Tennination reactions, chain polymerization

The polymerization can tenninate when the free-radical terminal C of a long chain forms a bond with the terminal C of another long chain (combination), RC- -t- CR — RC—CR. Termination may also occur when the terminal free-radical C s of two long chains disproportionate, in a sort of auto-redox reaction. One C picks off an H from the C of the other chain, to give an alkane at one chain end and an alkene group at the other chain end ... 

It is tempting to attribute problems in reconciling data from model studies and actual polymerizations to difficulties associated with data interpretation. The polymerization experiments are often complicated by other tennination pathways, in particular chain transfer, which must be allowed for when assessing the results. It is notable in this context that the discrepancies are most evident for reactions carried out at higher temperatures (Sections 5.2.2.1.1 and 5.2.2.1.2). 

Various side reactions that are likely to lead to a slow loss of "living" ends have been described. With disulfide initiators, one (initiation by the dithiocarbamyl radical) is unavoidable since tlie experiment relies on the same radical species to both initiate polymerization and tenninate chains. 

The basic mechanism of radical polymerization was enunciated in the 1950 s and 1960 s, and comprises tlie initiation, propagation and tennination steps of a chain reaction. ... 

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