The Chemistry of Polymer Molecules

There are numerous other organic groups, many of which are involved in polymer structures. Several of the more common groups are presented in Table 14.2, where R and R represent organic groups such as CH3, C2H5, and CgHs (methyl, ethyl, and phenyl). 

Concept Check 14,1 Differentiate between polymorphism (see Chapter 3) and isomerism. [The answer may be found at www.wiley.com/college/callister (Student Companion Site).] 

The molecules in polymers are gigantic in comparison to the hydrocarbon molecules macromolecule already discussed because of their size they are often referred to as macromolecules. 

Within each molecule, the atoms are bound together by covalent interatomic bonds. For carbon-chain polymers, the backbone of each chain is a string of carbon atoms. Many times each carbon atom singly bonds to two adjacent carbon atoms on either side, represented schematically in two dimensions as follows  

Each of the two remaining valence electrons for every carbon atom may be involved in side bonding with atoms or radicals that are positioned adjaeent to the chain. Of course, both chain and side double bonds are also possible. 

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If the chemistry of polymer molecules were different from that of simple compounds resembling the repeating units (model compounds), the study of the chemical resistance of organic polymers would be difficult. Fortunately, Nobel laureate Paul Flory found that the rate of esterification of molecules with terminal hydroxyl and carboxyl groups is essentially independent of the size of the molecules. Thus it is customary to assume that the rates of most reactions of organic molecules are similar regardless of the size of the molecule. 

The chemical resistance of a plastics material is as good as its weakest point. If it is intended that a plastics material is to be used in the presence of a certain chemical then each ingredient must be unaffected by the chemical. In the case of a polymer molecule, its chemical reactivity will be determined by the nature of chemical groups present. However, by its very nature there are aspects of chemical reactivity which find no parallel in the chemistry of small molecules and these will be considered in due course. 

The chemistry of synthetic polymers is similar to the chemistry of small molecules with the same functional groups, but the physical properties of polymers are greatly affected by size. Polymers can be classified by physical property into four groups thermoplastics, fibers, elastomers, and thermosetting resins. The properties of each group can be accounted for by the structure, the degree of crystallinity, and the amount of cross-Jinking they contain. 

While the chemistry of polymer synthesis can be explained by the usual themes of organic reactions, the large size of polymer molecules gives them some unique physical properties compared to small organic molecules. 

Gel permeation chromatography (GPC) is essentially a process for the separation of polymer molecules according to their size. The separation occurs as the solute molecules in a flowing liquid move through a stationary bed of porous particles. The method has been used extensively in biochemistry to separate biological polymer molecules from small molecule contaminants (with the use of Sephadex column). Application of the method to synthetic polymer chemistry in the 1970s has revolutionized the procedures for polymer characterization and molecular weight determination. 

An alternative to in situ polymerization involves direct intercalation of macromolecules into layered structures. Silicates are most often used. The insertion of polymer molecules into layered host lattices is of interest from different points of view. First, this insertion process leads to the construction of organic-inorganic polylayered composites. Second, the intercalation physical chemistry by itself and the role intercalation plays in the gain of electronic conductivity are of interest. This becomes important in the construction of reversible electrodes " or when improving the physicomechanical properties of nylon-layered silicate nanocomposites, hybrid epoxide clay composites," and nanomaterials based on hectorite and polyaniline, polythiophene or polypyrrole. ... 

This chapter has dealt with subjects concerned with synthesis and application of interlocked polymers. Marked progress in this area is evident in some aspects. In spite of the huge and wide progress in the chemistry of interlocked molecules such as rotaxane and catenane, the progress of the chemistry of the interlocked polymers has still... 

There are numerous references in the literature to irreversible adsorption from solution. Irreversible adsorption is defined as the lack of desotption from an adsoibed layer equilibrated with pure solvent. Often there is no evidence of strong surface-adsorbate bond formation, either in terms of the chemistry of the system or from direct calorimetric measurements of the heat of adsorption. It is also typical that if a better solvent is used, or a strongly competitive adsorbate, then desorption is rapid and complete. Adsorption irreversibility occurs quite frequently in polymers [4] and proteins [121-123] but has also been observed in small molecules and surfactants [124-128]. Each of these cases has a different explanation and discussion. 

Our purpose in this introduction is not to trace the history of polymer chemistry beyond the sketchy version above, instead, the objective is to introduce the concept of polymer chains which is the cornerstone of all polymer chemistry. In the next few sections we shall introduce some of the categories of chains, some of the reactions that produce them, and some aspects of isomerism which multiply their possibilities. A common feature of all of the synthetic polymerization reactions is the random nature of the polymerization steps. Likewise, the twists and turns the molecule can undergo along the backbone of the chain produce shapes which are only describable as averages. As a consequence of these considerations, another important part of this chapter is an introduction to some of the statistical concepts which also play a central role in polymer chemistry. 

Since the six carbons shown above have 10 additional bonds, the variety of substituents they carry or the structures they can be a part of is quite varied, making the Diels-Alder reaction a powerful synthetic tool in organic chemistry. A moment s reflection will convince us that a molecule like structure [XVI] is monofunctional from the point of view of the Diels-Alder condensation. If the Diels-Alder reaction is to be used for the preparation of polymers, the reactants must be bis-dienes and bis-dienophiles. If the diene, the dienophile, or both are part of a ring system to begin with, a polycyclic product results. One of the first high molecular weight polymers prepared by this synthetic route was the product resulting from the reaction of 2-vinyl butadiene [XIX] and benzoquinone [XX] ... 

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