Chain initiation initiator efficiency

Deming TJ, Curtin SA (2000) Chain initiation efficiency in cobalt- and nickel-mediated polypeptide synthesis. J Am Chem Soc 122 5710-5717... 

Calculated fi om XPS data, packing density of the initiator, and a chain initiation efficiency. 

In equations 8 and 9, is the initiator efficiency, the fraction of initiator radicals that actually initiates chain growth, + k, and is the number of... 

In ATRP, the initiator (RX) determines the number of growing chains. Ideally, the degree of polymerization is given by eq. 7 and the molecular weight by cq. 8. Note the appearance of the initiator efficiency (/ ) in the numerator of these expressions. In practice, the molecular weight is ofien higher than anticipated because the initiator efficiency is decreased by side reactions. In some cases, these take the form of heterolytic decomposition or elimination reactions. Further redox chemistry of the initially formed radicals is also known. The initiator efficiencies are dependent on the particular catalyst employed. 

The same conditions of initiator efficiency are to be observed when the living carban-ionic end of a polymer chain is used to initiate the polymerization of a second monomer 6,7,26.27) for tjjg synthesis of a block copolymer, the monomers are to be added... 

M] = monomer concentration [l] = initiator concentration [R ] = radical concentration f = initiator efficiency k-j g = chain transfer rate "constant" ktr = termination (combination or disproportionation) rate constant... 

The efficiency of the intitiator is a measure of the extent to which the number of radicals formed reflects the number of polymer chains formed. Typical initiator efficiencies for vinyl polymerisations lie between 0.6 and 1.0. Clearly the efficiency cannot exceed 1.0 but it may fall below this figure for a number of reasons, the most important being the tendency of the newly generated free radicals to recombine before they have time to move apart. This phenomenon is called the cage effect . 

In the above reactions, I signifies an initiator molecule, Rq the chain-initiating species, M a monomer molecule, R, a radical of chain length n, Pn a polymer molecule of chain length n, and f the initiator efficiency. The usual approximations for long chains and radical quasi-steady state (rate of initiation equals rate of termination) (2-6) are applied. Also applied is the assumption that the initiation step is much faster than initiator decomposition. ,1) With these assumptions, the monomer mass balance for a batch reactor is given by the following differential equation. 

The duration of the inhibition period of a chain-breaking inhibitor of autoxidation is proportional to its efficiency. Indeed, with an increasing rate of chain termination, the rates of hydroperoxide formation and, hence, chain initiation decrease, which results in the lengthening of the induction period (this problem will be considered in a more detailed manner later). It should be noted that when initiated oxidation occurs as a straight chain reaction, the induction period depends on the concentration of the inhibitor, its inhibitory capacity, and the rate of initiation, but does not depend on the inhibitor efficiency. 

Zhao and Brittain [280-282] reported the LCSIP of styrene on planar silicon wafers using surface modifications of 2-(4-(ll-triethoxysilylundecyl)phenyl-2-methoxy-propane or 2-(4-trichlorosilylphenyl)-2-methoxy-d3-propane respectively. Growth of PS brushes from these SAMs has been successfully achieved factors that influence PS thickness included solvent polarity, additives and TiC concentration. Sequential polymerization by monomer addition to the same silicate substrate bearing the Hving polymer chains resulted in thicker PS films. FTIR-ATR studies using a deuterated initiator indicated that the initiator efficiency is low, and the... 

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