Dendrimers polydispersity

Although branch defects may contribute in a minor way to dendrimer polydispersities, inter-dendrimer bridging/looping and extraneous Starburst dendrimer growth derived from unremoved propagating reagents are the major contributors. PAMAM dendrimers may be obtained as very monodispersed samples, if... 

As for conventional linear polymers, gel permeation chromatography (GPC) can be used to find information on the composition of dendrimers, including their polydispersities. Obtaining standards of known relative molar mass and polydispersity is a problem with dendrimers, so the approach that has been taken most often is to use polystyrene standards, as described in Chapter 6. 

The interest in hyperbranched polymers arises from the fact that they combine some features of dendrimers, for example, an increasing number of end groups and a compact structure in solution, with the ease of preparation of hn-ear polymers by means of a one-pot reaction. However, the polydispersities are usually high and their structures are less regular than those of dendrimers. Another important advantage is the extension of the concept of hyperbranched polymers towards vinyl monomers and chain growth processes, which opens unexpected possibilities. 

A way to narrow the MWD and to approach the structure of dendrimers is the addition of a small fraction of a/-functional initiator, to inimers [40,71]. In this process the obtainable degree of polymerization is limited by the ratio of inimer to initiator. It can be conducted in two ways (i) inimer molecules can be added so slowly to the initiator solution that they can only react with the initiator molecules or with the already formed macromolecules, but not with each other (semi-batch process). Thus, each macromolecule generated in such a process will contain one initiator core but no vinyl group. Then, the polydispersity index is quite low and decreases with / M /Mn l-i-l//. (ii) Alternatively, initiator and monomer molecules can be mixed instantaneously (batch process). Here, the normal SCVP process and the process shown above compete and both kinds of macromolecules will be formed. For this process the polydispersity index also decreases with/,but is higher than for the semi-batch process, M /Mn=Pn//. ... 

Alternatively, the one-step polymerization of branched monomers results in what is called a hyperbranched polymer [53] possessing a higher degree of polydispersity and lower degree of branching compared to the analogous dendrimer. 

Figure 10.3 Electrophoretogram of several generations of (EDA-core) PAMAM dendrimers (E 0-10). The unlabeled smear in the middle of the gel is a conventional polydispersed linear poly(lysine) sample as a comparison...

The development of mass spectroscopic techniques such as matrix assisted laser desorption (MALDI) and electrospray mass spectrometry has allowed the absolute determination of dendrimer perfection [7,8], For divergent dendrimers such as PAMAM and PPI, single flaws in the chemical structure can be measured as a function of generation to genealogically define an unreacted site of or a side reaction producing a loop at a particular generation level. Mass spectromet-ric results on dendrimers, not only demonstrate the extreme sensitivity of the technique, but also demonstrate the uniformity of the molecular mass. The polydispersity index of Mw/Mn for a G6 PAMAM dendrimer can be 1.0006 which is substantially narrower than that of living polymers of the same molecular mass [7],... 

Therefore, within the PAMAM family, the shapes of dendrimers span the range from stars to spheres. Small G3 dendrimers have a diffuse, open structure while large G10 dendrimers are spheres with a uniform interior, sharp outside transition, and low polydispersity. 

Dendrimers are nearly spherical in shape when compared to conventional linear polymers [ 17,34], The lower generation dendrimers have internal segment density distributions similar to star molecules, but even the smallest are more compact than stars. As the generation number increases, the sphere-like characteristics increase and the largest dendrimers form a population of spheres with uniform internal density (roughly half dendrimer units-half solvent) [47], sharp interfaces on the outside and a small polydispersity in shape. 

The distribution of molecular weights of each generation was determined from measurements on about 50 molecules, with results shown in Figure 12.19 (the weight fraction is the percent dendrimer in each interval of molecular weight under consideration). Based on these distributions, the polydispersity index (.MJMa) of G5 to G10 can be calculated, with results shown in Table 12.1 [39], They are all less than 1.08, which means that the particle size distribution is very uniform for each generation. 

PAMAM dendrimers No. of molecules MW (AFM) measured MW theoretical Relative error (%) Polydispersity... 

An interesting study that was performed on dendrimers is also applicable to hyperbranched polymers. Roberts et al. [134] studied the effect of the dendrim-er size when used inside the human body. They found that large dendrimers (M ca. 87,000) were passed into the urine and excreted within two days. Smaller dendrimers (M ca. 5,000), on the other hand, accumulated mostly in the liver, kidney and spleen with no urine excretion. Since most hyperbranched polymers are polydisperse, this might create a problem for in vivo applications. 

After activation with MAO (molar ratios [Al] [Zr] = 1000) the polymerization of ethylene has been successfully carried out using the zirconocene functionalized dendrimer at 40 bar ethylene pressure and 70 °C. We obtained high activity and productivity values for the ethylene polymerization and polymers with very high molecular masses in the range of 2 x 10 g/mol. The polydispersity of the polymer is quite low (3.0) indicating the single site character of the catalytically active species. Optimization of this system and study of the mechanism are stiU under investigation. Nevertheless, these preliminary results reveal the suitability of polyphenylene dendrimers as supports for zirconocene catalysts. 

For most purposes, polydispersity is not an obstacle, and currently the potentially cheaper hyperbranched polymers are successfully entering industrial research and application. Nevertheless, their success is based on the know-how built up in research on dendrimers. 

In contrast to the uniform dendrimer, a polydisperse hyperbranched polymer with OH-terminal groups showed only one transition corresponding to the collapse from the monolayer of flatly spread molecules to a disordered liquid. Dendrimer 1 is similar to dendrimer 2 in Fig. 8, except the difference in branch ends (CH3 vs OH), did not spread and exhibited no structural transitions upon compression. 

In the case of the bis-dendritic zirconium complex 91, a much smaller decrease in activity was observed relative to the reference zirconium complex, probably because of the bigger size of zirconium. Titanium dendrimers 88—90 gave polymers of higher molecular weights than those produced with CpTiCls. The polydispersity... 

How do dendrimers and hyperbranched polymers compare from an industrial viewpoint Dendrimers offer the potential for producing polymers whose molecular size and structure are more regular and less polydisperse. 1 lyperbranched polymers are easier and cheaper to synthsize—a one-pot synthesis compared to the multipot synthesis for dendrimers. However, not too many AB/ monomers are readily available, and this may modify the overall economics. Hyperbranched polymers will probably find use in larger-scale or commodity applications where lower cost is a necessity and dendrimers in specialty applications where higher cost is justified. 

---