Primary covalent bonding

Secondary Bonding. The atoms in a polymer molecule are held together by primary covalent bonds. Linear and branched chains are held together by secondary bonds hydrogen bonds, dipole interactions, and dispersion or van der Waal s forces. By copolymerization with minor amounts of acryhc (CH2=CHCOOH) or methacrylic acid followed by neutralization, ionic bonding can also be introduced between chains. Such polymers are known as ionomers (qv). 

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

Similarly, polymers dissolve when a solvent penetrates the mass and replaces the interchain secondary bonds with chain-solvent secondary bonds, separating the individual chains. This cannot happen when the chains are held together by primary covalent cross-links. Thus, linear and branched polymers dissolve in appropriate solvents, whereas cross-linked polymers are insoluble, although they may be swelled considerably by absorbed solvent. 

Primary bonds - ionic, covalent or metallic bonds, which are all relatively strong (they generally melt between 1(X)0 and 4000 K, and... 

German scientist Staudinger in 1920 stated that colloidal properties of organic materials are because of the large size of individual molecules. He also stated that macromolecules contain only primary covalent bonds. 

The flexibility of amorphous polymers is reduced drastically when they are cooled below a characteristic transition temperature called the glass transition temperature (Tg). At temperatures below Tg there is no ready segmental motion and any dimensional changes in the polymer chain are the result of temporary distortions of the primary covalent bonds. Amorphous plastics perform best below Tg but elastomers must be used above the brittle point, or they will act as a glass and be brittle and break when bent. 

Secondary forces operate at longer distances (.25-50 nm) than covalent bonds. These secondary forces are much weaker than primary covalent bonds, but the forces are cumulative. Thus the cohesive energy of a polymer is equal to the summation of the cohesive energy density (CED) values for each mole of repeating unit ( < 2 kcal) in the chain. The CED of a liquid is defined as the energy of vaporization per unit volume, / E/V. 

The requirement that AG be negative for spontaneous dissolution is readily met if the first term, AH, is less than the second term, — 7A5. Because the polymer chain units are bound through primary, covalent bonds, the units are not free to move independent of their neighboring units. Thus the — TAS term is lower for polymer solution than for the solution of smaller molecules. Most approaches for correlating structure with polymer solubility have focused on the AH term. Hildebrand has proposed the following relationship,... 

To solve this equation for as2 we need to know the relationship of crx and ctbi, that is, whether the primary covalent bond order is altered upon coordination. For halocarbons as ligands there is generally no detectable lengthening of the bond noted by X-ray crystallography, which might justify the conclusion that ax [Pg.6235]

When a sample of polymer is fractured, the creation of the new surfaces must necessarily involve the breakage of primary (covalent) bonds or secondary bonds (van de Waals interactions and hydrogen bonds), or both... 

The molecular forces are secondary or van der Waals forces. It is also conceivable that primary valence forces form chemical bonds, either covalent or ionic, between adhesive and adherend. The contribution of covalent bonds to bond strength is a subject of great, if sometimes controversial, interest (6). 

To effect a transformation, covalent-bond requires the use of specific methods. In particular, we need to establish generic methods that are responsible for primary transformations. The dominant method of covalent-bond is make-covalent-bond. Make-covalent-bond establishes a covalent-bond between two reaction centers when enabling conditions are achieved. It also performs all the necessary bookkeeping to ensure the bond is recognizable by the instance of abc. Expressed in... 

Engineers generally btiild things from a fimited menu of materials— namely, metals, polymers, and ceramics. This menu follows directly from the three types of primary chemical bonding metalfic, covalent, and ionic. 

In addition to these noncovalent interactions, disulfide bonds form covalent links between the side chains of cysteines. When such bonds form, they restrict the folding patterns available to polypeptide chains. There are specialized laboratory methods for determining the number and positions of disulfide links in a given protein. Information about the locations of disulfide links can then be combined with knowledge of the primary structure to give the complete covalent structure of the protein. Note the subtle difference here The primary structure is the order of amino acids, whereas the complete covalent structure also specifies the positions of the disulfide bonds (Figure 4.13). 

Chemical modification is generally taken to mean a process involving the creation of a chemical bond (usually covalent) between a surface cell wall polymer and an introduced reagent to form a novel adduct. This can be done through several approaches, including plasma activation and graft polymerization with vinyl monomers, which are very well described in literature. The primary... 

As mentioned earlier, biologies need to be treated with some extra consideration when addressing their pharmaceutical stability. The biological activity of a protein, for instance, comes not only from its covalently bonded primary structure, but also from the folded conformation that makes up the secondary and tertiary structure. The conformation can be easily altered without breaking any covalent bonds, and once in this denatured state, some or all of the biological activity that makes the protein a useful therapeutic medicine may be lost. 

A polymer molecule is defined as a molecule of relatively high molecular weight consisting of regularly repeating units, or chemically similar units, connected by primary covalent bonds. The word polymer was coined by the Swedish chemist Berzelius in 1833. It is derived from two Greek roots, poly which means many and meros which implies parts . 

One method of classifying plastics is by their response to heat. Thermoplasts, also known as thermoplastic polymers, soften and liquefy on heating and harden again when cooled. The process is reversible and can be repeated. On heating, the weak secondary bonds between polymer chains are broken, which facilitates relative movement between the chains. If the molten polymer is further heated until the primary covalent bonds also break, degradation of the thermoplast follows. Thermoplastic polymers are linear or exhibit branching with flexible chains and include polyethylene, polystyrene and polypropylene (Figure 4.10). 

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