Polyphenylenes

One of the first hyperbranched polymers described in the literature was polyphenylenes, which were presented by Kim et al. [30-32] who also coined the term hyperbranched . The polyphenylenes were prepared via Pd(0) or Ni(II) catalyzed coupling reactions of various dihalophenyl derivatives such as di-bromophenylboronic acid. The polymers were highly branched polyphenylenes with terminal bromine groups which could be further transformed into a variety of structures such as methylol, lithiate, or carboxylate (Fig. 5). 

The polyphenylenes were brittle and did not form self-standing films when cast from solution. Therefore, they were considered poor materials. The use of these polymers was instead investigated as additives in polystyrene to improve processing and mechanical properties. A mixture of polystyrene and hyperbranched polyphenylene (5%) was studied and the results showed that the melt viscosity, especially at high temperatures and shear rates, was reduced by up to 80% as compared to pure polystyrene. Also, the thermal stability of polystyrene 

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 common feature of the p-phenylene group stiffens the polymer backbone so that the polymers have higher TgS than similar polymers which lack the aromatic group. As a consequence the aromatic polymers tend to have high heat deformation temperatures, are rigid at room temperature and frequently require high processing temperatures. 

One disadvantage of many of these materials, however, is their rather poor electrical tracking resistance. 

Although the first two materials discussed in this chapter, the polyphenylenes and poly-p-xylylenes, have remained in the exotic category, most of the other materials have become important engineering materials. In many cases the basic patents have recently expired, leading to several manufacturers now producing a polymer where a few years ago there was only one supplier. Whilst such competition has led in some cases to overcapacity, it has also led to the introduction of new improved variants and materials more able to compete with older established plastics materials. 

Poly-p-phenylene has been prepared in the laboratory by a variety of methods, including the condensation of p-dichlorobenzene using the Wurtz-Fittig reaction. Although the polymer has a good heat resistance, with decomposition 

The first EL from PPV stimnlated the work of chemists to develop new alternative routes to improve the preparation of this polymer. The idea of having a polymer soluble in organic solvent already in its conjugated form led to the introduction of solubilising groups in the PPV backbone. Some of the most studied structures that were developed are reported in Table 5.1. A detailed description of the synthesis for their preparation is not given references are reported for each structure [70-85]. Some of the general criteria followed to prepare these materials are discussed. 

The synthesis of conjugated copolymers is another approach followed to prepare new structures useful for device preparation. Moreover, the copolymeric approach is a suitable method for further tailoring the electrooptical properties of the active layer. In fact, the combination of different monomeric units, in different ratios, in the same structure, allows a fine modulation of the properties of the materials [90]. 

In Table 5.2 some of the many soluble copolymers based on the PPV structure are reported as examples. The structures so far prepared are mainly amorphous the glass transitions have to be taken into account for the preparation of devices where the dimensional stability is very important. Some of the materials are block copolymeric structures (9,12 and 16), while in other structures, although a repeating unit can be identified, the term copolymer is preferred because more than one chemical function appears in the repeating unit. As in the previous table, for each structure references are reported [69, 91-101]. 

An even better balance in the injection of the opposite charges was found when another layer more suitable for hole transport, such as unsubstituted PPV, is used [70]. A doublelayer LED, formed by ITO/PPV/cyano-PPV/Al has an internal quantum efficiency as high as 4% with emission at 610 nm [70]. The cyano-PPV approach was extremely useful and other withdrawing substituents were introduced in the PPV backbone with the aim of increasing the electron affinity other electron-deficient nitrogen containing groups such as oxadiazole [102,103], triazole [94], pyridine [104,105] and quinoxaline [106] were also introduced in the polymer structure. 

Copolymer 10 was found to give bright EL either with a dc forward bias or with a reverse bias voltage, and also with alternate field. The EL spectra are almost equivalent for a simple ITO/lO/Al configuration. 

Poylphenylenes were originally developed by Maxdem in California and subsequently by Mississippi Polymer Technologies (MPT). More recently MPT was bought by Solvay and the product is now sold under the trade name of Primospire. A pure paraphenylene would lack processability and so has to be substituted with phenylketone and/or copolymerised with unsubstituted metaphenylene [4-6]. In this way it is possible to produce both extrusion and injection 

Tg is a disadvantage. With reinforcement (e.g., glass or carbon fibre) 

PAEK can match and exceed many of the properties of Primospire. 

Accordingly it is not difficult to identify niches where polyphenylenes would be favoured over PAEK. 

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Polyphenylene polymers can be prepared by this coupling. For example, the preparation of poly(/i-quaterphenylene-2,2 -dicarboxylic acid) (643) was carried out using aqueous NaHCO and a water-soluble phosphine ligand (DPMSPP)[5I I]. Branched polyphenylene was also prepared[5l2). 

