Para-phenylene unit

Four equivalents of complex 1 react with tBuC C—C=C C=C C CtBu to give complex 110 with an intact C4 chain. Under the same conditions, the corresponding zirconocene complex 2a forms complex 111 through twofold C—C single-bond cleavage. If two butadiyne moieties are separated by a para-phenylene unit, as in RC=C C=C C6H4 C=C—C=CR, the titanium complex 112 is obtained. Depending on the stoichiometry, the two zirconocene complexes 113 and 114 can also be obtained [56 b],... 

This article has been focussing on poly(phenylene)s with 1,4-(para-)phenylene units since these polymers play a key role in the synthesis-driven search for electronic materials. From this article it has become clear that poly(phenylene) chemistry has not restricted its attention to linear (ID-) structures, but has more recently developed into 2D- and 3D-structures as well, the latter serving as functional shape-persistent nanoparticles. 

Cram and co-workers described complexation of [2.2]paracyclophane or para-phenylene unit containing crown compounds with primary ammonium salts, diammonium salts and alkali metal salts [23, 24]. Compound 5 makes complexes with tert-butylammonium thiocyanate (1 2 stoichiometry) and hexa-methylenediammonium or decamethylenediammonium hexafluorophosphate (1 1 stoichiometry). Cyclophanes 5-7 (Structure 2) solubilized 2 molar equivalent of tert-butylammonium tetraphenylborate in CHCI3 due to the resulting lipophilic complexes. 

There is a further very important class of main-chain liquid-crystal polymers that differ from those discussed above by being random copolyesters based on polymers such as poly(hydroxybenzoic acid) -fO— —C=0), where — — represents a para-disubstituted benzene ring (para-phenylene unit). The random copolymers include units of various other forms containing phenylene or naphthylene rings and their introduction reduces the crystallinity and melting point considerably without necessarily lowering the value of Polymers of these types have... 

Fox, M. A. Howard, J. A. K. MacBride J.A.H. Mackinnon A. Wade K. Big macrocychc assembhes of carboranes (big MACs) synthesis and crystal structure of a macrocychc assembly of four carboranes containing alternate ortho- and meta-carborane icosahedra linked by para-phenylene units, J. Organomet. Chem. 2003, 680, 155. 

Concerning the nature of electronic traps for this class of ladder polymers, we would like to recall the experimental facts. On comparing the results of LPPP to those of poly(para-phenylene vinylene) (PPV) [38] it must be noted that the appearance of the maximum current at 167 K, for heating rates between 0.06 K/s and 0.25 K/s, can be attributed to monomolecular kinetics with non-retrapping traps [26]. In PPV the density of trap states is evaluated on the basis of a multiple trapping model [38], leading to a trap density which is comparable to the density of monomer units and very low mobilities of 10-8 cm2 V-1 s l. These values for PPV have to be compared to trap densities of 0.0002 and 0.00003 traps per monomer unit in the LPPP. As a consequence of the low trap densities, high mobility values of 0.1 cm2 V-1 s-1 for the LPPPs are obtained [39]. 

Beside the polymerization routes of 1,3-cyclohexadiene derivatives repetitive Diels-Alder polyadditions were widely used to prepare arylated PPPs. Stille et al. developed a set of suitable monomers (1,4-diethynylbenzene and 1,4-phenyl-ene-bis(triphenylcyclopentadienone) derivatives) to generate phenylated PPPs (e.g. 17) with molecular weights of 20,000-100,000 [31]. Unfortunately, the repetitive polyadditon does not proceed regioselectively polymers containing para-as well as mefa-phenylene units within the main chain skeleton are formed. 

The first series of soluble oligo(para-phenylene)s OPVs (20) were generated by Kern and Wirth 1331 and shortly thereafter by Heitz and Ulrich [34]. They introduced alkyl substituents (methyls) in each repeat unit and synthesized oligomers (20) up to the hexamer. Various synthetic methods, like copper-catalyzed Ullmann coupling, copper-catalyzed condensation of lithium aryls, and twofold additon of organometallic species to cyclohexan-l,4-dione, have been investigated. 

