Structural units Condensation

Pectins are heteropolysaccharides with axial-axial a(1, 4)-D galactopyranosyluronic acid units condensed in the Ci conformation and interrupted by a-(1,2)-L rhamnopyranosyl residues. The structure is based on blocks of galacturonan partly esterified (the smooth zones) and blocks of highly ramified rhamnogalacturonan regions (hairy zones). The exact structure depends on the sources but also on the methods used to isolate the pectins [1,5]. Few data are given in Table 1. 

The polymers listed above, and all other linear polymers as well, are formed from monomers which enter into two, and only two, linkages with other structural units. This statement corresponds to the previous remark that the structural units of linear polymers necessarily are bivalent. The interlinking capacity of a monomer ordinarily is apparent from its structure it is clearly prescribed by the presence of two condensable functional groups in each monomer in the third and fourth examples above. The ability of the extra electron pair of the ethylenic linkage to enter into the formation of two bonds endows styrene with the same interlinking capacity. In accordance with the functionality concept introduced by Carothers, all monomers which when polymerized may join with two, and only two, other monomers are termed bifunctional. Similarly, a hifunctional unit is one which is attached to two other units. It follows that linear polymers are composed exclusively (aside from terminal units) of bifunctional units. ... 

In 1929 Carothers proposed a generally useful differentiation between two broad classes of polymers condensation polymers in which the molecular formula of the structural unit (or units) lacks certain atoms present in the monomer from which it is formed, or to which it may be degraded by chemical means, and addition polymers, in which the molecular formula of the structural unit (or units) is identical with that of the monomer from which the polymer is derived. Condensation polymers may be formed from monomers bearing two or more reactive groups of such a character that they may condense intermolecu-larly with the elimination of a by-product, often water. The polyamides and polyesters referred to above afford prime examples of condensation polymers. The formation of a polyester from a suitable hydroxy acid takes place as follows ... 

Since Carothers introduced the distinction between condensation and addition polymers based on the composition relationship between monomer and structural unit, a number of examples of polymerization processes have been found which formally resemble the condensation type but which proceed without evolution of a by-product. For example, a glycol and a diisocyanate react intermolecularly as follows ... 

The process proceeds through the reaction of pairs of functional groups which combine to yield the urethane interunit linkage. From the standpoint of both the mechanism and the structure type produced, inclusion of this example with the condensation class clearly is desirable. Later in this chapter other examples will be cited of polymers formed by processes which must be regarded as addition polymerizations, but which possess within the polymer chain recurrent functional groups susceptible to hydrolysis. This situation arises most frequently where a cyclic compound consisting of one or more structural units may be converted to a polymer which is nominally identical with one obtained by intermolecular condensation of a bifunctional monomer e.g., lactide may be converted to a linear polymer... 

Representative condensation polymers are listed in Table I. The list is by no means exhaustive, but it serves to indicate the variety of condensation reactions which may be employed in the synthesis of polymers. Cellulose and proteins, although their syntheses have not been accomplished by condensation polymerization in the laboratory, nevertheless are included within the definition of condensation polymers on the ground that they can be degraded, hydrolytically, to monomers differing from the structural units by the addition of the elements of a molecule of water. This is denoted by the direction of the arrows in the table, indicating depolymerization. 

In Section 3 of this chapter it was mentioned that polymers obtained by intermolecular condensation of bifunctional monomers may often be prepared alternatively by an addition polymerization of a cyclic compound having the same composition as the structural unit. Typical examples are shown in Table III. The processes indicated are appropriately regarded as addition polymerizations. Each of these polymers may also be prepared through the condensation of suitable bifunctional monomers. The dimethylsiloxane polymer, for example, may be prepared, as indicated in Table I (p. 45), through the condensation of dimethyl dihydroxysilane formed by hydrolysis of the di-chlorosilane... 

The chemical and physical properties of the polymers obtained by these alternate methods are identical, except insofar as they are affected by differences in molecular weight. In order to avoid the confusion which would result if classification of the products were to be based on the method of synthesis actually employed in each case, it has been proposed that the substance be referred to as a condensation polymer in such instances, irrespective of whether a condensation or an addition polymerization process was used in its preparation. The cyclic compound is after all a condensation product of one or more bifunctional compounds, and in this sense the linear polymer obtained from the cyclic intermediate can be regarded as the polymeric derivative of the bifunctional monomer(s). Furthermore, each of the polymers listed in Table III may be degraded to bifunctional monomers differing in composition from the structural unit, although such degradation of polyethylene oxide and the polythioether may be difficult. Apart from the demands of any particular definition, it is clearly desirable to include all of these substances among the condensation... 

The free amino group of the amino ester may then react analogously with another molecule of the monomer, etc. The kinetics of the polymerization are in harmony with a mechanism of this sort. The final polypeptide may contain up to 300 or more structural units. While the polymerization of N-carboxyanhydrides is closely analogous to the addition polymerizations of ethylene oxide and of other cyclic substances, definition unfortunately classifies it as a condensation polymerization inasmuch as carbon dioxide is eliminated in the process. 

Type Structural unit Products of bifunctional condensation ... 

Silicone paints are formed by controlled hydrolysis and condensation of alkyl alkox-ysilanes, and may be encountered either alone or in formulations with other synthetic resins. The typical structural unit in the polymer chain is dimethyl siloxane, and pyrolysis of such resins takes place with random chain scission and the extended formation of stable cyclic fragments. In Figure 12.14 the pyrogram of a silicone resin is shown, with cyclic siloxane oligomers eluting at the shorter retention times, followed by the linear siloxane fragments. 

During the cell cycle, chromosome structures shuttle between de-condensed interphase and condensed mitosis states. Dynamic changes also occur at the lower levels of architectures, i.e., at the chromatin and nucleosome levels. Upon gene activation and inactivation, folding and unfolding of the nucleosome structure and the chromatin fibers occur at limited loci of the genome. Namely, the structures of the chromosome are dynamic and mobile. Nevertheless, there are basic structural units that remain stable and constitute the fundamental chromosome architecture. 

The bicyclic tropane ring of cocaine of course presented serious synthetic difficulties. In one attempt to find an appropriate substitute for this structural unit, a piperidine was prepared that contained methyl groups at the point of attachment of the deleted ring. Condensation of acetone with ammonia affords the piperidone, 17. Isophorone (15) may well be an intermediate in this process conjugate addition of ammonia would then give the aminoketone, 16. Further aldol reaction followed by ammonolysis would afford the observed product. Hydrogenation of the piperidone (18) followed then by reaction with benzoyl chloride gives the ester, 19. Ethanolysis of the nitrile (20) affords alpha-eucaine (21), an effective, albeit somewhat toxic, local anesthetic. 

Dithiazolidines containing o-quinonemethide structural units in the 3,5-positions (e.g. (99)) are converted by reagents with an activated CHj group into a range of condensed heterocyclic compounds not easily accessible otherwise (Scheme 14) <85S535,86PHA283>. 

---