Mixed termination

A handful of bis(hydroxy)-bridged dimeric compounds have been reported in which aluminium centres bond to two different terminal organic residues. While complexes with mixed terminal groups - that is Al2(/r-OH)2R2R 2 [64,... 

This is the only situation in which "mixed" termination (i.e., coupling of two different chain carriers) can dominate termination. 

Rice and Herzfeld [27], at a time when still little was known about free radicals and chain reactions, had tried to account for the observed first-order behavior by postulating a "mixed" termination C2H5- + H- — C2H6. However, since C2H5-outnumbers H by several orders of magnitude under typical reaction conditions, this assumption proved untenable [30]. Thereupon Kuchler and Theile [35] suggested that initiation is bimolecular in ethane provided termination occurs without a... 

Rates of thermal cracking are first-order in good approximation for propane, butane and still higher hydrocarbons [21], This is remarkable because chain mechanisms with initiation by break-up of a reactant normally result in reaction orders of one half or one-and-a-half, depending on which radical is consumed by termination. First-order behavior can result from "mixed" termination, which, however, can in most cases be ruled out as dominant mechanism (see Section 9.3). A more probable explanation is a combination of effects that key hydrocarbon radicals participate in several steps of different molecularities, that some steps are reversible, and that some unimolecular ones require collision partners. 

The kinetic parameters were determined under various reaction conditions (light intensity, atmosphere). The statistical analyses showed that the experimental data in all cases were best described by the mixed termination model. The results obtained from this model indicated also that during the polymerization of sulfur-containing dimethacrylates, radical trapping occurred in a much lesser degree in comparison to the oxygen-containing counterpart and that the contribution of bimolecular reaction to the overall termination... 

Figure 4.6. Effect of double-bond conversion on the monomolecular termination rate coefficient calculated from the monomolecular termination model (1) and mixed termination model (III). Monomer, diethylene glycol dimethacrylate. Conditions Ar, 40°C initiator, Irgacure 651,0.06 M. Data taken from Ref. [30].

When k,d >ktc, = 2, and when ktcprevious result, but Equation 9.27a can also handle various degrees of mixed termination. 

The couplings of 1,6- and 1,7-intemal diynes or oxygen-linkage mixed terminal/internal diyne with isocyanides provide useful entries for constructing bicydic cydopentenone skeletons [152]. The cyclizations of symmetric or unsymmetric diynes bearing amine moiety at the tethered chain proceeded with modest to good regioselectivity in acceptable isolated yield [151,153]. 

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