Chain termination by combination

Again, in contrast to radical polymerization, there is no chain termination by combination, since the growing chains (macroions) repel each other electrostatically because of their like charges. Chain termination occurs only by reaction of... 

The process of addition of monomer units to the growing chain can be interrupted in different ways. One is chain termination by combination or disproportionation of radicals. Explicitly, two growing-chain radicals can combine to form a carbon-carbon bond, or disproportionation can occur with a hydrogen atom being transferred from one chain to the other ... 

The hypothesis of multiple build-in where a chain can interact with a chain Cj leads one to reflect on the possibility of a chain termination by combination. If reactions were occurring in which termination could occur by simple desorption and also by combination, two peaks would be observed. The second maxima would have a center at approximately twice the value of the flrst, as doubling of the most prevalent adsorbed chain lengths is likely (25). Furthermore, secondary events such as those discussed above or chain transfer could cause the distributions of the two peaks to be different from one another. Thus the fact that secondary reactions during Fischer-Tropsch synthesis occur and that multiple build-in and termination by combination are viable propositions help rationalize distributions that do not follow the Schulz-Flory law and appear with more than a single maximum. 

Again, in contrast to radical polymerization, there is no chain termination by combination, since the growing chains (macroions) repel each other electrostatically because of their like charges. Chain termination occurs only by reaction of the growing chain ends with substances such as water, alcohols, acids, and amines. The ions produced by reaction of these substances can sometimes initiate new chains (chain transfer). Under certain conditions the ionic propagation species retain their ability to grow over extended periods of time, even after complete consumption of monomer ( living polymers , see Sect. 3.2.1). 

Initiator decomposition Chain initiation Chain propagation Chain termination By combination By disproportionation Chain transfer To monomer... 

Scheme 3.4 Free-radical chain termination by combination and disproportionation.

In treatments of polymerization reactions that concentrated on a single feature, the effect of molecular weight upon the termination rate constant has been deduced, the relative rates of initiation of two monomers in a copolymerization have been assessed, constants for chain transfer to monomer have been obtained in an emulsion copolymerization, the relative amounts of chain termination by combination and disproportionation have been discovered from a molecular weight distribution, and the rate constant for long-chain branch formation in the free-radical polymerization of ethylene has been found by fitting a probalistic model. ... 

As shown above, chains terminated by combination follow a different distribution than those terminated by disproportionation or chain transfer. If a material contains chains formed according to both distributions (as would happen, e.g., if a chain-transfer agent were added to a system that terminates inherently by combination), the distributions must... 

End group determination. Polymer chains terminated by combination possess two initiator-derived chain ends. Disproportionation affords chains with only one such end. The value of ktd/ktc can therefore be determined by evaluating the initiator-derived polymer end groups/molecule by applying eqn [53] ... 

As shown above, chains terminated by combination follow a different distribution than those terminated by disproportionation or chain transfer. If a material contains chains formed according to both distributions (as would happen, for example, if a chain-transfer agent were added to a system that terminates inherently by combination), the distributions must be summed according to the proportion (mole or mass fraction) of each, as detailed in Kenat et al. Let j/ = mole (number) fraction of the chains that have been terminated by combination. The weight fraction of chains terminated by combination is 2 / -h 1). In terms of kinetic parameters, is given by... 

Chain termination by combination. In the case of chain termination by combination, the chains will be longer and two Flory distributions (from the two combining chains) make the overall distribution sharper. It can be derived that the following holds ... 

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