Dendritic polymer structure

Dendrigrafted polymer Hypergrafted polymer Linear dendritic hybrid FIGURE 5.12 Examples of dendritic polymer structures. 

Mixed aromatic-aliphatic polyethers have received attention of late as a means of forming liquid-cr talline and dendritic polymer structures. Liquid-liquid phase-transfer conditions were used for the polymerization of... 

Due to dieir compact, branched structure and to die resulting lack of chain entanglement, dendritic polymers exhibit much lower melt and solution viscosity dian their lineal" counterparts. Low a-values in die Mark-Houwink-Sakurada intrinsic viscosity-molar mass equation have been reported for hyperbranched polyesters.198 199 Dendrimers do not obey diis equation, a maximum being observed in die corresponding log-log viscosity-molar mass curves.200 The lack of chain entanglements, which are responsible for most of the polymer mechanical properties, also explains why hyperbranched polymers cannot be used as diermoplastics for structural applications. Aldiough some crystalline or liquid... 

Reid et al. [ 1.12] described the effect of 1 % addition certain polymers on the heterogeneous nucleation rate at-18 °C the rate was 30 times greater than in distilled, microfiltered water and at -15 °C, the factor was still 10 fold hogher. All added polymers (1 %) influenced the nucleation rate in a more or less temperature-dependent manner. However, the authors could not identify a connection between the polymer structure and nucleation rate. None the less it became clear that the growth of dendritic ice crystals depended on to factors (i) the concentration of the solution (5 % to 30 % sucrose) and (ii) the rate at which the phase boundary water - ice crystals moved. However, the growth was found to be independent of the freezing rate. (Note of the author the freezing rate influences the boundary rate). 

Dendrons and dendrimers are the most intensely investigated subset of dendritic polymers. In the past decade over 2000 literature references have appeared on this unique class of structure controlled polymers. The term dendrimer was coined by Tomalia, et al. over 15 years ago in the first reports on poly(amidoamine) (PAMAM) dendrimers [75, 76]. It is derived from the Greek words dendri-(branch tree-like) and meros - part of). Poly(amidoamine) dendrimers constitute the first dendrimer family to be commercialized and undoubtedly represent the most extensively characterized and best understood series at this time. In view of the extensive literature information in this area, much of the remaining overview will focus on PAMAM dendrimers and will... 

Since that time, synthetic chemists have explored numerous routes to these statistically hyperbranched macromolecular structures. They are recognized to constitute the least controlled subset of structures in the major class of dendritic polymer architecture. In theory, all polymer-forming reactions can be utilized for the synthesis of hyperbranched polymers however, in practice some reactions are more suitable than others. 

Dendrimers, the most precise subset of structure-controlled dendritic polymers preceeded by nearly half a decade, the more recent attention focused on hyperbranched polymers. It is notable that literature reports describing dendrimers far exceed the number of investigations published on random hyperbranched polymers. 

Dendritic polymers, the fourth major architectural class of macromolecules, can be divided into three subclasses. These subclasses may be visualized according to the degree of structural perfection attained, namely (1) hyperbranched polymers (statistical structures, Chapter 7), (2) dendrigraft polymers (semi-controlled structures, reviewed in this chapter) and (3) dendrimers (controlled structures, Chapter 1). 

Structure controlled dendritic polymers that have been studied using gel electrophoresis generally behave as mimics of either proteins or nucleic acids, and possess similar ionic groups such as -NH3 , -COO , or P04 functionality. Dendrimer structures may be widely modified as a function of their interior composition and as well as the nature of their surface groups. Depending on their structure, the influence of pH may vary dramatically for different dendrimers. 

Gel electrophoresis, a common analytical method in biological and medical research, can be conveniently applied in the analysis of dendritic polymers, owing primarily to the unique structure of the dendrimer analytes. It has currently been used to assess purity, measure molecular weight of dendrimers,... 

Another important issue is the difference between various branching types such as random hyperbranched [31], dendrigrafts and dendrimers. The complexity of synthesis requirements manifested by the statistical dendritic polymers versus the more structurally controlled dendrimers could make the former orders of magnitude more expensive than hyperbranched. Are the structures as significantly unique and of sufficient value effectiveness to justify the higher costs ... 

Some 17 years later, many of these predictions are turning into experimental reality as many of these questions are being answered in each new publication or patent that appears on dendritic architecture. Presently, dendritic polymers are recognized as the fourth major class of polymeric architecture consisting of three subsets that are based on degree of structural control, namely (a) random hyperbranched polymers, (b) dendrigraft polymers and (c) dendrimers (Figure 6). 

If combs represent one extreme of the topological family of branched polymers, then another extreme is given by the case of dendritic polymers, which retain a branched structure at all timescales. The study of tree-like branched architectures is also motivated by the important commercial low density polyethylene (LDPE), which has remarkable rheological properties making it suitable for many processing operations [3]. 

A useful toy theoretical model which captures the essential features of self-entangled dendritic polymers is the monodisperse Cayley tree in which each chain segment branches with a fixed functionality z at each of its ends, except those at the extremity of the molecule (see Eig. 13). Smaller versions of these structures, too low in molecular weight to be entangled, have been synthesised and are usually referred-to as dendrimers [47]. 

Fig. 7.1 Schematic representation of linear versus dendritic polymers linear (left) and hyperbranched (middle) polymers, perfect dendrimer (right). The amount of terminal groups is indicated below each structure. These architectures can also be attached to a cross-linked polymer bead to obtain a high-loading hybrid material.

Lue L, Prausnitz JM. Structure and thermodynamics of homogeneous-dendritic-polymer solutions computer simulation, integral-equation, and lattice-cluster theory. Macromolecules 1997 30 6650-6657. 

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