142 Advances in Polymer Science Editorial Board: A. Abe. A.-C ...

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Asymmetric Star Polymers Synthesis and Properties 79 Scheme 3 tion using a suitable initiator. The living precursor then reacts in the second step with a small amount of DVB, leading to the formation of a star molecule bearing within its core a number of active sites, which is theoretically equal to the number of the A arms of the star polymer. Subsequent addition of another monomer, in the third step, results in the growth of B arms of the miktoarm stars, since the active star, prepared at the second step, acts as a multifunctional initiator for the polymerization of the second monomer. The growing B arms have anionic sites at their outer ends thus providing the possibility of reacting with electrophilic compounds or other monomers towards the preparation of end-functionalized stars or star-block copolymers. This method can be carried out in inert atmosphere, avoiding the use of the highly demanding and time consuming vacuum technique. It was first reported by Okay and Funke [11] and by Eschwey and Burchard [12] and developed by Rempp and collaborators [13–16]. Scheme 3 illustrates the DVB method. Despite the advantages mentioned above the DVB method is characterized by several disadvantages, the foremost being the architectural limitations. Only stars of the type A n B n can be prepared and even in this case there is no absolute control of the number of arms, n. In fact, n is an average value and is influenced by several parameters. Specifically, n is increased by decreasing molecular weight of the precursor A and by increasing the molar ratio of the DVB to living chains. Another major problem is that a fraction of the living chains A are not incorporated into the star structure due to accidental deactivation, the high molecular weight of the chains (steric reasons) or the low molar ratio of DVB to living chains. The unreacted living arms A can act as initiators after the addition of the second monomer. Another disadvantage is that the B arms cannot be isolated and characterized independently. Finally, reaction of the living ends with the remaining double bonds of the DVB nodule can lead to the formation of loops (intramolecular reaction) or networks (intermolecular reaction). From the above, it is clear that the miktoarm stars prepared by this method are characterized by rather poor molecular and compositional homogeneity.
80 N. Hadjichristidis, S. Pispas, M. Pitsikalis, H. Iatrou, C. Vlahos 2.2.1.1.2 Anionic Polymerization with Diphenylethylenes (DPE) 1,1-Diphenyl ethylene (DPE) derivatives were used for the synthesis of miktoarm stars according to the method developed by Quirk [17, 18]. Two moles of living polymer A react with one mole of 1,3-bis(1-phenylethenyl) benzene, DDPE, leading to the formation of the coupled product having two active sites. These active sites can act as initiators for the polymerization of another monomer, thus producing miktoarm stars of the type A 2 B 2 . The reaction sequence is given in Scheme 4. It is a three step procedure, using a divinyl compound in a similar manner as the DVB method. Stars of predetermined architectures can be prepared by this method but only polymers of the type A 2B 2 and ABC have been produced so far. More complicated structures such as AB 3, AB 5, A nB n (with n>2)or ABCD have not appeared in the literature. The crucial point of the procedure is the control of the stoichiometry of the reaction between the living A chains and the DPE derivative, otherwise a mixture of stars is produced. A major problem is the fact that the rate constants for the reaction of the first and second polymeric chain with the DPE derivative are different. This results in bimodal distributions because of the formation of both the monoanion and dianion. In order to overcome this problem polar compounds have to be added, but it is well known that they affect dramatically the microstructure of the polydienes that are formed in the last step. However the addition of lithium sec-butoxide to the living coupled DPE derivative, prior to the addition of the diene monomer, was found to produce monomodal well defined stars with high 1,4 content. Finally another weak point of the method is that, as in the case of the DVB route, the B arms cannot be isolated from the reaction mixture and characterized separately. It is therefore difficult to obtain unambiguous information about the formation of the desired products. Scheme 4
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142 Advances in Polymer Science Edi
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Branched Polymers I Volume Editor:
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Volume Editor Dr. Jacques Roovers I
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Preface While books have been writt
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Contents Synthesis of Branched Poly
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Synthesis of Branched Polymers by C
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Author Index Volumes 101-142 Author
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Author IndexVolumes 101-142 231 Dun
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Author Index Volumes 101-142 233 Ka
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Author Index Volumes 101-142 235 Og
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Author Index Volumes 101-142 237 Su
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Subject Index Addition-fragmentatio
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Subject Index 241 Microphases 11z,
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