In polymers such as polystyrene that do not readily undergo charring, phosphoms-based flame retardants tend to be less effective, and such polymers are often flame retarded by antimony—halogen combinations (see Styrene). However, even in such noncharring polymers, phosphoms additives exhibit some activity that suggests at least one other mode of action. Phosphoms compounds may produce a barrier layer of polyphosphoric acid on the burning polymer (4,5). Phosphoms-based flame retardants are more effective in styrenic polymers blended with a char-forming polymer such as polyphenylene oxide or polycarbonate. 

Triphenyl phosphate [115-86-6] C gH O P, is a colorless soHd, mp 48—49°C, usually produced in the form of flakes or shipped in heated vessels as a hquid. An early appHcation was as a flame retardant for cellulose acetate safety film. It is also used in cellulose nitrate, various coatings, triacetate film and sheet, and rigid urethane foam. It has been used as a flame-retardant additive for engineering thermoplastics such as polyphenylene oxide—high impact polystyrene and ABS—polycarbonate blends. 

Trilialophenols can be converted to poly(dihaloph.enylene oxide)s by a reaction that resembles radical-initiated displacement polymerization. In one procedure, either a copper or silver complex of the phenol is heated to produce a branched product (50). In another procedure, a catalytic quantity of an oxidizing agent and the dry sodium salt in dimethyl sulfoxide produces linear poly(2,6-dichloro-l,4-polyphenylene oxide) (51). The polymer can also be prepared by direct oxidation with a copper—amine catalyst, although branching in the ortho positions is indicated by chlorine analyses (52). 

Polyphenylene oxide)s. Properties Comparison Chart, General Electric Co., Pittsfield, Mass., 1969. 

Polymerization Solvent. Sulfolane can be used alone or in combination with a cosolvent as a polymerization solvent for polyureas, polysulfones, polysUoxanes, polyether polyols, polybenzimidazoles, polyphenylene ethers, poly(l,4-benzamide) (poly(imino-l,4-phenylenecarbonyl)), sUylated poly(amides), poly(arylene ether ketones), polythioamides, and poly(vinylnaphthalene/fumaronitrile) initiated by laser (134—144). Advantages of using sulfolane as a polymerization solvent include increased polymerization rate, ease of polymer purification, better solubilizing characteristics, and improved thermal stabUity. The increased polymerization rate has been attributed not only to an increase in the reaction temperature because of the higher boiling point of sulfolane, but also to a decrease in the activation energy of polymerization as a result of the contribution from the sulfonic group of the solvent. 

Common conductive polymers are poly acetylene, polyphenylene, poly-(phenylene sulfide), polypyrrole, and polyvinylcarba2ole (123) (see Electrically conductive polymers). A static-dissipative polymer based on a polyether copolymer has been aimounced (124). In general, electroconductive polymers have proven to be expensive and difficult to process. In most cases they are blended with another polymer to improve the processibiUty. Conductive polymers have met with limited commercial success. 

A-Substituted polypyrazoles can also be obtained by using A-alkylhydrazines, and it should be noted that these polymers consist of a random mixture of head-to-head and head-to-tail structures. Other syntheses of polypyrazoles have been described in the literature. Thus polyphenylene pyrazoles (742) and (743) occurred when m- or p-diethynyl-benzene (DEB) reacted with 1,3-dipoles such as sydnones or bis(nitrilimines) (Scheme 64). 

Polypropylene has a chemical resistance about the same as that of polyethylene, but it can be used at 120°C (250°F). Polycarbonate is a relatively high-temperature plastic. It can be used up to 150°C (300°F). Resistance to mineral acids is good. Strong alkalies slowly decompose it, but mild alkalies do not. It is partially soluble in aromatic solvents and soluble in chlorinated hydrocarbons. Polyphenylene oxide has good resistance to ahphatic solvents, acids, and bases but poor resistance to esters, ketones, and aromatic or chlorinated solvents. 

Polyphenylene suLtide (PPS) has no known solvents below 190 to 205°C (375 to400°F) mechanical properties of PPS are unaffected by exposures in air at 230°C (450°F). It is resistant to aqueous inorganic salts and bases. 

When chloroform or methanol is used as the solvent for the oxidation of phenols, other products, originating from coupling of aryloxy radicals, e.g., polyphenylene ethers and/or diphenoquinones, are also formed. ... 

Negative mass spectrum from polyphenylene sulfide, 0-250 amu. 

Polymers containing oxazoline groups are obtained either by grafting the 2-oxazoline onto a suitable existing polymer such as polyethylene or polyphenylene oxide or alternatively by copolymerising a monomer such as styrene or methyl methacrylate with a small quantity (<1%) of a 2-oxazoline. The grafting reaction may be carried out very rapidly (3-5 min) in an extruder at temperatures of about 200°C in the presence of a peroxide such as di-t-butyl peroxide Figure 7.13). 

The uses of blends of polystyrene with the so-called polyphenylene oxide polymers are discussed in Chapter 21. 

The polyetherimides are competitive not only with other high-performance polymers such as the polysulphones and polyketones but also with polyphenylene sulphides, polyarylates, polyamide-imides and the polycarbonates. 

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