Branched phenylenes

Two other types of mixed topology are the branched/linear and branched/angular motifs. To what extent can the bond localization of the central cyclohexatriene in 

The preparation of 64b relied on a modification of the iterative cocyclization strategy to linear [N]phenylenes (Section 4.2.1) [77]. Thus, diyne 61 [63] was co-cydized with bis(trisisopropylsilyl)-l,3,5-hexatriyne 62 [55]. The resulting 63 was 

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A mixed cyclothiophene including phenylene units was synthesized by Tobe et al. [423], Macrocycle 4.55 (Chart 1.56) was made, which includes four alternating 2,5-thienylene and 1,3-phenylene units and is similar in shape to cyclo [8]thiophene 4.31. The meto-branched phenylene units helped to form the macrocycle and reduced strain energy, but interrupted the overall conjugation. The synthesis of a corresponding pyridine-containing macrocycle 4.56 was attempted, but the ring-closure reactions of penta- and heptacyclic precursors failed [424]. 

The sulfonium precursor route may also be applied to alkoxy-substituted PPVs, but a dehydrohalogenation-condensation polymerization route, pioneered by Gilch, is favored 37]. The polymerization again proceeds via a quinomethide intermediate, but die syndicsis of the conjugated polymer requires only two steps and proceeds often in improved yields. The synthesis of the much-studied poly 2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylene vinylene], MEH-PPV 15 is outlined in Scheme 1-5 33, 35]. The solubility of MEH-PPV is believed to be enhanced by the branched nature of its side-chain. 

Dendrimers have structures similar to that of hyperbranched polymer and can be taken as the perfectly branched polymer with monodispersity. However, they need to be prepared by a multistep procedure. Therefore, very little work has been done on dendritic polyfarylcnc ether)s. Morikawa et al. prepared a series of monomers with a various number of phenylene units.164,165 These monomer were used to prepare poly(ether ketone) dendrons with graded structures (Scheme 6.24). 

Itarou H., Mays I.W., and Hadjichri-Stidis N., Regular comb polystyrene and graft polyisoprene/ polystyrene copolymers with double branches ( Centipedes ). Quality of (l,3-phenylene)bis(3-methyl-l-phenylpentylidene)dilithium initiator in the presence of polar additives. Macromolecules, 31,6697, 1998. 

Fig. 15. A branched [4]rotaxane from self-assembly of the triply-branched trisfbipyridini-um)derivative and bis-p-phenylene-34-crown-lO...

The chemical features that prohibit crystallinity are main chain flexibility (e.g., rotation), branching, random copolymers or low inter-polymer chain attraction. Normally, polymers are not miscible with each other and on cooling from the melt will separate into different phases. When miscibility is exhibited, e.g., poly(phenylene oxide) (PPO) and PS, crystallisation does not take place. 

One notable deviation from this trend of increased branched selectivity with increased 7i-accepting character has been reported. Nomura and coworkers reported allylic substitution reactions catalyzed by [lr(COD)Cl]2 and triphenylpho-sphine to form polymers linked by branched l,l -(l,4-phenylene)diprop-2-enyl units [49]. Despite this exception, most iridium-catalyzed allylic substitution... 

Synthesis of several bridged macrocylic poly( -phenylene-vinylene)s 44 is also reported by Naso. The presence of such a ring structure improves the emission properties of the synthesized polymer more than that of non-bridged and branched polymer 45 (Equations (21) and (22)). 

The same concept was used in polycondensation systems for the preparation of branched and cross-linked polymers [121]. Copolyurethanes 73 were prepared from tetra(ethylene glycol), bis(5-hydroxymethyl-l,3-phenylene)-32-crown-10 (74)... 

Schliiter et alJ65,66 have recently used rod-shaped polymers1673 such as poly(p-phe-nylene)s and poly([l.l.l]propellane)s as cores for the attachment of convergently generated dendrons. These new branched macromolecules possess a rigid backbone wrapped with structural wedges, which become increasingly more dense toward the outer cylindrical surface. The Pd-catalyzed copolymerization of dibromobiphenyl derivatives with the Frechet-type ethereal dendrons with a substituted aryl diboronic acid afforded the dendritic coated poly(p-phenylene) rod. 

By chain backbone is understood the polymer chain proper without side groups and branches. For instance, all vinyl polymers have two atoms per structural unit in the chain backbone. If an aromatic ring is part of the backbone, Z is counted as follows o-phenylene, Z = 2 m-phenylene, Z = 3 p-phenylene, Z = 4. For alicyclic rings the same rule is applied. 

Since the phenylene group is a rigid planar structure, both ends cannot be attached to the same silicon atom. The group therefore joins two separate silicon atoms, and some form of chain or branched polymer results. The silicon atoms still have an average functionality of two, however, and so they must be stopped off in some way. Monovalent organic groups may be used to accomplish this by means of a second Grignard reaction ... 

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