Multifunctional cores

A potential source of structural imperfection is the rapid increase of reactive groups as growth is pursued. Their incomplete conversion leads to defects inside the molecule [27]. In convergent-iterative syntheses these problems are avoided by directing the dendritic growth from the surface inwards to a focal point. In a final step several dendrons are connected with a multifunctional core to yield the desired dendrimer (Fig. 9). 

Its design versatility, as generic dendrons may be prepared to be used later as building blocks in conjunction with other reactive molecules, or coupled to a multifunctional core to afford functional dendrimers, dendritic-linear hybrids, dendronized polymers, etc. This may be a particularly significant advantage if the coupled reactive or core molecule is itself sensitive to the reaction conditions used in the multiple steps of the iterative synthesis of a dendrimer. 

As we conjectured in the introduction, the fundamental role of topology in this approach to entangled polymer dynamics would indicate that changes to the topology of the molecules themselves would radically affect the dynamic response of the melts. In fact rheological data on monodisperse star-branched polymers, in which a number of anionically-polymerised arms are coupled by a multifunctional core molecule, pre-dated the first application of tube theory in the presence of branching [22]. Just the addition of one branch point per molecule has a remarkable effect, as may be seen by comparing the dissipative moduli of comparable linear and star polymer melts in Fig. 5. 

The convergent synthesis strategy proceeds in the opposite direction, from the periphery to the core, that is from the outside inwards. (Functionalised) dendri-mer components ( dendrons ) are bonded to the reactive terminal groups linked to a focal point of a multifunctional core unit. The CFP symbolism is again used to illustrate the principle of the synthesis (Fig. 2.2). 

This synthetic strategy can be altered for the preparation of branched oligo-knotanes, which necessitate a multifunctional core and monofunctional branching units. Reaction of the monohydroxy-knotane 28 with an excess of biphenyl-4,4 -disulfony 1 chloride 42 readily gives a sulfonylated knotane 49 containing one reactive sulfonyl chloride unit. 

Besides the divergent synthesis where the molecules are built up shell-by-shell starting from the centre, a convergent approach has been developed In which wedge like subunits, so called dendrons , are employed. The dendrons are prepared first and become assembled to the final dendrlmer via a multifunctional core (Scheme 3) [15,359]. The dendrons are versatile building blocks for molecules with a distinct tertiary structure (Scheme 4). 

Alternative approaches involve the reaction together of preformed blocks, such as in telechelic polymers and coupling reactions. The latter allow highly branched architectures such as star polymers to be formed by linking living anionic chains to a multifunctional core such as SiCl4 (Cowie, 1989a). 

Sawamoto et al. investigated the use of di- and trifunctional chloracetate initiators to carry out ATRP of MMA using the RuCl2(PPh3)3/Al(OiPr)3 catalyst system, then extended this to prepare calix [n arene-based multifunctional cores [346,347]. These cores were synthesized through the reaction of dichloroacetyl chloride and the corresponding calix [n] arene (Scheme 56) [346]. 

Star polymers are composed of a small multifunctional core and a given number of arms [32, 36], The core determines the arm number, and the arms determine the size of the stars. In the ideal case, the number of arms is constant throughout the sample and the arm length is the same for all the arms. However, real stars may deviate from the ideal situation. Often synthesis hardly provides both defined arm number and arm length. Hence, it is important to determine both the distribution of the arm number and arm length. 

This approach involves growth of the side arms outward from the core molecule, but an alternative is to prepare monofunctional living chains that are then coupled with a multifunctional core molecule. Other core molecules can be used that are modified inorganic structures, such as cyclotetrasiloxanes (10) and cyclotriphos-phazenes. These can be used as the initiating core for ATR polymerization of styrene or acrylate chains to form four- or six-armed star molecules. 

Linear polymer chains with an active terminal unit can be prepared using ATRP and NMP. These can be isolated, stored, and used later to couple with multifunctional core molecules. The use of divinyl benzene as the core is illustrated in Figure 5.5. 

While not related exclusively to block copolymer synthesis, the formation of many of the more complex architectures available through RAFT polymerization - including those based on a single monomer - shares the characteristics and caveats of linear block copolymer formation. One technique to obtain such structures (aldn to the triblock synthesis mentioned above) is the use of higher-level, multifunctional RAFT agents. A synthetic approach with a multifunctional core or a RAFT agent-functionalized polymer backbone allows... 

A strong dependence of the molar mass distribution on DP was also found for hb polymers obtained by SCVP, with Mw/Mn DP [126]. The broad molar mass distribution may be influenced by the different reactivities of the functional groups of the monomers, using the method of slow monomer addition [57, 62, 127, 128] or by adding a multifunctional core molecule [63, 127]. In A2 + B3 systems, both the polydispersity and the development of the molar mass are very sensitive to the amount of the added A2 monomer and to the formation of cycles [129,130]. 

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