Star like branched polymer

Fig. 5.12a-d. Different types of branching in polymers a short chain branching b long chain branching c star like branching d hyperbranching (dendrimers)... 

Dendrimers are polymers forming a rigid star-like branched structure. Monomer units of this starshaped structure could vary widely from amino acids to polyesters, which also determines its characteristics. In addition breakthrough synthetic techniques such as lego (Brauge et al. 2001) and click (Wu et al. 2004) chemistries have advanced both the efficiency and innovation of possible dendrimer structures. Different combinations of the core, monomer units, and surface functionality have resulted in the emergence of more than hundreds of dendrimers. Table 85.1 lists major classes of dendrimers based on their chemical structure and also their use in drug delivery applications. 

Two types of well defined branched polymers are acessible anionically star-shaped polymers and comb-like polymers87 88). Such macromolecules are used to investigate the effect of branching on the properties, 4n solution as well as in the the bulk. Starshaped macromolecules contain a known number of identical chains which are linked at one end to a central nodule. The size of the latter should be small with respect to the overall molecular dimensions. Comb-like polymers comprise a linear backbone of given length fitted with a known number of randomly distributed branches of well defined size. They are similar to graft copolymers, except that backbone and branches are of identical chemical nature and do not exhibit repulsions. 

The synthesis of well-defined LCB polymers have progressed considerably beyond the original star polymers prepared by anionic polymerization between 1970 and 1980. Characterization of these new polymers has often been limited to NMR and SEC analysis. The physical properties of these polymers in dilute solution and in the bulk merit attention, especially in the case of completely new architectures such as the dendritic polymers. Many other branched polymers have been prepared, e.g. rigid polymers like nylon [123], polyimide [124] poly(aspartite) [125] and branched poly(thiophene) [126], There seems to be ample room for further development via the use of dendrimers and hyperbran-... 

At the same time, the macromolecules might be classified according to whether their chains have only one kind of atoms - like carbon - in the backbone (isochains) or different elements (heterochains). Concerning their chain architecture, polymers are subdivided into linear, branched, comb-like, crosslinked, dendritic, or star-like systems. 

Hyper-branched polymers are prepared in a single-step polymerization from ABX monomers. Thus, a perfectly branched structure is present in dendrimers, whereas irregular branching is present in hyper-branched polymers. Aluminum alkoxide-based initiators or tin-based catalysts have been successfully used for the preparation of, hyper-branched [160-162, 166-168], dendrimer-like star polymers [160], and star-shaped polymers. The first and second generations of the benzyl ester of 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) are effective initiators for the ROP of lactones (e-CL) in the presence of Sn(Oct)2. The... 

A macromonomer is a macromolecule with a reactive end group that can be homopolymerized or copolymerized with a small monomer by cationic, anionic, free-radical, or coordination polymerization (macromonomers for step-growth polymerization will not be considered here). The resulting species may be a star-like polymer (homopolymerization of the macromonomer), a comblike polymer (copolymerization with the same monomer), or a graft polymer (copolymerization with a different monomer) in which the branches are the macromonomer chains. 

Constraint release is likely to be very important in the relaxation of branched polymer liquids. However, if we ignore that complication, the stress relaxation modulus for a liquid of highly entan ed stars is given simply by Eq. 69 with v replaced by vi. The viscosity and recoverable compliance can then be calculated from Eq. 69 with Eqs. 25 and 26. 

Another important feature controlling the properties of polymeric systems is polymer architecture. Types of polymer architectures include linear, ring, star-branched, H-branched, comb, ladder, dendrimer, or randomly branched as sketched in Fig. 1.5. Random branching that leads to structures like Fig. 1.5(h) has particular industrial importance, for example in bottles and film for packaging. A high degree of crosslinking can lead to a macroscopic molecule, called a polymer network, sketched in Fig. 1.6. Randomly branched polymers and th formation of network polymers will be discussed in Chapter 6. The properties of networks that make them useful as soft solids (erasers, tires) will be discussed in Chapter 7. 

Figure C2.1.2. Polymers with linear and nonlinear ehain arehiteetures. The nonlinear polymers ean have branched chains. Short chains of oligomers can be grafted to the main chain. The chains may form a star-like structure. The chains can be cross-linked and form a network.

A very large number of morphologies can be found in the world of polymers and copolymers. Polymers can be linear, branched, comb-type, star-like, micelles, macrocyclic or cross-linked, when chains are linked together for copolymers the order can be random, alternating, in block or graft as illustrated in Figure 10.1. The order of the repeating units has to be specified, as different orders result in different properties. 

Dendrimers branch perfectly with star-like topologies whereas hyperbranched polymers have imperfectly branched structures. Dendritic polymers have numerous sites per molecule to couple to active species, making them ideal carriers for drug molecules or biomacromolecules [96]. 

A quantitative analysis of counterion localization in a salt-free solution of star-like PEs is carried out on the basis of an exact numerical solution of the corresponding Poisson-Boltzmann (PB) problem (Sect. 5). Here, the conformational degrees of freedom of the flexible branches are accounted for within the Scheutjens-Fleer self-consistent field (SF-SCF) framework. The latter is used to prove and to quantify the applicability of the concept of colloidal charge renormalization to PE stars, that exemplify soft charged colloidal objects. The predictions of analytical and numerical SCF-PB theories are complemented by results of Monte Carlo (MC) and molecular dynamics (MD) simulations. The available experimental data on solution properties of PE star polymers are discussed in the light of theoretical predictions (Sect. 6). 

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. 

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