Influence of Alkyl Side Chains

The effect of the alkyl side chain length on the physical and chemical properties of the PATs has been studied intensively by many authors and the results can be summarized as follows. 

With increasing alkyl side chain length  

The reduction potential (E i) of PAT films also increases (Table 11) [481], and 

These characteristics are similar to those of the N-alkyl substituted poly-(pyrrole)s [482]. The reason for these changes is the steric effect of the side chain. This leads to the changes in the interaction, the variation of the symmetry of the polymer chain, and the disturbance of the ring-ring planarity along the polymer chain [481]. The decrease in electrical conductivity with increasing interchain distance can also be observed by reducing the pressure [483,484]. 

However, electrochemically polymerized 3-alkyl-2,2 -bithiophenes (alkyl = hexyl, dodecyl) give conducting films whose electrical conductivities are independent of the length of the alkyl chains [485-487], 

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Table 8.4 Influence of the side chain on the alkylation of substrates 39a-g.

Table 1. Influence of the substituents on the oxidation of alkyl side chains.

Foster [32] studied the influence of polymer side-chain length by performing solution experiments on a homologous series of acrylate polymers. A regular decrease of stability in shear was observed as the side chain length was increased. The reason for this feature is unclear as yet. This point has been confirmed, however, in ultrasonic experiments [33]. Nogami [34], in contrast, showed that different alkyl ester groups on acrylate and methacrylate polymers had no effect on shear stability. 

Monomer Reactivity. The nature of the side chain R group exerts considerable influence on the reactivity of vinyl ethers toward cationic polymerization. The rate is fastest when the alkyl substituent is branched and electron-donating. Aromatic vinyl ethers are inherently less reactive and susceptible to side reactions. These observations are shown in Table 2. 

Sodium LAS synthesized via aluminum chloride catalysis dissolves better than sodium LAS from the hydrogen fluoride route. The main difference is the tetralin content. Dialkyltetralinsulfonates (DATs) function as hydrotropes and this influence can be larger than that of the 2-phenylalkane content. For a homolog (equal alkyl side chains), the higher the DAT content, the lower the... 

When alkylated with 3-(3-indolyl)ethyl bromide and subsequently treated with various aldehydes, AAs are transformed into a mixture of 121 and 122 having high to very high diastereoisomer ratios (from 72 28 to 98.5 1.5). The predominant isomer was shown to have the structure 121 from X-ray analysis. The diastereoselectivity is particularly influenced by the size of the side chain in the starting AA ester (93TL5867). 

Viscosity is often regulated by bubbling air through solutions of polysiloxanes or the liquid material itself. In this manner, alkyl side chains are oxidized and oxygen bridges set up between silicon atoms. Obviously, the greater the number of such cross links, other influences constant, the greater will be the viscosity... 

The electrolyte may also influence the product distribution. This can be demonstrated in the case of the side-chain oxidation of alkyl aromatics [73] ... 

Stability constants for Cu11 complexation of Gly—X and X—Gly dipeptides show that even relatively small alkyl side chains can influence coordination. 3 At low pH, for example, the bidentate chelates N(amino),0(peptide) of L-Leu-Gly and L-Ile-Gly are the thermodynamically less stable, presumably due to steric hindrance from the branched side chains. Similar coordination by Gly—X is virtually independent of the nature of X. This latter result is not unexpected since only the Gly moiety is coordinating. However, on raising the solution pH (> 4) the peptide deprotonation that occurs to give tridentate N(amino),N(peptide),0(carboxyl) chelation is slightly inhibited by the Leu and lie residues (by 0.7-0.8 log units). 

The ratio of ferrocene-modified siloxane subunits to unsubstituted siloxane subunits rrv.n ratio) was varied as was the length (a ) of the alkyl side chain onto which the ferrocene moiety was attached as shown in Fig. 3.3. The electrode containing co-polymer with m n ratio of 1 1 or 1 2 was the more efficient electron relay systems. The ferrocene-modified homopolymer on the other hand loses flexibility due to steric hindrance caused by the side chain substitution by ferrocene, preventing efficient electron transfer from the enzyme to the electrode. The length of the alkyl side chain onto which the ferrocene moiety is attached was also found to influence the electron transfer efficiency of the electron relay system. Maximal current density was measured... 

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