Star structure branched polymers

Highly branched polymers, polymer adsorption and the mesophases of block copolymers may seem weakly connected subjects. However, in this review we bring out some important common features related to the tethering experienced by the polymer chains in all of these structures. Tethered polymer chains, in our parlance, are chains attached to a point, a line, a surface or an interface by their ends. In this view, one may think of the arms of a star polymer as chains tethered to a point [1], or of polymerized macromonomers as chains tethered to a line [2-4]. Adsorption or grafting of end-functionalized polymers to a surface exemplifies a tethered surface layer [5] (a polymer brush ), whereas block copolymers straddling phase boundaries give rise to chains tethered to an interface [6],... 

The core first method has been applied to prepare four-arm star PMMA. In this case selective degradation of the core allowed unambiguous proof of the star structure. However, the MWD is a little too large to claim that only four-arm star polymers are present [81], Comb PMMAs with randomly placed branches have been prepared by anionic copolymerization of MMA and monodisperse PMMA macromonomers [82], A thorough dilute solution characterization revealed monodisperse samples with 2 to 13 branches. A certain polydispersity of the number of branches has to be expected. This was not detected because the branch length was very short relative to the length of the backbone [83]. Recently, PMMA stars (with 6 and 12 arms) have been prepared from dendritic... 

In 1959, Zimm and Kilb (34) made some calculations of the intrinsic viscosities of certain branched polymer molecules, taking into account the hydrodynamic interaction between portions of the polymer chain, using a modification of the Rouse procedure. They carried out these difficult calculations for a quite restricted range of models, obtaining numerical results for equalarmed stars with 3, 4, and 8 branches, and for one modified star with 2 long branches and 8 shorter branches. They found that their numerical results for this set of structures could be approximately represented by ... 

Expansions of the forms of Eqs. (6.1) and (6.6) hold also for branched polymers, though the numerical values of the coefficients are different, and depend on the type of branching. The coefficient al has been obtained (25) for regular star and symmetrical comb structures it is found that for these at is greater than the value 134/105 for linear polymers. This implies that as the temperature is increased above T= , branched molecules expand more rapidly than linear ones, so that for T> 0, g> g0. For molecules having many short branches on a long backbone, as the number of branches increases at approaches a limit ... 

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 number of well-defined macromonomers differing in the types of the monomer and the end functionality have been made available in these two decades. Their polymerization and copolymerization have provided a relatively easy access to a variety of branched polymers and copolymers, including comb-, star-, brush-, and graft-structures. Progress will no doubt continue to disclose further different types of macromonomers and branched polymers. 

Characterization of the poly(macromonomers) prepared by homopolymerization has proved that they provide a useful probe for discussing the structural characteristics of the star and brush polymers. Graft copolymers have been and will be a most important area of application of the macromonomer technique since a variety of multi-phased and microphase-separated systems can easily be designed just by an appropriate combination of a macromonomer and a conventional monomer. In general, however, characterization of their absolute MW, branch/backbone composition as well as their distributions remain to be studied in more detail. 

Aside from the linear architecture, BCs can be prepared with advanced architectures such as miktoarm star structures, i.e., BCs where arms of different chemical nature are linked to the same branch point [20]. Unique segregation properties are expected of these polymers [21, 22]. 

Investigations on star polymers have determined that they constitute unique three-dimensional structures among the branched polymers since there is a large number of arm chains radiating from the central core and each... 

The data shown in Fig. 10 a represent studies on unfractionated materials, possibly containing some linear chains as a low percentage contaminant. Values of g computed as g = pj(p - - 1) (/> + 2) for these polydisperse mixtures (JSS) may be slightly too low. In other instances g was calculated for the regularly branched polymer from the known structure g = (3p — 2)// for star-branched chains 233) ... 

The self-condensing copper-catalyzed polymerization of macromonomer of poly(tBA) with a reactive C—Br bond (H-6) affords hyperbranched or highly branched poly(tBA).447 Copolymerization of H-1 and TV-cyclohexylmaleimide induced alternating and self-condensing vinyl polymerization.448 The residual C—Cl bond was further employed for the copper-catalyzed radical homopolymerization of styrene to give star polymers with hyperbranched structures. Hyperbranched polymers of H-1 further serve as a complex multifunctionalized macroinitiator for the copper-catalyzed polymerization of a functional monomer with polar chromophores to yield possible second-order nonlinear optical materials.325... 

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. 

An excellent summary of — M data by Fetters et al. provides estimates for these parameters in both 6 and good solvents for a number of linear polymersJ The parameters in Eq. (19) are not only influenced by the experimental conditions (solvent, temperature) but are also affected by the polymer s structure. For monodisperse stars, it has been found that increasing the number of arms decreases K g while u remains identical to that of the linear polymer (see Fig. 1). However, for randomly branched polymers, it has been found that u is closer to 0.5 and in some cases much lower. Such low values of this exponent might be considered as an indication of the branched polymer being in an unperturbed state however, this is not the case. An explanation as to why u is so small for randomly branched polymers has been found using fractal behavior, and an overview is given by Burchard. ... 

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