Recombination of primary

The following conditions are stipulated the catalyst decomposition rate constant must be one hour or greater the residence time of the continuous reactor must be sufficient to decompose the catalyst to at least 50% of the feed level the catalyst concentration must be greater than or equal to 0.002 x Q, where the residence time, is expressed in hours. An upper limit on the rate of radical formation was also noted that is, when the rate of radical formation is greater than the addition rate of the primary radicals to the monomers, initiation efficiency is reduced by the recombination of primary radicals. 

Tunnelling recombination of primary F, H pairs can result either in closely spaced v+,i pairs (the so-called a, I centres) which annihilate immediately due to Coulomb interaction and a consequently large instability radius. However some i ions occur in crowdion configurations, and leave vacancy moving away up to 4-5 ao even at 4 K [31]. The distinctive feature of tunnelling recombination is its temperature independence, which makes it one of the major low-temperature secondary processes in insulating solids with defects. 

In order to inhibit degradation by the radical scavenging, two approaches were studied one is to let an excipient to play the role ofa radical scavenger, so protecting the drug from the radical attack, the second is to freeze the solution into a solid state where the cage effect favours the recombination of primary radicals ofthe solvent prior to the reaction with the drug. 

Now, this recombination of primary peroxy radicals is, according to our model, considered to be the immediate source for peracid radicals. This idea is-see Scheme II-that primary peroxy radicals recombine first to a tetroxide cage. Under steady state conditions, this tetroxide is supposed to decompose at the rate of its formation. One part of this decomposition, a, leads to peroxides. A second part, p, is, possibly by intermediate formation of aldehyde, further oxidized to peracid radicals and H00-. ... 

They seem to interfere, by slow reactions, with propagating macroperoxy radicals secondly, they might be capable of reducing the number of initiated chain reactions by fast inhibition of peracid radicals produced by non-terminating recombination of primary macroperoxy radicals within initiated radical pairs. 

Complex mechanism of class (ii) and (iii) reactions may account for the puzzling result in the studies on radiation-induced fluorescence in cis- and trans-decalins containing 3-100 mM of benzene [54], where it was concluded that on the time scale of geminate recombination of primary pairs in trans-decalin (< 1 ns), the hole is scavenged by benzene with rate constant of 7.7x1010 g-l (vs. (5-5.5)xl09 M l s-1 observed in the transient conductivity experiments [7,8,12,14]). This was taken as evidence for the involvement of short-lived, reactive excited solvent holes. 

The increase in viscosity associated with the formation of either an amorphous solid at the glass transition temperature or a crystalline lattice at the phase transition temperature will significantly reduce the mobility of most radicals. An increase in viscosity in rubbery liquids, due to decreases in temperature down towards the glass transition, will also reduce mobility. Consequently, recombination of primary radicals at the site of formation would be favored, lowering their yield. Other reaction pathways not involving diffusion and migration might also be favored. These effects are discernible and sometimes can be pronounced. 

The dependence of quantum yield on light intensity can be characterized by two limiting cases [64]. At high intensity [65] the quantum yield increases linearly with the square root of the reciprocal intensity. This dependence is typical for homogeneous and heterogeneous photoreactions where recombination of primary products predominates, It has been observed for the Ti02 catalyzed photo-... 

Not all activated species R succeed in Initiating polymer chains recombination of primary radicals in the "cage" decreases the efficiency f to a value lower than 1. It has also been found that an Increase of viscosity lowers the initiation efficiency. 

A recent study of iodine atom recombination in solution by Luther et al. [294] used a dye laser (wavelength 590nm, pulse duration 1.5ps) to photodissociate iodine molecules in n-heptane, -octane or methyl cyclohexane at pressures from 0.1 to 300 MPa. Over this pressure range, the viscosity increases four-fold. The rate of free-radical recombination was monitored and the second-order rate coefficient was found to be linearly dependent on inverse viscosity. This provides good reason to believe that the recombination of free iodine atoms is diffusion-limited, especially as the rate coefficient is typically 10 °dm mol s . The recombination of primary and secondary pairs is too rapid to be monitored by such equipment as was used by Luther etal. [294] (see below). Instead, the depletion of molecular iodine absorption just after the laser pulse was used to estimate the yield of (free) photodissociated iodine atoms in solution. They found that the photodissociation quantum yields (survival probability) were about 2.3 times smaller than had been measured by Noyes and co-workers [291, 292] and also by Strong and Willard [295]. This observation raises doubts as to the accuracy of the iodine atom scavenging method used by Noyes et al. or perhaps points to the inherent difficulties of doing steady-state measurements. In addition, Luther et al. 

Note 2 The recombination of primary radicals and their reactions with other species may lead to reduced initiator efficiency. 

---