Propagation cationic

By way of example, the distances and R2 can be assumed as variables describing a cationic propagation step in the model system shown in Fig. 1. 

Fig. 1. A simple model system of a cationic propagation step...

The model process Eq. (15) has been studied by means of the MINDO/3 method to clarify the energetic conditions during the formation of cyclic reactive intermediates in cationic propagation of alkoxy-substituted monomers. The enthalpies of formation in the gas phase AH°g of both the alternative structures e and /were supplemented by the solvation energies Eso]v for transition into solvent CH2C12 with the assistance of the continuum model of Huron and Claverie which leads to heats of formation in solution AH° s. Table 13 contains the calculated results. 

During the transfer from the gas phase to the solvent CH2C12, the formation, of NO is so strongly preferred that the following two events can take place the chain termination due to formation of carbinol end groups and nitrations as successive reactions, both of which could be experimentally detected to be disturbances of the cationic propagation 122). 

The propagation reactions of the growing cationic chain end with the monomer ethene have already been discussed in part 4.3. The reaction enthalpies of the corresponding propagation steps show different tendencies for the gas phase and solution, when the cationic chain end is lengthened. However, as the monomer is increased in size and the cationic chain end remains the same, then the tendencies for the gas phase and solution correspond to each other. This is an indication that the solvent influence on the cationic propagation reaction is determined by the nature of the cations in question and their solvation. 

M) the polymerization proceeds entirely by cationic propagation as evidenced by a unimodal MWD, which is characteristic of the cationic MWD. The extent of the ionic contribution relative to radical polymerization also increases with increasing radiation intensity, perhaps due to the rapid depletion of water at the higher intensities. Other evidence for the ionic contributions include kinetic and activation energy data. 

Transfer of a P-proton from the propagating carbocation is the most important chain-breaking reaction. It occurs readily because much of the positive charge of the cationic propagating center resides not on carbon, but on the P-hydrogens because of hyperconjugation. Monomer, counterion or any other basic species in the reaction mixture can abstract a P-proton. Chain transfer to monomer involves transfer of a P-proton to monomer with the formation of terminal unsaturation in the polymer. 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer of the cationic propagating center can occur either by electrophilic aromatic substituation or hydride transfer. Intramolecular electrophilic aromatic substituation (or backbiting) occurs in the polymerization of styrene as well as other aromatic monomers with the formation of... 

The effect of water is to be explained. Cationic polymerization of VCZ is generally not inhibited by water. The monomer is very basic and can well compete for the carbonium ion with water. Since the polymerization is readily initiated by a proton, water acts as chain transfer agent rather than inhibitor. Although the reactivity of the carbonium ion depends certainly on the nature of the counter-ion, as will be discussed later, water seems to act as an efficient chain transfer agent, at least in the present system. The Ion-radical might consequently be converted to a proton so that the cationic propagation could even be promoted in the presence of water. 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

A related pseudo-cationic propagation via a covalent perchlorate ester has bee proposed for the polymerisation of styrene by perchloric acid (135), though the situation in this system is extremely complex (136). 

A comparison of Figs. 5 and 6 shows that between the anionic polymerization of styrene and the transition metal polymerization of styrene there is a region of poor catalysts which produce either atactic or no polymers. It has been shown by the substitution effects of the reaction that both the styryl anion and the styryl cation propagate on the... 

Bestian and Clauss proposed that the polymerization occured with isomerization on a cationic alkyltitanium species or one of its associated forms. Propagation by anionic and cationic species accounts for their results more easily. Most of the oligomer low molecular weight product was from anionic type propagation (Equation 8). However, the 7.8% of the dimer and the 30% of the trimer fractions were produced by cationic propagation of the n-butyl group (Equation 9). 

Overberger and Jarovitzky (37) proposed the formation of cationic propagating species through diproportionation of the titanium aluminum complex into RjAlClJ and a positive titanium species. Propagation of this ion pair, before temporary chain termination by alkyl migration, produces the same possibility for steric control by the end of the polymer chain. ... 

Another type of ionic species was proposed by Uelzmann (92). Uelzmann suggested that from trialkylaluminum and titanium tetrachloride, TiClg)+ and (R3A1C1) caused cationic initiation on the titanium followed by an anionic propagation on the aluminum ion. Bestian and co-workers (70) proposed similar cationic intermediates which propagate by anionic shifts. These steps are the opposite of the anionic initiation and cationic propagation proposed in this review for the butene-1 polymerization. 

Artificial cation channels could give fundamental information on the mechanism of cation flow and channel conduction [6.69, 6.70]. A solid-state model of cation transfer inside a channel is provided by the crystal structure of the KBr complex of 27c (Y = Y = CH3), which contains stacks of macrocycles with cations located alternately inside and above a macrocyclic unit, like a frozen picture of cation propagation through the channel defined by the stack [6.71]. 

Cationic propagation involving mononuclear catalyst species is illustrated schematically as follows [1] ... 

The nature of the polymer is diagnostic of the nature of the initiating intermediate. The homopolymer of the donor olefin in the presence of an acceptor monomer can only be obtained by a cationic propagation mechanism, and not by a free-radical mechanism. Correspondingly, the homopolymer of the acceptor olefin... 

To broaden our understanding of the chemical behavior of these novel monomers, it would be appropriate to try anionic polymerizations of captodative monomers. Inasmuch as sulfur is able to stabilize adjacent carbanions, a-alkylthioacrylates and -acrylonitriles should respond well to anionic initiators. Cationic polymerization of certain captodative monomers may also be of interest as alkylthio- and cyano-substituents can stabilize a cationic propagating center. 

These are excellent initiators and centres of the cationic propagation of polysiloxane chains. 

Cationic propagation of siloxane monomers has been known for a long time [322] and has been studied extensively [323, 325]. However, the underlying mechanism is only just emerging. The complex with oxonium oxygen is almost certainly the transition form. Writing the reaction as... 

Table 13 Rate Constants of the Cationic Propagation of Various Styrenes Measured by Stopped-Flow Method ...

Table 15 Rate Constants of Cationic Propagation Estimated from y-Radiation Experiments...

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