Monomer concentration reversals

Primary radical termination is also of demonstrable significance when very high rates of initiation or very low monomer concentrations are employed. It should be noted that these conditions pertain in all polymerizations at high conversion and in starved feed processes. Some syntheses of telechelics are based on this process (Section 7.5.1). Reversible primary radical termination by combination with a persistent radical is the desired pathway in many forms of living radical polymerization (Section 9.3). 

Polymer growth J(c) showed nonlinear monomer concentration dependence in the presence of ATP (Carrier et al., 1984), while in the presence of ADP, the plot of J(c) versus monomer concentration for actin was a straight line, as expected for reversible polymerization. The data imply that newly incorporated subunits dissociate from the filament at a slower rate than internal ADP-subunits in other words, (a) the effect of nucleotide hydrolysis is to decrease the stability of the polymer by increasing k and (b) nucleotide hydrolysis is uncoupled from polymerization and occurs in a step that follows incorporation of a ATP-subunit in the polymer. Newly incorporated, slowly dissociating, terminal ATP-subunits form a stable cap at the ends of F-actin filaments. 

In summary, then, polymerization of ATP-actin under conditions of sonication displays two characteristic deviations from the simple law described by equation (4), which is only valid for reversible polymerization. These are (a) overshoot polymerization kinetics, and (b) the steady-state amount of polymer formed decreases, or the steady-state monomer concentration increases, with the number of filaments. These two features are the direct consequence of ATP hydrolysis accompanying the polymerization of ATP-actin, as will be explained now. 

Spurious Correlations. If the reagent F which is, or which may form, or may react with, a chain-breaking agent, is contained as an impurity in the solvent, then increasing the monomensolvent ratio will decrease / if it is contained in the monomer, the reverse will happen. In this way a spurious variation of DP with monomer concentration may arise, which will be superimposed upon the normal effects due to variations in the rate of monomer transfer and solvent transfer with changing monomer concentration. Such effects can only be elucidated by the use of monomer and solvent specimens purified in different ways, as has been demonstrated very effectively by Zlamal, Ambroz, and Vesely (see Example 1). 

The methods of gel synthesis, immobilization of monomer conjugated enzyme, assay of enzyme activity, and determination of gel water content have been published elsewhere (4,5). A schematic of the synthesis is shown in Fig. 1. The gel compositions are identified as NA-100" (100% NIPAAm), "NA-95" (95% NIPAAm, 5% AAm), NA-90 (90% NIPAAm, 10% AAm) and "NA-85" (85% NIPAAm, 15% AAm) all are based on mole percents of monomers. Total monomer concentration was always 1.75 M. The experiment to determine the temperature dependence of enzyme activity was carried out after the enzyme reversibility experiment. 

The [Co(mnt)2f ( = 1, 2, 3) redox series was reexamined with the aid of ESR by Vlcek and Vlcek.101 Upon chemical or electrochemical oxidation of [Co(mnt)2]2-, an initial spin-triplet monomer [Co(mnt)2] formed. Slow dimerization led to the final oxidation product [Co(mnt)2]2 , which in the solid state or in noncoordinating solvents was diamagnetic. Solution spectra in DMSO showed an ESR signal due to [Co(mnt)2] THF solution spectra were identical but indicated three times less monomer concentration. A THF solution of [Co(mnt)2]2- underwent a reversible one-electron reduction process localized on the metal to yield a green solution of a Co1 complex, [Co(mnt)2]3-, with no ESR signal. 

Whereas the two-tank arrangement permits monomer feed profiles which vary smoothly in one direction, the three-tank scheme leads to inflections and concentration reversals as illustrated in Figure 4. Such reversals are useful in preparing hard-soft-hard, hydrophilic-hydrophobic-hydrophilic polymer variations and the like. In addition, three tank power feed has been useful as a means of calculating monomer inventory in copolymerization experiments (4). 

The polymerization proceeds without termination or chain transfer to give poly-norbomene with a narrow molecular weight distribution [35], After an induction period, the rate of monomer consumption (rate of polymerization) becomes constant, indicating a zero-order dependence on the monomer concentration. The induction period is caused by part of titanacycle 7 undergoing non-productive, but rapidly reversible, cleavage to norbornene and the titanium methylene complex, Eq. (17 a). 

