Effect of Molecular Size

An example is the effect of DP on the rates of alkaline hydrolyses of poly (vinyl acetate) s. Rapid increases in the rates can be seen [19] in large, but not in small molecules, as the reactions progress. Solvents that are good for the products, like acetone–water mixtures, are used in these reactions. These are, however, poor solvents for the staring materials with high DP. Low molecular weight molecules are more soluble. This means that, at the start of the reaction, the large molecules are coiled up and the reactive sites not readily available. As the reactions progress, the chains unravel and the sites became more accessible with accompanying increases in the reaction rates. Because the small molecules are more soluble, the reactive sites are accessible from the start of the reactions, and the rates are constant. There are many report in the literature on the effects of chain conformation [19–25]. One example is radical bromination of poly (methyl styrene) [20] with N-bromosuccinimide-benzoyl peroxide or Br₂-K₂CO₃-light. ¹³C NMR spectroscopy shows differences in reactivities of the methyl groups in the 3 and 4 positions on the benzene rings between isotactic and atactic polystyrenes [20]. The differences in reactivities in poly (vinyl alcohol) s between isotactic (meso) and syndiotactic (dl-diol) portions of the polymers and between cis and trans acetals [26–28] is another example. In extending this to model compounds, reactions of stereo isomers of pentane-2,4-diol and heptane 1,4,6-triol with formaldehyde take place much faster for the meso than for the dl-diol portions [26–28]. Even more important are the steric effects imposed by restricted rotations. For instance, quaternizations of chloromethylated polyether sulfones exhibit decreasing rates at high degrees of substitution. This can be attributed to restricted rotations of the polymeric chains, because this phenomenon is not observed with more flexible chloromethylated polystyrene under identical conditions [23, 24].

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مواضيع ذات صلة

Halogenation

Reactions That Favor Large Molecules

Effects of Changes in Solubility

Paired Group and Neighboring Group Effects

Reactivity of Macromolecules

Polyalkanoates

Synthetic Methods for the Preparation of Nucleic Acids

DNA and RNA

Nucleic Acids

Chemical Modification of Proteins

Synthetic Methods for the Preparation of Polypeptides

Structures and Chemistry of Proteins