Radical chain polymerization definition

Once a compound has been shown to polymerise, the most interesting question for me is What is stopping the chains from growing When that question has been answered we must know much about the kinetics of the system and at least a little about its chemistry. Before entering into an account of the reactions which stop chains from growing, it is important to make once again a clear distinction between termination and transfer reactions. There is no reason for not adhering to the radical chemist s definition of termination a reaction in which the chain-carrier is destroyed. In cationic polymerizations there are two main types of termination reaction ... 

PMMA has been known, since the 1950s, to show the ESR spectrum with five intense and four weak hyperfine lines (see the insert of Fig. 9), when it is irradiated with y-rays at room temperature. After a long history of the study on this anomalous ESR spectrum [32-34], the interpretation is now almost settled that it is due to the propagating-type radical -CH2-C(CH3)COOCH3 (the radical expected to form during the radical polymerization of methyl methacrylate) [35, 36], Although the formation of this radical is a definite proof of the radiation-induced scission of the PMMA main-chain, the previous ESR studies have failed to elucidate the mechanism for the formation of this radical. 

This broad definition allows fw a variety of polymerization mechanisms, and it is formally possible for dispersion polymerizations to be anionically or cation-ically initiated, or to be stepwise rather than chain polymerizations. These types of dispersion polymerizatitm are less commcn and are outside the scope of this chapter, which is devoted largely to radically initiated systems. A recent review [2] covers polycondensations by dispersion methods. 

Modified from the definition in [1,2]. secondary radical (in a chain polymerization)... 

Note 6 There exist, exceptionally, some polymerizations that proceed via chain reactions that, according to the definition, are not chain polymerizations. For example, the polymerization HS-X-SH + H2C = CH-Y-CH = CHj (-S-X-S-CHj-CH2-Y-CH2-CH2-) proceeds via a radical chain reaction with intermolecular transfer of the radical center. 

If we multiply the time elapsed per monomer added to a radical by the number of monomers in the average chain, then we obtain the time during which the radical exists. This is the definition of the radical lifetime. The number of monomers in a polymer chain is, of course, the degree of polymerization. Therefore we write... 

Many emulsion polymerizations can be described by so-called zero-one kinetics. These systems are characterized by particle sizes that are sufficiently small dial entry of a radical into a particle already containing a propagating radical always causes instantaneous termination. Thus, a particle may contain either zero or one propagating radical. The value of n will usually be less than 0.4. In these systems, radical-radical termination is by definition not rate determining. Rates of polymerization are determined by the rates or particle entry and exit rather than by rates of initiation and termination. The main mechanism for exit is thought to be chain transfer to monomer. It follows that radical-radical termination, when it occurs in the particle phase, will usually be between a short species (one that lias just entered) and a long species. 

Such a mechanism is open to serious objections both on theoretical and experimental grounds. Cationic polymerizations usually are conducted in media of low dielectric constant in which the indicated separation of charge, and its subsequent increase as monomer adds to the chain, would require a considerable energy. Moreover, termination of chains growing in this manner would be a second-order process involving two independent centers such as occurs in free radical polymerizations. Experimental evidence indicates a termination process of lower order (see below). Finally, it appears doubtful that a halide catalyst is effective without a co-catalyst such as water, alcohol, or acetic acid. This is quite definitely true for isobutylene, and it may hold also for other monomers as well. 

A kinetic scheme is most easily worked out for the pure polymerization.106 It is useful first to make certain simplifying approximations and definitions. We replace Scheme 6 by Scheme 8, where In is an initiator producing radicals R , M is the monomer, Mn is the growing polymer chain, and M —Mm is combination product and Mn( H) are disproportionation products. If all radicals R produced by the initiator were available to start chains, we could write, from the first two reactions, kinetic Equation 9.40 for rate of change of R-concentration. Because of cage recombination, only some fraction f of the... 

In chain reactions involving short chains an increase of the initiator concentration above a definite limit is useless. In the limit, at a chain length equating 1, the reaction transforms from a chain to a free radical type, and H202 transforms from the initiator to one of the reagents. Dolgoplosk and Tinyakova [33] note that as H202 concentration increases, polymerization is terminated, whereas low concentrations of the substance initiate this reaction. 

Some quantum yields of radical polymerization (< >p for definition see Eq. (2b)) with the model monomer MM A are presented in Table 5. It is obvious that a variety of parameters acts on this value. Mainly, the participation of initiator molecules and of primary radicals on chain termination are responsible for the differences in the < >p-values. 

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