Dendrimer-like star-branched polymer

One of the ultimate branched polymers, as illustrated in Figure 5.11, has emerged since 1995 as a novel class of weU-defined hyperbranched polymers. Despite a variety of names having been proposed for these polymers, they have been termed recently as dendrimer-like star-branched polymers (DSPs), on the basis of their branched architectures, which are similar to those of well-established dendrimers. From a structural point of view, the DSPs represent promising specialty functional materials with many potential applications. 

Figure 5.n Sixth-generation dendrimer-like star branched polymer (DSP). 

Precise Synthesis of Dendrimer-Like Star-Branched Polymers, a New Class of Well-Defined Hyperbranched Polymers... 

Figure 5.1 3G and 5G dendrimer-like star-branched polymers and block copolymer. 

Precise Synthesis of Dendrimer-Like Star-Branched Polymers... 

As seen in Scheme 5.1, preparation of the IG polymer in this synthesis involved the ROP of cCL and subsequent chain-end modification. Conversion of the terminal hydroxyl group to two hydroxyl functions enabled further ROP to the 2G polymer. The IG polymer synthesized by this procedure was a 6-arm star-branched PcCL. The target dendrimer-like star-branched polymer was obtained as a 2G polymer by the second iteration and possessed a minimum architectural unit. One more repetition of the synthetic sequence involving the two reaction steps resulted in a 3G dendrimer-like star-branched PaCL. The 3G polymer possessed six branches at the core and two branches at the junctions in both the 2G- and 3G-based layers, composed of 42 arm segments (6 (IG) + 12 (2G) + 24 (3G) = 42). The observed M value was 96 000 g/mol, close to the theoretical value, and the molecular-weight distribution was not narrow, but an acceptable value of 1.14. 

The results of the 2G and 3G dendrimer-like star-branched polymers and block copolymers, after fractional precipitation, are summarized in Table 5.2. The resulting polymer all possessed the observed Mn values in good agreement with those calculated, and narrow molecular-weight distributions (M /Mn 1.1). Since the off-center living polymers and the 2G living dendrons used as subunits are sampled during the synthesis and well characterized prior to the synthesis and then reacted with a multifunctional core to synthesize the 2G and 3G polymers, this procedure corresponds to an example of an arm-first process. 

Table 5.2 Representative dendrimer-like star-branched polymers synthesized by the convergent iterative methodology...

Monteiro et al recently reported the convergent synthesis of 2G dendrimer-like star-branched polymers by combining click chemistry with ATRP, as shown in Scheme 5.8 (Urbani et al, 2008). This success indicates the possibility of click chemistry for the synthesis. However, as the molecular weights of the resulting polymers were not high ( 50 000 g/mol, MJM < 2) and the final click reactions proceeded incompletely, more optimization is needed for general use. 

As mentioned above, some difficulties arose in the synthesis of dendrimer-like star-branched polymers by the two aforementioned methodologies based on the core-first divergent and arm-first convergent approaches described in Sections 5.2.1 and 5.2.2. In fact, most of the polymers are limited to 4G stages and a few 10 g/mol orders in molecular weight except for two cases reported by Gnanou et al. (Tables 5.1 and 5.2). Moreover, the structures of the resulting polymers could not be well characterized and structural imperfections were indicated in several cases. 

Scheme 5.13 Introduction of functional groups at the periphery part (or termini) of 3G dendrimer-like star-branched polymer by the reaction of 3G brominated polymer with functionalized 1,1-diphenylalkyl anion.

Ree et al. recenfly reported synchrotron SAXS studies of 3G (A-A2-A4)4 and 4G (A-A2-Ag- 32)4 polymers showing that these polymers reveal ellipsoidal shapes composed of a core part and a shell part where the core part has a relatively higher density than the shell part (Jin et al., 2008 Jin et al., submitted). This indication is also quite different from the molecular shapes that would be imagined. Detailed SAXS study is expected to characterize the shapes of dendrimer-like star-branched polymers. 

In order to study the solution behavior of dendrimer-like star-branched polymers, the intrinsic... 

Since the molecular weight of dendrimer-like star-branched polymers can be made to vary widely by a change in the branched architecture, it is possible to compare the viscosity values between themselves, each in the same generation (Hirao et al, 2009b). The [t/]dendrimer-iike... 

Similarly, the g value was determined in each dendrimer-like star-branched polymer. The results are summarized in Table 5.8. As mentioned above, the [tjldendrimer-uke values were... 

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