Kinetics of Chain Growth Polymerization

The initiation step of chain growth creates a reactive site that can react wth other monomers, starting the polymerization process. Before the monomer forms the reactive site, the initiator (i) (which may be either a radical generator or an ionic species) first creates the polymerization activator (A) at a rate defined by the rate constant ky This process can be represented as shown in Eq. 4.7. 

The activator reacts with the monomer (M) present in the reaction vessel to create the propagating species, A4A, according to Eq. 4.8, with a rate constant 2  

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Why are the kinetics of chain growth polymerization more difficult to study than those of step growth polymerization What simplification do we use to treat the kinetics of the chain growth process How does this simplification reduce the complexity of the problem and what are the limitations of this method ... 

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. 

Chain growth differs from step growth in that it involves initiation and usually also termination reactions in addition to actual growth. This makes its kinetic behavior similar to that of chain reactions (see Chapter 9). However, the chain carriers in chain-growth polymerization need not be free radicals, as they are in ordinary chain reactions. Instead, they could be anions, cations, or metal-complex adducts. While the general structure of kinetics is similar in all types of chain-growth polymerizations, the details differ depending on the nature of the chain carriers. 

Coordination polymerization is another variant of chain-growth polymerization. The kinetic chain starts with addition of monomer to a metal complex, propagation is by successive insertion of monomer at the metal, and termination occurs when the metal complex separates from the polymer. Inasmuch as the complex is restored... 

Living polymerizations are limited to the realm of chain-growth polymerizations, in which a monomer is transformed to a polymer by a reactive species (an initiator, I) via a kinetic chain reaction (Scheme 15.1). An intrinsic limitation of a typical chain-growth process, such as free-radical polymerization, is the occurrence of termination reactions that lead to the formation of dead chains, chains that are incapable of further growth. 

ADMET has been shown to be a step-growth polycondensation reaction [31[. The kinetics of step-growth polymerization and consequences thereof are completely different than those of chain polymerizations. Since ROMP and many other single-site transition metal-catalyzed polymerizations discussed in this book proceed... 

Due to the high importance of chain growth polymerization for polymer production, huge efforts have been undertaken to understand and elucidate the kinetics involved in the chain growth process and the parameters influencing molar mass, dispersity D, and the nature of Ihe resulting macromolecules. In Figure 3.4, the kinetic steps, exemplified for a free radical process, are outlined. 

It is generally assumed that the stereosequence distributions, or tacticity, obtained in polymerization reactions of vinyl and related monomers is primarily a kinetically-controlled process (1). That is, as in other important aspects of chain-growth polymerization reactions (e.g., molecular weight and copolymer composition), tacticity is determined by the relative rates of competitive reactions, in this case the rates of isotatic, kj, and syndiotactic, ks, additions, as shown in Equation 1. 

Ethenic polymerization is an economically important class of polymer-forming reactions whose kinetics typifies chain-growth polymerization [1]. The terms vinyl, olefin, or addition polymerization are often used, although they are more restrictive. Usually three stages are essential to the formation of a useful high polymer. 

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

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. 

PHA is produced in Alcaligenes eutrophus from acetyl CoA in three steps and the last step is the chain growth polymerization of hydroxyalkanoate CoA esters catalyzed by PHA polymerase (synthase), yielding PHA of high molecular weight. Kinetics and mechanism of the polymerization of hydroxyalkanoyl CoA monomers with this bacterial polymerase have been investigated. 

Scheme 4 Mechanism of chain growth for a all Pd(II) polymerizations and ethylene polymerizations with Ni(II), and b a-olefin polymerizations with Ni(II). Specific kinetic data shown for Ni catalyst 1.15b [63]...

The production of hydrocarbons using traditional F-T catalysts is governed by chain growth (polymerization) kinetics. The theoretical equation describing the distribution of hydrocarbon products, commonly referred to as the Anderson-Schulz-Flory (ASF) equation, is... 

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