Terminal-penultimate

The various copolymerization models that appear in the literature (terminal, penultimate, complex dissociation, complex participation, etc.) should not be considered as alternative descriptions. They are approximations made through necessity to reduce complexity. They should, at best, be considered as a subset of some overall scheme for copolymerization. Any unified theory, if such is possible, would have to take into account all of the factors mentioned above. The models used to describe copolymerization reaction mechanisms arc normally chosen to be the simplest possible model capable of explaining a given set of experimental data. They do not necessarily provide, nor are they meant to be, a complete description of the mechanism. Much of the impetus for model development and drive for understanding of the mechanism of copolymerization conies from the need to predict composition and rates. Developments in models have followed the development and application of analytical techniques that demonstrate the inadequacy of an earlier model. 

The arrangement of monomer units in copolymer chains is determined by the monomer reactivity ratios which can be influenced by the reaction medium and various additives. The average sequence distribution to the triad level can often be measured by NMR (Section 7.3.3.2) and in special cases by other techniques.100 101 Longer sequences are usually difficult to determine experimentally, however, by assuming a model (terminal, penultimate, etc.) they can be predicted.7 102 Where sequence distributions can be accurately determined Lhey provide, in principle, a powerful method for determining monomer reactivity ratios. 

Tetrad level Erqierirr itel [2] Terminal Penultimate ROMM... 

Fig. 16a d. Fractions of isolated Mj(a), M2(c) units in copolymer of styrene with acrylonitrile M2, and fractions of isolated pairs of these units MjM, (b) and M2M2 (d) Experimental points are compared to the plots calculated via various models terminal (- -), penultimate (----------), CR-1 (---------), CR-2 (--------) [282]... 

N -Terminal Penultimate Ante-penultimate Penultimate C-Terminal amino acid amino acid amino acid amino acid amino acid residue residue residue residue residue... 

There are several cases where NMR spectroscopy has been used to investigate copolymers which deviate from the terminal model for copolymerisation (see also chapter 3). For example, Hill and co-workers [23, 24] have examined sequence distributions in a number of low conversion styrene/acrylonitrile (S/A) copolymers using carbon-13 NMR spectroscopy. Previous studies on this copolymer system, based on examination of the variation of copolymer composition with monomer feed ratio, indicated significant deviation from the terminal model. In order to explain this deviation, propagation conforming to the penultimate (second-order Markov) and antepenultimate (third-order Markov) models had been proposed [25-27]. Others had invoked the complex participation model as the cause of deviation [28]. From their own copolymer/comonomer composition data. Hill et al [23] obtained best-fit reactivity ratios for the terminal, penultimate, and the complex participation models using non-linear methods. After application of the statistical F-test, they rejected the terminal model as an inadequate description of the data in comparison to the other two models. However, they were unable to discriminate between the penultimate and complex participation models. Attention was therefore turned to the sequence distribution of the polymer. 

The study of the mechanisms of copolymerization reactions has attracted the attention of polymer scientists over many years. Copolymerization reactions have been described in terms of various mathematical models, terminal, penultimate, complex, etc., which are characterized by a particular set of reactivity ratios. These mathematical models can be used to make predictions about the variation of the copolymer composition or the copolymer microstructure (for example, the triad fractions, number average sequence lengths, etc.) with comonomer feed composition or monomer conversion (1). 

Terminal-penultimate chelation literature mechanism) r cemo propagation... 

Another crucial requirement of LA-mediated isotactic control is that the LA remains in the terminal-penultimate position during propagation that is, that the terminal-penultimate binding position is the most reactive in the polymerization. However, chelation can occur not only at the terminal-penultimate position but at any other point along the polymer chain, including the penultimate-antepenultimate and/or the terminal-incoming... 

Recently, we explicitly examined the energetics of Li" " coordination at different positions of the polymer terminus using high-level ab initio calculations. These calculations suggested that while there was a modest thermodynamic preference for Li chelation at the terminal-penultimate... 

