Step-growth polymerization requirements

Linear step-growth polymerizations require exceptionally pure monomers in order to ensure 1 1 stoichiometry for mutually reactive functional groups. For example, the synthesis of high-molecular-weight polyamides requires a 1 1 molar ratio of a dicarboxylic acid and a diamine. In many commercial processes, the polymerization process is designed to ensure perfect functional group stoichiometry. For example, commercial polyesterification processes often utilize dimethyl terephthalate (DMT) in the presence of excess ethylene glycol (EG) to form the stoichiometric precursor bis(hydroxyethyl)terephthalate (BHET) in situ. 

We can see from this simple example why step-growth polymerizations require very high conversions of functional groups compared to syntheses of the same linkages in conventional organic reactions. 

Polymerization reactions proceed either by the step growth or the chain addition mechanisms. Step-growth polymerizations require monomers with at least two functional groups and are involved in the manufacture of several industrially important polymers such as polyamides, polyesters, and in the formation of biopolymers such as polysaccharides, proteins and polypeptides in nature. 

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 ... 

Step-growth polymerization processes must be carefully designed in order to avoid reaction conditions that promote deleterious side reactions that may result in the loss of monomer functionality or the volatilization of monomers. For example, initial transesterification between DMT and EG is conducted in the presence of Lewis acid catalysts at temperatures (200°C) that do not result in the premature volatilization of EG (neat EG boiling point 197°C). In addition, polyurethane formation requires the absence of protic impurities such as water to avoid the premature formation of carbamic acids followed by decarboxylation and formation of the reactive amine.50 Thus, reaction conditions must be carefully chosen to avoid undesirable consumption of the functional groups, and 1 1 stoichiometry must be maintained throughout the polymerization process. 

It is unlikely that carbon dioxide would be produced from the degrading polymer since multiple bond fragmentation and reformation would be required. After repeated precipitation of the polymer, GICO ) decreased markedly while the yield of other products was not affected. It is proposed that potassium carbonate, used to generate reactive phenolate during the step growth polymerization, was occluded in the precipitated polymer and was the primary source of the observed carbon dioxide. 

The foregoing discussion illustrates one of the reasons why the great majority of step-growth polymerizations which can be written on paper are not used in fact the expenses involved in the purchase of monomers, carrying out the reaction, or preparing the polymer for further use. Tlie overall process which produces a final product of required quality at the lowest cost will be chosen. 

A satisfactory step-growth polymerization reaction must satisfy the following requirements ... 

It is necessary to understand these requirements in order to be able to carry out and control step-growth polymerizations. The requirements listed above are considered in more detail in the following sections of this chapter. 

It can be seen that it is relatively easy to carry out step-growth polymerizations inefficiently or entirely ineffectively if the requirements listed in Section 5.3 are not fully appreciated. 

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