Polymerization chain propagation stage

During this chain propagation stage of polymerization (whether free radical or ionic) there are four basic reactions between the active growing chains a aA and aaaB and the monomers A and B. 

As follows from the previous sub-section, polymerization of the epoxy compounds under the action of the TA is a particular case of their anionic polymerization induced by bases on the stage of chain propagation, this process obeys the well established principles of anionic polymerization of epoxy compounds. This problem has been discussed in a monograph 2) and two excellent reviews 25,146 ... 

Chain polymerization involves three stages initiation, propagation, and termination. The most important of the chain polymerization methods is free radical polymerization, in which the initiation step occurs by an attack on the monomer molecule by a free radical. A free radical is a reactive molecule possessing an unpaired electron and is usually formed by the decomposition of a relatively unstable molecule referred to as an initiator. In particular, those compounds containing peroxide bonds, (—O— O—), can produce free radicals by thermal decomposition, for example,... 

The beginning of the polymerization requires an activation of the monomers, whether it is thermal, photochemical, by free radicals, or ionic, which allows the binding of the activated monomer in the propagation stage this way, lineal or branched polymeric chains are produced, depending on the chemical characteristics of the monomer used. 

Transformation of Anionic Polymerization into Cationic Polymerization. Richards et al. (26. 27, 73-75) proposed several methods for the transformation of a living anionic polymeric chain end into a cationic one. Such a process requires three distinct stages polymerization of a monomer I by an anionic mechanism, and capping of the propagating end with a suitable but potentially reactive functional group isolation of polymer I, dissolution in a solvent suitable for mechanism (2), and addition of monomer II and reaction, or change of conditions, to transform the functionalized end into propagating species II that will polymerize monomer II by a cationic mechanism (73). 

As in the case of cationic polymerization, the presence of a metal atom can drastically change the electronic parameters of ring opening. In other words, the counterion may play the role of a symmetry switch , i.e. it can induce the reversion of stereospecificity of the active center. The possibility of anionic chain propagation on contact ion pairs via on intermediate stage of the formation of monomer-separated ion pairs was considered by Erusalimsky as far back as 1970 [61]. However, even 20 years later the author of the present paper does not attempt to discuss this problem in detail. 

It has been established [79, 80] that the initial stage of copolymerization in hydrocarbon media is determined only by the influence of diene. Styrene virtually does not take part in the reaction. As a result, the chain propagation rate is close to that in the polymerization of diene alone. Only after almost all the diene has been consumed, does the insertion of styrene into the growing chain begin. In this case the polymerization rate is higher than in styrene homopolymerization (presumably, because of the higher concentration of the monomer form of active centers). Finally, instead of the expected random copolymer, a block copolymer is formed in this system. Its formation mechanism has not yet been completely explained in the literature. 

After the production of the monosaccharide nucleotides which provides the basic material for biological polymerization of the glycosaminoglycan chain, the reactions to form the glycosaminoglycan chains (P4, S46) can be considered to occur in three stages chain initiation, chain propagation, and chain termination. 

There are several variants of the DCR theory differing from one another by the way in which account is taken of the physical factors influenced by the diffusion control on the description of elementary reactions rate. As a rule, the main factor influenced is the bimolecular chain termination process. The constant rate of chain termination is considered as a fnnction of the macroradical s mobility, their length [9-14], free volume [12,15-17] or characteristic viscosity of monomer-polymeric system. However, with the aim of explaining the auto deacceleration stage, the efficiency of initiation and constants of rate chain propagation are also considered to be functions of the macroradical s mobility [12,15,18]. 

Metal complexes bound to a polymer support most frequently induce ionic polymerization of olefins, dienes and acetylenes, and less commonly radical polymerization of vinyl-type monomers, acting at all reaction stages initiation, chain propagation and termination. Active sites for the addition of monomer molecules to the growing polymer chain can in many cases be regenerated yielding new polymer chains (catalysis via a polymer chain). 

Polymerization process consists of several consecutive stages. Initiation, chain propagation and termination are the main of them. Optimal conditions are necessary for proceeding of corresponding stages to produce polymer product of high quality. These conditions conclude... 

Chain-growth polymerization is the other important polymerization process besides the step reactions, discussed in Sect. 3.1. The condition for the reaction is given in Fig. 3.23. The mechanism of the reaction involves a small number of active molecules, M. Only these active molecules support further polymerization of the monomers, A. The chemical composition is maintained by a continued chain of the same reactions of AwithM. The illustration of the mechanism refers to a free radical polymerization. Initiation, propagation, and termination are the three stages of the reaction. 

Radical chain-growth polymerization consists of three stages chain initiation, chain propagation, and chain termination. 

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