Chain - direction initiation

However, as can also be seen in Fig. 11, primary and secondary amines do not perform very effectively as primers, compared to tertiary amines, even though they also contain long alkyl chains. It has been demonstrated that, instead of directly initiating ECA polymerization, primary and secondary amines first form aminocyanopropionate esters, 12, because proton transfer occurs after formation of the initial zwitterionic species, as shown in Eq. 7 [8,9]. 

It is therefore useful to distinguish between active and latent errors or failures. An active human error has an immediate effect in that it either directly causes a hazardous state of the system or is the direct initiator of a chain of events which rapidly leads to the imdesirable state. 

The crystalline lamellar thickness Dc obtained by StrobPs method is initially 1.4 nm and grows to about 2.0 nm, which is roughly equal to the crystallite size in the chain direction of 2.8 nm estimated from the wide-angle X-ray diffraction (WAXD) [7]. Interestingly, the persistence length /p = 1.45 nm just before crystallization measured by SANS (also see Fig. 11) [9,10] is almost equal to the crystal thickness. 

Fig. 15 Initiation of fibre fracture by a crack oriented parallel to the chain direction in a domain. It is proposed that a circular crack with a radius q releases the strain energy in a sphere around the crack with the same radius. Note that in this two-dimensional drawing only the circular crack is shown in perspective...

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

Generally speaking, the direct ozonation is important if the radical reactions are inhibited. That means that the water either does not contain compounds that initiate the chain reaction (initiators) or it contains many that terminate the chain reaction very quickly (scavengers). With increasing concentrations of scavengers the mechanism of oxidation tends to the direct pathway. Therefore, both inorganic carbon as well as the organic compounds play an important role. 

The mechanism of the aerobic oxidation of alcohols depends on the particular catalyst used. Two general mechanisms can be considered (1) the direct oxygenation of alcohols by 02 through a free-radical chain process initiated by the catalyst, and (2) the direct oxidation of the alcohol by the catalyst, which is then regenerated by 02. Both mechanisms are well illustrated [6] by the aerobic oxidations catalyzed by the persistent tetramethylpiperidine-N-oxyl (TEMPO) radical 1 and the nonpersis-tent phthalimide-N-oxyl (PINO) radical 2. 

Natta, Farina and Donati (123) found that optically active menthyl ethyl ether complexed with butyl lithium initiated asymmetric polymerization of optically inactive sorbic and /9-styryl acrylic esters. This strongly suggests that monomer orientation occurs during coordination with lithium by steric interaction with the asymmetric ligand. Similarly, asymmetric polymers were obtained using optically active 2-methyl-butyl lithium wherein the asymmetric polymer chain directed the monomer orientation. 

Under these reaction conditions, the use of ammonium and related onium salts with nucleophilic anions has been found effective at converting the HCl/SnCU-initiated, uncontrolled polymerizations into controlled/ living processes [105], Similar results are reported for TiCl4-based polymerizations [174,175], Effective salts include tetraalkylammonium and phosphonium salts R4N+Y and R4P+Y (Y = I, Br, Cl, CH3COO R = CH3, C2Hs, 71-C4H9, etc.). As added nucleophiles do in nonpolar solvents, the added salts retard the polymerization, narrow the MWD of the polymers, and render their M values directly proportional to conversion and close to the calculated values (one living chain per initiator molecule). 

A further observation was that the optical absorption by the N2O increased somewhat during the induction period. At the same time additional measurements of the IR emission at 4.75 pm showed a growth in the radiation from the oscillators CO, N2 0(1 3) and 02(1 3) before appreciable decomposition of the N2O had occurred. Both these effects can be associated with vibrational disequilibrium in the system (the absorption coefficient of the N2O being directly related to its vibrational temperature), and Zaslonko et al. use this evidence to support the view that the overall CO + N2O reaction occurs even at lower temperatures via a chain process initiated by the unimolecular dissociation of NjO- The overall CO + N2O reaction is strongly exothermic (AH — —87... 

Xanthate 446 undergoes cyclization in the presence of camphorsulfonic acid via a radical chain reaction initiated by a small amount of lauroyl peroxide to give pyrroloimidazoles 449 in 56% yield. The use of an acid and anhydrous medium inhibits nucleophilic attack of the basic heterocycles at the xanthate moiety and allows radical reactions to occur. Fused heteroaromatic compounds can also be prepared directly from benzimidazole carrying an N-alkenyl substituent and xanthates by a tandem radical addition/cyclization to provide, for example, pyrrolobenzimidazole 453 in 57% yield (Scheme 106) <2002OL4345>. 

This has two consequences (1) most importantly, direct initiation of radicals in lipids bound to the heme, and (2) assurance of lipid release as LOO rather than LOOH. Chain propagation may proceed through LOO directly or through epoxyallylic peroxyl radicals from LOO cyclization. 

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