Polymerizations, chain growth

Many polymerizations are initiated by free radicals, especially alkoxy radicals formed by thermal decomposition of peroxides. A general mechanism for olefin free radical polymerization with initiation, propagation, and termination is given in Fig. 14.1. 

After the initial reaction of a radical with the first monomer unit, a series of propagation steps follows, rapidly building up the molecular weight and degree of polymerization. The important part of this mechanism is therefore the (3), (3), etc. noted. This is what makes the polymer With unsymmetrical monomers the head-to-tail addition is preferred because whatever it is in the R group that stabilized the radical once will do so each time a propagation step happens. 

Finally, chain transfer is undesirable except when it is used intentionally to limit molecular weight by adding good chain transfer agents such as carbon tetrachloride. Here transfer of a chlorine atom limits the size of one chain and at the same time initiates formation of a new chain by the trichloromethyl radical. Instead of (3), (3), (3), etc., we get (3), (3), (7), (8), (3), (3), (7), (8), etc., with a lower average chain length. 

Mercaptans (R—S—H) and phenols (Ar—O—H) also make good chain transfer agents by breaking the S—H or O—H bonds. 

A wide variety of monomer olefins can be used in free radical polymerization. Common examples are given in Fig. 14.2. You should be able to furnish the starting monomer given the structure of the polymer or vice versa. 

In chain-growth polymerization, monomers can onlyjoin active chains. Monomers contain carbon-carbon double bonds (e.g., ethylene, propylene, styrene, vinyl chloride, butadiene, esters of (meth)acrylic acid). The activity of the chain is generated by either a catal) t or an initiator. Several classes of chain-growth polymerizations can be distinguished according to the type of active center  

Monomers Should contain at least a double bond 

Growing principle Reaction of the monomer with the active center. Chain 

Number of growing chains Small (10 -10 mo i Growing chain life time Very short (0.5-10 s) 

Reactions of the functional groups of either the monomers or the growing chains. No initiator required. Catalyst used to accelerate reactions 

The rate at which the molecular weight of a chain-growth polymerization increases d ends on three rates the rate of initiation, propagation, and termination (Ri, Rp, Rt). In many cases, once initiated, the pol)rmerization proceeds very quickly, and foe 

The initiation, propagation, and termination shorthand notations are shown in the following equations [13], 

The rate constant is k, M is the monomer, and R is the initiating radical. A polymer s reactivity depends on the last unit added to the growing chain rather than on the length of the chain or any of the other units on the chain [13]. Propagation is the 

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We shall have considerably more to say about this type of kinetic analysis when we discuss chain-growth polymerizations in Chap. 6. 

Our primary purpose in this section is to point out some of the similarities and differences between step-growth and chain-growth polymerizations. In so doing we shall also have the opportunity to indicate some of the different types of chain-growth polymerization systems. 

Step-growth polymerizations can be schematically represented by one of the individual reaction steps VA + B V —> Vab V with the realization that the species so connected can be any molecules containing A and B groups. Chain-growth polymerization, by contrast, requires at least three distinctly different kinds of reactions to describe the mechanism. These three types of reactions will be discussed in the following sections in considerable detail. For now our purpose is to introduce some vocabulary rather than develop any of these beyond mere definitions. The principal steps in the chain growth mechanism are the following ... 

Elsewhere in this chapter we shall see that other reactions-notably, chain transfer and chain inhibition-also need to be considered to give a more fully developed picture of chain-growth polymerization, but we shall omit these for the time being. Much of the argumentation of this chapter is based on the kinetics of these three mechanistic steps. We shall describe the rates of the three general kinds of reactions by the notation Rj, Rp, and R for initiation, propagation, and termination, respectively. 

Photoinitiation is not as important as thermal initiation in the overall picture of free-radical chain-growth polymerization. The foregoing discussion reveals, however, that the contrast between the two modes of initiation does provide insight into and confirmation of various aspects of addition polymerization. The most important application of photoinitiated polymerization is in providing a third experimental relationship among the kinetic parameters of the chain mechanism. We shall consider this in the next section. 

There is a great deal more that could be said about emulsion polymerization or, for that matter, about free-radical polymerization in general. We shall conclude our discussion of the free-radical aspect of chain-growth polymerization at this point, however. This is not the end of chain-growth polymerization, however. There are four additional topics to be considered ... 

Chain-growth polymerization through cationic active species. This is taken up in Sec. 6.11. 

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

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