Conventional chain transfer agent

Even in the presence of such aldehydes as acetaldehyde, propionaldehyde, butyraldehyde, and heptanol (all conventional chain-transfer agents for vinyl... 

To get scale inhibitors with improved biodegradability, during the radical polymerization, chain transfer agents have been used. Conventional chain transfer agents are halogen compoimds and thiols. 

Conventional chain transfer agents, normally active with Zie-... 

Aside from conventional chain-transfer agents for vinyl chloride polymerizations, such as chlorinated hydrocarbons or mercaptans, the molecular weight distribution may also be reduced with 2-iodopropane [81] and aldehydes such as propionaldehyde [82]. 

End-functional polymers, including telechelic and other di-end functional polymers, can be produced by conventional radical polymerization with the aid of functional initiators (Section 7,5.1), chain transfer agents (Section 7.5.2), monomers (Section 7.5.4) or inhibitors (Section 7.5.5). Recent advances in our understanding of radical polymerization offer greater control of these reactions and hence of the polymer functionality. Reviews on the synthesis of end-functional polymers include those by Colombani,188 Tezuka,1 9 Ebdon,190 Boutevin,191 Heitz,180 Nguyen and Marechal,192 Brosse et al.rm and French.194... 

The free-radical nature of ATRP is well established through a number of studies [Matyjaszewski et al., 2001], The effects of inhibitors and retarders, solvents, and chain-transfer agents are the same in ATRP as in conventional radical polymerization. The regio-selectivity, stereoselectivity, and copolymerization behaviors are also the same. 

The scope of the living cationic polymerizations and synthetic applications of these functionalized monomers will be treated in the next chapter on polymer synthesis (see Chapter 5, Section III.B). One should note that the feasibility of living processes for these polar monomers further attests to the formation of controlled and stabilized growing species. Conventional nonliving polymerizations, esters, ethers, and other nucleophiles are known to function as chain transfer agents and sometimes as terminators. In addition, the absence of other acid-catalyzed side reactions of the polar substituents, often sensitive to hydrolysis, acidolysis, etc., demonstrates that these polymerization systems are free from free protons that could arise either from incomplete initiation (via addition of protonic acids to monomer) or from chain transfer reactions (/3-proton elimination from the growing end). 

In ethylene—propene copolymerization the former monomer is greatly favoured and a value for r, of 72 was found. Hydrogen is particularly active as a chain transfer agent for this catalyst, a value of fetr,H,/ tr,M 3.8 X 10 being quoted, some ten times greater than that for a conventional Ziegler system [133b]. The active species in both these systems was ascribed to a low valence Cr complex. 

The effects of chain-transfer agents such as oc-tanethiol on copper-catalyzed polymerizations are similar to those on conventional radical polymerizations.295 These may also mean little difference among the growing species generated from the carbon—halo-... 

Termination reactions cannot be eliminated in radical polymerizations because termination reactions involve the same active radical species as propagation therefore, eliminating the species that participates in termination would also result in no polymerization. Termination between active propagating species in cationic or anionic processes does not occur to the same extent because of electrostatic repulsions. Equation (1) represents the rate of polymerization, Rp, which is first order with respect to the concentration of monomer, M, and radicals, P, while Eq. (2) defines the rate of termination, Rt, which is second order with respect to the concentration of radicals. To grow polymer chains with a degree of polymerization of 1000, the rate of propagation must be at least 1000 times faster than the rate of termination (which under steady state condition is equal to the rate of initiation). This requires a very low concentration of radicals to minimize the influence of termination. However, termination eventually prevails and all the polymer chains produced in a conventional free radical process will be dead chains. Therefore they cannot be used in further reactions unless they contain some functional unit from the initiator or a chain transfer agent. 

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