Random hyperbranched molecules

Dendrigraft molecules are star-like in the Oth generation, but have a uniform interior with an exterior transition zone for G3 [17,34], Random hyperbranched molecules have broad distributions both in molecular mass and in shape [17], Scattering from hyperbranched is closer to that of linear polymers than to spheres, indicating that there is a gradually tapering distribution on units from the center to the exterior. 

Figure 11.7 illustrates the three different subtypes of dendritically branched molecules that have been identified within the major architectural class of dendritic polymers. Random hyperbranched polymers, not only exhibit polydispersity in molecular mass between individual molecules, it should also be noted... 

Fig. 28. The ratio A2M [rj] at large for star molecules (symbols) and randomly branched structures [25,26,108,130,131]. The shaded area indicates the range of the experimental findings with randomly and hyperbranched samples [144]. The line was drawn to guide the eye...

A 50% functionalization evokes the interesting question, bearing in mind facile transesterification, of how the fluoroalkyl chains will be distributed over the molecules and how they will be distributed on one particular molecule This question has been examined in detail for dendrimers of the poly(propyleneimine) type functionalized with stearic acid [33]. It was proven that the compositional heterogeneity (distribution of degree of substitution) is random, but the positional heterogeneity (spatial distribution of the substituents over the dendrimer molecule) is not random. However, due to flexibility, no particular effect of the spatial distribution can be observed. Unlike the dendrimers, we expect the hyperbranched polyesteramides to be stiffer, so that spatial distribution could lead to interesting effects if the molecule were composed of a functionalized side and a non-func-tionalized side (Fig. 28), as shown possible for dendrimers via a convergent synthesis [34]. 

To complete our discussion of branching, we are going to consider two special cases. The first of these involves random branching without network formation. There is presently (circa. 2007) considerable interest in the potential uses of hyperbranched polymers, and this is what you get if you perform a polycondensation on an A-R-B, molecule, where, again, an A can only react with a B. 

Synthesis of complex polymeric molecules such as random and block copolymers, star and graft polymers, hyperbranched and dendritic structures by NMP and other CRP techniques has been reviewed several times. Good overviews of telechelic polymers or the coupling of NMP with other polymerization techniques are available. Synthesis of bioconjugates through CRDRP, including NMP, is treated by Nicolas et... 

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