Chains with direct branching

Kinetic study of chain reactions with direct branching... 

Figure 12.8. Reaction possibilities of an active center in a chain reaction with direct branching...

Star-shaped polymer molecules with long branches not only increase the viscosity in the molten state and the steady-state compliance, but the star polymers also decrease the rate of stress relaxation (and creep) compared to a linear polymer (169). The decrease in creep and relaxation rate of star-shaped molecules can be due to extra entanglements because of the many long branches, or the effect can be due to the suppression of reptation of the branches. Linear polymers can reptate, but the bulky center of the star and the different directions of the branch chains from the center make reptation difficult. 

This was qualitatively shown in investigations of conformational behaviour and intramolecular mobility (IMM) of cholesterol-containing polymers in dilute solutions as of a function of solvent quality 134-136,185-l88) and temperature. Polarization luminescence provides one of the most fruitful methods for the evaluation of IMM l75,176). The method permits to get direct information about rotational mobility of the macromolecule as a whole, as well as about the mobility of the main chains and side branches. This is achieved via the attachment to macromolecules of so called luminescent markers (LM) — anthracylacyloxymethane groups in the case reported. Below are shown the chain fragments with LM which give information on the mobility of main chains (LM-1) and of side groups (LM-2) ... 

Branched-chain aminotransferases (BCATs) evolved from aspartate aminotransferases (AATs) showed a record 105- to 2 x 106-fold improvement in catalytic efficiency (kcat/KM). Not only were the 13-17 amino acid substitutions concentrated in the most active mutants, but all but one mutated amino acid residues are located far from the active site. With directed evolution, enantioselectivities can be improved on enantiounspecific enzymes (from E = 1.1 to 25.8) and even inverted to yield the opposite enantiomer in comparison to the wild type (40% d- to both 90% d- and 20% L-). 

Triisobutylaluminum is a colourless transparent liquid (the boiling point is 138 °C at 6.6 GPa), which dissolves well in hydrocarbons. When heated up to 140-160 °C in vacuum (the residual pressure is 35 GPa), triisobutylaluminum disintegrates into isobutene and diisobutylaluminum hydride. Unlike lower trialkyl derivatives of aluminum with the direct carbon chain (e.g. triethylaluminum), aluminumtrialkyls with a branched chain (triisobutylaluminum is one of them) are monomeric. 

In both, the metal atoms form infinite channels containing zigzag B chains with C atom branches as represented schematically in Figure 7.26. The metal atoms and branched chains stack in one direction to give two-dimensional MBC slabs. In the structural pictures, the most obvious difference in the structures is the way these slabs stack in the third dimension. However, the crucial structural difference actually lies within the slabs themselves the B-C zigzag chains differ. The B chain in YBC is regular but it is alternating in ThBC. 

Intramolecular pair correlation function e( r I of directly connected monomers within a sphere of radius r of a given monomer, for randomly branched polymers with N monomers made from ulcanizing linear chains with degree of polymerization Nq. Both axes have logarithmic scales. 

This idea may be clarified by a comparison with other branches of physics. Every department of deductive science must necessarily be founded on certain postulates which are regarded as fundamental. Frequently these fundamental postulates are so closely related to experiment that their acceptance follows directly upon the acceptance- of the experiments upon which they are based, as, for example, the inverse-square law of electrical attraction. In other cases the primary postulates are not so directly obvious from experiment, but owe their acceptance to the fact that conclusions drawn from them, often by long chains of reasoning, agree with experiment in all of the tests which have been made. The second law of thermodynamics is representative of this type of postulate. It is not customary to attempt to derive the second law for general systems from anything more fundamental, nor is it obvious that it follows directly from some simple experiment nevertheless, it is accepted as correct because deductions made from it agree with experiment. It is an assumption, justified only by the success achieved by its consequences. 

Only in exceptional short-lived applications are volatile plasticizers useful. Certain dihexyl phthalates are sufficiently low in volatility to be satisfactory for use in noncritical applications. Volatility loss from vinyl compositions is directly related to a plasticizer s vapor pressure and oxidation resistance. These are dependent upon molecular weight and structure. The effect of molecular structure can be illustrated with two isomeric dioctyl phthalates. Figure 16 shows that the straight chain ester with half the volatility confers much improved low-temperature flexibility compared with the branched structure. 

A variety of methods have been developed for the selective installation of chirality at oxindole C3 that involve transfer of functionality from indole C2. The Vedejs group has prepared a small collection of chiral DMAP (4-(lV,A-dimethylamino) pyridine) derivatives bearing conformationally restricted side chains that have been employed as nucleophilic catalysts to direct the transfer of indolyl C2 acetate or carboxylate groups to oxindole C3 with excellent enantiofacial selectivity [15]. As illustrated in Scheme 1, indolyl acetate 1 was converted to the chiral oxindole 3 (94%, 91% ee) using DMAP catalyst 2, while the opposite sense of enantioselec-tivity was observed when indolyl carboxylate 4 was treated with DMAP catalyst 5 to afford 6 as the major oxindole adduct (99%, 94% ee). In addition to probing modifications of the DMAP side chains, it was demonstrated that the overall rate of the reaction catalyzed by DMAP 2 was decreased when the indolyl acetate 1 contained a branched isopropyl substituent at C3 however, good enantioselectivity was observed with the branching substituent (82%, 94% ee). It was also... 

Functionalized olefins can be classified in two groups the <5-fimctionalized olefins in which the functional group is not directly branched on the double bond but on an alkyl chain of the olefin as in the case of oct-7-en-l-al or linoleic alcohol, and the a-functionalized olefins in which the functional group is directly branched on the double bond as in the case of methyl acrylate or phenyl vinyl ether. The results described for these two groups will be discussed separately. Hydroformylation of water-soluble olefins in two-phase system with water-insoluble catalysts is far beyond the scope of this chapter and will not be discussed here (1, 2]. 

---