Degradation of Macromolecular Substances

The following example describes the synthesis of a superabsorbent via crosslinking copolymerization of acrylic acid (sodium salt) with Af/vf -methylenebis (acrylamide). 

In a 600 ml glass beaker equipped with a magnetic stirrer and a thermometer 25 g acrylic acid, 0.1 g Af,Af -methylene(bisacrylamide), and 0.12 g sodium carbonate are dissolved in 60 ml dist. water. 0.7 ml of an aqueous sodium peroxodisulfate solution (Na2S20g, 10 wt%) are added slowly at room temperature to the reaction mixture in the beaker. The mixture foams strongly and a temperature increase can be observed when the addition is too quick. One has to be careful that the temperature does not exceed 25°C. If necessary, cooling is applied briefly but removed afterwards. The mixture should be set to pH 4.9. 

In the presence of oxygen the thermal degradation of polymers is complicated by oxidation reactions, making the course of the reaction rather obscure. 

Hydrolytic degradation is especially important in polymers with hydrolyzable links between the CRUs. Thus, polyesters can be saponified to yield the starting materials from which they were formed. Acetal links in synthetic polymers such as polyoxymethylene, or in natural polymers such as cellulose, can be hydrolyzed with acids. However, the resistance to hydrolysis depends very much on the structure of the polymer for example, polyesters of terephthalic acid are very difficult to hydrolyze while aliphatic polyesters are generally easily hydrolyzed. Polyamides are normally much more resistant to hydrolysis than polyesters they may be cleaved by the methods usually employed for polypeptides and proteins. 

Grinding or milling causes degradation of many polymers. The process of mastication of natural rubber involves a mechanically initiated, autoxidative degradation which lowers the molecular weight to a level where the material is easier to process on a commercial scale. 

The radical chain fragments resulting from mechanical handling can initiate the formation of block and graft copolymers in the presence of polymerizable monomers. 

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TIMP). Both factors reduce further degradation of connective tissue. The hepatocytes are now multilayered instead of normal single-layered cell plates and lose their microvilh. Fenestration of the sinusoids disappears, whereas the sinusoidal extracellular matrix increases, leading to capillarization of the sinusoids, (s. pp 406, 526) In this way, the distance between the hepatocytes and the blood becomes greater, and the clearance of macromolecular substances is reduced. Stronger flow resistance in the liver leads to portal hypertension. Portoportal and portocentral bands of connective tissue form, in which portosystemic intrahepatic shunts develop. 

The third possibility for synthesizing polymeric substances is the modification of existant natural or synthetic macromolecules (see Chap. 5). These processes can either be chemical or physical. Chemical modifications are reactions on macromolecules without degradation of the main chain (macromolecular substitution routes, polymer-analogous reactions ) like, for example, hydrolysis. 

Insofar as humin is an insoluble macromolecular residue, it has mostly been examined by techniques amenable to solid materials (i.e., elemental analysis, infrared, solid-state NMR, and ESR spectroscopy). Degradative techniques such as oxidation, reduction, and pyrolysis have also been employed. All these methods have been used for the study of humic substances and excellent reviews of the various methods are provided by Schnitzer and Khan (1972, 1978) as well as by Stevenson (1982). 

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