Polymer main-chain extension

The hierarchical self-organization exhibited routinely by Nature and subsequently mimicked by chemists in the laboratory has opened up the field to a wide variety of potential applications and new methodology in the field of chemistry to which these concepts can be applied. Synthetic polymers can also exhibit several types and hierarchical levels of self-assembly, including (1) main-chain extension based on molecular self-assembly resulting in the formation of high molecular... 

Electron transport in polymers or doped polymers occurs by charge transfer between adjacent acceptor functionalities. As for hole transport, the functionalities can be associated with a dopant molecule, pendant groups of a polymer, or the polymer main chain. Most literature references are of doped polymers. Compared to the veiy extensive literature on hole transport, there have been relatively few references to electron transport. In part, this is due to difficulties related to trapping. To accurately measure the mobility, materials are required in which trapping can be neglected. This requires acceptor molecules with electron affinities that are large compared to potential traps. Since O2, a potential electron trap, is invariably present at high concentrations, the electron... 

Recently, an extension of the dehydrohalogenation process to the synthesis of a new class of heterobimetallic polyyne materials 174 which contain mixed metals with alternating Ru(dppe)2 and Pd(P Bu3)2 moieties (dppe = bis(diphenylphosphino)ethane) in the polymer backbone has been reported (Equation (67)), with 71/ =14,000 (by GPC measurements). The incorporation of ferrocene units into the polymer main chain and alternation between either Ni(PBu3)2 175 or Pd(PBu3)2 1 6 moieties has also been achieved (Equation (68)) and the molecular weights of the resulting polymers were 71/ = 26,100 and 21,400, respectively. 

As indicated earlier, another powerful tool for upgrading polymer properties is the postpolymerization reaction of preformed polymers. These reactions may occur on reactive sites dispersed in the polymer main chain. Such reactions include chain extensions, cross-linking, and graft and block copolymer formation. The reactions may also occur on reactive sites attached directly or via other groups/chains to the polymer backbone. Reactions of this type are halogenation, sulfonation, hydrolysis, epoxidation, surface, and other miscellaneous reactions of polymers. In both cases these types of reactions transform existing polymers into those with new and/or improved properties. 

For networks that exhibit a smectic phase, mechanical elongation always causes orientation of the director perpendicular to the axis of the stress [122, 125], as shown on Fig. 33. The mesogenic groups therefore become perpendicular to the polymer main chain if we make the reasonable assumption that the polymer chain is extended preferentially in the extension direction. Such an arrangement allows the polymer backbone to occupy the space between the layers. 

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

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. 

The successful development of polyfethylene terephthalate) fibres such as Dacron and Terylene stimulated extensive research into other polymers containing p-phenylene groups in the main chain. This led to not only the now well-established polycarbonates (see Chapter 20) but also to a wide range of other materials. These include the aromatic polyamides (already considered in Chapter 18), the polyphenylene ethers, the polyphenylene sulphides, the polysulphones and a range of linear aromatic polyesters. 

Solid state 2H NMR parameters are almost exclusively governed by the quadrupole interaction with the electric field gradient (EFG) tensor at the deuteron site.1 8 The EFG is entirely intramolecular in nature. Thus molecular order and mobility are monitored through the orientation of individual C-2H bond directions. Therefore, 2H NMR is a powerful technique for studying local molecular motions. It enables us to discriminate different types of motions and their correlation times over a wide frequency range. Dynamics of numerous polymers has been examined by solid state 2H NMR.1 3,7,9 Dynamic information on polypeptides by NMR is however limited,10 26 although the main-chain secondary structures of polypeptides in the solid have been extensively evaluated by 13C and 15N CP/MAS NMR.27,28... 

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