Star-shaped units

Star-shaped molecules with oligophenyl arms derived from 9,9-spirobifluorene as the central unit have been synthesized by Tour et al. [58] and Salbeck et al. [59] and have been suggested as potential emitter materials for blue LEDs. 

Star shaped macromolecules are polymers, where the one end of f > 2 (f functionality of the star) linear chains is chemically attached by covalent bonds to a small central linker unit, are the simplest form of branched polymers. Modern anionic polymerization techniques allow us to synthesize star systems with a large number of nearly monodisperse arms [133, 134],... 

The core-first approach is based on the initiation of polymerization by a multifunctional initiator. The number of arms is then defined by the number of functional units present on the core. In order to have a good control of the molecular structure of star-shaped polyesters, the initiation must be quantitative and fast. It is also mandatory to avoid possible side-reactions between the initiating species on the core. 

The most comprehensive studies on shape-persistent Hekates have been performed on stilbenoid star-shaped molecules. Structures and mesomorphic properties are collected in Table 1. Core building blocks with only one repeating unit per arm and one flexible chain 26a,b, 28a-d, 30a did not show any liquid crystal properties [56-58]. In the series of two chain derivatives 28e, 30b,c the formation of meso-phases depend on the core [57-59]. The electron deficient triazine and the dicyanopyridine building block induced obviously columnar mesophases. The pyridine derivative 30b showed only a crystalline phase [58]. In the series of nine chain stars 26c-g [60-62] and 28f-n [57], the compounds formed columnar phases depending on the chain length of the peripheral chains. Propyloxy chains are too short but hexyloxy and dodecyloxy chains are sufficient for the formation of liquid crystal phases by nanosegregation [60-62]. This can be rationalised by a dense... 

QuasicrystaUine phases form at compositions close to the related crystalline phases. When solidified, the resultant strucmre has icosahedra threaded by a network of wedge disclinations, having resisted reconstruction into crystalline units with three-dimensional translational periodicity. The most well-known examples of quasicrystals are inorganic phases from the ternary intermetallic systems Al-Li-Cu, Al-Pd-Mn, Zn-Mg-Ln, Al-Ni-Co, Al-Cu-Co, and Al-Mn-Pd. In 2007, certain blends of polyisoprene, polystyrene, and poly(2-vinylpyridine) were found to form star-shaped copolymers that assemble into the first known organic quasicrystals (Hayashida et al., 2007). 

Wnek and coworkers synthesized a four-arm star-shaped PMMA with a cyclic siloxane core, as shown in equation 63. Four ketene silylacetal units were first attached to 1,2,3,4-tetramethylcyclotetrasiloxane (78) to obtain the tetrafunctional core 79, which was used as an initiator of the GTP of MMA with formation of the targeted four-armed star-shaped PMMA (80). 

Complementary dendritic hexamers based on a central scaffold made up of linked pentaerythritol and tri(hydroxymethyl)amino methane units have been found to also exhibit liquid-crystalline properties. This star-shaped scaffold was used to create supermolecules containing two different hemispheres, referred to thereafter as Janus supermolecular Hquid crystals (Fig. 76) [323,324]. One of the hemisphere contains three cyanobiphenyl end-groups, whereas the other lobe consists of three chiral phenyl benzoate mesogenic moieties laterally attached. The type of mesophase observed (N ... 

The core-first method, which uses an active multifimctional core to initiate growth of polymer chains, was apphcable to make hybrid POSS-core star-shaped polyoxazohnes that showed an increase in Tg, compared to that of polyoxazohne initiated by methyl p-toluenesulfonate (MeOTs) with poly(2-methyl-2-oxazohne) (POZO) [76]. Other hybrid star-shaped polyoxazohnes initiated by cube-OTs or cube-benzyl revealed the same phenomenon. This was attributed to the reduction of segmental mobifity of POZO in starshaped polyoxazolines, which was caused by the incorporation of hard, compact POSS moiety to the core of star polymer with the core-first technique. The conclusions were drawn that the thermal stabilities of star-shaped polymers increased as the POSS wt % was increased, and this was used as a measure of the effect of the inorganic POSS unit on polymer thermal properties. 

Star-shape polymers. Star polymers contain three or more polymer chains emanating from a central structural unit (Fig. 14.2). 

It is plausible to suppose that the temperature dependence should not be affected by branching unless the branching frequency is unusually high, since the friction factor f is presumed to reflect the behavior of relatively short (ca. five to twenty mer units) segments of the chain. Results on star-shaped branched polymers support this view 101, 136, 196). For example, the ratio exhibited the same dependence... 

Star-shaped structures may also be obtained using hydroxylated fuUerenes, the so-caUed fuUerenoles, and, for example, isocyanate-terminated polymers. These are Unked by a condensation that yields urethane units (Figure 2.68b). The resulting material is highly viscous and soluble in common organic solvents. 

Star-shaped core with iron-sandwich units... 

Star-shaped core with iron-sandwich units 2,987 4,527 = 2.0" 1.3" H2O (basic) Electrochemical aqueous nitrate and nitrite reduction [152]... 

A similar approach can also be applied to create a nanoporous network having two different polymerizable units. Lee published an LC formed by H-bonds between phloroglucinol and pyridine derivatives [69]. These molecules were mixed in a 1 3 ratio to form a star-shaped discotic columnar phase. The pyridine derivative contains two different photoreactive polymerizable groups in the alkyl chain. These groups, diacetylene and acrylate, might be selectively polymerized along the vertical and horizontal axes. The core molecules are removed with a sodium hydroxide treatment, to create the nanoporous conjugated network. These pores have a pyridine interior and an estimate pore size of 1 mn. 

Fig. 5 Fraction P of the terminal monomer units located at a distance r from the center of a star-shaped PE at different contents of a linear PE. z is the degree of charge compensation, which reflects the linear PE content. Reprinted from [89] with permission from the American Chemical Society...

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