Similar reactions occur with polystyryllithium and are particularly noticeable at high dilution. Isomerization of polyisoprenyllithium is quite a rapid process in tetrahydrofuran even at moderate concentrations since the original absorption at 287 m/x is converted to absorption at 330 mu during the polymerization process at room temperature. It is seemingly a reversible reaction and becomes conspicuous at low monomer concentrations. The original absorption band is re-formed on the addition of more isoprene. It was suggested (3) that the observed changes are caused by isomerization to the more stable symmetrical anion shown below ... 

In principle, dimerization is reversible and the equilibrium is rapidly established. The equilibrium concentration of radical ions derived from vinyl monomers is so small that it cannot be detected by ESR (for monomer concentrations 10-1 mol dm-3 it is < 10-7 mol dm-3). The rate constant of dissociation of the dimeric dianion of a-methylstyrene (aMeS)... 

The reversibility of propagation, or more specifically, the position of the equilibrium as determined by the ratio of the rate constants of propagation and depropagation is also independent of the mechanism. The equilibrium monomer concentration of monosubstituted alkenes such as styrenes and vinyl ethers are so low ([M] < 10-6 mol/L) at temperatures used for carbocationic polymerizations that the reversibility of polymerization can be neglected. 

This equation describes the relation between two parameters characterizing the reversible polymerization ceiling temperature and equilibrium monomer concentration. For any temperature there is a certain value of [ML [Eq. (48)]. If the starting concentration of monomer is lower than this value, i.e., [M0] < [ML, the polymerization will not proceed. If [M]0 is higher than [ML, polymerization will proceed until concentration of monomer reaches [ML (i-e., consumption of monomer will be equal to [M]0 - [ML). 

More complex situations have been treated analytically, such as reversible deactivation of initially active catalyst either by dimerization (2C C2) or bimolecular reaction (C + C 2C) [99]. Approach to equilibrium concentration of active centres would be accompanied by a fall in rate to a steady value (assuming constant monomer concentration and a stable catalyst) and a rise in molecular weight with time either to a maximum value or to a steady rate of increase dependent on the presence or absence of transfer reactions. The effect on average molecular weight of transfer reactions in which the catalyst entities possess two active centres has been calculated [100]. Although some ionic catalysts may behave in this way there is no evidence to indicate that these mechanisms apply to any known coordination catalyst. 

In 1971 Lyudvig et al. [97] reported an investigation of the polymerization of 1,3-dioxolane using precision purification of reactants and Et3 0 SbCl6 as initiator. Under these conditions polymerization with different initial monomer concentrations are reported to take place without any induction period. Lyudvig et al. report that the polymerization is first order with respect to both monomer and initiator concentration. Polymerization of 1,3-dioxolane is a reversible process. The final kinetic equation takes a form similar to eqn. (6). A UV spectroscopic method was used to investigate the nature of the active centre in the polymerization. Very briefly, what these workers found was that the maximum they observed for the polymerizing mixture was different from that which could be attributed to a simple cyclic oxonium ion. Hence they propose that the active centre has the polymeric tertiary oxonium ion structure... 

Micelle formation has been studied for sodium salts of fatty acids containing terminal double bonds using electrical conductivity . Here two critical micelle concentration points were observed of which the first point at 0.044 moles litres" was critical in terms of the number average degree of polymerisation of the polymers produced. At concentrations up to the second point the molecular weight change was significantly smaller. The photopolymerisation of acrylamide in reverse micelles was found to be first order with respect to monomer concentration whilst the order was found to depend upon the oil concentration in the... 

Initiation by pyridine is much slower and its consumption is very low. The rate of the ensuing polymerization increases with decreasing temperature implying a reversible, exothermic addition of pyridine to the monomer. Plots of [monomer],/[monomer]o versus time are convex the slopes at t = 0 increase with increasing initial monomer concentration - evidence of a slow second step of initiation ... 

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