Equilibrium Step-Growth Polymerization

Effect of Impurity on Development of Molecular Weight in Equilibrium Step-Growth Polymerizations... 

The concepts we use to develop relations between the degree of conversion and molecular weight distribution of the reaction mixture in equilibrium step-growth polymerizations are most clearly illustrated with reference to the selfpolymerization of a monomer which contains two coreactive groups. An example would be a hydroxy acid that can undergo self-polymerization according to... 

Equation (5-22) is the differential number distribution function for equilibrium step-growth polymerizations in homogeneous systems. 

The breadth of the number distribution in equilibrium step-growth polymerization of linear polymers is indicated by... 

When an equilibrium step-growth polymerization is 99.5% complete what fraction of the reaction mixture is still monomer... 

It is not practical to conduct free-radical polymerizations under conditions where there is an equilibrium between polymerization and depolymerization processes. The polymer synthesis is effectively irreversible in normal radical polymerizations. The course of the reaction is then determined kinetically, and the molecular weight distribution cannot be predicted statistically as was done for equilibrium step-growth polymerizations described in Chapters. 

There is an important difference between the distributions calculated for equilibrium, bifunctional step-growth polymerization in Chapter 5 and for the free-radical polymerizations with termination by disproportionation or chain transfer that are being considered here. The distribution functions in the step-growth case apply to the whole reaction mixture in the free-radical polymerization this distribution describes only the polymer which has been formed. There is obviously a strong parallel between the probability S of this section and the extent of reaction p used in the step-growth calculations in Chapter 5. Many authors use the same symbol for both parameters. Different notations are used here, however, for clarity. 

The reaction continues until one of the reagents is almost completely used up equilibrium is established that can be shifted at will at high temperatures by controlling the amounts of reactants and products. In step-growth polymerization, the monomer molecules are consumed rapidly, and chains of any length x and y combine to form longer chains. 

Monomers can be joined by means of two principal methods to form polymers, and these methods are used as the broad basis for classification of synthetic polymers. The first of these, condensation, or step-growth polymerization, involves the use of functional group reactions such as esterification or amide formation to form polymers. When each of the molecules involved has only one functional group then the reaction between a carboxylic acid and an alcohol gives an ester (Eq. 20.3). In this equilibrium reaction water removal will help drive the reaction to the right. 

When an equilibrium step-growth polymerization is 99% complete, what fraction of the reaction mixture is still monomer (a) on mole basis and (b) on weight basis [Ans. (a) 0.01 (b) 0.0001]... 

Many step-growth polymerizations involve an equilibrium between reactants and products, the latter comprising macromolecular species and (usually) eliminated small molecules. 

Both polyesters and nylon 6,6 are prepared by step-growth polymerizations. The activation energies for such reactions are of the order of 84 kJ/mol. It is therefore usual to employ elevated temperatures to accelerate these reactions. The step-growth polymerizations shown are characterized by polymeiiza-tion-depolymerization equilibria, with equilibrium constants given by... 

Many step-growth polymerizations are carried out by mass or bulk-type polymerization. This is commonly done not only for convenience, but also because it results in minimum contamination. Few step-growth reactions are highly exothermic, so thermal control is not hard to maintain. Because equilibrium considerations are very important, the reactions are usually carried out in a way that allows continuous removal of the byproduct. Occasionally, the polymerizations are carried out in dispersion in some convenient carriers. Solution polymerizations are sometimes used as a way of moderating the reactions. 

In step-growth polymerization, the molecular weight continuously increases with time and the formation of polymer with sufficient high molecular weight for practical applications requires very high conversions of the reactive groups (>98-99%). This requirement imposes stringent conditions on the formation of polymers by step polymerization, such as the necessity for a favorable equilibrium and the absence of side reactions. 

The other common method is condensation polymerization (also called step-growth polymerization), in which the reaction between monomer units and the growing polymer chain end group releases a small molecule, often water, as shown in Figure 1.2. The monomers in this case have two reactive groups. This reversible reaction will reach equilibrium and halt unless this small molecular by-product is removed. Polyesters and polyamides are among the plastics made by this process. 

Important polymers that are produced by polyaddition are polyamide 6 (nylon) and all kinds of polyurethanes. In polycondensation one mol of a small molecule (typically H2O) is liberated per step of chain growths, important polymers that are produced by polycondensation are polyamide 6.6, poly(ethylene terephthalate) (PET), polycarbonate, polyarylate, and polysulfide. Step growth polymerization is usually slow, equilibrium limited and isothermal to slightly exothermic. Polyaddition and polycondensation reactions of monomers with three or more reactive end groups lead to three-dimensionally crosslinked resins. 

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