Chain kinetics

Starnes et al.hl have also suggested that the head adduct may undergo p-scission to eliminate a chlorine atom which in turn adds VC to initiate a new polymer chain. Kinetic data suggest that the chlorine atom does not have discrete existence. This addition-elimination process is proposed to he the principal mechanism for transfer to monomer during VC polymerization and it accounts for the reaction being much more important than in other polymerizations. The reaction gives rise to terminal chloroallyl and 1,2-dichlorocthyl groups as shown in Scheme 4.8. 

The As(III) reduction is also catalysed by Fe(III) with radical-chain kinetics . ... 

Kinetic vs. material chain. Kinetically, a chain reaction exists throughout the "life" of the radical, that is, from the initiation of a radical up to its termination by recombination or by disproportionation. The lifetime of a radical determines the so-called kinetic chain length Lp defined as the number of monomers consumed per initiating radical. Lp, by definition, can be calculated from the ratio between the propagation rate Rp to the initiation rate R, or, using steady-state hypothesis (Equation (1)), from the ratio between propagation rate to the termination rate Rt (Equation (3)). 

Sharpless epoxidation reactions are thoroughly discussed in Chapter 4. This section shows how this reaction is used in the asymmetric synthesis of PG side chains. Kinetic resolution of the allylic secondary alcohol ( )-82 allows the preparation of (R)-82 at about 50% yield with over 99% ee (Scheme 7-23).19... 

There is a large, but not total, overlap between the terms condensation polymers and stepwise kinetics and the terms addition (or vinyl) polymers and chain kinetics. In this section we will look at each of these four terms and illustrate the similarities and differences between them. 

Even so, there is no total overlap between the various characteristics of vinyl-chain kinetics and condensation-step kinetics. Following are examples illustrating the lack of adherence to this overlap ... 

The cyclopentadienyl radical, which is an important intermediate in the transition from cyclic to linear chain kinetics, is a resonance stabilized radical. It is presumably consumed mainly through fast radical-radical reactions,... 

Thermal aging has also been widely studied for its technological interest, but most of the studies are empirical or semiempirical (using, e.g., the Arrhenius law and an arbitrary end-life criteria, from mass-loss kinetic curves). Powerful conceptual tools (radical chain kinetics, coupled diffusion-reaction schemes, etc.) are available to avoid or, at least, to reduce significantly the empiric approaches. 

The mechanism can be best understood within the framework of the conventional theory of radical chain kinetics, provided that certain of the usual simplifying assumptions are omitted. A solution is given to the problem of steady-state polymerization rate as a function of monomer and initiator concentration, taking into account termination reactions of primary radicals and recombination of geminate chains arising from the same initiation event. This model is shown to account for the kinetic data reported herein. With appropriate rate constants it should be generally applicable to radical polymerizations. 

A further hypothesis, the Rice-Teller hypothesis of least motion, is a powerful tool in understanding the chain kinetics. This states that in a metathesis, that path is most likely which involves least concerted motion of nuclei. 

Chen, X.Y., Link, T.M. and Schramm, V.L. (1998) Ricin A-chain kinetics, mechanism, and RNA stem-loop inhibitors. Biochemistry, 37, 11605-11613. 

The synthetic strategies for the esters of cephalotaxine follow, for the most part, the partial esterification method, lire ideas that are exploited in the access to the fragments vary, but most use as the last step the anionic attachment of the acetate portion. The one method that remains to be exploited here is the enzymatic esterification of the natural enantiomer of cephalotaxine either with a racemate of the complete side chain (kinetic resolution) or with the appropriate enantiomer of the ester side chain. It is hoped that the solution to this particular problem in either enzymatic catalysis or plant tissue culture preparation will materialize in the near future. 

The time constant for exponential acceleration of the H2-O2 (and the H2-CO-O2) reaction toward the end of the induction period but prior to macroscopic reaction has been examined directly by quantitative emission spectrometry of radiation proportional to the exponentially growing concentration of one or more reaction products. - As we shall see in section 2.3.2, this time constant offers the most direct information yet obtainable about the elementary reaction rate coeflS-cients which govern the branching chain kinetics at high-temperatures. 

The explosion limit phenomenology observed in hydrogen-oxygen mixtures has been semi-quantitatively described in section 2.1.2. For an initial pressure below approximately one atmosphere there exists one threshold temperature for explosion, typically between 700 and 900 K, whose magnitude is determined by the appropriate isothermal branched chain kinetics. More specifically, this boundary, separating regimes of slow, quasi-steady state and explosively rapid reaction, is defined by an equality between the rates of chain centre formation by branching steps and destruction by termination steps. 

For sin s, monomer II or (prepolymer II) is added before step (b). Thus to a greater or lesser extent, the two networks are formed simultaneously. Network I chains are stretched and diluted by network II in a sequential IPN, but only diluted in an SIN, altering many morphological and physical properties. Of course it is required that the two polymerizations be non-interfering reactions, such as by stepwise and chain kinetics. 

Chain kinetics n. The study of rates and mechanisms of chemical reactions and of the factors on which they depend. 

Chain length n. The number of monomeric or structural units in a linear polymer, or the main chain in a branched polymer. This can also be written as the total length of chain molecule, measured from atom to atom along the chain. Kinetic chain length v. gives the average number of monomer molecules that can be added to a free radical before the free radical is destroyed by a termination reaction. Note—This term should not be used for the direct distance between the ends of the molecule. 

Structure and Nomenclature The principal method of polymerizing monomers by the chain kinetic scheme involves the opening of double bonds to form a linear molecule. In a reacting mixture, monomer, fully reacted polymer, and only a small amount of rapidly reacting species are present. Once the polymer terminates, it is dead and cannot react further by the synthesis scheme outlined previously. 

Chen X-Y, Berti PJ, Schramm VL (2000) Ricin a-chain kinetic isotope effects and transition state structure with stem-loop RNA f- J Am Chem Soc 122 1609-1617. doi 10.1021/ Ja992750i... 

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