Redistribution polymerizations

Several other synthetic techniques have also been described. Redistribution polymerization was outlined in COMC II (1995) (chapter Organopolysilanes, p 99) and proceeds by phosphonium salt-catalyzed redistribution of chlorodisilanes.133 Disproportionation polymerization, which is a similar process, has been described for the formation of polymers by ethoxide-catalyzed disproportionation of alkoxydisilanes via silyl anion intermediates.134 These procedures give rise to network polymeric products of rather low molecular weight (see below, Section 3.11.7.1). 

There are a number of published reviews on the synthesis of poly-silazanes. - " The present discussion is limited to a brief history and development of oligo- and polysilazane synthesis. The methods of synthesizing polysilazanes include (1) polymerization by aminolysis (or ammonolysis) of chlorosilanes, (2) ringopening polymerization of oligomeric cyclosilazanes, (3) deamination and condensation of bis- and tra-aminosilanes, (4) redistribution polymerization of chlorosilanes and aminosilanes, and (5) catalyzed dehydrocoupling of diamines and organosilanes. 

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

The halogen displacement polymerization proceeds by a combination of the redistribution steps described for oxidative coupling polymerization and a sequence in which a phenoxide ion couples with a phenoxy radical (eq. 11) and then expels a bromide ion. The resultant phenoxy radical can couple with another phenoxide in a manner that is analogous to equation 11 or it can redistribute with other aryloxy radicals in a process analogous to equations 7 and 8. 

Typical initiators for living anionic polymerization of siloxanes include conventional organoalkali compounds and lithium siloxanolates22). Initiators containing lithium counterions are preferable to sodium or potassium counterions due to the lower catalytic activity of lithium in siloxane redistribution reactions. Living anionic polymeriza-... 

The situation is quite different when actin is polymerized under sonication in the presence of ATP. In this case, the polymerization curve cannot be described by equation (4). At a high actin concentration, overshoot polymerization kinetics are observed, with a maximum and subsequent decrease to a lower stable plateau (Carlier et al., 1985). The final amount of polymer is the same as that obtained when sonication is applied to F-actin that had polymerized spontaneously without sonication. Conversely, when sonication is stopped, repolymerization accompanies the spontaneous length redistribution to a population of less numerous, but longer filaments. 

Others Chemical vapor deposition (CVD) polymerization, redistribution, and disproportionation 573... 

Fig. 4.7 A schematic representation of a cationic displacement along a polymeric chain above its Tg. (a) An initial activated step (ft) allows the formation of an interstitial pair, the migration of which (c) and (d) is assisted by local free volume redistribution.

Olefin metathesis (olefin disproportionation) is the reaction of two alkenes in which the redistribution of the olelinic bonds takes place with the aid of transition metal catalysts (Scheme 7.7). The reaction proceeds with an intermediate formation of a metallacyclobutene. This may either break down to provide two new olefins, or open up to generate a metal alkylidene species which -by multiple alkene insertion- may lead to formation of alkylidenes with a polymeric moiety [21]. Ring-opening metathesis polymerization (ROMP) is the reaction of cyclic olefins in which backbone-unsaturated polymers are obtained. The driving force of this process is obviously in the relief of the ring strain of the monomers. 

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