Branched chain stmctures

Butane and all larger alkanes exhibit stmctural isomerism. Structural isomerism occurs when two molecules have the same atoms but different bonds. That is, the molecules have the same formulas but different arrangements of the atoms. For example, butane can exist as a straight-chain molecule (normal butane, or u-butane) or with a branched-chain stmcture (called isobutane), as shown in Figure 20.4. Because of their different stmctures, these structural isomers have different properties. 

This will lead initially to branched chain stmctures such as indicated schematically in Figure 2.10, G indicating a glycerol residue and P a phthalic acid residue. In due course these branched molecules will join up, leading to a cross-linked three-dimensional product. 

Figure C2.1.2. Polymers witli linear and nonlinear chain architectures. The nonlinear polymers can have branched chains. Short chains of oligomers can be grafted to tire main chain. The chains may fonn a. stor-like stmcture. The chains can be cross-linked and fonn a network.

Properties provided by the branched hydrocarbon chain stmcture of these PAO fluids include high viscosity index in the 130—150 range, pour points of —50 to —60° C for ISO 32 to 68 viscosity range (SAE lOW and SAE 20W, respectively), and high temperature stabifity superior to commercial petroleum products. In their use in automotive oils such as Mobil 1, some ester synthetic fluid is normally included in the formulation to provide sufficient solubihty for the approximately 20% additives now employed in many automotive oils. 

Thiuram Sulfides. These compounds, (8) and (9), are an important class of accelerator. Thiurams are produced by the oxidation of sodium dithiocarbamates. The di- and polysulfides can donate one or more atoms of sulfur from their molecular stmcture for vulcanization. The use of these compounds at relatively high levels with litde or no elemental sulfur provides articles with improved heat resistance. The short-chain (methyl and ethyl) thiurams and dithiocarbamates ate priced 2/kg. Producers have introduced ultra-accelerators based on longer-chain and branched-chain amines that are less volatile and less toxic. This development is also motivated by a desire to rninirnize airborne nitrosamines. 

Syn- and anti-orientations are possible and there is evidence that the anti-orientation does not favor orbital overlap such an orientation is favored with larger branched-chain substituents. A C-nmr study found that the TT-electron density on the vinyl P-carbon is lower as the reactivity of the monomer increases (20). Methyl vinyl ether exists almost entirely ia the syn-stmcture, a favorable orbital overlap situation, and MVE for this reason is less reactive to both polymerization and hydrolysis (21). 

The rate of solvent diffusion through the film depends not only on the temperature and the T of the film but also on the solvent stmcture and solvent-polymer iuteractions. The solvent molecules move through free-volume holes iu the films and the rate of movement is more rapid for small molecules than for large ones. Additionally, linear molecules may diffuse more rapidly because their cross-sectional area is smaller than that of branched-chain isomers. Eor example, although isobutyl acetate (IBAc) [105-46-4] has a higher relative evaporation rate than -butyl acetate... 

How does the stmcture of a cycloalkane differ from that of a straight-chain or branched-chain alkane (22.2)... 

Fig. 1.1 Examples of straight- and branched-chain aliphatic, alkenes, ahcychc and aromatic hydrocarbon stmctures...

Although polyethyleneimine possesses moderate catalytic effects on several hydrolysis reactions, the polymer modified with long alkyl chains show much enhanced reactivity toward hydrophobic substrates. Remarkable catalytic effects were observed in the hydrolysis of phenyl esters and a sulfate ester catalyzed by the methylene-imidazole-modified polymer. These results were explained as arising from the hi y branched, compact stmcture of the polyethyleneimine derivative. Klotz called the imidazole-containing polyethyleneimine synzyme (s)mthetic enzyme), on the basis of its stmctural characteristics and catalytic activity which are comparable to hydrolytic enzymes. 

According to the steric stmcture there can be three major configurations—a linear chain, a branched chain, or a cross-linked (network or 3-dimensional) chain, as shown in Figure 3-2. The linear chain has no branches or chemical links with neighboring chains (albeit with the existence of physical links, called entanglements). 

The highly ordered planar stmctures, as obtained by stereospecific polymerization, were mentioned in Section 2.3. In contrast to the branched chains, we deal here with linear chains where side substitute groups appear regularly on the same side of the plane, or alternatively. These structures are named isotactic or syndiotactic, respectively, and the degree of order is termed "tacticity." In both cases, good conditions for crystallization exist, even when the polymer itself is known as difficult to crystallize (as in the case of polystyrene or polymethylmethacrylate). Any orderless structure is named atactic. 

Figure 11.2 shows the stmctures of the first four alkanes ( = 1 to n = 4). Nat-mal gas is a mixture of methane, ethane, and a small amount of propane. We discussed the bonding scheme of methane in Chapter 10. The carbon atoms in all the alkanes can be assumed to be -hybridized. The structures of ethane and propane are straightforward, for there is oidy one way to join the carbon atoms in these molecules. Butane, however, has two possible bonding schemes resirlting in different com-poimds called n-butane ( stands for normal) and isobirtane. n-Butane is a straight-chain alkane because the carbon atoms are joined in a continuoirs chain. In a branched-chain alkane like isobutane, one or more carbon atoms are bonded to a nonterminal carbon atom. Isomers that differ in the order in which atoms are connected are called structural isomers. 

The copolymerizations of VDF with a variety of monomers, as well as their applications, were reviewed in Ameduri s paper [37], The structural variations present in these polymers include monomer inversions, monomer/stereo-sequence effects (in copolymers containing monomers with branch-forming stmcture elements), chain ends, and short/LCBs. The characterization of these polymers in early studies predominantly depended on ID-NMR and empirical chemical shift additivity rules [38,39]. With the improvement in both NMR hardware and pulse sequence development, 2D-NMR characterization of these polymers has become more popular. 

Paraffins - saturated hydrocarbons, straight or branched chains but no ring stmctures ... 

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