Polymer the main-chain

Linear Werner-type coordination polymers are defined as linear polymers, the main chains of which contain coordinate covalent bonds of transition metals. In this article, we restrict the discussion to the linear coordination polymers which can exist as a stable form in solution and thus our purpose is somewhat complementary, for example, to review dimensional inorganic complexes such as those of Miller and Epsteinld). 

In the smectic polymers the main chain must be confined to parallel planes within the smectic layers since the observed periodic structure in the direction of the layer normal can only be obtained by this arrangement. The X-ray data do not yield information on the actual two dimensional conformation of the chain on these smectic planes. 

Such polymer topologies have not been utilized in DNA transfection studies except for one study by Liu et al. [68], who report on bmsh-like polymers with PEl-(>-PEO side chains. However, in these polymers the main chain length was hardly longer than the side chain length, resulting in a spherical rather than cylindrical brush structure. 

The chromophores are closely related to those in peptides. Dissymetric perturbations of the amide chromophores of the main chain arise solely from side chain effects the aliphatic polyisocyanate has enhanced rotational strength as compared to its model compound (5 )-(-)-A,A -diacetyl-2-methylbutylamine. In the aliphatic polymer, the main chain has inherently symmetric chromophores which acquire optical activity from dissymetric perturbation of their environment by the side chain. In the aromatic polymer an additional Cotton effect also arises from interactions among the aromatic side chains. This enhancement may be explained by a conformational preference resulting from favored spatial arrangements of the asymmetric side chain but the study was complicated by the fact that the polymer is insoluble in most organic solvents except in chloroforms and by the specific interactions between this solvent and the urea-like main chain (XIXc). 

In this subclass of side-chain polymers, the main chain always consists of a n-conjugated backbone with electron-donating characteristics, the so-called p-type cable, to which several electron-accephng fullerene cages, or an n-type cable, are covalently linked. Owing to its intrinsic electronic properties, numerous double-cable-polymers (D-C) have been employed in electro-optical devices, namely photovoltaic devices (Chapters 7 and 8) [50]. 

Incorporation of cyclic aliphatic (aUcycHc) side groups markedly improves the plasma etch resistance of acryhc polymers, without reduciag optical transparency at 193 nm (91). Figure 32 presents stmctures of some acryhc polymers currendy under study for use ia 193-nm CA resists (92—94). Recendy, polymers with main-chain aUcycHc stmctures have been described that offer similar properties (95,96). 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

The polyamides are soluble in high strength sulfuric acid or in mixtures of hexamethylphosphoramide, /V, /V- dim ethyl acetam i de and LiCl. In the latter, compHcated relationships exist between solvent composition and the temperature at which the Hquid crystal phase forms. The polyamide solutions show an abmpt decrease in viscosity which is characteristic of mesophase formation when a critical volume fraction of polymer ( ) is exceeded. The viscosity may decrease, however, in the Hquid crystal phase if the molecular ordering allows the rod-shaped entities to gHde past one another more easily despite the higher concentration. The Hquid crystal phase is optically anisotropic and the texture is nematic. The nematic texture can be transformed to a chiral nematic texture by adding chiral species as a dopant or incorporating a chiral unit in the main chain as a copolymer (30). 

Mechanical Properties Related to Polymer Structure. Methacrylates are harder polymers of higher tensile strength and lower elongation than thek acrylate counterparts because substitution of the methyl group for the a-hydrogen on the main chain restricts the freedom of rotation and motion of the polymer backbone. This is demonstrated in Table 3. 

The main chain of these polymers contains, as the principal component, five- or six-membered heteroaromatic rings, ie, imides, which are usually present as condensed aromatic systems, such as with benzene (phthalimides, 3) and naphthalene (naphthalimides, 4) rings. 

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. 

Hydrophilic spacer groups may be introduced into a polymer through the side chain, the main chain, or both. Films can be prepared using different values of monomer feed (62). 

The living polymerization process offers enormous flexibiUty in the design of polymers (40). It is possible to control terminal functional groups, pendant groups, monomer sequencing along the main chain (including the order of addition and blockiness), steric stmcture, and spatial shape. 

---