Silicone rubber for purposes of this review will cover materials based on the polymer polydimethylsiloxane (PDMS). Of necessity, groups other than methyl as substituents on silicon will be included, and changes in the backbone structure from that of pure siloxane will be discussed. Rubbers in which polydimethylsiloxane is only a minor constituent will not be a part of this paper. The definition of what constitutes a rubber becomes blurred as one considers elastoplastic materials and rubbery resins. In this review, a rubber is a material whose properties and function when cured depend principally upon an elastic response to stress. Curing means that a network of crosslinked polymer is established by any one of a number of different vulcanization reactions. Silicone rubber is unique in the large number of choices available for forming crosslinks. A crosslinked network of silicone rubber polymers is relatively weak; hence reinforcement by small-particle active fillers, such as silicas, is essential. The volume of literature, even in this specialized elastomer field, is large. This decade, centered on a date roughly 30 years after work began in this field, shows a high level of research and development activity. Computer searches of literature are common and, depending on the specific profile, the references emerging number around 2500. Clearly, even to annotate such a bibliography is beyond the scope of this paper. This review is aimed at a discussion of the significant developments of this past decade. Of necessity, this requires a judgment of the significance of each publication, a decision which could be debated. Many references will be omitted from consideration, but those wishing a more complete bibliography can obtain it from any of a number of commercial computer searches. Reviews of silicone rubber began to appear only 8 years after work began. Many of these were aimed at acquainting engineers with the properties of these newer rubbers and were not comprehensive reviews. Two reviews did cover the field effectively. Lewis gave a summary of the first 19 years of work and showed the state of the art at that time. Hunter gave a complete statement of the history of steps taken to improve the strength of silicone rubber. Other reviews have appeared in the decade of this review. Many of these, particularly in eastern European countries, are aimed at acquainting users with the properties and technology of silicone rubbers. Lewis updated his earlier review as a segment of a book. A number of reviews of specialized areas have appeared—e.g., encapsulating and potting, adhesives, radiation chemistry, biomedical applications, and elastoplastics. Another segment of a book by Bobear covered some of the same areas as Lewis but included applications of the elastomers. Polmanteer reviewed the chemistry and phenomenological behavior of silicone rubber. A specialized review of only patents in the field of silicones contains a segment on silicone rubber. The coverage is from 1970 forward to publication in 1977, and as a result fewer patents are mentioned than are covered in the present review. This review will attempt to cover the significant developments during the past decade in all of the above areas.

Cyclic structures in conventional organic chemistry have been well established. Early workers thought only five and siR membered carbon rings were possible, but it was later shown1 that strained rings of three and four members could exist.Continued studies with cyclic hydro-carbons2 led to the preparation of much larger unstrained rings containing up to eighteen carbon atoms.Carothers and Hill3 believed all the larger rings may not be entirely without strain, but they pointed out the effect of oxygen in the ring as a means of reducing strain.Cyclic structures have also been shown to exist in inorganic chemical systems. Particularly in the field of mineral silicates, Bragg4 has shown by X-ray analysis, cyclic structures containing six, * The material contained In this paper was presented in large part before the

Application of Bond Refractions to Organo-silicon Chemistry 2455 2.0 6.0 10.0 14.0 Total pressure, atm.Fig. 1.-Solubility of boron fluoride in re-pentane at constant temperature.however, clearly shown. Similar minima were reported by Wiebe, Gaddy and Heins3 for the solubility in nitrogen in water.

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A technique is described for quantitatively examining the diffraction pattern of polydimethylsiloxane at and below room temperature with the x‐ray diffractometer. The crystalline fraction of the unstretched material is determined by the classical method developed for natural rubber. In the case of the stretched elastomer, a modified procedure is required which takes into account the extent of preferred orientation of the molecular chains. As the extension ratio is increased, the crystallization temperature rises and preferential orientation of the crystallites with respect to the extension axis increases. The crystallinity increases with decreasing temperature and appears to be independent of the extension ratio below −60°C. An over‐all crystalline fraction of 0.42 was measured for a silicone rubber specimen at −60°C. and an extension ratio of 6.3.