Chain termination in free radical polymerization

The formation of polymers with terminal LM during chain termination in free-radical polymerization is based on the ability of anthracene and some of its derivatives to participate in homolytical reactions It was established that anthracene-containing compounds interact with macroradicals which are generated in free-radical... 

Problem 6.28 The bimolecular chain termination in free-radical polymerization is a diffusion-controlled reaction that can be treated as a three-stage process (North and Reid, 1963 Odian, 1991), described below. 

Fig. 57. Chain transfer in free radical polymerization with a side group of the-dead polymer. Note After termination of the radical one side chain and a free linear chain are obtained.

Nonlinear polymer formation in emulsion polymerization is a challenging topic. Reaction mechanisms that form long-chain branching in free-radical polymerizations include chain transfer to the polymer and terminal double bond polymerization. Polymerization reactions that involve multifunctional monomers such as vinyl/divinyl copolymerization reactions are discussed separately in Sect. 4.2.2. For simplicity, in this section we assume that both the radicals and the polymer molecules that formed are distributed homogeneously inside the polymer particle. 

Reaction (O 23.105) describes a termination reaction with an impurity X present. Impurity termination in free-radical polymerization can also take place, but in free-radical chain... 

If the reactions involved in free-radical polymerization proceeded precisely in the maimer outlined above (initiation, propagation, termination), in all instances, one would obtain long and linear polymer chains. However, in free radical polymerization, linear chains are, in fact, not formed. Instead, extensive branching is observed particularly with monomers such as ethylene. In this context a slight clarification of branching is required. Thus, for a polymer obtained from a substituted vinyl monomer regular side groups are present (Fig. 2.3). 

Chain-Length Dependent Bimolecular Termination in Free-Radical Polymerization Theory, Validation and Experimental Application of Novel Model-Independent Methods... 

Chain-Length Dependent Bimolecular Termination in Free-Radical Polymerization... 

Chain-length dependent bimolecular termination in free-radical polymerization ... 

Buback M, Muller E, Russell GT. SP-PLP-EPR smdy of chain-length-dependent termination in free-radical polymerization of n-dodecyl methacrylate, cyclohexyl methacrylate, and benzyl methacrylate evidence of composite behavior. J Phys Chem A 2006 110 3222-3230. 

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. 

Although this mechanism is an oversimplification, it does give the basic idea. Chain termination is more complicated than in free radical polymerization. Coupling and disproportionation are not possible since two negative ions cannot easily come together. Termination may result from a proton transfer from a solvent or weak acid, such as water, sometimes present in just trace amounts. 

As in so many things in this field, if you want to work through the arguments yourself, you cannot do better than go to Flory— see Principles of Polymer Chemistry, Chapter EX. Stockmayer s equation illustrates the point we wish to make with dazzling simplicity as f the number of branches, increases, the polydispersity decreases. Thus for values of/equal to 4, 5 and 10, the polydispersity values are 1.25, 1.20 and 1.10, respectively. Note also that for / = 2, where two independent chains are combined to form one linear molecule (Figure 5-28), the polydispersity is predicted to be 1.5. Incidentally, an analogous situation occurs in free radical polymerization when chain termination is exclusively by combination. 

The bimolecular termination reaction in free-radical polymerization is a typical example of a diffusion controlled reaction, and is chain-length-depen-dent [282-288]. When pseudobulk kinetics appUes, the MWD formed can be approximated by that resulting from bulk polymerization, and it can be solved numerically [289-291]. As in the other extreme case where no polymer particle contains more than one radical, the so-caUed zero-one system, the bimolecular termination reactions occur immediately after the entrance of second radical, so unique features of chain-length-dependence cannot be found. Assuming that the average time interval between radical entries is the same for all particles and that the weight contribution from ohgomeric chains formed... 

The number-average degree of polymerization can be obtained from the rates of propagation (eqn 10.65) and chain breaking (sum of eqns 10.66 and 10.67) as in free-radical polymerization with termination by chain transfer to a transfer agent (see eqn 10.42) ... 

The distribution is as in free-radical polymerization with termination by disproportionation or terminating chain transfer (eqn 10.45 in Section 10.3.4) and, with increase of the progression factor as conversion increases, in step-growth polymerization of bifimctional monomers (eqn 10.19 in Section 10.2.3). According to eqn 10.81, the progression factor is... 

