Reactions, classification chain polymerization

The study of the molecular weight of the intermediate course is an effective method for the classification of polymerization as chain or stepwise reaction. In Figure 3, the molecular weight of the obtained polymer is plotted against the yield, for the oxidative polymerization of dimethylphenol with the copper catalyst and for the electro-oxidative polymerization. The molecular weight rises sharply in the last stage of the reaction for the copper-catalyzed polymerization. This behavior is explained by a stepwise growth mechanism. 

There is an alternative, somewhat less meaningful system of classification addition polymerization, in which molecules of monomer are simply added together and con-iensation polymerization, in which monomer molecules combine with loss of some simple nolecules like water. As it happens, the two systems almost exactly coincide nearly all ases of chain-reaction polymerization involve addition polymerization nearly all cases pf step-reaction polymerization involve condensation polymerization. Indeed, some hemists use the term addition polymerization to mean polymerization via chain reactions. 

There are two fundamental polymerization mechanisms. Classically, they have been differentiated as addition polymerization and condensation polymerization. In the addition process, no by-product is evolved, as in the polymerization of vinyl chloride (see below) whereas in the condensation process, just as in various condensation reactions (e.g., esterification, etherification, amidation, etc.) of organic chemistry, a low-molecular-weight by-product (e.g., H2O, HCl, etc.) is evolved. Polymers formed by addition polymerization do so by the successive addition of unsaturated monomer units in a chain reaction promoted by the active center. Therefore, addition polymerization is called chain polymerization. Similarly, condensation polymerization is referred to as step polymerization since the polymers in this case are formed by stepwise, intermolecular condensation of reactive groups. (The terms condensation and step are commonly used synonymously, as we shall do in this book, and so are the terms addition and chain. However, as it will be shown later in this section, these terms cannot always be used synonymously. In fact, the condensation-addition classification is primarily applicable to the composition or structure of polymers, whereas the step-chain classification applies to the mechanism of polymerization reactions.)... 

The above classification of polymers according to polymerization mechanism, as shown by the variation of molecular weight with conversion [Figs. 1.2(a) and 1.2(b)], is not without its ambiguities. Certain polymerizations show a linear increase of molecular weight with conversion [Fig. 1.2(c)] when the polymerization mechanism deviates from the normal chain or step pathway. This is observed in certain ionic chain polymerizations, which involve a fast initiation process coupled with the absence of reactions that terminate the propagating reactive centers. Biological syntheses of proteins also show the behavior described by Fig. 1.2(c) because the various... 

The many ways to make polymers can be broken into two types of reactions based on the mechanisms of the polymerization, step and chain reactions (5). This reaction classification was termed condensation or addition reactions in the past but this archaic nomenclature is slowly dying away. The labels step and chain were developed for the two types of polymerizations by Flory and Mark (5, 6). All step reactions conduct the same stepwise reaction between all reactive entities in the reaction mixture. The... 

The present Report makes no pretence at covering the field in all its many facets. Only work related to copolymerization chemistry will be reported and that is taken from the scientific literature, ignoring multitudinous patents which have appeared. The Report is largely confined to addition polymerization processes, although the sharp demarcation between the traditional classifications of polymerization reactions is to some extent becoming blurred, particularly when attempts have been made to incorporate both non-polar and polar structural units within a polymer chain, as will be seen later. 

The three classifications just described are essentially based on monomer structure. Condensation polymerizations arise when the two monomers are stable but have functionalities that can react with each other. Addition polymerizations require unsaturation in the monomer that is vulnerable to attack by radicals or ions, and ring-opening polymerizations require cyclic monomers. An alternative and more modern classification emphasizes the differing mechanisms of polymerization, producing two classes step-growth reactions and chain-growth reactions. Figure 13.13 contrasts the two mechanisms. 

A corresponding anionic mechanism in the presence of a strong base (or electron donor) is plausible. Other cyclic compounds may be susceptible to polymerization by similar ionic mechanisms. Inasmuch as the growth step must be extremely rapid, a chain reaction is indicated and classification with vinyl-type addition polymerizations should be appropriate in such cases. 

Whether the formation of poly(p-xylylene) should be included in this chapter is not clear. Decisive data are not available to indicate the classification of this polymerization as a step or chain reaction. The formation of high polymer occurs instantaneously when p-xyly-lene contacts the cool surface, precluding the evaluation of polymer molecular weight versus conversion. Also, the mode of termination for this reaction is unknown. 

