Reaction Engineering of Chain-Growth Polymerization

This chapter discusses the reaction engineering of chain-growth polymerization. In order to form polymers of specified properties, we observe that reactor temperature is a very important variable. To find this, the energy balance equation must be solved, along with mole balance relations of various species. In the study of copolymers, the quantities of practical interest are the relative distributions of the monomers on polymer chains and the overall rates of copolymerization. With these, it is possible to carry out the reactor design. 

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In this chapter, we have considered the reaction engineering of chain-growth polymerization. In order to manufacture polymers of desired physical and mechanical properties, the performance of the reactors must be closely controlled. To do this, various transport equations governing their performance must be established, which, in principle, can be solved numerically. The usual Runge-Kutta technique takes considerable computational time and, at times, gives numerical instability. To overcome aU of these problems, a semianalytical approach can be used. 

There are some fundamental differences in the engineering of step-growth and chain-growth polymerizations because of basic distinctions in the mechanisms of these reactions. A propagation reaction (Section 6.3.2) in a kinetic chain sequence must be fast or the series of monomer additions will not be long enough to produce... 

This chapter will deal with ionic chain growth polymerization for monomers which are of some industrial importance. It will deal mainly with those polymer reaction engineering aspects which are relevant for designing processes and products. As it cannot cover the entire subject, the systems dealt with are chosen as examples of the most important features of ionic polymerization. 

The most important chain-growth polymers are polyolefins and other vinyl polymers. Examples of the former are polyethylene, and polypropylene, and examples of the latter are poly(vinyl chloride), polystyrene, poly(vinyl alcohol), polyacrylonitrile, and poly(methyl acrylates). The most common stepwise reactions are condensation polymerizations. Polyamides, such as nylon 6-6, which is poly(hexamethylene adipamide), and polyesters, such as poly(ethylene terephthalate), are the most important commercial condensation polymers. These polymers were originally developed for use in fiber manufacture because of their high melting points but are now used also as thermoplastics. Polycarbonate is an engineering plastic that is made from bisphenol A and phosgene by a stepwise reaction. 

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

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