The above kinetic expressions illustrate some basic differences between cationic and free radical processes. In the cationic polymerization, the propagation rate is of first order with respect to the initiator concentration, whereas in free radical polymerization it is proportional to the square root of initmtor concentration (Eq. [34]). Furthermore, the molecular weight (or DP) of the polymer synthesized by the cationic process is independent of the concentration of the initiator, regardless of how termination takes place, unlike free radical polymerization where DP is inversely proportional to [I] in the absence of chain transfer (Eq. [35]). 

The basic steps in free-radical polymerization are initiation, propagation, chain transfer, and termination. 

Catalytic chain transfer is a versatile tool that complements other means of polymerization. It allows the synthesis of the large variety of structured polymers shown in Figure 11. The primary outlet for CCT is to control molecular weight in free-radical polymerizations without the use of stoichiometric chain terminators (sections 1—3). All of the products can be considered to be monofunctional in that they are all terminated by unsaturation. The unsaturation... 

Scheme 15 summarizes the current understanding of LCo in free radically polymerized vinylic monomers. It differs from the previously available scheme25 in that the formation of the radical adduct with LCo is not the first step of CCT but rather is a reaction that poisons the catalyst. Second, the scheme includes catalytic termination arising through the reaction of LCo1 with radical chains. Its should be emphasized that essentially all of the reactions are substantially reversible. The directions of the arrows indicate the course of productive transformations. 

Both cationic and anionic polymerizations proceed via chain mechanisms, like free radical polymerizations. In free radical processes the initiator is incorporated into the polymer chains, as they are formed. But in ionic polymerizations the termination processes that produce dead polymer chains... 

The nature of free-radical polymerization has traditionally hindered attempts to produce an ideal living free radical polymerization technique. It is very difficult to prevent chain transfer and termination reactions in free-radical polymerizations and although several methods have afforded polymers with very low polydispersities < 1.1), these approaches are often referred... 

This relation is of fundamental importance in free-radical polymerization since the kinetic chain length decreases with an increase in the rate of initiation. Thus an attempt to accelerate polymerization by adding more initiator will produce a faster reaction but the polymer will have shorter chains. This can also be seen as a consequence of the steady-state approximation in a linear chain reaction since the rate of termination is equal to the rate of initiation and, if the rate of termination increases to match the rate of initiation, the chains must necessarily be shorter. 

Termination will occur when the carbocation undergoes reaction with nucleophilic species other than monomer to produce a dead chain and no re-initiation. Since cationic polymerizations are carried out with high-purity reagents and under rigorous conditions this reaction is much less likely than chain transfer to monomer. The mutual repulsion of the charged polymerization sites ensures that bimolecular termination cannot occur (unlike in free-radical polymerization, where this is the most probable termination route). Recombination of the cation with the counter-ion will occur, and these termination reactions are often very specific to the chemistry of the initiator. 

Though ionic polymerization resembles free-radical polymerization in terms of initiation, propagation, transfer, and termination reactions, the kinetics of ionic polymerizations are significantly diflFerent from free-radical polymerizations. In sharp contrast to free-radical polymerizations, the initiation reactions in ionic polymerizations have very low activation energies, chain termination by mutual destruction of growing species is nonexistent, and solvent effects are much more pronounced, as the nature of solvent determines whether the chain centers are ion pairs, free ions, or both. No such solvent role is encountered in free-radical polymerization. The overall result of these features is to make the kinetics of ionic polymerization much more complex than the kinetics of free-radical polymerization. 

Effect of Solvents and Reaction Conditions. The term "solvent" is customarily used rather loosely in polymerization reactions because such "solvents" may refer either to the actual medium in which the reaction is carried out, or to trace materials present in the medium. Hence, the term really encompasses any component other than monomer and initiator. Thus, in free-radical polymerization, the role of the solvent is limited to "interfering" with the normal propagation reaction, either through chain transfer or even by termination (inhibition or retardation). Either of these events can affect only the chain length or the overall rate, or both. 

In a recent publication Okamura et ah (12) describe similar results in a different system. It is believed that the unusual rate increase observed in these various systems which are chemically so different is caused by the physical state of the reaction medium at temperatures a few degrees above Tg. The high viscosity of this gel-like medium presumably favors chain propagation in its competition with termination. This effect, which is kinetically similar to the "gel-effect in free radical polymerizations, can only arise if the termination step (charge recombination) becomes diffusion controlled. The latter process would arise if both ionic species involved in the reaction were of macromolecular size. This is undoubtedly true for the growing chain, but the mobility of the counter ion should only be significantly reduced in such a medium if it is of a polymolecular structure, involving perhaps a voluminous solvation cluster. 

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