Polyacrylonitrile and related polymers

Grassie N, McGuchan R, Pyrolysis of polyacrylonitrile and related polymers, Europ Poly J, 8, 257, 1972. 

Polyacrylonitrile and polymethacrylonitrile make a fifth pair of this kind, but the viscosity relations for the latter are based on osmotic measurements for only four unfractionated polymers with molecular weights lying between 180,000 and 500,000 [Fuhrman and Mesrobian (115a)], so that only a crude estimate of a can be made for this polymer. Nevertheless, the effect of the methyl group is about the same as in the other, better established cases. 

Polymers containing long alkyl side chains are likely to have good resistance to water and alcohol since the solubility parameters of the polymer and solvents are quite different. Conversely, polar polymers, such as polyacrylonitrile, are predicted to show good resistance to attack by aliphatic hydrocarbons. By the same token, the longer alkyl chain acrylics are expected to be more soluble in aliphatic solvents since solubility parameter of the former polymers is more nearly equal to that of aliphatic hydrocarbons. These and related predictions of this theory have been experimentally verified innumerable times by coating chemists and formulators. 

Chapter 5 shows that the application of hydrolytic enzymes is a powerful yet mild strategy to directly improve polymer surface properties (i.e. hydrophilicity) or activate materials for further processing. The surface hydrolysis of polyamides (PA), polyethyleneterphthalates (PET) and polyacrylonitriles (PAN) is discussed, as well as the mechanistic details on the enzymatic surface hydrolysis. The mechanistic data, combined with advances in structural and molecular biology, help to explain different activities of closely related enzymes on polymer surfaces. 

It is also possible to prepare all-carbon polymers of closely related structure. For example, pyrolysis of polyacrylonitrile, (-CH2CHCN-)X, first results in cyclization of some of the -CN side chains.61 Prolonged pyrolysis yields very pure graphitic material. It is very strong and has high thermal stability. In the form of fibers, it can be used for reinforcement in high-performance composites. Additional information on pyrolysis is given in Chapter 9. 

In the present article an attempt is made to collect, and interpret on a molecular basis, what is known thus far about the interactions of polyacrylonitrile molecules with each other and with other molecules, and to relate such interactions to macroscopic properties of polymer and fiber. 

Initial achievements in this field were carried out by Kobayashi as early as 1978. Copolymerization of 71 with acrylonitrile produced linear polymers 72 (Scheme 10.14), which are soluble in polar aprotic solvents, that were assayed for different Michael additions. In general, good conversions were obtained, but enantioselec-tivities were always below 60% ee [204-206]. In a related approach by Oda, different spacers were introduced between the polyacrylonitrile backbone and the chiral fragment, which resulted in an increase of selectivity up to 65% ee [207]. Addition of thiol groups in PS-DVB resins to the double bond allowed the preparation of the corresponding insoluble polymers 73, which were assayed by Hodge for the addition of thiols to unsaturated ketones and nitrostyrene. Again, selectivities were... 

The solubility of chemically related compounds decreases with increasing molecular mass since the intermolecular forces of interaction increase. For example, benzene is completely miscible with ethanol, whereas anthracene and ethanol are only partially miscible. The influence of molecular mass on solubility is particularly evident in macromolecules. For example, alcohol, acetone, and acetic acid readily dissolve styrene, but not polystyrene vinyl aeetate dissolves in saturated hydrocarbons and ether, whereas poly(vinyl acetate) does not. Cellulose is insoluble in alcohols, polyfethylene glycol) is insoluble in ethers, poly(vinyl chloride) is insoluble in vinyl chloride, and polyacrylonitrile is insoluble in acetonitrile, even though good solubility would be expected on account of the chemical relation between the polymers and monomers. 

Polymer pyrolysis refers to the pyrolytic decomposition of metal-organic polymeric compounds to produce ceramics. The polymers used in this way are commonly referred to as preceramic polymers in that they form the precursors to ceramics. Unlike conventional organic polymers (e.g., polyethylene), which contain a chain of carbon atoms, the chain backbone in preceramic polymers contains elements other than carbon (e.g., Si, B, and N ) or in addition to carbon. The pyrolysis of the polymer produces a ceramic containing some of the elements present in the chain. Polymer pyrolysis is an extension of the well-known route for the production of carbon materials (e.g., fibers from pitch or polyacrylonitrile) by the pyrolysis of carbon-based polymers (54). It is also related to the solution sol-gel process described in the previous section where a metal-organic polymeric gel is synthesized and converted to an oxide. 

The principal forms of spinning are listed in Table 11-1, together with the phase-transformation process as well as the principal commercial fibers formed by each technique. Generally, the selection of a particular type of spinning process is related to the material being spun. For example, nylons are semicrystalline in the solid state and have a definite melting point. Whereas the nature of solid nylon makes it difficult to put it into a solution, it does not prevent a melt from being formed. Hence, nylon is a melt-spun fiber. On the other hand, polyacrylonitrile is an amorphous solid and, as such, is dissolved more readily than a semicrystalline polymer. Here, the result is that polyacrylonitrile is either dry-... 

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