Polymer, branched Ladder

This strategy may be realized by the use of reactive oligomers (RO), i.e. low-molecular weight compounds which may be converted to polymers of linear, branched, ladder and three-dimensional network structures. Of special importance are RO s which form cross-linked polymers since in this case materials with optimal values of heat and fire resistance, strength, chemical stability, atmospheric resistance, durability, etc. may be obtained. 

A linear high polymer can also be represented by the spaghetti-like strand shown in V and, in these terms, we can recognize branched polymers (VI) and random crosslinked polymers (VII). Ladder polymers (VIII) are also known ... 

FIGURE 1.1. Shapes of polymeric molecules, (a) linear polymer, (b) branched polymer, (c) star>shaped polymer, (d) comb-shaped polymer, (e) ladder polymei (f) semiladder polymer, and (g) network structure. 

Show examples of linear, branched, ladder, sheel, and cmsslinked polymer structures. 

Branched, ladder, and network polymers deviate from the linear macromolecules discussed up-to now. In order to remain fusible and plastic, the molecule must contain sufficient segments that are flexible and linear. Even without branches, linear molecules may have insufficient flexibility to melt. The linearpoly(p-phenylene), for example, is a rigid macromolecule (class 3 of Sect. 1.1.3), because rotation about its bonds does not change the molecular shape. One must thus watch in such molecules that sufficient flexibility exists for plastics applications. 

Some polymers are linear—a long chain of connected monomers. PE, PVC, Nylon 66, and polymethyl methacrylate (PMMA) are some linear commercial examples found in this book. Branched polymers can be visualized as a linear polymer with side chains of the same polymer attached to the main chain. While the branches may in turn be branched, they do not connect to another polymer chain. The ends of the branches are not connected to anything. Special types of branched polymers include star polymers, comb polymers, brush polymers, dendronized polymers [1], ladders, and dendrimers. A cross-linked polymer, sometimes called a network polymer, is one in which different chains are connected. Essentially the branches are connected to different polymer chains on the ends. These three polymer structures are shown in Figure 1.3. 

Figure 1 (a) Linear PE chain, (b) Polymer ring, (-CH2-)48- (c) PE chain with one branching point, (d) Comb-like polymer, (e) Star-like polymer, (f) Ladder polymer, (g) Randomly branched polymer. 

Synthesis of soluble, highly branched ladder polymers with long side chains was achieved via bridging of functionalized PPP precursors (Scheme 19) [49]. 

Depending on the reaction conditions, the solvent used and the length of the radical at the Si atom, the hydrolytic condensation of trifunctional halogen-containing compounds drastically changes the structure, composition and properties of polyorganosiloxanes formed as a result of hydrolytic condensation and polycondensation. This leads to the formation of branched (I), ladder (II) or cross-linked molecular structures in the polymer. 

At present polyorganosiloxanes are manufactured in the form of 1) oligomers with linear or cyclic chains (silicone liquids) 2) polymers with linear chains (silicone elastomers) 3) polymers with cyclolinear, ladder and branched chains. 

As we have already said, the reaction of the hydrolytic condensation of methyltrichlorosilane is largely determined by the type of the solvent used. In the presence of nonpolar solvents in water the process occurs at high speed the formed polymer precipitates. In case of polar solvents, which dissolve the monomer, polymer and water, the condensation takes place in a homogeneous medium, because methyltrichlorosilane is well-soluble in alcohols and ethers. It rules out the possibility of precipitation therefore, we obtain a soluble branched or ladder polymer. 

The distillation of toluene, which is accompanied by the condensation of trihydroxyphenylsilane, thus forming ladder polyphenylsilsesquioxane. Since the second stage of alcoholysis forms not only trihydroxyphenylsilane, but also acetoxydihydroxyphenylsilane, it is natural that the condensation forms ladder polyphenylsilsesquioxane with a defective structure (a link-varied polymer with a certain number of linear and branched elements in the chain). 

The polyalumophenylsiloxane obtained is a polydisperse mixture of polymer homologues consisting of the abovementioned ladder macromolecules and branched macromolecules of the following common formula [C6H5Si(OH)2.xO0.5x(O)]3AI n... 

The mechanisms of the condensation reactions of polysilanols have been the subject of numerous reports (for reviews, see References 181 and 182) and are catalysed by acids and by bases. A simple outline of the mechanisms is given in Scheme 10. The hydrolysis of RSiCh species where R is small and no precautions are taken to remove HC1 as it is formed leads to the formation of ladder and branched polymers via RSi(OH)3 or chlorosilanols. The presence of larger groups or the avoidance of acidic or basic conditions allows the intermediate silanols to be formed as outlined in Section n. 

Two types of new silicon-branched organosilicon polymers, linear and ladder polysilane structures, were produced from dihalo- and tetrahalodisilane, respectively, via alkali-metal-mediated reactions. Further investigations disclosed that the polymers may he useful as photoresists, semiconductors, ceramic precursors, and composite materials in high-technology fields. 

Polymers can be viewed as consisting of a backbone on which are attached atoms or groups of atoms. The polymer backbone may have a linear, branched, or network structure. More unusual polymer structures may have peculiar characteristics such as star, comb-like, ladder, or other structures. For linear polymers the backbone extends mainly in one dimension, for sheets in two dimensions, and for reticulate polymers in... 

From picosecond transient photoconductivity measurements on PPP films,22 we know that mobile charged states decay within 110 ps. In conventional routes to PPPs, defects like branched chains and large torsion angles of neighboring rings are known to occur. These defects act as shallow or deep traps for positive and negative polarons,38,39 which limit the mobility of charge carriers.40 The synthetic route toward the PPP-type ladder-polymers prevents the described defects and leads to a trap concentration of less than 1 trap per 1000 monomer units,28 whereas substi-... 

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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