Termination, of chain growth

Let us for the moment disregard chain transfer reactions. Radical polymerization then consists of three component reactions initiation, propagation of the polymer chains, and termination of chain growth. The rate of primary radical formation, v, by decomposition of the initiator I, may be written ... 

The termination of chain growth can also occur both in the gas phase and at the polymer surface. In the gas phase, free radicals are lost by reaction with both hydrogen atoms and other free radicals. The kinetics of these processes are given by... 

Stabilization and Termination of Chain Growth by Ring Formation... 

Because the purity of the monomers used affects the molecular weight of the polyester 39), we can assume that proton donors whether added to the monomer mixture or formed during copolymerization, participate in termination or transfer reactions. Termination of chain growth may occur by reaction of the growing chain end with proton donors according to Eqs. (77) and (78). 

Termination reactions are harder to define in cationic processes because they are easy to confuse with chain transfer. Termination of chain growth in cationic polymerization may take place in various ways. Many of the reactions that terminate the growth of a propagating chain do not, however, terminate the kinetic chain because a new propagating species is generated in the process. 

Equations (6.105) and (6.106) for apply to free-radical polymerization following ideal kinetics in which termination of chain growth occurs only by mutual reaction (coupling and disproportionation) of chain radicals. Combining Eqs. (6.100) and (6.105) one may write... 

The Oxoadd Chain Elongation Process Decarboxylation as a Driving Force in Biosynthesis Stabilization and Termination of Chain Growth by Ring Formation... 

These ddNTPs can be incorporated in a growing DNA chain, but they lack a 3 -hydroxyl group to form a bond to another nucleoside triphosphate. The incorporation of a ddNTP into the growing chain causes termination at that point. The presence of small amounts of ddNTPs in a replicating mixture causes random termination of chain growth. 

The central mechanism of chain formation involves the generation of free radicals (reaction (i)), the initiation (reaction (ii)), propagation (reaction (iii)), and termination (reactions (iv) and (v)) (Scheme 4.2). The radical generation and the first monomer addition to an initiating radical constitute the initiation step, whereas the successive monomer additions over a new free radical and the termination of chain growth by disproportionation and/or coupling actually constitute the formation of chains as represented in Scheme 4.2. 

Terminations of chain growths occur through recombinations, reactions with impurities, and by chain transfers ... 

Compared with chemical oxidation polymerization, electrochemical polymerization is performed at an electrode (conductive substrate) using the positive potential [97-104]. Whereas the powder forms are obtained by chemical polymerization, the electrochemical method leads to films deposited on the anode. When a positive potential is apphed at the electrode, pyrrole monomer such as a heterocychc compound is oxidized to form a delocalized radical cation, which includes the possible resonance forms. Radical-radical coupling reaction produces the dimerization of the monomer radicals at the a-position. Removal of 2H+ ions consequently forms the neutral dimer. Next step is chain propagation which includes the oxidation of the neutral dimer to form the dimer radical. The resultant radical can react with other monomer or dimer and this radical coupling and the electrochemical oxidation processes repeat in order to extend the polymer chain. The final step involves the termination of chain growth and the resultant PPy film is formed on the anodic electrode. 

The transition states for the steps of propagation are formed repeatedly in liquid medium systems, containing monomer, initiator, the formed polymer, and frequently a solvent. There are many different types of initiating reactions. These polymerization, however, never terminate by combination or by disproportionation as they do in free-radical chain-growth polymerizations. Instead, terminations of chain growths are results of unimolecular reactions, or transfers to other molecules, like monomers or solvents, or impurities, like moisture. They can also result from quenching by deliberate additions of reactive terminating species. 

The final step is termination of chain growth mostly by radical transfer reaction to monomer [306], whereas combination or disproportionation are observed only to a small extent. The monomer radical is able to start a new chain. The most widely used procedures for preparation of commercially PVC resins are, in order of their importance, suspension, emulsion, bulk, and solution polymerization. A common feature of the first three methods is that PVC precipitates in liquid VC at conversions below 1%. The free polymerization of VC in a precipitating medium exhibits an accelerating rate from the beginning of reaction up to high conversion [307]. This behavior is called autoacceleration and is typical for heterogeneous polymerization of halogenated vinyls and acrylonitrile [308]. 

The Mayo-Lewis equation is general for all chain reaction polymerizations regardless of whether the mechanism involves anionic, cationic, radical, or organometallic reactive chain species furthermore, it is independent of the mode of termination of chain growth. 

In the Sanger method, the piece of DNA to be seqnenced, the template strand, is subjected to an enzyme, DNA polymerase, that replicates it many times (as the complementary strand. Section 26-9), starting from one end (the 3 end). Replication requires addition of the four ingredient nucleotides (abbreviated as N below) A, G, T, and C [in the form of their reactive triphosphates (TP)], respectively, for chain buildup. The trick is to add very small amounts of a modified nucleotide to the mixture, which causes termination of chain growth at the very spot at which the normal nucleotide would have been incorporated. The modification is simple Instead of the usual 2 -deoxyribose (d), we use 2, 3 -dideoxyribose (dd). Moreover, to visualize the end result, the base attached to the dideoxyribose is labeled with a fluorescent dye molecule that can be detected by exposure to light of the appropriate wavelength. 

Evidence for termination of chain growth by transfer of a hydrogen in a-position to an in-chain ester group to the chain-end of poly(alkyl acrylate)s was seen in some cases where the obtained polymers had olefinic unsaturation. ... 

The side reactions lead to loss of initiator, termination of chain growth and formation of polymers with broad molar mass distributions. For methyl methacrylate (R = Ri = CH3) the side reactions essentially can be eliminated by using polar solvents, low temperatures, bulky initiators (for which reaction with the C=0 group is sterically-hindered) and large counter-ions, e.g. by polymerization in tetrahydrofuran at —75°C using cumylcaesium as initiator. 

The mechanism for the polymerization of VCM in the presence of TBPO-stannous octoate may be described as follows. The t-butoxy radical adds to VCM and initiates polymerization, in lieu of hydrogen abstraction. The reaction between the substrate radical (i.e., a VCM radical or the propagating chain radical) and the stannic ion results in the termination of the radical chain reaction and regeneration of the stannous ion. Thus, the latter is available for decomposition of BPO to generate additional chain-mitiating t-butoxy radicals. However, the termination of chain growth results in an inefficient consumption of radicals ... 

Complete termination of chain growth is brought about when two radicals, at least one of which is at an active chain end, meet and quench each other. The quenching radical can be another growing chain end, an initiator fragment, or an ethylene radical. Various impurity molecules can prematurely terminate the growth of a chain, so great care is taken to ensure that all reactants are extremely pure. 

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