Polymerization, activation equilibrium

In this equilibrium the Nd-species to which a diene is coordinated is active in polymerization, whereas the Nd-species to which an arene is coordinated is inactive. According to the authors the experimentally determined ranking of activities toluene > mesitylene > toluene (+ 7% hexamethylbenzene) correlates with the electron richness (i.e. Lewis basicity) of the aromatic compounds. The polymerization activity decreases with increasing Lewis basicity of the aromatic compound as the equilibrium is shifted and the concentration of the active species is reduced. These considerations were supported by the following experimental results (Table 18). 

Table III shows the relationship between polymerization activity and the evolution of methane as a function of reaction time. Nearly 15 min after mixing of the metallocene and MAO, the catalyst attains its maximum activity the production of methane is low. After 2 h at 10°C, the equilibrium mentioned previously is reached. This corresponding activity is then nearly constant for 17 h. This result shows that another important function of MAO is the reactivation of inactive complexes formed by hydrogen-transfer reactions.

Cleavage of formaldehyde from the active centers and polymerization of formaldehyde at the same cationic chain ends (polymerization-depolymerization equilibrium of formaldehyde) (9). 

Already the equilibrium (77) has described the loss of the polymerization activity of a certain part of the centres by interaction of growing ends of equal basicity. Therefore we have called this reaction isobasic termination , An addition of alkali metal hydroxides to the initiator (potassium hydroxide)... 

In all polymerizations an equilibrium exists between active chain end and monomer. This is not usually noticeable because the equilibrium is overwhelmingly in favour of the chain end, and hence, polymer formation. However, the poly(oxymethylene) cation appears to be in equilibrium with a significant concentration of formaldehyde. 

Preference for neutral metallocene dimethyl coordination over anion coordination to form /(-Me di-nuclear cationic complexes (e.g., 43) is observed for those truly weakly coordinating anions such as B(C6F5)4 and MePBB . Stabilization of highly reactive and unstable metallocenium cations by /(-Me coordination allows isolation and characterization of such species in the pure state, yet affords excellent polymerization activity in solution, presumably via dissociation to a more reactive monomeric form, as indicated by NMR studies.For more coordinating anions such as CH3B(C6F5)3, /(-Me bimetallic cationic complexes are not detected, except when an excess of neutral metallocene dimethyl is employed (Scheme 25) 143,315 equilibrium can be utilized to stabilize... 

The cationic pohmierizations of cyclic acetals are different from the polymerizations of the rest of the cyclic ethers. The differences arise from greater nucleophilicity of the cyclic ethers as compared to that of the acetals. In addition, cyclic ether monomers, epirane, tetrahydrofuran, and oxepane, are stronger bases than their corresponding polymers. The opposite is true of the acetals. As a result, in acetal polymerizations, active species like those of 1,3-dioxolane may exist in equilibrium with macroalkoxy carbon cations and tertiary oxonium ions. By comparison, the active propagating species in polymerizations of cyclic ethers, like tetrahydrofuran, are only terdaiy oxonium ions. The properties of the equilibrium of the active species in acetal polymerizations depend very much upon polymerization conditions and upon the structures of the individual monomers. 

Chemically activated equilibrium polymers (CAEPs). If the monomers are inactive unless activated through chemical reaction with initiator molecules, it obeys a chemically activated equilibrium polymerization. The amount of initiator added determines the number of activated monomers that in turn fixes the number of EPs in the system. The living polymerization of poly(a-methylstyrene) is an example of a CAEP, provided the polymerization reaction is not (chemically) terminated [18]. 

Chemically activated equilibrium polymerization of material in a good solvent was recently investigated by means of a scaling theory similar to that described for EPs [40], For this type of supramolecular polymer, the effect of excluded-volume interactions on the growth seems to be even more subtle than for EPs, and to be the most prominent very close to the polymerization transition. The location of the polymerization transition remains unaffected by the interactions but it becomes slightly more rounded because the initiator fraction r in Eq. (18) is replaced by yr 1.2r. For CAEPs the polymerization transition (at temperature Tp) and the crossover to the semidilute regime (at temperature T ) turn out to be quite close to each other, quite unlike the situation for EPs,... 

Fig. 1. Change in energy of a polymerization/depolymerization equilibrium along the reaction coordinate, showing the relation of the activation energies of propagation and depropagation and the heat of polymerization.

The mechanism of ion polymerization in formaldehyde crystals proposed by Basilevskii et al. [1982] rests on Semenov s [1960] assumption that solid-phase chain reactions are possible when the arrangement of the reactants in the crystal prepares the configuration of the future chain. The monomer crystals capable of low-temperature polymerization fulfill this condition. In the initial equilibrium state the monomer molecules are located in the lattice sites and the creation of a chemical bond requires surmounting a high barrier. However, upon creation of the primary dimer cation, the active center shifts to the intersite, and the barrier for the addition of the next link... 

In systems of LP the dynamic response to a temperature quench is characterized by a different mechanism, namely monomer-mediated equilibrium polymerization (MMEP) in which only single monomers may participate in the mass exchange. For this no analytic solution, even in terms of MFA, seems to exist yet [70]. Monomer-mediated equilibrium polymerization (MMEP) is typical of systems like poly(a-methylstyrene) [5-7] in which a reaction proceeds by the addition or removal of a single monomer at the active end of a polymer chain after a radical initiator has been added to the system so as to start the polymerization. The attachment/detachment of single monomers at chain ends is believed to be the mechanism of equilibrium polymerization also for certain liquid sulphur systems [8] as well as for self-assembled aggregates of certain dyes [9] where chain ends are thermally activated radicals with no initiators needed. 

In animals, acetyl-CoA carboxylase (ACC) is a filamentous polymer (4 to 8 X 10 D) composed of 230-kD protomers. Each of these subunits contains the biotin carboxyl carrier moiety, biotin carboxylase, and transcarboxylase activities, as well as allosteric regulatory sites. Animal ACC is thus a multifunctional protein. The polymeric form is active, but the 230-kD protomers are inactive. The activity of ACC is thus dependent upon the position of the equilibrium between these two forms ... 

Because this enzyme catalyzes the committed step in fatty acid biosynthesis, it is carefully regulated. Palmitoyl-CoA, the final product of fatty acid biosynthesis, shifts the equilibrium toward the inactive protomers, whereas citrate, an important allosteric activator of this enzyme, shifts the equilibrium toward the active polymeric form of the enzyme. Acetyl-CoA carboxylase shows the kinetic behavior of a Monod-Wyman-Changeux V-system allosteric enzyme (Chapter 15). 

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