Equation (7.32) shows that pjj is constant for a particular copolymer if the terminal model applies therefore the ratio NmjMi/Nmi also equals this constant. Equation (7.49) shows that Pj u is constant for a particular copolymer if the penultimate model applies therefore the ratio NmiMiMi/NmiMi equals this constant, but the ratio NmjMj/Nmj does not have the same value. 

These observations suggest how the terminal mechanism can be proved to apply to a copolymerization reaction if experiments exist which permit the number of sequences of a particular length to be determined. If this is possible, we should count the number of Mi s (this is given by the copolymer composition) and the number of Mi Mi and Mi Mi Mi sequences. Specified sequences, of any definite composition, of two units are called dyads those of three units, triads those of four units, tetrads those of five units, pentads and so on. Next we examine the ratio NmjMi/Nmi nd NmjMiMi/NmiMi If these are the same, then the mechanism is shown to have terminal control if not, it may be penultimate control. To prove the penultimate model it would also be necessary to count the number of Mi tetrads. If the tetrad/triad ratio were the same as the triad/dyad ratio, the penultimate model is proved. 

From appropriate ratios of these sequence lengths, what conclusions can be drawn concerning terminal versus penultimate control of addition The following are experimental tacticity fractions of polymers prepared from different monomers and with various catalysts. On the basis of Fig. 7.9, decide whether these preparations are adequately described (remember to make some allowance for experimental error) by a single parameter p or whether some other type of statistical description is required ... 

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). 

Rea.CtlVltyRa.tlO Scheme. The composition of a copolymer at any point in time depends on the relative rates that each monomer can add to a chain end. If it is assumed that the chemical reactivity of a propagating chain depends only on the terminal unit and is not affected by any penultimate units, then four possible propagation steps in the copolymerisation of two monomers, and M2, with two growing chain ends, M and M2, can be written as follows ... 

The incretin effect is reduced in type 2 diabetes, and this is attributed, at least in part, to reduced secretion of GLP-1. The biological actions of GLP-1 remain essentially intact in type 2 diabetes, but administration of extra GLP-1 is not a practical therapeutic option because the peptide is degraded rapidly if A < 2 min) by the enzyme dipeptidyl peptidase IV (DPP-4). DPP-4 cleaves the N-terminal dipeptide from many of the peptides that have either an alanine or a proline residue penultimate to the N-terminus (Fig. 6). 

Primary radical termination may involve combination or disproportionation with the propagating radical. It is often assumed that small radicals give mainly combination even though direct evidence for this is lacking. Both pathways are observed for reaction of eyanoisopropyl radicals with PS (Scheme 3.14) (Section 7.4.3.2). The end group formed by combination is similar to that formed by head addition to monomer differing only in the orientation of the penultimate monomer unit. 

For radicals 1, ktA/ktc shows a marked dependence on the bulk of the substituent (R2). While phenylethyl radicals (2) and cumyl radicals (5) afford predominantly combination, there are indications of a substantial penultimate unit effect. The radicals 6, with an a-neopentyl substituent, give predominantly disproportionation, Termination in AMS polymerization might therefore also give substantial... 

Cases have been reported where the application of the penultimate model provides a significantly better fit to experimental composition or monomer sequence distribution data. In these copolymerizations raab "bab and/or C BA rBBA- These include many copolymerizations of AN, 4 26 B,"7 MAH28" 5 and VC.30 In these cases, there is no doubt that the penultimate model (or some scheme other than the terminal model) is required. These systems arc said to show an explicit penultimate effect. In binary copolynierizations where the explicit penultimate model applies there may be between zero and three azeotropic compositions depending on the values of the reactivity ratios.31... 

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). 

Triad information is more powerful, but typically is subject to more experimental error and signal assignments are often ambiguous (Section 7.3.3.12). Triad data for the MMA-S system are consistent with the terminal model and support the view that any penultimate unit effects on specificity are small.Mv lS... 

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