Permeability substituted polyacetylenes

Table 9 Oxygen permeability coefficients (P02) and P02/PN2 of gas permeable substituted polyacetylenes...

Table 4. Oxygen Permeability Coefficients (PO2) and PO2/PN2 of Highly Permeable Substituted Polyacetylenes...

Substituted polyacetylenes has been most intensively examined as gas-permeable materials aiming at practical application. These studies are motivated by the extremely high gas permeability of poly(TMSP), ... 

The second key factor determining permeability in polymers is the sorption coefficient. The data in Figure 2.18 show that sorption coefficients for a particular gas are relatively constant within a single family of related materials. In fact, sorption coefficients of gases in polymers are relatively constant for a wide range of chemically different polymers. Figure 2.25 plots sorption and diffusion coefficients of methane in Tanaka s fluorinated polyimides [23], carboxylated polyvinyl trimethylsiloxane [37] and substituted polyacetylenes [38], all amorphous glassy polymers, and a variety of substituted siloxanes [39], all rubbers. The diffusion... 

In contrast, organophilic PV membranes are used for removal of (volatile) organic compounds from aqueous solutions. They are typically made of rubbery polymers (elastomers). Cross-linked silicone rubber (PDMS) is the state-of-the-art for the selective barrier [1, 43, 44]. Nevertheless, glassy polymers (e.g., substituted polyacetylene or poly(l-(trimethylsilyl)-l-propyne, PTMSP) were also observed to be preferentially permeable for organics from water. Polyether-polyamide block-copolymers, combining permeable hydrophilic and stabilizing hydrophobic domains within one material, are also successfully used as a selective barrier. 

Membranes for gas separation have recently attracted much attention since membrane separation is one of promising energy-saving operations 104). For example, membranes for the oxygen enrichment of air, the isolation of helium, and the removal of C02 and S02 from natural gas are being intensively studied. The high gas permeability of substituted polyacetylenes, especially of poly[l-(triinethylsilyl)-l-propyne], is described below. 

Fig. 6. Oxygen permeability coefficient (Po2) and selectivity (Po2/Pn2) of typical polymers and substituted polyacetylenes (membrane thickness 30 pm, 25 °C)...

Typical functions of substituted poly acetylenes are based on their (i) high gas permeability and (ii) electronic and photonic properties. The former originates from the rigid main chain and bulky substituents. Though electrical insulators, substituted polyacetylenes are more or less conjugated polymers, and this feature has been utilized to develop their electronic and photonic functions such as photoconductivity, electrochromism, optical nonlinearity and ferromagnetism. 

Since many substituted polyacetylenes have unique properties (high 02-permeability, high Tg, good thermostability, etc. [18]), we became interested in the development of novel photoinitiators for the polymerization of substituted acetylenes (Scheme 9). It is known that certain substituted acetylenes can be polymerized upon UV irradiation of Mo(CO)6 or W(CO)6 [19]. However, the reaction can only be performed in CCI4, which, most probably, acts as a co-catalyst. Our goal was to develop a storage stable... 

Terminal alkynes substituted with chiral substituents have been polymerized by using a rhodium catalyst, [RhCl(NBD)]2 (NBD = norbomadiene) [6]. As shown in Scheme 3, polymerization of a chiral (carbamoyloxy)phenylacetylene 4 forms a cis-substituted polyacetylene 5. Due to the bulkiness of the substituents, these polymers show a helical conformation with no extended conjugation in the polymer chain. These materials are potentially useful as enantioselective permeable membranes to separate racemic amino acids and alcohols in water or in methanol. They can be also used as chiral stationary phase for enantioselective high-performance liquid chromatography (HPLC) analysis. 

Since polymers of substituted PA have good solubility and good air stability, they make good membranes. Even though substituted PA do not possess very high conductivity, some of them exhibit excellent gas and liquid permeability. These two factors combined imply that substituted polyacetylenes could potentially be used for the oxygen enrichment of air and the separation of ethanol-water mixtures [111]. 

Poly(diphenylacetylene) [poly(DPA)] exhibits the highest thermal stability among substituted polyacetylenes. In this polymer, weight loss is observed only above 500 C. However, insolubility of the polymer in any solvent has prevented detailed studies of its gas permeability 16-17). On the other hand, the introduction of sub-... 

Most properties of the substituted polyacetylenes (PA), such as permeability to penetrants, solubility, and thermal stability are strongly related to the number and/or steric hindrance of die substituents 1-4). However, these properties cannot be related in a strai tforward fashion to the kind of substituents or to the mono- or doublesubstitution along the main chain. In fact, the configuration of the repeat units and, consequently, the chain conformation and packing have to be taken into account and might be responsible for the properties of Afferent polymers. For instance, it has been shown that structural differences originating from different cis/trans tactic ratio in... 

Poly(DPA) is, thermally, the most stable substituted polyacetylene, but it is insoluble in any solvent, and hence it is impossible to fabricate a membrane from it by solution casting. In order to prepare this polymer membrane, desilylation of poly[l-phenyl-2-p-(trimethylsilyl)phenylacetylene] (PTMSDPA) membrane has been examined by use of trifluoroacetic acid as catalyst in hexane. The desilylation reaction proceeds to completion, as evidenced by IR spectroscopy and the anticipated weight decrease (eqn [a] in Scheme 4). As expected, the poly (DPA) membrane produced is insoluble in any solvent and possesses high thermal stability. This polymer shows fairly high gas permeability irrespective of the absence of any spherical substituent, as described below. The pinanylsilyl-containing poly(DPA) in eqn [b] is soluble, membrane-forming, and CD-... 

Helix is the most common higher-order structure of synthetic polymers such as peptides, polymethacrylates, polychloral, polyisocyanides, polyisocyanates, and polysilanes. Polyacetylenes bearing appropriate substituents also form a helix. Substituted helical polyacetylenes are promising candidates for enantioselective permeable materials, polarization-sensitive electro-optical materials, asymmetric electrodes, and hence their synthesis is currently under intensive research. This section overviews the synthesis and properties of helical polyacetylenes recently reported. 

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