Chain initiation probability

Studies with isotopic labeling of intact cells and with glycosyl-transferases in vitro all indicate diat the polypeptide backbones of most glycoproteins are completely assembled before Ac first sugar is incorporated. A possible exception to diis mechanism is the incorporation in some tissues of N-acetylglucosamine into nascent polypeptides still attached to polyribosomes although ribosomal chain initiation probably occurs in liver (see Section IV,A,4,b), its occurrence in other organs is still controversial. 

As the result of theoretical consideration of polycondensation of an arbitrary mixture of such monomers it was proved [55,56] that the alternation of monomeric units along polymer molecules obey the Markovian statistics. If all initial monomers are symmetric, i.e. they resemble AaScrAa, units Sa(a=l,...,m) will correspond to the transient states of the Markov chain. The probability vap of transition from state Sa to is the ratio Q /v of two quantities Qa/9 and va which represent, respectively, the number of dyads (SaSp) and monads (Sa) per one monomeric unit. Clearly, Qa(S is merely a ratio of the concentration of chemical bonds of the u/i-ih type, formed as a result of the reaction between group Aa and Ap, to the overall concentration of monomeric units. The probability va0 of a transition from the transient state Sa to an absorbing state S0 equals l-pa where pa represents the conversion of groups Aa. 

It hag been shown that transition of a backbone carbon from the sp to sp state is promoted by tensile stresses and inhibited by compressive strains (10,44). The acceleration of the process of ozone oxidation of the polymers under load is not associated with the changes in supramolecular structure or segmental mobility of the chain. The probably reason of this effect is a decreasing of the activation energy for hydrogen abstraction (44). The mechanism of initial stages of the reaction of ozone with PP can be represented as ... 

Probabilities of configurations conducive to the intramolecular back-biting abstraction of a hydrogen atom are evaluated for growing unperturbed PVAc chains. A realistic RIS model is used for the chain statistics, Probabilities are found to be smaller than those seen in an earlier treatment of the polyethylene chain. The smaller probabilities of PVAc contribute to the virtual absence of short branches. The present study therefore provides support for the validity of the Roedel mechanism for the formation of short branches in the free radical initiated polymerization of ethylene. 

An analogous homopolymerisation can be initiated by strong bases, including for example tert-amines. In this case chain propagation probably proceeds through an oxyanion ... 

As mentioned in the preceding section, when vinyl or other alkenyl groups are present as substituents, photolysis leads to cross-linking, which dominates over chain scission. Probably the cross-linking results when silyl radicals formed in the initial photolysis add to vinyl carbon atoms on neighboring chains, but other mechanisms are also possible. 

The results of studies of this reaction in a hollow reactor show high selectivity up to 640 °C in the range of 4-EP volume rate from 0.065 to 0.78 h 1 and at 4-EP 20% aqueous H202 = 1 3 [94], Under optimal conditions at 620 °C, 4-VP yield equals 20.9% with 92% selectivity. Injection of quartz granules to the reactor raises the yield to 44.3% and selectivity to 96%. This is because the total surface on which, probably, the chain initiation reaction ... 

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

The effective diffusivity Dn decreases rapidly as carbon number increases. The readsorption rate constant kr n depends on the intrinsic chemistry of the catalytic site and on experimental conditions but not on chain size. The rest of the equation contains only structural catalyst properties pellet size (L), porosity (e), active site density (0), and pore radius (Rp). High values of the Damkohler number lead to transport-enhanced a-olefin readsorption and chain initiation. The structural parameters in the Damkohler number account for two phenomena that control the extent of an intrapellet secondary reaction the intrapellet residence time of a-olefins and the number of readsorption sites (0) that they encounter as they diffuse through a catalyst particle. For example, high site densities can compensate for low catalyst surface areas, small pellets, and large pores by increasing the probability of readsorption even at short residence times. This is the case, for example, for unsupported Ru, Co, and Fe powders. 

Termination. Two radicals react to form one or two molecules having no unpaired spins. If the radicals are assumed randomly distributed in space, the termination rate is second order in radical concentration. Under conditions where diffusion is restricted, however, the twin chains initiated by radicals from the same initiator molecule may remain close together for a time, so that termination has a higher probability than in the case of random distribution of radicals. This case has been discussed by Allen and Patrick (I, 3). It results in a rate expression formally equivalent to a reaction first order in radical concentration, with half-life t. Termination by initiator radicals is also taken into account here. 

In lithium alkyl-initiated polymerizations only chain initiation and propagation steps need be considered in hydrocarbon solvents. Both reactions are strongly influenced by extensive association of all lithium compounds. The reactive species in chain propagation is the small amount of dissociated material which probably exists as an ion pair. Association phenomena disappear on adding small amounts of polar additives, and the aggregates are replaced by solvated ion pairs. In polar solvents of relatively high dielectric constant (e.g. tetrahydrofuran), some dissociation of the ion pairs to free ions occurs, and both species contribute to the propagation step. The polymerizations are often complicated in tetrahydrofuran by two side reactions, namely carbanion isomerization and reaction with the solvent. 

Figure 3.5. Reversible and irreversible inhibition of a-adren-ergic receptors in the spleen, a Contractile tension developed by spleen slices in response to norepinephrine, in the presence of tolazoline and phenoxybenzamine. b Stmctures of norepinephrine, tolazoline, and phenoxybenzamine. c Reaction of phenoxybenzamine with the a-adrenergic receptor. The initial formation of the aziridine ring occurs in solution. The aziridine then reacts with a nucleophilic amino acid side chain (most probably a cysteine) in the binding site of the receptor.

Jaacks40) has suggested that, while most chains initiated must be ion pairs, zwitterions could make up most of the polymer. An alkoxide ion will react rapidly with a tertiary ammonium counter ion and consequently the molecular weigth of chains initiated by hydroxide ion is probably quite low. 

Chain termination probabilities initially decrease with increasing chain size (Fig. 2b) product distributions are non-Flory on all catalysts. This reflects an increase in readsorption rate as larger a-olefins become increasingly difficult to remove from liquid-filled catalyst pellets (4,5,14,40,41,44). Large olefins readsorb extensively and leave catalyst pellets predominantly after they form n-paraffins in sequential chain initiation and termination steps. As larger olefins (n > 30) disappear from the products, the chain termination probability reaches a constant value and product distributions become predominantly paraffinic and obey Flory kinetics (Fig. 2b). The asymptotic termination probability (/3=o) reflects the intrinsic probability of... 

Our cofeed studies were carried out at typical FT synthesis conditions and often in the added presence of water, an indigenous product of the FT reaction that also inhibits the rate of olefin hydrogenation and of other secondary reactions (4,30). In our studies, the addition of a-olefins to the H2/CO feed did not affect the rate of CO conversion also, at low concentrations (<5 mol%), added a-olefins did not affect the value of the chain growth probability. Thus, a-olefins act predominantly as chain initiators in our studies of FT synthesis on Co and Ru catalysts. 

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