Effect of Different Side Groups

The side groups attached to a polymer chain can have a more profound effect on the polymer properties than the skeleton itself. Side groups are responsible for several properties such as protecting the skeleton against chain cleavage reactions, solubility properties, steric and polar interactions between side groups on the same chain or different chains which determine the Tg, crystallinity and surface properties of the material. 

It is also possible to have halogen side groups, such as fluorine and chlorine. Polymers with fluorine in their side groups have extreme hydrophobicity and water insolubility. This raises the thermal and oxidative stability and confers solvent, fuel and oil resistance. Some examples of fluoro polymers are poly(tetrafluoroethylene) (Teflon), 

The Tg of carbon backbone polymers with ester side groups vary widely with the tacticity and the nature of the R groups. Atactic PMMA has a Tg of 105 °C while PVAc has a Tg of 28-31 °C. They are all nontoxic [6]. 

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Enough is now known about the effect of different side groups attached to a polyphosphazene chain to allow some general structure-property relationships to be understood. To a limited extent, these relationships allow the prediction of the properties of polymers not yet synthesized. Some general relationships will be described in the following sections, but specific properties associated with certain side groups are summarized in Table 3.1. 

Many investigators have studied dissolved solutions of poly(organosiloxanes) with different side groups at silicon atoms in the macromolecule backbones [32 - 35], These works show results of the studies of the effect of the side groups origin, their disposition and the influence of hydrodynamic and conformation parameters of macromolecules. 

Stolte, S., Arning, J., Bottin-Weber, U., Muller, A., Pimer, W.R., Welz-Biermann, U., Jastorff, B., Ranke, J., Effects of different head groups and functionalised side chains on the cytotoxicity of ionic liquids. Green Chem. 9, 760-767 (2007). 

Table VI shows the relative reactivities of various asym DAMs. An equi-molecular mixture of two kinds of asym DAMs was fed as a 5% benzene solution and hydrogenolyzed in order to check the effect of the methyl group on the reactivity. Two kinds of asym DAMs having similar reactivities were selected as a combination. The reaction conditions were temperature, 400°C H2/hydrocarbons molar ratio, 2. The contact time was changed since the reactivities of asym DAMs differed considerably according to their structures this made it possible to evaluate the different reactivities. Side reactions such as demethylation, isomerization, and disproportionation were negligible under these reaction conditions. The relative values for the reactivities of the asym DAMs shown in Table VI are determined when the value of 2,5-DMeDPM as a standard material is fixed at 100.

These results can be interpreted as the sum of the steric effects of the different substituents as depicted in 41. The effects of the two substituents R and phenyl more or less cancel each other out and what remains is the shielding effect of the alkoxycarbonyl group. Thus, a possible un-directing stereoelectronic effect by an electrophilic substituent which is perpendicular to the plane of the enolate (see COOCH3 in 41) has not been observed and instead substitution takes place from the less shielded side. 

Electrochemical oxidation of some catecholamines such as dopamine, L-dopa, and methyldopa has been studied using cyclic voltammetry. The catecholamines undergo intramolecular cyclization to form the corresponding o-quinone derivatives. The significant differences in the electrochemical behaviour of the catecholamines have been attributed to the effects of the side-chain carboxyl group.253 Electron-transfer reactions of 2,-deoxyguanosine-5,-monophosphate (dGMP) in phosphate buffers by cyclic... 

Interactions can be decreased by increasing the distance between the chains, for instance with long side chains, which lower Tg. This effect appears to be greater than the increase of chain stiffness, as shown in the examples below with PMP the more compact structure of the side group gives rise to a different balance of these counteracting effects. 

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