Asymmetric stars

Hadjichristidis, N, Pispas, S.t Pitsikalis, MIatrou, H Vlahos, C. Asymmetric Star Polymers Synthesis and Properties, VoL 142, pp, 71428. 

A very good example is the conductance of a dianthra[a,c]naphtacene starphenelike molecule presented in Fig. 20, interacting with three metallic nano-pads. The EHMO-NESQC T(E) transmission spectrum per tunnel junction looks like a standard conjugated molecule T(E) with well-identified molecular orbitals and their resonances. For the Fig. 20 case all the T(E) are the same. One can note a small deviation after the LUMO resonance, due to a little asymmetry in the adsorption site between the three branches on the nano-pads [127]. A lot of asymmetric star-like three-molecular-branches system can be constructed, in particular in reference to chemical composition of the central node. This had been analyzed in detail [60]. But in this case, each molecule becomes a peculiar case. The next section presents one application of this central-node case to construct molecule OR and molecule XOR logic gates. 

Different types of LCB are distinguished. Star polymers are the simplest branched polymers because they have only one branch point. Regular star polymers have a branch point with a constant number (functionality,/) of arms and every arm has the same molecular weight. They are therefore monodisperse polymers. Star polymers may also have arms with a most probable distribution [5], Star polymers can also be polydisperse due to a variable functionality. Palm tree [6] or umbrella polymers [7] that contain a single arm with different molecular weight (MW) than the other arms are classified under the asymmetric star [8] polymers, see Figure 3.2. 

Fig. 33. Struciures of the asymmetric star cluster Ni2W4Pt2(/i-CR)(ji3-CR)3(ji-CO)-(CO)7Cp4 (147) and the symmetric star" cluster Ni2W4Pt2( 3-CR)4(CO)8Cp4 (148).

Synthesis of AA B, ABB, and ABC Asymmetric Star-Block Copolymers Using Furan Derivatives. 126... 

AA B asymmetric star-block copolymer Scheme 12 Synthesis of AA B asymmetric star-block copolymer... 

The synthesis and the properties, both in bulk and in solution, of asymmetric star polymers are reviewed. Asymmetry is introduced when arms of different molecular weight, chemical nature or topology are incorporated into the same molecule. The phase separation, aggregation phenomena, dilute solution properties etc. are examined from a theoretical and experimental point of view. Recent applications of these materials show their importance in modern technologies. 

Asymmetric star polymers are megamolecules [1] emanating from a central core. In contrast to the symmetric stars very little was known, until recently, about the properties of the asymmetric stars. This was due to the difficulties associated with the synthesis of well-defined architectures of this class of polymeric materials. The synthesis, solution and bulk properties, experimental and theoretical, of the following categories of asymmetric stars will be considered in this review ... 

Fig. la-c. Asymmetric stars with a molecular weight asymmetry b chemical asymmetry c topological asymmetry... 

Asymmetric PS stars of the type (PSA)n(PSB)n were also prepared by the divinyl-benzene (DVB) method [9]. Living PS chains, prepared by sec-BuLi initiation, were reacted with a small amount of DVB producing star homopolymers. The DVB core of the stars contains active anions which, if no accidental deactivation occurs, are equal to the number of the arms that have been linked to this core. These active sites are available for the polymerization of an additional quantity of monomer. Consequently further addition of styrene produced asymmetric star polymers... 

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