Polyphenylenes building blocks

Fig. 78. Polyphenylenes building blocks used by Sakamoto and Mullen for glucosamine dendrimers.437... 

Certain polymers have come to be considered standard building blocks of the polyblends. For example, impact strength may be improved by using polycarbonate, ABS and polyurethanes. Heat resistance is improved by using polyphenylene oxide, polysulphone, PVC, polyester (PET and PBT) and acrylic. Barrier properties are improved by using plastics such as ethylene vinyl alchol (EVA). Some modem plastic alloys and their main characteristics are given in Table 1.2. 

Scheme 9. Three-dimensional stick-and-ball model of the first four generations of a polyphenylene dendrimer built from the biphenyl core and the A2B building blocks...

The achiral network of tetraphenylenes served as an inspiration for the design of a it-conjugated double helix. Double helical polymers, in which two polyphenylene helices are intertwined or tetraphenylenes are sequentially annelated, were recognized as the chiral building block of the network (Fig. 15.24) [107]. 

Fig. 15.24 Double helical polyphenylenes as building blocks of three-dimensional graphite".

Fig. 15. (a) Normalized pure-exchange CODEX intensities E(tm) as a function of tm for the aromatic ternary CH and the quaternary Cquat in Td-G2(-Me),6 dendrimer (T=363K). The fit curve for the ternary carbons is a stretched exponential cxp[—(rln/rcyi with /I = 0.51 and tc = 401 ms. The dotted line indicates the final CODEX exchange intensities, (b) Motional model of the localized, cooperative dynamics in polyphenylene dendrimers, including two-site jumps of all phenyl substituents of a pentaphenyl benzene building block. As indicated by X-ray analysis and computer simulations, the peripheral aromatic rings are inclined by 30° with respect to an axis normal to the face of the central benzene ring. For details, see ref. 44. 

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. 

Figure 8.6 Divergent synthesis of polyphenylene dendrimers through Diels-Alder reaction (Morgenroth et al, 1998). (Reproduced from F. Morgenroth, A.J. Berresheim, M. Wagner and K. Mullen, Spherical polyphenylene dendrimers via Diels-Alder reactions The first example of an A(4)B building block in dendrimer chemistry, Chemical Communications, 1139-1140,1998, by permission of the Royal Society of Chemistry.

Special emphasis is paid to a modular approach in which functionalized benzene moieties serve as building blocks for chain-type (ID), disc-type (2D), and dendrimer-type (3D) graphene derivatives (Fig. 1) [104,105]. The critical question, when taking on graphene as a challenge for polymer synthesis, is whether any of these polyphenylenes can serve as precursors for graphenes and how the necessary chemical transformation to flat graphene sheets can be accomplished. 

Scheme 2 Synthesis of ladder-type polyphenylenes using para- and meto-dibenzoyl benzene building blocks, and dehydrogenation of 3b to polymer 3 with para-quinodimethane repeat units. No control over the configuration at the bridging methylene carbons could be achieved...

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