Dendritic monomers

Fig. 42. Combinatorial tier construction with isocyanate based dendritic monomers...

In THF at 20°C, dendritic monomers 3a-3c show absorption bands at 335 and 278 nm, due to the focal 1,4-diethynylbenzene unit and the dendritic wedges, respectively. On the other hand, dendritic macromolecular rods 2a-2c display a strong absorption band in the visible region (400-460 nm), characteristic of an extended electronic conjugation in the backbone. Upon excitation of the conjugated backbone at 425 nm in THF (abs425 nm = 0.01) at 20°C, 2a-2c show a strong blue fluorescence at 454 nm, where the quantum yield ( FL) has been evaluated to be virtually 100%. Of much interest is the fact that the fl>FL value of 2c stays at nearly 100%, even when the solution is concentrated until the... 

Branched phenylacetylene monomers can be used to construct dendrimers on supports. This was accomplished using a triazene tether as a focal point, the reaction triad outlined above, and an AB2 monomer, as seen in Scheme 7.16 As in similar solution-phase convergent dendrimer syntheses, the first step was to prime the periphery of the dendrimer with t-butyl groups by di-coupling of a triazene-tethered dibromoaryl monomer with (di-f-butyl-phenyl)acetylene. In this and all subsequent coupling steps, an excess amount of the monodendron acetylene was used to drive the reaction to completion. The reagents and catalysts were washed away and the excess monodendron was recovered. At the end of the first step, the tri-aryl dendron I-M3-(t-Bu)4 was cleaved from the support (M represents the dendritic monomer unit). Two equivalents of this product were then coupled with the triazene-tethered di-acetylene ary] monomer to produce the heptaaryl dendron I-M7-(r-Bu)8. 

IT number of terminal monomer units ID number of dendritic monomer units IL number of linear monomer units... 

In contrast to these successful efforts to link dendrimers and linear polymer chains in the manner of bloek copolymers, only very recently has a breakthrough been achieved in the synthesis of linear polymers with dendritic side groups, and this will be highlighted in this contribution (Scheme 1). Considerable synthetic effort is required in preparing suitable dendritic monomers. The amazing kinetic and structural features observed for such monomers and the respective polymers are nevertheless highly rewarding. 

Using Pd-catalyzed polycondensation reactions poly(p-phenylenes) with protected hydroxyl groups were prepared. These groups were used for the attachment of dendritic groups, in a similar way to the one referred to above. Dendritic monomers can also be used for the direct preparation of the desired structures (Scheme 106). 

The different types of dendritic monomer units are shown in Figure 30.5, where the dendrimer (Fig. 30.5a) contains three dendritic and six terminal groups, while the hyperbranched stmcture (Fig. 30.5b) has two dendritic, three linear, and five terminal units, yielding branching functionalities (as defined by Frdchet [11]) of 1 and 0.7, respectively. 

The hypermonomer strategy exploits the use of monomers that display higher funrtional group density (AB where n > 3, typically AB4 or ABg) than traditional dendritic monomer building... 

Figure 31 (a) A cyclic hexamer formed by the self-assembly of preorgahized dendritic monomers (54) containing complementary DDA and AAD hydrogen-bonding arrays [107,108] (b) a cyclic hexamer formed by self-assembly of a ditopic monomer (55) containing ureidopyridopyrimidine 6 [26]. 

Figure 35).Another alternative to dendritic monomers involves the preparation of a dendritic core terminated in olefins which are available for metathesis with a catalyst s alkylidene such that the catalyst becomes incorporated at the ends of the dendrimer. From there, ROMP of monomers such as norbomene elongates the dendritic ends into linear arms to yield a star polymer with a dendritic core (Figure 36). Interestingly, prior to ROMP, the catalysts attached to the dendrimer reversibly dimerized at low temperatures and high concentrations. 

J.M.J. Frechet (C. J. Hawker, 1990) replaced the divergent synthesis by a convergent growth of a dendritic polymer. The repeatedly employed monomer, 5-hydroxymethyl-l, 3-benzenediol, was 1,3-O-dibenzylatcd with 3,5-bis(benzyloxy)benzyl bromide. The resulting benzyl alcohol containing 7 benzene rings was converted to the benzyl bromide which was... 

In a second novel approach, Mullen and co-workers further developed an inter-molecular repetitive Diels-Aldcr procedure for the generation of dendritic and hy-perbranched poly(phenylcnc)s. Hereby, they applied the concept of reacting A2B-type monomers, in this case monomers containing both cyclopcnladienone (dien-... 

Hyperbranched polymers are characterized by their degree of branching (DB). Hie DB of polymers obtained by the step-growth polymerization of AB2-type monomers is defined by Eq. (2.1) in which dendritic units have two reacted B-groups, linear units have one reacted B-group, and terminal units have two unreacted B-groups191 ... 

Another definition, taking into account polymerization conversion, has been more recently proposed.192 Perfect dendrimers present only terminal- and dendritic-type units and therefore have DB = 1, while linear polymers have DB = 0. Linear units do not contribute to branching and can be considered as structural defects present in hyperbranched polymers but not in dendrimers. For most hyperbranched polymers, nuclear magnetic resonance (NMR) spectroscopy determinations lead to DB values close to 0.5, that is, close to the theoretical value for randomly branched polymers. Slow monomer addition193 194 or polycondensations with nonequal reactivity of functional groups195 have been reported to yield polymers with higher DBs (0.6-0.66 range). 

Dendrimers have structures similar to that of hyperbranched polymer and can be taken as the perfectly branched polymer with monodispersity. However, they need to be prepared by a multistep procedure. Therefore, very little work has been done on dendritic polyfarylcnc ether)s. Morikawa et al. prepared a series of monomers with a various number of phenylene units.164,165 These monomer were used to prepare poly(ether ketone) dendrons with graded structures (Scheme 6.24). 

Hyperbranched polymers are generally composed of branched (dendritic), Hn-ear, and terminal units. In contrast to AB2 systems, there are two different types of linear units in SCVP one resembles a repeat unit of a polycondensate (----A -b----) and one a monomer unit of a vinyl polymer (--a(B )---). 

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