Kinetic penultimate effect

What are the differences between the two treatments (kinetic penultimate effect and depropagation) used to account for the deviations observed in the copolymer composition equation ... 

Following a procedure similar to that used in deriving Eq. (7.11), the copolymer composition equation with a kinetic penultimate effect present is obtained as... 

For many systems, the copolymer composition appears to be adequately described by the terminal model yet the polymerization kinetics demand application of the penultimate model. These systems where rAAB=rliAR and aha bba hut sAfsB are said to show an implicit penultimate effect. The most famous system of this class is MMA-S copolymerization (Section 7.3.1.2.3). 

Termination scheme 11 applies to the geometric mean and phi factor models and scheme 12 Is required for the penultimate effect model. All the above reaction models were used In attempts to simulate kinetic data. 

Anionic copolymerizations have been investigated by applying the classical Mayo-Lewis treatment which was originally developed for free-radical chain reaction polymerization [198]. The copolymerization of two monomers (Mj and M2) can be uniquely defined by the following the four elementary kinetic steps in Scheme 7.21, assuming that the reactivity of the chain end (Mj" or ) depends only on the last unit added to the chain end, that is, there are no penultimate effects. 

Guillot, J. Penultimate Effects in Radical Copolymerization I - Kinetical Study. 

The penultimate effect on the kinetics of the activation/deactivation processes may be quite significant. For example, model studies of activation of various dimers in ATRP copolymerization of methyl acrylate (MA) and methyl methacrylate (MMA) showed the following relative values of act for dimeric species H-MA-MA-Br, H-MMA-MA-Br, H-MA-MMA-Br, and H-MMA-MMA-Br to be 1, 4.6, 19, and 96, respectively. The back-strain effect resulting from the presence of a MMA penultimate unit, and formation of a thermodynamically more stable radical from a MMA-Br terminal unit, increased the values of act by 5 and 20 times, respectively, in comparison to MA penultimate/terminal imits. The combined effects resulted in a 100-fold increase of k ct for H-MMA-MMA-Br relative to H-MA-MA-Br (116). 

From kinetical statement, penultimate effects correspond to the influence of units preceding the radical unit on the reactivity of the growing radical. In a more precise way, one speaks about penultimate effect if there is an influence of the unit immediately before the radical unit, and pen-penultimate effect if there is an influence of the unit preceding this last one... 

Penultimate Effects in Radical Gopolymerization I-Kinetical Study 111... 

Experimental Determination of Kinetical Parameters when Penultimate Effects are Taken in Account... 

It was shown that penultimate effect modify the kinetics of copolymerization and given a method which allows to express the copolymerization equation, in the general case. 

A kinetical study of the radical polymerization of acrylonitrile-methyl methacrylate has revealed the existence of a penultimate effect. The calculated reactivity ratios are ... 

The basic kinetic equations for chain addition copolymerization are given in Table I for three termination models geometric mean (GM), phi factor (PF) and penultimate effect (PE). It Is important to note the symmetry in form created by confining the effect of choice of termination model to a single factorable function H. 

The utility of the runaway and sensitivity parameters a,B, and b has been demonstrated through both numerical simulation and experimentation (3, h) Numerical simulations employed literature values for the kinetic constants for the monomer pairs of Styrene-Methyl Methacrylate (SMMA), Styrene-Acrylonitrile (SAN) and, AcrylonItrIle-Methyl Methacrylate (ANMMA). Phi-factor kinetics were generally used, however both geometric mean and recently-advanced penultimate effect kinetics (6) were tested as well. Experiments were confined to the Styrene-Acrylonitrile comonomer system, however, the full range of compositions was studied. An extensive initial rate study was performed on this system to develop the kinetic constants needed to evaluate the runaway parameters. ... 

Sato T, Kawasaki S, Seno M, Tanaka H, Kato K. Kinetic and ESR studies on radical polymerization. ESR and kinetic evidences for the penultimate effect in the radical-initiated copolymerization of N-cyclohexyhnaleimide and bis(2-ethylhexyl) itaconate in benzene. Makromol Chem 1993 194 2247-2256. 

The early kinetic models for copolymerization, Mayo s terminal mechanism (41) and Alfrey s penultimate model (42), did not adequately predict the behavior of SAN systems. Copolymerizations in DMF and toluene indicated that both penultimate and antepenultimate effects had to be considered (43,44). The resulting reactivity model is somewhat compHcated, since there are eight reactivity ratios to consider. 

The first quantitative model, which appeared in 1971, also accounted for possible charge-transfer complex formation (45). Deviation from the terminal model for bulk polymerization was shown to be due to antepenultimate effects (46). Mote recent work with numerical computation and C-nmr spectroscopy data on SAN sequence distributions indicates that the penultimate model is the most appropriate for bulk SAN copolymerization (47,48). A kinetic model for azeotropic SAN copolymerization in toluene has been developed that successfully predicts conversion, rate, and average molecular weight for conversions up to 50% (49). 

The preparation of homochiral compounds by formation and separation of diastereoisomers or by kinetic resolution of racemates, at or near the end of a total synthesis, has been a method of choice. This avoids the possibility of racemization should chirality be introduced earlier. However, the costs are high because only half by weight of the homochiral compound is theoretically possible from the racemate unless the optical antipode can also be easily inverted to the desired product. Indeed, previous methods for producing levomethadone based on the classical resolution at the end, or at the penultimate stage of the synthesis, were costly and not very effective. Levomethadone hydrochloride has previously been marketed as L-Polamidon and Levadone16 but was subsequently withdrawn because of the high cost of production. 

Variants of the above have been used to explain unusual data where it was suspected that monomer units penultimate to the chain end (or even further back) were affecting the reaction rate constants (6). Such treatments suffer from a degree of arbitrariness in that the experimental data may not provide an adequate test of the kinetic model (3). In some cases, penultimate unit effect models have been used to interpret unusual data where one might expect depropagation to be important. 

Penultimate and similar kinetic models are used nowadays principally for the treatment of the experimental data, obtained from the copolymerization of certain monomers like fumaronitrile or maleic anhydride which are characterized by rather strong steric and polar effects. 

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