POLYMERIZATION, in chemistry, is the process whereby two or more atoms or molecules of the same substance unite to give a more complex molecule; the resulting "polymeride" there f ore has the same percentage composition as the original sub stance, but a molecular weight which is 2, 3 or 4, etc., times as great. This is the sense in which the term was originally used (see the articles ISOMERISM and ASSOCIATION) . But there is a tendency to restrict its use to cases in which the chemical and physical properties of the substance are altered in the process, the term association being conveniently applied to cases in which it has not hitherto been possible to distinguish clearly between the properties of the simple and the more complex substance. More over, once a substance has polymerized it is not readily recon verted to the simpler compound, whereas in association the two are in a constant state of change into one another, i.e., they are in equilibrium. Water (q.v.) is undoubtedly a mixture of (prob ably) (H20)2, and molecules which have not yet been obtained as separate liquids; it is therefore said to be asso ciated. On the other hand when acetylene gas, C2H2, is passed through a red-hot tube, some of it is polymerized into liquid benzene, ; or when the volatile, mobile liquid isoprene, C5H8, is warmed, or even kept for a short time, it polymerizes first to dipentene, CioHis, and then to a rubber-like substance, (C5H8)n, of very high molecular weight (see TERPENES) . In these two cases the properties of the polymerides are very different from those of the original substances, and separation of the two is easy. It should be emphasized, however, that there is no very clear line of demarcation between association and polymerization, and the distinction is chiefly one of convenience.
Cases of polymerization among inorganic substances are dealt with under the heading of ASSOCIATION, since it is only in a few cases, such as sulphur and phosphorus, that different polymerides, e.g., 54, and 58, can be identified and shown to have different properties (see ALLOTROPY). The following examples are there fore restricted to organic compounds.
Formaldehyde (q.v.), a gas which is usually sold as a 40% aqueous solution ("formalin"), polymerizes to paraf ormal dehyde, probably when the solution is concentrated. When this is carefully heated it gives rise to trioxymethylene, a white crystalline solid which is also formed spon taneously and rapidly by anhydrous liquid formaldehyde when kept below its boiling point ( —2 I C) . Further, when formalin is
kept with milk of lime at the ordinary temperature, it gradually changes to "formose," "methylenitan," or " a -acrose," a com plex mixture of sugars of the formula or Acetaldehyde (q.v.), also gives rise to a number of polymerides, e.g., (I) aldol, which is pro duced by the action of dilute hydrochloric acid or of zinc chloride at the ordinary temperature, (2) paraldehyde, formed by the addition of one drop of concentrated sulphuric acid ; it has a much higher boiling point than acetaldehyde (124° C as compared with 21° C), and is used as a soporific, (3) metalde hyde, also produced by the action of acids at low temp erature, a crystalline solid which sublimes unchanged but is re converted to acetaldehyde on long heating. Acrolein, a volatile liquid, polymerizes slowly to disacryl, a white amorphous mass; or when warmed with alkalis it gives metacrolein, a solid melting at 45° C which is of use in the manufacture of synthetic resins (q.v.).
Cyanic acid, cyanamide and their derivatives furnish examples of ready and complex polymerization. The acid is N C•OH (or HN:CO), isomeric with fulminic acid (C:N•OH); at ordinary temperatures its aqueous solutions rapidly give cyamelide, a white, porcelain-like insoluble mass. The esters of cyanic acid are unknown, but the isomeric isocyanic esters, e.g., rapidly polymerize to isocyanuric esters heating at 150° C. Further, just as acetylene gives benzene, so bromoacetylene, CH:. CBr, gives tribromobenzene, r3, but even more readily, since the change in this case occurs at the ordinary temperature. Many of the higher olefines (see CHEM ISTRY, ORGANIC: Aliphatic Division) polymerize readily under the influence of dilute acids; thus isoamylene, gives di- and tri-isoamylenes, and C15H30. Esters of cinnamic acid slowly polymerize on keeping, but more rapidly in sunlight, to truxillic and truxinic esters: The foregoing examples illustrate the great variety of possibili ties in polymerization. Many synthetic resins which are finding increasing industrial application, owe their characteristic proper ties to the fact that their high molecular weight is the result of polymerization. (A. D. M.) POLYMETHYLENES: see CHEMISTRY, ORGANIC, Homo cyclic Compounds.