Reactivity of Macromolecules

In consideration of various chemical reactions of macromolecules, the reactivity of their functional groups must be compared to those of small molecules. The comparisons have stimulated many investigations and led to conclusions that functional groups exhibit equal reactivity in both large and small molecules, if the conditions are identical. These conclusions are supported by theoretical evidence [1, 2]. Specifically, they apply to the following situations [1]:

1. Reactions that take place in homogeneous fluid media with all reactants, intermediates, and end products fully soluble. These conditions exist from the start to the end of the reactions.

 2. All elementary steps involve only individual functional groups. The other reacting species are small and mobile.

3. The steric factors in the low molecular weight compounds selected for comparison must be similar to those of the large molecules. The above can be illustrated by a few examples. For instance, the rates of photochemical cis–trans isomerization of azobenzene residues on the backbones of flexible polymeric chains are analogous to those of small molecules [3]. Another example is the activation energy for cis–trans isomerization of azo-aromatic polymers. It is the same for low molecular weight analogs [4]. A third example is an experiment in comparing conformational transitions of some eximers in large and small molecules. A sandwich complex forms between an excited aromatic chromophore, Ar, and a similar chromo phore in the ground state when irradiated with light of an appropriate wavelength. The conformation required by such an excimer can correspond to a prohibitive energy requirement for the unexcited molecule. All conformational transitions must take place during the lifetime of the excited state of the chromophore that is of the type [5]:

The ratio of the fluorescence intensity of an excimer and a normal molecule is a measure of the probability that the conformational transition takes place during the excited lifetime. A polyamide with only a small proportion of the following units was used for comparison:

Emissionspectraofdilutesolutionsoftheabovepolyamideanditslowmolecularweightanalogwere measured over a range of temperatures. They showed that the activation energies of the conformational transitions required for excimer formations are essentially the same for both materials [5]. In addition, all bimolecular activation-controlled reactions are independent of the degree of polymerization [6]. Simple SN2 reactions between reactive groups attached to chain ends of mono disperse macromolecules in a wide range of molecular weights are independent of the DP [7, 8] in the range of 20–2,000 [7]. This was shown on three different reactions. In the first one, the reactivities of chlorine-terminated low and high molecular weight polystyrenes towards polystyryllithium are equal in benzene and cyclohexane solvents:

In the second one, the reactivity of primary amine-terminated polyoxyethylenes with sulfonyl chloride-ended polyoxyethylenes in chloroform is also the same:

In the third one, chain length dependence of the propagation rates was measured in polymerizations of methyl methacrylate. In the range of DPs from 130 to 14,200, the propagation constant was shown to be independent of the chain length [8]. On the other hand, unequal reactivity was observed:

1. In bimolecular reactions that are diffusion-controlled.

2. When neighboring group participations become significant.

3. When the properties of the polymers in solution are altered by gelation.

4. When the tacticities of the polymers affect neighboring group interactions.

 5. When heterogeneous conditions affect accessibility to the reactive sites.

 There are special situations that can occur. For instance, electrostatic charges carried by the polymers may extend over long distances in solutions and may manifest themselves in reactions with charged reagents. Sometimes, chain flexibility or folding can cause functional groups to come together and interact, though they may be located well apart on the polymer backbone. Polymer solutions of this type are comparable to dispersions of individual droplets of concentrated solutions. Some statements above may require additional clarification. An elaboration, therefore, follows.

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مواضيع ذات صلة

Halogenation

Reactions That Favor Large Molecules

Effects of Changes in Solubility

Effect of Molecular Size

Paired Group and Neighboring Group Effects

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