Small molecules are joined

Another important reaction involving certain unsaturated hydrocarbons is polymerization, a process in which many small molecules are joined together to form a large molecule. Polymerization will be discussed in Section 22.5. 

In a chemical reaction, two small molecules are joined, and a water molecule is produced. What type of reaction took place ... 

Polymers are large molecules (macromolecules) that consist of one or two small molecules (monomers) joined to each other in long, often highly branched, chains in a process called polymerization. Both natural and synthetic polymers exist. Some examples of natural polymers are starch, cellulose, chitin (the material of which shells are made), nucleic acids, and proteins. Synthetic polymers, the subject of this chapter, include polyethylene, polypropylene, polystyrene, polyesters, polycarbonates, and polyurethanes. In their raw, unprocessed form, synthetic polymers are sometimes referred to as resins. Polymers are formed in two general ways by addition or by condensation. 

Condensation polymerization a type of polymerization in which the formation of a small molecule, such as water, accompanies the extension of the polymer chain. (22.5) Condensation reaction a reaction in which two molecules are joined, accompanied by the elimination of a water molecule. 

Condensation reaction A reaction in which a small molecule, such as water or hydrogen chloride, is eliminated and two molecules are joined. 

Generally speaking, the idea of flexible polymer chains may appear rather surprising. At school one is taught that the atoms in a molecule are joined together by covalent bonds in some specific order. Therefore their positions in space with respect to each other must be fixed too — just following from the chemical formula for the structure. And if one looks at a small strand of the chain only, this argument will be quite correct. 

A second general reaction used to synthesize commercially important polymers is condensation polymerization. In a condensation reaction two molecules are joined to form a larger molecule by elimination of a small molecule, such as H2O. For example, an amine (a compound containing —NH2) reacts with a carboxylic acid (a compound containing —COOH) to form a bond between N and C plus an H2O molecule (T FIGURE 12.37). 

Addition polymers Polymers made from monomers (usually containing carbon-carbon double bonds) joined directly with one another Condensation polymers Polymers made from monomers (each with two functional groups) that combine so that small molecules are split out between their functional groups... 

The formation of ethers is an example of a condensation reaction, in which two molecules are joined together and a small molecule, usually water, is eliminated. 

One characteristic of phosphoric and phosphorous acids is their tendency to undergo condensation reactions when heated. A cotidensatiotl reaction is one in which two or more molecules combine to form a larger molecule by eliminating a small molecule, such as H2O. (Section 12.2) The reaction in which two H3PO4 molecules are joined by the elimination of one H2O molecule to form H4P2O7 is represented in Equation 22.75. 

The aldol reaction (aldol condensation) is one of the fundamental reactions of organic chemistry, since it leads to the formation of a new carbon-carbon bond and is broadly applicable. A condensation reaction is one in which two molecules are joined with the concomitant expulsion of a small stable molecule, usually water or an alcohol. The aldol reaction may be used to condense various combinations of aldehydes and ketones. The mixed aldol condensation of an aldehyde having no a-hydrogen atom with a ketone is specifically known as the Claisen-Schmidt reaction. This variation of the aldol condensation is illustrated here in the synthesis of dibenzalacetone. 

A condensation polymer is a polymer formed by linking together many molecules by condensation reactions. A condensation reaction is one in which two molecules are joined by the elimination of a small molecule such as water. Wallace Carothers (1896-1937), a chemist at E. I. du Pont de Nemours and Company, realized that polymers could be prepared by condensation reactions. He and his coworkers produced a number of polymers this way, including polyamides (nylon) and polyesters. The first synthetic fibers, produced from nylon, were announced by du Pont in 1938. ... 

If you have studied Chapter 21, especially Sections 21.15 and 21.16, you are ready to tackle this chapter. If you haven t studied Chapter 21 yet, read at least those last two sections. Return herewith two ideas (1) Large molecules can be made by joining together many smaller molecules, like links in a chain, and (2) physical and chemical properties of large molecules can be studied and predicted, just as the properties of small molecules are studied and predicted. 

Processes such as imine formation from a primary amine and an aldehyde or ketone, in which two molecules are joined with the elimination of water (or other small molecules such as alcohols), are called condensations. 

Synthetic large molecules are made by joining together thousands of small molecular units known as monomers. The process of joining the molecules is called polymerisation and the number of these units in the long molecule is known as the degree of polymerisation. The names of many polymers consist of the name of the monomer with the suffix poly-. For example, the polymers polypropylene and polystryene are produced from propylene and styrene respectively. Names, and symbols for common polymers are given in Appendix F. 

Fig. 8.1 Biosynthesis of peptidoglycan. The large circles represent A -acetylglucosamine orN-acetylmuramic acid to the latter is linked initially a pentapeptide chain comprising L-alanine, D-glutamic acid and meso-diaminopiraelic acid (small circles) terminating in two D-alanine residues (small, darker circles). The lipid molecule is undecaprenyl phosphate. In the initial (cytoplasm) stage where inhibition by the antibiotic D-cycloserine is shown, two molecules of Dalanine (small circles) are converted by an isomerase to the D-forms (small, darker circles), alter which a ligase joins the two D-alanines together to produce a D-alanyl-D-alanine dipeptide.

The membrane surface facing the lumen is called the apical surface, and the membrane surface on the side facing blood is called the basolateral surface. The intestinal cells are joined at the tight junctions [63,75]. These junctions have pores that can allow small molecules (MW < 200 Da) to diffuse through in aqueous solution. In the jejunum, the pores are 7-9 A in size. In the ileum the junctions are tighter, and pores are 3-4 A in size (i.e., dimensions of mannitol) [63]. 

A chemical reaction by which small molecules (of monomer) are joined together to form large molecules (of polymer). Polymerisation may be effected by (a) addition, in which the polymer molecule is a multiple of the monomer molecule, (b) by condensation, in which the empirical formula of the polymer differs from that of the monomer, and (c) by copolymerisation, in which the polymer molecule is built up from two or more different monomers... 

---