Star-shaped architecture linear polymers

Homopolymerization of macromonomer provides regular star- or comb-shaped polymers with a very high branch density as shown in Fig. 1 a,c,e. Such polymacromonomers, therefore, are considered to be one of the best models for understanding of branched architecture-property relationships. Their properties are expected to be very different from the corresponding linear polymers of the same MW both in solution and the bulk state. Indeed, during the past decade, remarkable progress has been accomplished in the field of static, dynamic, and hydrodynamic properties of the polymacromonomers in dilute and concentrated solutions, as well as by direct observation of the polymers in bulk. 

The branched architecture has great influence on the packing of molecular chains. In general, dendrimers have smaller hydrodynamic radius and the melt and solution viscosity of a hyperbranched polymer is expected to be lower than that of a parent linear polymer. Viscosity measurements performed with a cone viscometer confirmed the decrease of viscosity of star-shape polymers compared to the respective high molecular weight arms (polymers B-R-4 and C-R-4, Tables 1 and 2). This observation is consistent with the decrease of hydrodynamic volume observed for... 

Polymers are normally classified into four main architectural types linear (which includes rigid rod, flexible coil, cyclic, and polyrotaxane structures) branched (including random, regular comb-like, and star shaped) cross-linked (which includes the interpenetrating networks (IPNs)) and fairly recently the dendritic or hyperbranched polymers. I shall cover in some detail the first three types, but as we went to press very little DM work has been performed yet on the hyperbranched ones, which show some interesting properties. (Compared to linear polymers, solutions show a much lower viscosity and appear to be Newtonian rather than shear thinning [134].) Johansson [135] compares DM properties of some hyperbranched acrylates, alkyds. and unsaturated polyesters and notes that the properties of his cured resins so far are rather similar to conventional polyester systems. 

Fig. 4 Polymer architectures (a) linear polymers, (b) ring polymers, (c-f) branched polymers (c) graft polymers, (d) star-shaped polymers, (e) hyperbranched polymers, and (1) dendrimers...

Discriminating branched and star polymers from linear ones can always be achieved by measuring the properties in dilute solution. In fact, molecules having the same molar mass but different macromolecular architectures exhibit different transport and light scattering properties. More specifically, a branched macromolecule is more compact than a linear molecule having the same molar mass, and therefore it will display less friction and will diffuse more easily in the solvent. Viscometry can be used to detect branched structures, since the Mark-Houwink-Sakurada exponent (Eq. 2.23) for branched and star-shaped polymers is lower tiian that for the corresponding linear chain. Unfortunately, in order to measure the difference, one must have a sample made exclusively... 

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. 

Polymers are macromolecules based on a large number of repeating units (monomers) covalently bound in a chain-like molecular architecture with a variety of compositions, structures, and properties. A large diversity of synthetic strategies and monomer combinations can be used to generate linear or nonlinear polymers, such as star-shaped, comb (brush), or branched polymers, and dendrimers (Fig. 11.1). 

The field of dendritic architectures, as a general class of macromolecules, has found widespread interest in the past decades. Much has been achieved in the preparation of three-dimensional stractures such as comb- and star-shaped polymers and dendrimers. These materials have comparable physical and chemical properties to their linear analogous that make them very attractive for numerous applications [1-4]. 

The spherical architectures of highly branched macromolecules, such as dendrimers, star-shaped polymers, and hyperbranched polymers, have attracted much attention from the viewpoint of nanotechnology, because their numerous terminal units can be converted into various functional groups leading to novel nanomaterials (Zeng and Zimmerman, 1997 Hirao etal., 2007 Satoh, 2009). Thus, various types of dendrimers, star polymers, and hyperbranched polymers have been synthesized and their properties were compared to the linear analogues (Stiriba et al., 2002 Hirao et al., 2005). 

Architectural polymers (Hirao etal, 2005 Hadjichristidis etai, 2006) refer, in this chapter, to polymers that have finite molecular weights and are architecturally more complex than linear chains. They can be cyclic, star-shaped, combed, balloon-shaped, and H-shaped. Illustrated in Figure 24.1 are the structures of cyclic, balloon-shaped, and H-shaped polymers, as well as of a miktoarm star ABC triblock copolymer. 

Star polymers consist of several linear polymer chains connected at one point. Prior to the development of CRP, star molecules prepared by anionic polymerization had heen examined. However, due to the scope of ionic polymerization, the composition and functionality of the materials were limited. The compact structure and globular shape of stars provide them with low solution viscosity and the core-shell architecture facilitates entry into several applications spanning a range from thermoplastic elastomers (TPEs) to dmg carriers. Based on the chemical compositions of the arm species, star polymers can be classified into two categories homoarm star polymers and miktoarm (or heteroarm) star copolymers... 

Nowadays, the PDC seems to he experiencing a certain renaissance as it might enable the fractionation of polymers with the same chemistry hut differing in the molecular architecture. In this case, the difference in solubility, that is, of branched or star-shaped material in comparison to linear chains, is used to archive a separation as schematically depicted in Figure 16. 

The most popular click reaction is Huisgen 1,3-dipolar cycloaddition of azides to alkynes applicable to a very wide range of macro molecular architecture. It has been employed for the preparation of various polymer topologies including linear, star, hyperbranched, and H-shaped polymers. The general approach is illustrated in Scheme 70 for the preparation of linear block copolymer of EO with MMA and St. Anionically prepared PEO was functionalized with azide and used in copper-catalyzed click reaction with PMMA or PSt with alkyne moiety synthesized by using alkyne-flinctional ATRP initiator. It should be noted that alkyne functionality of hetero-functional ATRP initiator was protected with a trimethylsilyl... 

The polymer architecture affects the demixing behaviour of thermoresponsive polymers [562], On the basis of theoretical studies it is expected that, as a rule, branched macromolecules are more soluble than their linear analogues [563-565]. This prediction was confirmed experimentally in the case of a solution of star-like polystyrene in cyclohexane (an UCST-type phase separation) for which an increase in the degree of branching resulted in a decrease in the temperature of demixing [566, 567], On the basis of a review of water-soluble polymers of various shapes by Aoshima and Kanaoka [30], it appears that water-soluble polymers do not offer a uniform tendency in their LCST-type phase behaviour. 

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