Star type architectures

The cationic polymerization method of Cl3P=NSiMe3 has many new applications. The preparation of dendrimeric polymers with star-type architecture illustrates the versatility of this method (Fig. 3.29) [21]. 

There are many polymer architectures beyond chains such as stars, combs, and brushes. An example of a star-type oligophenylene is structure 100.276 It can be described as possessing three oli-... 

Star-type and Bottle-Brush-type Architectures... 

It therefore becomes possible to take advantage of this peculiar mechanism to prepare palm-tree like architectures where the C ) core bears six PS chains of the same length and an additional seventh chain of a different length or/and a different chemical nature (e.g., polyisoprene). Furthermore, as the out-growing chain bears a terminal carbanion more reactive than the five remaining on the core, it becomes possible to specifically couple two palm-trees with, for example, dibromo-p-xylene to form dumbbell type architectures where two six-arm PS stars with a fullerene core are linked together by a PS or a polyisoprene chain [97, 98]. Scheme 5.17 gives a schematic representation of these palm-tree and dumbbell architectures. 

Monomers of die type Aa B. are used in step-growth polymerization to produce a variety of polymer architectures, including stars, dendrimers, and hyperbranched polymers.26 28 The unique architecture imparts properties distinctly different from linear polymers of similar compositions. These materials are finding applications in areas such as resin modification, micelles and encapsulation, liquid crystals, pharmaceuticals, catalysis, electroluminescent devices, and analytical chemistry. 

In a seminal and seemingly forgotten paper, Burchard et al. " discussed the analysis of various polymer architectures based on integrated light scattering (LS) and quasielastic light scattering (QELS). They considered mono- and polydisperse linear and star-branched polymers with/number of arms ( rays ), and random polycondensates of Af or ABC type (identical or different... 

A number of different types of copolymers are possible with ATRP—statistical (random), gradient, block, and graft copolymers [Matyjaszewski, 2001]. Other polymer architectures are also possible—hyperbranched, star, and brush polymers, and functionalized polymers. Statistical and gradient copolymers are discussed in Chap. 6. Functionalized polymers are discussed in Sec. 3-16b. 

Cationic synthesis of block copolymers with non-linear architectures has been reviewed recently [72]. These block copolymers have served as model materials for systematic studies on architecture/property relationships of macromolecules. (AB)n type star-block copolymers, where n represents the number of arms, have been prepared by the living cationic polymerization using three different methods (i) via multifunctional initiators, (ii) via multifunctional coupling agents, and (iii) via linking agents. 

The synthesis of hetero-arm star-block copolymers with well-controlled architecture such as AnBn- or ABC-type star-block copolymers, recently accomplished by a living anionic process utilizing a novel concept of the living coupling reaction [84], however, has been a challenge by a living cationic process. 

It is a three step procedure, using a divinyl compound in a similar manner as the DVB method. Stars of predetermined architectures can be prepared by this method but only polymers of the type A2B2 and ABC have been produced so far. More complicated structures such as AB3, AB5, AnBn (with n>2)or ABCD have not appeared in the literature. 

Depending on the composition of the monomer feed and the polymerization procedure, different types of heterogeneities may become important. For example, in the synthesis of tailor-made polymers telechelics or macromonomers are frequently used. These oligomers or polymers usually contain functional groups at the polymer chain end. Depending on the preparation procedure, they can have a different number of functional end groups, i.e. be mono-, bifunctional, etc. In addition, polymers can have different architectures, i.e. they can be branched (star- or comb-like), and they can be cyclic. 

Living polymerization processes pave the way to the macromolecular engineering, because the reactivity that persists at the chain ends allows (i) a variety of reactive groups to be attached at that position, thus (semi-)telechelic polymers to be synthesized, (ii) the polymerization of a second type of monomer to be resumed with formation of block copolymers, (iii) star-shaped (co)polymers to be prepared by addition of the living chains onto a multifunctional compound. A combination of these strategies with the use of multifunctional initiators andtor macromonomers can increase further the range of polymer architectures and properties. 

ABA tribiock, or all three can be different, as in an ABC triblock copolymer. Obviously, the number of possible block sequences increases rapidly with the number of blocks and the number of different types of block in the chain. One can also synthesize block copolymers with branched architecture, such as star-branched block copolymers, in which each of the arms of the star contains either the same or different block sequences (see Fig. 13-1). One or more of the blocks could also be stiff or liquid crystalline (Chiellini et al. 1994 Chen et al. 1996 Radzilowski et al. 1997 Jenekhe and Chen 1998). For a given type of block copolymer, the degree of polymerization N of the whole molecule, or the degree of polymerization Ni of one or more of the blocks, can be varied. Thus, the number of different types of block copolymers is practically endless. 

A special type of network architecture that deserves mention is the Attached Resource Computer Network (ARCNet). Developed in 1977, it was not based on any existing IEEE 802 model. However, ARCNet is important to mention because of its ties to IBM mainframe networks and also because of its popularity. Its popularity comes from its flexibility and price. It is flexible because its cabling uses large trunks and physical star configurations, so if a cable comes loose or is disconnected, the network will not fail. Additionally, since it used cheap, coaxial cable, networks could be installed fairly cheaply. 

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... 

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. 

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