Chain terminating agent

In addition, hydroperoxyl radicals become the active chain termination agents in the presence of ions and salts of transition metals (see Chapter 17). 

Polar solvents block the O—H bond of phenols in the reaction with peroxyl radicals due to the formation of hydrogen bond and decrease the activity of phenols as chain terminating agents [1,9,10]. 

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. 

Medium molar mass polyisobutenes are obtained by using higher purity isobutene, by lowering the temperature to — -25 to -40° C, and eventually by operating in the presence of a chain-terminating agent (diisobutenes) [12]. The semiliquid or soft-resin polymers present typical Mv in the range 40-85 103 (M = 8-13-103). 

The antioxidant acts as a chain terminating agent. The antioxidants react faster with peroxy radicals (ROO) than with macroradicals (R) and the activity depends upon their structure [114-116]. 

In fact, MWD obtained at different deuterium-ethylene ratios are not influenced by the amount of the chain terminating agent Similar results have been found by Berger and Grieveson in ethylene polymerization, using different hydrogen concentrations. 

Use Intermediate for silicone fluids, as a chain-terminating agent, imparting water repellency. 

The hydroxide ion is usually not sufficiently nucleophilic to reinitiate polymerization and the kinetic chain is thus broken. Water is an especially effective chain terminating agent. For example, Ctr,s is approximately 10 in the polymerization of styrene at 25° C with sodium naphthalene. Thus the presence of even small concentrations of water can greatly limit the polymer molecular weight and polymerization rate. 

FIGURE 17 MW distributions of polymers made with Cr/silica catalyst, showing its response to H2 as a chain terminating agent. The catalyst was tested at 95 °C with, or without, 3.5 bar H2 in 34.4 bar ethylene. 

FIGURE 150 Change in the MW distribution of polymer made with Cr/alumina when H2 is added to the reactor as a chain terminating agent. After activation at 700 °C, the catalyst was tested at 95 °C, with and without the addition of 0.34 MPa H2 in 3.8 MPa ethylene. 

Like Cr/alumina, Cr/AIPCL is considerably more responsive to H2 as a chain-terminating agent than is Cr/silica [332], Figure 163 is a plot showing the HLMI increase with H2 addition, as a function of the P/Al ratio in the support. Although 0.35 MPa of H2 added to the reactor doubles the HLMI from polymer made with Cr/silica, there is around 40-60-fold improvement in HLMI when Cr/aluminophosphate is the catalyst. However, the response to H2 is not much affected by the P/Al ratio of the support. 

Uses Silylation reagent intermediate for silicone fluids chain terminating agent imparts water repellency as reagent for temporary protection of reactive sites in mfg. of betalactum antibiotics org. synthesis of carbohydrates, nucleosides, dyes, perfumes, agrochems. solubilizer in polar/nonpolar soivs. 

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 rubber industries use alkanolamines as secondary vulcanization accelerators to give improved mechanical properties and better heat resistance. Alkanolamines help stabilize rubber dispersions and may also act as an antioxidant for some rubbers. Alkanolamines are activators for the dielectric heating and microwave vulcanization processes. The various alkanolamines are used in the cold cure of certain rubbers and can act as chain termination agents in the chloroprene polymerization. 

Of some interest is the fact that many solvents act as chain transfer agents and are used to control the molar mass of the product. Also Kalb et al. [456] mentioned the possibility of adding telogens (chain-terminating agents) to reduce the intrinsic viscosity of the product. Fie studied the effect using methanol or other alcohols. Polymerization with solvents such as toluene [452] and benzene [456] yielded a low-molar-mass polymer, and the conversion did not increase over 17% even when the pressure was raised to 9 x 10 Pa. 

The absence of any fortuitous chain termination step is an established feature of homogeneous anionic polymerization of diene and styrene in hydrocarbon solvent with organolithiums. Similarly, in the absence of adventitious chain terminating agents, the chain ends formed with Ba/Mg/Al catalysts are stable to an appreciable extent. 

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