For the synthesis of the target structures, it is absolutely necessary to introduce solubilizing substituents in the positions peripheral to the benzoyl substituents. The primary coupling product, 117, a poly(2,5-dibenzoyl-l,4-phenyl-ene) derivative - a poly(para-phenylene) with two benzoyl substituents in each structural unit - is, as expected, very poorly soluble. Highly substituted monomers (2,5-dibromo-l,4-bis(3,4-dihexyloxy-benzoyl)benzene), containing four solubilizing alkoxy groups per monomer unit, allow the synthesis of polymeric materials with M of about 12,000 and M, of about 22,000 [139]. 

The anion-radicals depicted in Scheme 3.62 were investigated by ESR and electron adsorption spectroscopy (Gregorius et al. 1992). The para isomer appears to behave completely different from the meta isomer. In full agreement with the results from MO theoretical calculations, the unpaired electron is delocalized over the whole para isomer, but confined to a stilbene unit in the meta isomer. The remaining parts in the meta isomer are uncharged. This spontaneous charge localization is not a consequence of steric hindrance, but follows from the role of the m-phenylene unit as a conjuga-tional barrier. 

Navarro [36] has reported several soluble, low transition temperature MCLC polymers containing ortho- or para-linked units and cinnamates or phenylene-fcis-acrylates in the main chain. One series of cinnamate-containing polymers is shown as 9. Some of the polymers seemed to be partially crystalline in the solid state. All had N and/or S mesophases which were retained in the solid glasses upon cooling to room temperature. Some of the polymers underwent thermal degradation and cross-linking above 300°C. Irradiation ( max = 300 nm)... 

Mohamed, N. A., and Al-Dossary, A. O. H. (2003), Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and mefa-phenylene units. Part III Preparation and properties of semi-permeable membranes for water desalination by reverse osmosis separation performance, Eur. Polym. /., 39,1653-1667. 

In many other cases polymers also contain benzene units as part of the chain structure (sections 2.3, 2.4 and 2.7). Poly-para-phenylene (PPP) 16 is the classical case of a conjugated polymer and of a rigid-rod species whose chain stiffness plays an important role in controlling supramolecular architecture (see... 

More tractable and potentially useful polyether ketones, incorporating phenylene-carborane-phenylene units, and with properties suitable for high temperature applications, have been prepared by acylation reactions (using trifluoromethanesulfonic acid as both medium and catalyst) between appropriate dicarboxylic acids and phenoxyphenylcarboranes. For example, the polyetherketone 20 (Scheme 3.6), derived from bis(4-phenoxyphenyl)-para-carborane and biphenyl-4,4 -dicarboxylic acid, is essentially amorphous on isolation from trifluoromethanesulfonic acid, but crystallizes when heated above its glass transition temperature (267°C) before hnally melting at about 390 0... 

Fig. 14.5. Sketch of repeat unit of PET. A 180° flip motion of the phenylene unit changes the orientation between the C—H bond of a protonated aromatic carbon atom and the applied magnetic field. Bo (labeled as sites 2 and 3). Note the difference in orientation of approximately 18° between the fiber axis and the para-axis of the aromatic ring.

Fig. 6.6 PAVs with meta-, para- and ortho-phenylene units.

Figure 1,59, Poly(para-phenylene sulphide) (PPS) (b) dibenzothiophene sulphide unit formed from (a) by an intramolecular phenyl-phenyl link (c) polybenzothiophene carrying a positive polaron.

Also phenylene units have been introduced into polycarbosilane chains either by polymerization of a para-bromo phenylsilane [103] ... 

Synthesis of Monodlsperse 01Igo(para-phenylene-ethynylene)s Using Orthogonal Protecting Groups with Different Polarity for Terminal Acetylene Units... 

---