Free radicals chain-terminating agents

Both the initiation and continuation of the oxidation are materially affected by temperature (oxidation rates are doubled for each 10°C rise in temperature), but may also be catalyzed by the presence of various metals or by light. The termination of the oxidation reaction may result from the exhaustion of the oxygen supply in lubrication systems or from the formation of stable products R + R - R-R) in the oxidation chain reaction. Antioxidant or oxidation inhibitors may function as chain terminating agents by reacting with free radicals to form stable products, by acting as peroxide decomposers, or they may act as metal passivators to prevent catalytic effects. 

The mechanism of the Ziegler polymerization has been the subject of many experiments and much speculation. It is certainly not of free radical nature, since hydrogen acts as a chain transfer agent. When tritiated alcohols are added as chain-terminating agents, tritium is found in the polymer. If ( C2H5)3A1 is used as a starter, the polymer is radioactive. Therefore the overall reaction can be formulated only in terms of metal-carbon bond participation ... 

The compound R H in the above reaction is known as a chain-transfer agent. They are also sometimes referred to as chain terminating agents, because the free-radical on the growing polymer Px is removed. Under appropriate conditions, almost anything in the reaction mass may act as a chain-transfer agent, including initiator, monomer, solvent, and dead polymer. 

Lipid peroxidation (see Fig. 17.2) is a chain reaction that can be attacked in many ways. The chain reaction can be inhibited by use of radical scavengers (chain termination). Initiation of the chain reaction can be blocked by either inhibiting synthesis. of reactive oxygen species (ROS) or by use of antioxidant enzymes like superoxide dismutase (SOD), complexes of SOD and catalase. Finally, agents that chelate iron can remove free iron and thus reduce Flaber-Weiss-mediated iron/oxygen injury. 

Although there are other unsaturated compounds that will undergo addition-elimination with free radicals, the benzyl ketene acetal XIV appears to be the most active double bond as far as rate of addition is concerned and the most efficient as far as regards to the extent of elimination is concerned. A comparison with the list of chain transfer agents listed in the Polymer Handbook (23) indicated that only the sulfur compounds appear to be more effective than XIV. Hydrolysis of the end-capped oligomer gives a macromer that is terminated with a carboxylic acid group. 

The free radical polymerization of HPMA in the presence of mercaptans involves two different initiation mechanisms (Scheme 2) [26]. One is the initiation by RS radicals from chain transfer agent the other appears to be the direct initiation by the primary isobutyronitrile (IBN) radicals formed by the decomposition of AIBN [27]. The RS are formed by either the free radical transfer reaction of alkyl mercaptans with the IBN radicals or the chain transfer reaction of an active polymer chain with the mercaptans. The initiation by the RS radicals produces the ST polymers with a functional group at one end of the polymer chain. The initiation by IBN radicals leads to nonfunctional polymer chains with an IBN end group. The presence of the polymers with IBN end groups effects the purity and the functionality of ST polymers. As expected, the production of nonfunctionalized polymer chains is affected by reaction conditions. The polymerization is mainly terminated by chain transfer reaction with the mercaptans, but other termination mechanisms, such as disproportionation and recombination, take place depending on the reaction conditions [26]. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

Polymerization of emulsion SBR is started by free radicals generated by the redox system in cold SBR and by persulfate or other initiator in hot SBR. The initiators are not involved in the molecular structure of the polymers. Almost all molecules are terminated by fragments of the chain transfer agent (a mercaptan). Schematically, the molecules are RSM H. where RS is the C H S pan of a dodccyl mercaptan molecule M is the monomer involved n is the degree of polymerization, and H is a hydrogen atom formerly attached to the sulfur of a mercaptan. In the case of free-radical-initiated polymerization of butadiene, by itself to form homopolymers or with other monomers for fonn copolymers, the butadiene will be about 18% 16% fix-1.4 and 66% trms-1,4-... 

Formulas for emulsion polymerization also include buffers, free radical initiators, such as potassium persulfate (KiSiOs), chain transfer agents, such as dodecyl mercaptan (G sSTT). The system is agitated continuously at temperatures below 100°C until polymerization is essentially complete or is terminated by the addition of compounds such as dimethyl dithiocarbamate to prevent the formation of undesirable products such as cross-linked polymers. Stabilizers such as phenyl Beta-naphthylamine are added to latices of elastomers. 

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