Propagating site, active

The first viewpoint contradicts the autocatalytic character of the reaction, conductometric measurements in the polymerization system and some other facts (see below). Scheme (33) can be considered as completely experimentally substantiated. The following important proofs were obtained A direct experimental discovery of a quaternary ammonium alcoholate in the reaction system, 42) a full agreement of the nature of the active propagating site with all the existing kinetic and structural data l4,149 153 157 I58) establishment of the ionic behaviour of the propagating sites by comparison of the kinetic curves of the process with the character of the electric... 

Thus, in the presence of alcohols or other proton donors the polymerization of epoxy compounds under the action of TA proceeds according to the anionic mechanism to give quaternary ammonium alcoholate as the active propagating site [Scheme (33)]. 

This means that the active propagating sites such as free alkoxy ions and ion pairs are solvated with the hydroxyl groups. This must lead to an increase in the reactivity of the solvated ion pair as compared with that of the contact ion pair and decrease in the free ion reactivity, i.e. ultimately to the levelling off of the reactivity differences of these particles. 

Both reactions lead to the appearance of end hydroxyl and unsaturated groups in the polymer. The accumulation rate of these groups is expected to be controlled by the concentration of the active propagating sites (see Fig. 13) in accordance with Scheme (43). 

The presence of DMAP is essential to control the polymerization process. In the absence of DMAP, the polydispersity index of the polymer formed increases to 1.3, and the gel permeation chromatography (GPC) profile gives a small new peak whose MW is twice that of the major product. The high stability of the propagation centers allows the isolation of poly(PA) having active propagation sites, which can sequentially polymerize different monomers to give precisely controlled block copolymers. 

Model based on the variation of the number of active" coordination sites at the catalyst surface. The growth of tubules during the decomposition of acetylene can be explained in three steps, which are the decomposition of acetylene, the initiation reaction and the propagation reaction. This is illustrated in Fig. 14 by the model of a (5,5) tubule growing on a catalyst particle ... 

Figure 3. Termination of Pu(IV) polymer network propagation by attachment of V02 r to active -OH sites.

In an aftermath of large scale natural or man-made disasters there is likely to be lack of donor sites for SSG harvest to treat extensive bums. Use of isolated keratinocytes which can be actively propagated in vitro [24] and applied in vivo along with dermal substitutes may potentially solve the problem of epithelial deficiency and scarring complications, thus reducing the rates of morbidity and mortality in bums critical care. 

At the end of considerations of the polymerisation of cyclic ethers with coordination catalysts, one should emphasise the versatility of catalysts used for polymerisations, which is manifested in the formation of various active polymerisation sites capable of promoting propagation according to various mechanisms. 

Determination of propagation rate constants in cationic (and in anionic) systems is complicated by the simultaneous occurrence of different types of propagating sites. In olefin polymerizations, some portion of the active centers may exist as free ions and others as ion pairs of varying degrees of solvation. In the solvents in which cationic polymerizations are normally carried out, the polymerization is mainly due to free ions. In low dielectric constant media like benzene or hydrocarbon monomers, however, ion pairs will dominate the reaction. 

With the continual discovery of new reactions leading to the formation of polymers, and with the increased understanding of the mechanisms of those reactions, it is natural that certain definitions of descriptive terms will change with time. The term polycondensation is used here to denote those polymerization reactions which proceed by a propagation mechanism in which an active polymerization site disappears every time one monomer equivalent reacts. This may be illustrated for one propagation step in polyesterification by... 

This type of initiation is known as monofunctional initiation as it produces one active (ionic) site for propagation. Propagation takes place by the addition of monomer to the ionic site ... 

The chain can be terminated by a chain transfer reaction with the solvent or by reaction with an impurity in the reaction mixture. If the solvent cannot donate a proton to terminate the chain and if all impurities that can react with a carbanion are rigorously excluded, chain propagation will continue until all the monomer has been consumed. At this point, the propagating site will still be active, so the polymerization reaction will continue if more monomer is added to the system. Such nonterminated chains are called living polymers because the chains remain active until they are killed. Living polymers usually result from anionic polymerization because the chains caimot be terminated by proton loss from the polymer, as they can in cationic polymerization, or by disproportionation or radical recombination, as they can in radical polymerization. 

Chain-growth polymers are made by chain reactions— by the addition of monomers to the end of a growing chain. These reactions take place by one of three mechanisms radical polymerization, cationic polymerization, or anionic polymerization. Each mechanism has an initiation step that starts the polymerization, propagation steps that allow the chain to grow at the propagating site, and termination steps that stop the growth of the chain. The choice of mechanism depends on the stmcture of the monomer and the initiator used to activate the monomer. In radical polymerization, the initiator is a radical in cationic polymerization, it is an electrophile and in cationic polymerization, it is a nucleophile. Nonterminated polymer chains are called living polymers. 

Initiation and propagation take place by electrophilic addition of the monomers to the active cationic sites. Therefore, the monomer must be nucleophilic and its substituents should be able to stabilize the resulting positive charge. As a result, the reactivity of monomers is roughly proportional to the electron-donating ability of their substituents, as can be seen below. 

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