Coupling, termination

The Heck-Cassar-Sonogashira-Hagihara system, which couples terminal alkynes and vinylic or aromatic halides in an amine solvent, typically piperi-... 

The distribution of the coupling-terminated molecules will be represented by the sharper dichain distribution discussed above, its degree of polymerization being given by... 

Termination is a head-to-head configuration at the juncture of the two macroradicals. The extent of coupling termination can then be obtained by determining the extent of head-to-head configuration in the product. The kinetic equation for coupling termination is shown in the following equation ... 

Using the methods described, the values of Cm and Ci in the benzoyl peroxide polymerization of styrene have been found to be 0.00006 and 0.055 respectively [Mayo et al., 1951]. The amount of chain transfer to monomer that occurs is negligible in this polymerization. The chain-transfer constant for benzoyl peroxide is appreciable, and chain transfer with initiator becomes increasingly important as the initiator concentration increases. These effects are shown in Fig. 3-7, where the contributions of the various sources of chain ends are indicated. The topmost plot shows the total number of polymer molecules per 105 styrene monomer units. The difference between successive plots gives the number of polymer molecules terminated by normal coupling termination, transfer to benzoyl peroxide, and transfer to styrene. 

The preparation of trisaccharide 63 illustrates the activation and enzymic coupling of the 9-acetate of vV-acetylneuraminic acid, This involves the utilization of enzymes in a cascade of reactions which probably do not occur in cells (a) synthesis of Neu5,9Ac2 from the 6-acetate of vV-acetylmannosa-mine with the catabolic sialyl aldolase, (b) activation with CMPNeu5Ac synthetase, and (c) coupling. Acetylation in cells seems posterior to coupling. Terminal nonreducing vV-acetyl-9-O-acetylneuraminic acid residues appear... 

To determine the proportions of unimolecular (chain transfer) and bimolecular (coupling) termination, let y fraction of molecules be terminated by the former mechanism. Therefore,... 

Alkynylarenes. The catalyst system, usually in combination with a mild base (trialkylamine or KjCOj), is used to couple terminal alkynes with aryl iodides and triflates. A large rate enhancement is observed when an iodide salt is added, as shown in a synthesis of 1,2-bis(trimethylsilylethynyl)benzene. ... 

This reaction has wide application in coupling terminal alkynes, especially in the formation of cyclic alkynes. 

For a monodisperse polymer sample, d = 1. The ranges of d values change drastically with the different mechanisms of polymerization. The values of d are 1.01-1.05 in living polymerization (anionic, cationic, living free radical, etc.), around 1.5 in condensation polymerization or coupling termination of polymerization, around 2 in disproportionation reactions on polymerization, 2-5 for high-conversion olefins, 5-10 in self-acceleration on common free radical polymerization, 8-30 in coordination polymerization, and 20-50 in branching reactions on polymerization. 

Coupling of any two radicals depletes the supply of reactive intermediates and terminates the chain. Several pairings are possible for radical coupling termination steps (see the text). 

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