Polyphenylenes characterization

To the best of our knowledge, apart from the above-mentioned systems of Miller and Neenan and the dendrimers presented by our group, no further work on large, highly branched polyphenylene dendrimers has been reported. This chapter will start therefore with the different ways of synthesizing these dendrimers, followed by their functionalization, characterization, and some applications. Finally, we present an outlook describing future work. 

The characterization of our polyphenylene dendrimers via mass spectrometry is particularly valuable because it allows the detection of potential growth imperfections during the [2+4] cycloaddition, even at the higher generations with molecular masses above 20,000 g/mol. In this way, incompletely reacted products give signals at lower molecular masses with characteristic mass differences in comparison to a perfectly reacted dendrimer. 

Beside mass spectrometry, NMR spectroscopy is a useful and widely applied tool to characterize the structure of dendrimers and estimate their purity [37]. In the case of polyphenylene dendrimers NMR spectroscopy is of only limited value for determining structure and purity. This is due to the large number of... 

Kimura et al. also presented the synthesis and characterization of a second-generation ruthenium(II) bis(terphenyl)polyphenylene dendrimer 52, an attractive molecule with regard to its electrochemical and photochemical pro-... 

Koch [82] and optimized further by us. The polyphenylene 28, which represents a first-generation dendrimer, forms 28 new bonds yielding the planar polycyclic aromatic hydrocarbon 56 (Scheme 23). Owing to its extreme insolubility in all common solvents, PAH 56 is characterized by laser desorption mass spectrometry based on its M+ peak at miz =1621. 

Other macromolecules are formed by condensing their monomers to form a repeat functional group (e.g., esters, amides, ethers) interspersed by alkyl chains, aromatic rings, or combinations of both. These condensations are characterized frequently, although not always by the loss of some by product (e.g., water, alcohol). The methods of formation of these polymers are far more varied than those of addition polymers. Examples of condensation polymers are (a) poly(esters), (b) poly(urethanes), (c) poly (carbonate), and (d) polyphenylene oxide). 

Eq. (5) in conjunction with Eqs. (8) and (9) have, so far, provided adequate representation of experimental isotherms6 32, which are characterized by an initial con vex-upward portion but tend to become linear at high pressures. Values of K, K2 and s0 have been deduced by appropriate curve-fitting procedures for a wide variety of polymer-gas systems. Among the polymers involved in recent studies of this kind, one may cite polyethylene terephthalate (PET) l2 I4), polycarbonate (PC) 19 22,27), a polyimide l6,17), polymethyl and polyethyl methacrylates (PMMA and PEMA)l8), polyacrylonitrile (PAN)15), a copolyester 26), a polysulphone 23), polyphenylene oxide (PPO)25), polystyrene (PS) 27 28), polyvinyl acetate 29) and chloride 32) (PVAc and PVC), ethyl cellulose 24) (EC) and cellulose acetate (CA) 30,3I>. A considerable number of gases have been used as penetrants, notably He, Ar, N2, C02, S02 and light hydrocarbons. 

Waters (39) has described the oxidation of 2.6-dimethylphenol with alkaline ferricyanide. The products he obtained were the diphenoquinone (VIII R=R,=CH3) and an amorphous material (M.W. 800) which he did not further characterize. In retrospect, it would appear that this product was a low molecular weight polyphenylene ether (VII R=R1=CH3). 

Preparation and characterization of highly branched aromatic polymers, polyphenylenes, polyesters, polyethers, and polyamides, were reviewed. These polymers were prepared from condensation of AB -type monomers, which gave noncrosslinked, highly branched polymers. The polymer properties are vastly different compared to their linear analogs due to their resistance to chain entanglement and crystallization. 

Even though there are several claims to the preparation of highly branched polyphenylene ethers, the characterization of these polymers has not been substantiated.35... 

In the Phillips process, polyphenylene sulfide (PPS) is obtained from the polymerization mixture in the form of a fine white powder, which, after purification, is designated Ryton V PPS. Characterization of this polymer is complicated by its extreme insolubility in most solvents. At elevated temperatures, however, Ryton V PPS is soluble to a limited extent in some aromatic and chlorinated aromatic solvents and in certain heterocyclic compounds. The inherent viscosity, measured at 206°C in 1-chloronaphthalene, is generally 0.16, indicating only moderate molecular weight. The polymer is highly crystalline, as shown by x-ray diffraction studies (9). The crystalline melting point determined by differential thermal analysis is about 285°C. 

Large polybenzenoids have been extensively studied by Mullen et al. [ 143 -145]. Among various sizes and shapes of these superacenes , as he named them, are 107,108, and the 50-benzene-unit PAH (109). These compounds are generally constructed from their dendritic polyphenylene precursors in good yields, albeit difficult to characterize conclusively. Even larger sheets have been proposed and where the limits lie is still unknown. 

Spin relaxation in dilute solution has been employed to characterize local chain motion in several polymers with aromatic backbone units. The two general types examined so far are polyphenylene oxides (1-2) and aromatic polycarbonates (3-5) and these two types are the most common high impact resistant engineering plastics. The polymer considered in this report is an aromatic polyformal (see Figure 1) where the aromatic unit is identical to that of one of the polycarbonates. This polymer has a similar dynamic mechanical spectrum to the impact resistant polycarbonates (6 ) and is therefore an interesting system for comparison of chain dynamics. 

Ryton Polyphenylene Sulfide is a new commercial plastic which is characterized by good thermal stability, retention of mechanical properties at elevated temperatures, excellent chemical resistance, a high level of mechanical properties, and an affinity for a variety of fillers. It is produced from sodium sulfide and dichlorobenzene. Its unusual combination of properties suggests applications in a variety of molded parts such as non-lubricated bearings, seals, pistons, impellers, pump vanes, and electronic components. Tough coatings of polyphenylene sulfide can be applied to metals or ceramics by a variety of techniques and are used as protective, corrosion-resistant coatings in the chemical and petroleum industries. Incorporation of small amounts of polytetrafluoroethylene provides excellent non-stick properties in both cookware and industrial applications. 

Brown, C. E., Kovacic, R, Wilkie, C. A., Hein, R. E., Yaniger, S. I., and Cody, R. B. J., "Polynuclear and Halogenated Structures in Polyphenylenes Synthesized from Benzene, Biphenyl, and p-Terphenyl under Various Conditions Characterization by Laser Desorption/Fourier Transform Mass Spectrometry,". Polym. Sci. Polym. Chem, Ed, 24, 255-267,1986. 

Similar to the other rigid aromatic polymers, the insolubility of polyphenylenes constitutes the major problem associated with their synthesis and characterization [5] indeed, this is the very reason why most polyphenylenes synthesized to date have been decorated with solubilizing side chains. Such substituents not only cause the dihedral angles to be detrimental for conjugation when connected at ortho positions (see Section 22.5.1), but they also dilute the main-chain properties, particularly in bulk. Consequently, the aim has been to reduce the side chain content as much as possible for certain applications. Bearing this in mind, Schliiter and coworkers have synthesized a variety of alkyl chain-substituted PPPs with systematically decreased densities of the side chains (Figure 22.22) [34]. 

A. Tran, B. Krnczek, Development and characterization of homopolymers and copolymers from the family of polyphenylene oxides, J. Appl. Polym. ScL, 106, 2140 (2007). 

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