Carothers classification (condensation vs. addition) is primarily based on the composition or structure of polymers. The second classification (chainwise vs. stepwise) was proposed by P. J. Floiy, and is based on the kinetic scheme or mechanism governing the polymerization reactions. Step reactions are those in which the chain growth occurs in a slow, stepwise manner. Two monomer molecules react to form a dimer. The dimer can then react with another monomer to form a trimer, or with another dimer to form tetramer. Thus, the average molecular weight of the system increases slowly over a period of time. This is exemplified by the following polyesterification ... 

Polymer degradation reactions are frequently categorized based on the site in the macromolecule structure where the reaction occurs. This leads to the following classification of scission reactions a) polymeric chain scission, b) side group reactions, c) combined reactions [5, 3]. These reactions follow one of the mechanisms described previously, but this different classification allows a better correlation of the nature of the reaction products with the structure of the polymer and provides more understanding regarding the expected pyrolysis products. 

In contrast with the usually slow progress of condensation polymerization the second major classification, addition, or vinyl-type polymerizations, usually proceed very rapidly, so rapidly that they are referred to as chain reaction polymerizations. This method of producing synthetic polymers uses the potential dual functionality present in a carbon-carbon double bond. The process is initiated by the use of radical or charged initiator species to form new sigma bonds from the carbon-carbon double bonds of the monomer, to link the monomer units (Eq. 20.6). 

We will discuss the various polymerization mechanisms in greater detail in Chapter 2. The original classification of polymers as either condensation or addition polymers as proposed by Carothers does not permit a complete differentiation between the two classes or polymers, particularly in view of the new polymerization processes that have been developed in recent years. Consequently, this classification has been replaced by the terms step-reaction (condensation) and chain-reaction (addition) polymerization. These terms focus more on the manner in which the monomers are linked together during polymerization. 

As disciissed in Chapter 1, under a scheme proposed by Carothers, polymers are classified as addition or condensation polymers depending on the type of polymerization reaction involved in their synthesis. This classification scheme, however, does not permit a complete difierentiation between the two classes of polymers. A more complete but still oversimplified scheme that is still based on the dilTerent polymerization processes places polymers into three classes condensation, addition, and ring-opening polymers. This scheme reflects the stractures of the starting monomers. Probably the most general classification scheme is based on the polymerization mechanism involved in polymer synthesis. Under this scheme, polymerization processes are classified as step-reaction (condensation) or chain-reaction (addition) polymerization. In this chapter, we will discuss the different types of polymers based on the different polymerization mechanisms. 

Later, in 1953, P.J. Flory divided the polymers by their reaction mechanism into chain-reaction and step-reaction, rather than by comparing the polymer s constitutional unit and the monomer. The addition polymers are generally produced by a chain reaction mechanism, and the condensation polymers produced by a step-reaction mechanism. Currently it is customary, though not scientifically correct, to refer to addition or chain-reaction polymerization and to condensation or step-reaction polymerization. Some have suggested that the classification of polymers... 

Although these definitions were perfectly adequate at the time, it soon became obvious that notable exceptions existed and that a fundamentally sounder classification should be based on a description of the chain-growth mechanism. It is preferable to replace the term condensation with step-growth or step-reaction. Reclassification as step-growth polymerization now logically includes polymers such as polyurethanes, which grow by a step-reaction mechanism without elimination of a small molecule. 

Although these earlier definitions were based on the chain structure of the polymers, they were closely related, as just described, to the mode of formation as well. It soon became apparent that such a classification has serious shortcomings, as so-called polycondensates could result from addition polymerization reactions. For example, although Nylon 6 can be prepared by the polycondensation reaction of e-aminocaproic acid (Braun et al., 1984), it is now synthesized by the ring-opening addition polymerization of e-caprolactam (Sandler and Karo, 1992), and this process has a profound effect on the... 

Many authors divide polymerizations into chain reactions and stepwise reactions. Of course, all reactions proceed in steps, that is, one reaction step follows another. Termolecular reactions are rare but in the organic chemistry sense, the term stepwise reaction signifies that intermediate products can be isolated and subsequently again made to react. This means that, in the absence of impurities, certain reactions can be frozen. In actual fact, anionic addition polymerizations can be frozen at low temperatures and made to run again at higher temperatures. That this procedure is not possible in the presence of water or carbon dioxide is an experimental consideration and not a conceptual difficulty. If we lived in an isocyanate atmosphere, no steps could be isolated in the polyamide synthesis. Thus, such classifications are based on experimental expertise, which can never be the basis of a physical definition. 

There are three general classifications of living radical polymerization based on differences in the reversible activation reaction step described in the previous section. These three mechanisms are termed dissociation-combination, atom transfer and degenerative chain transfer, respectively [17, 18]. 

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