The mechanism is most probably of electron transfer nature as suggested in Eq. (5.22) and (5.23)... 

The hexacumyl methyl ether functional initiator 19 was synthesized by Cloutet et al. [36] and used for the living cationic polymerization of IB in conjunction with TiCl4 in CH2Cl2/methylcyclohexane (40/60 v/v) at -80 °C in the presence of a proton trap. The star sample obtained exhibited Mn=13,000 gmol-1 and Mw/Mn=1.27. 

Dihaloketones. A number of studies of the electroreduction of a,a -dibromoketones have been reportedly directed to obtaining evidence for the the intermediacy of cyclo-propanone-derived intermediates (186). Reduction of 2,4-dibromo-2,4-dimethyl-3-pentanone (185) consumed two Faradays of current and gave a species which is briefly stable at -32 °C. The intermediate (186) could be intercepted by subsequent addition of a protonic trapping agent to the low-temperature catholyte (equation 100). Results have... 

Rich (1995) has proposed that the proton pump in cytochrome c oxidase is driven mainly by electrostatic interactions (or repulsion) between protons in a proton trap and protons transferred from the matrix side to the O2 reduction site for neutralizing oxides (O ) are produced by O2 reduction. In this mechanism, a structural change for gating proton transfer from the matrix side to the proton trap is required for a complete cycle of redox-coupled proton pumping. However, no such structural change has been detected. 

In addition to protonic acids, Lewis acids are the most common initiators of carbocationic polymerizations. Two mechanisms are possible. Direct initiation is rare and usually slow. The more prevalent mechanism is by cocatalysis in binary systems, with the Lewis acid acting as a coinitiator or catalyst rather than as initiator. Cationating or protonating species are the true initiators, which are therefore the species incorporated at the polymer s end group. The most common initiator is adventitious water in insufficiently dried systems. Thus, mechanistic studies should be performed under stringently dry conditions or in the presence of proton traps such as hindered pyridines. In addition to water, the protonating reagent may be an alcohol, carboxylic acid, amine, or amide [Eq. (28)]. 

More recently, Kennedy reported another initiating system that controls styrene polymerization with an added nucleophile 2,2,4-trimeth-ylpentyl chloride (TMP-Cl)/TiCl4 with N,N-dimethylacetamide (DMA) in CH3Cl/methylcyclohexane (4 6 v/v) mixture at -80° C [165]. The use of another additive, 2,6-di-ferf-butylpyr idine (proton trap), is described as beneficial. The molecular weight and MWD are controlled in this system, but the role of the added DMA is still ambiguous [166]. This system with the aliphatic (erf-chloride was designed to extend to the synthesis of isobutene-styrene block copolymers via sequential cationic polymerization (Chapter 5). 

Indeed, at least in one case (polymerization of 1,3,5-trioxane), it has been shown that BF3 does not initiate the polymerization of rigorously purified monomer, although polymerization proceeds in less thoroughly dried systems [38], Also in the presence of proton trap, BF3 does not initiate the polymerization of 1,3,5-trioxane [39]. 

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