Branched and hyperbranched polymer

Fig. 14. Surface-grafted hyperbranched, branched, and linear polymers from ID to 3D...

The chain architecture and chemical structure could be modified by SCVCP leading to a facile, one-pot synthesis of surface-grafted branched polymers. The copolymerization gave an intermediate surface topography and film thickness between the polymer protrusions obtained from SCVP of an AB inimer and the polymer brushes obtained by ATRP of a conventional monomer. The difference in the Br content at the surface between hyperbranched, branched, and linear polymers was confirmed by XPS, suggesting the feasibility to control the surface chemical functionality. The principal result of the works is a demonstration of utility of the surface-initiated SCVP via ATRP to prepare surface-grafted hyperbranched and branched polymers with characteristic architecture and topography. 

Fig. 15. Schematic representation of the synthesis of hyperbranched, branched, and linear polymers grafted from functionalized silicon wafers SFM images and XPS spectra of the surface-grafted polymers. (Reproduced with permission from [48],Copyright 2001 American Chemical Society.)...

Keywords. Solution properties. Regularly branched structures. Randomly and hyperbranched polymers. Shrinking factors. Fractal dimensions. Osmotic modulus of semi-di-lute solutions. Molar mass distributions, SEC/MALLS/VISC chromatography... 

Dendrimers and hyperbranched polymers are two groups of materials resembling each other. The architectural difference is that dendrimers are perfectly branched structures, while hyperbranched polymers contain defects. Dendrimers are mono-dispersed while hyperbranched polymers are more dispersed which can be an advantage in some applications. 

The third International Dendrimer Symposium took place at Berlin Technical University in 2003. Interdisciplinary lectures demonstrated the extent to which dendritic molecules branch ouf into other areas of science, such as physics, biology, medicine, and engineering. The possibilities of functionalisation and resulting applications in industry were at the focus of this symposium. For example, nano-dimensioned dendrimer-based contrast agents were presented as multilabels for visualisation of blood vessels (see Chapter 8). Potential applications of dendritic materials as luminescence markers in diagnostics attracted lively interest (see Chapter 8). Consideration of the differences between dendrimers and hyperbranched polymers from the viewpoint of their cost-favourable application was also a topic of discussion [18]. 

This review covered recent developments in the synthesis of branched (star, comb, graft, and hyperbranched) polymers by cationic polymerization. It should be noted that although current examples in some areas may be limited, the general synthetic strategies presented could be extended to other monomers, initiating systems etc. Particularly promising areas to obtain materials formerly unavailable by conventional techniques are heteroarm star-block copolymers and hyperbranched polymers. Even without further examples the number and variety of well-defined branched polymers obtained by cationic polymerization should convince the reader that cationic polymerization has become one of the most important methods in branched polymer synthesis in terms of scope, versatility, and utility. 

The syntheses and applications of dendrimers and hyperbranched polymers have attracted considerable scientific interest in recent years. These highly branched polymers possess... 

In this review, complete coverage of the syntheses and applications of silicon-based dendrimers and hyperbranched polymers will be provided. For organizational purposes, dendrimers and hyperbranched polymers will be reviewed in separate sections, and each section will be further divided according to polymer class. For this review, the various classes will be defined according to the branch backbone substituents as follows ... 

Although linear polymers make up the bulk of commercial materials, alternate architectures such as crosslinked materials (thermosets), branched or graft copolymers, or dendrimers and hyperbranched polymers are used because of their unusual properties. 

The continuing developments in the preparation of dendrimers and hyper-branched polymers ° have moved this field into consideration by the polymer colloids community. As dendrimers and hyperbranched polymers get ever larger, their dimensions become sufficient to consider their solutions as molecular polymer colloids. Inevitably, therefore, research into such polymers will feature in new directions for polymer colloids research, both as polymer colloids in their own right and as additives for modification of the properties of existing polymer colloids. Such research will be most effective if carried out through collaborations of organic and polymer chemists with colloid scientists. 

Dendrimers and hyperbranched polymers are globular macromolecules having a highly branched structure, in which all bonds converge to a focal point or core, and a multiplicity of reactive chain ends. Because of the obvious similarity of their building blocks, many assume that the properties of these two families of dendritic macromolecules are almost identical and that the terms dendrimer and hyperbranched polymer can be used interchangeably. These assumptions are incorrect because only dendrimers have a precise end-group multiplicity and functionality. Furthermore, they exhibit properties totally unlike that of other families of macromolecules. 

Puskas and Grasmtiller characterized the synthesized star-branched and hyperbranched polyisobutylenes (PIBs) by SEC-light scattering in tetrahydrofuran (THF), with the dnidc measured as 0.09 mL/g. The radius of gyration gave a slope of 0.3, demonstrating the formation of a star-branched polymer [10]. 

Recently the development of dendritic and hyperbranched polymers (HBPs) has attracted much attention (Tomalia, 1985, Newkome et al, 1985, Webster, 1991, Chu and Hawker, 1993, Wooley et al, 1994, Feast and Stanton, 1995, Malmstrom et al, 1995, Kim, 1998). The key features of the macromolecular architecture of dendrimers and HBPs are given in Section 1.2, and their synthesis by stepwise polymerization is discussed in Section 1.2.1. Dendrimers and HBPs are globular macromolecules that have a highly branched structure with multiple reactive chain ends (shell), which converge to a central focal point (core) see Figure 5.1, where I is the core, 11 is the structure and 111 is the shell. 

For the propylene polymerization catalyzed by the complexes 1-7 (Scheme 2) the simulations were performed [27] based on the calculated energetics of the elementary reactions [ 13c-d]. For system 6 of Scheme 2, the calculated average number of branches is 238 br./lOO C, which is slightly larger than the experimental value of 213 br./lOO C. However, the temperature and pressure dependence of the number of branches and the polymer microstructure are in-line with experimental observations [21] 1) an increase in polymerization temperature leads to a decrease in the number of branches 2) olefin pressure does not affect the branching number, but affects the topology, leading to hyperbranched structures at lowp. 

Theoretical description of dendritic structures is an active but difficult topic because the length between branched points is close to or even smaller than the Kuhn length, above which the segments can be thought of as if they are freely jointed with each other. Most modeling or simulation work has focused on the quasi-static and rheological properties of hyperbranched polymers and dendrimers in shear flow [240, 244, 255-263]. Limited amounts of data were published for dendrimers and hyperbranched polymers within elongational flow field. 

The itmer part of the dendrimer and hyperbranched polymers is more extended than the outer part. If researchers want to incorporated mechanophores into dendrimers and hyperbranched polymers, we infer the most probable positions are the branch units close to the core. 

Perfectly branched dendrimers have potentially better properties for applications in the field of biomedicine than hyperbranched polymers due to their well-defined and predictable structure and narrow mass distribution, which is important for in vitro and in vivo applications. However, hyperbranched polymers have one very significant advantage, which is their easier preparation by a one-step synthesis. Therefore, hyperbranched polymers are also used in technical applications, for example, as additives, blends, or coating components and as multifunctional cross-linkers. But both, dendrimers and hyperbranched polymers, have been extensively studied in the fields of encapsulation and delivery of drugs, dyes, and genes because of their original branched architecture (Fig. 5.15). Small molecules of interest can be incorporated in the interior cavities of dendritic molecules or bound to their outer functional groups. 

---