Asymmetric star architectures

Asymmetric star polymers are megamolecules [1] emanating from a central core. In contrast to the symmetric stars very little was known, until recently, about the properties of the asymmetric stars. This was due to the difficulties associated with the synthesis of well-defined architectures of this class of polymeric materials. The synthesis, solution and bulk properties, experimental and theoretical, of the following categories of asymmetric stars will be considered in this review ... 

Only a few studies have been devoted to the bulk properties of asymmetric homopolymer stars. The main issue under investigation was, up to now, the selfdiffusion and viscoelastic behavior of three-arm stars where the molecular weight of the third arm was varied in order to observe the transition in the diffusion from linear polymer to a polymer with a star architecture. 

The drawbacks associated with this method have been already mentioned. Probably the most important is the architectural limitation, i.e., only asymmetric stars of the type AnA n can be prepared by this method. However, even these structures are not unambiguously characterized. A fraction of the living arms A is not incorporated in the star structure, probably due to steric hindrance effects. These living chains may act as initiators for the polymerization of the monomer that is added for the preparation of the asymmetric star. Another problem is that the active sites of the living An star are not equally accessible to the newly added A monomers, due to steric hindrance effects. Furthermore, the rate of initiation is not the same for these active sites. For all these reasons, it is obvious that the final products are structurally ill-defined with a rather great dis-persity of the n values and are characterized by broad molecular weight distributions. Nevertheless, this method is technically important, since it can be applied on an industrial scale and also provides the... 

A different approach was used by Milner [326] in order to predict the phase diagram for asymmetric copolymer architectures (for example A2B, A3B etc. types of miktoarm stars). The free energy of the system can be calculated by summing the free energies of the polymer brushes existing on the two sides of the interphase. Milner described the effects of both chain architecture (i.e., number of arms) and elastic (conformational) asymmetry of the dissimilar chains, in the strong segregation limit, by the parameter... 

Adhikari R, Michler GH, Lebek W, Goerhtz S, Weidisch R, Knoll K. Morphology and micromechanical deformation behavior of SB-block copolymers. II. Influence of molecular architecture of asymmetric star block copolymers. J Appl Polym Sci 2002 85(4) 701-713. 

The success of the slip link model for symmetric and asymmetric star polymers inspires its application to more complex architectures, such as H polymers. The mechanisms of relaxation and branch point motion were established in studies of symmetric and asymmetric star polymers, as were the parameters (Mf,, and Tg) that allow the simulations to be compared... 

FIGURE 2.1 Various possible architectures for amphiphilic copolymer (a) linear block copolymers with different numbers of A and B blocks, (b) cyclic block copolymers, (c) star block copolymers, (d) graft block copolymers, (e) block copolymers with dendritic or hyper-branched blocks, and (f) semitelechelic polymer (upper), telechelic polymer (middle), and asymmetrical telechelic polymer with different hydrophobic chain ends [9]. 

In this case, chlorosilane reagents (XVIII, XIX), halomethyl benzene reagents (XX), and 1,1-diphenylethylene (DPE) derivatives (XXI) carrying aUsyUialide functions are typical coupling agents used to deactivate anionically derived polymers, mostly PS, PB, and PI. A variety of star-like polymers of precise functionality, including multiarm stars, star-block copolymers, asymmetric and miktoarm stars, and other branched architectures, are accessible in this way [1, 12-14, 35]. This... 

Another significant application of the ADMET polymerisation relates to the preparation of star-shaped polymers, which are branched macromolecules in which several linear polymer chains are attached to a unique branching point or core [48]. Montero de Espinosa, Winkler and Meier [49] described an ADMET approach to obtain those architectures (three- and four-arm) using small tri-acrylates and tetra-acrylates. More recently, Unverferth and Meier [50] reported the synthesis of well-defined star-shaped polymers via a head-to-tail ADMET polymerisation whereby di(trimethylolpropane)tetra-acrylate (four-arm) and dipentaerythritol hexa-acrylate (six-arm) served as core units, and fatty acid-derived 10-undecenyl acrylate as asymmetric a,(0-diene monomers. In this case, star-shaped polymers containing arms of 10 or 20 monomer units with an a,(0-unsaturated ester backbone and their subsequent post-polymerisation via a base-catalysed Thia-Michael addition were prepared. 

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