Chain ends, active

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acryflc and methacryflc anionic polymerizations. Complex formation with the cation of the initiator (146) and enoflzation of the active chain end are among the more popular hypotheses (147). Unlike free-radical polymerizations, copolymerizations between acrylates and methacrylates are not observed in anionic polymerizations however, good copolymerizations within each class are reported (148). 

The apparent lack of dependence of the propagation reaction on the surface area of the sodium suggests that the reaction of a chlorine ended chain with sodium is probably fast and not the rate determining step. The rate determining process is probably the reaction of the sodium ended chain with the dichloride. This latter reaction is presumably not on the sodium surface because of the lack of dependence on the surface area. This is supported by the observation that if the sodium is allowed to settle part way through the reaction most of the polymer appears to be in the solution and not absorbed on the sodium surface via the longlived active chain ends. 

In the co-end of the chain, the dissociation always occurs at the bond which is indicated by the arrow A. The dissociation of this C-S bond at the A position gives a more-reactive carbon-centered radical and a less-reactive polymer thiyl radical, which leads to the termination of the active chain ends. In the case of the a-chain end, however, there is a possibility that the bond at the C position dissociates to produce a diethylaminothiocarbonyl radical and a thiyl radical in addition to the preferable bond scission at B. Such dissociation at C may not induce living radical polymerization [76]. 

We have therefore modified the kinetic expression to handle the 13c-NMR data. For the special case where the concentration of active chain ends Jpf] is constant, the derived kinetic expression is reduced to a very simple form ... 

Polymerization of a mixture of oligo(3-tolylsilsesquioxanes) involves consecutive addition of the oligomer to an active chain end of the polymer molecule. This is a reversible process, the equilibrium depending greatly on temperature. Thus, above 300 °C, oligo(3-tolylsilsesquioxanes) are mainly formed An electron-microscopic examination of the polymer structure has revealed the formation of a specifically... 

One additional item of experimental evidence for the dimeric association of polyisoprenyl lithium was provided by a light scattering study (21), in n-hexane at 25°C., where it was found that the molecular weight of the terminated polymer was very close to one-half that of the active polymer. All of these data seem to leave no doubt that the active chain ends in the organo-lithium polymerization of styrene, isoprene and butadiene, in non-polar solvents, are associated as pairs, at least at chain-end concentrations of 10 2 M or less. This conclusion has also been supported by data obtained in four other laboratories (22,... 

Ionic polymerizations are remarkable in the variety of polymer steric structures that are produced by variation of the solvent or the counter ion. The long lived nature of the active chain ends in the anionic polymerization of diene and styrene type monomers lends itself to studies of their structure and properties which might have relevance to the structure of the polymer produced when these chain ends add further monomer. One of the tools that, may be used in the characterization of these ion pairs is the NMR spectrometer. However, it should always be appreciated that, the conditions in the NMR tube are frequently far removed from those in the actual polymerization. Furthermore NMR observes the equilibrium form on a long time scale, and this is not necessarily that form present at the moment of polymerization. 

The most studied activated chain ends are the macromolecular free radicals. Vast numbers of monomers have been investigated... 

It is generally agreed that propagation in the cationic polymerization of cyclic ethers occurs after nucleophilic attack by the monomer oxygen atom (equation 3). Therefore, many authors attempt to explain their copolymerization data by noting that the more basic monomer has the higher reactivity with the active chain end. The order of basicity which has been established (36, 38) is ... 

The concentrated solution viscosity measurement yields the weight-average degree of association of active chain ends rather than the more conventional number-average (mole fraction) value. However, the calculation of the equilibrium constant for association, K, can be accomplished if Mw and the heterogeneity index of the polymer sample are known. The latter parameter can be determined via postpolymerization characterization. 

The presence of a hydrophobic-hydrophilic interface can dramatically change the reaction conditions. The hydrophobic core will selectively absorb hydrophobic species from the solution (Fig. 12), and this will result in a redistribution of monomer concentrations between the core and bulk solution. Because the probability of attachment for each comonomer is determined by its concentration in a relatively small reaction volume near an active chain end, the active center inside the hydrophobic core will mainly attach more hydrophobic species on the other hand, when the active center is located on the globule surface, it will mainly attach polar (soluble) monomers. In this way, the two-layer globule will grow, retaining its core-shell structure with a predominantly hydrophobic core and a hydrophilic outer envelope (see Fig. 12). 

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