ASSOCIATION AND POLYMERIZATION S.
T. BOWDEN, UNrVZRSIlY COLLEGE OP SOUm WALES AND MONMOUTHSHIRE, CARDIP*, WALES
As a result of recent work on the molecular complexity of liquids and gases the phenomena of association and polymerization have obtained an added importance and significance. However, a considerable diversity of opinion is evidenced in the literature and standard works of chemistry as to the precise method of specifying the various types of combination which obtain between molecules of the same kind. It is the purpose of the present paper to discuss briefly the chemical combination of like molecules with a view to a more precise classification of the fundamental features. According to Mellorl the doubling or trebling of molecules as a result of chemical combination is called polymerization, and polymerism is the term applied to the special case of isomerism when the percentage composition of two or more substances is the same, but the molecular weight is different. Nernst2 employs the expression polymerization of liquids and reserves the term association to specify the combination of a solute ion with a neutral molecule of solvent. Many a u t h o ~ showever, ,~ use the terms synonymously, but Alexander Smith4 refers to mixtures of gaseous molecules which are multiples of the simplest possible as associated vapors, and liquids composed of more complex aggregates than the simplest possible as associated liquids. Moreover, he points out that in all such cases dissociation into simpler molecules takes place gradually as the temperature is raised. On the contrary, when a substance changes sharply into another substance, as formaldehyde into para-formaldehyde, tpe phenomenon is called polymerization. Although the terms polymerization and association are frequently applied to the formation of more complex aggregates of elements, it will be convenient to confine the present discussion to the consideration of the molecular complexity of compounds as distinct from that of the elements. The combination between like molecules may be broadly divided into two classes: (a) the combination between "odd" molecules, i. e., molecules of odd molecnlar number, indicating a free bond or free valence electron, and (b) the combination between molecules of even molecular number. In class (a) reversibility is the salient feature, an equilibrium being produced depending on the temperature, pressure, and the nature of the solvent. This class comprises the so-called "free radicals" ; the triarylmethyls of Gomberg, the divalent nitrogen compounds of Wieland, the metal ketyls "Modern Inorganic Chemistry," Longmans. Green and Co.. 1919. p. 549; "Treatise on Inorganic andYTheoretical Chemistry," Longmans, Green and Co., 1927, Vol. I, p. 856; Vol. V, p. 722.
' "Theoretical Chemistry," Macmillan and Co., 1923. Whadwell, Chem. Reviews, 4, 375 (1927). ' "Introduction to Inorganic Chemistry," The Century Co., 1322
1923, p. 282.
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of Schlenk, and the simple inorganic odd molecules like chlorine peroxide and nitrogen peroxide. This type of combination is to be regarded as association, and results in the formation of a potentially non-polar, covalent linkage. The reversal of the process involves the generation of a free valence and is thus correctly designated molecular dissociation. On the other hand, in class (b) the combination occurs between molecules of even molecular number and two fundamentally different types of union may be recognized: (1) irreversible combination, as for example the union of three molecules of acetylene to give benzene, or the formation of para-formaldehyde from three molecules of formaldehyde, and (2) reversible combination, as occurs in the case of (HeO), in the liquid state, or (AlCl& in the gaseous state. In the latter case an equilibrium is set up between molecular complexes of even molecular number, whereas i n the free radical systems the equilibrium obtains between odd and even numbered molecular entities. If we examine the three above-mentioned types of union in the light of the modern conception of valence, i t will he evident that the chemical character of the combination is different in each case, as indicated in the following scheme: Example
1. Odd Molecule Combination
2PhaC
* P h C . CPh3
2. Even Molecule Combination 3C2Hz -r CsHs (Irreversible) 3. Even Molecule Combination 2H10 (H20)2 (Reversible)
*
Type of linkage
Normal Association covalent Normal Polymerization covalent Semi-polar Semi-polar or association coordinate
On the premises of this classification, the first type of combination is termed association, the second type polymerization, while the third type may be conveniently designated snni-polar association. In the third type of union, one of the atoms of the molecule functions as donor and another as acceptor atom.526 The term semi-polar association gives expression to the fact that the combination of the like molecules occurs through the formation of the coordinate or semi-polar link. 6
Sidgwick, "The Electronic Theory of Valency." Oxford University Presr;, 1927,
p. 52.
q u g d e n , "The Parachor and Valency," Routledge and Sons, Ltd., 1930, p. 114.
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