Polymeric covalent substances

There is an ill-defined boundary between molecular and polymeric covalent substances. It is often possible to recognise discrete molecules in a solid-state structure, but closer scrutiny may reveal intermolecular attractions which are rather stronger than would be consistent with Van der Waals interactions. For example, in crystalline iodine each I atom has as its nearest neighbour another I atom at a distance of 272 pm, a little longer than the I-I distance in the gas-phase molecule (267 pm). However, each I atom has two next-nearest neighbours at 350 and 397 pm. The Van der Waals radius of the I atom is about 215 pm at 430 pm, the optimum balance is struck between the London attraction between two I atoms and their mutual repulsion, in the absence of any other source of bonding. There is therefore some reason to believe that the intermolecular interaction amounts to a degree of polymerisation, and the structure can be viewed as a two-dimensional layer lattice. The shortest I-I distance between layers is 427 pm, consistent with the Van der Waals radius. Elemental iodine behaves in most respects - in its volatility and solubility, for example - as a molecular solid, but it does exhibit incipient metallic properties. 

Gels are viscoelastic bodies that have intercoimected pores of submicrometric dimensions. A gel typically consists of at least two phases, a soHd network that entraps a Hquid phase. The term gel embraces numerous combinations of substances, which can be classified into the following categories (2) (/) weU-ordered lamellar stmctures (2) covalent polymeric networks that are completely disordered (2) polymer networks formed through physical aggregation that are predominantly disordered and (4) particular disordered stmctures. 

By far the majority of carbohydrate material in nature occurs in the form of polysaccharides. By our definition, polysaccharides include not only those substances composed only of glycosidically linked sugar residues but also molecules that contain polymeric saccharide structures linked via covalent bonds to amino acids, peptides, proteins, lipids, and other structures. 

Thus, aside from the covalently polymerized a-chain itself, the majority of protein structure is determined by weaker, noncovalent interactions that potentially can be disturbed by environmental changes. It is for this reason that protein structure can be easily disrupted or denatured by fluctuations in pH, temperature, or by substances that can alter the structure of water, such as detergents or chaotropes. 

The combining of two or more substances or molecular entities to yield a single substance or molecular entity, a process that involves either covalent or noncovalent bonding. Included in this definition is the formation of ion pairs from free ions, the noncovalent aggregation of monomers to form polymeric structures or complexes, as well as colligation. The opposite of association is dissociation. 

As already shown, conventional macromolecules (or polymers) consist of a minimum of a several hundred covalently linked atoms and have molar masses clearly above 10 g/mol. The degree of polymerization, P, and the molecular weight, M, are the most important characteristics of macromolecular substances because nearly all properties in solution and in bulk depend on them. The degree of polymerization indicates how many monomer units are linked to form the polymer chain. The molecular weight of a homopolymer is given by Eq. 1.1. 

Besides the polymeric drugs described so far other systems have been and still are intensively investigated - examining the possibility of polymeric deposit forms for low molecular drugs having no covalent bond to the polymer, e.g. by admixing of the active substance into the polymer or by imbeding between foils. 

Azobenzenes have been utilized to measure the free volume in polymers and the speed of polymeric segmental motion [42, 43], Azobenzenes that are covalently bonded to a polymer backbone may influence various properties of the macromolecule. Photoisomerization of such substances will cause changes in wettability [44], viscosity [45], solubility [46], membrane properties [47], and swelling properties [48]. 

Many groups have studied the grafting of different functionalities onto the backbone structure of different polymers. Some of these have involved covalent or ionic coupling of bio-active compounds to inert substances (22,23) while others have been concerned with exposing the monomer and polymeric substrate to ionizing radiation. (24,25,26,27)... 

Polystyrene For polystyrene, both covalently and noncovalently bound composite materials with carbon nanotubes are known. Polystyrene, like the methacrylates, can be generated by surface-initiated radical polymerization on nanotubes functionalized with initiator molecules (in analogy to Figure 3.86). Suitable substances then actually include analogous compounds like in the case of polymethacrylates. 

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