Chain transfer termination

In contrast to chain transfer, termination reactions destroy free radicals. Two mechanisms are considered. Termination by combination produces a single molecule of dead polymer ... 

The above scheme cannot explain some experimental facts such as effect of cocatalysis, chain transfer termination, etc. 

III. Chain Transfer, Termination Processes, and Molecular Weight of the Polymers. 23... 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

Chain transfer/termination steps involve reactions with methanol [schemes (84) to (86)], spontaneous transfer [scheme (87)] or reaction with acids [scheme (88)] [107] ... 

Further evidence of the chain transfer (termination) involving polymer chains comes from the polymerization of a given monomer A in the presence of a preformed polymer, e.g. poly-B ... 

Intermolecular Chain Transfer (Termination) to Polymer Chains... 

Fig. 16. Dependence of In (kp/kt) on the effective volume (-Eg) for N-substituted aziridines 5.3.6 Intramolecular Chain Transfer (Termination) Followed by Reinitiation...

C5Mes)2Lu(R) complexes undergo all of the key reactions involved in olefin polymerization including olefin insertion (chain propagation), )7-H and ) -alkyl elimination, and Lu—R bond hydrogenolysis (chain transfer/ termination) (23). 

The polymer produced by 3 under the conditions noted above is essentially linear polyethylene with a molecular weight in the range of 30,000, with an MJM ratio of 2.5. End group analysis reveals approximately 25% vinyl end groups, implying that chain transfer/termination occurs by both y -H elimination and net Zr—R bond protonolysis processes. The latter... 

In chain transfer/terminations illustrated in eq 3.7-3.10, the component containing the transition metal is still an active catalyst. Thus, each active center may produce hundreds or thousands of polymer chains. 

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. 

The intermolecular chain transfer (termination) reaction was demonstrated by polymerizing a given monomer in the presence of another preformed polymer. A copolymer is obtained in this case... 

Since chain transfer terminates a growing chain, its effect is a lower molecular weight than would result in its absence. The effect of any chain transfer on the rate of polymerization will, however, vary depending on the relative rates of the transfer [Eq. (6.111)] and reinitiation [Eq. (6.112)], as compared to the rate of normal propagation reaction [Eq. (6.6)]. 

The results of the dilute solution polymerization suggested that solvent chain transfer was a governing factor In the termination step. By the same token, the mechanism of solvent chain transfer termination forms free radicals which do not carry carboxyl groups, and serve as potent initiator species In the polymerization media. It was concluded that in order to obtain higher functionality of the PnBA the solvent had to be eliminated from the polymerization reactor. 

The propagation mechanism for both iron and cobalt catalysts is considered to follow a Cossee-type mechanism [130] (pathway A in Scheme 5.10), in which sequential ethylene coordination and migratory insertion of the bound ethylene into a metal alkyl bond occur before chain transfer terminates the reaction. The... 

The preceding sections have dealt with polymerization by either insertion or GTP mechanisms. Of course, vinyl monomers are also polymerizable by radical, anionic, or cationic mechanisms. In this short section, we summarize the processes which are reasonably well understood from a mechanistic viewpoint, and which involve the intervention of transition metal alkyls (or hydrides), either during initiation, propagation, or chain transfer/termination. A much larger class of polymerization reactions where redox-active transition metal complexes are used to mediate radical polymerizations by reversible atom transfer (ATRP) or other means has been extensively and recently reviewed from a mechanistic perspective and will only be briefly mentioned here. 

The chains terminated by recombination are larger than those terminated by disproportionation. Deactivation of the growing chain radical always occurs through chain transfer termination, i.e., by abstraction of an atom from another molecule such as the initiator, monomer, solvent, completed polymer chain, modifier, or impurities, and thus the propagating chain radical becomes saturated. The molecule from which the atom has been abstracted will then become a free radical and may or may not start a new chain. Accordingly, the rate of polymerization does not decrease but the molecular weight decreases. 

Here, the CTC agent acts as a chain transfer terminator but does not initiate new chain in the classical manner. Similarly, Haddleton and co-workers (129) used methyl(2-bromomethyl)acrylate in transition-metal-mediated controlled radical polymerization to replace the -halogen end group via addition-fragmentation to yield a methacrylate-based macromonomer. 

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