Polyphenylene chain

Weber and Thomas [270] prepared rigid poly(p-phenylene)-type polymeric networks by reacting tetrafunctional building blocks of tetrabrominated 9,9 -spirobifluorene with benzene-1,4-diboronic acid or 4,4 -biphenyldiboronic acid in a mixed organic solvent. Here, the contorted unit of spirobifluorene forces the four polyphenylene chains to grow in four different directions. 

Well developed fibrillar structures were formed when the polymerization was carried out with M0CI5 [27], whereas PPP aggregates of indefinite shape were observed with FeCE as catalyst. The formation of the fibrils was interpreted as resulting from simultaneous polymerization and crystallization, and was attributed to the fact that benzene reacts with polyphenylene chain ends aligned on the crystalline surface of already precipitated PPP [27]. In the case of FeCl3, the crystallinity of the PPP is less pronounced, and it was suggested that benzene could form polyphenyl side-chains or polynuclear structures, which would prevent the formation of the fibrillar structures [24]. 

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. 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

LC polyesters belong to the class of thermotropic main-chain LCPs, which also comprises polymers such as polycarbonates, polyethers, polyphenylenes, polyester-imides, polymers containing azo- or azo V-oxide linking groups, some cellulose derivatives, and polypeptides such as po 1 y (y - be n zy 1 -1. - g 1 u tamate). Both from the academic and industrial points of view, polyesters are by far the most important representatives of this class of polymers. 

As shown in Eq. 2.59, zirconacyclopentadienes can be inserted into such a polymer chain (lq) from the beginning. The polymer can then be converted into polyphenylenes 88 in good yields [43]. Silyl-bridged diynes react with Cp2ZrBu2 to afford a macrocyclic tri-mer (89), as shown in Eq. 2.60 [43]. 

S. Setayesh, A.C. Grimsdale, T. Weil, V. Enkelmann, K. Mullen, F. Meghdadi, E.J.W. List, and G. Leising, Polyfluorenes with polyphenylene dendron side chains toward non-aggregating, light-emitting polymers, J. Am. Chem. Soc., 123 946-953, 2001. 

J.L. Bredas, R. Silbey, D.S. Boudreaux, and R.R. Chance, Chain-length dependence of electronic and electrochemical properties of conjugated systems polyacetylene, polyphenylene, polythiophene, and polypyrrole, J. Am. Chem. Soc., 105 6555-6559, 1983. 

Another approach to CPL is the synthesis of conjugated polymers with intrinsic chiro-optical properties. A variety of polymers with CPPL have been synthesized so far. Most of them are based upon well-known conjugated polymers such as poly(thiophene)s [4,111], polyphenylene vinylene)s [123], poly(thienylene vinylene)s [124], ladder polymers [125], PPPs [126], polyphenylene ethynylene)s, [127] and poly(fluorenes) [128]. All of them have been modified with chiral side-chains, which induce the chiro-optical properties. 

In contrast to dendrimers built up from aliphatic chains, polyphenylene dendrimers exist as shape persistent nanoparticles as we have demonstrated above. The preparation of functionalized dendrimers is the key step towards various applications. 

In contrast to dendrimers built up from aliphatic chains, polyphenylene dendrimer micelles possess shape and size persistent cavities due to their rigid scaffold which strongly depends on the type of dendrimer. In this case a selective incorporation of guest molecules, e.g., fluorescent dyes, should be possible, dependent on the size of the guest molecule and the cavity of the host. The non-covalent uptake of dyes with an appropriate size thus allows the investigation of their interactions within the dendritic micelle. In our case we made the second-generation polyphenylene dendrimer 48, which bears 16 carboxy-functions at the periphery, by starting from a tetrahedral core and an appropriately... 

Scheme 22. Second generation polyphenylene dendrimer surrounded by 16 dodecyl chains used as a hard-core-soft-shell system... 

As the laser beam can be focused to a small diameter, the Raman technique can be used to analyze materials as small as one micron in diameter. This technique has been often used with high performance fibers for composite applications in recent years. This technique is proven to be a powerful tool to probe the deformation behavior of high molecular polymer fibers (e.g. aramid and polyphenylene benzobisthiazole (PBT) fibers) at the molecular level (Robinson et al., 1986 Day et al., 1987). This work stems from the principle established earlier by Tuinstra and Koenig (1970) that the peak frequencies of the Raman-active bands of certain fibers are sensitive to the level of applied stress or strain. The rate of frequency shift is found to be proportional to the fiber modulus, which is a direct reflection of the high degree of stress experienced by the longitudinally oriented polymer chains in the stiff fibers. 

Stiff polymers, such as polyphenylene, nylon 66, polyphenylene sulfone, and polyarylether ketone (PEEK), have relatively high Tg values because of the presence of phenylene and sulfone or carbonyl stiffening groups in the chain. 

Aromatic cyclic chains are more stable than aliphatic catenated carbon chains at elevated temperatures. Thus linear phenolic and melamine polymers are more stable at elevated temperatures than polyethylene, and the corresponding cross-linked polymers are even more stable. In spite of the presence of an oxygen or a sulfur atom in the backbones of polyphenylene oxide (PPO), polyphenylene sulfide (PPS), and polyphenylene sulfone, these polymers are... 

Amide urethane, and ester groups in the polymer chain, such as those present in nylons and polyesters may be hydrolyzed by acids to produce lower-molecular-weight products. Polyacetals are also degraded by acid hydrolysis, but ethers, such as polyphenylene oxide (PPO), are resistant to attack by acids. 

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). 

The coupling can be catalysed by the presence of Fe(III) or Co(ll) but is most efficiently induced by the dipyridyl complex of Ni(II) chloride, as described by Yamamoto et al.52). It is also efficiently promoted by l,4-dichoro-2-butene, as described by Taylor et al.53). The structures of these Yamamoto polyphenylenes are discussed in Sect. 3 at this point it is sufficient to note that they are yellow/brown infusible powders, apparently having a high degree of p-coupling but relatively short (10—12 rings) chain lengths. 

In contrast to polymers produced by chain reactions, those derived from step-reaction routes are expected to have lower molecular weights. Naarman327 has used ir spectroscopy to estimate the phenyl end-groups, and so the molecular weights, of polyphenylenes produced by the Kovacik method. By increasing the reaction temperature from 5 to 36 °C, he was able to increase the degree of polymerization from 10 to 45. Kovacik and co-workers 328) used a similar approach and have estimated... 

Kovacic polyphenylene is brown with about 1 spin/chain detectable by ESR. Yamamoto polymer is yellow with a shorter chain length and fewer spins. In Kovacic polymer the spins and colour may both be due to polynuclear species. Polyphenylene produced from the poly(dihydrocatechol) precursor 249) is also yellow, but has a high molecular weight, of the order of 10s. It contains about 15% o-linkages, and the aromatization procedure may leave a high level of twists in the chain originating from the flexible precursor. This material dopes only to low levels of conductivity with sodium naphthalide (6x 10 3 Scm-1) and iron chloride (1.5 x 10-2 Scm-1) but reaches a level comparable to Kovacic and Yamamoto polyphenylenes with AsFs (102 S cm-1). 

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