Termination reactions in free-radical polymerization

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 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 section emphasizes initiation and termination steps in alkene polymerization. The main terminating reactions in free-radical polymerization are the coupling of two radicals and disproportionation. Coupling of two radicals pairs the odd electrons and stops chain growth. 

The termination reaction in free-radical polymerizations of the esters of acrylic and methacrylic acids takes place by recombination and by disproportionation. Methyl methacrylate polymerizations, however, terminate at 25 °C predominantly by disproportionation. ... 

The radical-radical termination reaction in free radical polymerization is the most complex reaction in the polymerization process (298,299). Its termination rate coefficient kt is influenced by a multitude of different factors, which are not easily separated. Only within the last 15 years new methods have become available that allow for an accurate measurement of this rate coefficient. The scatter of the termination rate coefficients given in the Polymer Handbook (173) reported for the same monomer at the same reaction temperature is a direct manifestation of the influence of these various parameters on kt. This appreciable disagreement is partly explainable by the frequent use of incorrect values for the propagation rate coefficient k, which is always needed to determine kt from the coupled form of the two coefficients. However, the situation has improved greatly with the invention of the PLP-SEC method. 

The termination reaction of free radical polymerization is a typical example of an intermacromolecular diffusion controlled reaction.3 Photophysical studies carried out in the 1980 s demonstrated for the first time that the reaction is solvent- and molecular weight-dependent. The experiments involved triplet quenching of probes attached to polymer chain ends. A benzil group was linked to the end of one PS sample (PS-B) and an anthryl group was linked to the end of a second PS sample (PS-A). The quenching rate coefficient kq of the benzil phosphorescence by anthryl groups is given by Eq. (3.26), where r0 is the lifetime of benzil phosphorescence in the absence of anthryl and ris the benzil phosphorescence lifetime in the presence of anthryl in concentration [A],... 

The termination process in free-radical polymerization is caused, as was shown early in this chapter, by one of three types of reactions (1) a second order radical-radical reaction, (2) a second order radical-molecule reaction, and (3) a first order loss of radical activity. 

In this thesis the chain-length dependence of termination reactions during free-radical polymerization has been investigated. Interest in the kinetic parameters describing this process, primarily arises from the profound effect that termination reactions have on the overall kinetics of polymerization processes, on the MWD of the final product and therefore also on the final product properties. 

Herein is reported a summary of recent studies of diffusion-controlled termination and propagation reactions in free-radical polymerization which are pertinent to emulsion polymerization kinetics. Also included are discussions of the effects of diffusion-controlled termination and propagation on molecular weight and branching development with particular reference to the synthesis of poly (vinyl chloride) at high conversions and to the significant reduction of thermal stability of PVC which occurs. 

We now know that the termination reaction in free-radical addition polymerization is diffusion controlled right down to 0% conversion. This is not really surprising given the extremely high chemical reactivity of free-radical pairs and the size of the reacting species. 

Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

Radicals being neutral species tend to react together. Indeed, the most common side reactions in free-radical processes involve the formation of adducts between two radicals, via combination or disproportionation. These unwanted termination steps usually occur much faster than the desired reactions between radicals and substrates. Thus, the key to control in both radical addition and polymerization procedures consists in lowering the concentration of transient radical species. This will minimize the side reactions between radical species, yet the kinetics of the useful reactions will also be affected. 

These early examples of macromonomer synthesis illustrate some of the chief principles that have subsequently led to a great variety of methods. Ionic polymerization is often preferred because of the long lifetime of the active sites. Transfer reactions in free-radical processes are also used quite often, yielding both acceptable molecular weights and an adequate proportion of terminal functions. 

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. 

Although ATRP behaves differently from conventional free radical polymerization, the fundamental reactions involved are very similar and include initiation, propagation, transfer and termination (see Scheme 6). Since chain termination does not occur in a truly living polymerization, the living character of the chains in ATRP derives from the fact that chain propagation is first order with respect to radical concentration and irreversible bi-molecular termination is second order. As such, the concentration of the radicals is kept very low, the rate of bi-molecular termination is greatly reduced, and typically less than 10% of all of the chains will terminate. Unlike conventional free radical polymerization, where the rate is dictated by a steady state between the initiation and termination rates, the rate and concentration of propagating radicals in ATRP is controlled entirely by the equilibrium between activation and deactivation [255]. 

In free-radical polymerization with termination by coupling, there are three possible termination steps reaction of end groups -MA with -MA, of -MA with —Mb, and of -MB with — MB. Each eliminates two reactive end groups. Leaving the possibility open that all steps contribute significantly, the termination rntc is... 

Initiation is the slow reaction in the initiation-propagalion-termination sequence in free radical reactions whereas selected initiation reactions in anionic systems can be very rapid compared to the subsequent propagation reaction. This facilitates the preparation of anionic polymers with narrow molecular weight distributions, if the polymerization is conducted carefully. 

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... 

The contribution of the homolytical substitution to the total sum of reactions occuning in free-radical polymerization in the presence of anthracene of its derivatives depends on the nature of the monomer and the anthracene-containing compound and on the polymerization conditions. It has been diown that the macroradicals of the methacrylic esters do not interact with anthracene under usual conditions On the other hand, in the polymerization of styrene, substitution with the formation of terminal LM of type II (Scheme 1) proceeds only if one or both meso-positions of the anthracene ring are free ... 

A limited number of attempts have been made to set up a general mechanistic scheme describing cationic systems in terms of fundamental reactions, in a similar manner to that used in free radical polymerizations, and to derive generally applicable kinetic equations [3—4]. Because of the individuality of each cationic system, however, this approach has met with little success, and there has been a greater tendency towards treating each polymerization in isolation for detailed kinetic analysis. It is possible, however, to postulate at least token schemes which can be used as a guide. After the pre-initiation equilibria, polymerization can be considered in terms of classical initiation, propagation, transfer and termination reactions, i.e. for vinyl monomers... 

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. 

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. 

Kinetics models are useful for designing commercial reactors and for studying the fundamental mechanisms of the important reactions. The free-radical polymerization that takes place in emulsion systems is characterized by three main reactions initiation, propagation, and termination. Various radical transfer reactions can also be important. The rate of polymerization for bulk, solution, and suspension processes can be expressed as shown by Equation 2 ... 

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. 

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. 

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